Regeneration of the semitendinosus tendon and muscle after isolated semitendinosus harvest for primary anterior cruciate ligament reconstruction: A systematic review

PURPOSE: To systematically evaluate the morphology, temporal evolution and musculoskeletal adaptations associated with semitendinosus tendon (ST) and muscle (SM) regeneration after isolated ST autograft harvest for primary anterior cruciate ligament reconstruction (ACLR). METHODS: A systematic review was conducted according to PRISMA 2020 guidelines. PubMed, Embase, Scopus and Web of Science were searched from inception to 20 December 2025. Eligible studies included primary ACLR with isolated ST autografts and objective post-operative assessment of ST or SM regeneration using imaging (ultrasound, magnetic resonance imaging and computed tomography) or histology. Methodological quality was evaluated using the JBI checklist for randomized controlled trials and the Methodological Index for Non-Randomized Studies instrument for non-randomized studies. Due to heterogeneity in surgical techniques, imaging protocols, follow-up intervals and regeneration definitions, results were synthesized narratively according to synthesis without meta-analysis principles, with regeneration frequencies reported by modality and predefined time windows. RESULTS: Sixteen studies comprising 376 patients (mean age 25.5 years) were included. Across studies, tendon-like continuity was frequently detected between 6 and 12 months post-operatively, although reported regeneration rates varied depending on imaging modality and definitional criteria. The regenerated neotendon (NT) typically demonstrated non-anatomical distal attachment or fusion patterns, accompanied by proximal migration of the musculotendinous junction, permanent shortening and atrophy of the SM. CONCLUSION: The ST and SM exhibit robust biological potential for regeneration after isolated harvest, typically forming a viable NT within the first post-operative year. However, regeneration does not restore native anatomy; significant muscle shortening, atrophy and proximal insertion shifts are consistent findings. Despite these morphological deficits, the high rate of tissue continuity supports the isolated ST graft as a low-morbidity option that preserves overall hamstring complex integrity. LEVEL OF EVIDENCE: Level IV.