A carbon fiber-reinforced polyetheretherketone (PEEK) composite acetabular cup was fabricated by injection molding. An extensive hip joint simulator test was conducted to evaluate the tribological performance of this CF-PEEK composite cup as a potential orthopaedic bearing surface. Hip joint simulator testing up to 10 million cycles showed that the wear of the composite cups is about 1% that of the UHMWPE cups. Hertzian contact stress analysis indicated that the CF-PEEK material had a lower maximum contact stress/yield strength ratio compared to air-irradiated UHMWPE acetabular cups. With prudent application of composite design principles, it is possible to engineer a composite insert that shows promise as an orthopaedic implant bearing surface in total hip replacements.
Linear reciprocating pin-on-plate-type wear testing has been a standard technique for the screening of orthopaedic implant materials since the early 1980s. This investigation compares a wear screening technique based on linear motion with a modern hip joint simulator based on multi-axial motion. Two groups of differently sterilized UHMWPE samples were tested. The first group of samples was sterilized by ethylene oxide (EtO) gas that caused no structural changes in the UHMWPE. The second group of samples was sterilized in nitrogen by gamma-irradiation and then subjected to a stabilization treatment that resulted in a significant level of crosslinking in the UHMWPE. When tested on the linear reciprocating wear machine, the EtO sterilized specimens (non-crosslinked linear polyethylene) showed an approximately 30% lower wear rate than the gamma-irradiated and stabilized specimens (crosslinked polyethylene). When tested on the hip simulator, the EtO sterilized specimens exhibited two to three times higher wear rates than the gamma irradiated and stabilized specimens. The ranking of wear resistance obtained with the hip simulator was strikingly different than that obtained with the linear reciprocating wear machine. This study indicates that screening wear machines based on linear motion do not correlate with multi-axial joint simulators and may produce misleading results in the prediction of clinical wear performance of UHMWPE bearing materials.
The wear of ultra-high molecular weight polyethylene is one of the most important factors in the longevity of total joint replacement prostheses. This paper investigates the morphological changes in UHMWPE induced by plastic deformation and wear. A plasma etching technique was used to reveal the lamellar structure and its crystalline orientation in UHMWPE. Specimens under examination included unconsolidated UHMWPE powder, freeze fractured surfaces, microtomed slices, surfaces of tensile stretched specimens, shear fracture surfaces, and worn surfaces of acetabular and tibial components that had been tested on hip and knee joint simulators. For the unconsolidated powder and the freeze fracture surfaces, the orientation of the lamellae on the surface was random. For the surfaces of microtomed slices and tension specimens, the orientation of the lamellae was perpendicular to the direction of microtoming and tensile deformation. X-ray diffraction of the tension specimen indicated preferential orientation of the c-axis of the orthorhombic structure in the stretching direction. A similar orientation phenomenon was observed on the worn surfaces of acetabular and tibial components. A sequential biaxial tension test was conducted to study the effect of molecular orientation on the longitudinal and transverse strength of UHMWPE. An orientation-induced softening phenomenon was observed when the UHMWPE was stretched in a direction perpendicular to the molecular chains. Based on these observations, an orientation softening wear model was proposed. The model states that molecular orientation induced by plastic deformation during joint articulation is undesirable for the wear resistance of UHMWPE acetabular and tibial components.
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