Objective: In order to evaluate the influence of kinematics by two different design concepts of cruciate-retaining total knee arthroplasties (TKAs), we compared intra-operative findings of compressive forces on the tibial joint surface in terminal procedure after assembling each component.
Methods: We developed a tibial metal tray containing six load cells to directly measure compressive forces on the tibio-femoral joint surface. Utilizing both a navigation system and our developed force sensor, 13 NexGen® CR TKAs (Z group) and 9 NRG® CR TKAs (S group) were compared with axial rotation, distribution of six local forces, subtotal forces of lateral/medial tibial compartment and kinematic pathway (gravity axes) which jointed a lateral/medial gravity point to be made by three local forces.
Results: The two groups showed different findings except the axial rotation pattern. Z group's subtotal force had a higher lateral force than the medial force (P < 0.05), but lateral roll back (6 mm) which was similar to medial roll back in the kinematic pathway. These findings indicated an undesirable influence of asymmetric condyle and dual contact design in the NexGen® TKA. Conversely, S group's subtotal force showed a similar lateral force compared to the medial force and internal rotation with medial pivot which indicated a more lateral roll back (7 mm) than medial roll back (4 mm) in the kinematic pathway. These findings support the superiority of single radius and spherical arc design in the NRG® TKA.
Moreover, well gap balances in Z group were shown at 20° and 110° of knee flexion. Because the spacer block was inserted into the gap spaces between the aspect of the femoral bone cut and the tibial bone cut with 6.5° of posterior slope, this finding indicated that gap balances were not adjusted at full extension and 90° knee flexion, and this led us to observe the gap balance was inclined to laxity of the medial cruciate ligament.
Conclusions: Our developed force sensor showed that two different design concepts of TKAs enable there to be different ligament balance, gap balance and kinematic pathways, except in axial rotation.
