Abstract
Background:
Total knee arthroplasty (TKA) has achieved excellent clinical outcomes and functional performances. However, younger and Asian patients require even greater implant longevity and higher flexion. It is necessary for much further functional upgrading to design TKA with mobility and stability. Therefore, we determined the relationship between mobility and stability of TKA.
Methods:
We evaluated the force of constraint of femorotibial surfaces in two types of designs in order to measure the property of femorotibial surfaces. The anatomical geometry knee (AGK) has an asymmetrical design, which restores the anatomical jointline in both sagittal and coronal planes, and is expected to permit normal kinematics, with cruciate-retaining fixed type. The functional designed knee (FDK) has a symmetrical design, and enhances concave femorotibial surfaces with cruciate-retaining mobile type.
We performed mechanical tests 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. 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 710N has been loaded on the femoral component during this test.
Results:
Regarding AP direction, both designs showed about 400N as the maximum load for anterior direction in all position. For posterior, AGK showed about 100N in all position, FDK showed 400N (0-degree), 350N (90-degree), and 300N (140-degree). As the maximum moment load to IE rotational direction, FDK showed free because it was the mobile type. AGK showed 4.5N-m to tibial internal rotation of femoral component in all position, 8.6N-m (0-degree), 6.5N-m (90-degree), and 5.2N-m (140-degree) to tibial external rotation of femoral component.
Conclusions:
The force to AP direction of constraint for posterior was 1/4 compared with one for anterior in AGK. The force to IE rotation for tibial internal rotation was lower than tibial external rotation. It is suggested that AGK permitted femoral rollback and rotation with medial pivot pattern easily than FDK. We evaluated the geometry characteristics of femorotibial surfaces quantitatively by measuring their force of constraint. These results suggest that the anatomical geometry knee permits femoral rollback and tibial internal rotation with medial pivot pattern, which is required to achieve high functional performance.
