INTRODUCTION. The increasing incidence of periprosthetic femoral fractures (PFF) after total hip arthroplasty presents growing concerns due to challenges in treatment and increased mortality. PFF are often observed when the prosthesis is implanted in varus, especially with blade-type stems. To help elucidate its impact on the PFF risk, the specific research question is: What is the effect of misalignment of a blade-type stem (resulting in down-sized prosthesis) on 1)the distribution and magnitude of cortical stresses and 2)implant-bone micromotion. METHOD. We developed two finite element models consisting of an average female femur implanted within a generic blade-type stem prosthesis, (i)in neutral alignment, and (ii)oriented in 5° of varus, coupled with corresponding down-sizing of the prosthesis. Each model consisted of 1.1million elements, while the average mesh length at the implant-bone interface was 0.4mm. Elastic moduli of 15GPa(cortex), 150MPa(trabecular bone), and 121GPa(implant), and Poisson's ratio of 0.3 were assumed. The distal end was fixed and the interface was defined as a surface-to-surface contact with friction coefficients (dynamic 0.3; static 0.4). Walking and stair-climbing were simulated by loading the joint contact and muscle forces after scaling to the subjects’ body weight. The peak von Mises stress and the average stress within the surface having 1cm diameter and the center at where the peak stress occurred at each contacting area, the
Introduction. 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
Introduction. Good outcomes in reverse shoulder arthroplasty (RSA) rely in part on stability of the humeral component. Traditionally humeral components have been cemented, however there has been recent interest in press-fit fixation of humeral components in RSA. Lateralization of the head center in RSA can impart larger moments on the humeral component than for anatomic reconstructions, increasing the importance of distal humeral canal preparation for implant stability. To date, the primary stability of any type of press-fit humeral prosthesis has been largely unexplored. The goal of this study is to evaluate the effect of over-reaming the distal humeral canal in a press-fit humeral component in RSA. Methods. Computed tomography (CT) data of the shoulder were obtained from 55 shoulders. Images were segmented to produce digital models of the humerus. Humeral components for RSA (2mm diameter size increments) were sized and placed per the surgical technique, including preparation of the humerus with the appropriate reamers (1mm increments). Finite element models for each specimen were created with heterogeneous bone properties derived from the CT scan. Pressfit between the bone and stem was resolved to quantify the initial contact pressure on the stem; each stem was then loaded at 566N oriented 20° lateral and 45° anterior. Overall motion of the stem was measured, as well as
Introduction. Long-term success of the cementless acetabular component has been depends on amount of bone ingrowth around porous coated surface of the implant, which is mainly depends on primary stability, i.e. amount of micromotion at the implant-bone interface. The accurate positioning of the uncemented acetabular component and amount of interference fit (press-fit) at the rim of the acetabulum are necessary to reduce the implant-bone micromotion and that can be enhancing the bone ingrowth around the uncemented acetabular component. However, the effect of implant orientations and amount of press-fit on implant-bone micromotion around uncemented acetabular component has been relatively under investigated. The aim of the study is to identify the effect of acetabular component orientation on implant-bone relative micromotion around cementless metallic acetabular component. Materials and Method. Three-dimensional finite element (FE) model of the intact and implanted pelvises were developed using CT-scan data [1]. Five implanted pelvises model, having fixed antiversion angle (25°) and different acetabular inclination angle (30°, 35°, 40°, 45° and 50°), were generated in order to understand the effect of implant orientation on implant-bone micromotion around uncemented metallic acetabular component. The CoCrMo alloy was chosen for the implant material, having 54 mm outer diameter and 48 mm bearing diameter [1]. Heterogeneous cancellous bone material properties were assigned using CT-scan data and power law relationship [1], whereas, the cortical bone was assumed homogeneous and isotropic [1]. In the implanted pelvises models, 1 mm diametric press-fit was simulated between the rim of the implant and surrounding bone. Six nodded surface-to-surface contact elements with coefficient of friction of 0.5 were assigned at the remaining portion of the implant–bone interface [1]. Twenty-one muscle forces and hip-joint forces corresponds to peak hip-joint force of a normal walking cycle (13%) were used for the applied loading condition. Fixed constrained was prescribed at the sacroiliac joint and pubis-symphysis [1]. A submodelling technique was implemented, in order to get more accurate result around implant-bone interface [1]. Results and Discussions. The peak implant-bone sliding
Introduction. Modeling the press-fit that occurs in Total Hip Arthroplasty (THA) cementless implants is crucial for the prediction of micromotion using finite element analysis (FEA). Some studies investigated the effect of the press-fit magnitude and found a direct influence on the micromotion [1,2]. They assumed in their model that press-fit occurs throughout the prosthesis. However [3] found using computed tomography measurement that only 43% of the stem-bone interfaces is really in contact. The aim of this study is to investigate the press-fit effect at the stem-bone interface on the implant micromotion. Methods. Finite element analysis (FEA) was performed on a Profemur® TL implanted into a Sawbones®. The implant orientation was validated in a previous study [4]. All materials were defined as linear isotropic homogeneous. FEA was carried out for the static loading conditions defined by [5] simulating walking fastly. Frictional contact between the bone and the prosthesis was assumed all along the prosthesis with a coefficient μ set to 0, 63 for the plasma spray (Fig. 1a) and 0,39 for the polished surface (Fig. 1b) [6]. Firstly, FEA was performed without press-fit (Fig. 2a) and then press-fit was simulated with an interference of 0,05 mm [2] between stem and bone in specific areas: superior (Fig. 2b), intermediate (Fig. 2c), inferior (Fig. 2d), and cortical alone (Fig. 2e) and finally over the entire surface in contact with the bone. The press-fit effect at the stem-bone interface on the micromotion was investigated. Measurement of the micromotion was realised on different points located on the plasma spray surface by calculating the difference between the final displacement of the prosthesis and the final displacement of the bone. Results. When press-fit is applied along the entire stem-bone interface, micromotion is lower than 10 μm. In the case when no press fit is simulated, micromotion is in the range of 11 μm and 48 μm. When press-fit is included where only cortical bone is (small areas mid-way proximal and medial part), micromotion is in the range of 17 μm and 30 μm. When the press-fit is included where inferior cancellous bone is (more distal), micromotion is between 9 μm and 38 μm. When the press-fit is included in the intermediate cancellous bone (mid-way), micromotion is between 1 μm and 47 μm. Finally, when press-fit is involved in the superior cancellous bone (more proximal) alone, micromotion is in the range of 4 μm and 12 μm. The results are shown on Fig. 3. Discussion. The maximum stem-bone interface micromotions calculated in this study always remain lower than 50 μm. [7] shows that