Initial Tendon Retraction is Associated With Muscle Degeneration After Nonoperatively Treated Proximal Hamstring Avulsions

1.1-Background and purpose Magnetic resonance imaging (MRI) is crucial for diagnosing proximal hamstring avulsions (PHA) by assessing tendon rupture and retraction, thereby guiding treatment decisions alongside clinical and patient data. However, the prognostic value of pre-treatment MRI for predicting muscle degeneration and clinical outcomes remains unclear. This study aimed to determine whether pre-treatment MRI findings—particularly tendon retraction—predict subsequent muscle degeneration and clinical outcomes in nonoperatively treated PHA patients. 1.2-Methods This study is a post hoc analysis of nonoperatively treated patients (n = 95) from the Proximal Hamstring Avulsion Clinical Trial (PHACT). Diagnostic MRIs were reassessed for tendon retraction, Wood classification, number of tendons avulsed, and hematoma size. The primary outcome was muscle degeneration, defined by the loss of lean muscle volume (LMV) and an increase in muscle fat fraction (MFF) quantified by DIXON MRI at 24 months. The secondary outcome was maximum hamstring muscle isometric force at 24 months. Outcome data was expressed as the limb symmetry index (LSI), which was the measurement of the injured hamstring expressed as a percentage of measurement of the uninjured hamstring. Linear regression was used to analyze the association between diagnostic MRI measurements, patient factors, and LSIs for LMV, MFF, and maximum isometric force. 1.3-Results The median LSIs of the LMV, MFF and maximum strength were 78%, 139%, and 84%, respectively, at 24-month follow-up. A multivariate linear regression model including tendon retraction, age, sex, hematoma size and whether the dominant limb was injured explained 48%, 48% and 23% of the variance in the LSIs of LMV, MFF and maximum force, respectively. Tendon retraction was the strongest explanatory factor for the variance of muscle degeneration observed in patients with nonoperatively treated PHA. 1.4-Interpretation Greater initial tendon retraction is associated with increased muscle atrophy and fat infiltration in the hamstring muscles in patients with nonoperatively treated PHA. ClinicalTrials.gov number, NCT03311997