[A new mechanism of injury of the anterior cruciate ligament].

In alpine skiing various mechanisms of injury to the anterior cruciate ligament have been described. External rotation-flexion-valgus stress and hyper-extension are considered the classic mechanisms. Rupture of the anterior cruciate ligament without falling has rarely been reported. In ski racers competing in the World Cup downhill races an accumulation of isolated anterior cruciate ruptures following landing after a jump in a more or less backward position without falling has become evident in recent years. We recorded this mechanism for the first time in 1986. A ski racer landing in a bent position on flat ground after a jump documented on video tape felt a sharp pain and immediate instability, though he did not fall. Clinically hemarthrosis and a positive Lachman sign were found, and intraoperatively an isolated rupture of the anterior cruciate ligament was confirmed. Analysis of the video tape and biomechanical considerations showed that even a minimal shift of the center of body mass dorsal to the axis of the lower leg leads to an acceleration of the thigh and body oriented centrifugally, vertically and in a plantar direction against the rotational axis of the knee and to a forward rotational acceleration of the tibia by the dorsal shaft of the ski boot. This acceleration produces a massive compensatory quadriceps contraction to prevent a backward fall, followed by an "anteroposterior shift" of the femur on the tibia in the sense of an anterior drawer, which in association with other factors leads to an ACL rupture. This "anteroposterior shift mechanism" is only possible in excellent skiers with the muscular control and coordination to prevent a backward fall.