Achieving specified laxity in a non-cruciate total knee: a laboratory design study

Background Non-cruciate total knee arthroplasty designs, including ultracongruent, medially congruent, and medial pivot, are gaining increasing attention in total knee arthroplasty surgery. However there is no consensus for the bearing surface design, whether there should be different medial, lateral, anterior and posterior laxities, or whether the medial side should be a medial pivot. This study proposes the criterion of reproducing the laxity of the anatomic knee, defined as the displacements and rotations of the femur on the tibia in the loaded knee when shear and torque are applied. The purpose of this study was to determine the ideal tibial radii to achieve that goal. Methods The femoral component was based on the average knee from 100 mild arthritic knee scans. There were eight tibial components that were designed with different sagittal radii: antero-medial, antero-lateral, postero-medial, and postero-lateral. Radii were defined as the percent height reduction from full conformity with the femoral profile. Components were 3D printed. A test rig was constructed where the tibial component was fixed and shear and torque were applied to the femoral component. Displacements and rotations of the femoral component were measured at 0 and 45 degrees of flexion, the latter representing any flexion angle due to the constant femoral sagittal radius. Results Displacements ranged from 0 to 11mm, and rotations ranged from 1 to 11 degrees. Anterior femoral displacements were higher than posterior due to the shallow distal-anterior femoral profile. The final femoral and tibial components with the most closely matched anatomic laxity values, were designed and tested. Conclusions A steeper distal-anterior femoral radius was an advantage. High medial-anterior tibial conformity was important. However, on the lateral side, the posterior sagittal tibial radius had to be shallower than ideal to allow femoral rollback in high flexion. This meant that the posterior laxity displacements on the lateral side were higher than anatomic, and there was no guidance for lateral femoral rollback.