Summary

Despite the fact that overall HKA did not change by increasing tibial component varus, we found a significant proportion of knees with a combination of valgus and internal rotation of the femoral component of more than 3°.

Abstract

Introduction.
Modern total knee arthroplasty (TKA) alignment techniques often accept some degree of tibial varus to balance the knee in extension without ligament releases. However, when using a tibia-first anatomic balancing technique (eg iKA), tibial varus will impact femoral coronal and axial component alignment. This study investigated the resulting femoral alignment and hip-knee-ankle angle (HKA) when tibial alignment is restricted.

Methods.
A retrospective review of a prospective database of robot-assisted TKAs using a digital joint tensioning device was performed (BalanceBot, Corin, UK). 692 TKAs with correctable deformity were extracted. An anatomical tibial varus resection was simulated to recreate the native tibial joint line within boundaries (A) 0–3°, B) 0–4°, C) -2–6°) while accounting for wear. The femoral component coronal and axial angles to balance the knee in flexion and extension were calculated.

Results.
Knees with mild varus deformity (HKA: 3°-10°) showed no change in post-op alignment between groups (2.2±2.2° varus in all groups, p=0.999).
The mean femoral component coronal angle increased from 0.9±3.4° to 1.4±3.8° valgus from groups A to C (p=0.010) but showed high variability (min: 10° varus, max: 10° valgus). Mean femoral rotation did not change with group (A: 0.1±3.0° int. C: 0.4±3.5° int., p=0.087), however, the proportion of knees with neutral femoral component (±3° coronal, 0°-6° rotation) decreased from group A to C (53.5% vs 43.0%, p=0.0005). Combined femoral component valgus (>3°) and internal rotation (>3°) however, more than doubled (6% vs 13%, p<0.0001) from group A to C.

Discussion.
Despite the fact that overall HKA did not change by increasing tibial component varus, we found a significant proportion of knees with a combination of valgus and internal rotation of the femoral component of more than 3°. A combination of valgus and internal rotation of the femoral component might negatively impact patellofemoral tracking and stability.