THE “CAPSULAR INSERTION LINE” CAN BE A SAFE AND RELIABLE LANDMARK FOR THE TIBIAL CUT IN UNICOMPARTMENTAL KNEE ARTHROPLASTY: A RADIOLOGICAL AND CLINICAL STUDY AT A MINIMUM THREE-YEAR FOLLOW-UP

Introduction The extramedullary guides for the tibial resection during medial unicompartmental kneearthroplasty (UKA) are inaccurate, with an error risk in coronal and sagittal planes and in cut thickness. A previously published demonstrated the radiological accuracy of a simple anatomical landmark to improve the tibial cut in UKA. To comfirm the clinical reliability to this landmark, This study aimed to: 1) assess radiographic precision and alignment, 2) evaluate the clinical outcomes using Patient-Reported Outcome Measures (PROMs), and 3) determine the survivorship at a minimum follow-up of 3-years. Material and Methods A retrospective analysis was conducted on 123 (70 females, 53 males; mean age 59.3 years; mean BMI 30.5 kg/m²; international demographic) consecutive primary fixed-bearing UKAs performed between 2020 and 2023 in the same department. All procedures were performed using a manual instrumentation and a standardized surgical technique based on the use of a simple landmark: “the capsular line” to guide the varus-valgus, slope and thickness of resection of the tibial cut. Radiographic precision was evaluated via pre- and post-operative HKA, tibial slope, and posterior offset measurements. Clinical success was assessed at a minimum FU of 3-years follow-up using OKS, KOOS, and KSS 2011 scores. A standard survivorship analysis at a minimum of 3 years was performed. Results Radiographically, the surgical technique achieved high precision in coronal correction, with mean HKA changing from 174.1° to 178.0° ( p < 0.001) post-operatively and the mean JLCA was reduced from 3.5° to 1.8° (p < 0.001) and a preservation of the proximal tibia Vara for 98% of the patients within 1.5° from the target. Tibial slope outliers (> 12°) were significantly reduced from 28.8% to 5.4%. 95% of the PE were 8 mm and 5 % 9 mm. Significant clinical improvements were observed across all PROMs (p < 0.001), with mean OKS increasing from 13.8 to 41.1 and from 21.9 to 86.3 for the functional segment of the KOOS. The 3-years implant survivorship was 100%. Discussion and conclusion the results of the study demonstrated that a simple anatomical landmark, the “capsular line” can be reliable, accurate and safe for the tibial cut in fixed-bearing UKA. This simple landmark can be used daily regardless of the gender, BMI and ethnicity of the patient and might be helpful for surgeons across the world welling to perform UKA safely without having access to a robotic platform.