Thigh Muscle Changes in the ACL-Deficient Knee: A 4-Year Longitudinal MRI Study of 1,207 Patients

Anterior cruciate ligament (ACL) injuries may lead to long-term neuromuscular and structural adaptations in thigh muscles. Although quadriceps dysfunction is well reported, chronic changes in other muscle groups, especially in nonoperatively managed ACL-deficient individuals, remain poorly understood.

The present cohort study utilized Osteoarthritis Initiative data to assess longitudinal thigh muscle changes in individuals with ACL tears confirmed on magnetic resonance imaging (MRI) and no history of reconstruction. A validated deep-learning model segmented muscle cross-sectional area and quantified intra-muscular adipose tissue and contractile percentage. Quantitative MRI data were obtained at baseline and at 4-year follow-up. Propensity score matching (1:2 to 1:3) controlled for baseline covariates. Strength was assessed with use of standardized Osteoarthritis Initiative protocols. Linear mixed-effects models compared longitudinal changes between ACL-deficient and ACL-intact thighs.

A total of 1,207 thighs were analyzed, including 92 with ACL tears and 1,115 controls, with a mean patient age of 61 ± 9 years. Over 4 years, ACL-deficient thighs exhibited progressive hamstring atrophy (-28.18 mm 2 /year; 95% confidence interval, -42.43 to -13.92; p < 0.001) and sartorius atrophy (-3.02 mm 2 /year; 95% confidence interval, -5.15 to -0.89; p = 0.006). No significant differences were observed in quadriceps or adductor cross-sectional area. Hamstring force decreased significantly (-3.49 N/year; 95% confidence interval, -6.62 to -0.36; p = 0.029), whereas quadriceps force and specific force showed no significant changes. Intra-muscular adipose tissue and contractile percentage did not significantly differ between groups. Findings had been similar in unmatched patients.

The present findings highlight selective muscle deterioration in the posterior thigh muscles following ACL injury, with minimal changes in quadriceps morphology, over time. These results underscore the importance of long-term, targeted rehabilitation strategies focusing on hamstring preservation. This study represents the first longitudinal matched-cohort analysis of muscle morphology and fat infiltration in unreconstructed ACL-deficient knees.

Prognostic Level II . See Instructions for Authors for a complete description of levels of evidence.