A Nano-Composite Poly(Methyl-Methacrylate) Bone Cement
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components results in the formation of particle agglomerates of 50-200 ptm diameter. These large defects are sites of high stress concentration that reduce the fracture toughness of PMMA cements, leading to early fracture of the cement [1, 2] and loosening of the implant, ultimately necessitating early revision surgery to replace the implant. Previous studies have investigated this mechanism of failure by demonstrating a two-fold decrease in impact strength of bone cement with the addition of micro-meter size radiopacifying particles [3]. In this study, the micrometer size radiopacifying particles in PMMA bone cements were replaced by commercially available nanophase aluminum oxide particles. These nanocomposite PMMA bone cements were thereafter characterized using ultra-small angle x-ray scattering (USAXS) at the UNICAT beamline of the Advanced Photon Source, Argonne National Laboratory. Low-voltage scanning electron microscopy (LVSEM) was performed on the fracture surfaces of tensile specimens and on both micro- and nano-sized filler particles. These experiments provided the average inter-particle or inter-agglomerate distance between adjacent radiopacifier particles present in the cured cements. The nanocomposite and control PMMA bone cements were also subjected to ASTM standard tensile tests to determine their mechanical properties.
399 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
EXPERIMENT Specimen composition and preparation Commercially available PMMA bone cement used in orthopaedic surgery is commonly provided as a two component system; a powder component consisting of pre-polymerized PMMA, radiopacifying particles (Barium Sulfate or Zirconium Oxide, 10 wt.%) and Benzoyl Peroxide (BPO, 2 wt.%) as initiator; and a liquid component consisting of Methyl-Methacrylate (MMA) monomer, Hydroquinone (25 ppm) as stabilizer and N,N-Dimethyl-p-toluidine (DMPT, 2 wt.%) as reaction promoter. Both components are mixed, commonly in a vacuum mixing device to reduce air introduction, initiating a polymerization process with an average setting time of 10-15 min. For the experiment, OsteobondTM [t] (Zimmer Inc, Warsaw, IN) bone cement containing BaSO 4 radiopacifying particles of 1-3 pgm diameter was purchased as control. A separate PMMA powder component without radiopacifiers (Zimmer Inc, Warsaw, IN) was used for the nano-composites. Acrylic-coated aluminum oxide particles of 60 nm average diameter dispersed in ethanol were purchased (Nanophase Technologies Inc, Burr Ridge, IL). The particles were dried, re-dispersed in MMA, and mixed with PMMA for a final volume fraction of filler particles identical to that of radiopaque cements. Specimens for USAXS were molded as thin sheets of 0.5 mm thickness, while tensile testing utilized Type V dumbbell shaped samples as specified in ASTM D638-97 [4]. Mechanical Testing An Instron 4201 tensile tester was used to perform ASTM D638-97 standard tensile tests on all cement samples to determine their ultimate stress, ultimate strain and work-of-fracture (define
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