Tribological evaluation of total shoulder arthroplasty implants : advanced in vitro wear testing and retrieval analysis

Abstract

The glenohumeral joint is subject to dynamic and cyclic loading and motion during activities of daily life (ADLs), which can promote the wear of the articulating surfaces of shoulder joint replacements. The adverse reaction to the generated wear debris can influence the long-term survival of the implant and is considered an issue of contemporary concern. Therefore, it is important to be able to measure the wear and surface characteristics of anatomical and reversed total shoulder replacements in order to understand the complex wear mechanisms that occurs in vivo and predict their performance. To date, there is no consensus in the literature regarding the methods to test the in vitro wear of shoulder replacements. Hence, the aim of this research was to develop a novel protocol based on the loading and motion of relevant ADLs. This protocol subjects the shoulder prosthesis to cyclic loading and intermittent motion in a multi-station shoulder wear simulator. After five million cycles, the polymeric volumetric wear was 58.8 mm3 for reverse shoulder components against metallic glenospheres. The three-dimensional surface roughness (Sa) values of the polyethylene humeral components fell from 692 ± 132 nm to 42 ± 29 nm. Comparison with an earlier wear tests revealed a non-statically significant difference in wear, suggesting that the addition of intermittent loading is unnecessary. This could help those wear testing shoulder implants in future. When an anatomic shoulder design consisting of a PyroCarbon humeral heads (ceramic-like alternative bearing material) and a polyethylene glenoid inserts were tested, the polyethylene wear was 90.6 mm3 . The surface roughness of the polyethylene components fell from 296 ± 28 nm to 32 ± 8 nm. Interestingly, PyroCarbon components did not exhibit a measurable loss in mass or change in the surface roughness. Explant analysis can give key insights into how artificial joints have performed in the body. Therefore, and for the first time, the surface characteristics of retrieved metal– on–polyethylene reverse shoulder prostheses were assessed. From a heterogeneous cohort of thirteen explants with a mean time in vivo of 16 ± 11 months, results indicated - viii - no correlation between the surface roughness and duration of implantation. However, tantalisingly among the metallic components available, low surface roughness values were found (ρ = 0.032) when the bearing material was inverted and used as a humeral component. If shown to be true in larger explant studies, this could suggest that such inverted materials could be associated with a superior tribological performance compared with a conventional arrangement. Predominant surface damage modes observed on most of the conventional reverse total shoulder arthroplasty implants during the analysis of the surface topography were similar to those obtained from the in vitro wear test, and consistent with previous observations on retrieved components; the similitude between these results helped to validate the Newcastle Shoulder Wear Simulator. The collection of results and findings obtained in this research at Newcastle University may help others who test artificial shoulder joints to validate the design of their simulators, improve mathematical wear model predictions, optimize the selection of bearing materials, and contribute to the ongoing efforts to produce the first international standard for in vitro wear testing of shoulder prostheses