Can we rebuild what has been taken? A narrative review of biological and functional consequences of hamstring tendon harvest in ACL surgery

This narrative review aimed to summarize the consequences of hamstring tendon (HT) harvest for anterior cruciate ligament reconstruction (ACLR), with reference to tendon regeneration, knee flexor morphology, clinical assessment of knee flexion strength, and rehabilitation strategies for optimizing recovery. The available evidence suggests that the harvested semitendinosus (ST) and gracilis (GR) tendons regenerate in ∼70-86% of cases after ACLR. Despite tendon regeneration, ACLR with HT autograft results in morphological alterations in both the ST and GR. These changes include tendon elongation, proximal migration of the musculotendinous junction, muscle atrophy, shortened fascicles, and increased fatty infiltration, which may persist for several years after ACLR. Growing evidence also suggests that assessment of knee flexion strength after ACLR cannot rely on a single test position, and that knee flexion strength should be evaluated in both hip-flexed and hip-extended positions to provide a more comprehensive evaluation of the recovery. We propose that optimizing the recovery of knee flexion strength may benefit from a multifaceted approach: (1) early, symptom-guided initiation of isometric and low-load exercises to minimize early strength deficits, (2) progressive loading of the knee flexors across multiple hip-and knee-joint angles, including the seated leg-curl to promote a high ST activation, (3) subsequent incorporation of eccentric loading as part of progressive strengthening strategies, with the aim of enhancing ST and GR muscle volumes and fascicle lengths, and (4) strength assessments across multiple hip and knee-joint positions to monitor recovery, as testing in a seated position may underestimate residual deficits in the ST and GR.