Medial unicompartmental knee arthroplasty improves congruence and restores joint space width of the lateral compartment

Background: Osteoarthritic progression of the lateral compartment remains a leading indication for medial unicompartmental knee arthroplasty (UKA) revision. Therefore, the purpose of this study was to evaluate the alterations of the lateral compartment congruence and joint space width (JSW) following medial UKA. Methods: Retrospectively, lateral compartment congruence and JSW were evaluated in 174 knees (74 females, 85 males, mean age 65.5 years; SD ± 10.1) preoperatively and six weeks postoperatively, and compared to 41 healthy knees (26 men, 15 women, mean age 33.7 years; SD ± 6.4). Congruence (CI) was calculated using validated software that evaluates the geometric relationship between surfaces and calculates a congruence index (CI). JSW was measured on three sides (inner, middle, outer) by subdividing the lateral compartment into four quarters. Results: The CI of the control group was 0.98 (SD ± 0.01). The preoperative CI was 0.88 (SD ± 0.01), which improved significantly to 0.93 (SD ± 0.03) postoperatively (p < 0.001). In 82% of knees, CI improved after surgery, while in 18% it decreased. The preoperative significant JSW differences of the inner (p < 0.001) and outer JSW (p < 0.001) were absent postoperatively. Conclusion: Our data suggests that a well-conducted medial UKA not only resurfaces the medial compartment but also improves congruence and restores the JSW of the lateral compartment.