Reconstructing Stability: Isolated Late MCL Rupture after UKR Managed without Revision Arthroplasty

Background: Unicondylar knee replacement (UKR) is increasingly preferred for patients with isolated anteromedial osteoarthritis, offering rapid recovery and functional benefits. Success relies heavily on the integrity of the medial collateral ligament (MCL). While intraoperative MCL injury is a known complication, late-onset MCL rupture following UKR is extremely rare and typically managed with revision to constrained total knee arthroplasty (TKA). Case Report: A 63-year-old woman with bilateral mobile-bearing Oxford UKR presented ten months postoperatively with right knee instability after a twisting injury. Clinical examination revealed grade III medial laxity with preserved range of motion. Radiographs showed well-aligned, well-fixed UKR components, and valgus stress imaging confirmed isolated MCL rupture. Given implant stability, MCL reconstruction was performed using a semitendinosus autograft harvested from the ipsilateral leg. Anatomical fixation was achieved with bioabsorbable screws and titanium ligament anchors with fiber wire augmentation, preserving existing implants. Postoperative rehabilitation included bracing, early mobilization, and progressive strengthening. At two-year follow-up, the patient reported full pain relief, restored function, and no valgus laxity. Radiographs confirmed implant stability and intact reconstruction. Conclusion: This case highlights the viability of isolated MCL reconstruction with semitendinosus autograft in managing late MCL rupture after UKR. Preserving original components avoids constrained revision, reduces surgical morbidity, and maintains excellent functional outcomes. This technique may be considered in similar rare cases of post-UKR ligamentous failure.