Quadriceps muscle status after medial-pivot versus ultracongruent total knee arthroplasty using localized bioimpedance analysis: a randomized exploratory study

BACKGROUND: Implant design in total knee arthroplasty (TKA) may influence postoperative biomechanics and muscle recovery. Ultracongruent (UC) and medial pivot (MP) designs aim to enhance anteroposterior stability and reproduce physiological knee kinematics, yet comparative data remain scarce, and muscle recovery has rarely been directly evaluated. Localized bioelectrical impedance analysis (L-BIA) provides an objective method to assess skeletal muscle status, but its application to compare muscle recovery between different TKA implant designs has not been previously investigated. METHODS: A prospective, randomized, patient-blinded study was conducted between 2022 and 2024, including 40 patients undergoing primary TKA for advanced knee osteoarthritis. Patients were randomly assigned to receive either a medial-pivot (MP) or an ultracongruent (UC) prosthesis. The final analysis included 30 patients (15 per group). The primary outcome was postoperative quadriceps muscle recovery between implant groups, assessed longitudinally by reactance (Xc) of the rectus femoris, vastus medialis, and vastus lateralis using localized bioelectrical impedance analysis (L-BIA). Assessments were performed preoperatively and at 6 and 12 months postoperatively. Secondary outcomes included resistance (R), WOMAC, Knee Society Score, range of motion, and radiographic parameters. Temporal changes and between-group differences were analysed using repeated-measures and covariance-based statistical models. RESULTS: Demographic and preoperative characteristics were comparable between groups. Resistance values remained stable across all time points with no differences between implant designs. Reactance showed a transient decrease at 6 months followed by recovery at 12 months in both groups, with no significant differences between groups. ANCOVA demonstrated that baseline reactance was associated with 12-month reactance values (all p < 0.001), whereas implant design had no significant effect. The clinical relevance of this finding remains uncertain. Radiographic parameters, postoperative range of motion, and clinical outcomes improved significantly after surgery in both groups without significant differences between MP and UC designs. CONCLUSION: No significant differences in quadriceps recovery were observed between MP and UC implants during the first postoperative year. Baseline muscle status was associated with postoperative L-BIA outcomes, whereas no significant differences were observed between implant designs.