Functional knee phenotypes of OA patients undergoing total knee arthroplasty are significantly more varus or valgus than in a non-OA control group

Purpose The aim of this study was to analyse the coronal alignment of a large population of patients undergoing total knee arthroplasty using a modern classification of the knee phenotypes found in a population of non-osteoarthritic individuals. Methods Five hundred and four navigated total knee arthroplasties were included in the OA group. The following angles were measured with a computer image-free navigation system: mechanical femorotibial angle measured on the medial side without stress and with maximum manual stress to reduce deformation, and medial distal femoral mechanical angle. The native medial distal femoral and medial proximal tibial angles (coronal orientation of the femoral or tibial joint line after correction of wear) were calculated. The data were analysed as categorical data. These data were then compared with those published in a non-arthritic population, considered as a control non-OA group. The main criterion was the percentage of subjects with normal overall coronal alignment, defined by the association of a normal native medial distal femoral angle and a normal native medial proximal tibial angle. The secondary criteria were the percentages of subjects with normal medial femorotibial mechanical angle, normal native medial distal femoral angle and normal native medial proximal tibial angle. The influence of gender on primary and secondary criteria in the study group was analysed. The most frequent phenotypes in the study group were identified. Results Normal overall coronal alignment was found in 66 patients in the OA group (12.7%) and 76 patients in the non-OA-group (24.7%) ( p < 0.01 after adjustment by gender). There were fewer normal patients in the OA-group than in the non-OA-group for medial femorotibial mechanical angle, native medial distal femoral angle and native medial proximal tibial angle. In females, there were significantly fewer normal medial femorotibial mechanical angle. In males, there were significantly more cases with native medial distal femoral varus and in females more cases with native medial distal femoral valgus. There was no significant influence of gender on native medial proximal tibial angle. There was a wider distribution of the phenotypes in the OA-group than in the non-OA-group. Conclusion The distribution of functional phenotypes of the knee in patients undergoing total knee arthroplasty is different from those found in a reference non-osteoarthritic population. Level of evidence Level III—retrospective cohort study.