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Better insights needed into failure mechanisms of hip replacements

If a hip prosthesis implanted to replace a worn-out joint itself fails, then what are the reasons? Until now this problem has been little understood: partly due to incomplete records, partly due to insufficient knowledge of the forces acting on the hip replacement.

Computation models developed by the University of Twente and UMC St. Radboud can make an important contribution to our understanding in this field, says Prof. Nico Verdonschot in his inaugural lecture as Professor of Implantation Biomechanics at the University of Twente. Such models can, for instance, predict the strength of a bone much better than a specialist can do visually using an X-ray image.

Prosthetic replacements for joints such as the hip and the knee are available in many types and sizes. But which prosthesis is the best for which patient? Verdonschot aims to find answers to this question by combining his experience at the Orthopaedic Research Lab in Nijmegen with the considerable expertise in mechanics present at the University of Twente. Indeed, Twente researchers have advanced computation tools for calculating the strength of constructions. They have also developed a sophisticated muscle-skeleton model in which all the forces in play can be simulated. By combining this model with experience from the clinic, Verdonschot expects to achieve a leading position in research into the functioning and the lifespan of joint replacements.

At the same time he points to the lack of proper records in the Netherlands: it was only in spring of this year that medical authorities began to record how long a joint replacement lasts. These data are not yet linked to death registers, meaning that they do not always provide adequate information. Minister of Health Abraham Klink, in answer to a parliamentary question, has already stated that he is not aware that poor-quality prosthetic joints are on the market. ‘Not aware’ is indeed the only correct term, remarks Verdonschot: the records simply doesn’t exist. Sweden, by contrast, does have a comprehensive registration system from which much can be learned.

Bone strength

The type of calculation model that calculates the forces acting on joint prostheses is also suitable for predicting the strength of a bone itself, for instance if the bone is subject to metastases and weaker points in the structure that can lead to a spontaneous fracture. The models can make this prediction much more effectively than a specialist who makes a visual estimation based on an X-ray image: the model generates 86 percent correct predictions, while the doctors score between 25 and 50 percent. The predictions can then be used to make a recommendation on, for instance, a surgical operation to reinforce the bone. Verdonschot also discusses the trend which involves using the body’s own processes instead of a prosthesis. In regenerative medicine, for instance, stem cells are cultivated to create new bone tissue: to date this approach certainly cannot replace an artificial joint, but it can help the prosthesis to connect better with the existing bone. The stem cells can be manipulated to create bone tissue that has the same preferred direction as the bone in which it is inserted.

The appointment of Verdonschot, who is himself a Mechanical Engineering alumnus of the UT, is in line with the strengthening of the university’s health care profile and the close collaboration with hospitals and other care institutions. Technology can be successfully integrated in the clinic only if all parties work closely together right from the start. The combination with the calculation models may seem a small step but, according to the new professor, it may bring great consequences.

Prof. Nico Verdonschot held his inaugural lecture ‘De reis van techniek naar kliniek’ (‘The path from the technological to the clinical’) on 25 September 2008.

Wiebe van der Veen | alfa
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