Abstract
Introduction
Total knee arthroplasty (TKA) has achieved excellent clinical outcomes and functional performances. However, there is a need for greater implant longevity and higher flexion by younger and Asian patients. We determined the relationship between mobility and stability of TKA product because they are essential for much further functional upgrading. This research evaluated the geometry characteristics of femorotibial surfaces quantitatively by measuring their force of constraint by computer simulation and mechanical test.
Methods
We measured the force of constraint of femorotibial surfaces in order to evaluate the property of femorotibial surfaces. A total knee system was used for this evaluation, and has an asymmetrical joint surface, which restores the anatomical jointline in both sagittal and coronal planes, and is expected to permit normal kinematics, with cruciate-retaining fixed type.
We performed computer simulation using finite element analyses (FEA) and mechanical tests using knee simulator to measure the force of constraint regarding anterior-posterior (AP) and internal-external (IE) rotational direction in extension position, 90-degree flexion and a maximum flexion of 140-degree. In the FEA, Young's modulus and Poisson's ratio were set to 213 GPa and 0.3 for Co-Cr-Mo alloy as the femoral component, and 1 GPa and 0.3 for UHMWPe as the tibial insert, respectively. The force load to AP direction of tibial tray was measured when the femoral component moved plus or minus 10 millimeters. The moment load to IE rotational direction of tibial tray was measured when the femoral component moved plus or minus 20 degrees. The vertical load of 710 N was loaded on the femoral component during these measurements.
Results
Regarding AP direction, the results of FEA showed 506 N (0-degree), 421 N (90-degree), and 389 N (140-degree) as the maximum load for anterior direction, and 152 N (0-degree), 166 N (90-degree), and 174 N (140-degree) for posterior direction. The results of mechanical tests showed 463 N (0-degree), 387 N (90-degree), and 332 N (140-degree) as the maximum load for anterior direction, and 108 N (0-degree), 121 N (90-degree), and 197 N (140-degree) for posterior direction [Fig. 1]. As the maximum moment load to IE rotational direction, the results of FEA showed 7.0 N-m (0-degree), 6.6 N-m (90-degree), and 5.5 N-m (140-degree) to tibial internal rotation of femoral component, and 9.5 N-m (0-degree), 8.1 N-m (90-degree), and 5.5 N-m (140-degree) to tibial external rotation of femoral component. The results of mechanical tests showed 4.5 N-m to tibial internal rotation of femoral component in all position, 8.6 N-m (0-degree), 6.5 N-m (90-degree), and 5.2 N-m (140-degree) to tibial external rotation of femoral component [Fig. 2].
Discussion
The force to AP direction of constraint for posterior was obviously lower than one for anterior. The torque to IE rotation for tibial internal rotation was lower or equal than tibial external rotation. These results suggest that this total knee system permits femoral rollback and tibial internal rotation with medial pivot pattern, which is required to achieve high functional performance. Furthermore, computer simulation can be a good method in this evaluation for their consistency.
