Development and evaluation of a polyethylene knee insert with integrated wear sensors

Abstract

Currently, measuring wear of the insert of knee implants can only be accurately done in vitro. There are yet no accurate in vivo techniques that can capture and deliver data in real time. Therefore, the main objective of the research reported in this thesis was to introduce and validate a polyethylene insert with integrated wear sensors into knee implants that could be used in vivo after total knee replacement surgery. The instrumented insert was expected to help physicians and patients to acquire accurate wear information about the knee implant for an objective outcome evaluation of the intervention and to help to prevent excessive implant deterioration over time.
A mechanical test setup with different parts of a knee implant was built. First, the positions where the highest forces would act on the implants, were determined by integrating resistance sensors in the test setup. The results of this test were used to determine the positions of the wear sensors, since the tibiofemoral forces correlate with the amount of the wear of the insert. Then, the insert with integrated wear sensors was produced by filling six cutouts in the insert with silver conductive paint and creating six tracks in three different layers. When the resistance of the track would be infinite, the track should be worn-out. This prototype was tested in the same test setup.
The resistance values of the conductive tracks were measured to range from 40.3 to 128.4 Ω. After 6732 and 9974 cycles, the highest track on the medial and lateral sides, respectively, became damaged by the moving femoral head. The resistance of the tracks rose immediately to be infinite. Measuring the wear track with a caliper gave a value of 0.65 mms, which was deeper than the pre-determined track depth. The following tracks gave similar results.
One of the limitations of the present study concerned the movement of the knee. Since the test setup was only made of a hinge, it could not be compared with a whole knee movement that also contains translation. Thereby was the prototype only suited for in vitro tests due to the materials choice and lack of a data transmitter. Since the prototype was made by hand, the layer thickness could not exceed the minimum of 0.6 mm and were the layers glued on each other. This made the wear measuring method not as accurate as wished. Nevertheless, the wear pattern corresponded well with the expected wear caused by the tibiofemoral forces.
All in all, the insert with integrated silver conductive tracks is a concept that can be translated into a good solution to measuring wear in vivo in the near future. For the further development of this concept, the insert should be printed completely on a 3D printer. Thereby, more research should be performed on materials that could be used as conductive tracks, the possibilities to integrate more wear sensors to measure wear more accurately and the transmission of signals.