Arthrogenic muscle inhibition (AMI) is a process which results in quadriceps activation failure and knee extension deficit. The primary aim of this study was to determine the incidence and severity of AMI following acute anterior cruciate ligament (ACL) injury. The secondary aim was to investigate important risk factors.

Consecutive patients who presented with an acute ACL injury between October 2021 and February 2022 were considered for study inclusion. Eligible patients underwent a standardized physical examination at their first outpatient appointment. This included an assessment of quadriceps inhibition, identification of any extension deficits, grading of AMI and its reversibility according to the AMI classification.

Trois cents consecutive patients with acute ACL ruptures were prospectively enrolled in the study. 170 patients (56.7%) presented with AMI. Patients presenting with AMI displayed significantly inferior Lysholm, IKDC, SKV and KOOS scores than patients without AMI (p<0.0001). Multivariate analysis revealed that the presence of effusion, concomitant injuries and high pain scores were associated with a significantly greater risk of AMI. Additional associations with the presence of AMI included a short duration between injury and evaluation, the use of crutches, and using a pillow as a support at night. In contrast, a previous ACL injury was associated with significantly lower odds of developing AMI (OR: 0.025, 95% CI [0 to 0.2], p=0.014). Inter-observer reliability of the classification system was almost perfect (0.86–0.99 CI 95%).

AMI occurs in over half of patients with acute anterior cruciate ligament injuries. When it occurs, it is easily reversible in the majority of patients with simple exercises targeted at abolishing AMI. The presence of “red flags” should increase the index of suspicion for the presence of AMI and these include presence of an effusion, high pain scores, multiligament injuries, use of crutches, and using a pillow as a support at night. Patients with a prior history of ipsilateral or contralateral ACL injury are at significantly lower risk of AMI than those with a first time ACL-injury.