Monitoring Broken Bones Using a Wireless Bone Monitor

Depending on the values of the forces measured by the sensor, they can decide whether the healing process is progressing normally or whether there is a risk that the fracture or implants might be overloaded.

Until now, clinicians have used expensive and complicated electronic devices that transmitted the measured data as radio signals. According to Dr. Felix Gattiker, from EMPA's (Swiss Federal Laboratories for Materials Testing and Research) Electronics, Metrology and Reliability Laboratory (Dubendorf, Switzerland), an electronics-free sensor offers many advantages--not least of the financial sort. In the new EMPA sensor, the data are read out by means of an ultrasonic scanner.

The solution comes in the form of a small, hollow spiral that sits on the implant together with a fluid reservoir. When the implant is subjected to compression or tension, the fluid level in the spiral changes. This level is measured with the help of an ultrasonic device, and the resulting data allows mechanical loading on the implant to be calculated.

The ultrasonic image is, however, too indistinct to allow the fluid level to be determined visually, so the researchers decided to analyze the ultrasonic signal in more detail. They quickly found a dependence between the ultrasonic echo generated over the complete spiral and the actual fill level--the weaker the ultrasonic echo measured, the higher the level, and therefore the greater the force acting on the sensor.

The sensor produces effective measurement data, as numerous experiments with artificial tissues--mixtures of gelling agent, glass balloting, and graphite powder, which, depending on the mixing ratios allow different types of tissue to be simulated--have demonstrated. In addition, the sensor is also economic to manufacture, being less expensive than the existing electronic versions.

The next step is to test the accuracy of the new method using various animal tissues, since each material has its own acoustic signature because it reflects and absorbs ultrasonic energy differently. Moreover, the EMPA scientists are investigating the proposal of making the sensor out of biodegradable materials, in which case the device would simply dissolve away in the patient's body after completing its task.

The surgeon, therefore, need not sharpen his or her scalpel a second time, there being no need to remove the sensor when the fracture has healed. Finally, there is the outstanding matter of finding an industrial partner to manufacture the sensors and integrate them into the implants.

Related Links:
EMPA - Swiss Federal Laboratories for Materials Testing and Research