Low femoral component prominence negatively influences early revision rate in robotic unicompartmental knee arthroplasty

Abstract Purpose The revision rate of unicompartmental knee arthroplasty (UKA) is higher than in total knee arthroplasty (TKA), and implant positioning may play a role. In combination with a pre‐operative CT, robotic UKA may provide the ability to position the implants more precisely. The aim of this study was to investigate the influence of component prominence relative to the native joint surface on early outcomes and revisions. The hypothesis was that aiming for restoration of joint space to 0.5–1.5 mm will improve outcomes. Methods Retrospective analysis of prospectively collected data of 94 patients undergoing robotic‐assisted UKA (Mako, Stryker) was performed. The ‘prominence’ of the implant surface relative to the native bony surface in sagittal plane, hip–knee–ankle (HKA) correction in coronal plane was documented intraoperatively. The mean achieved gap between two components under valgus stress captured in at least 5 different flexion angles was calculated. These were then analysed for impact on early revision rate and outcomes, stratified by gender. Results Median HKA correction was 3.5° (range 0°–9.5°). Median femoral prominence was 1.5 mm (range − 0.6 to 4 mm) and median tibial prominence was 4.3 mm (2–7 mm). The median achieved gap was 1.0 mm (− 1.2 to 2.8 mm). There was no difference in achieved correction between men and women ( p = n.s.) but men had a higher achieved combined prominence than women ( p < 0.001). PROMs did not correlate with the average gap ( p = n.s.) nor with combined prominence ( p = n.s.). Two patients underwent an early revision. Lower femoral prominence was a significant predictor of revision ( p = 0.045; OR = 0.21; 95% CI 0.000–0.918). Conclusion Female patients need less component prominence to achieve the same average gap balance through a range of motion, without correlation with patient’s height. Intraoperatively low femoral prominence could be a reason for early revision. Level of evidence IV.