Artificial hip joints are firmly anchored to the patient's damaged bone by screws. But which parts of the bone will safely hold the screws in place? A simulation model is to calculate the strength of the bone from computer tomography images.

Hip prostheses do not hold forever. If an implant comes loose, the doctors have to replace it. Most patients need this second operation after about 15 years. By then, the first prosthesis has often worn down the pelvic bone in several places. Moreover, the bone density, and thus also its strength, changes with increasing age. Medics therefore have to work out where best to place the screws that connect the artificial joint to the bone, and what shape the hip prosthesis needs to be in order to fit the surrounding bones as well as possible. At present, doctors examine patients using computer tomography (CT), and determine the rough density of the bones from the images. On the basis of various assumptions, they then calculate how strong the bones are in different places. The problem is that, although there are various theories on which the simulations can be based, the results often deviate significantly from reality. The consistency of the damaged bones is usually different from what the simulation leads to believe.

This is set to be changed by researchers at the Fraunhofer Institute for Machine Tools and Forming Technology IWU in Dresden and their colleagues at the biomechanics laboratory of the University of Leipzig. They are developing a model with which doctors can reliably and realistically calculate the density and elasticity of the bone from the CT scanner images. To this end, the researchers are transferring methods usually used for component testing to human hip bones, which involve inducing oscillations in the bone. This type of examination cannot be carried out on the patient. The bone has to be clamped into an apparatus. "The nature of the oscillations enables us to deduce local properties of the bone - such as its density and elasticity," explains IWU group manager Martin Quickert. The researchers compare these results with scanned images of the bone and describe the correlations on the basis of a mathematical model. This should make it possible in future to determine the strength of a bone directly from the CT scanner images. The scientists have already performed the first examinations on prepared and thus preserved bones, and plan to induce oscillations in unprepared bones left in their natural state over the coming months. The researchers hope that in about two years' time, doctors will be able to obtain a realistic simulation model of unprecedented quality from computer tomography data. The prostheses can then be perfectly anchored, and will be held safely in place for longer.

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Source: Martin Quickert
Fraunhofer-Gesellschaft