Preoperative age, BMI and KL grade predict long‐term functional outcome of knee arthroplasty better than CPAK‐classification

Purpose To assess whether coronal plane alignment of the knee (CPAK) is associated with functional outcome of total knee arthroplasty (TKA) and identify factors predicting functional outcomes during 10 years of follow‐up in a prospective cohort. Methods In total, 250 patients aged 65 or younger were enrolled. Primary knee arthroplasty was performed using mechanical alignment (MA). Baseline variables, including joint line obliquity (JLO) and arithmetic hip‐knee angle (aHKA), were used to predict Oxford knee score (OKS) at 1, 2, 5, 8 and 10 years postoperatively using ordinary least squares regression analysis. Variable importance and overall predictive ability were assessed. Results In total, 203 patients were included in the analyses. At 1 year, the mean for OKS was 39.7 (SD = 7.3), and was 0.247, showing moderate predictive performance. Higher age and higher Kellgren–Lawrence (KL) grade predicted higher OKS at 1 year. At 2 years, the mean for OKS was 40.5 (SD = 7.1), and was 0.298. Higher age, higher preoperative OKS, lower BMI and higher KL grade predicted better OKS at 2 years. At 5 years, the mean for OKS was 41.2 (SD = 6.7), and was 0.219. None of the variables were statistically significant at 5 years. At 8 years, the mean OKS was 41.5 (SD = 6.9), and decreased to 0.074. Higher BMI predicted lower, though not significant, OKS. At 10 years, the mean for OKS was 36.1 (SD = 6.6), and was 0.233. Variables that predicted higher OKS at 10 years were valgus mechanical axis, valgus aHKA and higher KL grade. JLO and aHKA had a negligible effect on OKS at 1, 2 and 8 years. At 5 and 10 years, aHKA and JLO influenced the OKS, but clinical significance is uncertain. Conclusion Predictive ability of preoperative variables was moderate. CPAK variables were uncertain predictors at every follow‐up in relation to age, BMI and KL grade. Level of Evidence Level II.