Evaluation of knee prostheses range of motion using low-cost sensors

Total knee replacement is a common type of surgery used to treat diseases such as osteoarthritis. It consists on the implantation of a prosthesis that substitutes the knee joint of the patient. Between the different types of prosthesis used, hinge prosthesis and condylar constrained knee (CCK) prostheses are the more movement restricting. Although it was believed that hinge prostheses allowed for a lower range of motion, a recent article by TL Malcolm et al. (1) proves similar outcomes in terms of functionality and stability for both types of prosthesis. Following this idea, it can be interesting to quantitatively compare the range of motion of both prostheses to test the hypothesis
that hinge prostheses leave at least the same freedom of movement as CCK ones. This will be done by measurements of tibial rotation range at different knee flexion angles.

In order to perform the measurements, a system has been developed that is able to measure knee flexion and tibial rotation. It is based on two inertial sensors, each composed by a set of accelerometers, gyroscopes and magnetometers. A module developed in 3D Slicer medical imaging platform receives the data from the sensor and processes it to yield measurements of the angles of interest. Low cost of the device has been a design criterion, using free software and 3D printed pieces as cheap alternatives to other commercial products.

3D printed models to simulate both movements (knee flexion and tibial rotation) have been created and used to test the initial performance of the system. The device has been tested as well against an optical tracking system as gold standard. Results show a measurement error of 2.94o ± 1.27o for tibial rotation and 0.005o ± 0.005o for knee flexion. It has been proved thus that the system presents a high accuracy when measuring knee flexion and a low error in tibial rotation, provided that it is intended for comparison between subjects.

The device presents the advantage of being easily operable and not requiring a complicated setup. This, combined with its low cost, defines it as a useful alternative for physicians as a tool for joint angle estimation.