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Mechanics for Medical Device Development 2005
The meeting was organised by the Stress and Vibration Group and the Polymer Physics Group of the Institute of Physics. The conference was attended by around 25 delegates representing mainly academia but with some input from industry.
The conference opened with an invited lecture from Dr Jun Je Wu of the University of Durham on the Mechanical Integrity of Ultra-High Weight Polyethlene for Joint Prosthesis. Jun Je introduced the types of ultra-high weight polyethylene for knee joint prostheses used in knee replacement. She provided information on the failure of the knee and discussed the factors that influence failure. She highlighted the presence of fusion defects in the polymers. Wear resistance has been improved by reinforcing with carbon but this modification decreases the fatigue life. A popular improvement is using a chemical cross linking but this reduces the fatigue threshold and other mechanical properties. Jun Jo’s approach is to introduce physical entanglements in the processing. This means that the polymer chains are constrained by the high density of the chains in their neighbourhood. The molecular weight of the material is high and hence the viscosity is such that does not flow during processing so it is necessary to use compression moulding or ram extrusion of melted powder. A specially design computer controlled compression moulding device was described, along with the processing parameters. The influence of the dwell temperature and time was studied; 175ºC temperature produced the highest elongation to break. Micrographs taken after tests shows there are two different types of fusion defects – voids prior to complete consolidation and shear defects caused by inhomogenity of grain bonding after consolidation. The first type can be eliminated by proper processing but the second is the most probable cause of failure in service. The question is how to remove the memory of the grain boundary by complete molecular diffusion. The necessary mechanism is called reptation and the time necessary to do this is proportional to the molecular weight to the power of 3.4 for high density polythene. A theoretical approach based on heat conduction implemented in FEA was used to model the system and calculate the maximum related molecular weight (MRMW). The important question that ended Jun Je’s presentation is should a minimum MPMW be defined for all implants?
Alex Page from University College London was the second speaker of the day. The topic of the presentation was an investigation of the angle of insertion of a conservative hip replacement. As the femoral bone stock is preserved in a conservative hip replacement it has a longer life than a conventional hip replacement and makes it more suitable for younger patients. The hip replacement in the study was uncemented titanium alloy. An FEA study was carried out using MARC. Imaging of a goat femur was used to provide a detailed model of the femur. The implant and femur models were brought together to provide a 3-D mesh. The hip forces were provided from the literature. A number of contact analyses were carried out and the maximum and minimum strains were calculated from the FEA assuming isotropic and homogeneous properties. The strain energy densities were calculated to input into a strain adaptive remodelling paradigm. The highest SED was found in a femur with an angle of 145º. The results from the FEA correlated well with the animal study.
The third talk of the day was on getting medical devices to market faster by John Clark from Fene R C D Ltd. John showed that new drugs take around 11.8 years to get a new drug to market at a cost of £300 million. The result is increased costs, so medical devices need to be developed faster. Numerical simulations such as FEA can be used to help the situation. John presented a series of case studies that he has worked on that utilised FEA to improve design. These included a dose counter for an asthma inhaler, and automatic optimisation for design using FEA for soft tissue replacements.
The fourth presentation of the day compared FEA and photoelastic analysis of hip prosthetics. There are certain patients in which a conservative hip replacement is not possible. A new conservative hip replacement has been designed, this has a higher femoral cut. The fit between the implant and the femur is very important close fits develop preferential strains. The implant/femur was modelled in MARC in a similar way to that described in Alex Page’s presentation. Three orientations were studied, 125, 135 and 145º. Reflection photoelasticity was used on saw bones. The experimental results were correlated with the FEA. This demonstrated that 135º insert produced results that modelled the intact condition best.
Next were two brief poster presentations by Pamela Byrne from UCL on hip prosthetics and the other by Christopher Sarran from Tayside Flow Technologies on CFD in graft design.
The first speaker in the afternoon was Darren Hodson, an invited speaker from Astra Zeneca. The presentation concentrated on the use of computational techniques for the development of medical devices. As in John Clark’s talk the importance of accurate and validated models for the analysis of non-linear dynamic systems was emphasised. Examples of both structural and fluid applications were provided. CFD was used to simulate particles in the airflow introduced propellent. Darren said the only way to model this is using CFD, as experiments are not possible that can provide the actual conditions. Darren moved on to describe a phased approach to research and development. A case study of a polymeric spring was presented to illustrate the development process. The issues for modelling are:- linear elastic isotropic, boundary conditions, manufacturing variabilities, assembly processes. An explanation of how each of these could be dealt with to take the spring from an initial design to a final design was provided. In the design assessment a probabilistic variance analysis was carried out in Ansys. To highlight best practice convergence and the mesh density were examined. As the predicted moment did not change and that was within the range given in experiments, the model was considered to be validated. So the message is to consider statistical variations in the analysis. The second case study was a multi component system idealised as a compound cylinder. Lamé’s equations were used to obtain the hoop and radial stresses. A variance analysis was carried out in Ansys. Issues such as stress relaxation and creep were considered. One of Darren’s main conclusions was that in developing high performance complex geometry multi-part plastic components careful statistical analysis is required as there is little information on how the materials behave in combination.
Athena Markaki of the University of Cambridge gave a talk on magneto-mechanical bone growth stimulated by actuation of bonded ferro-magnetic arrays. The purpose of
developing this technique is to reduce loosening of implants. So making porous metallic implants will allow bone growth into implant. The implant should be highly permeable, adequate strength in both tension and compression and the stiffness should be that of the critical bore. The approach is to take ferrous fibres and use a magnetic field to produce a bending moment causing the fibre to move. Various stages of experimentation were presented to validate the approach ending with a 3-D random fibre array. To model the bone fibre interaction various different bone stiffnesses were taken and it was shown that sufficient strain could be generated to facilitate movement into bone.
The third speaker of the afternoon was James Eaton Evans from the University of Limerick. The topic of the presentation was experimental stress analysis of angioplasty balloons using thermoelastic stress analysis. The manufacture and application of the balloons was describes along with a description of the experimental technique. It was shown that undesirable failures could be predicted using the technique. The technique was validated against simple thin walled cylinder theory and it was shown that the balloons behaved in this manner at low loads but at higher loads departed from this behaviour. The onset of plasticity in the balloons was detected by a reduction in the thermoelastic response of the material.
The fourth speaker of the afternoon was Joseph Franks from Brunel University on the use of diamond like carbon (DLC) for coatings. Joseph described the benefits of using diamond like carbon including demonstrating its biocompatibility. Heomocombatibility was demonstrated in an artery. Wear resistance was also evaluated. The coating process was described using plasma assisted vapour deposition. DLC coatings have been used in knee implants, on dental and surgical tools. Can work at room temperatures so can be used on plastics. Collagen has also been coated; if collagen is exposed to blood it forms clots. A thick coating can be applied in multiple stages so that collagen is integrated into the body without entering the blood.
The first speaker of the last session was Tony Anson an invited speaker from Brunel University. The topic of Tony’s talk was the use of Mechanical Engineering in Medical Device Development and examined devices used in cardiovascular treatment. Tony demonstrated graphically the results of occluded arteries. Tony pointed out that replacement of cells is a long way in the future. Tony described balloon angioplasty and stenting using a guide wire. Tony then demonstrated how the system works, and described the different types of stents that are available. Current coronary by-pass procedures were described. Tony described two new procedures that allow the heart to continue beating during the surgery. These mean that the heart and lungs are working normally and less traumatic for the body. A special tool kit is required via an endoscopic like device. The new device that Tony is in the process of developing is for sutureless anastomisis which uses a silicone artery that is placed into the incision and suture line reverts to the outside of the coronary artery making the artery suitable for the blood flow. Tony then moved on to explain how he had optimised the device using Fluent. The results of the modelling allowed the optimization of the geometric parameters that enabled an initial design of the device.
The final speaker of the day was Arindam Chaudhuric an invited speaker from the Vascular Endovascular Unit at University College London. The title of Arindam’s
talk was on Aneurysm Walls stresses. Arindam described the nature of the problem and its serious impact on the health of the nation. EVAR is Endovascular Aneurysm Repair and first performed in 1991. Two trials have taken place in the UK. The results are 1/3 of the mortality rate compared with that of the traditional techniques. Failure commonly due to ‘endoleaks’. The mechanics of aneurysm was described in terms of the blood pressure. The aim of the study was to look at the changes in wall stress after the implant repair. A test rig that used a blood analogue and latex to model the arteries. An aneurysm was measured from a CT scan and a metal rapid prototyping model produced which was dipped in latex to produce the test model. This was put into the rig and covered with strain gauges. The stent graft was deployed inside the model, and an ungrafted model tested as a control. The peak wall stresses were at the inflection point. Once the graft was introduced there was a large reduction in the stresses. Introducing the endoleak causes significant increases in stress. Type 2 endoleaks cause a reduction in stresses as there is a route out as well as a route in. Wall stress analysis is not applied in the UK at the moment, currently we rely on pressure measurements but a better approach would be nanobots that take strain readings.
The meeting closed with a plenary discussion in which all the speakers commented on questions raised by the audience. The outcome of the discussion was that there is clear need for accurate validated mathematical models in this sector.