RESUMEN
PURPOSE: The aim of this prospective study was to assess the natural history of the donor hamstrings, compared to the contralateral knee and thigh. METHODS: In 47 patients who had ACL reconstruction (hamstrings technique), bilateral knees and thighs were investigated using MRI scans, up to 2 years post-operatively. RESULTS: Tendon regeneration below the knee joint line of the semitendinosus occurred in 30/47 (64 %) patients and of the gracilis tendon in 17/47 (36 %) patients. Insertion of both tendons at the "pes anserinus", with normal anatomic relations, was observed in 5/47 (11 %) patients. Semitendinosus and gracilis donor muscles were found retracted in 41/47 (87 %) patients. In 25/47 (53 %) patients, there was evidence of fatty infiltration of the semitendinosus muscle. All (47/47) (100 %) patients showed semitendinosus muscle atrophy and 41/47 (87 %) patients showed gracilis muscle atrophy, compared to the contralateral limb. High signal intensity on STIR images was observed in the semitendinosus in 22/47 (46 %) patients and in the gracilis muscle in 30/47 (64 %) patients, on MRI examination performed up to 24 months post-operatively. CONCLUSIONS: The "regenerated" hamstrings tendons most commonly do not regenerate fully anatomically, so they should not be considered as a first choice for re-harvesting in revision ACL reconstruction. The "donor hamstrings muscles" are found to be shorter, contracted, thinner and with a long-term abnormal MRI signal, features which are considered to be related to reduced muscle performance (knee flexion and internal rotation) post-operatively. LEVEL OF EVIDENCE: II.