Thigh Muscle Changes in the ACL-Deficient Knee: A 4-Year Longitudinal MRI Study of 1,207 Patients.
Journal:
The Journal of bone and joint surgery. American volume
Published Date:
Dec 10, 2025
Abstract
BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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. LEVEL OF EVIDENCE: Prognostic Level II . See Instructions for Authors for a complete description of levels of evidence.
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