Summary

Post operative saggital alignment is affected by various factors in robotic total knee arthroplasty and surgeons should be aware of them

Abstract

Introduction

Navigation assisted/robotic total knee arthroplasty improves accuracy in knee joint arthroplasty and enables intraoperative visualization of the mechanical axis. Instrumented knee replacement meant taking bony resections relative to the anatomical axis of the femur and tibia using intramedullary rods. Both axes have similar outcomes in the coronal plane, however in the sagittal plane there appears to be a discrepancy between the two, where when the leg appears straight to the eye, navigation determines some degree of flexion. Our objective was to determine if there is a discrepancy between both axes, quantify it and identify its implications on final implant position. This will provide users of navigation and robotic arthroplasty systems a better understanding of the sagittal plane variability between the anatomic axis (AA) and navigational axis (NA).

Methods

Using 30 preoperative long leg computed tomography scans, 3- dimensional lower limb models were developed using Mimics ( Materialise, Leuven, Belgium). The NA was plotted according to company recommended workflow protocols. In short, the centre of the femoral head using a best fit sphere. The distal femoral point was plotted at the deepest point of the trochlea at the centre of the intercondylar notch. The footprint of the ACL was plotted as the proximal tibial point. The intermalleolar point was marked on an axis connecting the lateral and medial malleolus, 60% more medially than the centre point of the line. The AA was plotted using best fit long axis cylinders matched to the cortices of the long bones to determine their exact anatomical centred axis. Assuming the distal femoral and proximal tibial cuts are perpendicular each axis angles were measured referencing the NA first and then the anatomical. Their variations were measured and compared. Next, the distal femoral point and proximal tibial point were plotted in the anterior posterior plane to study the effect it had on the NA. Additionally, the effects of femoral anteversion (FA), anterior femoral bow (AFB) and anterior tibial bow (ATB) was studied.

Results

In the sagittal plane NA of the femur was on average is 2.9±1.7 degrees more posterior to the AA. In the Tibia the NA was on average 2.6±1.3 degrees more posterior to the AA. Moving the distal femoral and proximal tibial point anteriorly anteriorised the NA and brought the two axes closer together. When there was increased AFB and ATB the AA was anteriorised and brought closer to the NA. Increased FA anteriorised the NA.

Conclusions

The discrepancy between NA and AA in the sagittal plane is dependent on the intra-articular distal femur and proximal tibia point plotting. Additionally, patient osteology can contribute to discrepancy between the NA and AA in the form of AFB, ATB, and FA. Surgeons should be aware of this when trying to achieve a straight knee in the sagittal plane as NA tends to overestimate knee and component flexion.