Meng Deng

Study of creep behavior of ultra-high-molecular-weight polyethylene systems

The short- and long-term creep behaviors of ultra-high-molecular-weight polyethylene (UHMWPE) systems (compression-molded UHMWPE sheets and self-reinforced UHMWPE composites) have been investigated. The short-term (30-120 min) creep experiment was conducted at a load of 1 MPa and a temperature range of 37-62 degrees C. Based on short-term creep data, the long-term creep behavior of UHMWPE systems at 1 MPa and 37 degrees C was predicted using time-temperature superposition and analytical formulas. Compared to actual long-term creep experiments of up to 110 days, the predicted creep values were found to well describe the creep properties of the materials. The creep behaviors of the UHMWPE systems were then evaluated for a creep time of longer than 10 years, and it was found that most creep deformation occurs in the early periods. The shift factors associated with time-temperature superposition were found to increase with increasing temperature, as per the Arrhenius equation. The effects of temperature, materials, and load on the shift factors could be explained by the classical free volume theory.

Long‐term γ irradiation effects on ultrahigh molecular weight polyethylene

Effects of γ irradiation, irradiation environment, and long-term postirradiation aging on thermal behavior, crystallinity, mechanical properties, and weight change of ultrahigh molecular weight polyethylene (UHMWPE) were investigated. The γ irradiation was conducted at 2.5 Mrad under four different environments, i.e., air, nitrogen, acetylene, and vacuum. The postirradiation aging effects were monitored over a period of 5.5 years. The results showed that γ irradiation, irradiation environments, and postirradiation aging significantly changed the properties of UHMWPE. After γ irradiation, the melting temperature and crystallinity of UHMWPE were increased in all cases, with the highest increase at longest postirradiation aging time. γ irradiation and postirradiation aging caused weight gain of UHMWPE in all conditions. The tensile and flexural properties were significantly affected by γ irradiation. At 5.5 years postirradiation, tensile-tested UHMWPE specimens fractured in brittle form, indicating dramatic reduction in the materials toughness. This kind of behavior may be detrimental to UHMWPE load-bearing orthopedic devices for their intended long-term applications.

A Differential Scanning Calorimetry Study of Retrieved Orthopedic Implants Made of Ultrahigh Molecular Weight Polyethylene

Differential scanning calorimetry (DSC) was used to examine thermal and thermooxidative properties of ultrahigh molecular weight polyethylene (UHMW-PE) of five acetabular components of failed orthopedic implants retrieved at revision of total hip arthoplasty. The results were compared with controls (unimplanted acetabular cups, a 20-year-old slab of UHMW-PE, and raw material). Profiles of exothermic peaks indicated increased levels of oxidation in all retrieved cups. In three retrieved cups, DSC revealed an additional peak of endotherm that was not seen in control samples. The additional endotherm peaks were not artifacts due to oxidation during scanning, heat buildup during cutting of the samples, or the sterilization method after retrieval. The additional peak was associated with the bulk of the polymer that was extracted with hexane. It varied in relative area, depending on its original location of the sample in a cup, implicating local variability in the extent of changes in material property. The distribution of the changes suggests that, during implantation, tissue exposure and friction affected the level of oxidation and degree of crystallinity in the UHMW-PE to a greater degree than did loading alone. Overall results showed that DSC may be a useful tool in evaluating changes in the properties of UHMW-PE orthopedic components in vivo.

Effects of gamma irradiation, irradiation environment, and postirradiation aging on thermal and tensile properties of ultrahigh molecular weight polyethylene fibers

The effects of gamma irradiation in four types of irradiation environment on the thermal and tensile properties of gel-spun, ultrahigh molecular weight polyethylene fibers (Spectra™ 1000) have been investigated. The gamma irradiation was conducted at 2.5 Mrad and in air, nitrogen, acetylene, and vacuum to study the effects of irradiation media on the aforementioned properties. Thermal and tensile properties of virgin and irradiated fiber samples were examined using differential scanning calorimetry and an Instron tensile tester, respectively. The results indicate that both gamma irradiation and irradiation environment affected the properties of the polyethylene fibers, and substantial changes were observed for the oxygen-containing environment. The tensile-fractured surfaces of the fibers were examined by scanning electron microscopy. The properties of irradiated fibers were further evaluated at 160 days postirradiation and found to be affected, substantially. The postirradiation aging significantly decreased the tensile strength and elongation of the irradiated fibers, indicating that polyethylene fibers should not be exposed to gamma irradiation.