Hamstring muscle strength recovery remains incomplete at 2 years after harvest for ACL reconstruction and does not correlate with radiological tendon regeneration at 6 months: A prospective study.

PURPOSE: The aim was to assess the effect of the degree of hamstring tendon regeneration on isometric flexion strength of the knee at short-term (6 months) and long-term (2 years) follow-up. METHODS: A prospective cohort study was conducted from January 2021 to December 2023 at our tertiary care centre, including 48 patients (mean age 27.69 ± 9.48 years) who underwent anterior cruciate ligament (ACL) reconstruction using hamstring autografts. Hamstring strength was measured by digital spring balance at 90° knee flexion at 6 months and 2 years. A magnetic resonance imaging (MRI) was done at 6-month postoperative follow-up to assess tendon regeneration. Patients were grouped based on regeneration status: none, partial, or complete. Functional scores (Visual Analogue Scale VAS, International Knee Documentation Committee IKDC, Knee Injury and Osteoarthritis Outcome Score KOOS, Lysholm Tegner) were evaluated preoperatively and 6 months postoperatively. RESULTS: At 6 months, 50% patients showed complete tendon regeneration, while 29.2% had no regeneration. Mean strength recovery did not significantly differ among regeneration groups (p = 0.783). Patients with semitendinosus (ST) grafts had slightly higher strength recovery than those with ST + gracilis (G) grafts, though not significant (p = 0.145). Functional outcomes were similar across graft types and regeneration status. At 2 years, significant improvement in hamstring strength was observed compared to 6 months with mean strength of 83.5% (p < 0.001). CONCLUSION: Tendon regeneration was frequently observed on MRI at 6 months, but knee flexion strength deficits were similar across regeneration groups and graft types. Strength recovery improved by 2 years but did not reach that of the contralateral limb, highlighting the need for ongoing rehabilitation focussed on hamstring strengthening. LEVEL OF EVIDENCE: Level II, prospective comparative study.