Isolated Polyethylene Insert Exchange for Flexion Instability After Primary Total Knee Arthroplasty Demonstrated Excellent Results in Properly Selected Patients.

Background: Historically, isolated polyethylene exchange (IPE) for flexion instability after total knee arthroplasty (TKA) has demonstrated generally poor and unpredictable results. The purpose of this study is to evaluate the results of a single surgeon's strict indications and protocol for IPE for flexion instability after primary TKA.

Methods: Between 2009 and 2016, 418 revision TKAs were performed by the senior author. Patients were considered for IPE if they demonstrated excellent radiographic alignment and component positioning preoperatively. Intraoperatively, if component rotation, sizing, and fixation were all excellent and the flexion and extension gaps could be balanced, then IPE was performed. We retrospectively reviewed 31 knees in 30 patients who were treated with IPE specifically for flexion instability after primary TKA. The mean follow-up was 41 months (range, 24-85 months). Nineteen knees were cruciate-retaining TKAs revised to a more constrained "deep-dish" ultracongruent insert, and 12 posterior-stabilized TKAs were revised to thicker posterior-stabilized insert.

Results: At a mean follow-up of 41 months, only 2 of 31 knees (6.5%) required subsequent component revision surgery for recurrent instability. Knee Society pain scores improved from 70 preoperatively to 86 postoperatively (P < .0001), and function scores improved from 39 points preoperatively to 44 points postoperatively (P = .015).

Conclusion: IPE for flexion instability in carefully selected patients was successful in over 90% of patients for a mean follow-up of 41 months. Pain and function scores significantly improved. Longer-term follow-up is necessary to determine whether these results are durable over time.