Abnormally high dislocation rate following constrained condylar knee arthroplasty for valgus knee: a case-control study

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Abstract
With the use of constrained condylar knee (CCK) prostheses, dislocation of the knee following total knee arthroplasty (TKA) with valgus deformity is rare. In our practice with such patients, however, an abnormally high dislocation rate was noted. It appeared to be associated with the extent of soft-tissue release which varied among surgeons following different sequences of release. We asked in CCK TKA with valgus deformity is releasing both the lateral collateral ligament (LCL) and popliteus tendon (PT) associated with the occurrence of dislocation. This is a case-control study of consecutive patients with valgus deformity who underwent primary CCK TKA between July 2008 and October 2015. The cases and controls were patients with and without postoperative dislocation of the knee, respectively. The extent of the release of lateral soft-tissue structures was compared between the two groups. Other patient characteristics including age, body mass index, pre- and post-operative valgus deformity, preoperative flexion-contracture, and height of the polyethylene insert were compared as well to reduce confounding. Forty-three consecutive patients with a minimum 2-year follow-up were enrolled. 9.3% (4/43) of the patients had postoperative dislocation of the knee. While the dislocated patients did not significantly differ from the controls on most characteristics, they were more likely to have both the LCL and PT released together during the surgery [100% (4/4) vs. 2.6% (1/39), P < 0.001]. Releasing both LCL and PT in CCK TKA with valgus deformity may increase the risk of dislocation, and need to be performed with some caution.

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