Sensitivity of wear coefficient to dimensional tolerance in shoulder implants

The geometry of joint implants is surely one of the factors that mostly influence the wear process, as it directly affects contact and lubrication conditions. Usually only the implant size and the nominal clearance are considered in the literature. However, as recently demonstrated by the authors for reverse shoulder implants [1] the dimensional tolerance can play a major role. Nevertheless, the real implant geometry is generally not reported in research studies or in standards. The present study aims to investigate the sensitivity of wear and, particularly, of the wear coefficient (k) to implant dimensional tolerance. Five samples of shoulder prosthesis were tested in a joint simulator [2,3]. The initial and worn geometry of components were measured using a coordinate measuring machine. The wear model presented in [1,2] was used to estimate specific k according both to the Archard law and to the wear law for UHMWPE. Experimental and numerical wear maps were compared in order to validate the wear laws, a point still largely debated in the literature. Preliminary results suggest that the dimensional tolerance can explain the high dispersion (up to 25%) of wear volumes for implants with the same nominal size. [1] JMBBM,61(2016)455-463. [2] JMBBM,55(2016):53-66. [3] J Biomech,48(2015):3072-9.