Ultrasound Evaluation of the Regenerating Tendon of the Semitendinosus After Harvest for Anterior Cruciate Ligament Reconstruction

<b>Objectives</b>: Previous studies have suggested the potential for semitendinosus (ST) tendon regeneration following harvesting for anterior cruciate ligament (ACL) reconstruction. This retrospective cross-sectional (observational) study aims to evaluate ST tendon regeneration using high-resolution ultrasound (US), with special reference to morphological and structural changes in the muscle-tendon unit. <b>Methods</b>: Twenty-four patients who had undergone ST tendon harvesting at least 24 months prior were evaluated. Ultrasound assessment included neotendon (NT) detection, thickness, echotexture, insertion site, dynamic gliding, myotendinous junction (MTJ) shifting, and muscle cross-sectional area (CSA), compared with the healthy contralateral side. <b>Results</b>: ST tendon regeneration was detected in 19/24 patients (79%). In regenerated tendons, NT thickness was significantly greater than the native tendon (3.40 ± 1.38 mm vs. 2.40 ± 0.27 mm; mean difference 0.98 mm; <i>p</i> = 0.005. Subgroup analysis revealed that fibrillar-like NTs were associated with a smaller MTJ shift (3.91 ± 1.14 cm vs. 7.75 ± 2.43 cm; <i>p</i> = 0.001) and higher muscle CSA preservation (0.85 ± 0.10 vs. 0.55 ± 0.09; <i>p</i> < 0.001). A strong inverse correlation was found between MTJ displacement and muscle CSA (ρ = -0.96; <i>p</i> < 0.001). Patients without NT regeneration (n = 5) exhibited more pronounced MTJ retraction (11.0 ± 1.0 cm) and muscle hypotrophy (CSA ratio 0.41 ± 0.07), although these results were descriptive. <b>Conclusions</b>: High-resolution US is an effective, non-invasive method for assessing ST tendon regeneration from a qualitative and quantitative perspective. Our findings indicate possible changes in the architecture and position of the regenerated tendon, the MTJ, and the muscle belly, which may reflect structural remodeling of the muscle-tendon unit.