Originally, the vertical expandable titanium rib (VEPTR™) was developed to treat children with Thoracic insufficiency syndrome secondary to fused ribs and congenital scoliosis. Over the years its usage has widen and is currently being used to treat all etiology of early onset scoliosis (EOS). A major draw back remains the size of the titanium VEPTR™ implant. In keeping with the new trend of chrome-cobalt alloy (CoCr). spinal implants, we set out to explore if redesigning the VEPTR™ was mechanically sound. The aim of this study was twofold. Firstly, we investigate the mechanical properties of a VEPTR™ made with CoCr alloy compared to that of titanium alloy. Secondly we investigated how much we could down size the VEPTR™. Finite element analyses were performed on 3 different VEPTR™ designs (rod diameter of 6mm, 5mm and 4mm) subjected to a compressive load of 500N (equivalent to a 50Kg child). For each configuration, two materials, titanium alloy and chrome-cobalt alloy, were used. Maximum Von Mises stress distribution (VMSD), plastic strain (PS) and total displacement (TD) of the VEPTR™ were measured as indicators of mechanical properties of the implant.Introduction
Materials & Methods
After arthroplasty, stress shielding and high shear stresses at the bone-implant interface are common problems of load bearing implants (e.g. hip prostheses). Stiff implants cause stress shielding, which is thought to contribute to bone resorption1. High shear stresses, originated by low-stiffness implants, have been related to pain and interfacial micro-movements², prohibiting adequate implant initial fixation. A non-homogeneous distribution of mechanical properties within the implant could reduce the stress shielding and interfacial shear stresses3. Such an implant is called “functionally graded implant” (FGI). FGI require porous materials with well-controlled micro-architecture, which can now be obtained with new additive manufacturing technologies (e.g. Electron Beam Melting). Finite element (FE) simulations in ANSYS-v14.5 are used to develop an optimization methodology to design a hip FGI. A coronal cut was performed on a femur model (Sawbones®) with an implanted Profemur®TL (Wright Medical Inc.) stem to obtain the 2D-geometry for FE simulations. The central part of the FGI stem was made porous, the neck and inferior tip were solid. Ti6Al4V elastic material was assumed (E=120 GPa, v=0.3). Three bone qualities were considered for the optimization: poor (E=6GPa; v=0.3); good (E=12GPa; v=0.3); excellent (E=30GPa; v=0.3). The structure of bone evolves to maintain a reasonable level of the strains. Similarly in the proposed algorithm, the strut sections of the porous material evolve to keep stresses (proportional to strains) at a reasonable level. Starting with a very small strut section, resulting in an almost zero-rigidity stem, strut sections are increased or decreased as a function of the stresses they support. This is done incrementally, until force values corresponding to normal walking of an 80 kg person (1867 N)4 are reached. Force direction was vertical and no action of the abductors was considered, to analyze the worst case scenario. The optimized FGI microstructure is defined by the strut diameter distributions. Since the distance between struts remain constant, variations in strut diameters result in variations in density. Optimized FGI porous structure was compared for the three bone qualities considered and with a solid stem in terms of bone stresses.Introduction
Methodology
Typical failure of cementless total hip arthroplasty is the lack of initial stability. Indeed, presence of motion at the bone implant-interface leads to formation of fibrous tissue that prevents bone ingrowth, which in turn may lead to loosening of the implant. It has been shown that interfacial micromotion around 40 produces partial ingrowth, while micromotion exceeding 150 completely inhibits bone ingrowth. Finite element analyses (FEA) are widely used to evaluate the initial stability of cementless THA in pre-clinical validation. Untill now, most FE models developed to predict initial stability of cementless implants were performed based on static load, by selecting the greatest load at a particular time of the cycle activity, but in fact the hip is exposed to varied load during the activity. The aim of this study is to investigate the difference in the predicted micromotion induced by static, quasi-static and dynamic loading conditions. Finite element analysis (FEA) was performed on a Profemur®TL implanted into a composite bone. The implant orientation was validated in a previous study [3]. All materials were defined as linear isotropic homogeneous. Static and dynamic FEA was performed for the loading conditions defined by simulating stair-climbing. In the static analysis, the applied resultant force (calculated with a body weight of 836N) were 951N and 2107N to simulate the abductor muscle and the hip joint contact forces, respectively [4]. In the dynamic analysis, the applied resultant force can be seen on Fig. 1. The initial stability was extracted on 54 points (Fig. 2) located on the plasma spray surface by calculating the difference between the final displacement of the prosthesis and the final displacement of the composite bone.Introduction
Materials & Methods
Porous metallic materials, due to their capability of tailoring their mechanical properties to those of bone, have been suggested to be utilized in prosthesis to avoid the stress shielding phenomenon1, believed to increase the risk of implant loosening2. The aim of this work is to obtain the most simplified model possible to simulate the mechanical behavior of a Ti6Al4V porous structure. For this purpose, a beam element model was analyzed and the results were then compared to a 3D-solid model. Two computational models of the porous structure were developed: a 3D solid model, considered as the reference for comparison, and a beam model as a simplified and computationally inexpensive approximation (Fig. 1). CATIA V5R20 (3D modelling) and ANSYS V13 (simulations) were used. Isotropic elastic material model was used. Strut diameter (ϕb) was set to 450 μm, pore diameter (ϕp) was varied between 600 and 5000 μm, and pore number (np) between 2 and 9. Structures sizes varied from 2.1 × 2.1 × 2.1 mm3 to 49.05 × 49.05 × 49.05 mm3. Apparent elastic modulus (Eap) and its difference between both models (error) were analyzed for the different values of ϕp and slenderness ratio (SR). In addition, the influence of loading direction was analyzed with the beam model for cubic and diamond cell geometries. Eap variations were compared.INTRODUCTION
EXPERIMENTAL METHODS
To compare the volume of acetabular bone resection after primary hip arthroplasty with different cup designs and technique of implantation using a computer model. The factors influencing acetabular bone resection during acetabular cup implantation in THA or hip resurfacing (SRA) include the design of the component and technique of implantation. The impact of these variables on bone resection was simulated with a computer model. A 3-D pelvis was reconstructed from CT scan images. The bony acetabulum circumference was 52.5mm. Implantation of pressfit acetabular component sustaining angles of 165°, 170° and 180° with different wall thicknesses (3.5, 4.0, 5.0mm) at various depths was simulated. Bone loss of 2742mm3 was calculated for the 165°, 4mm thick, 54mm cup, and deepening of reaming by 1 and 2mm would result in bone loss of 3780mm3 (+38%) and 5076mm3 (+85%), respectively. When oversizing to a 56mm 165° component, 4998mm3 (+82%) of bone was removed. For a 54mm, 5 mm thick component sustaining an angle of 180°, the bone loss would reach 12 410mm3 (+450%). Acetabular component design has a significant influence on the amount of acetabular bone resection. The surgical technique (avoiding over deepening and oversised components) should minimise bone loss. This knowledge is of particular importance in hip resurfacing since the acetabular component size depends on the selected femoral component size. The knowledge is is also important in THA to minimise bone loss at primary implantation.
