Functional ankle instability (FAI) may involve abnormal kinematics. However, reliable quantitative data for kinematics of FAI have not been reported. The objective of this study was to determine if the abnormal kinematics exist in the talocrural and subtalar joints in patients with FAI. Five male subjects with unilateral FAI (a mean age of 33.4 ± 13.2 years) were enrolled. All subjects were examined with stress radiography and found to have no mechanical ankle instability (MAI). Lateral radiography at weight-bearing ankle internal rotation of 0° and 20° was taken with the ankle at 30° dorsiflexion and 30° plantar flexion. Patients underwent computed tomography scan at 1.0 mm slice pitch spanning distal one third of the lower leg and the distal end of the calcaneus. Three-dimensional (3D) kinematics of the talocrural and subtalar joints as well as the ankle joint complex (AJC) were determined using a 3D-to-2D registration technique using a 3D-to-2D registration technique with 3D bone models and plain radiography. FAI joints in ankle dorsiflexion demonstrated significantly greater subtalar internal rotation from 0° to 20° internal rotation. No statistical differences in plantar flexion were detected in talocrural, subtalar or ankle joint complex kinematics between the FAI and contralateral healthy joints. During ankle internal rotation in dorsiflexion, FAI joints demonstrated greater subtalar internal rotation. The FAI joints without mechanical instability presented abnormal kinematics. This suggests that abnormal kinematics of the FAI joints may contribute to chronic instability. FAI joints may involve unrecognized abnormal subtalar kinematics during internal rotation in ankle dorsiflexion which may contribute to chronic instability and frequent feelings of instability.

Objectives: This study was aimed to reveal the differences in knee valgus angle at landing as a static indicator and wobbling movement of the knee during landing as a dynamic indicator between ACL injury and uninjured athletes. Methods: This study was case-control study. There were 6 female basketball players with ACL injuries and 38 female basketball players without them, whose knee kinematics were measured using 2-dimensional video cameras during single-leg jump landings. The task was performed from 30cm-box. Knee kinematics and wobbling of the knee which was calculated by relative frontal motion to the flexion movement were compared between knees with ACL-injured and uninjured. Results: Six athletes who had confirmed ACL injuries, did not demonstrate significantly different knee valgus angle at initial contact and maximum knee flexion during landing, compared to 38 uninjured athletes. The knee valgus angles at initial contact for injured and uninjured athletes were 12.3° and 14.8° (p = 0.15), respectively. Five of six anterior cruciate ligament injured knees presented knee wobbling during landing. Relative frontal motion at 18° knee flexion was significantly greater in athletes with ACL-injured (p = 0.02). Conclusions: 84% of ACL injury presented with the knee wobbling and the frontal knee motion was greater with low knee flexion during knee wobbling, while the knee valgus angle was not significantly different. This study suggests that knee wobbling may be a biomechanical and dynamic risk factor for ACL injury in female basketball players.