A numerical model of the ankle to analyze joint prostheses

The ankle is one of the most injured joint in the body. However, it remains a new field of study in biomedical orthopedics. With the increased life span, more people suffer from arthosis and joint problems. That is why the improvements done in arthroplasty become important for the future. The goal of this project is to develop a numerical model for the ankle. This modelling will serve for future analysis, improvements for implants and developments of new prostheses. This model is build in two parts. First, a simple cylinder is considered instead of the talus for the finite elements analysis and then,another one with numerical representation of the tibia. This 3D representation is obtained by performing a segmentation on computer tomography scans. Both modellings will be tested in simulations with particular loading cases as the maximal force during gait or physiological load, as body weight. The outcome of this study leads to these conclusions. With the assumptions made in both modellings, the bone with the implant resists to the applied physiological loads. However, The model with the tibia needs more improvements in order to be more realistic, as inhomogeneous properties for bone tissue instead of the homogeneous ones or quadratic meshing definition. This model needs to be increased by adding the other components of ankle after the arthroplasty; that means the talus, its implant and the polyethylene part. The relations between the parts and the materials for the entire model of the ankle can be constructed based on the outcome of this project.

Relationship between interfacial micromotion and bone density around cementless hip implant

Valérie Parvex

The hip is one of the most injured joint in the body with ageing. Osteoarthritis and degenerative diseases often lead to the necessity of a total hip arthroplasty in order to reduce pain and increase mobility. The advantage of cementless implants resides in the natural stem anchorage by osseointegration instead of the cement use. Nevertheless, the related stability is highly dependent on the micromotion between the implant and the bone. As more and more arthroplasties are performed each year, it is important to control the outcome and to reduce the risks linked with implant loosening due to excessive micromotion. The goal of this study is to improve a micromotion measurement method based on compression and torsion tests on cadaveric femurs. Moreover, in order to help the surgeon to position the implant to limit high micromotion, density around the implant is studied in order to compare with micromovements distribution. Markers made of stainless steel are put on the internal bone surface. The compression and torsion tests are performed under micro-computed tomography environment. The locations of markers are visible on scans performed before, during and after the tests. Post-processing based on micro-CT scans allows the extraction of micromotion, subsidence and gap values between loading and rest states. Contrary to other studies, which use LVDT to measure micromotion on the outer bone surface, this method allows local measurement. Thanks to micromotion values projection on the implant, it is possible to see the local distribution around the stem. The amplitude of measured micromotion remains under the limit value of 150 [µm] that is supposed to lead to the formation of brous tissue, except when bone quality is damaged. The improved method is accurate and fast. Nevertheless, it is highly dependent on the image acquisition quality. However, no general statistical correlation has been found between density and micromotion. Nonetheless, based on the rendering of the postoperative planning software HipPlan (Symbios, Switzerland), it seems that few support zones present similarities between low micromotion and high bone density.

2013

A numerical model to reproduce an experimental design to detect strain in cement

Christina Maria Bauer

An experimental set-up consisting of a loading device inserted in a μCT scanner was numerically simulated to predict the strain at the bone-implant interface of the scapula after applying various loading conditions. The aim of the project was to determine which applied force is required to induce strain measurable by the μCT scanner. First, a 3D model of the bone was obtained by scanning a cadaveric scapula using CT scanner. Second, cortical and trabecular bone were segmented according to Hounsfield Units using Amira software. Extracted 3D scapula was assembled with a glenoid implant and cement and was cut to fit into the loading device. Final, a finite element model was created using Abaqus software. A mesh convergence study was undertaken to set the local element size to 2 mm around the peri-implant area. Strain – volume distribution in the cement was investigated. For an axial loading of 2.4 kN, 60% of the volume achieved a maximum principal strain value lying between 0.15% - 0.29% strain. Previous studies on DVC approach showed strain errors of 6 to 8 με. Such a low error percentage would be suitable for this study and hence the strain level should be detectable.

2016

Cement stress predictions after anatomic total shoulder arthroplasty are correlated with preoperative glenoid bone quality

Alain Farron, Raphaël Rodolphe Obrist, Alexandre Terrier

Hypothesis: We hypothesized that biomechanical parameters typically associated with glenoid implant failure after anatomic total shoulder arthroplasty (aTSA) would be correlated with preoperative glenoid bone quality. Methods: We developed an objective automated method to quantify preoperative glenoid bone quality in different volumes of interest (VOIs): cortical bone, subchondral cortical plate, subchondral bone after reaming, subchondral trabecular bone, and successive layers of trabecular bone. Average computed tomography (CT) numbers (in Hounsfield units [HU]) were measured in each VOI from preoperative CT scans. In parallel, we built patient-specific finite element models of simulated aTSAs to predict cement stress, bone-cement interfacial stress, and bone strain around the glenoid implant. CT measurements and finite element predictions were obtained for 20 patients undergoing aTSA for primary glenohumeral osteoarthritis. We tested all linear correlations between preoperative patient characteristics (age, sex, height, weight, glenoid bone quality) and biomechanical predictions (cement stress, bone-cement interfacial stress, bone strain). Results: Average CT numbers gradually decreased from cortical (717 HU) to subchondral and trabecular (362 HU) bone. Peak cement stress (4-10 MPa) was located within the keel hole, above the keel, or behind the glenoid implant backside. Cement stress, bone-cement interfacial stress, and bone strain were strongly negatively correlated with preoperative glenoid bone quality, particularly in VOIs behind the implant backside (subchondral trabecular bone) but also in deeper trabecular VOIs. Conclusion: Our numerical study suggests that preoperative glenoid bone quality is an important parameter to consider in aTSA, which may be associated with aseptic loosening of the glenoid implant. These initial results should now be confronted with clinical and radiologic outcomes. (C) 2017 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.