One of many tricky elements of mounting prosthetics correctly, is managing the weight. Usually, creation of a prosthetic limb errs on the lighter side, sacrificing capability for lightness. This is because usually it is not really possible to examine the surviving bone in enough detail to safely screw a prosthetic into the bone.

When it is possible, in a large percentage of cases the prosthetic slowly works its way loose, rubbing and pulling against and sometimes breaking the bone around it, resulting in more surgery. This is especially true of hip prosthesis, which are typically replaced every fifteen 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 as the patient ages. Screw holes that held tightly fifteen years ago, are no-longer tight, and surgeons have to take their best guess where next to drill.

. 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. Of course, piling assumptions on top of assumptions, leads to errors. In some cases, it is little more than guesswork in finding places to screw into that are secure. This is again why soft tissue mounting is used whenever possible.

A new approach, hopes to change that. 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 are developing a model with which doctors can reliably and realistically calculate the density and elasticity of the bone from the CT scanner images.

Basically, the model uses machine vision and an expert system to analyse the CT, and highlight the best areas to drill, for the surgeons. Medical staff still have to work out the best shape of prosthetic in order to connect with the bone, and fit in with the surrounding bones as well as possible. However, there again a 3D simulation and analysis can help, showing simulated connections and the stress levels on the bone.

Much of the revolutionary work already exists in industry. Component testing to human hip bones, which involve inducing oscillations in the bone to see how much it can safely take, cannot, for obvious reasons be done on the patient. However, it can be performed innumerable times upon a simulated version of the patient's body, garnered from CT data. To aid in reinforcing the data, a database of human bone safe oscillations at various bone densities has been built up from donated bone matter. Prepared and preserved skeletal bones at a wide range of ages have been used, giving a baseline for oscillation tolerances for a given density. As a side effect of this work, prosthetics are much less likely to ever work their way loose, meaning reimplant surgeries will become a lot less common.

The research will continue for the next two years, before clinical application of this technology will begin in Europe.

References

Safely fixed hip prostheses - Press Release