Patient-specific instrumentation in Oxford unicompartmental knee arthroplasty is reliable and accurate except for the tibial rotation

Patient-specific instrumentation (PSI) is a technique to plan and position the prosthesis components in unicompartmental knee arthroplasty (UKA) surgery. This study assesses whether the definitive component position in the frontal, sagittal and axial plane is according to the preoperative plan, based on the hypothesis that PSI is accurate. Twenty-six patients who had PSI Oxford UKA surgery were included prospectively. The component position in vivo was determined with a postoperative CT-scan and compared with the planned component position using MRI-based digital 3D imaging. Adjustments to the preoperative plan and implanted component sizes during surgery were recorded. Intraoperatively, no femoral adjustments were performed; 12 tibial re-resections were necessary. The median absolute deviation from the plan in degrees (range) in the frontal, sagittal and axial plane was 1.8A degrees (- 1.5A degrees-6.5A degrees), 2.0A degrees (- 6.5A degrees-8.0A degrees) and 1.0A degrees (- 1.5A degrees-5.0A degrees) for the femoral component, and 2.5A degrees (- 1.0A degrees-6.0A degrees), 3.0A degrees (- 1.0A degrees-5.0A degrees) and 5.0A degrees (- 6.5A degrees-12.5A degrees) for the tibial component. The femoral component is positioned 0.5 (- 1A degrees-2.5A degrees) mm more lateral and 0.8 (- 1.0A degrees-2.5A degrees) mm more anterior. The tibial component is positioned 2.0 (- 5.0-0.0) mm more lateral and 1.3 (- 3.0-6.0) mm more distal. The femoral and tibial default plans were changed four times (15.4%) and nine times (34.6%), respectively, before approval by the surgeon. PSI in Oxford UKA surgery is reliable and accurately translates the preoperative plan into the in vivo situation, except for the tibial rotational position. The preoperative planning is a crucial step in avoiding re-resections that can cause angular deviations in prosthesis position, especially in tibial component rotational position. It is advised to avoid re-resections and to consider this while planning the PSI procedure. Prospective comparative study Level II.