Risk factors of post-operative malalignment in fixed-bearing medial unicompartmental knee arthroplasty.

PURPOSE

The purpose of this study was to identify risk factors of post-operative malalignment in medial unicompartmental knee arthroplasty (UKA) using multivariate logistic regression.

METHODS

We retrospectively enrolled 92 patients who had 127 medial UKAs. According to post-operative limb mechanical axis (hip-knee-ankle [HKA] angle), 127 enrolled knees were sorted into acceptable alignment with HKA angle within the conventional ± 3 degree range from a neutral alignment (n = 73) and outlier with HKA angle outside ± 3 degree range (n = 54) groups. Multivariate logistic regression was used to analyse risk factors including age, gender, body mass index, thickness of polyethylene tibial insert, pre-operative HKA angle, distal femoral varus angle (DFVA), femoral bowing angle (FBA), tibial bone varus angle (TBVA), mechanical distal femoral and proximal tibial angles, varus and valgus stress angles, size of femoral and tibial osteophytes, and femoral and tibial component alignment angles.

RESULTS

Pre-operative DFVA, TBVA and valgus stress angle were identified as significant risk factors. As DFVA increased by one degree, malalignment was about 45 times probable (adjusted OR 44.871, 95 % CI 2.608-771.904). Shift of TBVA and valgus stress angle to a more varus direction were also significant risk factors (adjusted OR 13.001, 95 % CI 1.754-96.376 and adjusted OR 2.669, 95 % CI 1.054-6.760).

CONCLUSIONS

Attention should be given to the possibility of post-operative malalignment during medial UKA in patients with a greater varus angle in pre-operative DFVA, TBVA and valgus stress angle, especially with a greater varus DFVA, which was the strongest predictor for malalignment.