No association between fibrosis on magnetic resonance imaging at return to play and hamstring reinjury risk

Background: Connective tissue scar (fibrosis) is a common finding on magnetic resonance imaging (MRI) after recovery from acute hamstring injuries. Fibrosis has been suggested as a predisposing factor for reinjury, but evidence from clinical studies is lacking.

Purpose/hypothesis: The aim of this study was to examine the association between the presence of fibrosis on MRI at return to play after an acute hamstring injury and the risk of reinjury. The hypothesis was that fibrous tissue on MRI was associated with an increased reinjury risk.

Study design: Cohort study; Level of evidence, 3.

Methods: Magnetic resonance images were obtained from 108 consecutive athletes with modified Peetrons classification grade 1 or 2 hamstring injuries within 5 days of injury and within 7 days of return to play. The presence and extent of abnormally low signal intensity in the intramuscular tissue on MRI, suggestive of fibrosis, were assessed on both T1- and T2-weighted images. Reinjuries were recorded over a 1-year follow-up period. The association between fibrosis and reinjury risk was analyzed with a Cox proportional hazards model.

Results: The MRIs of the initial injury showed 45 (43%) grade 1 and 63 (57%) grade 2 injuries. Median time of return to play was 30 days (interquartile range [IQR], 22-42 days). At return to play, 41 athletes (38%) had fibrosis on MRI with a median longitudinal length of 5.8 cm (IQR, 3.3-12.5 cm) and a median volume of 1.5 cm3 (IQR, 1.5-3.9 cm3). In athletes with fibrosis, 24% (10/41) sustained a reinjury, and in the subjects without fibrosis, 24% (16/67) had a reinjury, resulting in a hazard ratio of 0.95 (95% CI, 0.43-2.1; P=.898).

Conclusion: Fibrosis is commonly seen on MRI at return to play after grade 1 or 2 hamstring injuries but is not associated with reinjury risk.