Abstract
Introduction
Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. Femorotibial constraint is a key property of a TKA prosthesis and should be designed to match the device's intended function. Cruciate Retaining (CR) prostheses are usually used for patients with a functioning posterior cruciate ligament (PCL). For patients without a fully functioning PCL, CR-Constrained (CRC) prostheses may be used. A CRC tibial insert usually has a more conforming sagittal profile especially in the anterior aspect to provide increased constraint to prevent paradoxical femoral translation during knee flexion. A quantitative understanding of the constraint behavior of a prosthesis design is critical to ensure its functional outcome. Using a validated computer simulation, this study evaluated the anterior-posterior (AP) constraint of two types of tibial inserts (CR and CRC) from a same TKA product family.
Methods
Both the CR and CRC prostheses are from the same TKA product family (Optetrak Logic, Exactech, USA). Three sizes (sizes 1, 3, and 5) from each product line were included in this study. Computer simulations using finite element analysis (FEA) were performed at 0° flexion per ASTM F1223 standard [1] (Figure 1). The simulation has been validated with physical testing (more details submitted in a separate abstract to ISTA 2013). Briefly, FEA models were created with all materials considered linear elastic. The tibial baseplate was distally fixed and a constant compressive force (710 N) was applied to the femoral component. Nonlinear Surface-Surface-Contact was established at the articulating surfaces. A coefficient of friction of 0.1 was assumed for all articulations [2]. The femoral component was driven under a displacement-controlled scheme to slide along AP direction on the tibial insert. Constraint force occurring at the articulation was derived from the reaction force at the distal fixation. A nonlinear FEA solver was used to solve the simulations.
Results
The force-displacement curves predicted by the simulation exhibited the hysteresis loop appearance for both CR and CRC inserts (Figure 2). The anterior aspect of the CRC curves showed a steeper raise than the CR curves, and the trend was consistent across sizes. Taking the slope from 0 to 5 mm range, the anterior constraint of the CRC insert was significantly greater than the CR insert, while the posterior constraint of the CRC insert was also slightly greater (Figure 3).
Discussion/Conclusion
The increased AP constraint of the CRC insert revealed in the study is consistent with the design geometry and functional intent of the device. With a much increased anterior lip, the CRC insert is expected to provide substantially greater anterior constraint than the CR insert to prevent paradoxical femoral translation for patients without a fully functioning PCL. The CRC insert is also expected to provide slightly increased posterior constraint due to its gently elevated posterior lip. This study quantitatively demonstrated the effect of design geometry on the outcome constraint function of different TKA prostheses.
