Charts were reviewed to identify patients with early postoperative dislocations, as well as dislocation timing, recurrence, and other characteristics. We defined early dislocation as instability occurring within 6 months after surgery. Revision surgery for instability was also identified.
For the statistical analysis, orientation error was defined as the absolute value of the difference between target orientation (40° inclination, 25° anteversion) and radiographic measurements. Repeated-measures multiple regression with the generalized estimating equations approach was used to identify baseline patient characteristics (age, sex, BMI, primary diagnosis, laterality) associated with component positioning outside of our targeted ranges for inclination and anteversion. Fisher exact tests were used to examine the relationship between dislocation and component placement in either the Lewinnek safe zone or our targeted zone. All tests were 2-sided with a significance level of .05. All analyses were performed with SAS for Windows 9.3 (SAS Institute, Cary, North Carolina).
Results
Mean (SD) acetabular inclination was 42.2° (4.9°) (range, 27.6°-65.0°), with a mean (SD) orientation error of 4.2° (3.4°) (Figure 3A). Mean (SD) anteversion was 23.9° (6.5°) (range, 6.2°-48.0°), with a mean (SD) orientation error of 5.2° (4.1°) (Figure 3B). Components were placed outside the Lewinnek safe zone for inclination or anteversion in 46.5% of cases and outside the target zone in 17.7% of cases (Figure 4). Variation in acetabular anteversion alone accounted for 67.3% of target zone outliers (Table 2). Only 0.9% of components were placed outside the target ranges for both inclination and anteversion.
Regression analysis was performed separately for inclination and anteversion to determine the risk factors for placing the acetabular component outside the target orientation ranges. Only higher BMI was associated with malposition with respect to inclination (hazard ratio [HR], 1.059; 95% confidence interval [CI], 1.011-1.111; P = .017). Of obese patients with inclination outside the target range, 90.9% had an inclination angle of more than 50°. Associations between inclination outside the target range and age (P = .769), sex (P = .217), preoperative diagnosis (P > .99), and laterality (P = .106) were statistically insignificant. Only female sex was associated with position of the acetabular component outside the target range for anteversion (HR, 1.871; 95% CI, 1.061-3.299; P = .030). Of female patients with anteversion outside the target range, 70.0% had anteversion of less than 15°. Associations between anteversion outside the target range and age (P = .762), BMI (P = .583), preoperative diagnosis (P > .99), and laterality (P = .235) were statistically insignificant.
Six THAs (1.1%) in 6 patients experienced dislocation within 6 months after surgery (Table 3); mean (SD) time of dislocation was 58.3 (13.8) days after surgery. There was no relationship between dislocation incidence and component placement in the Lewinnek zone (P = .224) or our target zone (P = .287). Of the dislocation cases, 50% involved female patients, and 50% involved right hips. Mean (SD) age of these patients was 53.3 (7.6) years. Mean (SD) BMI was 25.4 (0.9) kg/m2. Osteoarthritis was the primary diagnosis for all patients with early dislocation; 32- or 36-mm femoral heads were used in these cases. Two patients had acetabular components placed outside of our target zone. One patient, who had abnormal pelvic obliquity and sagittal tilt from scoliosis (Figures 5A, 5B), had an acetabular component placed outside both the target zone and the Lewinnek safe zone. Mean (SD) acetabular inclination was 39.8° (3.6°), and mean (SD) anteversion was 21.8° (7.3°) (Figure 5C). Two dislocations resulted from trauma, 1 dislocation was related to hyperlaxity, 1 patient had cerebral palsy, and 1 patient had no evident predisposing risk factors. Three patients (0.54%) had multiple episodes of instability requiring revision during the follow-up period.
Discussion
To our knowledge, this study represents the largest cohort of primary THAs performed with an imageless navigation system. Our results showed that increasing targeted acetabular anteversion to 25° using a posterolateral surgical approach and modern implants resulted in a 1.1% incidence of early dislocation and a 0.54% incidence of recurrent instability requiring reoperation. Of the patients with a dislocation, only 1 did not experience trauma and did not have a risk factor for dislocation. Only 1 patient with a dislocation had acetabular components positioned outside both the target zone and the Lewinnek safe zone. The acetabular component was placed within the target zone in 82.3% of cases in which the imageless navigation system was used. In our cohort, BMI was the only risk factor for placement of the acetabular component outside our target range for inclination, and sex was associated with components outside the target range for anteversion.