Effect of Limb Alignment in Medial Unicompartmental Knee Arthroplasty on Functional Outcomes and Patient Satisfaction

Background: Unicompartmental knee arthroplasty (UKA) is effective in the treatment of symptomatic unicompartmental knee osteoarthritis. However, ideal limb alignment and acceptable changes of alignment remains contentious. Purpose: The purpose of this study is to look at the impact of post-operative mechanical limb alignment on short term functional outcomes in an Asian population. Methods: We performed a retrospective review of all mobile bearing, Oxford UKAs performed from July 2016 to January 2019 by a single-surgeon (43 patients, 47 knees). Pre- and post-operative weight bearing radiographic parameters were measured, as well as functional assessments via Oxford knee society (OKS) scores and the original Knee society score (KSS). Results: The post-operative mechanical axis mean was varus 4.3° (range valgus 2° to varus 10°), with a mean correction of 4.4° towards valgus. All our patients had an improvement in functional scores and range of movement post-operatively, however, patients with pre-operative alignment of varus 15° did not meet the cut off for minimal important change (MIC) in OKS scores. More varus pre-operative alignment resulted in a larger degree of change in limb alignment post-operatively. A larger change in alignment was also associated with higher satisfaction scores in patients. Conclusion: Post-operative limb alignment of up to 10° varus does not compromise short term functional outcome scores in medial UKA. Caution is advised when selecting patients with varus 15° and above for UKA as they may not be able to achieve a minimal important change in functional scores post-operatively.