Effect of extent of motion and type of load on the wear of polyethylene in a biaxial hip simulator

The most commonly used wear test device for prosthetic hip joints is the so-called biaxial rocking motion (BRM) design. The design has been criticized for its excessive sliding distance per cycle. The design was modified so that the extent of motion was reduced from 46 to 23 degrees, and comparative tests were run with the use of 1-kN static load. The present authors have earlier used 1-kN static load with good results. To further confirm this finding, additional, comparative tests were run with double-peak dynamic load and 46 degrees extent of motion. All three tests (46 degrees /static, 23 degrees /static and 46 degrees /dynamic), were done with conventional ultra-high-molecular-weight polyethylene acetabular cups against polished CoCr femoral heads in diluted calf serum lubricant. In all tests, the principal cup wear mechanism was adhesive, manifested as burnishing. With respect to wear particles, those produced in the 46 degrees /dynamic test showed the lowest resemblance to particles isolated from periprosthetic tissues. The 46 degrees /dynamic test produced a mean wear rate 43% higher than 46 degrees /static, whereas the 46 degrees /static and 23 degrees /static tests produced almost identical values. The results indicated that the 46 degrees extent of motion and dynamic load may not always be the optimal combination in BRM tests.