Finite Element Analysis of Bone and Experimental Validation

This chapter describes the application of the finite element (FE) method to bone tissues. The aspects that differ the most between bone and other materials’ FE analysis are the type of elements used, constitutive models, and experimental validation. These aspects are looked at from a historical evolution stand point. Several types of elements can be used to simulate similar bone structures and within the same analysis many types of elements may be needed to realistically simulate an anatomical part. Special attention is made to constitutive models, including the use of density-elasticity relationships made possible through CT-scanned images. Other more complex models are also described that include viscoelasticity and anisotropy. The importance of experimental validation is discussed, describing several methods used by different authors in this challenging field. The use of cadaveric human bones is not always possible or desirable and other options are described, as the use of animal or artificial bones. Strain and strain rate measuring methods are also discussed, such as rosette strain gauges and optical devices.publishe

Safety of a decentralized scheme for Free-Flight ATMS using Mixed Integer Linear Programming

In this paper we consider policies for free-flight management of air traffic. We consider instantaneous and bounded heading angle deviation as conflict avoidance maneuvers. The corresponding model, resulting in a Mixed Integer Linear Programming (MILP) problem allow to solve both conflict detection and conflict resolution problems. The developed algorithm proved successful in a centralized implementation with a large number of cooperating aircraft. However, the application of such algorithm to a Free Flight environment, where cooperation can only be expected from neighboring aircraft, poses many challenges. We consider a model of the decentralized conflict resolution strategy that is based on a hybrid system, and sufficient conditions under which a 3-aircraft Free Flight MILP-based scheme guarantees safety of flight are provided

An improved method for the automatic mapping of computed tomography numbers onto finite element models.

The assignment of bone tissue material properties is a fundamental step in the generation of subject-specific finite element models from computed tomography data. Aim of the present work is to investigate the influence of the material mapping algorithm on the results predicted by the finite element analysis. Two models, a coarse and a refined one, of a human ileum, femur and tibia, were generated from CT data and used for the tests. In addition a convergence analysis was carried out for the femur model, using six refinement levels, to verify whether the inclusion of the material properties would significantly alter the convergence behaviour of the mesh. The results showed that the choice of the mapping algorithm influences the material distribution. However, this did not always propagate into the finite element results. The difference between the maximum Von Mises stress remained always lower than 10%, apart one case when it reached the 13%. However, the global behaviour of the meshes showed more marked differences between the two algorithms: in the finer meshes of the two long bones 20-30% of the bone volume showed differences in the predicted Von Mises stresses greater than 10%. The convergence behaviour of the model was not worsened by the introduction of inhomogeneous material properties. The software was made available in the public domain

Primary stability of an anatomical cementless hip stem: a statistical analysis

The primary stability that the surgeon can achieve during surgery is a determinant of the clinical success of cementless implants. Thus, estimating what level of primary stability can be obtained with a new design is an important aspect of pre-clinical evaluation. The primary stability of a cementless hip stem is not only affected by the implant design, but also by other factors such as the mechanical quality of the host bone, the presence of gaps around the bone\u2013implant interface, the body weight of the patient, and the size of the implant. Even the most extensive experimental study can only explore a small sub-set of all possible combinations found in vivo. To overcome this limitation, we propose a combination of experimental and numerical methods. The primary stability of a cementless anatomical stem is assessed in vitro. A finite element model is developed to accurately replicate the same experiment. The model is then parameterised over the various factors that affect the primary stability, and used in a Monte Carlo scheme to assess the primary stability over a simulated population. In this study, the method was used to investigate the mechanical stability of an anatomical cementless stem over more than 1000 simulated cases. Twenty cases were found macroscopically unstable, due to a combination of unfavourable conditions. The rest of the Monte Carlo sample showed on average a peak micromotion under stair climbing loading of 206\ub1159\u3bcm. The proposed method can be used to evaluate new designs in conditions more representative of the variability in clinical practic

On the biomechanical stability of cementless straight conical hip stems

The aim of the present study was to investigation in vitro the effect of deficient bone-implant contact on the primary stability of a straight conical stem. Various possible deficient contact patterns were derived from surgical simulations. The effect of stair climbing loads on the bone-implant micromotion was firstly investigated using a finite element model and then an in vitro test aimed at assessing primary stability. It was found that if the surface features are prevented from biting dense bone in a few small but critical regions, stem primary stability is completely lost. These results suggest that the surface features used in the axisymmetric stem under investigation can be too sensitive to deficient contact conditions, and thus should be augmented with additional antirotational fins. Preliminary tests showed that a stem with the addition of such fins presents good primary stability in all tested conditions

Sensor/actuator abstractions for symbolic embedded control design

www.nd.edu/∼ptabuada Abstract. In this paper we consider the problem of developing sensor/actuator abstractions for embedded control design. These abstractions take the form of inequalities relating sensor/actuator characteristics with the continuous dynamics ’ output. When satisfied, they allow to decouple control design from the choice of sensor/actuators, thus simplifying control design while ensuring implementability.