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Measuring body forces with sensors contained in the implant

Medical engineers have been working on the development of implantable intramedullary systems to control the distraction of regenerated bone (callus) during limb lengthening processes for many years. In collaboration with WITTENSTEIN intens GmbH and Prof. Rainer Baumgart (ZEM Munich), researchers from the University of Konstanz have further developed the fully implantable, telemetrically-controlled distraction nail FITBONE©. They have equipped the nail with sensors and energy- and data transmission modules to carry out biomechanical movement and load-bearing analyses of the locomotor system in vivo. The researchers’ goal is to develop a system to optimise limb lengthening through additional rehabilitative and therapeutic measures.

FITBONE enables biomechanical movement and load analyses of the locomotor system to be carried out. © Markus Glaser

The measurement of internal forces acting on joints, bones, muscles, tendons and ligaments is one of a number of challenging tasks in the field of biomechanics. Both in vitro and in vivo measurements are time-consuming and complicated. In clinical and sports settings, precise information about the load on biological material is needed in order to put in place preventive, curative, rehabilitative and therapeutic measures.  Prof. Hartmut Riehle and Markus Glaser from the University of Konstanz have joined forces with the medical technology company WITTENSTEIN intens GmbH and Prof. Rainer Baumgart (ZEM Munich) to further develop an implantable, intramedullary distraction nail known as FITBONE©, by the addition of sensors and energy and data transmission modules. "In contrast to external bone fixation systems, FITBONE© systems are inserted into the medullary cavity rather than being fixed outside the body," said Prof. Hartmunt Riehle.

The distraction nail can be compared to an extendable car aerial. The implant is activated by a computerised control unit, which separates the bone segments at a rate of approximately one millimetre per day. Bone mass regenerates in the distraction cleft (referred to as a callus in the field of medicine) and solidifies during a consolidation phase of four to nine months. The application of FITBONE© is indicated in people who have different leg lengths, dwarfism or excessive height and polio, and also for the treatment of limbs affected by tumours, sarcomas and pseudoarthroses.

Validation of biomechanical simulation

In a study to be carried out in the near future in cooperation with an implant clinic, the researchers will combine the intramedullary FITBONE© distraction nail with sensors and microelectronic modules to record the forces and moments in the implant. “The measurement values will be telemetrically transmitted to an external control device and stored for subsequent analysis,” said Markus Glaser. The researchers will also validate biomechanical simulation models based on load measurements on the femur when walking, standing, going up- and downstairs and during physiotherapy. Information about the distraction forces occurring during the limb lengthening process will help the researchers to determine and improve treatment parameters such as daily bone separation width, frequency and stimulation. “Once this is done, we will also analyse the features in terms of the consolidation of the callus tissue,” said Prof. Hartmut Riehle.

The course of motion will be determined with a 3D movement analysis system (LUCOTRONIC) and a force measurement plate. Optical markers that light up at regular intervals will be spatially recorded with a camera system consisting of three or six cameras. “The software uses the data to calculate the spatial coordinates, which are then entered into a simulation,” said Glaser.

Schematic showing the measurement of forces using the callus distraction nail FITBONE®
The callus distraction nail is equipped with sensors as well as energy and data transmission modules. © Markus Glaser / WITTENSTEIN intens GmbH

Patient safety is paramount

As far as the determination of the biomechanical load during different activities is concerned, the investigations need to be carried out at the highest possible levels of sensitivity. “Our initial plans are to carry out these investigations after the completion of the distraction process in order to preclude damage occurring to the system as a result of high forces which might make it necessary to terminate treatment prematurely,” said Markus Glaser. If the measurement results are within the range of the implant’s operating capacity, the researchers will be able to extend measurements to the active part of the distraction. Up until now, no FITBONE© surgery has been associated with any infections. “It has been shown that conventional systems involving external fixation are associated with a higher risk of infection at the sites where connective elements such as wires or screws are used, especially after several months of treatment. In the worst case, systems of this kind can even lead to severe bone infections,” said Prof. Hartmut Riehle.

External fixation systems for securing two bone segments in a fixed relationship use bolts and pins pierced through the skin. Holes are drilled into uninjured bone areas and bolts are screwed into the holes. The bolts are joined with a rod outside the body to make a rigid support.

Further information:
Prof. Dr. Hartmut Riehle
Sports Sciences
University of Konstanz
Tel.: +49 7531 88-2743
E-mail: hartmut.riehle(at)uni-konstanz.de

Markus Glaser
Sports Sciences
University of Konstanz
Tel.: +49 7531 883627
E-mail: glaser.markus(at)gmx.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/measuring-body-forces-with-sensors-contained-in-the-implant