Summary

Three distinct play types, minimal knee flexion upon initial ground contact, trunk tilt toward one side, and an increase in knee valgus predominate in ACL tears in NBA players.

Abstract

Introduction

Anterior cruciate ligament (ACL) injuries in professional basketball players are devastating injuries that can affect athletic performance and shorten career length. In this study, we performed a comprehensive video review of all ACL injuries in NBA athletes from 2006-2022 to determine associated biomechanics, injury mechanism, and game situation.

Methods

NBA players diagnosed with an ACL tear between the years 2006-2022 were identified using official team injury reports. Videos of each injury were obtained and processed to synchronize multiple camera angles and obtain frame-by-frame screenshots of each injury. Two independent reviewers evaluated each video. Visual evaluation included assessment of joint kinematics of the hip, knees, and ankles using a protractor software. Three predefined time-points were measured for biomechanical angles: initial contact of the injured leg with the ground (IC), 33 ms later (IC+33), and 66 ms later (IC+66). Additionally, game situation, player behavior, and location on court were assessed qualitatively.

Results

Videos of 38 in-game ACL tears in NBA players between October 2006-February 2022 were obtained. 9 injuries (24%) were non-contact, 29 (76%) injuries involved indirect contact, and 0 injuries involved direct contact with the lower leg. Of the indirect contact injuries, 19 (66%) involved arm to trunk contact, 5 (17%) involved arm to arm contact, and 5 (17%) involved trunk to trunk contact. 32 (84%) injuries occurred on offense, 3 (8%) on defense, 2 (5%) on rebounds, and 1 (3%) during a dead-ball. Common situational patterns for injury included gather steps when attacking the basket following picking up the ball (n=16), landing following a jump (n=14), and jump stops (n=5). 31 (82%) injuries occurred within the paint or out of bounds behind the basket. At IC, on average, athletes were found to have minimal knee flexion (12.58°), an ankle in extension (31.44°), a flexed hip (39.29°), and a knee in valgus (5.09°). Between IC and IC+33, knee valgus increased from 5.09° to 11.98° and knee flexion increased from 12.58° to 32.64°, while there was minimal change in hip flexion. On IC, IC+33, and IC+66, the majority of injuries involved the trunk being tilted toward the injured leg, hip abduction, trunk rotation toward injured leg, and neutral foot rotation.

Conclusion

To our knowledge, this is the first study to use video analysis to look at ACL injury mechanisms in NBA players over multiple seasons. Three major mechanisms predominate ACL tears in NBA players: the first step following picking up the ball when attacking the basket, landing following a jump, and jump stops. No injuries examined involve direct contact to the knee, emphasizing the importance of bodily kinematics in this injury pattern. The increase in knee valgus angle between IC and IC+33 should be noted as a possible precipitant to injury. This data may be helpful in identifying players at increased risk for ACL injury due to play style or body kinematics.