Characterization of Mineral Deposits on Plasma-Sprayed Ha-Coated Ti-6Al-4V
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*Massachusetts Institute of Technology, Cambridge, MA 02139 "*Brigham and Women's Hospital, Boston, MA 02115
ABSTRACT Plasma-sprayed hydroxyapatite (PSHA) coatings are often used as the attachment vehicle for the femoral component of a total hip prosthesis. The key to successful hip replacement is the attachment of the femoral stem to the surrounding bone. Within hours of implantation of PSHA-coated Ti-6AI-4V rods in canine bone, bone mineral deposits on the HA coating. These deposits take the form of globular networks of fibrous crystallites. The crystallinity of these deposits was confirmed via high-resolution transmission electron microscopy (TEM) and electron diffraction. We have also characterized the composition of these deposits using x-ray energy-dispersive spectroscopy (XEDS) in the scanning TEM. Finally, we have studied the morphology of the fiber-like crystallites by stereography in the TEM. INTRODUCTION The attachment of the prosthesis stem to surrounding bone is crucial to the success of a hip replacement. Some femoral stems currently utilize plasma-sprayed hydroxyapatite (PSHA) as an attachment vehicle. Prior studies have shown that bone has grown over 90% percent of the PSHA coated surface of a Ti-6A1-4V rod within 14 days of implantation in
beagles, while only 20% of the surface was bone bonded after 6 days [1]. In this study, we characterized the early deposits (3 hours and 6 days post-implantation) of mineral on the surface of a PSHA coated Tialloy rod using TEM. Selected area electron diffraction (SAED), x-ray energy-dispersive spectroscopy (XEDS), stereography, and high resolution electron microscopy (HREM) were used to characterize the morphology and crystal structure of the deposits and compare them to the mineral deposits in mature bone. Electron microscopic and x-ray diffraction studies of bone have demonstrated that bone mineral appears as both needle-like and plate-like crystals 5 to 10 nm wide, 1.5-5 nm thick, and 50-100 nm in length [2,3]. EXPERIMENT PSHA coated (Bio-coat, Southfield, MI) titanium alloy cylindrical rods, 6 mm in diameter, were implanted into cancellous bone in the distal
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Mat. Res. Soc. Symp. Proc. Vol. 414 01996 Materials Research Society
femur and proximal tibia of adult mongrel dogs. Animals were sacrificed at 3 hours, 2, 6, and 14 days. Bone containing the implant was removed, fixed in formalin, and embedded in PMMA. Unimplanted PSHA coated rods were embedded in Araldite resin and prepared for TEM in the same manner as the implanted specimens. Three-millimeter diameter grooves were machined into 1 mm thick transverse sections of the implants. Steel tubing was epoxied into the grooves and sections polished until the 3 mm diameter sections were freed. The sections were mechanically thinned to 3 0-40 l.tm with a 1 l~tm finish on both sides using 600 grit paper, 1000 grit aqueous Si-C slurry, and 6 and 1 •tm diamond pastes. A single slot copper TEM grid was epoxied to the sample, and the specimen was ion-milled to electron transparency on a cold-stage. The sampl
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