Abstract: Recent advancements in additive manufacturing have enabled the design of tailored lattice structures, suitable for a range of applications, including biomedical implants. The design of lattice structures for biomedical applications requires accurate prediction of both stiffness and strength. This study presents a numerical framework to estimate the effective elastic properties and limit stresses of periodic lattice structures using a strain energy-based homogenization method combined with an efficient method to assess limit stresses along the different directions of the lattice structures. Four cell topologies (ACC, BCC, SC-BCC, F2CC) with varying strut diameters were analysed. The effective elastic constants were computed by applying periodic boundary conditions to the representative volume element of the lattice structure. To estimate the effective limit stresses, a method based on the volume-averaged stress over a subset of the most stressed elements (Ω) was proposed. Three selection strategies for Ω were developed and evaluated through sensitivity analyses. Results show that the estimated limit stresses strongly depends on the definition of Ω. The proposed method enables early-stage design of lattice-based implants with tailored mechanical behaviour and controlled failure limits, enhancing safety and biological integration, contributing to the development of computational tools for patient-specific implant design.

Keywords: Additive manufacturing | Finite Element Analysis | Homogenization | Lattice structure | Osseointegration

Abstract: The long-term effectiveness of osseointegrated implants is heavily dependent on the short-term stability, primarily achieved immediately after surgery through a mechanical connection between the bone and the implant. The most common implant designs nowadays are straight and rely on screw or press-fit fixtures. Despite the promising results achieved by current transfemoral implants, the incidence of early failures and complications is still high. Starting from the hypothesis that a patient-specific approach could lead to better primary stability immediately post-surgery, this study aims to investigate the effect of implant design on primary stability. This was performed by analyzing two patient-specific implants, customized according to the medullary canal morphology, and a simple straight implant as the reference standard. To quantitatively assess the primary stability, a comparative computational analysis was conducted to examine the effective contact area, the relative micromotion, and the stress distribution at the interface between the bone and the implant stem during a static load-bearing exercise. The results showed that implants that follow the curvature of the residual femur provide lower micromotion values and a wider contact area, with a reduction of up to 30.4% and an increase of 10.8%, respectively, compared to the straight design, leading to a more homogeneous load distribution.

Keywords: finite element analysis | load-bearing exercise | osseointegration | patient-specific design | primary stability | transfemoral implant

Abstract: The use of osseointegrated implants as a solution for amputation treatment, providing a stable connection between the stump and prosthetic limb, is expanding its application to different anatomical districts. A particular focus is put on upper limb amputations, where socket-suspended systems often lead to high abandonment rates of prostheses and osseointegrated implants can enhance prosthetic-limb use, and improve bone perception and range of motion, leading to a better quality of life. This study aims to evaluate the effect of bone-implant interference and implant length of a straight implant, on the primary stability of a humerus CAD model through finite element analysis. The results suggest that a compromise can be made between interference and length to accommodate varying bone morphology and stump length. Additionally, the study results may suggest re-evaluating the prescribed weight limits for transhumeral implants based on their actual load-bearing capacity.

Keywords: <h3 class="h4">Author keywords</h3> | finite element method | osseointegration | primary stability

Abstract: Osseointegrated implant is a promising solution for limb amputations, but its widespread use is limited by risks such as bone resorption, infections, and strict patient requirements. Typically, the bone and prosthesis are coupled using a press-fit condition, providing short-term stability, or primary stability (PS), which leads to bone in-growth and long-term stability, or secondary stability (SS). However, the greater stiffness of the implant compared to the bone is a concern for SS. Currently, osseointegrated implants are commercially available only in fixed configurations, with a limited use of customization. This study aims to compare the contact effectiveness of three press-fitted intramedullary stems for femoral amputations, developed using three designs (straight, standard curvature, and patient-specific curvature). Moreover, a novel implant design methodology is reported, such is an easy way to develop a patient-specific design. The von Mises stress distribution at the bone-implant interface was analyzed. The study uses CAD models of a femur acquired through CT scans. A FEA was conducted to evaluate the elastic behavior of the bone when the implant is press-fitted with an interference of 0.1 mm. The outcomes show how the patient-specific implant result in a more physiological distribution of the load in the bone. This study could be used as a starting point for further studies on primary and secondary stabilities.

Keywords: 3D Modelling | Finite Element Model | Osseointegration | Patient-Specific Design | Transfemoral Implant

Abstract: Total elbow arthroplasty (TEA) is an effective and frequently used treatment for patients with debilitating elbow pathology. Total elbow prostheses have lagged behind those of the knee, hip and shoulder for different reasons, such as the high failure rate of the early designs. Concern remains regarding the longevity of TEA implants, especially in younger patients. The main cause of revision of the implant is usually related to the phenomenon of aseptic loosening mainly due to the cement-bone interface failure. Aim of this work is the biomechanical analysis of a new elbow prosthesis to investigate the mechanical behaviour at the cement-bone interface. For this reason, a musculoskeletal model has been developed by modelling the forces of the muscles and after FEM analyses have been performed. Obtained results confirm the validity of the implemented model and can provide guidelines for surgeons regarding the implant configurations with the aim to reduce the aseptic loosening.

Keywords: CAD | FEM | Reverse engineering | Total elbow arthroplasty