Dr Mark Taylor from the School of Engineering Sciences of Southampton University recently delivered a lecture at the Lodge on Finite Element Modelling of bones and prostheses. Mark graduated from the University of Wales in Cardiff as a Mechanical Engineer and carried out a PhD at the University of London on bio-engineering applications. He gained considerable practical experience before joining the University of Southampton where he is responsible for a growing research team working on problems related to bones and prostheses.
An important research topic is the modelling of the bone and stem of the prosthesis. In order to understand the forces acting on that system, it is essential to develop accurate stress analysis models. One of the main problems in this regard is the magnitude of the muscle forces which can only be estimated at present as it has proved to be impossible to measure them 'in vivo'.
Mark's group has studied the problem of rim damage due to the stresses produced on the bone surface where the implant rests. This is a region where cracks can occur as a result of the difference in the rigidity of the bone and the implant. A better understanding of the biomechanics of the stress in the region and how different types of cement behave can lead to more satisfactory and durable implants.
The research of Mark's group has been extended to the study of other joints in the human body, in particular those in the knee which are more difficult to replace satisfactorily. There is a growing demand for knee replacement, due not only to trauma but to arthritis and other illnesses affecting a large section of the population. Modelling total knee joint replacement is one of the most challenging fields of research at present. The kinetics and state of stress depend on the implant design and the surrounding soft tissue structure. The new models presented by Mark allow for dynamic behaviour rather than the usual static only analysis.
The research has now progressed to modelling specific patient knee replacements using finite elements to investigate variation between different individuals and correlate results with direct data on a patient by patient basis. The results obtained so far are interesting as the comparison of stress distribution in the bone for several patients have proved to be very different. The differences were not only due to the various types of joints used but also to the type of cement applied.
This work has led to interesting conclusions regarding the durability of joints. The next stage of the research is simulating better the dynamic environment and modelling the wear of the joints, including the whole of the joint which comprises part of the bone plus the ligaments and soft tissue contributions.
Another area of research in which the Southampton group is currently working is the long term behaviour of the bone as it changes and develops. The work here concentrates on the fatigue properties of cortical and cancellous bone tissues which are very complex and include the effect of accumulated strain. The objective is to gain a better understanding of the cancellous bone fatigue and to assess the material degradation on fatigue life. This is difficult in view of the configuration of those tissues. Because of that, the idea has been put forward to relate the properties of cortical bones which are easier to determine, with those of cancellous bone.
The work of the Group also includes experimental work on carbon fibre composite implants. They have been developed using techniques to provide better load transfer facilities in younger patients but many of the carbon fibre implants that have appeared in the market for them have failed. This is in part due to the need to develop a better coating for them. Currently a series of mechanical and clinical tests are under way for this new type of prosthesis. New measurement techniques have been developed based on acoustic measurements which allow one to determine microcracks, delamination and fibre breakage. Progressive failure shows up as different peaks of activity using acoustic emission technique. These techniques are now being extended to study bone cement failure to understand better the cement/prosthesis interface failure behaviour. The acoustic measurement technique has proven to be a powerful tool for the detection of cracks in all these cases.
The migration of implants is an important research topic which can lead to a better understanding of their failure and durability. Mark is trying to develop a mechanical model which can follow the process.
Mark's talk was followed with great interest by the audience and a lively discussion ensued. It was followed by a short presentation by Dr John Baynham of the Industrial Division of WIT exploring the capabilities of the BEASY (Boundary Elements Analysis System) programme, particularly those related to the modelling of prostheses and knees. Special facilities in the BEASY code allow for wear, fracture and crack propagation analysis. The code has been applied by different medical researchers for modelling prosthesis among others bio-engineering problems.
Wessex Institute