A New Diagnostic Technique to Detect Early Migration of Joint Prostheses

Migration of an implanted prosthesis due to aseptic loosening is difficult to detect without regular imaging. In this paper, a low-cost, non-radiographic, robust diagnostic technique is presented, which can detect the migration of the humeral component of an elbow prosthesis. The system consists of a single magnet single sensor configuration and migration data are based on the variation in the magnetic field. The magnetic sensor was enclosed in titanium alloy and a magnet was embedded at a reference point in the humeral bone enclosed in ultra-high molecular weight polyethylene (UHMWPE). A layer of bone cement was then placed between the enclosed magnet and sensor. An algorithm linked with a Savitzky- Golay (SG) filter was developed that could decouple, convert, and filter the magnetic field signal to provide both the linear and angular displacement. The system was also designed to eliminate the cross talk and non-linearity effect of the magnet. The highest resolution the sensor achieved was 0.3 mm with a detectable linear migration range of 0.3 mm to 4 mm in the x/y axis and between 8-20 mm in the z-axis (along the humeral canal). The detectable rotational range was 0.5 to 3.0 degrees in the x/y axis. The repeatability of the calibrated sensor was analysed and showed a standard deviation of 0.05 mm over 150 cycles. The resolution was dependent upon the operating conditions and sensor positioning. There was no interference from the titanium alloy, bone cement nor the UHMWPE. This sensor system offers an alternative non-radiographic option for measuring migration of implanted prostheses.