Impact of severe varus deformity on the outcome of mobile-bearing unicompartmental knee arthroplasty: a comparison of patients with varus deformity of less than and more than 15 degrees using matched-pair analysis

INTRODUCTION: The outcome of patients undergoing UKA with larger preoperative deformity (> 15º varus) and lesser deformity is sparsely compared in the literature. This matched-pair retrospective study attempts to fill this research gap by comparing the effect of preoperative and postoperative residual varus on outcome. METHODS: Patients undergoing UKA with preoperative varus deformity > 15º (HKA > 15º group) were selected and matched for age and gender with those with < 15º of preoperative varus deformity (HKA < 15º group). There were 72 age and gender-matched pairs knees. HKA was measured and Forgotten Joint Score, OKS, VAS and WOMAC scores were obtained at a minimum follow up of 5 years. Postoperative residual varus was classified into 3 groups based on postoperative HKA: varus < 3º, varus 3 to 7º and varus > 7º. RESULTS: Mean correction of deformity was significantly more (p < 0.001) in the HKA > 15º group (10.5º) when compared to HKA < 15º group (4.5º). There was no significant difference in outcome measures OKS (p = 0.21), FJS (p = 0.35) and VAS (p = 0.6) between patients in the two groups. There was no significant correlation between the preoperative and postoperative HKA of the patient with postoperative OKS, FJS and VAS in the two groups. There was no statistically significant difference between FJS (p = 0.73) of the three postoperative varus groups. Excellent to good OKS scores were obtained in 98% & 96% of patients in HKA > 15° and HKA < 15° groups respectively. CONCLUSION: UKA performed in selected patients with large preoperative deformities (> 15º varus) fulfilling Oxford criteria can give satisfactory outcomes in the short to medium term comparable with those with lesser deformity. There was no association between postoperative residual varus and patient reported outcome scores.