Finite Element Analysis of Medical Devices: WS Atkins Perspective

Transcription

1 Industry Sector RTD Thematic Area Date Biomedical 13 th Nov 2001 Finite Element Analysis of Medical Devices: WS Atkins Perspective Stuart Kelly WS Atkins Consultants Ltd, Bristol, UK Summary: This presentation summarises WS Atkins perspective on the current and future use of finite element stress analysis and other computational analysis methods in medical device design and development, based on our experience of providing consultancy services to the industry: Contents: 1 Introduction 2 WS Atkins Experience 3 Case Study Synthetic Heart Valve Finite Element Analysis 4 Benefits of FE for Medical Device Development 5 Barriers to use of FE in Medical Device Development 6 Future Developments Required 1

2 1 Introduction WS Atkins Consultants supplies a range of technical services to support the design and development of medical and drug delivery devices. We have worked on both implantable and non-implantable devices, and particular areas of expertise include stress analysis, fluid flow analysis, device safety and management of testing programmes. This presentation summarises WS Atkins perspective on the current and future use of finite element stress analysis and other computational analysis methods in medical device design and development, based on our experience of providing consultancy services to the industry. 2 WS Atkins Experience Examples of projects completed in the past 3 years include: - Development of non-newtonian blood models for CFD, and application to mechanical heart valves. - Carbon sorbent column design to optimise mass transfer. - Development of methods for FE analysis of long bones (ABAQUS/Standard). - FE contact analysis of total knee replacement (ABAQUS/Standard). - Drug capsule stress and fracture analysis (ABAQUS/Standard & LS DYNA3D). - Pharmaceutical device latch analysis and design (LS DYNA3D). - Synthetic heart valve design to optimise durability (LS DYNA3D). - Football shin guard appraisal and design (LS DYNA3D). Fig. 1 Analysis of human femur Fig. 2 Biomet Merck DA2000 Knee 2

3 3 Case Study Synthetic Heart Valve Finite Element Analysis Surgery to replace patients diseased heart valves with man-made replacements is now a relatively common procedure. There are currently two distinct types of replacement heart valve available; bio-prosthetic valves made from treated animal tissue or mechanical valves constructed from metal and ceramic components. Unfortunately both types of valve have inherent problems. In the case of mechanical valves, daily drug therapy to prevent blood clotting is required, which is expensive and can lead to secondary complications. On the other hand, bio-prosthetic valves can have a limited lifespan, and therefore the patient will often require re-operation. AorTech Europe Ltd (Bellshill, Scotland, UK) is in the advanced stages of developing an innovative replacement heart valve. The universities of Glasgow, Leeds and Liverpool were also involved in the initial design and development of the new valve. The AorTech valve, which is constructed entirely from synthetic polymers, has the potential to overcome the main problems with existing valves by achieving long-term durability with minimal anti-clotting drug therapy. By early 2000, a valve design had been developed with a proven life-time of over 700 million fatigue cycles in experimental fatigue tests, equivalent to around 17 years in the body. Subsequent commercial development by AorTech seeks to optimise the valve performance, without compromising valve life-time. WS Atkins is supporting AorTech in optimising the design of the synthetic valve. As the design has progressed, WS Atkins has completed a programme of computational stress analysis to optimise the distribution of stress between the valve components. Using explicit finite element packages such as LS-DYNA3D, we can model the complex deformations of the non-linear, highly elastic valve materials during opening and closing of the valve. The use of computational models provides the opportunity to investigate sensitivity to key design parameters in an efficient and cost-effective way. We have also worked to significantly reduce the analysis time for each new design iteration - a key factor in helping the customer to meet tight development timescales. Fig. 3 AorTech Synthetic Valve Prototype Fig. 4 Closed Shape (130 mmhg pressure) 3

4 4 Benefits of FE for Medical Device Development In common with other manufacturing sectors, the ability to develop new medical devices more quickly and cheaply can offer a competitive advantage. For many medical devices, the only way to predict effectiveness with a high level of confidence is through clinical trials, and therefore progressing to commence clinical trials as quickly as possible can reduce development risks and costs significantly. In principle, the use of computational analysis to evaluate initial design iterations can reduce physical testing requirements, and ensure that designs meet fundamental functional design requirements, thereby reducing risks as early as possible in the design cycle, with a consequent reduction in overall costs and timescales. This approach requires full integration of FE analysis into the design cycle. However, at present FE models of medical devices are more often developed at a later stage in the development process, when the design is already highly constrained by materials, manufacturing and commercial commitments. Nevertheless, such FE analyses can and do provide valuable insights into the design trade-offs for devices, and can help ensure that late design changes do not have unexpected knock-on effects. A particular benefit of computational analysis for medical devices is the ability to carry out sensitivity studies on uncertain parameters, which can be used to study variations in physiology or implantation in a very efficient way. Significant parameters can be quickly isolated from those which have minimal effect on device performance, potentially reducing the scale of subsequent experimental studies or leading to a more robust redesign. 5 Barriers to use of FE in Medical Device Development In our experience, several major barriers and objections to using computational analysis in device design are due to the limited information available to decision makers, for example: - Low awareness of capabilities and benefits. - Perception of FE as a research tool only. - Past experience of inappropriate use, leading to either incorrect results or results which add minimal value to the design cycle. - Seen as expensive technology, i.e. as an additional cost rather than a tool for reducing costs and risks. Other, practical obstacles to widespread use of computational design tools include: - Integration with current design processes (e.g. CAD, design philosophy). - Lack of sufficient and validated material models and data. - Availability of representative load data (e.g. muscle loads). - Limited understanding of physical processes and their interactions, such that modelling assumptions can be difficult to justify. 4

5 6 Future Developments Required Future work should seek to build on the considerable body of existing published research work, with a focus towards practical and relatively inexpensive design tools. Major areas for future work include: - Communication of the capabilities and benefits of using computational analysis, and the contexts in which use is most appropriate and valuable, to the medical device industry and wider biomechanics community. - Development of finite element models as practical design tools, including: - Establishing best practice regarding assumptions & simplifications (e.g. muscle loading for hip implants, blood models for mechanical heart valves, etc.), based on a combination of experimental and computational work. - Further awareness/development of methods for parametric model definition and design optimisation. - Development of validated material models and measurement of material properties. - Increase awareness, use and availability of validated methods for solving coupled fluid-structure interaction problems, to establish the importance of coupled effects in device design, and to account for them where required. 5