Unrestricted kinematic alignment in varus total knee arthroplasty outperforms mechanical alignment in CPAK I phenotype, but yields comparable outcomes in CPAK IV: A retrospective analysis from the FP-UCBM Knee Study Group.

Abstract

To compare clinical outcomes in varus-aligned patients undergoing cruciate-retaining total knee arthroplasty (TKA) with mechanical alignment (MA) versus unrestricted kinematic alignment (KA). The hypothesis was that KA would yield superior outcomes, and that preserving joint line obliquity-regardless of alignment technique-would be associated with better results. A retrospective analysis of prospectively collected data from 140 KA and 209 MA TKA cases was performed. Inclusion criteria were: end-stage varus osteoarthritis (aHKA < 178°), MA or unrestricted KA TKA and ≥ 1-year follow-up. Exclusion criteria included prior major surgery (osteotomies, fractures) on the affected limb, inadequate preoperative full-length radiographs, or post-operative complications unrelated to alignment strategy. Patients were categorised by CPAK phenotype (I, IV and VII) based on the joint line obliquity. Clinical outcomes at 1-year follow-up were assessed using the Knee Society Score (KSS) pt.1 and 2, Oxford Knee Score (OKS), SF-12, and Forgotten Joint Score (FJS). ANOVA was used to compare results of MA and KA in the overall varus population, and in CPAK subgroups. Statistical significance was set at p < 0.05. KA led to significantly higher KSS pt.1 (84.6 ± 15.3 vs. 73.9 ± 18.9; p < 0.001) and FJS (90.5 ± 15.3 vs. 80.4 ± 15.8; p < 0.001) than MA. In CPAK I patients, KA outperformed MA in KSS pt.1 (83.5 ± 16.2 vs. 74.9 ± 19.1; p < 0.001) and FJS (89.8 ± 15.5 vs. 80.7 ± 17.8; p < 0.001). No differences were found between KA and MA in CPAK IV patients (p > 0.05). KSS pt.1 (80.1 ± 13.8) and FJS (86.5 ± 18.1) achieved with MA in CPAK IV were significantly higher than in both the overall varus aHKA group and CPAK I patients treated with MA (p < 0.05). Cruciate-retaining unrestricted KA yields better clinical outcomes compared to MA in varus aHKA patients, and in the CPAK I subgroup. In CPAK IV, preserving joint line obliquity leads to similar outcomes with both MA and KA. Level IV.