Anatomical landmarks allow for accurate tibial component positioning in kinematically aligned mobile bearing medial unicompartimental knee arthroplasty.

Background: The kinematic alignment (KA) technique for medial unicompartmental knee arthroplasty (UKA) aims at preserving the native anatomy, laxity, and biomechanics of the medial compartment, ensuring optimal implants interaction throughout the knee's range of motion. The accuracy of the manual implantation of a medial KA UKA remains unexplored. This retrospective cohort study aimed to evaluate the accuracy of tibial component implantation, using three intra-operative anatomical landmarks-the 'medial wall of the medial femoral condyle' (MWMC), a joint line-inserted 2.5 mm K-wire, and the 'deep MCL line' - with the hypothesis that their use would allow to accurately establish the orientation of the tibial cut in both the frontal and sagittal planes, as well as its thickness.

Patients and methods: Operative notes and pre/postoperative radiographs from a cohort of 90 consecutive KA mobile bearing medial UKAs (Oxford®) were retrospectively reviewed. Coronal and sagittal alignment accuracy were assessed by measuring the post-operative 'Tibial line to Femoral line Angle' (TFA) and 'Medial Sagittal Slope Difference' (MSSD), respectively. Based on these values, cases were divided into inliers (≤±3 °) and outliers (>±3 °). Polyethylene insert thickness, indicative of tibial cut accuracy, was extracted from operative notes.

Results: The mean TFA was -0.2 ° (SD 2.6; range -9 to 6.8), with 87.4% of inliers. The mean MSSD was -0.7 ° (SD 1.9; range -6.6 to 3.7), with 85.1% of inliers. The median insert thickness was 4 mm (range 3 to 6 mm), with 96.7% of cases within 4 ± 1 mm DISCUSSION: The use of straightforward anatomical landmarks - MWMC, K-wire, and deep MCL line - ensure accurate kinematic alignment of the tibial component of a medial UKA LEVEL OF EVIDENCE: IV.