Acquired valgus instability after knee replacement.

: Four cases of acquired valgus instability occurred after knee resurfacing operations; all four were initially stable. Three cases originally had a varus deformity which required extensive releases for its correction. All of these patients fell, disrupting the medial joint capsule and medial collateral ligament. This complication in the two early cases could have been prevented by adequate bracing until full recovery of the quadriceps had occurred. In one case, attribution due to projection medially of the tibial component may have played a role. In the fourth case, the tibial component was inserted with the tibia in external rotation, thus advancing the tibial attachment of the medial collateral ligament, which was disrupted during manipulation to gain flexion. The simplest treatment for this complication is probably to brace the knee until the joint reaction settles and then revise to a fully constrained prosthesis.