Abstract
Introduction
Comprehensive research and retrieval analyses of metal on metal / metal on polyethylene hip fretting and corrosion have been reported. Design choices such as modularity, material couples, geometry and offsets, as well as surgical variability and patient sensitivity have been cited as factors contributing to revision. Findings are informing new designs, surgical techniques and patient testing. However, similar efforts have not been performed on the shoulder. Do reduced joint reaction forces imply lower risk of fretting and corrosion? In this study we designed an accelerated corrosion fatigue (ACF) test specific for the shoulder to allow for evaluation of varying designs, and compared results to a reported shoulder retrieval study [Day ORS 2015].
Methods
Anatomic configuration and reverse shoulder ACF tests were developed with loads and orientations determined from instrumented shoulder data and reported literature. Scaled loads of 1480 N and 962 N were applied to anatomic (Fig 1.A) and reverse (Fig 1.B) prostheses, respectively (n=5 each, with additional assembly control), in potential worse case loading directions (α=25°, β=20°: anatomic; α=0°, β=0°: reverse), at 5 Hz for 3.0 Mc with R=0.1. Test environment included 0.9% NaCl solution at elevated temperature (50° C) and a decreased pH (3.5). Mass, roughness (Ra) and taper damage (modified Goldberg scoring system) measures were taken before and after testing. Taper connections were assembled at impact loads of 3600 N +/− 20% based on cadaveric studies. Goldberg scores for 79 humeral heads and 61 stems from an IRB approved collection served as the comparator.
Results
The ACF test methodology caused only fretting in anatomic testing (Fig 2) but both fretting and corrosion in reverse tests (Fig 3). Alloys of Titanium (Ti-6V-4Al and Ti-6Al-7Nb) exhibited higher levels of fretting than CoCr alloys in both test configurations. Significant Ra decreases post-test were noted on all Ti based components, with Ra increases on CoCr based heads and glenospheres (p<0.05). The average mass loss measured in anatomical testing was 1.44 ± 0.35 mg with an average Goldberg score of 1.8 ± 0.3. In the reverse test mass loss was 1.51 ± 0.36 mg with a Goldberg score of 2.4 ± 0.3. Retrievals (implanted 4.6 ± 4.3 years) had weighted average Goldberg scores of 1.6 for heads compared to 2.0 for stems, with the majority of damage from fretting.
Discussion
Anatomic and reverse shoulder ACF tests were created that result in levels of taper damage similar to those noted in clinical retrievals (Fig 2, Fig 3). Reverse implant tapers, under reduced loads compared to anatomic but with higher humeral load offset, tend to show higher levels of taper damage in vitro. Under the same assembly conditions, offset is a large contributor to micromotion, fretting, and likely corrosion. With offset and modularity as design options for the shoulder surgeon, care must be taken not to introduce offerings which may lead to significant metal and particle release in vivo. The proposed ACF testing may be one method to evaluate current and proposed shoulder designs.
