Wear Rates of 32 mm and 40 mm Glenospheres In A Reverse Total Shoulder Arthroplasty In Vitro Wear Simulation Model
Purpose: Larger glenosphere diameters have been shown to increase prosthesis stability and impingement-free range of motion in reverse total shoulder arthroplasty (rTSA). Higher wear rates increase the potential for polyethylene wear-induced osteolysis and aseptic component loosening. The goal of this study is to evaluate the rate of polyethylene wear for 32 mm and 40 mm glenosphere sizes utilized in rTSA. It is hypothesized that increased polyethylene wear occurs with larger glenosphere diameters.
Methods: Twelve commercially available CoCrMo glenospheres (size 32 mm and 40 mm, n = 6/group) and their respective humeral liners were subjected to 5 million cycles (MC) of a previously established wear simulation protocol, representing 5-7 years of device life. Two motion profiles, abduction-addiction (20-618N) and flexionextension (20-927N), were alternated every 250,000 cycles. Individual stations were enclosed in bowls filled with bovine calf serum as lubrication. Every 250,000 cycles, liners were removed and mass loss was determined gravimetrically according to ISO Standard 14242-2 and converted to volume loss and volumetric wear rate (VWR). At 0, 2.5, and 5 MC, liners were imaged using micro-computer tomography (CT) to determine linear surface deviation of the bearing surface due to wear. Liner volumes were isolated from CT scans, volumes were co-registered, and surface deviations between time points were mapped across the entire bearing surface and by quadrant. White light interferometry was also performed at five locations of the humeral liners at 0, 2.5, and 5 MC to determine changes in micro-scale surface roughness. Wear particles were isolated from test serum from both motion profiles at the beginning and conclusion of testing and imaged via scanning electron microscopy (SEM) for characterization of wear particle morphology
Results: Total volume loss was significantly higher in 40 mm liners at 0.5 MC and all time points from 1 MC onward (Fig. 1, A). VWR was significantly higher in 40 mm liners for all flexion-extension time points (0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 MC) as well as at 1.25 and 2.75 MC abduction-adduction time points (Fig. 1, B). Overall, the flexion-extension motion profile induced higher VWRs than abduction-adduction for both liner geometries. Representative CT surface deviation maps are shown in Figure 1, C and demonstrate concentration of bearing surface deviation on the inferior aspect of the liner. Mean bearing surface deviation between 0 and 5 MC was significantly higher on 32 mm liners (0.35 ± 0.03 mm) compared to 40 mm liners (0.28 ± 0.01 mm, P = .002). Surface roughness (Sa) was 0.033 ± 0.003 m and 0.025 ± 0.003 m for 32 mm and 40 mm humeral liners, respectively, at 0 cycles and 0.022 ± 0.003 m and 0.022 ± 0.003 m at 5 MC cycles, respectively
Conclusion: Larger glenospheres result in significantly greater polyethylene volume loss and volumetric wear rates, while smaller glenospheres lead to greater polyethylene surface deviations. These results are consistent with the previously established relationships between femoral head size and volumetric and linear wear rates in total hip arthroplasty. The enhanced stability provided by larger glenospheres must be weighed against the potential for increased polyethylene wear and wear-induced osteolysis to reduce complication rates in rTSA.
Haggart JR, Newton MD, Ho A, Hartner S, Baker KC, Kurdziel MD, et al, [Wiater JM]. Wear rates of 32mm and 40mm glenospheres in a reverse total shoulder arthroplasty in vitro wear simulation model. Specialty Day Meeting of the American Shoulder and Elbow Surgeons, San Diego, CA. March 11-14, 2017.