No abstract

The wear, delamination, and fatigue resistance of artificially aged gamma irradiation-sterilized conventional polyethylene (CPE) and gas-plasma-sterilized melt-annealed highly crosslinked polyethylene tibial inserts (HXPE) were compared. Six CPE and 12 HXPE (six irradiated at 58 kGy and six at 72 kGy) left knee inserts were wear tested for 5.5 million cycles (Mc) under loads and motions that mimic activities of daily living, such as walking, chair rise, stair ascent, and deep squatting. Another six HXPE (72 kGy) and six CPE inserts were also tested under conditions that could produce severe delamination for 8 Mc. Ten other knees (five 72 kGy HXPE and five CPE) were subjected to posterior edge loading fatigue testing for 5 Mc. The HXPE inserts had an average wear rate reduction of about 80% relative to their CPE counterparts during all activities. All of the CPE inserts delaminated and fractured during high cycle deep squat (1528 flexion) motions, while all the HXPE remained intact. None of the HXPE inserts delaminated after 8 Mc, while all of the CPE inserts developed delamination damage within 1.5-5. Keywords: wear; delamination; oxidation; knee; highly crosslinked UHMWPE Total knee arthroplasty (TKA) has been successful in restoring function and relieving pain that results from osteoarthritis. A consensus statement developed by the National Institutes of Health reports that success of TKA is supported by >20 years of follow-up data with about 90% of patients experiencing an improvement in pain and functional status. 1 Many authors report 10-15-year implant survival rates of >90%. 2 However, wear, delamination, material degradation, and the potential for subsurface cracking of ultra high molecular weight polyethylene (UHMWPE) in combination with patient weight, activity level, and surgical technique issues are factors leading to surgical revision. 34 Major improvements have included sterilization of polyethylene components in oxygen-free environments, net-shape compression molding, and, most recently, the introduction of highly crosslinked materials to improve wear and delamination resistance. Despite these improvements, concerns still persist because radiation sterilization generates macroradicals that are precursors of oxidation. 5 Polyethylene acetabular liners and tibial inserts that had been radiation-sterilized in a low-oxygen environment underwent similar in vivo oxidative mechanisms as polyethylene components that had been radiation-sterilized in air. 56 Wear accounted for several surgical revisions of oxygen-free sterilized tibial bearings. 7 Electron beam irradiation crosslinking and melt annealing of UHMWPE has been shown to reduce the wear of total hip arthroplasty (THA) in both in vitro simulator tests [8][9][10] and in vivo clinical studies. 1112 This reduction was attributed to the fact that crosslinking reduces the strain softening phenomenon that arises from multidirectional shear stresses acting on UHMWPE during articulation. 13 UHMWPE tibial knee inserts are also subjected to mult...

The microstructure of a macrodefect-free (MDF) cement has been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and highresolution electron microscopy (HREM). The microchemistry of ultramicrotomed samples has been studied by energy dispersive spectrometry (EDS) and parallel electron energy loss spectrometry (PEELS). MDF cement consists of CaAI2O4 and CaAI4O7 grains randomly distributed in a polymer matrix. The ceramic/polymer interface contains an amorphous interphase inside of which are distributed very fine crystallites of the metastable hydration product Ca2AI2O5 -8 H 2 0 . PEELS analyses of the interphase revealed the presence of carbon, indicating that Ca2A1205 * 8 H 2 0 was most likely stabilized by the intercalation of polymeric chains into its basal interlayers. The polymer phase preferably cross-links with Al. In situ environmental cell electron microscopy showed that moisture uptake of MDF cements occurred by polymer swelling and interphase dissolution. The role of the interfacial interphase in dry and wet mechanical properties is discussed. [

The Pechini process was used to produce high-purity, monocalcium aluminate (CaA1,O.J powders at temperatures as low as 900°C. Absorption spectrometry and BET measurements revealed particles with sizes ranging from submicrometer to 100 pm, with specific surface areas as high as 10 m'/g. Auger electron spectrometry (AES) was used to study the progressive elimination of surface carbon from the organic burnout as a function of temperature. The growth kinetics of calcium aluminate crystallites from a polymeric precursor were studied during calcination as a function of temperature, using transmission electron microscopy (TEM). At the early stages of crystallization, the activation energy for crystallite growth was found to be 118 kJ/mol. This is substantially less than the 356 kJ/mol previously reported. A growth kinetic exponent of n = 1.68 was determined for the amorphous-to-crystalline transformation in the temperature range 700" to 850°C. These values were consistent with growth by short-range diffusion.