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A new strategy for improving bone implants

Biomechanics is an interdisciplinary field of research and is also of great interest to aerospace engineers. Aerospace engineers at the University of Stuttgart develop equations and models in order to gain a better understanding of the interaction between bones and implants. In addition, they believe that their results will contribute to developing new implants and implant structures.

Gunter Faust, an engineer at the University of Stuttgart, has been dealing with the biomechanics of bones, implants and muscular strengths since the mid-1980s. He develops systems of equations to calculate the interaction between bones, implants and muscular strengths as precisely as possible. Faust bases his calculations on the finite element method (IFE) known from numerical mathematics, which has a long-standing tradition at the Institute for Statics and Dynamics of Aero- and Astronautics Structures (ISD). “John Argyris, who founded our institute, was one of the creators of the finite element method. This method has turned out to be a very good choice for solving differential equations in many areas, not only for obtaining more accurate results when constructing aeroplanes,” said Faust.

In the 1980s, a dentist contacted the ISD for help. This was not really surprising as dentistry also relies on robustness, stability, etc., just like aircraft parts and bones and implants. This particular dentist had the goal of developing more efficient methods of fitting dental implants into the jawbone for a longer time than previously possible. “In common with other bone implants, we focused a lot of attention on achieving the long-term stability of the dental implant,” said Faust.

Looking for a suitable equation

Faust started work on the dental implant by intensively studying the biomechanics of bone growth and remodelling. “We are studying the structure of the cancellous bone, in particular the structure at the site where the bone melds to the implant, for example artificial hip joints,” said Faust who is very much focused on developing systems of equations that help him calculate the remodelling of the bone at these sites.

Migration process of the femoral neck bolt in the opposite direction to that of static strain (first two photos in the bottom row) © Faust/University of Stuttgart

"Our major problem is to find out how we can mathematically model bone remodelling phenomena. We know that natural processes are non-linear. Therefore, the equations that describe bone remodelling phenomena in adults also have to be non-linear," said Faust summarising his approach. The researchers had previously found out that linear equations were unsuitable for their purposes. "Initially, we used linear systems of equations, but this regularly led to the bone structure breaking up," said Faust who has since realised "that a linear model of the system "human being" would not be able to survive for more than a minute."

The experimental validation of the results is the most difficult obstacle Faust has to overcome. During his search for suitable objects, he made contact with an abattoir from where he obtained cattle bones. "Dead, moist bones only maintain the biological structures of living bones for around eight hours," said Faust who nevertheless found dead bones to be an excellent way to develop the bone remodelling model. "We measured the bone structure, transferred it into coordinates and developed an initial geometry which we could then use for further investigations."

 

Dynamic hip joint screw (here Targon PF of the company AESCULAP) © AESCULAP

This geometry can be used for further calculations and to develop equations which might subsequently be integrated into software for practical applications. Faust envisages using the calculations for personalised medicine applications: CT images of patients' bones reveal a bone structure for which the software can already successfully determine the geometry. "The images provide us with data about the patient's individual bone structure. We then use an implant to simulate the remodelling of the bone in silico. We base our simulations on the assumption that the implant is a living structure. The model equations can then be used to add or remove the metal of the implant. The combination of information about the bone remodelling process and the information obtained from the physicians treating the patient, can lead to implant structures that are adapted to the requirements of individual patients. The final choice of the implant structure will be taken jointly by the physician and engineer," said Faust explaining his vision.

The calculations of the aero- and astronautics engineers are also envisaged to contribute to the composition of implant materials, for example taking into account the implants' porosity. These calculations can then be used for implant constructions based on biological information. "Implants that are not only developed on the basis of mechanical, but even more so on the basis of biological principles, stand a much better chance of long-term stability," concluded Faust.

Further information:

University of Stuttgart
Institute for Statics and Dynamics of Aero- and Astronautics Structures (ISD)
Dipl.-Ing. Gunter Faust
Pfaffenwaldring 27
70569 Stuttgart
Tel.: +49 (0)711 685-67097
E-mail: faust[at]isd.uni-stuttgart.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/a-new-strategy-for-improving-bone-implants