The NFL’s Chop-Block Rule Change: Does It Prevent Knee Injuries in Defensive Players?

BACKGROUND: The chop block, a football maneuver in which an offensive player blocks an opponent around the thigh while another offensive player engages the same opponent above the waist, was declared illegal by the National Football League (NFL) before the 2016-2017 season. Chop blocks have been hypothesized to be associated with medial collateral ligament and anterior cruciate ligament injury, especially in offensive/defensive linemen. PURPOSE: To quantify the impact that the chop-block rule change had on the incidence of knee injuries to defensive players in the NFL over 4 seasons (2014-2018). STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: NFL injury data for all defensive players from regular-season games played from 2014 through 2018 were collected. For this study, all knee injuries were attributed to competitive game play. Injury rates were reported as the number of injuries per 1000 athletic exposures (with 95% CIs). RESULTS: A total of 256 games were played during the 2014-2015, 2015-2016, 2016-2017, and 2017-2018 NFL regular seasons, and all were included in this study. Among defensive players, the relative risk for a knee injury per 1000 athletic exposures was 0.84 (95% CI, 0.75-0.96) for the 2 seasons after the chop-block rule change (2016-2017 and 2017-2018) versus the 2 seasons before (2014-2015 and 2015-2016) (P = .009). Thus, the relative risk reduction was 16%. The relative risk for a defensive player to be placed on injured reserve per season was 0.90 (95% CI, 0.72-1.13) for the 2 seasons after the rule change versus the 2 seasons before (P = .39). CONCLUSION: The NFL’s recent ruling against in-game chop blocks may have reduced the incidence of knee injuries among defensive players.