Correlation of Femoral Component Micromotion to a Physical Test Using an FEA Model.

Transcription

1 Correlation of Femoral Component Micromotion to a Physical Test Using an FEA Model. Robert Davignon, Ananthkrishnan Gopalakrishnan. Stryker Corporation, Parsippany, NJ, USA. Disclosures: R. Davignon: 3A; Stryker Corporation. 4; Stryker Stock. A. Gopalakrishnan: 3A; Stryker Corporation. 4; Stryker Stock. Introduction: In total knee arthroplasty (TKA), effective femoral component fixation and placement are key factors in preventing femoral component loosening postoperatively. Femoral component loosening due to insufficient fixation is an uncommon, but potentially significant complication in TKA. Previous studies have shown femoral loosening resulting from insufficient bone stock or inaccurate surgical bone cuts 1. Due to the severity of these failures, it is important to accurately assess micromotion and liftoff for implant designs. The purpose of this study was to develop an FEA analog to correlate results seen in a physiologically relevant physical test. Methods: In a previous study, a physical test was developed to assess the effect bone prep has on femoral micromotion using Sawbones composite femurs (Pacific Research, Vashon, WA). The physical test assessed three press-fit femoral bone preparations and three intentionally gapped femoral bone preparations to study the effect of bone preparation on femoral component micromotion. Bones were loaded via the ISO load profile and micromotion was measured using an Aramis optical system 2. For the purposes of this study, an FEA analog of the physical test was created to simulate the gapped bone preparation at the final time point of 154,000 cycles, representing 8 weeks of loading 3. In order to replicate the physical test with an FEA simulation, 3D CAD models of the femoral component and Sawbones femur tested were virtually implanted with the same intentional gap preparation (Figure 1). The peg holes were created as outer diameter line to line, assuming that the press-fit had relaxed by the final time point. The ISO load profile was simplified into 12 snapshots to represent the most extreme loads, flexion angles, IE rotations, and AP translations described in the load profile (Figure 2). Each snapshot was run with a static FE analysis (ANSYS 14.0). Force was applied normal to the ground; flexion was about the femoral components flexion axis, rotation was applied about the tibial insert, while anterior-posterior motion was applied via the femur. For each test the components were oriented to their prescribed positions, the femur was held fixed at its proximal end and the tibia was free to move superior/inferior, varus/valgus and medial/lateral. Patches were imprinted on the implant and the bone at the center of each implant flat, offset from the cutting plane (anterior chamfer, anterior flange, posterior chamfer, and posterior condyle). Each bone cut surface had a local coordinate system defined to orient flexion/extension shear (X), deformation normal to the bone cut surface (Y), and medial/lateral shear (Z) vectors relative to each cut surface. Local relative deflection was measured between the imprinted faces at each bone cut surface (Figure 1). The results for each snapshot from the load cycle were used to calculate peak to peak (Pk-pk) micromotion for comparison to physical results. Results: The Pk-pk results for each test setup are displayed in figure 3. X represents shear micromotion in a flexion/extension direction. Y represents micromotion normal to each cut surface. Z represents shear micromotion in the medial/lateral direction oriented to each cut surface. The average difference at all measurement locations between the simulation and physical test results in X are 1.86 standard deviations of the physical test, standard deviations for Y, and standard deviations for Z. The simulation accurately estimated micromotion normal to the implant/bone interface (Y), as measured in the physical test. The simulation underestimated micromotion in the shear direction (X, Z) as measured in the physical test. Discussion: Micromotion normal to the implant/bone interface (Y) is a key factor in bone ingrowth into cementless components. Therefore, it is important to be able to determine relative levels of micromotion between specific designs. The purpose of this study was to create a simulation that correlates to micromotion as measured in a physical test, allowing fast iterations of design feature effectiveness during implant development. With our current model assumptions, the simulation shows a good correlation with micromotion normal to the cut surface (difference of Standard Deviations), which is a critical factor in bone ingrowth. Applied force components in the X direction are higher than applied force components in the Z direction. This would lead to the assumption that at the end of the load cycle, geometry resisting X micromotion would be damaged to a higher degree than geometry resisting Z micromotion. The simulation of an undamaged Sawbones femur underestimated micromotion in the X direction to a higher degree than in the Z direction, thus supporting this assumption. Future studies can account for accumulated damage to the Sawbones femur by measuring bone cut surfaces at the end of testing with a coordinate

2 measurement machine. In addition, the peg holes should be inspected at the end of the physical test to better understand the final peg hole geometry and remaining peg press-fit for more accurate simulations. By accounting for accumulated damage in future tests, shear results should be more accurate. Significance: The method developed allows for virtual assessment of micromotion with results that correlate to a physiologically relevant physical test. By assessing iterative implant designs, fixation features can be optimized to minimize micromotion under physiologically relevant loading in this simulation faster than in a physical test. Acknowledgments: References: [1] King, T.V, et al, 1984, Clin. Orth. and Related Res., 194, [2] Gopalakrishnan, A, et al, 2013, Orthopaedics Research Society (submitted). [3] Bobyn, JD, Pilliar, RM, Cameron, HU, and Weatherly, GC: Clin. Orthop Relat Res , 1980.

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4 ORS 2014 Annual Meeting Poster No: 1863

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