Rui B. Ruben

On the optimal shape of hip implants

The success of a total hip arthroplasty is strongly related to the initial stability of the femoral component and to the stress shielding effect. In fact, for cementless stems, initial stability is essential to promote bone ingrowth into the stem coating. An inefficient primary stability is also a cause of thigh pain. In addition, the bone adaptation after the surgery can lead to an excessive bone loss and, consequently, can compromise the success of the implant. These factors depend on prosthesis design, namely on material, interface conditions and shape. Although, surgeons use stems with very different geometries, new computational tools using structural optimization methods have been used to achieve a better design in order to improve initial stability and therefore, the implant durability. In this work, a multi-criteria shape optimization process is developed to study the relationship between implants performance and geometry. The multi-criteria objective function takes into account the initial stability of the femoral stem and the effect of stress shielding on bone adaptation after the surgery. Then, the optimized stems are tested using a concurrent model for bone remodeling and osseointegration to evaluate long-term performance. Additionally, the sensitivity to misalignments is analyzed, since femoral stems are often placed in varus or valgus position. Results show that the different criteria are contradictory resulting in different characteristics for the hip stem. However, the multi-criteria formulation leads to compromise solutions, with a combination of the geometric characteristics obtained for each criterion separately.

Shape Optimization of a Cementless Hip Stem for a Minimum of Interface Stress and Displacement

The primary stem stability is an essential factor for success of cementless hip stems. A correct choice of the stem geometry can improve the stem stability and, consequently, increase the life time of a hip implant. In this work, it is proposed a computational model for shape optimization of cementless hip stems. The optimization problem is formulated by the minimization of relative displacement and stress on bone/stem interface using a multi-criteria objective function. Also multiple loads are considered to incorporate several daily life activities. Design variables are parameters that characterize the geometry of selected cross sections, which are subject to geometric constraints to ensure a clinically admissible shape. The stem/bone set is considered a structure in equilibrium with contact conditions on interface. The contact formulation allows us to analyze different lengths of porous coating. The optimization problem is solved numerically by a steepest descent method. The interface stress and relative displacement are obtained solving the contact problem by the finite element method. Numerical examples are presented for a two-dimensional model of a hip stem, however, the formulation is general and can be applied to the three-dimensional case. The model gives indications about the relation between shape, porous coating and prosthesis stability.

Fully automatic segmentation of femurs with medullary canal definition in high and in low resolution CT scans

Femur segmentation can be an important tool in orthopedic surgical planning. However, in order to overcome the need of an experienced user with extensive knowledge on the techniques, segmentation should be fully automatic. In this paper a new fully automatic femur segmentation method for CT images is presented. This method is also able to define automatically the medullary canal and performs well even in low resolution CT scans. Fully automatic femoral segmentation was performed adapting a template mesh of the femoral volume to medical images. In order to achieve this, an adaptation of the active shape model (ASM) technique based on the statistical shape model (SSM) and local appearance model (LAM) of the femur with a novel initialization method was used, to drive the template mesh deformation in order to fit the in-image femoral shape in a time effective approach. With the proposed method a 98% convergence rate was achieved. For high resolution CT images group the average error is less than 1mm. For the low resolution image group the results are also accurate and the average error is less than 1.5mm. The proposed segmentation pipeline is accurate, robust and completely user free. The method is robust to patient orientation, image artifacts and poorly defined edges. The results excelled even in CT images with a significant slice thickness, i.e., above 5mm. Medullary canal segmentation increases the geometric information that can be used in orthopedic surgical planning or in finite element analysis.

Shape optimization of uncemented hip prostheses

A computational model to obtain optimized geometries for the femoral component of hip prosthesis is presented. Using structural optimization techniques, the objective is to determine the shape of uncemented stems that maximize initial stability and improve performance. To accomplish this, the optimization problem is formulated by the minimization of the contact stresses and relative displacement on bone–stem interface. Design variables are geometric parameters that characterize selected cross sections. These parameters are subject to a set of linear geometric constraints in order to obtain clinically admissible geometries. Furthermore, a multiple load formulation is used to incorporate different daily life activities. Optimization results are useful to design new stems or, if integrated in an appropriate computer-aided design (CAD) system, to design custom-made hip prostheses. In the later case, the model is able to include personalized information such as patients femur geometry and therefore personalized geometric constraints and optimization parameters.

Hip Prosthesis Design Using a Multi-Criteria Formulation

Just after a Total Hip Arthrosplasty, the fixation of the cementless stem mainly depends on mechanical factors. If a good initial mechanical stability is achieved, new bone growths into the stem porous coating, leading to a desirable long-term biologic fixation. On the other hand, an inefficient primary stability could be painful for the patient and promotes aseptic loosening, the main cause of cementless stem failure. In fact, “small” relative displacements between bone and stem as well as “small” contact stresses promote bone ingrowth, while larger displacement and stress values can destroy it. Thus, the primary stability can be improved controlling the displacement and stress level at the bone-implant interface. Such improvement can be performed actuating on stem geometry, friction coefficient between bone and stem, surgery technique and biologic response of bone to stem material. This interface phenomenon occurs simultaneously with bone adaptation process due to stress shield effect. The bone remodeling around the stem is also decisive for prosthesis success and it is itself related with initial conditions, such as primary stability and host-bone quality.