Patient monitoring system for load measurement with spinal fixation devices

Fractures of the spine can be stabilized by different implants. Their stiffness varies widely and only little is known about the loads acting on these devices. In order to measure the forces and moments in the implant, the internal fixator after Dick was modified. An inductively powered telemetry unit is placed inside the fixator and is hermetically sealed against body fluids. An integrated eight channel telemetry chip was developed to measure the signals of six strain gauge sensors, the implant temperature and the power supply. Because two fixators are implanted together, two telemetry transmitters run at the same time. This paper describes the function of the instrumented implant and the external system components.