Single flexion-axis selection influences femoral component alignment and kinematics during knee simulation

The objective of this paper is to investigate the effect of the implant geometry and alignment on its surface displacement, and to show that the proposed method of alignment can be used to improve the consistency of total knee replacement alignment for knee simulation. Poor femoral flexion-axis selection in the alignment process can possibly alter the intended design's functionality and introduce significant anterior/posterior (A/P), and proximal/distal (P/D) displacements. In the study, four multi-axis femoral components from each of two different manufacturers, NKII and 3DKnee, were used. A custom-built femoral alignment and surface measurement instrument was used to adjust and locate the single femoral axis position for each implant, which would optimally minimize their P/D displacements and A/P translations. The aligned NKII implants yielded a mean implant maximum P/D and A/P contact point shifts of 0.577 +/- 0.078 mm (+/- std. dev.) and of 2.325 +/- 0.243 mm between 0 and 60 degrees of flexion, which was significantly different from the aligned 3DKnee, 0.415 +/- 0.157 mm and 0.800 +/- 0.1512mm (p<0.0001, p<0.0001). Future work is needed to quantify the effect of femoral flexion axis selection on resulting long-term wear, damage areas, and soft tissue loading during simulation.