Does the preoperative CPAK classification influence clinical outcomes after medial and lateral unicompartmental knee arthroplasty? Short-term outcomes and phenotype distribution in a retrospective series of consecutive cases

Recent evidence suggest that mechanically aligned TKAs may yield heterogeneous outcomes across CPAK categories, but whether this applies to unicompartmental knee arthroplasty (UKA) remains unclear. This retrospective study evaluated the influence of preoperative CPAK category on postoperative clinical outcomes after UKA, with additional analyses assessing CPAK distribution, residual varus alignment, and their relationship with patient-reported outcomes. A total of 312 consecutive UKAs (280 patients) with ≥ 1-year follow-up were analyzed. Preoperative HKA and JLO were measured on weight bearing long-leg radiographs, and patients were classified according to the CPAK system. Outcomes were compared across the six most represented CPAK groups (I – VI) in 253 patients with complete PROM data. Mean postoperative OKS was 40.5 ± 5.3, with a mean ΔOKS of 21.3 ± 8.7 and 97% patients’ satisfaction. No significant differences were observed among CPAK categories for OKS (p = 0.80), ΔOKS (p = 0.90), or satisfaction (p = 0.70), and alignment-based grouping (varus, neutral, valgus) showed similarly homogeneous outcomes. Residual varus ≥ 5° did not adversely affect results (p > 0.10), and no meaningful correlation was found between aHKA and postoperative OKS (r = − 0.003, p = 0.96). CPAK distribution differed significantly by sex (p < 0.001). Five patients were revised (2 for persistent pain, 1 for PCL instability, 2 for infection). Preoperative CPAK phenotype did not influence short-term clinical outcomes after UKA, and no heterogeneity in PROMs was detected across phenotypes. These findings support UKA as a reliable and alignment-tolerant procedure across a wide range of coronal morphotypes. Prospective studies with postoperative long-leg radiographs and longer follow-up are warranted to determine whether CPAK preservation influences long-term survivorship.