Automated In-Vitro Testing of Orthopaedic Implants: A Case Study in Shoulder Joint Replacement

Engineering of a multi-station shoulder simulator

This work aimed to engineer a multi-station shoulder simulator in order to wear test shoulder prostheses using recognized shoulder activities of daily living. A bespoke simulator was designed, built and subject to commissioning trials before a first wear test was conducted. Five JRI Orthopaedics Reverse Shoulder VAIOS 42 mm prostheses were tested for 2.0 million cycles and a mean wear rate and standard deviation of 14.2 ± 2.1 mm(3)/10(6) cycles measured for the polymeric glenoid components. This result when adjusted for prostheses diameters and test conditions showed excellent agreement with results from hip simulator studies of similar materials in a lubricant of bovine serum. The Newcastle Shoulder Simulator is the first multi-station shoulder simulator capable of applying physiological motion and loading for typical activities of daily living.

In vitro wear testing of a contemporary design of reverse shoulder prosthesis

Reverse shoulder arthroplasty is an increasingly common surgical intervention. However there are concerns and known limitations in relation to such joint replacement, while novel designs of reverse shoulder prostheses continue to appear on the market. Many claim to offer improvements over older designs but such assertions are difficult to validate when there is no consensus as to how such implants should be tested in vitro or even if such testing is necessary. In order to permit appropriate in vitro testing of reverse shoulder prostheses a unique, multi-station test rig was designed which was capable of applying motion in three axes to test prostheses. The shoulder simulator can apply up to 110° of motion in the flexion-extension and abduction-adduction axes and up to 90° in the internal-external rotation axis. Dynamic loading of up to 1500 N can be provided. The simulator is computer controlled so that the motions and loading associated with particular activities of daily living can be applied. A 4.5 million cycle wear test of commercially available reverse shoulder prostheses was undertaken using a 'mug to mouth' activity of daily living. Gravimetric analysis was used to characterise wear. After 4.5 million cycles of 'mug to mouth', the average wear rate of the test components was 14.3mm(3)/million cycles. Polyethylene test components showed a reduction in roughness and the median wear particle diameter was 167 nm. A three axis shoulder simulator has been designed and used to successfully test multiple samples of a commercially available reverse shoulder prosthesis.