Knee joint line orientation after total knee arthroplasty is affected by the mechanical axis inclination of the lower limb according to foot position

Purpose

Knee joint line orientation (KJLO), an essential component of knee kinematics, has recently been of interest as a clinical parameter, but the factors that determine this are unknown. This study aims to clarify the effect of mechanical axis inclination of the lower limb on KJLO after total knee arthroplasty.

Methods

The study included 95 knees of 95 patients who underwent kinematically aligned total knee arthroplasty in which two postoperative whole-leg standing radiographs (open and closed stance) were evaluated. We measured hip-knee-ankle angle, mechanical lateral distal femoral angle, medial proximal tibial angle, KJLO, joint line convergence angle, ankle joint line orientation and the inclination angle of lower limb mechanical axis (θ); the amount of change in each measurement was defined as delta (Δ). The correlation between ΔKJLO and each measured value was analysed, and a multiple linear regression analysis was performed to identify significant factors related to ΔKJLO.

Results

The open and closed stance radiographs had differences in KJLO (3.4 ​± ​2.2° vs. −0.2 ​± ​2.3°,p ​< ​0.001), ankle joint line orientation (−3.3 ​± ​4.4° vs. −6.8 ​± ​4.1°, p ​< ​0.001) and θ (−0.3 ​± ​1.6 vs. 3.3 ​± ​1.5, p ​< ​0.001). We found that Δθ determined ΔKJLO, with a strong negative correlation (r ​= ​−0.847, p ​< ​0.001). According to the ΔKJLO equation, ΔKJLO (°) ​= ​0.361 ​− ​0.898 ​× ​θ (°), KJLO changed by 0.9° per 1° of θ.

Conclusion

The KJLO after total knee arthroplasty is affected by the mechanical axis inclination of the lower limb; KJLO changed by 0.9° per 1° of θ.