Dual mobility bearings withstand loading from steeper cup-inclinations without substantial wear.

Steep cup abduction angles with adverse joint loading may increase traditional polyethylene bearing wear in total hip arthroplasties. However, there have been few reports evaluating the effect of cup inclination on the wear of dual-mobility devices. In a hip joint simulation, we compared the short-term wear of two-sizes of modular highly cross-linked dual-mobility bearings (28 mm femoral head diameter/42 mm polyethylene insert outer diameter/54 mm acetabular shell diameter; 22.2 mm femoral head diameter/36 mm polyethylene insert outer diameter/48 mm acetabular shell diameter) at 50 and 65° of cup inclination with modular 28 mm femoral head on 54 mm cup diameter metal-on-highly cross-linked polyethylene bearings. Increasing inclination from 50-65° had no changes in volumetric wear of 28/42/54 mm (mean, 1.7 vs. 1.2 mm3 /million cycles, respectively; p = 0.50) and 22.2/36/48 mm (mean, 1.7 vs. 1.2 mm3/million cycles, respectively; p = 0.48) dual mobility bearings. At 65°, 22.2/36/48 mm dual-mobility bearings had lower volumetric loss (mean, 2.2 vs. 6.3 mm(3) ; p = 0.03) and wear rates (mean, 1.2 vs. 2.7 mm3/million cycles; p = 0.02) compared to metal-on-highly cross-linked polyethylene bearings. Modern-generation dual-mobility designs with highly cross-linked polyethylenes may potentially withstand edge-loading from steeper cup-inclinations without substantial decreases in wear.

Short-term wear evaluation of thin acetabular liners on 36-mm femoral heads.

BACKGROUND: Dislocation remains the leading cause of revision THA. One approach to decreasing prosthetic dislocation risk has been the use of larger femoral head component sizes. The upper limit of head size in metal-on-polyethylene hip arthroplasty has historically been limited because of concerns about increased wear on thin polyethylene components. It is not known to what degree this concern should apply to more wear-resistant polyethylene components. QUESTIONS/PURPOSES: We therefore determined (1) in vitro wear rates of polyethylene liners of varying thicknesses, (2) whether there were differences in the microscopic wear characteristics as a function of polyethylene thickness, and (3) whether thin polyethylene components resulted in early catastrophic failures. METHODS: We used a hip wear simulator to compare the wear performance of 12 highly crosslinked polyethylene acetabular inserts. The internal diameter of all components was 36 mm, and there were three samples tested of each thickness (1.9, 3.9, 5.9, or 7.9 mm). Testing was conducted for 2.4 million cycles. Gravimetric mass loss was converted to volumetric loss, which was subsequently converted to theoretical linear penetration rates. RESULTS: Wear rates decreased with increasing polyethylene thickness. Mean ± SD wear rates for the 1.9-, 3.9-, 5.9-, and 7.9-mm groups were 5.0 ± 0.5, 3.2 ± 0.3, 2.5 ± 1.1, and 2.2 ± 1.3 mm(3)/million cycles, respectively (p < 0.016). Calculated penetration rates were 0.015, 0.012, 0.011, and 0.010 mm/million cycles, respectively (p < 0.016). There were no catastrophic failures in any group. CONCLUSIONS: Thinner polyethylene components demonstrated higher wear rates, although even the highest wear rate observed in the thinnest polyethylene specimen was lower than that commonly reported for noncrosslinked polyethylene components. While encouraging, these findings should be validated in vivo before clinical recommendations can be made.

Wear performance evaluation of a contemporary dual mobility hip bearing using multiple hip simulator testing conditions.

The dual mobility hip bearing concept combines a small bearing with a large diameter bearing through a dual articulation system, potentially increasing the stability of the hip. Bearings with two articulations introduce concerns of whether or not wear might be increased compared to a conventional bearing. We therefore evaluated the wear performance of a dual mobility hip bearing using sequentially cross-linked and annealed polyethylene under the conditions of impingement, abrasion, and when the mobile liner becomes immobilized at either the inner or outer diameter. We found the wear performance of this dual mobility hip is dictated by the conditions experienced by the smaller inner articulation and by the polyethylene material. The highest wearing group wore 75% less than a single articulating conventional gamma/inert polyethylene bearing.