Abstract
Introduction
Despite consequent advancement in Total Knee Arthroplasty (TKA) up to 20% of patients are not satisfied after having been operated. Beside correct implantation, the design of the TKA-system is supposed to be a key factor of a successful TKA. Consequently it has been tried to restore natural kinematics by the design of the prosthesis. A medially stabilized design therefore is supposed to allow a lateral translation with a medial pivot.
Objectives
Our study compared posterior stabilized (PS) with medially stabilized (MS) TKA-design in terms of kinematics, femorotibial and patellofemoral contact patterns in vitro.
Methods
Twelve fresh frozen human knee specimen (8 male, 4 female, Ø 63.9 y) were tested in a knee rig under natural condition and after TKA with the two different types of TKA systems (GMK PS; GMK MS Sphere; Medacta International, Switzerland). The knee rig simulated a loaded squat from approximately 20°−120° of flexion with a ground reaction force of 50 N. An ultrasound 3D-motion analysis system (Zebris, CMS 10, Germany) measured complete knee kinematics and pressure sensitive foils (Tekscan Inc. US) analysed the femorotibial and patellofemoral contact patterns. By having the same bone cuts for the PS and MS knee system the direct comparison within every single knee specimen could be performed. For stastics mixed effects models were applied by utilizing random intercept per specimen to compare PS and MS during the whole squat. P < 0.05 represented statistical differences.
Results
Femorotibial kinematics of MS design showed a significant reduction of medial femorotibial translation compared to PS design. At 120° of flexion the PS design displayed a mean posterior translation of the femur related to the tibia of 15.62 mm, while the MS design translated medially only an average of 3.45 mm in the same direction. In the lateral compartment both designs showed a posterior translation of the femur in relation to the tibia. At 120° of flexion the femur of the PS design translated 19.00 mm posteriorly compared to the MS design with 14.73 mm (p<0.01).
In the medial femorotibial compartment of MS design we observed an enlarged contact area (+175.6 mm2) and lower peak pressure (−1.83 MPa), in contrast in the lateral compartment there was a reduced contact area (−19.68mm2) and increased peak pressure (+0.63 MPa) compared to PS design (p<0.01).
The retropatellar peak pressure increased significantly after TKA. The contact area at 120° flexion showed no difference between GMK Sphere and the natural knees. The PS had a lower contact area. Retropatellar mean pressure increased for the PS compared to MS and natural knee. There was no significant difference of the retropatellar peak pressure between MS and PS.
Conclusion
Neither of the two designs seems to replicate completely the physiological movement of the knee. However, the MS TKA system provided a lateral translation with a medial stable compartment, which is supposed to provide a-p stability and restore the physiological knee kinematics in the active arc of flexion. In terms of retropatellar contact patterns MS restored natural conditions better than PS.
