Revision total knee arthroplasty (TKA) is frequently complicated by bone loss and ligament instability, necessitating specialized implants to increase constraint and transmit forces away from the joint surface. Femoral stems are commonly used to enhance fixation and distribute force from the condyles to the metaphysis or diaphysis, to higher-quality bone capable of sustaining the forces at the knee joint.
Modular implants are now commonplace in revision surgery, because they allow intraoperative customization of the implant to the patient’s anatomy, degree of bone loss, and need for metaphyseal or diaphyseal fixation. However, these advantages are not without a downside. The modular junction introduces potential weaknesses in the implant, which may lead to early failure.
We report a case of loosening of a Triathlon TS (Stryker) femoral component that was not evident on preoperative radiographs. To our knowledge, this complication has not been reported with this particular revision knee system. The patient provided written informed consent for print and electronic publication of this case report.
Case Report
A 56-year-old woman underwent 2-stage revision left TKA secondary to infection at an outside institution. She had undergone 17 prior knee surgeries with multiple revisions prior to this most recent revision surgery. A constrained implant was used at her last reimplantation secondary to ligamentous laxity after extensive débridement for infection. A Triathlon TS revision knee system with cemented stemmed tibial and femoral components was implanted; stems designed for uncemented fixation were cemented. She had a history of a quadriceps tendon tear, which was repaired prior to her revision, and quadricepsplasty was performed at the time of revision.
Seven years after this revision surgery, the patient presented to our clinic with progressive global instability, occasional effusions, and 2 documented episodes of frank dislocation. On examination, she was unstable in flexion and extension. Her extensor mechanism was intact, although with 7º active lag. She had a palpable quadriceps tendon defect. Her passive range of motion was 0º to 130º. Her active range of motion was 7º to 130º. Her erythrocyte sedimentation rate and C-reactive protein levels were within normal limits, and aspiration was negative for infection. Radiographs showed apparently well-fixed components with cemented femoral and tibial stems (Figures 1A, 1B).
The patient underwent revision surgery for global instability with the surgical goal to upsize the polyethylene insert and advance the quadriceps to improve stability. In the operating room, a defect in the quadriceps mechanism was seen between the vastus medialis obliquus (VMO) and the patella, as well as a large effusion. Upon removal of the polyethylene insert, the tibial and patellar components were examined and found to be well fixed. The femoral component was grossly loose. On closer inspection, the condylar portion was found to be rotating in the axial plane freely on the well-fixed cemented stem in the femoral canal (Figures 2A-2D). The entire femoral component was removed with some difficulty because the well-fixed uncemented stem design was cemented in place. This required a small, anterior episiotomy of the femur. Reconstruction of the femur was performed using a trabecular metal cone, a cemented stem, and condylar component with distal and posterior augments (Figures 3A, 3B). A shorter, thinner stem was implanted and cemented into the previous cement mantle. A 19-mm constrained polyethylene liner was selected (the prior liner was 13 mm), which gave adequate stability with range of motion 0º to 130º. The VMO was advanced approximately 1.5 cm at the time of closure of the arthrotomy. The patient was implanted with the same Triathlon TS system, because the tibial component was well fixed, well positioned, and did not require revision.
Discussion
The need and use of stemmed, modular femoral components for revision TKA is neither questioned nor a novel concept in arthroplasty.1 Femoral bone defects encountered in revision arthroplasty generally lack sufficient cortical integrity to support an unstemmed component. Biomechanical analyses have reliably demonstrated improved initial stability and reduced relative motion provided by femoral stem extension.2,3 Correspondingly, significant translational and rotational movements of the femoral component when disconnected from the stem presumably correspond with clinical observations of instability.3 We report a unique case of failure of the modular junction of a stemmed femoral component in revision TKA that was not readily apparent on plain radiographs.
Dissociation of a cemented stem from the condylar portion of the component has been described at our institution with a different implant design.4 To our knowledge, we describe the first report of failure at the modular junction of the Triathlon TS femoral component.