Mont MA

Over the past 3 decades, total knee arthroplasty (TKA) has been considered a safe and effective treatment for end-stage knee arthritis.¹ However, as the population, the incidence of obesity, and life expectancy continue to increase, the number of TKAs will rise as well.^2,3 It is expected that over the next 16 years, the number of TKAs performed annually will exceed 3 million in the United States alone.⁴ This projection represents an over 600% increase from 2005 figures.⁵ Given the demographic shift expected over the next 2 decades, patients are anticipated to undergo these procedures at younger ages compared with previous generations, such that those age 65 years or younger will account for more than 55% of primary TKAs.⁶ More important, given this exponential growth in primary TKAs, there will be a concordant rise in revision procedures. It is expected that, the annual number has roughly doubled from that recorded for 2005.⁴

Compared with primary TKAs, however, revision TKAs have had less promising results, with survivorship as low as 60% over shorter periods.^7,8 In addition, recent studies have found an even higher degree of dissatisfaction and functional limitations among revision TKA patients than among primary TKA patients, 15% to 30% of whom are unhappy with their procedures.^9-11 These shortcomings of revision TKAs are thought to result from several factors, including poor bone quality, insufficient bone stock, ligamentous instability, soft-tissue incompetence, infection, malalignment, problems with extensor mechanisms, and substantial pain of uncertain etiology.

Despite there being several complex factors that can lead to worse outcomes with revision TKAs, surgeons are expected to produce results equivalent to those of primary TKAs. It is therefore imperative to delineate the objective and subjective outcomes of revision techniques to identify areas in need of improvement. In this article, we provide a concise overview of revision TKA outcomes in order to stimulate manufacturers, surgeons, and hospitals to improve on implant designs, surgical techniques, and care guidelines for revision TKA. We review the evidence on 5 points: aseptic survivorship, functional outcomes, patient satisfaction, quality of life (QOL), and economic impact. In addition, we compare available outcome data for revision and primary TKAs.

1. Aseptic survivorship

Fehring and colleagues¹² in 2001 and Sharkey and colleagues¹³ in 2002 evaluated mechanisms of failure for revision TKA and reported many failures resulted from infection or were associated with the implant, and occurred within 2 years after the primary procedure. More recently, Dy and colleagues¹⁴ found the most common reason for revision was aseptic loosening, followed by infection. The present review focuses on aseptic femoral and tibial revision.

The failure rate for revision TKA is substantially higher than for primary TKA with the same type of prosthesis because of the complexity of the revision procedure, the increasing constraint of the implant design, and the higher degree of bone loss. (Appendix 1 lists risk factors for revision surgery. Appendix 2 is a complete list of survivorship outcomes of revision TKA.)

Sheng and colleagues¹⁵ in 2006 and Koskinen and colleagues¹⁶ in 2008 analyzed Finnish Arthroplasty Register data to determine failure rates for revision and primary TKA. Sheng and colleagues¹⁵ examined survivorship of 2637 revision TKAs (performed between 1990 and 2002) for all-cause endpoints after first revision procedure. Survivorship rates were 89% (5 years) and 79% (10 years), while Koskinen and colleagues¹⁶ noted all-cause survival rates of 80% at 15 years. More recently, in 2013, the New Zealand Orthopaedic Association¹⁷ analyzed New Zealand Joint Registry data for revision and re-revision rates (rates of revision per 100 component years) for 64,556 primary TKAs performed between 1999 and 2012. During the period studied, 1684 revisions were performed, reflecting a 2.6% revision rate, a 0.50% rate of revision per 100 component years, and a 13-year Kaplan-Meier survivorship of 94.5%. The most common reasons for revision were pain, deep infection, and tibial component loosening (Table 1).

Posterior stabilized implants

Laskin and Ohnsorge¹⁸ retrospectively reviewed the cases of 58 patients who underwent unilateral revision TKA (with a posterior stabilized implant), of which 42% were for coronal instability and 44% for a loose tibial component. At minimum 4-year follow-up, 52 of the 58 patients had anteroposterior instability of less than 5 mm. In addition, 5 years after surgery, aseptic survivorship was 96%. Meijer and colleagues¹⁹ conducted a retrospective comparative study of 69 revision TKAs (65 patients) in which 9 knees received a primary implant and 60 received a revision implant with stems and augmentation (60 = 37 posterior stabilized, 20 constrained, 3 rotating hinge). Survival rates for the primary implants were 100% (1 year), 73% (2 years), and 44% (5 years), and survival rates for the revision implants were significantly better: 95% (1 year), 92% (2 years), and 92% (5 years) (hazard ratio, 5.87; P = .008). The authors therefore indicated that it was unclear whether using a primary implant should still be an option in revision TKA and, if it is used, whether it should be limited to less complex situations in which bone loss and ligament damage are minimal (Table 2).

Constrained and semiconstrained implants

In a study of 234 knees (209 patients) with soft-tissue deficiency, Wilke and colleagues²⁰ evaluated the long-term survivorship of revision TKA with use of a semiconstrained modular fixed-bearing implant system. Overall Kaplan-Meier survival rates were 91% (5 years) and 81% (10 years) at a mean follow-up of 9 years. When aseptic revision was evaluated, however, the survival rates increased to 95% (5 years) and 90% (10 years). The authors noted that male sex was the only variable that significantly increased the risk for re-revision (hazard ratio, 2.07; P = .02), which they attributed to potentially higher activity levels. In 2006 and 2011, Lachiewicz and Soileau^21,22 evaluated the survival of first- and second-generation constrained condylar prostheses in primary TKA cases with severe valgus deformities, incompetent collateral ligaments, or severe flexion contractures. Of the 54 knees (44 patients) with first-generation prostheses, 42 (34 patients) had a mean follow-up of 9 years (range, 5-16 years). Ten-year survival with failure, defined as component revision for loosening, was 96%. The 27 TKAs using second-generation prostheses had a mean follow-up of about 5 years (range, 2-12 years). At final follow-up, there were no revisions for loosening or patellar problems, but 6 knees (22%) required lateral retinacular release of the patella (Table 3).

Rotating hinge implants

Neumann and colleagues²³ evaluated the clinical and radiographic outcomes of 24 rotating hinge prostheses used for aseptic loosening with substantial bone loss and collateral ligament instability. At a mean follow-up of 56 months (range, 3-5 years), there was no evidence of loosening of any implants, and nonprogressive radiolucent lines were found in only 2 tibial components. Kowalczewski and colleagues²⁴ evaluated the clinical and radiologic outcomes of 12 primary TKAs using a rotating hinge knee prosthesis at a minimum follow-up of 10 years. By most recent follow-up, no implants had been revised for loosening, and only 3 had nonprogressive radiolucent lines (Table 4).

Endoprostheses (modular segmental implants)

In a systematic review of 9 studies, Korim and colleagues²⁵ evaluated 241 endoprostheses used for limb salvage under nononcologic conditions. Mean follow-up was about 3 years (range, 1-5 years). The devices were used to treat various conditions, including periprosthetic fracture, bone loss with aseptic loosening, and ligament insufficiency. The overall reoperation rate was 17% (41/241 cases). Mechanical failures were less frequent (6%-19%) (Table 5).

2. Functional outcomes

The goal in both primary and revision TKA is to restore the function and mobility of the knee and to alleviate pain. Whereas primary TKAs are realistically predictable and reproducible in their outcomes, revision TKAs are vastly more complicated, which can result in worse postoperative outcomes and function. In addition, revision TKAs may require extensive surgical exposure, which causes more tissue and muscle damage, prolonging rehabilitation. (Appendix 3 is a complete list of studies of functional outcomes of revision TKA.)

This discrepancy in functional outcomes between primary and revision TKA begins as early as the postoperative inpatient rehabilitation period. Using the functional independence measurement (FIM), which estimates performance of activities of daily living, mobility, and cognition, Vincent and colleagues²⁶ evaluated the functional improvement produced by revision versus primary TKA during inpatient rehabilitation. They compared 424 consecutive primary TKAs with 138 revision TKAs. For both groups, FIM scores increased significantly (P = .015) between admission and discharge. On discharge, however, FIM scores were significantly (P = .01) higher for the primary group than the revision group (29 and 27 points, respectively). Furthermore, in the evaluation of mechanisms of failure, patients who had revision TKA for mechanical or pain-related problems did markedly better than those who had revision TKA for infection.

Compared with primary knee implants, revision implants require increasing constraint. We assume increasing constraint affects knee biomechanics, leading to worsening functional outcomes. In a study of 60 revision TKAs (57 patients) using posterior stabilized, condylar constrained, or rotating hinge prostheses, Vasso and colleagues²⁷ examined functional outcomes at a median follow-up of 9 years (range, 4-12 years). At most recent follow-up, mean International Knee Society (IKS) Knee and Function scores were 81 (range, 48-97) and 79 (range, 56-92), mean Hospital for Special Surgery (HSS) score was 84 (range, 62-98), and mean range of motion (ROM) was 121° (range, 98°-132°) (P < .001). Although there were no significant differences in IKS and HSS scores between prosthesis types, ROM was significantly (P < .01) wider in the posterior stabilized group than in the condylar constrained and rotating hinge groups (127° vs 112° and 108°), suggesting increasing constraint resulted in decreased ROM. Several studies have found increasing constraint might lead to reduced function.^28-30

However, Hwang and colleagues³¹ evaluated functional outcomes in 36 revision TKAs and noted that the cemented posterior stabilized (n = 8), condylar constrained (n = 25), and rotating hinge (n = 13) prostheses used did not differ in their mean Knee Society scores (78, 81, and 83, respectively).

There remains a marked disparity in patient limitations seen after revision versus primary TKA. Given the positive results being obtained with newer implants, studies might suggest recent generations of prostheses have allowed designs to be comparable. As design development continues, we may come closer to achieving outcomes comparable to those of primary TKA.

3. Patient satisfaction

Several recent reports have shown that 10% to 25% of patients who underwent primary TKA were dissatisfied with their surgery^30,32; other studies have found patient satisfaction often correlating to function and pain.^33-35 Given the worse outcomes for revision TKA (outlined in the preceding section), the substantial pain accompanying a second, more complex procedure, and the extensive rehabilitation expected, we suspect patients who undergo revision TKA are even less satisfied with their surgery than their primary counterparts are. (See Appendix 4 for a complete list of studies of patient satisfaction after revision TKA.)

Barrack and colleagues³² evaluated a consecutive series of 238 patients followed up for at least 1 year after revision TKA. Patients were asked to rate their degree of satisfaction with both their primary procedure and the revision and to indicate their expectations regarding their revision prosthesis. Mean satisfaction score was 7.4 (maximum = 10), with 13% of patients dissatisfied, 18% somewhat satisfied, and 69% satisfied. Seventy-four percent of patients expected their revision prosthesis to last longer than the primary prosthesis.

Greidanus and colleagues³⁶ evaluated patient satisfaction in 60 revision TKA cases and 199 primary TKA cases at 2-year follow-up. The primary TKA group had significantly (P < .01) higher satisfaction scores in a comparison with the revision TKA group: Global (86 vs 73), Pain Relief (88 vs 70), Function (83 vs 67), and Recreation (77 vs 62). These findings support the satisfaction rates reported by Dahm and colleagues^33,34: 91% for primary TKA patients and 77% for revision TKA patients.

4. Quality of life

Procedure complexity leads to reduced survivorship, function, and mobility, longer rehabilitation, and decreased QOL for revision TKA patients relative to primary TKA patients.³⁷ (See Appendix 5 for a complete list of studies of QOL outcomes of revision TKA.)

Greidanus and colleagues³⁶ evaluated joint-specific QOL (using the 12-item Oxford Knee Score; OKS) and generic QOL (using the 12-Item Short Form Health Survey; SF-12) in 60 revision TKA cases and 199 primary TKA cases at a mean follow-up of 2 years. (The OKS survey is used to evaluate patient perspectives on TKA outcomes,³⁸ and the multipurpose SF-12 questionnaire is used to assess mental and physical function and general health-related QOL.³⁹) Compared with the revision TKA group, the primary TKA group had significantly higher OKS after surgery (78 vs 68; P = .01) as well as significantly higher SF-12 scores: Global (84 vs 72; P = .01), Mental (54 vs 50; P = .03), and Physical (43 vs 37; P = .01). Similarly, Ghomrawi and colleagues⁴⁰ evaluated patterns of improvement in 308 patients (318 knees) who had revision TKA. At 24-month follow-up, mean SF-36 Physical and Mental scores were 35 and 52, respectively.

Deehan and colleagues⁴¹ used the Nottingham Health Profile (NHP) to compare 94 patients’ health-related QOL scores before revision TKA with their scores 3 months, 1 year, and 5 years after revision. NHP Pain subscale scores were significantly lower 3 and 12 months after surgery than before surgery, but this difference was no longer seen at the 5-year follow-up. There was no significant improvement in scores on the other 5 NHP subscales (Sleep, Energy, Emotion, Mobility, Social Isolation) at any time points.

As shown in the literature, patients’ QOL outcomes improve after revision TKA, but these gains are not at the level of patients who undergo primary TKA.^36,41 Given that revision surgery is more extensive, and that perhaps revision patients have poorer muscle function, they usually do not return to the level they attained after their index procedure.

5. Economic impact

Consistent with the outcomes already described, the economic impact of revision TKAs is excess expenditures and costs to patients and health care institutions.⁴² The sources of this impact are higher implant costs, extra operative trays and times, longer hospital stays, more rehabilitation, and increased medication use.⁴³ Revision TKA costs range from $49,000 to more than $100,000—a tremendous increase over primary TKA costs ($25,000-$30,000).^43-45 Furthermore, the annual economic burden associated with revision TKA, now $2.7 billion, is expected to exceed $13 billion by 2030.⁴⁶ In the United States, about $23.2 billion will be spent on 926,527 primary TKAs in 2015; significantly, the costs associated with revising just 10% of these cases account for almost 50% of the total cost of the primary procedures.⁴⁶

In a retrospective cost-identification multicenter cohort study, Bozic and colleagues⁴⁷ found that both-component and single-component revisions, compared with primary procedures, were associated with significantly increased operative time (~265 and 221 minutes vs 200 minutes), use of allograft bone (23% and 14% vs 1%), length of stay (5.4 and 5.7 days vs 5.0 days), and percentage of patients discharged to extended-care facilities (26% and 26% vs 25%) (P < .0001). Hospital costs for both- and single-component revisions were 138% and 114% higher than costs for primary procedures (P < .0001). More recently, Kallala and colleagues⁴⁴ analyzed UK National Health Service data and compared the costs of revision for infection with revision for other causes (pain, instability, aseptic loosening, fracture). Mean length of stay associated with revision for infection (21.5 days) was more than double that associated with revision for aseptic loosening (9.5 days; P < .0001), and mean cost of revision for septic causes (£30,011) was more than 3 times that of revision for other causes (£9655; P < .0001). The authors concluded that the higher costs of revision knee surgery have a considerable economic impact, especially in infection cases.

With more extensive procedures, long-stem or more constrained prostheses are often needed to obtain adequate fixation and stability. The resulting increased, substantial economic burden is felt by patients and the health care system. Given that health care reimbursements are declining, hospitals that perform revision TKAs can sustain marked financial losses. Some centers are asking whether it is cost-effective to continue to perform these types of procedures. We must find new ways to provide revision procedures using less costly implants and tools so that centers will continue to make these procedures available to patients.

Conclusion

Given the exponential growth in primary TKAs, there will be a concordant increase in revision TKAs in the decades to come. This review provides a concise overview of revision TKA outcomes. Given the low level of evidence regarding revision TKAs, we need further higher quality studies of their prostheses and outcomes. Specifically, we need systematic reviews and meta-analyses to provide higher quality evidence regarding outcomes of using individual prosthetic designs.

Over the past 3 decades, total knee arthroplasty (TKA) has been considered a safe and effective treatment for end-stage knee arthritis.¹ However, as the population, the incidence of obesity, and life expectancy continue to increase, the number of TKAs will rise as well.^2,3 It is expected that over the next 16 years, the number of TKAs performed annually will exceed 3 million in the United States alone.⁴ This projection represents an over 600% increase from 2005 figures.⁵ Given the demographic shift expected over the next 2 decades, patients are anticipated to undergo these procedures at younger ages compared with previous generations, such that those age 65 years or younger will account for more than 55% of primary TKAs.⁶ More important, given this exponential growth in primary TKAs, there will be a concordant rise in revision procedures. It is expected that, the annual number has roughly doubled from that recorded for 2005.⁴

Compared with primary TKAs, however, revision TKAs have had less promising results, with survivorship as low as 60% over shorter periods.^7,8 In addition, recent studies have found an even higher degree of dissatisfaction and functional limitations among revision TKA patients than among primary TKA patients, 15% to 30% of whom are unhappy with their procedures.^9-11 These shortcomings of revision TKAs are thought to result from several factors, including poor bone quality, insufficient bone stock, ligamentous instability, soft-tissue incompetence, infection, malalignment, problems with extensor mechanisms, and substantial pain of uncertain etiology.

Despite there being several complex factors that can lead to worse outcomes with revision TKAs, surgeons are expected to produce results equivalent to those of primary TKAs. It is therefore imperative to delineate the objective and subjective outcomes of revision techniques to identify areas in need of improvement. In this article, we provide a concise overview of revision TKA outcomes in order to stimulate manufacturers, surgeons, and hospitals to improve on implant designs, surgical techniques, and care guidelines for revision TKA. We review the evidence on 5 points: aseptic survivorship, functional outcomes, patient satisfaction, quality of life (QOL), and economic impact. In addition, we compare available outcome data for revision and primary TKAs.

1. Aseptic survivorship

Fehring and colleagues¹² in 2001 and Sharkey and colleagues¹³ in 2002 evaluated mechanisms of failure for revision TKA and reported many failures resulted from infection or were associated with the implant, and occurred within 2 years after the primary procedure. More recently, Dy and colleagues¹⁴ found the most common reason for revision was aseptic loosening, followed by infection. The present review focuses on aseptic femoral and tibial revision.

The failure rate for revision TKA is substantially higher than for primary TKA with the same type of prosthesis because of the complexity of the revision procedure, the increasing constraint of the implant design, and the higher degree of bone loss. (Appendix 1 lists risk factors for revision surgery. Appendix 2 is a complete list of survivorship outcomes of revision TKA.)

Sheng and colleagues¹⁵ in 2006 and Koskinen and colleagues¹⁶ in 2008 analyzed Finnish Arthroplasty Register data to determine failure rates for revision and primary TKA. Sheng and colleagues¹⁵ examined survivorship of 2637 revision TKAs (performed between 1990 and 2002) for all-cause endpoints after first revision procedure. Survivorship rates were 89% (5 years) and 79% (10 years), while Koskinen and colleagues¹⁶ noted all-cause survival rates of 80% at 15 years. More recently, in 2013, the New Zealand Orthopaedic Association¹⁷ analyzed New Zealand Joint Registry data for revision and re-revision rates (rates of revision per 100 component years) for 64,556 primary TKAs performed between 1999 and 2012. During the period studied, 1684 revisions were performed, reflecting a 2.6% revision rate, a 0.50% rate of revision per 100 component years, and a 13-year Kaplan-Meier survivorship of 94.5%. The most common reasons for revision were pain, deep infection, and tibial component loosening (Table 1).

Posterior stabilized implants

Laskin and Ohnsorge¹⁸ retrospectively reviewed the cases of 58 patients who underwent unilateral revision TKA (with a posterior stabilized implant), of which 42% were for coronal instability and 44% for a loose tibial component. At minimum 4-year follow-up, 52 of the 58 patients had anteroposterior instability of less than 5 mm. In addition, 5 years after surgery, aseptic survivorship was 96%. Meijer and colleagues¹⁹ conducted a retrospective comparative study of 69 revision TKAs (65 patients) in which 9 knees received a primary implant and 60 received a revision implant with stems and augmentation (60 = 37 posterior stabilized, 20 constrained, 3 rotating hinge). Survival rates for the primary implants were 100% (1 year), 73% (2 years), and 44% (5 years), and survival rates for the revision implants were significantly better: 95% (1 year), 92% (2 years), and 92% (5 years) (hazard ratio, 5.87; P = .008). The authors therefore indicated that it was unclear whether using a primary implant should still be an option in revision TKA and, if it is used, whether it should be limited to less complex situations in which bone loss and ligament damage are minimal (Table 2).

Constrained and semiconstrained implants

In a study of 234 knees (209 patients) with soft-tissue deficiency, Wilke and colleagues²⁰ evaluated the long-term survivorship of revision TKA with use of a semiconstrained modular fixed-bearing implant system. Overall Kaplan-Meier survival rates were 91% (5 years) and 81% (10 years) at a mean follow-up of 9 years. When aseptic revision was evaluated, however, the survival rates increased to 95% (5 years) and 90% (10 years). The authors noted that male sex was the only variable that significantly increased the risk for re-revision (hazard ratio, 2.07; P = .02), which they attributed to potentially higher activity levels. In 2006 and 2011, Lachiewicz and Soileau^21,22 evaluated the survival of first- and second-generation constrained condylar prostheses in primary TKA cases with severe valgus deformities, incompetent collateral ligaments, or severe flexion contractures. Of the 54 knees (44 patients) with first-generation prostheses, 42 (34 patients) had a mean follow-up of 9 years (range, 5-16 years). Ten-year survival with failure, defined as component revision for loosening, was 96%. The 27 TKAs using second-generation prostheses had a mean follow-up of about 5 years (range, 2-12 years). At final follow-up, there were no revisions for loosening or patellar problems, but 6 knees (22%) required lateral retinacular release of the patella (Table 3).

Rotating hinge implants

Neumann and colleagues²³ evaluated the clinical and radiographic outcomes of 24 rotating hinge prostheses used for aseptic loosening with substantial bone loss and collateral ligament instability. At a mean follow-up of 56 months (range, 3-5 years), there was no evidence of loosening of any implants, and nonprogressive radiolucent lines were found in only 2 tibial components. Kowalczewski and colleagues²⁴ evaluated the clinical and radiologic outcomes of 12 primary TKAs using a rotating hinge knee prosthesis at a minimum follow-up of 10 years. By most recent follow-up, no implants had been revised for loosening, and only 3 had nonprogressive radiolucent lines (Table 4).

Endoprostheses (modular segmental implants)

In a systematic review of 9 studies, Korim and colleagues²⁵ evaluated 241 endoprostheses used for limb salvage under nononcologic conditions. Mean follow-up was about 3 years (range, 1-5 years). The devices were used to treat various conditions, including periprosthetic fracture, bone loss with aseptic loosening, and ligament insufficiency. The overall reoperation rate was 17% (41/241 cases). Mechanical failures were less frequent (6%-19%) (Table 5).

2. Functional outcomes

The goal in both primary and revision TKA is to restore the function and mobility of the knee and to alleviate pain. Whereas primary TKAs are realistically predictable and reproducible in their outcomes, revision TKAs are vastly more complicated, which can result in worse postoperative outcomes and function. In addition, revision TKAs may require extensive surgical exposure, which causes more tissue and muscle damage, prolonging rehabilitation. (Appendix 3 is a complete list of studies of functional outcomes of revision TKA.)

This discrepancy in functional outcomes between primary and revision TKA begins as early as the postoperative inpatient rehabilitation period. Using the functional independence measurement (FIM), which estimates performance of activities of daily living, mobility, and cognition, Vincent and colleagues²⁶ evaluated the functional improvement produced by revision versus primary TKA during inpatient rehabilitation. They compared 424 consecutive primary TKAs with 138 revision TKAs. For both groups, FIM scores increased significantly (P = .015) between admission and discharge. On discharge, however, FIM scores were significantly (P = .01) higher for the primary group than the revision group (29 and 27 points, respectively). Furthermore, in the evaluation of mechanisms of failure, patients who had revision TKA for mechanical or pain-related problems did markedly better than those who had revision TKA for infection.

Compared with primary knee implants, revision implants require increasing constraint. We assume increasing constraint affects knee biomechanics, leading to worsening functional outcomes. In a study of 60 revision TKAs (57 patients) using posterior stabilized, condylar constrained, or rotating hinge prostheses, Vasso and colleagues²⁷ examined functional outcomes at a median follow-up of 9 years (range, 4-12 years). At most recent follow-up, mean International Knee Society (IKS) Knee and Function scores were 81 (range, 48-97) and 79 (range, 56-92), mean Hospital for Special Surgery (HSS) score was 84 (range, 62-98), and mean range of motion (ROM) was 121° (range, 98°-132°) (P < .001). Although there were no significant differences in IKS and HSS scores between prosthesis types, ROM was significantly (P < .01) wider in the posterior stabilized group than in the condylar constrained and rotating hinge groups (127° vs 112° and 108°), suggesting increasing constraint resulted in decreased ROM. Several studies have found increasing constraint might lead to reduced function.^28-30

However, Hwang and colleagues³¹ evaluated functional outcomes in 36 revision TKAs and noted that the cemented posterior stabilized (n = 8), condylar constrained (n = 25), and rotating hinge (n = 13) prostheses used did not differ in their mean Knee Society scores (78, 81, and 83, respectively).

There remains a marked disparity in patient limitations seen after revision versus primary TKA. Given the positive results being obtained with newer implants, studies might suggest recent generations of prostheses have allowed designs to be comparable. As design development continues, we may come closer to achieving outcomes comparable to those of primary TKA.

3. Patient satisfaction

Several recent reports have shown that 10% to 25% of patients who underwent primary TKA were dissatisfied with their surgery^30,32; other studies have found patient satisfaction often correlating to function and pain.^33-35 Given the worse outcomes for revision TKA (outlined in the preceding section), the substantial pain accompanying a second, more complex procedure, and the extensive rehabilitation expected, we suspect patients who undergo revision TKA are even less satisfied with their surgery than their primary counterparts are. (See Appendix 4 for a complete list of studies of patient satisfaction after revision TKA.)

Barrack and colleagues³² evaluated a consecutive series of 238 patients followed up for at least 1 year after revision TKA. Patients were asked to rate their degree of satisfaction with both their primary procedure and the revision and to indicate their expectations regarding their revision prosthesis. Mean satisfaction score was 7.4 (maximum = 10), with 13% of patients dissatisfied, 18% somewhat satisfied, and 69% satisfied. Seventy-four percent of patients expected their revision prosthesis to last longer than the primary prosthesis.

Greidanus and colleagues³⁶ evaluated patient satisfaction in 60 revision TKA cases and 199 primary TKA cases at 2-year follow-up. The primary TKA group had significantly (P < .01) higher satisfaction scores in a comparison with the revision TKA group: Global (86 vs 73), Pain Relief (88 vs 70), Function (83 vs 67), and Recreation (77 vs 62). These findings support the satisfaction rates reported by Dahm and colleagues^33,34: 91% for primary TKA patients and 77% for revision TKA patients.

4. Quality of life

Procedure complexity leads to reduced survivorship, function, and mobility, longer rehabilitation, and decreased QOL for revision TKA patients relative to primary TKA patients.³⁷ (See Appendix 5 for a complete list of studies of QOL outcomes of revision TKA.)

Greidanus and colleagues³⁶ evaluated joint-specific QOL (using the 12-item Oxford Knee Score; OKS) and generic QOL (using the 12-Item Short Form Health Survey; SF-12) in 60 revision TKA cases and 199 primary TKA cases at a mean follow-up of 2 years. (The OKS survey is used to evaluate patient perspectives on TKA outcomes,³⁸ and the multipurpose SF-12 questionnaire is used to assess mental and physical function and general health-related QOL.³⁹) Compared with the revision TKA group, the primary TKA group had significantly higher OKS after surgery (78 vs 68; P = .01) as well as significantly higher SF-12 scores: Global (84 vs 72; P = .01), Mental (54 vs 50; P = .03), and Physical (43 vs 37; P = .01). Similarly, Ghomrawi and colleagues⁴⁰ evaluated patterns of improvement in 308 patients (318 knees) who had revision TKA. At 24-month follow-up, mean SF-36 Physical and Mental scores were 35 and 52, respectively.

Deehan and colleagues⁴¹ used the Nottingham Health Profile (NHP) to compare 94 patients’ health-related QOL scores before revision TKA with their scores 3 months, 1 year, and 5 years after revision. NHP Pain subscale scores were significantly lower 3 and 12 months after surgery than before surgery, but this difference was no longer seen at the 5-year follow-up. There was no significant improvement in scores on the other 5 NHP subscales (Sleep, Energy, Emotion, Mobility, Social Isolation) at any time points.

As shown in the literature, patients’ QOL outcomes improve after revision TKA, but these gains are not at the level of patients who undergo primary TKA.^36,41 Given that revision surgery is more extensive, and that perhaps revision patients have poorer muscle function, they usually do not return to the level they attained after their index procedure.

5. Economic impact

Consistent with the outcomes already described, the economic impact of revision TKAs is excess expenditures and costs to patients and health care institutions.⁴² The sources of this impact are higher implant costs, extra operative trays and times, longer hospital stays, more rehabilitation, and increased medication use.⁴³ Revision TKA costs range from $49,000 to more than $100,000—a tremendous increase over primary TKA costs ($25,000-$30,000).^43-45 Furthermore, the annual economic burden associated with revision TKA, now $2.7 billion, is expected to exceed $13 billion by 2030.⁴⁶ In the United States, about $23.2 billion will be spent on 926,527 primary TKAs in 2015; significantly, the costs associated with revising just 10% of these cases account for almost 50% of the total cost of the primary procedures.⁴⁶

In a retrospective cost-identification multicenter cohort study, Bozic and colleagues⁴⁷ found that both-component and single-component revisions, compared with primary procedures, were associated with significantly increased operative time (~265 and 221 minutes vs 200 minutes), use of allograft bone (23% and 14% vs 1%), length of stay (5.4 and 5.7 days vs 5.0 days), and percentage of patients discharged to extended-care facilities (26% and 26% vs 25%) (P < .0001). Hospital costs for both- and single-component revisions were 138% and 114% higher than costs for primary procedures (P < .0001). More recently, Kallala and colleagues⁴⁴ analyzed UK National Health Service data and compared the costs of revision for infection with revision for other causes (pain, instability, aseptic loosening, fracture). Mean length of stay associated with revision for infection (21.5 days) was more than double that associated with revision for aseptic loosening (9.5 days; P < .0001), and mean cost of revision for septic causes (£30,011) was more than 3 times that of revision for other causes (£9655; P < .0001). The authors concluded that the higher costs of revision knee surgery have a considerable economic impact, especially in infection cases.

With more extensive procedures, long-stem or more constrained prostheses are often needed to obtain adequate fixation and stability. The resulting increased, substantial economic burden is felt by patients and the health care system. Given that health care reimbursements are declining, hospitals that perform revision TKAs can sustain marked financial losses. Some centers are asking whether it is cost-effective to continue to perform these types of procedures. We must find new ways to provide revision procedures using less costly implants and tools so that centers will continue to make these procedures available to patients.

Conclusion

Given the exponential growth in primary TKAs, there will be a concordant increase in revision TKAs in the decades to come. This review provides a concise overview of revision TKA outcomes. Given the low level of evidence regarding revision TKAs, we need further higher quality studies of their prostheses and outcomes. Specifically, we need systematic reviews and meta-analyses to provide higher quality evidence regarding outcomes of using individual prosthetic designs.

Over the past 3 decades, total knee arthroplasty (TKA) has been considered a safe and effective treatment for end-stage knee arthritis.¹ However, as the population, the incidence of obesity, and life expectancy continue to increase, the number of TKAs will rise as well.^2,3 It is expected that over the next 16 years, the number of TKAs performed annually will exceed 3 million in the United States alone.⁴ This projection represents an over 600% increase from 2005 figures.⁵ Given the demographic shift expected over the next 2 decades, patients are anticipated to undergo these procedures at younger ages compared with previous generations, such that those age 65 years or younger will account for more than 55% of primary TKAs.⁶ More important, given this exponential growth in primary TKAs, there will be a concordant rise in revision procedures. It is expected that, the annual number has roughly doubled from that recorded for 2005.⁴

Compared with primary TKAs, however, revision TKAs have had less promising results, with survivorship as low as 60% over shorter periods.^7,8 In addition, recent studies have found an even higher degree of dissatisfaction and functional limitations among revision TKA patients than among primary TKA patients, 15% to 30% of whom are unhappy with their procedures.^9-11 These shortcomings of revision TKAs are thought to result from several factors, including poor bone quality, insufficient bone stock, ligamentous instability, soft-tissue incompetence, infection, malalignment, problems with extensor mechanisms, and substantial pain of uncertain etiology.

Despite there being several complex factors that can lead to worse outcomes with revision TKAs, surgeons are expected to produce results equivalent to those of primary TKAs. It is therefore imperative to delineate the objective and subjective outcomes of revision techniques to identify areas in need of improvement. In this article, we provide a concise overview of revision TKA outcomes in order to stimulate manufacturers, surgeons, and hospitals to improve on implant designs, surgical techniques, and care guidelines for revision TKA. We review the evidence on 5 points: aseptic survivorship, functional outcomes, patient satisfaction, quality of life (QOL), and economic impact. In addition, we compare available outcome data for revision and primary TKAs.

1. Aseptic survivorship

Fehring and colleagues¹² in 2001 and Sharkey and colleagues¹³ in 2002 evaluated mechanisms of failure for revision TKA and reported many failures resulted from infection or were associated with the implant, and occurred within 2 years after the primary procedure. More recently, Dy and colleagues¹⁴ found the most common reason for revision was aseptic loosening, followed by infection. The present review focuses on aseptic femoral and tibial revision.

The failure rate for revision TKA is substantially higher than for primary TKA with the same type of prosthesis because of the complexity of the revision procedure, the increasing constraint of the implant design, and the higher degree of bone loss. (Appendix 1 lists risk factors for revision surgery. Appendix 2 is a complete list of survivorship outcomes of revision TKA.)

Sheng and colleagues¹⁵ in 2006 and Koskinen and colleagues¹⁶ in 2008 analyzed Finnish Arthroplasty Register data to determine failure rates for revision and primary TKA. Sheng and colleagues¹⁵ examined survivorship of 2637 revision TKAs (performed between 1990 and 2002) for all-cause endpoints after first revision procedure. Survivorship rates were 89% (5 years) and 79% (10 years), while Koskinen and colleagues¹⁶ noted all-cause survival rates of 80% at 15 years. More recently, in 2013, the New Zealand Orthopaedic Association¹⁷ analyzed New Zealand Joint Registry data for revision and re-revision rates (rates of revision per 100 component years) for 64,556 primary TKAs performed between 1999 and 2012. During the period studied, 1684 revisions were performed, reflecting a 2.6% revision rate, a 0.50% rate of revision per 100 component years, and a 13-year Kaplan-Meier survivorship of 94.5%. The most common reasons for revision were pain, deep infection, and tibial component loosening (Table 1).

Posterior stabilized implants

Laskin and Ohnsorge¹⁸ retrospectively reviewed the cases of 58 patients who underwent unilateral revision TKA (with a posterior stabilized implant), of which 42% were for coronal instability and 44% for a loose tibial component. At minimum 4-year follow-up, 52 of the 58 patients had anteroposterior instability of less than 5 mm. In addition, 5 years after surgery, aseptic survivorship was 96%. Meijer and colleagues¹⁹ conducted a retrospective comparative study of 69 revision TKAs (65 patients) in which 9 knees received a primary implant and 60 received a revision implant with stems and augmentation (60 = 37 posterior stabilized, 20 constrained, 3 rotating hinge). Survival rates for the primary implants were 100% (1 year), 73% (2 years), and 44% (5 years), and survival rates for the revision implants were significantly better: 95% (1 year), 92% (2 years), and 92% (5 years) (hazard ratio, 5.87; P = .008). The authors therefore indicated that it was unclear whether using a primary implant should still be an option in revision TKA and, if it is used, whether it should be limited to less complex situations in which bone loss and ligament damage are minimal (Table 2).

Constrained and semiconstrained implants

In a study of 234 knees (209 patients) with soft-tissue deficiency, Wilke and colleagues²⁰ evaluated the long-term survivorship of revision TKA with use of a semiconstrained modular fixed-bearing implant system. Overall Kaplan-Meier survival rates were 91% (5 years) and 81% (10 years) at a mean follow-up of 9 years. When aseptic revision was evaluated, however, the survival rates increased to 95% (5 years) and 90% (10 years). The authors noted that male sex was the only variable that significantly increased the risk for re-revision (hazard ratio, 2.07; P = .02), which they attributed to potentially higher activity levels. In 2006 and 2011, Lachiewicz and Soileau^21,22 evaluated the survival of first- and second-generation constrained condylar prostheses in primary TKA cases with severe valgus deformities, incompetent collateral ligaments, or severe flexion contractures. Of the 54 knees (44 patients) with first-generation prostheses, 42 (34 patients) had a mean follow-up of 9 years (range, 5-16 years). Ten-year survival with failure, defined as component revision for loosening, was 96%. The 27 TKAs using second-generation prostheses had a mean follow-up of about 5 years (range, 2-12 years). At final follow-up, there were no revisions for loosening or patellar problems, but 6 knees (22%) required lateral retinacular release of the patella (Table 3).

Rotating hinge implants

Neumann and colleagues²³ evaluated the clinical and radiographic outcomes of 24 rotating hinge prostheses used for aseptic loosening with substantial bone loss and collateral ligament instability. At a mean follow-up of 56 months (range, 3-5 years), there was no evidence of loosening of any implants, and nonprogressive radiolucent lines were found in only 2 tibial components. Kowalczewski and colleagues²⁴ evaluated the clinical and radiologic outcomes of 12 primary TKAs using a rotating hinge knee prosthesis at a minimum follow-up of 10 years. By most recent follow-up, no implants had been revised for loosening, and only 3 had nonprogressive radiolucent lines (Table 4).

Endoprostheses (modular segmental implants)

In a systematic review of 9 studies, Korim and colleagues²⁵ evaluated 241 endoprostheses used for limb salvage under nononcologic conditions. Mean follow-up was about 3 years (range, 1-5 years). The devices were used to treat various conditions, including periprosthetic fracture, bone loss with aseptic loosening, and ligament insufficiency. The overall reoperation rate was 17% (41/241 cases). Mechanical failures were less frequent (6%-19%) (Table 5).

2. Functional outcomes

The goal in both primary and revision TKA is to restore the function and mobility of the knee and to alleviate pain. Whereas primary TKAs are realistically predictable and reproducible in their outcomes, revision TKAs are vastly more complicated, which can result in worse postoperative outcomes and function. In addition, revision TKAs may require extensive surgical exposure, which causes more tissue and muscle damage, prolonging rehabilitation. (Appendix 3 is a complete list of studies of functional outcomes of revision TKA.)

This discrepancy in functional outcomes between primary and revision TKA begins as early as the postoperative inpatient rehabilitation period. Using the functional independence measurement (FIM), which estimates performance of activities of daily living, mobility, and cognition, Vincent and colleagues²⁶ evaluated the functional improvement produced by revision versus primary TKA during inpatient rehabilitation. They compared 424 consecutive primary TKAs with 138 revision TKAs. For both groups, FIM scores increased significantly (P = .015) between admission and discharge. On discharge, however, FIM scores were significantly (P = .01) higher for the primary group than the revision group (29 and 27 points, respectively). Furthermore, in the evaluation of mechanisms of failure, patients who had revision TKA for mechanical or pain-related problems did markedly better than those who had revision TKA for infection.

Compared with primary knee implants, revision implants require increasing constraint. We assume increasing constraint affects knee biomechanics, leading to worsening functional outcomes. In a study of 60 revision TKAs (57 patients) using posterior stabilized, condylar constrained, or rotating hinge prostheses, Vasso and colleagues²⁷ examined functional outcomes at a median follow-up of 9 years (range, 4-12 years). At most recent follow-up, mean International Knee Society (IKS) Knee and Function scores were 81 (range, 48-97) and 79 (range, 56-92), mean Hospital for Special Surgery (HSS) score was 84 (range, 62-98), and mean range of motion (ROM) was 121° (range, 98°-132°) (P < .001). Although there were no significant differences in IKS and HSS scores between prosthesis types, ROM was significantly (P < .01) wider in the posterior stabilized group than in the condylar constrained and rotating hinge groups (127° vs 112° and 108°), suggesting increasing constraint resulted in decreased ROM. Several studies have found increasing constraint might lead to reduced function.^28-30

However, Hwang and colleagues³¹ evaluated functional outcomes in 36 revision TKAs and noted that the cemented posterior stabilized (n = 8), condylar constrained (n = 25), and rotating hinge (n = 13) prostheses used did not differ in their mean Knee Society scores (78, 81, and 83, respectively).

There remains a marked disparity in patient limitations seen after revision versus primary TKA. Given the positive results being obtained with newer implants, studies might suggest recent generations of prostheses have allowed designs to be comparable. As design development continues, we may come closer to achieving outcomes comparable to those of primary TKA.

3. Patient satisfaction

Several recent reports have shown that 10% to 25% of patients who underwent primary TKA were dissatisfied with their surgery^30,32; other studies have found patient satisfaction often correlating to function and pain.^33-35 Given the worse outcomes for revision TKA (outlined in the preceding section), the substantial pain accompanying a second, more complex procedure, and the extensive rehabilitation expected, we suspect patients who undergo revision TKA are even less satisfied with their surgery than their primary counterparts are. (See Appendix 4 for a complete list of studies of patient satisfaction after revision TKA.)

Barrack and colleagues³² evaluated a consecutive series of 238 patients followed up for at least 1 year after revision TKA. Patients were asked to rate their degree of satisfaction with both their primary procedure and the revision and to indicate their expectations regarding their revision prosthesis. Mean satisfaction score was 7.4 (maximum = 10), with 13% of patients dissatisfied, 18% somewhat satisfied, and 69% satisfied. Seventy-four percent of patients expected their revision prosthesis to last longer than the primary prosthesis.

Greidanus and colleagues³⁶ evaluated patient satisfaction in 60 revision TKA cases and 199 primary TKA cases at 2-year follow-up. The primary TKA group had significantly (P < .01) higher satisfaction scores in a comparison with the revision TKA group: Global (86 vs 73), Pain Relief (88 vs 70), Function (83 vs 67), and Recreation (77 vs 62). These findings support the satisfaction rates reported by Dahm and colleagues^33,34: 91% for primary TKA patients and 77% for revision TKA patients.

4. Quality of life

Procedure complexity leads to reduced survivorship, function, and mobility, longer rehabilitation, and decreased QOL for revision TKA patients relative to primary TKA patients.³⁷ (See Appendix 5 for a complete list of studies of QOL outcomes of revision TKA.)

Greidanus and colleagues³⁶ evaluated joint-specific QOL (using the 12-item Oxford Knee Score; OKS) and generic QOL (using the 12-Item Short Form Health Survey; SF-12) in 60 revision TKA cases and 199 primary TKA cases at a mean follow-up of 2 years. (The OKS survey is used to evaluate patient perspectives on TKA outcomes,³⁸ and the multipurpose SF-12 questionnaire is used to assess mental and physical function and general health-related QOL.³⁹) Compared with the revision TKA group, the primary TKA group had significantly higher OKS after surgery (78 vs 68; P = .01) as well as significantly higher SF-12 scores: Global (84 vs 72; P = .01), Mental (54 vs 50; P = .03), and Physical (43 vs 37; P = .01). Similarly, Ghomrawi and colleagues⁴⁰ evaluated patterns of improvement in 308 patients (318 knees) who had revision TKA. At 24-month follow-up, mean SF-36 Physical and Mental scores were 35 and 52, respectively.

Deehan and colleagues⁴¹ used the Nottingham Health Profile (NHP) to compare 94 patients’ health-related QOL scores before revision TKA with their scores 3 months, 1 year, and 5 years after revision. NHP Pain subscale scores were significantly lower 3 and 12 months after surgery than before surgery, but this difference was no longer seen at the 5-year follow-up. There was no significant improvement in scores on the other 5 NHP subscales (Sleep, Energy, Emotion, Mobility, Social Isolation) at any time points.

As shown in the literature, patients’ QOL outcomes improve after revision TKA, but these gains are not at the level of patients who undergo primary TKA.^36,41 Given that revision surgery is more extensive, and that perhaps revision patients have poorer muscle function, they usually do not return to the level they attained after their index procedure.

5. Economic impact

Consistent with the outcomes already described, the economic impact of revision TKAs is excess expenditures and costs to patients and health care institutions.⁴² The sources of this impact are higher implant costs, extra operative trays and times, longer hospital stays, more rehabilitation, and increased medication use.⁴³ Revision TKA costs range from $49,000 to more than $100,000—a tremendous increase over primary TKA costs ($25,000-$30,000).^43-45 Furthermore, the annual economic burden associated with revision TKA, now $2.7 billion, is expected to exceed $13 billion by 2030.⁴⁶ In the United States, about $23.2 billion will be spent on 926,527 primary TKAs in 2015; significantly, the costs associated with revising just 10% of these cases account for almost 50% of the total cost of the primary procedures.⁴⁶

In a retrospective cost-identification multicenter cohort study, Bozic and colleagues⁴⁷ found that both-component and single-component revisions, compared with primary procedures, were associated with significantly increased operative time (~265 and 221 minutes vs 200 minutes), use of allograft bone (23% and 14% vs 1%), length of stay (5.4 and 5.7 days vs 5.0 days), and percentage of patients discharged to extended-care facilities (26% and 26% vs 25%) (P < .0001). Hospital costs for both- and single-component revisions were 138% and 114% higher than costs for primary procedures (P < .0001). More recently, Kallala and colleagues⁴⁴ analyzed UK National Health Service data and compared the costs of revision for infection with revision for other causes (pain, instability, aseptic loosening, fracture). Mean length of stay associated with revision for infection (21.5 days) was more than double that associated with revision for aseptic loosening (9.5 days; P < .0001), and mean cost of revision for septic causes (£30,011) was more than 3 times that of revision for other causes (£9655; P < .0001). The authors concluded that the higher costs of revision knee surgery have a considerable economic impact, especially in infection cases.

With more extensive procedures, long-stem or more constrained prostheses are often needed to obtain adequate fixation and stability. The resulting increased, substantial economic burden is felt by patients and the health care system. Given that health care reimbursements are declining, hospitals that perform revision TKAs can sustain marked financial losses. Some centers are asking whether it is cost-effective to continue to perform these types of procedures. We must find new ways to provide revision procedures using less costly implants and tools so that centers will continue to make these procedures available to patients.

Conclusion

Given the exponential growth in primary TKAs, there will be a concordant increase in revision TKAs in the decades to come. This review provides a concise overview of revision TKA outcomes. Given the low level of evidence regarding revision TKAs, we need further higher quality studies of their prostheses and outcomes. Specifically, we need systematic reviews and meta-analyses to provide higher quality evidence regarding outcomes of using individual prosthetic designs.

Before 1990, a considerable number of revisions were performed, largely for implant-associated failures, in the first few years after index primary knee arthroplasties.^1,2 Since then, surgeons, manufacturers, and hospitals have collaborated to improve implant designs, techniques, and care guidelines.^3,4 Despite the substantial improvements in designs, which led to implant longevity of more than 15 years in many cases, these devices still have limited life spans. Large studies have estimated that the risk for revision required after primary knee arthroplasty ranges from as low as 5% at 15 years to up to 9% at 10 years.^4,5

The surgical goals of revision total knee arthroplasty (TKA) are to obtain stable fixation of the prosthesis to host bone, to obtain a stable range of motion compatible with the patient’s activities of daily living, and to achieve these goals while using the smallest amount of prosthetic augments and constraint so that the soft tissues may share in load transfer.⁶ As prosthetic constraint increases, the soft tissues participate less in load sharing, and increasing stresses are put on the implant–bone interface, which further increases the risk for early implant loosening.⁷ Hence, as characteristics of a revision implant become more constrained, there is often a higher rate of aseptic loosening expected.⁸

Controversy remains regarding the ideal implant type for revision TKA. To ensure the success of revision surgery and to reduce the risks for postoperative dissatisfaction, complications, and re-revision, orthopedists must understand the types of revision implant designs available, particularly as each has its own indications and potential complications.

In this article, we review the classification systems used for revision TKA as well as the types of prosthetic designs that can be used: posterior stabilized, nonlinked constrained, rotating hinge, and modular segmental.

1. Classification of bone loss and soft-tissue integrity

To further understand revision TKA, we must consider the complexity level of these cases, particularly by evaluating degree of bone loss and soft-tissue deficiency. The most accepted way to assess bone loss both before and during surgery is to use the AORI (Anderson Orthopaedic Research Institute) classification system.⁹ Bone loss can be classified into 3 types: I, in which metaphyseal bone is intact and small bone defects do not compromise component stability; II, in which metaphyseal bone is damaged and cancellous bone loss requires cement fill, augments, or bone graft; and III, in which metaphyseal bone is deficient, and lost bone comprises a major portion of condyle or plateau and occasionally requires bone grafts or custom implants (Table 1). These patterns of bone loss are occasionally associated with detachment of the collateral ligament or patellar tendon.

In addition to understanding bone loss in revision TKA, surgeons must be aware of soft-tissue deficiencies (eg, collateral ligaments, extensor mechanism), which also influence type and amount of prosthesis constraint. Specifically, constraint choice depends on amount of bone loss and on the condition of stabilizing tissues, such as the collateral ligaments. Under conditions of minimal bone loss and intact peripheral ligaments, a less constrained device, such as a primary posterior stabilized system, can be considered. When ligaments are present but insufficient, a semiconstrained device is recommended. In the presence of medial collateral ligament attenuation or complete medial or lateral collateral ligament dysfunction, a fully constrained prosthesis is required.⁸ Therefore, amount of bone loss or soft-tissue deficiency often dictates which prosthesis to use.

For radiographic classification, the Knee Society roentgenographic evaluation and scoring system¹⁰ has been implemented to allow for uniform reporting of radiographic results and to ensure adequate preoperative planning and postoperative assessment of component alignment. This system incorporates the evaluation of alignment in the coronal, sagittal, and patellofemoral planes and assesses radiolucency using zones dividing the implant–bone interface into segments to allow for easier classification of areas of lucency. More recently, a modified version of the Knee Society system was constructed.¹¹ This modification simplifies zone classifications and accommodates more complex revision knee designs and stem extensions.

2. Posterior stabilized designs

Cruciate-retaining prostheses are seldom applicable in the revision TKA setting because of frequent damage to the posterior cruciate ligament, except in the case of simple polyethylene exchanges or, potentially, revisions of failed unicompartmental TKAs. Thus, posterior stabilized designs are the first-line choice for revision TKA (Figure 1). These prostheses are indicated only when the posterior cruciate ligament is incompetent and in the setting of adequate flexion and extension and medial and lateral collateral ligament balancing.

However, studies have shown that posterior stabilized TKAs have a limited role in revision TKAs, as the amount of ligamentous and bony damage is often underestimated in these patients, and use of a primary implant in a revision setting often requires additional augments, all of which may have contributed to the high failure rate. Thus, this design should be used only when the patient has adequate bone stock (AORI type I) and collateral ligament tension. This situation further emphasizes the importance of performing intraoperative testing for ligamentous balance and bone deficit evaluation in order to determine the most appropriate implant (Table 2).

3. Nonlinked constrained designs

Nonlinked constrained (condylar constrained) designs are the devices most commonly used for revision TKAs (>50% of revision knees). These prostheses provide increased articular constraint, which is required in patients with persistent instability, despite appropriate soft-tissue balancing. Increased articular constraint allows for more knee stability by providing progressive varus-valgus, coronal, and rotational stability with the aid of taller and wider tibial posts.¹² Specifically, these implants incorporate a tibial post that fits closely between the femoral condyles, allowing for less motion compared with a standard posterior stabilized design.¹²

In addition, these designs may be used with augments, stems, and allografts when bone loss is more substantial. In particular, stem extensions allow for load distribution to the diaphyseal regions of the tibia and femur and thereby aid in reducing the increased stress at the bone–implant interface, which is a common concern with these implants. However, these extensions cost more, require intramedullary invasion, and are associated with higher rates of leg and thigh pain.¹²

These prostheses are often implicated in cases involving a high degree of bone loss (eg, AORI type II or III). They are ideally used in cases in which complete revision of both tibial and femoral components is needed and are indicated in cases of incompetent posterior cruciate ligament, partial functional loss of medial or lateral collateral ligaments, or flexion-extension mismatch.¹³ Furthermore, use of a constrained prosthesis is recommended in the setting of varus or valgus instability, or repeated dislocations of a posterior stabilized design (Table 2).

Ten-year survivorship ranges from 85% to 96%, but this is substantially lower than the 95% to 96% for condylar constrained prostheses used in primary TKAs.^14-17 Moreover, the large discrepancy between survivorship of primary TKA and revision TKA with a constrained prosthesis further affirms that the complexity of revision surgery, rather than the prosthesis used, may have more deleterious effects on outcomes. However, surgeons must be aware that increased constraint leads to increased stress on the prosthetic interfaces with associated aseptic loosening and early failure, and this continues to be a legitimate concern.

4. Rotating hinge designs

Many patients who undergo revision TKA can be managed with a posterior stabilizing or nonlinked constrained design. However, in patients who present with severe ligamentous instability and bone loss (AORI type II or III), a rotating hinge prostheses, or highly constrained device, is often recommended (Figure 2).¹⁸ By using a rotating mobile-bearing platform, this prosthesis permits axial rotation through a metal-reinforced polyethylene-post articulation in the tibial tray. In addition, it involves use of modular diaphyseal-engaging stems and diaphyseal sleeves, which allow for the bypass of bony defects and areas of bone loss (Table 2).

However, the rigid biomechanics of hinged prostheses is associated with increased risk for aseptic loosening (aseptic 10-year survival, 60%-80%), imparted by the transfer of stresses across the bone. The higher risk for early loosening, osteolysis, and excessive wear—caused by the highly restricted biomechanics of early generations of fixed hinged designs—has led to the development of new devices with mobile mechanics. Prosthetic designs have been improved with an added rotational axis to reduce torsional stress, a patellar resurfacing option, and better stem fixation and patellofemoral kinematics. Overall, these are aimed to improve rates of instability and aseptic loosening, with promising results demonstrated in the literature.

5. Modular segmental arthroplasty designs

Segmental arthroplasty prostheses, which typically are end-of-the-line revision TKA options, are applicable only in cases of extensive bone loss (more than can be treated with allografts or augments; AORI type 3), complete ligamentous disruption/absence, loss of periprosthetic soft tissue, and multiple previous revision procedures (Figure 3). Despite the limited indications for these prostheses, they yield quick return to function without graft nonunion or resorption, and they augment ingrowth/ongrowth. Furthermore, the next surgical option could be fusion or amputation. When failures were specifically evaluated for aseptic loosening across 4 studies, the survival rate ranged from 83% to 99.5%, with the most frequent complication being infection (up to 33% in one series).^6,19-21

The major roles for segmental arthroplasty prostheses in primary TKAs are in the setting of oncologic conditions that require bony excision, or unreconstuctable fractures about the knee. Used after ancillary metastatic disease, these prostheses demonstrate positive results, according to several reports.^22,23 In the setting of revision TKA, however, these prostheses should be used only when other surgical options are unfeasible, given the high risk for infection and the re-revision rates. Currently, revision TKAs with tumor prostheses have a high failure rate (up to 50%) because of the extensive surgery and the lack of bony and soft-tissue support (Table 2).

Conclusion

Orthopedists performing revision TKAs must consider bone stock and remaining ligament stability. In particular, they should choose implants for least constraint and adequate knee stability, as these are essential in minimizing the stresses on the implant–bone interface. Ultimately, functional outcomes, survivorship, and postoperative satisfaction determine the success of these designs. However, predictors of outcomes of revision surgery are often multifactorial, and surgeons must also consider procedure complexity and patient-specific characteristics.

Before 1990, a considerable number of revisions were performed, largely for implant-associated failures, in the first few years after index primary knee arthroplasties.^1,2 Since then, surgeons, manufacturers, and hospitals have collaborated to improve implant designs, techniques, and care guidelines.^3,4 Despite the substantial improvements in designs, which led to implant longevity of more than 15 years in many cases, these devices still have limited life spans. Large studies have estimated that the risk for revision required after primary knee arthroplasty ranges from as low as 5% at 15 years to up to 9% at 10 years.^4,5

The surgical goals of revision total knee arthroplasty (TKA) are to obtain stable fixation of the prosthesis to host bone, to obtain a stable range of motion compatible with the patient’s activities of daily living, and to achieve these goals while using the smallest amount of prosthetic augments and constraint so that the soft tissues may share in load transfer.⁶ As prosthetic constraint increases, the soft tissues participate less in load sharing, and increasing stresses are put on the implant–bone interface, which further increases the risk for early implant loosening.⁷ Hence, as characteristics of a revision implant become more constrained, there is often a higher rate of aseptic loosening expected.⁸

Controversy remains regarding the ideal implant type for revision TKA. To ensure the success of revision surgery and to reduce the risks for postoperative dissatisfaction, complications, and re-revision, orthopedists must understand the types of revision implant designs available, particularly as each has its own indications and potential complications.

In this article, we review the classification systems used for revision TKA as well as the types of prosthetic designs that can be used: posterior stabilized, nonlinked constrained, rotating hinge, and modular segmental.

1. Classification of bone loss and soft-tissue integrity

To further understand revision TKA, we must consider the complexity level of these cases, particularly by evaluating degree of bone loss and soft-tissue deficiency. The most accepted way to assess bone loss both before and during surgery is to use the AORI (Anderson Orthopaedic Research Institute) classification system.⁹ Bone loss can be classified into 3 types: I, in which metaphyseal bone is intact and small bone defects do not compromise component stability; II, in which metaphyseal bone is damaged and cancellous bone loss requires cement fill, augments, or bone graft; and III, in which metaphyseal bone is deficient, and lost bone comprises a major portion of condyle or plateau and occasionally requires bone grafts or custom implants (Table 1). These patterns of bone loss are occasionally associated with detachment of the collateral ligament or patellar tendon.

In addition to understanding bone loss in revision TKA, surgeons must be aware of soft-tissue deficiencies (eg, collateral ligaments, extensor mechanism), which also influence type and amount of prosthesis constraint. Specifically, constraint choice depends on amount of bone loss and on the condition of stabilizing tissues, such as the collateral ligaments. Under conditions of minimal bone loss and intact peripheral ligaments, a less constrained device, such as a primary posterior stabilized system, can be considered. When ligaments are present but insufficient, a semiconstrained device is recommended. In the presence of medial collateral ligament attenuation or complete medial or lateral collateral ligament dysfunction, a fully constrained prosthesis is required.⁸ Therefore, amount of bone loss or soft-tissue deficiency often dictates which prosthesis to use.

For radiographic classification, the Knee Society roentgenographic evaluation and scoring system¹⁰ has been implemented to allow for uniform reporting of radiographic results and to ensure adequate preoperative planning and postoperative assessment of component alignment. This system incorporates the evaluation of alignment in the coronal, sagittal, and patellofemoral planes and assesses radiolucency using zones dividing the implant–bone interface into segments to allow for easier classification of areas of lucency. More recently, a modified version of the Knee Society system was constructed.¹¹ This modification simplifies zone classifications and accommodates more complex revision knee designs and stem extensions.

2. Posterior stabilized designs

Cruciate-retaining prostheses are seldom applicable in the revision TKA setting because of frequent damage to the posterior cruciate ligament, except in the case of simple polyethylene exchanges or, potentially, revisions of failed unicompartmental TKAs. Thus, posterior stabilized designs are the first-line choice for revision TKA (Figure 1). These prostheses are indicated only when the posterior cruciate ligament is incompetent and in the setting of adequate flexion and extension and medial and lateral collateral ligament balancing.

However, studies have shown that posterior stabilized TKAs have a limited role in revision TKAs, as the amount of ligamentous and bony damage is often underestimated in these patients, and use of a primary implant in a revision setting often requires additional augments, all of which may have contributed to the high failure rate. Thus, this design should be used only when the patient has adequate bone stock (AORI type I) and collateral ligament tension. This situation further emphasizes the importance of performing intraoperative testing for ligamentous balance and bone deficit evaluation in order to determine the most appropriate implant (Table 2).

3. Nonlinked constrained designs

Nonlinked constrained (condylar constrained) designs are the devices most commonly used for revision TKAs (>50% of revision knees). These prostheses provide increased articular constraint, which is required in patients with persistent instability, despite appropriate soft-tissue balancing. Increased articular constraint allows for more knee stability by providing progressive varus-valgus, coronal, and rotational stability with the aid of taller and wider tibial posts.¹² Specifically, these implants incorporate a tibial post that fits closely between the femoral condyles, allowing for less motion compared with a standard posterior stabilized design.¹²

In addition, these designs may be used with augments, stems, and allografts when bone loss is more substantial. In particular, stem extensions allow for load distribution to the diaphyseal regions of the tibia and femur and thereby aid in reducing the increased stress at the bone–implant interface, which is a common concern with these implants. However, these extensions cost more, require intramedullary invasion, and are associated with higher rates of leg and thigh pain.¹²

These prostheses are often implicated in cases involving a high degree of bone loss (eg, AORI type II or III). They are ideally used in cases in which complete revision of both tibial and femoral components is needed and are indicated in cases of incompetent posterior cruciate ligament, partial functional loss of medial or lateral collateral ligaments, or flexion-extension mismatch.¹³ Furthermore, use of a constrained prosthesis is recommended in the setting of varus or valgus instability, or repeated dislocations of a posterior stabilized design (Table 2).

Ten-year survivorship ranges from 85% to 96%, but this is substantially lower than the 95% to 96% for condylar constrained prostheses used in primary TKAs.^14-17 Moreover, the large discrepancy between survivorship of primary TKA and revision TKA with a constrained prosthesis further affirms that the complexity of revision surgery, rather than the prosthesis used, may have more deleterious effects on outcomes. However, surgeons must be aware that increased constraint leads to increased stress on the prosthetic interfaces with associated aseptic loosening and early failure, and this continues to be a legitimate concern.

4. Rotating hinge designs

Many patients who undergo revision TKA can be managed with a posterior stabilizing or nonlinked constrained design. However, in patients who present with severe ligamentous instability and bone loss (AORI type II or III), a rotating hinge prostheses, or highly constrained device, is often recommended (Figure 2).¹⁸ By using a rotating mobile-bearing platform, this prosthesis permits axial rotation through a metal-reinforced polyethylene-post articulation in the tibial tray. In addition, it involves use of modular diaphyseal-engaging stems and diaphyseal sleeves, which allow for the bypass of bony defects and areas of bone loss (Table 2).

However, the rigid biomechanics of hinged prostheses is associated with increased risk for aseptic loosening (aseptic 10-year survival, 60%-80%), imparted by the transfer of stresses across the bone. The higher risk for early loosening, osteolysis, and excessive wear—caused by the highly restricted biomechanics of early generations of fixed hinged designs—has led to the development of new devices with mobile mechanics. Prosthetic designs have been improved with an added rotational axis to reduce torsional stress, a patellar resurfacing option, and better stem fixation and patellofemoral kinematics. Overall, these are aimed to improve rates of instability and aseptic loosening, with promising results demonstrated in the literature.

5. Modular segmental arthroplasty designs

Segmental arthroplasty prostheses, which typically are end-of-the-line revision TKA options, are applicable only in cases of extensive bone loss (more than can be treated with allografts or augments; AORI type 3), complete ligamentous disruption/absence, loss of periprosthetic soft tissue, and multiple previous revision procedures (Figure 3). Despite the limited indications for these prostheses, they yield quick return to function without graft nonunion or resorption, and they augment ingrowth/ongrowth. Furthermore, the next surgical option could be fusion or amputation. When failures were specifically evaluated for aseptic loosening across 4 studies, the survival rate ranged from 83% to 99.5%, with the most frequent complication being infection (up to 33% in one series).^6,19-21

The major roles for segmental arthroplasty prostheses in primary TKAs are in the setting of oncologic conditions that require bony excision, or unreconstuctable fractures about the knee. Used after ancillary metastatic disease, these prostheses demonstrate positive results, according to several reports.^22,23 In the setting of revision TKA, however, these prostheses should be used only when other surgical options are unfeasible, given the high risk for infection and the re-revision rates. Currently, revision TKAs with tumor prostheses have a high failure rate (up to 50%) because of the extensive surgery and the lack of bony and soft-tissue support (Table 2).

Conclusion

Orthopedists performing revision TKAs must consider bone stock and remaining ligament stability. In particular, they should choose implants for least constraint and adequate knee stability, as these are essential in minimizing the stresses on the implant–bone interface. Ultimately, functional outcomes, survivorship, and postoperative satisfaction determine the success of these designs. However, predictors of outcomes of revision surgery are often multifactorial, and surgeons must also consider procedure complexity and patient-specific characteristics.

Before 1990, a considerable number of revisions were performed, largely for implant-associated failures, in the first few years after index primary knee arthroplasties.^1,2 Since then, surgeons, manufacturers, and hospitals have collaborated to improve implant designs, techniques, and care guidelines.^3,4 Despite the substantial improvements in designs, which led to implant longevity of more than 15 years in many cases, these devices still have limited life spans. Large studies have estimated that the risk for revision required after primary knee arthroplasty ranges from as low as 5% at 15 years to up to 9% at 10 years.^4,5

The surgical goals of revision total knee arthroplasty (TKA) are to obtain stable fixation of the prosthesis to host bone, to obtain a stable range of motion compatible with the patient’s activities of daily living, and to achieve these goals while using the smallest amount of prosthetic augments and constraint so that the soft tissues may share in load transfer.⁶ As prosthetic constraint increases, the soft tissues participate less in load sharing, and increasing stresses are put on the implant–bone interface, which further increases the risk for early implant loosening.⁷ Hence, as characteristics of a revision implant become more constrained, there is often a higher rate of aseptic loosening expected.⁸

Controversy remains regarding the ideal implant type for revision TKA. To ensure the success of revision surgery and to reduce the risks for postoperative dissatisfaction, complications, and re-revision, orthopedists must understand the types of revision implant designs available, particularly as each has its own indications and potential complications.

In this article, we review the classification systems used for revision TKA as well as the types of prosthetic designs that can be used: posterior stabilized, nonlinked constrained, rotating hinge, and modular segmental.

1. Classification of bone loss and soft-tissue integrity

To further understand revision TKA, we must consider the complexity level of these cases, particularly by evaluating degree of bone loss and soft-tissue deficiency. The most accepted way to assess bone loss both before and during surgery is to use the AORI (Anderson Orthopaedic Research Institute) classification system.⁹ Bone loss can be classified into 3 types: I, in which metaphyseal bone is intact and small bone defects do not compromise component stability; II, in which metaphyseal bone is damaged and cancellous bone loss requires cement fill, augments, or bone graft; and III, in which metaphyseal bone is deficient, and lost bone comprises a major portion of condyle or plateau and occasionally requires bone grafts or custom implants (Table 1). These patterns of bone loss are occasionally associated with detachment of the collateral ligament or patellar tendon.

In addition to understanding bone loss in revision TKA, surgeons must be aware of soft-tissue deficiencies (eg, collateral ligaments, extensor mechanism), which also influence type and amount of prosthesis constraint. Specifically, constraint choice depends on amount of bone loss and on the condition of stabilizing tissues, such as the collateral ligaments. Under conditions of minimal bone loss and intact peripheral ligaments, a less constrained device, such as a primary posterior stabilized system, can be considered. When ligaments are present but insufficient, a semiconstrained device is recommended. In the presence of medial collateral ligament attenuation or complete medial or lateral collateral ligament dysfunction, a fully constrained prosthesis is required.⁸ Therefore, amount of bone loss or soft-tissue deficiency often dictates which prosthesis to use.

For radiographic classification, the Knee Society roentgenographic evaluation and scoring system¹⁰ has been implemented to allow for uniform reporting of radiographic results and to ensure adequate preoperative planning and postoperative assessment of component alignment. This system incorporates the evaluation of alignment in the coronal, sagittal, and patellofemoral planes and assesses radiolucency using zones dividing the implant–bone interface into segments to allow for easier classification of areas of lucency. More recently, a modified version of the Knee Society system was constructed.¹¹ This modification simplifies zone classifications and accommodates more complex revision knee designs and stem extensions.

2. Posterior stabilized designs

Cruciate-retaining prostheses are seldom applicable in the revision TKA setting because of frequent damage to the posterior cruciate ligament, except in the case of simple polyethylene exchanges or, potentially, revisions of failed unicompartmental TKAs. Thus, posterior stabilized designs are the first-line choice for revision TKA (Figure 1). These prostheses are indicated only when the posterior cruciate ligament is incompetent and in the setting of adequate flexion and extension and medial and lateral collateral ligament balancing.

However, studies have shown that posterior stabilized TKAs have a limited role in revision TKAs, as the amount of ligamentous and bony damage is often underestimated in these patients, and use of a primary implant in a revision setting often requires additional augments, all of which may have contributed to the high failure rate. Thus, this design should be used only when the patient has adequate bone stock (AORI type I) and collateral ligament tension. This situation further emphasizes the importance of performing intraoperative testing for ligamentous balance and bone deficit evaluation in order to determine the most appropriate implant (Table 2).

3. Nonlinked constrained designs

Nonlinked constrained (condylar constrained) designs are the devices most commonly used for revision TKAs (>50% of revision knees). These prostheses provide increased articular constraint, which is required in patients with persistent instability, despite appropriate soft-tissue balancing. Increased articular constraint allows for more knee stability by providing progressive varus-valgus, coronal, and rotational stability with the aid of taller and wider tibial posts.¹² Specifically, these implants incorporate a tibial post that fits closely between the femoral condyles, allowing for less motion compared with a standard posterior stabilized design.¹²

In addition, these designs may be used with augments, stems, and allografts when bone loss is more substantial. In particular, stem extensions allow for load distribution to the diaphyseal regions of the tibia and femur and thereby aid in reducing the increased stress at the bone–implant interface, which is a common concern with these implants. However, these extensions cost more, require intramedullary invasion, and are associated with higher rates of leg and thigh pain.¹²

These prostheses are often implicated in cases involving a high degree of bone loss (eg, AORI type II or III). They are ideally used in cases in which complete revision of both tibial and femoral components is needed and are indicated in cases of incompetent posterior cruciate ligament, partial functional loss of medial or lateral collateral ligaments, or flexion-extension mismatch.¹³ Furthermore, use of a constrained prosthesis is recommended in the setting of varus or valgus instability, or repeated dislocations of a posterior stabilized design (Table 2).

Ten-year survivorship ranges from 85% to 96%, but this is substantially lower than the 95% to 96% for condylar constrained prostheses used in primary TKAs.^14-17 Moreover, the large discrepancy between survivorship of primary TKA and revision TKA with a constrained prosthesis further affirms that the complexity of revision surgery, rather than the prosthesis used, may have more deleterious effects on outcomes. However, surgeons must be aware that increased constraint leads to increased stress on the prosthetic interfaces with associated aseptic loosening and early failure, and this continues to be a legitimate concern.

4. Rotating hinge designs

Many patients who undergo revision TKA can be managed with a posterior stabilizing or nonlinked constrained design. However, in patients who present with severe ligamentous instability and bone loss (AORI type II or III), a rotating hinge prostheses, or highly constrained device, is often recommended (Figure 2).¹⁸ By using a rotating mobile-bearing platform, this prosthesis permits axial rotation through a metal-reinforced polyethylene-post articulation in the tibial tray. In addition, it involves use of modular diaphyseal-engaging stems and diaphyseal sleeves, which allow for the bypass of bony defects and areas of bone loss (Table 2).

However, the rigid biomechanics of hinged prostheses is associated with increased risk for aseptic loosening (aseptic 10-year survival, 60%-80%), imparted by the transfer of stresses across the bone. The higher risk for early loosening, osteolysis, and excessive wear—caused by the highly restricted biomechanics of early generations of fixed hinged designs—has led to the development of new devices with mobile mechanics. Prosthetic designs have been improved with an added rotational axis to reduce torsional stress, a patellar resurfacing option, and better stem fixation and patellofemoral kinematics. Overall, these are aimed to improve rates of instability and aseptic loosening, with promising results demonstrated in the literature.

5. Modular segmental arthroplasty designs

Segmental arthroplasty prostheses, which typically are end-of-the-line revision TKA options, are applicable only in cases of extensive bone loss (more than can be treated with allografts or augments; AORI type 3), complete ligamentous disruption/absence, loss of periprosthetic soft tissue, and multiple previous revision procedures (Figure 3). Despite the limited indications for these prostheses, they yield quick return to function without graft nonunion or resorption, and they augment ingrowth/ongrowth. Furthermore, the next surgical option could be fusion or amputation. When failures were specifically evaluated for aseptic loosening across 4 studies, the survival rate ranged from 83% to 99.5%, with the most frequent complication being infection (up to 33% in one series).^6,19-21

The major roles for segmental arthroplasty prostheses in primary TKAs are in the setting of oncologic conditions that require bony excision, or unreconstuctable fractures about the knee. Used after ancillary metastatic disease, these prostheses demonstrate positive results, according to several reports.^22,23 In the setting of revision TKA, however, these prostheses should be used only when other surgical options are unfeasible, given the high risk for infection and the re-revision rates. Currently, revision TKAs with tumor prostheses have a high failure rate (up to 50%) because of the extensive surgery and the lack of bony and soft-tissue support (Table 2).

Conclusion

Orthopedists performing revision TKAs must consider bone stock and remaining ligament stability. In particular, they should choose implants for least constraint and adequate knee stability, as these are essential in minimizing the stresses on the implant–bone interface. Ultimately, functional outcomes, survivorship, and postoperative satisfaction determine the success of these designs. However, predictors of outcomes of revision surgery are often multifactorial, and surgeons must also consider procedure complexity and patient-specific characteristics.

The overall success of total knee arthroplasty (TKA) depends on proper implant choice, meticulous surgical technique, appropriate patient selection, and
effective postoperative rehabilitation. Inappropriate technique leads to suboptimal placement of implants in coronal, sagittal, or axial planes.^1-3 This results in eccentric prosthetic loading, which may contribute to accelerated polyethylene wear, early component loosening, higher rates of revision surgery, and unsatisfactory clinical outcomes. The need to optimize component positioning during TKA stimulated the development of computer-assisted navigation in TKA in the late 1990’s. Proponents of this technology believe that it helps to reduce outliers, improves coronal, sagittal, and rotational alignment, and optimizes flexion and extension gap-balancing. This is believed to result in improved implant survival and better functional outcomes. However, despite these postulated advantages, less than 5% of surgeons in the United States currently use navigation during TKA perhaps due to concerns of costs, increased operating time, learn- ing curve issues, and lack of improvement in functional outcomes at mid-term follow-up.
Navigated TKA, due to its accuracy and low margins of error, has the potential to reduce component malalignment to within 1o to 2o of neutral mechanical axis.⁴ However, others have reported that alignment of the femoral and tibial components achieved with computer navigation is not different than TKA using conventional techniques.^5-12 This lack of improvement reported in these studies may be due to a number of potential sources of errors, which can be either surgeon- or device-related. These errors may pre-dispose to discrepancies between alignments calculated by the computer and the actual position of the implants. Apart from software- and hardware-related calibration issues, the majority of inaccuracies, which are often surgeon-related, result from registration of anatomical landmarks, pin array movements after registration, incorrect bone cuts despite accurate jig placement, and incorrect placement of final components during cementation. Of these surgeon-related factors, variability in the identification of the anatomical landmarks appears to be critical and occurs due to anatomical variations or from inaccurate recognition of intraoperative bony landmarks. A recent study found that registration of the distal femoral epicondyles was more likely to be inaccurate than other anatomical landmarks, as it was found that a small change of 2 mm in the sagittal plane can lead to a 1o change in the femoral component rotation.¹³

Nevertheless, the general consensus from recent high- level evidence (Level I and II) suggests that navigated TKA leads to improved coronal-alignment outcomes and reduced numbers of outliers.^14-18 In a recent systematic review of 27 randomized controlled trials of 2541 patients, Hetaimish and colleagues19 compared the alignment outcomes of navigated with conventional TKA. The authors found that the navigated cohort had a significantly lower risk of producing a mechanical axis deviation of greater than 3o, compared with conventional TKA (relative risk [RR] = 0.37; P<.001). The femoral and tibial, coronal and sagittal malalignment (>3o) were also found to be significantly lower with navigated TKA, compared with conventional techniques. However, no substantial differences were found in the rotation alignment of the femoral component between the 2 comparison cohorts (navigated group, 18.8%; conventional group, 14.5%).

Advocates of navigation believe that improved component alignment would lead to better functional outcomes and lower revision rates.^20,21 However, at short- to mid-term follow-up, most studies have failed to show any substantial benefits in terms of functional outcomes, revision rates, patient satisfaction, or patient-perceived quality-of-life, when comparing computer-assisted navigation to conventional techniques.^11,22-25 Recent systematic reviews by Zamora et al²⁴ and Burnett et al25 found no significant differences in the functional outcomes between navigated and conventional TKA (P>.05). This lack of the expected improvement in functional outcomes reported in various studies with navigation could be due to variability in registration of anatomical landmarks leading to errors in the rotational axis, or a lack of complete understanding of the interplay of alignment, ligament balance, in vivo joint loading and kinematics. In a report from the Mayo Clinic,²⁶ the authors believed that there may be little practical value in relying on a mechanical alignment of ±3o from neutral as an isolated variable in predicting the longevity of modern TKA. In addition, they suggested that factors apart from mechanical alignment may have a more profound impact on implant durability.

Several studies^27-31 that compared the joint line changes or ligament balance between navigated and conventional TKA, report no substantial differences in the maintenance of the joint line, quality of life, and functional outcomes. Despite claims of decreased blood loss, length-of stay, cardiac complications, and lower risks of fat embolism with computer-assisted navigation by some authors, other reports have failed to demonstrate any substantial advantages, therefore, it is controversial if any clear benefit exists.^6,32-34 It is postulated that the high initial institutional costs of navigation can even out in the long run if the goals of improved survivorship and functional outcomes are achieved.³⁵ However, as mid-term follow-up studies have failed to show a survival or functional benefit, the purported costs savings from computer navigation may not be accurate. Navigated TKA has been reported to increase operative time by about 15 to 20 minutes, compared with conventional TKA. Although, this increases operative time, it has not been reported to increase the risk of deep prosthetic joint infections.

Navigation provides some benefits in terms of radiological alignment. However, the clinical advantages are yet to be defined. Currently, there are many unanswered ques- tions concerning alignment in TKA, such as having a more individual approach based on the patients’ own anatomic variations including considerations about the presence of constitutional varus in patients. Navigation may have a role when TKA is performed for complex deformities, fractures, or in the presence of retained implants that prevent the use of conventional guides. Nevertheless, one should always keep in mind cost considerations. This has been true with any technological advancement we have had in the past and will be of concern in the future as well, especially with rising healthcare costs. When analyzing costs with naviga- tion, one must take in to account not only the overall costs of technology, but also the added costs of training, increased operating room times, and disposables when performing these procedures. Although we are advocates of change and are excited about this technology, the cost-benefit ratio for computer navigated TKA needs to be reconciled.

The overall success of total knee arthroplasty (TKA) depends on proper implant choice, meticulous surgical technique, appropriate patient selection, and
effective postoperative rehabilitation. Inappropriate technique leads to suboptimal placement of implants in coronal, sagittal, or axial planes.^1-3 This results in eccentric prosthetic loading, which may contribute to accelerated polyethylene wear, early component loosening, higher rates of revision surgery, and unsatisfactory clinical outcomes. The need to optimize component positioning during TKA stimulated the development of computer-assisted navigation in TKA in the late 1990’s. Proponents of this technology believe that it helps to reduce outliers, improves coronal, sagittal, and rotational alignment, and optimizes flexion and extension gap-balancing. This is believed to result in improved implant survival and better functional outcomes. However, despite these postulated advantages, less than 5% of surgeons in the United States currently use navigation during TKA perhaps due to concerns of costs, increased operating time, learn- ing curve issues, and lack of improvement in functional outcomes at mid-term follow-up.
Navigated TKA, due to its accuracy and low margins of error, has the potential to reduce component malalignment to within 1o to 2o of neutral mechanical axis.⁴ However, others have reported that alignment of the femoral and tibial components achieved with computer navigation is not different than TKA using conventional techniques.^5-12 This lack of improvement reported in these studies may be due to a number of potential sources of errors, which can be either surgeon- or device-related. These errors may pre-dispose to discrepancies between alignments calculated by the computer and the actual position of the implants. Apart from software- and hardware-related calibration issues, the majority of inaccuracies, which are often surgeon-related, result from registration of anatomical landmarks, pin array movements after registration, incorrect bone cuts despite accurate jig placement, and incorrect placement of final components during cementation. Of these surgeon-related factors, variability in the identification of the anatomical landmarks appears to be critical and occurs due to anatomical variations or from inaccurate recognition of intraoperative bony landmarks. A recent study found that registration of the distal femoral epicondyles was more likely to be inaccurate than other anatomical landmarks, as it was found that a small change of 2 mm in the sagittal plane can lead to a 1o change in the femoral component rotation.¹³

Nevertheless, the general consensus from recent high- level evidence (Level I and II) suggests that navigated TKA leads to improved coronal-alignment outcomes and reduced numbers of outliers.^14-18 In a recent systematic review of 27 randomized controlled trials of 2541 patients, Hetaimish and colleagues19 compared the alignment outcomes of navigated with conventional TKA. The authors found that the navigated cohort had a significantly lower risk of producing a mechanical axis deviation of greater than 3o, compared with conventional TKA (relative risk [RR] = 0.37; P<.001). The femoral and tibial, coronal and sagittal malalignment (>3o) were also found to be significantly lower with navigated TKA, compared with conventional techniques. However, no substantial differences were found in the rotation alignment of the femoral component between the 2 comparison cohorts (navigated group, 18.8%; conventional group, 14.5%).

Advocates of navigation believe that improved component alignment would lead to better functional outcomes and lower revision rates.^20,21 However, at short- to mid-term follow-up, most studies have failed to show any substantial benefits in terms of functional outcomes, revision rates, patient satisfaction, or patient-perceived quality-of-life, when comparing computer-assisted navigation to conventional techniques.^11,22-25 Recent systematic reviews by Zamora et al²⁴ and Burnett et al25 found no significant differences in the functional outcomes between navigated and conventional TKA (P>.05). This lack of the expected improvement in functional outcomes reported in various studies with navigation could be due to variability in registration of anatomical landmarks leading to errors in the rotational axis, or a lack of complete understanding of the interplay of alignment, ligament balance, in vivo joint loading and kinematics. In a report from the Mayo Clinic,²⁶ the authors believed that there may be little practical value in relying on a mechanical alignment of ±3o from neutral as an isolated variable in predicting the longevity of modern TKA. In addition, they suggested that factors apart from mechanical alignment may have a more profound impact on implant durability.

Several studies^27-31 that compared the joint line changes or ligament balance between navigated and conventional TKA, report no substantial differences in the maintenance of the joint line, quality of life, and functional outcomes. Despite claims of decreased blood loss, length-of stay, cardiac complications, and lower risks of fat embolism with computer-assisted navigation by some authors, other reports have failed to demonstrate any substantial advantages, therefore, it is controversial if any clear benefit exists.^6,32-34 It is postulated that the high initial institutional costs of navigation can even out in the long run if the goals of improved survivorship and functional outcomes are achieved.³⁵ However, as mid-term follow-up studies have failed to show a survival or functional benefit, the purported costs savings from computer navigation may not be accurate. Navigated TKA has been reported to increase operative time by about 15 to 20 minutes, compared with conventional TKA. Although, this increases operative time, it has not been reported to increase the risk of deep prosthetic joint infections.

Navigation provides some benefits in terms of radiological alignment. However, the clinical advantages are yet to be defined. Currently, there are many unanswered ques- tions concerning alignment in TKA, such as having a more individual approach based on the patients’ own anatomic variations including considerations about the presence of constitutional varus in patients. Navigation may have a role when TKA is performed for complex deformities, fractures, or in the presence of retained implants that prevent the use of conventional guides. Nevertheless, one should always keep in mind cost considerations. This has been true with any technological advancement we have had in the past and will be of concern in the future as well, especially with rising healthcare costs. When analyzing costs with naviga- tion, one must take in to account not only the overall costs of technology, but also the added costs of training, increased operating room times, and disposables when performing these procedures. Although we are advocates of change and are excited about this technology, the cost-benefit ratio for computer navigated TKA needs to be reconciled.

The overall success of total knee arthroplasty (TKA) depends on proper implant choice, meticulous surgical technique, appropriate patient selection, and
effective postoperative rehabilitation. Inappropriate technique leads to suboptimal placement of implants in coronal, sagittal, or axial planes.^1-3 This results in eccentric prosthetic loading, which may contribute to accelerated polyethylene wear, early component loosening, higher rates of revision surgery, and unsatisfactory clinical outcomes. The need to optimize component positioning during TKA stimulated the development of computer-assisted navigation in TKA in the late 1990’s. Proponents of this technology believe that it helps to reduce outliers, improves coronal, sagittal, and rotational alignment, and optimizes flexion and extension gap-balancing. This is believed to result in improved implant survival and better functional outcomes. However, despite these postulated advantages, less than 5% of surgeons in the United States currently use navigation during TKA perhaps due to concerns of costs, increased operating time, learn- ing curve issues, and lack of improvement in functional outcomes at mid-term follow-up.
Navigated TKA, due to its accuracy and low margins of error, has the potential to reduce component malalignment to within 1o to 2o of neutral mechanical axis.⁴ However, others have reported that alignment of the femoral and tibial components achieved with computer navigation is not different than TKA using conventional techniques.^5-12 This lack of improvement reported in these studies may be due to a number of potential sources of errors, which can be either surgeon- or device-related. These errors may pre-dispose to discrepancies between alignments calculated by the computer and the actual position of the implants. Apart from software- and hardware-related calibration issues, the majority of inaccuracies, which are often surgeon-related, result from registration of anatomical landmarks, pin array movements after registration, incorrect bone cuts despite accurate jig placement, and incorrect placement of final components during cementation. Of these surgeon-related factors, variability in the identification of the anatomical landmarks appears to be critical and occurs due to anatomical variations or from inaccurate recognition of intraoperative bony landmarks. A recent study found that registration of the distal femoral epicondyles was more likely to be inaccurate than other anatomical landmarks, as it was found that a small change of 2 mm in the sagittal plane can lead to a 1o change in the femoral component rotation.¹³

Nevertheless, the general consensus from recent high- level evidence (Level I and II) suggests that navigated TKA leads to improved coronal-alignment outcomes and reduced numbers of outliers.^14-18 In a recent systematic review of 27 randomized controlled trials of 2541 patients, Hetaimish and colleagues19 compared the alignment outcomes of navigated with conventional TKA. The authors found that the navigated cohort had a significantly lower risk of producing a mechanical axis deviation of greater than 3o, compared with conventional TKA (relative risk [RR] = 0.37; P<.001). The femoral and tibial, coronal and sagittal malalignment (>3o) were also found to be significantly lower with navigated TKA, compared with conventional techniques. However, no substantial differences were found in the rotation alignment of the femoral component between the 2 comparison cohorts (navigated group, 18.8%; conventional group, 14.5%).

Advocates of navigation believe that improved component alignment would lead to better functional outcomes and lower revision rates.^20,21 However, at short- to mid-term follow-up, most studies have failed to show any substantial benefits in terms of functional outcomes, revision rates, patient satisfaction, or patient-perceived quality-of-life, when comparing computer-assisted navigation to conventional techniques.^11,22-25 Recent systematic reviews by Zamora et al²⁴ and Burnett et al25 found no significant differences in the functional outcomes between navigated and conventional TKA (P>.05). This lack of the expected improvement in functional outcomes reported in various studies with navigation could be due to variability in registration of anatomical landmarks leading to errors in the rotational axis, or a lack of complete understanding of the interplay of alignment, ligament balance, in vivo joint loading and kinematics. In a report from the Mayo Clinic,²⁶ the authors believed that there may be little practical value in relying on a mechanical alignment of ±3o from neutral as an isolated variable in predicting the longevity of modern TKA. In addition, they suggested that factors apart from mechanical alignment may have a more profound impact on implant durability.

Several studies^27-31 that compared the joint line changes or ligament balance between navigated and conventional TKA, report no substantial differences in the maintenance of the joint line, quality of life, and functional outcomes. Despite claims of decreased blood loss, length-of stay, cardiac complications, and lower risks of fat embolism with computer-assisted navigation by some authors, other reports have failed to demonstrate any substantial advantages, therefore, it is controversial if any clear benefit exists.^6,32-34 It is postulated that the high initial institutional costs of navigation can even out in the long run if the goals of improved survivorship and functional outcomes are achieved.³⁵ However, as mid-term follow-up studies have failed to show a survival or functional benefit, the purported costs savings from computer navigation may not be accurate. Navigated TKA has been reported to increase operative time by about 15 to 20 minutes, compared with conventional TKA. Although, this increases operative time, it has not been reported to increase the risk of deep prosthetic joint infections.

Navigation provides some benefits in terms of radiological alignment. However, the clinical advantages are yet to be defined. Currently, there are many unanswered ques- tions concerning alignment in TKA, such as having a more individual approach based on the patients’ own anatomic variations including considerations about the presence of constitutional varus in patients. Navigation may have a role when TKA is performed for complex deformities, fractures, or in the presence of retained implants that prevent the use of conventional guides. Nevertheless, one should always keep in mind cost considerations. This has been true with any technological advancement we have had in the past and will be of concern in the future as well, especially with rising healthcare costs. When analyzing costs with naviga- tion, one must take in to account not only the overall costs of technology, but also the added costs of training, increased operating room times, and disposables when performing these procedures. Although we are advocates of change and are excited about this technology, the cost-benefit ratio for computer navigated TKA needs to be reconciled.