Thigh Muscle Changes at Long-Term Evaluation Following ACL Injury Differ from Early Changes Associated with ACL Injury and Reconstruction

The study by Alzobi et al. utilized demographic data, strength measurements, and magnetic resonance imaging (MRI) from the Osteoarthritis Initiative (OAI) to evaluate muscle function following anterior cruciate ligament (ACL) injury. The authors added deep learning-based segmentation of the MRI to quantify parameters related to the function of individual muscle groups. The analysis included knees with nonoperatively treated ACL tears that were longitudinally evaluated over 4 years. Propensity-score matching applied to the OAI population allowed comparisons relative to a group with an intact ACL that controlled for age, sex, body mass index, Physical Activity Scale for the Elderly score, race/ethnicity, and abdominal obesity. The analysis was based on longitudinal isometric quadriceps and hamstrings strength measurements plus anatomical measures, including cross-sectional areas (CSAs) of the quadriceps, hamstrings, adductors, and sartorius. The analysis included baseline comparisons between the ACL-deficient and ACL-intact groups and longitudinal evaluation of the ACL-injured group over 4 years. Longitudinal changes were directly related to ACL deficiency by quantifying them relative to the matched ACL-intact group, reducing confounding from aging and progressive osteoarthritis. The OAI data set allowed a unique longitudinal analysis of the influence of ACL injury on quadriceps and hamstrings function. Uncommon features of this study include analysis of knees with nonoperatively treated ACL injuries after many years of function instead of prior to ACL reconstruction, analysis of the size and strength of the quadriceps and hamstrings, comparison with a control group of ACL-intact knees, and longitudinal analysis over 4 years. The results can be summarized with a few key baseline comparisons between the groups and longitudinal changes. At baseline, quadriceps and adductor CSAs were significantly smaller for ACL-deficient knees than ACL-intact knees, whereas hamstrings CSA was slightly, but still significantly, larger for ACL-deficient knees. Although not specifically addressed within the text of the manuscript, quadriceps and hamstrings strength measurements were significantly lower for ACL-deficient compared to ACL-intact knees. For ACL-deficient knees, CSA and strength of the hamstrings decreased significantly over 4 years, with no significant changes identified for the quadriceps or adductors. Sartorius CSA also decreased over 4 years. The authors concluded that ACL injury led to atrophy and strength decline of the hamstrings, with minimal changes to the quadriceps. The authors suggested that rehabilitation protocols should increase emphasis on hamstrings preservation and strengthening to improve long-term functional outcomes and joint stability. The secondary analysis of data from the OAI creates limitations that should be considered when interpreting the data. The ACL-deficient population was relatively old (average age, 63 ± 9 years) compared to patients typically evaluated following ACL injury, and had a lower percentage of female patients (31.5%) than a typical post-ACL-injury cohort. No data were provided on the time since ACL injury, but on the basis of the typical age at the time of an ACL tear, most knees were likely evaluated multiple decades following injury. As the authors noted, ACL reconstruction is currently more popular than nonoperative treatment for ACL tears, especially in young, active individuals. The conclusions may be specific to a population with the demographic characteristics and time from injury represented by the study, and less applicable to recently injured, active individuals hoping to regain knee function. Low quadriceps CSA in the ACL-deficient group at the baseline evaluation may indicate quadriceps atrophy over the initial decades following injury, which is generally consistent with prior studies on the quadriceps following ACL reconstruction1. Baseline quadriceps and hamstrings strength deficits for the ACL-deficient group may indicate general strength deficits for nonoperatively treated ACL tears, which is generally consistent with prior studies that evaluated knees prior to and following ACL reconstruction2–4. The deterioration related to decreasing hamstrings CSA and strength and sartorius CSA at long-term evaluation is certainly relevant for maintaining long-term function and may indicate a late decline that could contribute to impaired function and osteoarthritis. On the basis of the data presented, factors that influence muscle properties in ACL-deficient knees may change over time, and treatment strategies may need to be adjusted accordingly to preserve long-term function.