Anterior-posterior stability in partially conforming condylar knee replacement.

The purpose of the study was to estimate the anterior-posterior displacements that would occur with total knee prosthetic designs of different sagittal radii under functional conditions. A key question was whether some designs provided excessive constraint. Patients who had different total knee prosthetic designs were tested in a special rig where the AP drawer was measured under loaded and unloaded conditions. These same designs of total knee prostheses were then tested in a rig under different conditions of compressive and shear loads. Using these experiments as verification, a theoretical treatment was developed to predict the AP displacement for different activities. It was concluded that designs with low to moderate conformity required soft tissue restraint under more strenuous activities, or where high shear forces occurred under low compressive loads. Designs of moderate to high conformity could be fully self-stabilizing under all realistic conditions. However, for all designs, the AP displacement was only a few millimeters under functional loads, indicating that special care must be taken during surgery to avoid conflict between the condylar surfaces and the retained cruciate ligaments so that restriction of motion and excessive wear do not occur.