Maintaining Joint Line Obliquity Optimizes Outcomes of Functional Alignment in Total Knee Arthroplasty in Patients With Constitutionally Varus Knees

Abstract

BackgroundFunctional alignment (FA) strives to balance the knee soft-tissue envelope during total knee arthroplasty (TKA) using implant alignment adjustments rather than soft-tissue releases. There is a debate on how best to achieve FA. We compared minimum two-year outcomes between FA with a mechanical alignment plan [FA(m)] and FA with a kinematic alignment plan [FA(k)]. The null hypothesis was that there would be no difference in outcomes between FA(m) and FA(k). MethodsProspective data was collected from 300 consecutive robotic-assisted FA TKAs [135 FA(m) and 165 FA(k)]. Patient reported outcomes were obtained preoperatively and 2 years postoperatively. The coronal plane alignment of the knee classification was used to classify knee alignment phenotypes. ResultsOverall limb alignment was equivalent between groups. Final implant alignment was different between FA(m) and FA(k) groups, with FA(k) TKAs having higher tibial varus (P < .01), higher femoral valgus (P < .01), and higher joint line obliquity (P < .01). Patients reported higher Forgotten Joint Score-12 scores with FA(k) TKAs (79.4 versus 71.6, P = .018) and greater range of motion (125 versus 121°; P = .003). Patients who had constitutional varus reported the greatest improvement with FA(k) technique (Forgotten Joint Score at minimum 2 years of 89 versus 65; P < .001). ConclusionUtilizing an individualized alignment plan [FA(k)] led to a final implant position with greater joint line obliquity, yet the same overall limb alignment. This was associated with improved outcomes at 2 years post-TKA in patients who had constitutional varus. Three-dimensional component position and joint line obliquity affect the outcomes following TKA independently of coronal limb alignment.