S.V.N. Jaecques

Individualised, micro CT-based finite element modelling as a tool for biomechanical analysis related to tissue engineering of bone

Load-bearing tissues, like bone, can be replaced by engineered tissues or tissue constructs. For the success of this treatment, a profound understanding is needed of the mechanical properties of both the native bone tissue and the construct. Also, the interaction between mechanical loading and bone regeneration and adaptation should be well understood. This paper demonstrates that microfocus computer tomography (microCT) based finite element modelling (FEM) can have an important contribution to the field of functional bone engineering as a biomechanical analysis tool to quantify the stress and strain state in native bone tissue and in tissue constructs. Its value is illustrated by two cases: (1) in vivo microCT-based FEM for the analysis of peri-implant bone adaptation and (2) design of biomechanically optimised bone scaffolds. The first case involves a combined animal experimental and numerical study, in which the peri-implant bone adaptive response is monitored by means of in vivo microCT scanning. In the second case microCT-based finite element models were created of native trabecular bone and bone scaffolds and a mechanical analysis of both structures was performed. Procedures to optimise the mechanical properties of bone scaffolds, in relation to those of native trabecular bone are discussed.

A fast convolution-based methodology to simulate 2-Dd/3-D cardiac ultrasound images

This paper describes a fast convolution-based methodology for simulating ultrasound images in a 2-D/3-D sector format as typically used in cardiac ultrasound. The conventional convolution model is based on the assumption of a space-invariant point spread function (PSF) and typically results in linear images. These characteristics are not representative for cardiac data sets. The spatial impulse response method (IRM) has excellent accuracy in the linear domain; however, calculation time can become an issue when scatterer numbers become significant and when 3-D volumetric data sets need to be computed. As a solution to these problems, the current manuscript proposes a new convolution-based methodology in which the data sets are produced by reducing the conventional 2-D/3-D convolution model to multiple 1-D convolutions (one for each image line). As an example, simulated 2-D/3-D phantom images are presented along with their gray scale histogram statistics. In addition, the computation time is recorded and contrasted to a commonly used implementation of IRM (Field II). It is shown that COLE can produce anatomically plausible images with local Rayleigh statistics but at improved calculation time (1200 times faster than the reference method).

A finite element analysis of the vibrational behaviour of the intra-operatively manufactured prosthesis–femur system

In total hip replacement (THR) a good initial stability of the prosthetic stem in the femur, which corresponds to a good overall initial contact, will help assure a good long-term result. During the insertion the implant stability increases and, as a consequence, the resonance frequencies increase, allowing the assessment of the implant fixation by vibration analysis. The influence of changing contact conditions on the resonance frequencies was however not yet quantitatively understood and therefore a finite element analysis (FEA) was set up. Modal analyses on the hip stem-femur system were performed in various contact situations. By modelling the contact changes by means of the contact tolerance options in the finite element software, contact could be varied over the entire hip stem surface or only in specific zones (proximal, central, distal) while keeping other system parameters constant. The results are in agreement with previous observations: contact increase causes positive resonance frequency shifts and the dynamic behaviour is most influenced by contact changes in the proximal zone. Although the finite element analysis did not establish a monotonous relationship between the vibrational mode number and the magnitude of the resonance frequency shift, in general the higher modes are more sensitive to the contact change.

Early periprosthetic bone remodelling around cemented and uncemented custom-made femoral components and their uncemented acetabular cups

IntroductionPeriprosthetic bone remodelling after total hip replacement may contribute to aseptic loosening of the prosthesis. The selection between cemented and uncemented fixation of the stem is mainly determined by patient’s age, general constitution and CT scan-estimated bone quality; intra-operative observation may ultimately influence the choice of the fixation method. The influence of cemented versus uncemented stem fixation on periprosthetic bone remodelling around the uncemented cup has, to our knowledge, never been studied until now.MethodsA total of 75 patients received intra-operatively manufactured stem prostheses and a standard hydroxy apatite-coated pinnacle cup. The pre-operative CT scans provides guidance for the bone quality and hence the type of stem fixation: cemented or uncemented. The influence of either type of stem fixation on periprosthetic bone remodelling around the cup and the stem was measured by bone mineral density at 6xa0weeks, and 3, 6 and 12xa0months after surgery.ResultsEarly changes in bone mineral density were noted. The type of stem fixation had an influence on the bone remodelling of the femur and also of the pelvis. The caudal part of the acetabulum was subject to a greater loss in BMD at 12xa0months in the group with cemented stem fixation. Changes at 12xa0months correlated with the changes measured at any time point.ConclusionsThe selection of the stem implant and its type of fixation in the femoral cavity (cemented or uncemented fixation) seems to have an impact on the bone mineral density of the acetabulum. Long-term clinical follow-up is required to draw conclusions regarding the influence on prosthesis survival.

The correlation between the SOS in trabecular bone and stiffness and density studied by finite-element analysis

For the clinical assessment of osteoporosis (i.e., a degenerative bone disease associated with increased fracture risk), ultrasound has been proposed as an alternative or supplement to the dual-energy X-ray absorptiometry (DEXA) technique. However, the interaction of ultrasound waves with (trabecular) bone remains relatively poorly understood. The present study aimed to improve this understanding by simulating ultrasound wave propagation in 15 trabecular bone samples from the human lumbar spine, using microcomputed tomography-based finite-element modeling. The model included only the solid bone, without the bone marrow. Two structural parameters were calculated: the bone volume fraction (BV/TV) and the structural (apparent) elastic modulus (Es), and the ultrasound propagation parameter speed of sound (SOS). Relations between BV/TV and Es were similar to published experimental relations. At 1 MHz, correlations between SOS and the structural parameters BV/TV and Es were rather weak, but the results can be explained from the specific features of the trabecular structure and the intrinsic material elastic modulus Ei. In particular, the systematic differences between the three main directions provide information on the trabecular structure. In addition, at 1 MHz the correlation found between the simulated SOS values and those calculated from the simple bar equation was poor when the three directions are considered separately. Hence, under these conditions, the homogenization approach - including the bar equation - is not valid. However, at lower frequencies (50-300 kHz) this correlation significantly improved. It is concluded that detailed analysis of ultrasound wave propagation through the solid structure in various directions and with various frequencies, can yield much information on the structural and mechanical properties of trabecular bone.

Short Communication: Three-dimensional finite element models based on in vivo microfocus computed tomography: Elimination of metal artefacts in a small laboratory animal model by registration with artefact-free reference images

A method is presented to generate metal-artefact-free finite element (FE) models based on in vivo micro-CT images of bone-implant structures in the case of the Guinea Pigs tibiae. A bone-implant composite FE model was constructed based on both pre- (just before implant insertion) and post-operative (4days after implant insertion) sets of micro-CT scans. Definition of bone geometry and attribution of material properties to the volumetric elements was based on pre-operative images while post-operative scans were mainly used for registration. Standard Triangulation Language (STL) representations of implant and bone were generated after segmentation of CT images in MIMICS^(R) (Materialise). Registration was performed in 3-matic^(R) (Materialise). By taking two sets of scans at a 4days interval, undisturbed bone geometry before implant insertion and implant position after insertion were recorded. After adequate validation, FE models constructed with this method can be used to study bone adaptive response to controlled mechanical loading.

Feasibility of monitoring bone remodelling around loaded percutaneous tibial implants in guinea pigs by in vivo microfocus computed tomography

Abstract Guinea pigs were selected as an animal model for studying bone remodelling around loaded percutaneous implants. The feasibility of monitoring bone remodelling in vivo by microfocus computed tomography was assessed experimentally. A microfocus computed tomography (μCT) protocol was developed to minimise the side effects (i.e. radionecrosis) while maintaining sufficient image quality to allow segmentation of the μCT slices for subsequent processing into finite element models.

Bioactive glass coating for hard and soft tissue bonding on Ti6Al4V and silicone rubber using electron beam ablation

The adhesion of soft tissue to any commonly used implant material is poor. Therefore percutaneous implants are an infectious passage of bacteria into the body. To address this problem, a method that allows the growth of living soft tissue on medical implants ranging from metals to temperature sensitive polymers is developed. The implant surface is to be coated with bioactive glass (BAG) by applying electron beam ablation (ELBA). A proof of principle of the ELBA potential is given by coating Ti6Al4V and silicone rubber (poly-dimethylsiloxane, PDMS) substrates with thin BAG layers (~10 -m thickness). The BAG coatings are amorphous, sufficiently adherent and showed the desired bioactive in vitro dissolution behaviour in Hanksx92 solution. Finally, in vitro fibroblast cell adhesion experiments were performed. Compared to an uncoated material both the coated PDMS and Ti6Al4V samples showed good cell adhesion and proliferation. These results support the hypothesis that a BAG coating deposited by ELBA can enhance the tissue bonding potential of many types of implants, including those made of temperature sensitive materials. Introduction In contrast to the bone bonding potential of existing implants, the adhesion of soft tissue to any commonly used implant material is poor. Therefore an important problem of percutaneous implants is the infectious passage of bacteria into the body at the interface between tissue and implant [1]. Subcutaneous implants, e.g. access ports for injections used in chemotherapy, suffer under low fixation and should be anchored with the surrounding soft tissue. The main objective of this research is to develop a method, which allows the growth of living soft tissue on medical implants ranging from metals to temperature sensitive polymers. The surface of the implant is to be coated with bioactive glass (BAG), which has proven bonding potential to hard and soft tissue [2]. A new coating procedure based on electron beam ablation (ELBA) will be used [3]. A first goal was to deliver a proof of principle of the ELBA technique by coating Ti6Al4V and silicone rubber (poly-dimethylsiloxane, PDMS) substrates with micrometer thin BAG layers. Materials and Methods The ELBA coating technique [3] uses a pulsed high power electron beam that hits a BAG target and evaporates a thin layer in an explosive manner, which is called ablation. Vapour and fine molten particles condense at the surface of the substrate (Ti6Al4V or PDMS) and build an adherent and Key Engineering Materials Online: 2003-12-15 ISSN: 1662-9795, Vols. 254-256, pp 427-430 doi:10.4028/www.scientific.net/KEM.254-256.427

Evaluation of an elastomer for biomedical use in load-bearing applications

The fatigue behaviour of a medical-grade thermoplastic polyolefin (TPO) was investigated with the objective of using TPOs for endoprosthetical aims in dental and orthopaedic applications. For this purpose, a TPO was subjected to the combined action of shear and compression fatigue while in contact with Hanks solution. Cyclic loading between 150 and 1500 kPa shear stress and a number of cycles up to 9×106 was applied. The potential fatigue damage was assessed by in-fatigue-recording of the mechanical hysteresis curves and, after fatigue, by swelling tests in acetic acid solutions, dynamic mechanical thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The combined and critical application of these techniques allowed us to conclude that this material exhibits a very satisfactory fatigue behaviour under the loads investigated, making this material potentially useful for endoprostheses. Moreover, biomedical prescreening tests (according to USP Class VI) were all negative, stimulating continued research on this type of material.

Microstructural quality of vertebral trabecular bone can be assessed from ultrasonic wave propagation

Ultrasound is extensively studied as an alternative diagnostic “screening” tool for osteoporosis. However, only a few aspects about the interaction of ultrasonic waves with bone tissue are understood, primarily based on statistical correlations. Therefore, our long-term aim is to obtain a more mechanistic interpretation of ultrasonic wave propagation through bone microstructure. For this study our aim was to quantify the interplay between ultrasound frequency and bone microstructure in determining wave propagation. Three-dimensional (3D) numerical simulations were performed on 15 human trabecular bone samples from the lumbar spine (4×4×4 mm3). The 3D representation of trabecular bone architecture was obtained using microcomputed tomography at a resolution of 14 micrometer. The simulations were performed using commercial finite element (FE) software (MSC/Nastran), validated for this application. Three frequencies (1 MHz, 300 kHz and 50 kHz) were analyzed, and the velocity of wave propagation (Speed of Sound, SOS) was calculated. SOS showed no correlation with bone volume fraction (BV/TV) and a better correlation with the apparent elastic modulus. The variables were calculated in the three main directions of the bone sample. Our simulations at 50 kHz and 300 kHz frequency showed an excellent agreement between FE-calculated SOS and the SOS estimated with the bar velocity equation (R2 = 0.93 and R2 = 0.85 respectively). For 1 MHz, this correlation was significantly lower (R2 = 0.66), but could be substantially improved by including several morphometric parameters in a multiple regression (R2 = 0.79).