Anterior cruciate ligament (ACL) injuries can impact thigh muscle musculature, leading to alterations in muscle size and quality, which can affect strength and function. Longitudinal MRI biomarkers can track these changes in muscle groups over time, but most prior research has been cross-sectional or compared injured limbs with uninjured ones, which often undergo compensatory changes.
Our study aimed to evaluate four-year longitudinal changes in thigh muscle size, composition, and quality in individuals with unrepaired ACL ruptures, compared to matched individuals with intact ACLs. We used data from the Osteoarthritis Initiative (OAI) and excluded participants lacking baseline or longitudinal MRI data. Knees with partial or complete ACL tears, with no history of repair, were classified as having torn ACLs. We used multiple imputation to estimate missing data and performed 1:2/3 propensity-score matching to adjust for confounders such as risk factors and comorbidities.
We assessed muscle changes using a deep learning method for thigh segmentation, focusing on cross-sectional area (CSA), intra-muscular adipose tissue (intra-MAT), contractile percentage (non-fat muscle CSA/total muscle CSA), and specific force (force per CSA). Analyses were adjusted for baseline muscle values, Kellgren-Lawrence grade, and age at first knee injury. Sensitivity to data imputation and matching methods was evaluated.
The study included 348 propensity-score matched thighs (92 with torn ACL, 256 with intact ACL; average age 62 ± 9 years). At baseline, thighs with torn ACLs showed smaller CSA in all muscle groups and lower knee flexor and extensor forces, though they had higher specific force in quadriceps and lower in flexors. Longitudinally, ACL tears were linked to decreased CSA in flexors and Sartorius muscles, as well as decreased flexor muscle force, but not specific force. These results were consistent regardless of the imputation or matching methods used. No significant differences in muscle composition biomarkers, including intra-MAT or contractile percentage, were observed at baseline or follow-up.
In summary, ACL tears are associated with reductions in flexor muscle size and force and Sartorius muscle atrophy. However, there were no longitudinal changes in muscle composition or quality attributable to ACL tears. Contrary to previous studies suggesting smaller quadriceps sizes in ACL-injured knees, our study did not find longitudinal changes in quadriceps size.