Non-Invasive Measurement of Stress Levels in Knee Implants Using a Magnetic-Based Detection Method

A knee replacement surgery (arthroplasty) has become prevalent worldwide and has a high success rate over the short to medium term. In some cases, especially over the longer term, implant degradation can develop due to the deterioration of the ultra-high molecular weight polyethylene (UHMWPE) tibial insert. Unfortunately, there are no satisfactory techniques currently available for assessing implant integrity and predicting failure. This paper describes a possible solution to this problem by using a non-invasive, electromagnetic method for monitoring implant integrity. This approach utilizes the magnetoelastic property of amorphous ribbon, which when stressed causes an inductance change in a nearby magnetizing winding. Amorphous ribbons encased in UHMWPE disks, to simulate a knee insert, were subjected to varying tensile stresses under an applied ac magnetic field. A correlation between total circuit impedance and applied stress was observed. The results obtained demonstrate that the proposed sensor has sufficient sensitivity for measuring typical stress levels associated with the axial forces in tibial inserts.