When fitting a leg prosthesis on a patient, clinicians typically have to use a gait laboratory to analyze patient’s natural steps. The problem is that only one or two steps can be recorded by the lab, which provides too little information for a comprehensive fitting. Now researchers at the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig, Germany have developed a sensor system that fits into a prosthesis for a more long term analysis.
The adapter measures 4 x 4 x 3 centimeters and sits at the ankle joint or above the knee. It measure the applied forces in three spatial dimensions and three torque moments. A miniature data logger near the sensor reads out the data and stores them. “This adapter makes it possible to continuously measure the load on a leg prosthesis during different routine activities throughout an entire day,” says IST team leader Dr. Ralf Bandorf. The adapter has eight measuring bridges, each with four strain gauges. These consist of a sputtered insulating layer covered with a metal film. When the patient walks, the layer stretches according to the type of movement performed, and this changes the electrical resistance of the metal film. The 32 strain gauges are placed at a number of different points and in different orientations, so the data provide a complete picture of the load acting on the prosthesis. Strain gauges used in sensor systems normally consist of adhesive films, but in this case the layers are sputtered directly onto the surface. This means they can also be applied to the complex geometries of the adapter, for instance its edges, which would be difficult in the case of adhesive films. Moreover, the film is insensitive to moisture and does not require the use of adhesives.
“The main challenge was to design a suitable geometry for the adapter,” says Dr. Ralf Bandorf. It mustn’t be too large, as there is only limited space available inside the prosthesis, but it has to be large enough to accommodate the strain gauges. The developers are already testing a prototype of the adapter on the first patients, and will present it at the Hannover Messe from April 20 to 24.
Press release: Sensitive fitting process for leg prostheses