Kinematics of a slip-catch mechanism for anterior cruciate ligament injury in world cup alpine skiing

Background We have identified that a slip-catch situation is the main mechanism for anterior cruciate ligament (ACL) injuries in World Cup alpine skiing by simple visual inspection of injury videos. This occurs when the skier loses pressure on the outer ski during turning, and while extending the outer knee to regain grip, the inside edge of the outer ski catches abruptly, forcing the knee into valgus and internal rotation. To develop injury prevention methods based on this mechanism, a more precise description is needed. Objective To describe the detailed joint kinematics in a slip-catch situation using a model-based image-matching (MBIM) technique. Design Case study. Setting Men9s World Cup alpine skiing. Methods Video of a slip-catch injury situation in slalom captured with two nearly perpendicular camera views was analyzed using the MBIM method to obtain the time course of knee and hip joint kinematics. Results Knee valgus and internal rotation angle increased by 13° and 13°, respectively, in the last 60 ms before the time of ACL rupture, which was estimated to occur 100 ms after the catching the edge. Knee flexion angle increased by 32° in the last 20 ms from 22° at snow contact to a final position. Hip flexion and adduction angle increased by 18° and 13°, respectively, in the last 20 ms and the hip stayed at approximately 30° of internal rotation in the last 60 ms. Conclusion Abrupt compression, valgus and internal rotation loading seems to be the main mechanism of slip-catch injury situations in ACL injuries in World Cup alpine skiing. The knee loading pattern described from the previous simple visual inspection study matched well with the more detailed analysis using the MBIM technique. Prevention efforts should focus on avoiding valgus and internal rotation loading on the knee.