Ebook: Medicine Meets Engineering

Biomedical Engineering is defined as the science that integrates medical and engineering sciences to improve diagnosis and treatment of patients. Only by this integration can progress be achieved. Both medical and engineering sciences comprise a huge diversity in topics, so understandably, in combining these two areas of science, Biomedical Engineering is even more huge. If research between several medical disciplines is called multidisciplinary it is rational to call research in Biomedical Engineering megadisciplinary. Thanks to this megadisciplinary approach many breakthroughs can be achieved. More and more research groups are realising this and starting new research projects, resulting in a rapid increase in knowledge, which can only benefit the main aim of Biomedical Engineering, improving the diagnosis and treatment of patients when it is spread and applied.

Conferences are a valuable means in distributing knowledge. Since Biomedical Engineering is a multidisciplinary science it is important to reach both medical and engineering specialists. This requirement is very difficult to realise as both research groups often focus only on their own research field, which hinders the essential integration of knowledge.

The 2nd Regensburg Applied Biomechanics conference is special in that it realised both the distribution of new knowledge and the essential integration of medical and engineering specialists. The first step for that was to have not one, but two, congress chairmen, one medical and one technical: Prof. Nehrlich and Prof. Hammer. They made a unique program around the central topic ‘Applied Biomechanics’. This topic was well chosen, because it was challenging for and could be understood by both groups, which is not obvious, since both groups have a different culture and language. It also attracted many young scientists and since they are the future, this was very good to note.

The conference dealt with the latest results in applied biomechanics, ranging from fundamental bone strength properties via bone remodelling phenomena to new implants that replace lost human functions. Also, new research areas like robot surgery and tissue engineering were discussed.

This conference is an excellent example of the activities of ESEM, the European Society for Engineering and Medicine that aims at stimulating and integrating research in Biomedical Engineering. One of the ways ESEM is stimulating research is by awarding excellent presentations, and during this conference an ESEM scientific award was issued.

The only drawback in organising a successful conference is that everybody expects that next year it will be organised again…

Prof. Dr. Bart Verkerke, ESEM president

Proximal femur fractures are of main concern for elderly and especially osteoporotic patients. Despite advanced implant modifications and surgical techniques, serious mechanical complication rates between 4–18% are found in conventional osteosyntheses of proximal femur fractures. Clinical complications such as the rotation of the femoral head and the cut-out phenomenon of the fracture fixation bolt are often diagnosed during post-operative treatments. Therefore, efforts in new intramedulary techniques focus on the load bearing characteristics of the implant by developing new geometries to improve the implant-tissue interface. The objective of this investigation was to analyse the osteosynthesis/femur head interaction of two commonly used osteosyntheses, one with a helical blade and the other one with a screw design under different loading conditions.

For the comparative investigation the helical blade of the Proximal Femur Nail Antirotation was investigated versus the screw system of the Dynamic Hip Screw. After implantation in a femoral head the loads for rotational overwinding of the implants were analysed. Pull-out forces with suppressed rotation were investigated with analysis of the influence of the previous overwinding. All investigations were performed on human femoral heads taken of patients with average age of 70.3±11.8. The bone mineral densities of the human specimens were detected by QCT-scans (average BMD: 338.9±61.3) Prior to cadaveric testing the experimental set-up was validated and special influences were analysed by the use of synthetic foam blocks (Sawbone). The helical blade showed a significant higher torque for the rotation of the femoral head compared to the screw system. The pull-out forces of the blade were substantially lower than of the comparative screw.

Taken together the helical blade showed a higher potential of rotational stability, but after a rotation the lower pull-out forces demonstrate a higher degree of damage to the femoral head.

Previous investigations have shown that collagen shows excellent biological performance as a scaffold for tissue engineering. As a primary constituent of bone and cartilage, it demonstrates excellent cell adhesion and proliferation. However, in bone tissue engineering, it has insufficient mechanical properties for implantation in a load-bearing defect. The objective of this preliminary study was to investigate the possibility of developing a collagen/calcium-phosphate composite scaffold which would combine the biological performance and the high porosity of a collagen scaffold with the high mechanical stiffness of a calcium-phosphate scaffold.

Collagen scaffolds were produced by a lyophilisation process from a collagen slurry. The scaffolds were soaked for different exposure times in solutions of 0.1 M, 0.5 M or 1.0 M NaNH₄HPO₄ followed by 0.1 M, 0.5 M or 1.0 M CaCl₂. Mechanical tests of each scaffold were performed on a uniaxial testing system. Young's moduli were determined from stress-strain curves. The pore structure and porosity of the scaffolds were investigated using micro-computed tomography. A pure collagen scaffold served as a control.

All scaffolds showed a significantly increased compressive stiffness relative to the pure collagen scaffolds. The exposure to the 0.5 M solutions showed significantly superior results compared to the other groups. Analysis of the pore structure indicated a decrease in the overall porosity of the composite scaffolds relative to the controls. Regarding mechanical stiffness and porosity, scaffolds after 1 hour exposure to the 0.5 M solutions showed the best properties for bone tissue engineering. Further work will involve producing a scaffold with a more homogeneous calcium phosphate distribution.

The overall thrust of this work is concerned with the performance of the adhesives used to simulate cementation of gold crowns onto nickel chromium dies under static and dynamic compression. A measurement system, based on the mounting of strain gauges on the outer surface of the crowns, has been developed allowing an indirect semi-quantitative estimate of the state of adhesion.

This paper reports an investigation of the effect of increased total occlusal convergence (TOC) of the nickel chromium dies from 12° to 24° with different degrees of cementation, a) un-cemented, b) partially cemented and c) fully cemented.

Four nickel chromium dies (12°TOC) and five nickel chromium dies (24°TOC) for each convergence were fabricated using the lost wax technique. The axial height of all dies was 6mm. Two miniature gauges were installed on opposing axial surfaces of each gold crown 1 mm above the crown margin. Axial loading and unloading of the crowns was repeated five times for each crown and the values for strain recorded.

The results showed an increase in strain at the axial surfaces with increasing TOC, providing useful design information for the durability of restorative crowns. These findings, along with the findings of earlier work are consistent with a simple model of load transfer between the crown and the die.

Total hip arthroplasties (THA) can be performed with cemented and uncemented femoral components. Aseptic loosening of the joint replacement still illustrates a problem for both implantation techniques. This loosening can be caused, among other factors, by resorption of the bone surrounding the implant due to stress shielding. In order to analyse the absolute influence of the implantation technique on the bone degeneration in the periprosthetic femur, the strain adaptive bone remodelling after THA was investigated in a three-dimensional finite element (FE) simulation of a femur provided with a cemented and uncemented BICONTACT (Aesculap, Tuttlingen, Germany) femoral component. For this, a bone density evolution theory was implemented in the FE code MSC.MARC^®. In these static FE simulations, the muscle and hip resultant forces represent the maximum loading situation in the normal walking cycle. To describe the mechanical properties of the bone, an isotropic material law dependent upon density was used. The situation directly after implantation without any bone ingrowth was simulated. The cemented femoral component was bonded to the bone by a homogenous cement mantle. The numerical results show that proximally, the bone resorption areas surrounding the BICONTACT stem are heavily dependent upon anchoring technique. Furthermore, no significant bone remodelling is calculated in the distal periprosthetic femur in both models.

Soft tissues are pseudoelastic anisotropic materials; various formulas for their strain energy density have been proposed for modelling of their constitutive behaviour. However, the individual variance of elastic parameters is often more pronounced than their anisotropy, so that their constitutive relations can be modelled as either isotropic or orthotropic. Any hyperelastic model requires more mechanical tests to be input for an identification of its parameters than mere uniaxial tension tests; especially biaxial tension tests are very important also for isotropic hyperelastic materials. A design of a testing rig produced in cooperation of our institute with some local companies is presented. It enables us to carry out not only equibiaxial tension tests, but also some other biaxial tensile tests, because displacements in both mutually perpendicular directions can be controlled independently. The proposal of various types of biaxial tests is presented in the paper, with examples of their realization with porcine aortic wall tissue. The contribution focuses on ways of evaluation of the results and on identification of parameters of various constitutive models. The use of more mechanical tests in identification of constitutive parameters can improve the predictive capability of the models substantially.

A brief overview is given in this article on the main design philosophies and the resulting description concepts used for components which undergo monotonic and cyclic loading. Emphasis is put on a mechanistic approach avoiding a plain reproduction of empirical laws. After a short consideration of fracture as a result of monotonic loading using fracture mechanics basics, the phenomena taking place as a consequence of cyclic plasticity are introduced. The development of fatigue damage is treated by introducing the physical processes which (i) are responsible for microstructural changes, (ii) lead to crack initiation and (iii) determine crack propagation. From the current research topics within the area of metal fatigue, two aspects are dealt with in more detail because of their relevance to biomechanics. The first one is the growth behaviour of microstructural short cracks, which controls cyclic life of smooth parts at low stress amplitudes. The second issue addresses the question of the existence of a true fatigue limit and is of particular interest for components which must sustain a very high number of loading cycles (very high cycle fatigue).

The fatigue behaviour of materials is of particular interest for the failure prediction of materials and structures exposed to cyclic loading. For trabecular bone structures only a few sets of lifetime data have been reported in the literature and structural measures are commonly not considered. The influence of load contributions not aligned with the main physiological axis remains unclear. Furthermore age effects on the fatigue behaviour are not well described. In the present study, different groups of human vertebral cancellous bone were exposed to cyclic compression. The inital modulus and therefore lifetimes were found to be highly dependent on age. The decrease in both with increasing age was much more pronounced in specimens which were not aligned with the main physiological axis. This implies that old bone is much more sensitive to (cyclic) failure loads in general but particularly to loads which are not coincident with the physiological main axis.

A primary cause for revision operations of joint replacements is the implant loosening, due to immune reactions resulting from the agglomeration of polyethylene wear debris. Motivated by the successful application of bioceramic materials in hip joint prostheses, a trend towards the development of hard implant materials has occurred. Nonetheless in the area of total knee arthroplasty (TKA), modern efforts have still utilized polyethylene as the tibial-inlay joint component. The use of bioceramic hard-hard-pairings for total knee arthroplasty has been prevented by the complex kinematics and geometries required. Ceramics cannot cope with non-uniform loads, which suggests the need for new designs appropriate to the material. Furthermore, biomechanical requirements should be considered. A rolling-gliding wear simulator, which reproduces the movements and stresses of the knee joint on specimens of simplified geometry, has therefore been developed. High-precision machining processes for free formed bioceramic surfaces, with suitable grinding and polishing tools which adjust to constantly changing contact conditions, are essential. The goal is to put automated finishing in one clamping with five simultaneous controlled axes into practice. The developed manufacturing technologies will allow the advantageous bioceramic materials to be applied and accepted for more complex joint replacements such as knee prostheses.

A description of the hearing process is given using three-dimensional mechanical models. By means of simulation, normal, pathological and reconstructed situations can be investigated. The development of new concepts and prototypes as well as the optimization and the way of insertion of passive and active implants is facilitated by carrying out virtual tests. Mechanical models of spatial structures of the middle ear and its adjacent regions are established by applying multibody systems and finite element modeling approach. In particular, the nonlinear behavior of the elements is taken into account. For the determination of parameters such as coupling parameters in reconstructed ears, measurements using Laser Doppler Vibrometry (LDV) were carried out. The governing differential equations of motion allow the investigation of transient and steady state behavior by time integration and frequency domain methods. Optimization methods can be applied for determination of design parameters such as coupling stiffness and damping, the characteristics of actuator, the position of attachment and direction of actuation. Mechanical models enable non-invasive interpretation of dynamical behavior based on measurements such as LDV from umbo or multifrequency tympanometry. It is shown: The transfer behavior is depending on static pressures in the ear canal, tympanic cavity or cochlea. For reconstructed ears, the coupling conditions are governing the sound transfer substantially. Due to restricted coupling forces, the excitation of inner ear is limited and the sound transfer is distorted. Other sources of distortion are nonlinear coupling mechanisms. In reconstructions with active implants, the actuator excites the microphone whereby feedback effects may occur.

A central aim of current research is to determine the molecular mechanisms of articular cartilage repair. One major issue of articular cartilage repair is the achievable mechanical strength which has been correlated with the collagen metabolism, deposition and collagen cross-linking [1–3]. Current in vitro techniques, leading to cartilage integration used a shear test to failure [4–6]. Another well established in vitro method to investigate articular cartilage integration is the insert-ring push out model which is mainly utilized investigating the integration of tissue engineered cartilage to native cartilage [7–11]. Finite element modeling illustrates at least for the shear test to failure that the contact area is not homogeneously loaded [12]. For the mechanical analysis of articular cartilage integration in regard to its inhomogeneous integration a higher mechanical resolution method is needed.

Furthermore the shear test to failure as well as the ring-insert model lacks a comparison to in situ trauma situation, where ruptured or fractured articular cartilage surfaces are opposed after surgical reduction. Considering all these a T-peel test has been introduced in literature [13] but never been experimentally performed. This project deals with the establishment of a T-peel test as a topographical sensitive tool in mechanical analysis of T-peel data and its potential to investigate articular cartilage in vitro integration in comparison to articular cartilage rupture strength.

We report an in-vitro pilot study to assess the ability of a new impact test machine to evaluate bond strength of orthodontic brackets to tooth enamel. A total of 37 extracted premolar teeth were bonded with APC Plus MBT Victory orthodontic brackets. Bond strength was tested using a new pendulum-based instrumented impact test machine. The maximum stress, the impact energy and interaction time required to debond the brackets were recorded. Of the total tested, 9 samples were successfully debonded with no obvious damage to the tooth surface although 28 samples fractured through the enamel and dentine. There was a statistically significant difference between the maximum stress required to debond the bracket and that required to fracture the tooth, a higher stress being required to debond the bracket. Significantly less stress was required to fracture older teeth. The high incidence of tooth fracture suggests a need to modify the impact test protocol. The lack of a simulated periodontal ligament, which is present clinically and acts as a shock absorber, may have contributed to the high failure rate, although the striking position of the pendulum also needs to be considered.

Heightened intraocular pressure (IOP) is not always indicative of glaucoma, but is an important risk factor for the progression of certain types of eye damage not obviously felt by the sufferer. A number of measurement systems have been devised in the past to measure the IOP by applying force or pressure to the cornea, but past studies have shown that the cornea thickness and its curvature have a significant effect on measurements which may ultimately lead to clinical misdiagnosis. A cyclic strain controlled dynamic probing measurement system has been developed using an indenter of diameter 3.06 mm operating at actuation frequencies of between 0.1 Hz and 4 Hz and displacements up to 1 mm. The cyclic strain is actuated by a linear stage with a load cell and indenter coupled in series. The load cell records the resultant cyclic force where the dynamic modulus is expressed as amplitude ratio and phase lag. The mechanical eye model consists of a silicone membrane that can be varied in thickness and it is distended hydraulically to simulate a range of IOP. A pressure sensor measures the dynamic IOP within the system which will be compared against the dynamic modulus. The relationship between the mechanical properties and the physical properties of the membrane will be established in order to develop a probe which can be used clinically taking into account the effects of corneal stiffness and hydraulic behaviour of the eye. The preliminary study reported here a significant increase in amplitude ratio and mean ratio with increasing the frequency similar to the behaviour found in biological materials and gelatin.

This paper concerns the operation of the actuator for a prototype micro-engineered mechanical palpation device for deployment via a cystoscope to measure the dynamic mechanical properties of the prostate gland in vivo. The subassembly consists of a 400×200 μm silicon (Si) piston manufactured using deep reactive ion etching (DRIE) housed within an anodically bonded glass-Si-glass sandwiched housing. The micro-channel on the Si layer was formed by powder blasting and contains the micro-piston with one end pointing to the side of the housing and the other facing a via hole leading to a capillary tube. The opening on the side of the housing was sealed by a 5 μm thick silicone membrane which acts to retain the micro-piston and act as a return spring. A 320 μm diameter capillary forms the connection between the micro-channel and a micro-syringe which is operated by a programmable syringe pump to produce a reciprocating action. A pressure sensor is connected along the capillary tube to measure the dynamic pressure within the system. The micro-piston has already been used, separately actuated to measure the dynamic mechanical properties of known viscoelastic materials and prostate tissue. The purpose of the present work is to assess the functionality of the actuator assembly.

Improving the process of designing modern tools and devices for rehabilitation of both muscular and neural disabilities requires such intelligent systems that together with organizing intensive therapy could have an effective role in returning some of the patient's abilities and encourage them to complete the course of the training in order to get back to normal daily life. In this paper, first properties and laws governing the behavior of the electro-rheological fluid (ERF) as a smart material are briefly described. Then the principles of designing a two-degree-of-freedom intelligent damping system based on an electro-rheological fluid for the application in rehabilitation of human hands are explained. This mechanism provides the capability to create a virtual environment in which the training can be intelligently manipulated on the basis of body's feedback system. Also it can encourage the patient to continue with the therapy. Modeling and simulation of the electro-rheological (ER) force element is presented and results are compared with the available experimental data obtained, by other researchers, from a prototype system with relatively similar geometry but for other applications. Very good agreement is being noted between the theoretical model and the experimental data for different test configurations.

To validate the hypothesis that healing of fractures can be accelerated by oral administered L-arginine a guinea-pig model was chosen. A diaphyseal defect fracture was established in the right femur of each of the 32 small animals and stabilized. According to randomization groups the oral administration was realized (2 or 4 weeks medication / solvent). The following biomechanical variables were measured after 4 weeeks in 32 right femora and the corresponding uninjured left femora. The measurement for the healed femur was individually compared with that of the uninjured femur in each animal; bending, force (necessary for refracture) and energy (necessary for refracture). To apply the bending moment in a measurable and reproducible way each end of the femur was secured using a special device. For each femur a strain/momentum graph of the measurements and the essential parameters were drawn (stiffness, end of the linear range, and failure-point). The bending moment was always applied with the same loading rate.

The following three variables were used for the biomechanical evaluation; bending stiffness, force until failure and energy necessary for refracture. The bending stiffness reached 73% by the control group and 88% by the 4-week treatment group. The force necessary for refracture was 52% in the control compared with 65% in the 4-week treatment group. The energy necessary for refracture was 36% in the control compared with 73% in the group treated for 4 weeks. The 2 week treatment group showed no statistical significant differences to the control, but the femora from the 4 week treatment group required statistically significant higher energy for refracture than the femora from the control.

This study investigates the effect of microdamage on bone quality in osteoporosis using an ovariectomised (OVX) sheep model of osteoporosis. Thirty-four sheep were divided into an OVX group (n=16) and a control group (n=18). Fluorochromes were administered intravenously at 3 monthly intervals after surgery to label bone turnover. After sacrifice, beams were removed from the metatarsal and tested in three-point bending. Following failure, microcracks were identified and quantified in terms of region, location and interaction with osteons.

Number of cycles to failure (Nf) was lower in the OVX group relative to controls by approximately 7%. Crack density (CrDn) was higher in the OVX group compared to controls. CrDn was 2.5 and 3.5 times greater in the compressive region compared to tensile in control and OVX bone respectively. Combined results from both groups showed that 91% of cracks remained in interstitial bone, approximately 8% of cracks penetrated unlabelled osteons and less than 1% penetrated into labelled osteons. All cases of labelled osteon penetration occurred in controls. Crack surface density (CrSDn), was 25% higher in the control group compared to OVX.

It is known that crack behaviour on meeting microstructural features such as osteons will depend on crack length. We have shown that osteon age also affects crack propagation. Long cracks penetrated unlabelled osteons but not labelled ones. Some cracks in the control group did penetrate labelled osteons. This may be due the fact that control bone is more highly mineralized. CrSDn was increased by 25% in the control group compared to OVX. Further study of these fracture mechanisms will help determine the effect of microdamage on bone quality and how this contributes to bone fragility.

Biomechanical properties of soft tissue are important not only during computer simulation for medical training but also for systems where tissue deformation must be estimated in real-time, for example, Robot Assisted Surgery. The purpose of this paper is to describe some biomechanical tests consisting in the measurement of contact forces and deformations in tissue phantoms and porcine soft tissues (liver, brain, stomach and intestine). During the measurements two different procedures were applied. First, we have used a 5DOF micromanipulator instrumented with a spherical probe and a 6-axis force/torque ATI sensor. In the second procedure instead of the micromanipulator a Stäubli RX60 robot was used to apply the force over the samples. During this last test a high noise-signal relationship was detected and in order to improve the accuracy of the experiments some results were obtained using a Stäubli TX40 robot. Major accuracy in research in the field of soft tissue could be reached using standard procedures. Robotic systems allow precise movements to carry on biomechanical tests, and also permit a wide range of tasks to be implemented.

The overall aim of this work is to determine the quality of biological tissues based on the relationship between the dynamic mechanical properties and their histology. Two sets of rigs have been developed for dynamic mechanical measurement, one for micro-scale testing and the other for macro-scale testing. Preliminary results using the macro-scale measurement system only are reported here. This system uses strain-controlled cyclic probing actuated by a linear stepper motor operating at actuation frequencies between 0.5Hz and 20Hz. A 1mm diameter indenter probes the specimen up to a displacement of 0.2mm and a load cell measures the resultant cyclic force. A series of tissue mimics were prepared using various formulations of gelatin and safflower oil and preliminary tests carried out to determine a suitable range of experimental variables and to establish the repeatability of the tests. The dynamic mechanical properties are expressed as amplitude ratio, phase difference and mean ratios of stress and strain, and the behaviour of these measurands with actuation frequency, mean strain and strain amplitude was observed. Results consistent with the literature were found which form a foundation for measurements on collagen-lipid biological tissues.

The anatomical structure of biological tissues and their mechanical function are closely related. Forces have a decisive influence on growth and remodeling of tissues; furthermore, intra- and extravascular transport processes are mostly controlled mechanically and the metabolism of many cells is influenced by flow-induced shear stresses. In order to facilitate a mechanical analysis of biological systems, the anatomical tissue structure has to be determined with the aid of 3D imaging methods. In particular, the anisotropic fibrous architecture of the organs involved along with appropriate constitutive relations have to be considered. Examples of structure-(mechanical) function relationships are discussed in an exemplary fashion for bone, the heart and the uterus. The behavior of biological structures under unphysiological loading situations, such as they may occur in accidents, is addressed.

The contemporary virtual engineering environment enables a wide range of optimization procedures. The geometrical shape optimization is one of the most important tasks in this area. In the paper the trabecular bone surface remodeling process is formulated in terms of structural optimization and on the basis of this formulation the optimization algorithm useful in mechanical design is proposed. The developed system, based on the functional adaptation phenomenon is able to mimic trabeculae topology evolution, and enables investigations concerning different scenarios of bone remodeling. Some computation results of trabecular bone functional adaptation as well as mechanical design optimization, using the developed system are presented.