Transposition of the Semimembranosus as an Augmentation Technique for Anteromedial Rotatory Instability of the Knee: A Retrospective Case Series Study

Anteromedial rotatory instability (AMRI) resulting from medial collateral ligament (MCL) injuries, often combined with anterior cruciate ligament (ACL) tears, poses a significant challenge in knee surgery. This study evaluates the use of a novel technique-semimembranosus (SM) tendon transposition-as an augmentation to ACL and superficial MCL (sMCL) reconstruction, reducing medial knee opening and AMRI. A case series of 15 patients with AMRI underwent ACL and sMCL reconstruction with SM tendon transposition between January 2017 and July 2021, with a minimum follow-up of 24 months. Inclusion criteria included age 18 to 50 years, AMRI diagnosed clinically, and a minimum 24-month follow-up. Exclusion criteria included high-grade osteoarthritis, knee dislocations, and revision surgeries. Outcomes were assessed using the Lysholm Knee Score (LKS) and stress radiographs, manually performed at 0 and 30 degrees of knee flexion, to measure medial compartment gapping preoperatively and at 12 and 24 months postoperatively. Significant improvements were observed in LKS, with mean scores increasing by 121% from 42.8 ± 5.9 preoperatively to 97 ± 2.8 at 12 months and by 132% to 99.2 ± 1.8 at 24 months (<i>p</i> < 0.001). Radiographic medial opening decreased by 81% from 5.46 ± 0.74 mm preoperatively to 1.05 ± 0.9 mm at 12 months and by 83% to 0.92 ± 0.92 mm at 24 months (<i>p</i> < 0.001). All patients (100%) exceeded the minimal clinically important difference for LKS, and no residual instability was observed at final follow-up. The complication rate was 13% (arthrofibrosis), within the expected range for knee reconstructions. SM tendon transposition effectively restored medial stability and improved functional outcomes in AMRI patients, without the need for additional grafts or tunnels, presenting a low complication rate.