Implantable sensor system for the monitoring of bone healing

A continuous monitoring of the healing process of bone fractures by active, microelectronic implants is a potential measure to prevent complications. So far bone healing is monitored by diagnostic methods such as X-ray, CT and MRT. For ascertaining the permitted stress of implants, e. g. in dental surgery, heretofore clinical experiences are consulted. For these and further parameters a capture by measurement engineering would be preferable. An active, autonomously working implant for direct mounting at bone fractures was developed. It contains a sensor, an electronic control circuit, a radio system and a battery for power supply. The oxygen saturation of the extracellular tissue fluids at the site of fracture is measured periodically. Via a radio transmitter in the implant, an external receiver and a data transmission computer with wireless LAN connection to the internet, the measured values are sent to a web server with a marginal time lag. From there the data are retrieved with a web browser. The implant is coated with a biocompatible synthetic resin in order to avoid any contact between the electronic circuitry and the body tissue. The system was tested in-vivo by implantation into artificial bone defect at the cranial calvarium of sheep. The preliminary test validated the functionality of the data transmission system and the biocompatibility of the implant. A close contact between implant and surrounding tissue was verified by visual inspection after sacrificing the animal. However, the correlation between the dissolved oxygen saturation and the healing process could not be confirmed as yet. Because the measured values were subject to drift and the used sensor could not be calibrated in-situ, a physiological interpretation of the data is not possible.