Influence of Assembly Force and Taper-Trunnion Surface Topographies on the Initial Stability of Modular Hip Junctions

Transcription

1 Influence of Assembly Force and - Surface Topographies on the Initial Stability of Modular Hip Junctions Hannah J. Lundberg, Deborah J. Hall, Robert M. Urban, Brett R. Levine, Robin Pourzal. Rush University Medical Center, Chicago, IL, USA. Disclosures: H.J. Lundberg: None. D.J. Hall: None. R.M. Urban: 3B; Wright Medical, AgNovos Health, Spinal Motion, DePuy. 5; Zimmer, Wright Medical, AgNovos Health. B.R. Levine: 1; Human Kinetics. 3B; Zimmer, Biomet, ConMed. 5; Zimmer, Biomet. R. Pourzal: None. Introduction: Total hip arthroplasty (THA) is one of the most common surgical procedures with currently more than 250,000 per year and rising in the US alone. For several decades surgeons have relied on the concept of modular taper junctions to mount the femoral head to the femoral stem. Recently there are increasing reports of failed modular junctions due to fretting wear and corrosion. Although this problem is not new, its recent re-occurrence is of great concern and is a multifactorial problem. It can be related to design issues (taper-trunnion length, width, angle and angular mismatch), material, and surgical procedures. Rehmer et al. recently found that the optimal force for taper-trunnion fusion during surgical assembly is approximately 4 kn [1]. Although bench top tests and finite element analyses (FEA) have been used to investigate assembly and disassembly of modular femoral heads, it is not known whether the surface topography of the taper and trunnion change this behavior. It is therefore imperative that the taper-trunnion design result in sufficient locking over a range of assembly forces, and be forgiving to small misalignment during surgery. We hypothesize that the assembly load will have a significant impact on the deformation behavior of the topographies of the taper and trunnion. To test this hypothesis we have developed a novel FEA model to simulate the topography deformation process and to compute the resulting contact area under different assembly forces. A retrieval study was carried out to determine relevant input data for the model. Methods: 32 retrieved THAs from 8 different manufacturers were analyzed. All femoral heads were of CoCrMo alloy. Head sizes ranged from 28 to 50 mm (34 ± 5.72 mm). 15 femoral stems were CoCrMo, 16 were Ti-alloy, and 1 was stainless steel. The articulating couple was metal-on-polyethylene in 24, metal-on-metal in 5, and a hemiarthroplasty in 3. The average time in situ was 19 months. The damage to both trunnion and taper surfaces was evaluated in an earlier study [2]. For the present study only implants with the lowest trunnion damage (degree 1, minimal) were chosen. 31 taper surfaces were of degree 1, and 1 was degree 2 (mild). The valley to peak height and spacing between the circumferential machining marks of the trunnion were measured with white light interferometry (Zygo Corporation, Middlefield, CT). The mismatch between taper and trunnion angle was measured with a coordinate measuring machine (Smartscope, OGP Inc., Rochester, NY). An axisymmetric 2D FEA model was created in ABAQUS v6.13 (Dassault Systemes, Waltham, MA). A global model was created representing a half cross section of the femoral head, stem, and impactor. A local model was created representing the topography of the trunnion and taper surfaces (Fig. 1). machining mark height and spacing, femoral head size, and taper-trunnion angle combination in the local model was chosen from the mean retrieval measurements (Table 1). The CoCrMo femoral head and stem was modeled as elastic-perfectly plastic (E=200 GPa, ν=0.33, ρ=8.3 g/cm 3, and yield stress 450 MPa). The polyacetyl impactor was modeled as linearly elastic (E=2.9 GPa, ν=0.33, ρ=1.4 g/cm 3 ). The local model was driven by the global model through submodel boundary conditions at the edges interior from the surface of the trunnion and taper: displacements predicted in the global model were applied to the local model. Contact was modeled between the trunnion and taper surfaces (0.2 friction coefficient). To simulate implant assembly during surgery, the femoral head was moved into contact with the stem with a 100N load. Next a 2 kn, 4 kn, or 6 kn impaction force was applied over 0.1 seconds [2]. The amount of contact area between the trunnion and taper surfaces of the local models, Von Mises equivalent stress, and accumulated plastic equivalent strain was compared.

2 Results: The height of machining marks on the trunnion varied between 4.30 and 83.0 μm (mean 15.0±15.1 μm ) and the spacing between 39.5 and 228 μm (mean 188 ± 36.0 μm) (Table 1). The taper angles ranged from 5.30 to 6.08 degrees, and the trunnion angles ranged from 5.59 to 5.78 degrees. The range of angular mismatch between the trunnion and taper was from to degrees where a positive number indicates that the trunnion has a larger angle than the taper. Retrieval measurements (n=32). Positive angular mismatch indicates larger trunnion than taper angle. Table 1. Topography Height Topography Spacing Angle Angle Angular Mismatch Length Diameter Length Diameter (μm) (μm) (degree) (degree) (degree) (mm) (mm) (mm) (mm) Minimum Maximum

3 Mean Standard Deviation Impaction of the femoral head onto the stem resulted in plastic deformation of the trunnion and taper machining marks at all three assembly loads. The femoral head displaced 86.0, 110, and 159 μm for 2, 4, and 6 kn assembly loads, respectively. In the local model, which represented approximately a quarter of the entire trunnion surface area, contact area after initial contact was 26.5 mm 2. Contact area increased to a maximum of 53.1 mm 2 as the femoral head initially contacted the femoral stem. In steady state (achieved by 0.5 s after application of the assembly force) contact area decreased to 30.3 mm 2 for the 6kN assembly force, 43.4 mm 2 for the 4kN assembly force, but was maintained at 53.1 mm 2 for the 2kN assembly force. This decrease in contact area under higher assembly loads occurred as the machining marks in contact plastically deformed and slid past each other. The residual Von Mises stress at steady state within the taper and trunnion was dependent on assembly force: lower assembly force resulted in larger regions of higher stress (Fig. 2). Both the area and peak accumulated plastic equivalent strain increased with higher assembly force (Fig. 2).

4 Discussion: This study represents the first attempt to understand the role of taper-trunnion surface topography in the stability of the modular junction, and thus prevention of micromotion and subsequent fretting-corrosion. It was our hypothesis that the applied force during the assembly of trunnion and taper has a significant influence on the deformation behavior of the initial topography. Although the results of this study show different deformation behavior under varying assembly forces, it is unknown how the surface topography affects initial stability after surgical assembly. Rehmer et al [1] have shown during experimental tests that a load of 4KN provide sufficient stability. However, the initial topography has not been characterized. The wide range of topographies measured in the retrievals suggest that this threshold may differ between designs. On the other hand Panagiotidou et al [3] pointed out the importance of the contact area, showing that small trunnions, and thus smaller nominal contact areas, result in lower stability. The results of our study suggest that there may be an ideal ratio between contact area, residual stresses, and plastic strain. This ratio can be provided by adjusting the surface topography for each alloy combination. The novel model presented in this study will help to determine these topographies. Future steps will include experimental validation and consideration of other factors (angular mismatch, neck length, and frequency of hits). Significance: Surface topography of contemporary trunnion and taper surfaces of modular THA taper junctions has not been well characterized, and the influence of the surface topography on the stability of the construct is unknown. Knowledge of the deformation behavior of the surface topography during surgical assembly is essential to predicting implant stability in vivo and to prevent micromotion and subsequent fretting-corrosion. Acknowledgments: The authors would like to thank Dr. Markus Wimmer and Erica Dahlmeier for their contributions. References: [1] Rehmer et al. 2012, Clin Biomech 27: [2] Hall et al. 2013, Trans ORS 38:1796. [3] Panagiotidou 2013, JOR doi: /jor ORS 2014 Annual Meeting Poster No: 0185

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