Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine http://pih.sagepub.com/
Reduced stress shielding with limited micromotions using a carbon fibre composite biomimetic hip stem: a finite element model C Caouette, L'H Yahia and M N Bureau Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine published online 30 June 2011 DOI: 10.1177/0954411911412465 The online version of this article can be found at: http://pih.sagepub.com/content/early/2011/06/30/0954411911412465
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Reduced stress shielding with limited micromotions using a carbon fibre composite biomimetic hip stem: a finite element model C Caouette1, L’H Yahia1, and M N Bureau2* ´ cole Polytechnique de Montre´al, Montre´al, Laboratory of Innovation and Analysis of Bioperformance (LIAB), E Quebec, Canada
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Industrial Materials Institute, National Research Council of Canada, Boucherville, Quebec, Canada
The manuscript was received on 22 November 2010 and was accepted after revision for publication on 13 April 2011. DOI: 10.1177/0954411911412465
Abstract: Total hip arthroplasty (THA) enjoys excellent rates of success in older patients, but younger patients are still at risk of aseptic loosening and bone resorption from stress shielding. One solution to the stress shielding problem is to use a hip stem with mechanical properties matching those of cortical bone. The objective of the present study was to investigate numerically the biomechanical performance of such a biomimetic hip stem based on a hydroxyapatite (HA)-coated carbon fibre composite. A finite element model (FEM) of the biomimetic stem was constructed. Contact elements were studied to model the bone–implant interface in a non-osseointegrated and osseointegrated state in the best way. Three static load cases representing slow walking, stair climbing, and gait in a healthy individual were considered. Stress shielding and bone–implant interface micromotions were evaluated and compared with the results of a similar FEM based on titanium alloy (Ti–6Al–4V). The composite stems allowed for reduced stress shielding when compared with a traditional Ti–6Al–4V stem. Micromotions were slightly higher with the composite stem, but remained below 40 mm on most of the HAcoated surface. It is concluded that a biomimetic composite stem might offer a better compromise between stress shielding and micromotions than the Ti–6Al–4V stem with the same external geometry. Keywords: hip prosthesis, biomimetic material, composite material, finite element analysis
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INTRODUCTION
1.1 Total hip arthroplast modelling Despite a high clinical success rate, total hip arthroplasties (THAs) present a number of problems, especially for younger patients with active lifestyles who are likely to suffer aseptic loosening due to bone resorption mainly caused by osteolysis and stress shielding [1]. The use of composite materials in orthopaedics is an attempt to solve some of these problems. The so-called ‘isoelastic stems’ developed in the 1980s were aimed at providing a more natural *Corresponding author: Industrial Materials Institute, National Research Council of Canada, Boucherville QC, J4B 6Y4, Canada. email:
[email protected]
stress distribution in the proximal femur, in the hope of reducing stress shielding and bone resorption [2– 4]. Other, more contemporary studies have attempted to create stems made of composite materials (e.g. carbon fibre and polyetheretherketone (PEEK) [5], carbon or glass fibre and polyethylenimine (PEI) [6], and metallic core with a flexible composite outer layer [7]). One of these studies aimed to mimic the flexural rigidity of the natural femur [6], another study sought to reproduce the strain energy density observed in the natural femur by optimizing ply orientation within the material [5], and another attempted to axially modulate the elastic modulus of the stem [7]. However, high micromotions at the bone–implant interface prevented long-term bone fixation of isoelastic stems, leading to clinical failures [8, 9]. Proc. IMechE Vol. 225 Part H: J. Engineering in Medicine
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C Caouette, M N Bureau, and L’H Yahia
For successful osseointegration and good primary stability, threshold values of micromotions varying between 30 mm [10–12], 40 mm [13], and 100– 200 mm [14–16] have been proposed. Higher micromotions will lead to the formation of a fibrous layer rather than proper osseointegration. Besides the use of new materials, other avenues have been explored to improve stress distribution within the implanted femur. Improvement of crosssectional stem shape is one such avenue: a recent study by Sabatini and Goswami [17] concluded that anatomical circular and elliptical cross-sections produce a smoother stress field than trapezoidal crosssections and therefore produce less stress shielding. Another study by Gross and Abel [18] also showed that a hollow stem led to less stress shielding than its solid counterpart; it was also found that reducing the elastic modulus would also reduce stress shielding. However, it is noted in the literature that optimizing the shape of a stem made of a homogeneous metallic material is not sufficient to create a stem that reproduces the stiffness of the natural femur [6]. Clinical failures of more flexible implants underlined the importance of proper pre-clinical testing of new designs, for which finite element analysis (FEA) is a most widely used tool [19–23] and the method of choice to evaluate new stem concepts [18, 24, 25] both from an economical and an ethical point of view. Femoral bone modelling is a subject well covered in the literature [26–30]. Composite femur models are considered an acceptable substitute for pre-clinical testing of implants [31]. FEA boundary conditions for hip implants have been validated experimentally for metallic implants [16, 32, 33] and are indiscriminately used for healthy femurs or other types of implants. Most authors use bonded implants and only specify a friction coefficient [34] and contact surface behaviour [20, 35] and avoid or neglect a key aspect to finite element method (FEM) modelling of a cementless implant: bone–implant interface modelling [24]. Also, very few authors specify their complete contact conditions or justify their choices [16, 36, 37]. 1.2 Biomimetic concept Preliminary finite element studies were conducted to assess the validity and theoretical performance [38, 39] of a biomimetic stem concept previously developed. Unlike isoelastic stems that simply tried to match the bending stiffness of the femoral bone and were generally made of metallic materials, the current biomimetic stem is made of a polymer composite material that mimics the modulus of elasticity of bone. It therefore does not introduce a soft surface
layer apposed to the host bone, as was the case with some isoelastic stem designs. The biomimetic stem was constructed and mechanically tested for partial validation of the preliminary finite element model [40]; this step insured that the finite element model represented the real material and not a mathematical optimization of a theoretical material, as is sometimes the case with FEM studies on composite materials [41]. The purpose of this study is to investigate numerically the performance of the biomimetic hip stem using this partially validated finite element model, and to assess its performance when compared with a conventional metallic stem with the same external geometry. Prior to these simulations, a realistic bone–implant interface is described.
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MATERIALS AND METHODS
2.1 Stem concept The biomimetic stem concept is aimed at reproducing the natural physical structure of the femur, with its cortical outer shell and its weaker interior of trabecular bone. The stem is made of a biocompatible material, as indicated by in vitro cytotoxicity and in vivo tissue response, showing good osseointegration and proliferation of osteoblasts and their progenitors within the hydroxyapatite (HA) coating in histological sections of a rabbit model [42], and by good bone apposition with minimal inflammation (haematoxylin and eosin (H&E) staining) and no osteolysis when exposed to purposely produced debris from grit blasting in a rat model [43]. This material consisted of a continuous carbon-fibrereinforced polyamide 12 (CF/PA12) moulded into a hollow stem structure by an inflatable bladder moulding process. The carbon fibre volume fraction is 0.55; the moulding process has been optimized to obtain compressive properties (refer to Table 1) close to those of cortical bone [44, 45]. In the initial biomimetic concept, a polymeric core was included in the hollow stem to mimic trabecular bone, but this core was deemed unnecessary after a finite element study showed it had negligible influence on principal stresses in the femur [38]. The femoral stem geometry is inspired from an anatomical cementless design (see Fig. 1) and is modified from commercial designs. It has an oval cross-section, follows the ante curvature of the femoral shaft, and has a neck-shaft angle of 135°. It consists of a CF/PA12 hollow stem (wall thickness of 3 mm) coated in the proximal region with a semicrystalline HA layer [46] to facilitate bone growth and integration. The titanium (Ti) stem used for
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Reduced stress shielding using a biomimetic hip stem
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Table 1 Material properties of the composite material structure compared with bone and other traditional stem materials Material/tissue
Density (g/cm3)
Modulus (GPa)
Strength (MPa)
Poisson’s ratio
Trabecular bone Cortical bone Ti–6Al–4V Composite (compression) Composite (tension)
0.03–0.12 1.6–2.0 4.4–4.7 1.2–1.6
0.04–1.0 12–20 106 5–14 12–30
1.0–7.0 150 780–1050 53–220 70–250
0.01–0.35 0.28–0.45 0.33 0.3 0.36
Canonsburg, USA) were used to model the bone– implant interface. The force vector for these contact elements is 8 9
