EDGE LOADING WEAR DUE TO INCLINATION ANGLE FOR THREE CONTEMPORARY HIP BEARINGS

Abstract

Steep angles (> 55°) reduce femoral head coverage decreasing contact area and can subject the acetabular rim to excessive stresses. In the case of metal-metal implants it has been shown that at steep angles there is no bedding-in of the implants and run-away wear occurs. The dual mobility bearing concept mates a metal femoral head with a polyethylene liner that is free to articulate inside a polished metal shell. Previous work has shown acetabular wear can be minimized with this design, possibly through reduction of total amount of cross-shear motion in the joint. An additional potential benefit may exist through the maintenance of conforming contact and head coverage even under high inclination angle. This study evaluates the influence of inclination angle on the wear performance of three hip bearing designs. Four sets of dual mobility implants, three sets of metal-on-metal hip implants, and five sets of fixed hip implants were evaluated per inclination angle. All polyethylene components were made of GUR 1020 UHMWPE that was sequentially crosslinked and annealed three times (X3). The MoM components were fabricated from high carbon cast CoCr as per ASTM F75 (no heat treatment). A hip joint simulator was used for testing for a total of 2.5 million cycles with the cups oriented at either 35° or 65° of abduction. Testing was run at 1Hz following Paul curve physiologic loading and statistical analysis was performed using the Student’s t-test (p< 0.05). results for the 35 degrees of inclination angle condition show no statistical difference between any of the testing combinations with X3 polyethylene showing immeasurable wear. At this angle wear of the MoM devices was similar, although ion levels were not measured. results for the 65 degree condition showed an increase for the fixed PE and MoM systems. The increase in fixed PE bearing wear is consistent with previous findings and still within noise level values. The increase in MoM wear was substantial with both heads and cups showing scratches and abrasion damage related to edge contact. There is a statistically significant wear rate reduction (p< 0.05) of over 94% for both the dual mobility and fixed bearing PE constructs when compared to MoM. When comparing wear rates of the dual mobility system to the standard fixed acetabular bearing, the dual mobility device exhibited an 85% (p< 0.05) reduction in wear rate. The results of this study support our hypothesis that acetabular wear at high angles can be diminished through design. This is likely due to maintenance of the nature of the primary inner bearing contact regardless of shell positioning. Based on these results this dual mobility construct can be expected to outperform a conventional fixed construct and a metal-on-metal construct in terms of wear at high inclination angles, without any of the metal ion release concerns.