Evaluation of changes in fixed flexion deformity following medial unicompartmental knee arthroplasty

Aims While residual fixed flexion deformity (FFD) in unicompartmental knee arthroplasty (UKA) has been associated with worse functional outcomes, limited evidence exists regarding FFD changes. The objective of this study was to quantify FFD changes in patients with medial unicompartmental knee arthritis undergoing UKA, and investigate any correlation with clinical outcomes. Methods This study included 136 patients undergoing robotic arm-assisted medial UKA between January 2018 and December 2022. The study included 75 males (55.1%) and 61 (44.9%) females, with a mean age of 67.1 years (45 to 90). Patients were divided into three study groups based on the degree of preoperative FFD: <= 5 degrees, 5 degrees to <= 10 degrees, and > 10 degrees. Intraoperative optical motion capture technology was used to assess pre- and postoperative FFD. Clinical FFD was measured pre- and postoperatively at six weeks and one year following surgery. Preoperative and one-year postoperative Oxford Knee Scores (OKS) were collected. Results Overall, the median preoperative navigated (NAV) FFD measured 6.0 degrees (IQR 3.1 to 8), while the median postoperative NAV FFD was 3.0 degrees (IQR 1 degrees to 4.4 degrees), representing a mean correction of 49.2%. The median preoperative clinical FFD was 5 degrees (IQR 0 degrees to 9.75 degrees) for the entire cohort, which decreased to 3.0 degrees (IQR 0 degrees to 5 degrees) and 2 degrees (IQR 0 degrees to 3 degrees) at six weeks and one year postoperatively, respectively. A statistically significant improvement in PROMs compared with baseline was evident in all groups (p < 0.001). Regression analyses showed that participants who experienced a larger FFD correction, showed greater improvement in PROMs (beta= 0.609, p = 0.049; 95% CI 0.002 to 1.216). Conclusion This study found that UKA was associated with an approximately 50% improvement in preoperative FFD across all three examined groups. Participants with greater correction of FFD also demonstrated larger OKS gains. These findings could prove a useful augment to clinical decision-making regarding candidacy for UKA and anticipated improvements in FFD.