INTRODUCTION: There are only a few scientific reports which describe the kinematics of anterior cruciate ligament (ACL) ruptures. The aim of this study was to describe the knee kinematics of ACL ruptures with a single-angle video. METHODS: To collect kinematic data, a 3DCAD model was developed for both skis and a skier`s entire body. The model was manually matched to each video flame using 3DCAD software (Inventor Professional 2022, Autodesk, USA). A 21-year-old female alpine skier suffered an ACL tear of the left knee in a Far-East-Cup giant-slalom race in Nozawa, Japan on March 6, 2015. Prior to the injury, the skier lost snow contact of the left ski, but regained contact 20 ms prior to the injury. A video of the injury occurrence was filmed by a skiing coach with a video camera with a sampling rate of 30 fps. The moment of the injury situation was determined by hearing for the skier and by checking with the video. The knee flexion angle, knee internal rotation angle, and knee valgus angle for the injured side (left) leg were calculated using 3DCAD software. RESULTS: At the moment of the injury, the knee flexion angle was 36.9 degrees, internal rotation angle was 18.1 degrees, and valgus angle was 11.2 degrees. At 20 ms before the injury, the knee was extending at 3.5 degrees, internally rotating at 8.0 degrees, and valgus rotating with 3.3 degrees. DISCUSSION/CONCLUSION: Compared to previous reports, the characteristics of knee angles in this study shows a different trend. Using 3DCAD matching on a single-angle video may help to describe the knee kinematics in the moment of ACL injury.
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