Kinematics of anterior cruciate ligament ruptures in World Cup alpine skiing: 2 case reports of the slip-catch mechanism.

BACKGROUND Based on visual video analyses of 20 injury situations, the main mechanism of anterior cruciate ligament (ACL) injury in World Cup alpine skiing, termed the "slip-catch" mechanism, was identified. This situation is characterized by a common pattern in which the inside edge of the outer ski catches the snow surface while turning, forcing the knee into valgus and tibial internal rotation. To describe the exact joint kinematics at the time of injury, a more sophisticated approach is needed. PURPOSE To describe the knee and hip kinematics in 2 slip-catch situations utilizing a model-based image-matching (MBIM) technique. STUDY DESIGN Descriptive laboratory study. METHODS Two typical slip-catch situations in World Cup alpine skiing reported through the International Ski Federation (FIS) Injury Surveillance System were captured on video with several camera views and high video quality. The injury situations were analyzed using the MBIM technique to produce continuous measurements of knee and hip joint kinematics. RESULTS Within 60 milliseconds, the knee flexion angle increased rapidly from 26° to 63° in case 1 and from 39° to 69° in case 2. In the same period, we observed a rapid increase in internal rotation of the tibia with a peak of 12° and 9°, respectively. The knee valgus angle changed less markedly in both cases. We also observed a rapid increase of hip flexion as well as substantial hip internal rotation. CONCLUSION Knee compression and knee internal rotation and abduction torque are important components of the injury mechanism in a slip-catch situation. CLINICAL RELEVANCE Prevention efforts should focus on avoiding a forceful tibial internal rotation in combination with knee valgus.