Engineering in Medicine
History of Joint Replacement
Total joint replacement is now regarded as a highly successful treatment of arthritic joints. However, with increasing numbers of implants being used and with larger follow-up data becoming available, clinical problems associated with both materials and designs are continuing to challenge engineers and clinicians. Following the success of joint replacement in the hip, the attention of orthopaedic surgeons moved predictably enough to the other major synovial joint in the human body, the knee.
The knee is frequently attacked by rheumatoid arthritis and also by osteoarthritis, both of which constitute painful and crippling medical disorders. In common with arthritis of the hip, these disorders can cause the patient to be confined to bed or a wheelchair and any surgical procedure which can help alleviate the pain and enable the patient to remain active is of great value. The early total knee joint replacements were single axis hinge-type designs, which were introduced clinically in the early 1970s and while they provided good levels of pain relief they did not fully take account of the exceptionally complex engineering situation in the human knee. The twisting action of the muscles and ligaments gave rise to loosening forces on the implants during movement. This, coupled with the fact that the components had metal on metal surfaces rubbing against each other producing wear debris, gave rise to some clinical problems.
By 1976 designs which used metal on plastic bearings and which replaced the bone surfaces, were introduced to attempt to overcome some of the limitations of the previously described hinges. These designs required more precise surgery as well as medical indications, where ligaments and muscles had not suffered irreparable damage caused by the disease. The second generation of surface replacement type total knee replacements had improved fixation, improved instrumentation and allowed a greater degree of joint laxity. This led to improved motion and better long-term results. Typical design improvements were the addition of enhanced fixation features between the tibial component and the tibia and reduction of conformity to reduce cold flow and creep of the polymer. By the late 1980s and early 1990s designs had evolved from highly conforming surface type designs to highly non-conforming, surface type designs with metal reinforced tibial components as shown in Figures 1 and 2 Despite this remarkable progress significant engineering problems still remain in the search for implants which provide long term trouble-free performance in the human body.
The Design of Prosthetic Knee Implants
During the design and development of a new prosthetic knee implant each of these issues must be analysed from both engineering and medical perspectives. Considerable debate revolves around the question of component geometry, since the implants are not made of materials which deform under pressure to reduce stress levels, as the menisci do in the normal knee joint. A variety of designs exist which reduce contact stresses in the polyethylene and thereby reduce wear rate following implantation, however there remains considerable scope for engineering ingenuity to overcome the shortcomings of the implant materials. Some recent research has been conducted on materials which have damping characteristics and it is hoped that such materials can more closely mimic normal knee joint behaviour. Geometrical considerations lead naturally to the question of instrumentation, which is very important for accuracy and alignment during surgery. Certainly if the prosthetic geometries give rise to questions of contact stress and associated wear problems, so too do questions of precision and accuracy in surgery. The early hinge-type devices could be implanted with relatively few additional instruments but modern surface type prosthetic components, which are not intrinsically self aligning, need a much more complex instrument kit. Since there are limitations on what can be achieved in the operating theatre in terms of accuracy, the question of instrumentation cost and complexity needs to be considered. Again solutions to this type of problem can be found when bioengineers and medical practitioners collaborate.
Research
Extensive studies of the mechanics of polymer wear have found that slight surface imperfections can dramatically affect wear rates. The surface of the counterface, which is rubbing against the polymer, is one of the most important factors controlling the wear rate of the polymer. This has meant that manufacturers of implants must pay special attention to the methods of polishing and machining the implant components and this poses further engineering challenges. A wide range of techniques exist for stress analysis of the complex situation which exists, at the knee joint. While there is general agreement concerning the magnitudes and direction of the loads at the joint, much remains to be done to understand the complex stress distribution which exists both in the bony structure and in the implant structure. Two broad strategies for examining such situations exist, namely, experimental stress analysis and finite element analysis. Perhaps the most difficult area of stress analysis from the point of view of implant engineering is experimental analysis. Such experimental stress analysis is an essential tool for verification of theoretically based models. Design of tests and interpretation of experimental results obtained from such tests is a demanding task; not least of which is the care required in the setting up of experimental apparatus, but also in understanding the extreme limitations and shortcomings of such tests when it comes to drawing conclusions from the results.
The Future
A number of techniques have been used at the University of Limerick, the most significant of which is strain gauging. Strain gauge analysis using large scale Araldite models of the implant components has been conducted. The results have been useful in obtaining information about the stresses in the components in the body and also for making recommendations regarding new designs. (Strain gauges are electrical devices which give an output signal when they are deformed and they are widely used in examining structures which are subjected to large forces.) Modern computer technology has brought a transformation to the techniques of stress analysis with the advent of Finite Element Analysis. This technology allows the designer to subdivide the component into a very fine mesh and this enables the computer to analyse the structure at every node in the mesh, yielding a very accurate picture of the stresses and strains in the part being examined. Extensive research using this analysis technique for knee and hip implant devices is ongoing at the College of Engineering at the University of Limerick and such work is expected to increase as computer power improves. Testing of new designs of prosthetic knee components (Figure 3) is a complex affair and involves analysis of both the motions at the joint and the associated forces. While comparative data for knee designs can be obtained from simulator studies, care must be taken in applying such results directly to implant designs. Recent work in this area in the College of Engineering has included the design and construction of a knee joint simulator for wear testing of knee implants. The earlier reference to surface finish has clear implications for the manufacturing of knee implant components. The manufacturers of medical implants are subjected to very stringent national and international regulations. They are continuously engaged in engineering research to further improve their products by developing new methods of production, as the product requirements become increasingly more complex and demanding. Many design questions remain unanswered. Progress towards improved knee implants continues apace and with widespread collection of device histories by many researchers and national agencies, we can be confident that these questions will provide scope for further development of even more successful implants. In addition, with the increased interest being shown in collaboration between doctors and engineers, this area of human endeavour provides great new opportunities for young engineers to become involved in the development of new and improved methods of patient care in orthopaedics and other areas of medicine.
Tim McGloughlin is a Lecturer in Applied Mechanics and Thermodynamics. He has research interests in Bioengineering with a particular interest in orthopaedic implants for joint replacement.
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Last updated 13th March 1996 by Stephen Childs