Predict TKA Prosthesis Constraint Behavior Using Simulation—Validation and Applications

Abstract

Total knee arthroplasty (TKA) prostheses are semi-constrained artificial joints. A prosthesis design should have a level of femorotibial constraint that matches the device's clinical indication. For example, a cruciate retaining (CR) prosthesis is used for patients who have a fully functioning posterior cruciate ligament (PCL), while a CR-constrained (CRC) prosthesis that compensates for PCL function by offering additional anterior stability might be indicated for those who do not.To assess the constraint behavior of a TKA prosthesis, physical testing is usually required, and an industrial test standard has been developed for this purpose (ASTM F1223-08). Finite element analysis (FEA) has been widely used in stress analysis and structural evaluation of various orthopaedic medical devices, but has not been commonly used for joint constraint assessment. This study presents an FEA-based simulation to evaluate the constraint behavior of TKA prostheses. The effectiveness of the methodology was demonstrated by comparing it with results from physical testing, and then the simulation was also used to compare anterior-posterior (AP) constraint behavior of two lines of prostheses (CR and CRC) in the same TKA product family.A CR prosthesis (Optetrak Logic CR, size 3, Exactech) was tested using both simulation and physical testing. The prosthesis system consists of a CoCr femoral component, an UHMWPE tibial insert, and a CoCr tibial baseplate. CAD models assembled at 0 deg of flexion were used. Finite element models were created using 10-node tetrahedral elements, with all materials considered linear elastic. Boundary conditions were set up according to the ASTM F1223 standard (Fig. 1). Nonlinear surface-surface-contact was defined at the femorotibial interface and the insert-baseplate interface. Coefficient of friction was determined from physical testing. The femoral component was driven under a displacement-controlled scheme to slide on the tibial insert. At each time step, constraint force occurring at the articulating surface was derived from the reaction force at the distal fixation. A nonlinear FEA solver (NX Nastran SOL601, Siemens, TX) was used to resolve the simulation. For the physical testing, five prosthesis samples were tested under the same ASTM F1223 setup.AP constraint for two products (CR and CRC) from the same TKA family (Optetrak Logic, Exactech) was evaluated using the simulation method. Three sizes (sizes 1, 3 and 5) from each product line were analyzed to represent the common anotomical range.The simulation successfully captured contact location and pressure along the movement of the femoral component (Fig. 2). The force-displacement curve predicted by the simulation showed a hysteresis loop profile similar to that produced by the physical test (Fig. 3). Taking the curve slope from 0 mm to 5 mm, the simulation predicted 45.7 N/mm anteriorly and 36.4 N/mm posteriorly, which are less than 10% difference from the physical test results (46.4 N/mm anteriorly and 39.6 N/mm posteriorly).The force-displacement curves exhibited hysteresis appearance for both CR and CRC prostheses (Fig. 4). The profile of the curves was generally consistent across different sizes. The anterior constraint of the CRC prosthesis was significantly greater than that of the CR prosthesis, while the posterior constraint was slightly higher. Larger sizes exhibited slightly lower constraint than smaller sizes.This study demonstrated that the simulation was able to closely predict the femorotibial constraint behavior of the TKA prosthesis under ASTM F1223 testing. The simulation results resembled the physical testing results not only in general curve profile but also in the magnitude of slope values. One limitation of the current simulation is that no material nonlinearity was assumed, which could account for some of the difference between simulation and physical test.As an application, the CR and the CRC product lines were compared. The increased constraint of the CRC prosthesis is consistent with its geometrical characteristics and functional intention. The CRC insert is expected to provide significantly greater anterior constraint to prevent paradoxical femoral translation when the patient's PCL is not functioning. The CRC insert is also expected to provide slightly increased posterior constraint due to the elevated posterior lip. The reduced constraint on larger sizes is functionally desired to offer proportional translation freedom. The results demonstrated the effectiveness of the simulation in distinguishing the constraint behavior of different TKA prostheses.A validated simulation method could be very useful in TKA prosthesis design. Because no actual prototypes are required, design evaluation and optimization can be more quickly and easily achieved than using physical testing.