Performance of an athlete with straight posterior knee instability.

Isolated posterior ligamentous insufficiency of the knee frequently terminates an athlete's career and may even adversely affect the performance of tasks of daily living. Return to competitive athletics following surgical reconstruction for posterior instability is unusual. The pathophysiology of the lesion and a biomechanical analysis are discussed. A case report of an athlete with a ruptured posterior cruciate ligament who is presently successfully competing in professional football is also presented. Rigorous quantitative techniques were employed to document the compensatory mechanisms involved. These consisted of extremely strong quadriceps muscles contracting much earlier than normal in the gait cycle. Earlier quadriceps contraction to increase dynamic stability in the knee is able to overcome the instability resulting from the incompetent posterior cruciate ligament.