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Objectives and questions: This study aimed to evaluate the impact of patient-specific anatomical characteristics and implant size on endoprosthesis fit in total knee arthroplasty (TKA), and establish a preoperative protocol integrating 3D anatomical assessment and fit simulations to optimize prosthesis selection for improved fit to the bone surface, potentially enhancing functional outcomes and patient satisfaction.

Material and methods: Virtual implantation of three commercially available off-the-shelf TKA prostheses (DePuy Synthes, Stryker, Zimmer Biomet) was simulated in 92 patients who earlier received custom-designed implants. Fit accuracy was quantified in sagittal, coronal, and axial planes using newly defined measurement parameters.

Results: The mean deviation from optimal fit across all prostheses was 3.05±0.17 mm (p<0.01), with significant inter-brand differences (p=0.001): DePuy implants exhibited the largest deviations (3.13±0.19 mm), followed by Stryker (3.06±0.17 mm) and Zimmer (2.95±0.16 mm). Deviations varied significantly (p<0.05) across anatomical subregions and measurement planes.

Discussion and conclusions: Conventional TKA systems demonstrated clinically relevant fit deviations (>3 mm) across newly defined anatomical subregions, with implant size and patient-specific anatomy being key determinants. These findings underscore the need for preoperative 3D assessment protocols to guide prosthesis selection based on individual morphological characteristics.