Abstract
Introduction
Ability to accommodate increased range of motion is a design objective of many modern TKA prostheses. One challenge that any “high-flex friendly” prosthesis has to overcome is to manage the femorotibial contact stress at higher flexion angle, especially in the polyethylene tibial insert. When knee flexion angle increases, the femorotibial contact area tends to decrease thus the contact stress increases. For a high-flex design, considerations should be taken to control the contact stress to reduce the risk of early damage or failure on the tibial insert. This study evaluated the effect of femoral implant design on high flexion contact stress. Two prostheses from a same TKA family were compared – one as a conventional design and the other as a high-flex design.
Methods
Two cruciate retaining (CR) prostheses from a same TKA product family were included in this study. The first is a conventional design for up to 125° of flexion (Optetrak CR, Exactech, USA). The second is a high-flex design for up to 145° of flexion (Logic CR, Exactech, USA). The high-flex design has a femoral component which has modified posterior condyle geometry (Figure 1), with the intent to increase femorotibial contact area and decrease contact stress at high flexion. Three sizes (sizes 1, 3, and 5) from each prosthesis line were included to represent the commonly used size spectrum. Contact stress was evaluated at 135° of flexion using finite element analysis (FEA). The CAD models were simplified and finite element models were created assuming all materials as linear elastic (Figure 2). For comparison purpose, a compressive force of 20% body weight was applied to the femoral component. The average body masses of sizes 1, 3 and 5 patients are 69.6 kg, 89.9 kg, and 106.3 kg based on the manufacture's clinical database. A nonlinear FEA solver was used to solve the simulation. Von Mises stress in the tibial insert was examined and compared between the two prostheses.
Results
The high-flex design demonstrated lower tibial insert stresses compared to the conventional design, and the stress reduction is consistent across different sizes (Figure 3). The peak von Mises stress of the high-flex design was 8.6 MPa, 10.8 MPa, and 11.9 MPa for sizes 1, 3 and 5, representing a 40% to 60% decrease compared to those of the conventional design (14.3 MPa, 26.5 MPa, and 25.6 MPa respectively).
Discussion/Conclusion
One limitation of the study was that no material nonlinearity was considered in the FEA, thus stress values above the yield strength of polyethylene could be over-estimated. However, as a qualitative comparison, the analysis demonstrated the effectiveness of the high-flex design on reducing tibial insert contact stress. Although the actual flexion angle of a CR TKA patient is not fully defined by the prosthesis and largely affected by the patient's anatomy and pre-operative range of motion, a lower contact stress at high flexion indicates a more forgiving mechanical structure and less risk for polyethylene damage when the patient is able to perform high flexion activities.
