Does the Magnitude of Change in Coronal Plane Alignment of the Knee Affect Clinical Outcomes in Functional and Mechanical Alignment Total Knee Arthroplasty?

BACKGROUND:Changes to native coronal plane alignment of the knee (CPAK) phenotypes following total knee arthroplasty may influence outcomes, particularly with mechanical alignment (MA). This relationship has not been evaluated with functional alignment (FA). This study compared the magnitude of coronal alignment change between MA and FA and whether this influenced patient-reported outcomes. METHODS:This substudy analyzed data from a prospective, single-center, single-blinded randomized controlled trial involving 236 patients (119 FA and 117 MA). Long-leg radiographs were used to calculate the arithmetic hip-knee-ankle angle (aHKA), joint line obliquity (JLO), and CPAK phenotype preoperatively and postoperatively. Alignment change was assessed using absolute differences (change in aHKA, change in JLO), and a dose-response analysis was conducted. Clinical outcomes included the Forgotten Joint Score (FJS) and Oxford Knee Score at 1- and 2-yeas follow-up. RESULTS:The FA resulted in smaller changes in alignment compared to MA (Δ aHKA P = 0.01; change in JLO P = 0.01) and more frequent restoration of the preoperative CPAK phenotype (42.9 versus 10.3%, P = 0.01). In the MA group, restoration of the CPAK phenotype was associated with significantly higher FJS, whereas no such relationship was observed in the FA group. A dose-response relationship was identified in the MA group: greater changes in JLO were negatively correlated with FJS. This correlation was not seen with FA. CONCLUSIONS:Functional alignment more closely preserved native coronal alignment and restored CPAK phenotypes compared to MA. In MA cases, larger deviations from the native JLO and CPAK phenotypes were associated with poorer patient-reported outcomes. These findings suggest anticipating alignment changes may support optimal strategy selection in total knee arthroplasty. LEVEL OF EVIDENCE:Level I prospective randomized study.