Hydroxyapatite/diamondlike Carbon Nanocomposites: A Novel Surface Modification to Extend Orthopaedic Prosthesis Lifetime
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Conventional plasma-sprayed hydroxyapatite coatings suffer from many difficulties that have limited their use in orthopaedic implants, including uneven resorption rates, poor fracture toughness, and poor adhesion to medical alloys. The placement of a diamondlike carbon buffer layer may overcome these obstacles by providing unique chemical inertness, hardness, and cell-interaction properties at the implant–tissue interface. Nanocrystalline hydroxyapatite and amorphous diamondlike carbon films were prepared by room-temperature pulsed-laser deposition of hydroxyapatite and graphite targets, respectively. Scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and microscratch adhesion testing were used to determine surface morphology, interfacial structure, and adhesion of the bilayer coatings. Nanocrystalline hydroxyapatite/diamondlike carbon coatings have several potential orthopaedic applications, including use in hip prostheses.
I. INTRODUCTION
The implantation of hip prostheses over the past 40 years has revolutionized the treatment of osteoarthritis, rheumatoid arthritis, ankylosing spondylitis, large joint trauma, and other orthopedic conditions.1 In conventional hip prostheses, a nearly inert metal [cobalt– chromium–molybdenum alloy (ASTM F75), cobalt– nickel–chromium–molybdenum alloy (ASTM F562), or titanium–aluminum–vanadium alloy (ASTM F136)] surface articulates against an ultrahigh-molecular-weight polyethylene (UHMWPE) surface.2 These components are fixed in place using polymethylmethacrylate (PMMA) bone cement, which undergoes in situ polymerization during and immediately after implantation. According to the American Academy of Orthopedic Surgeons, approximately 120,000 hip replacement operations are currently performed each year in the United States.3 An inflammatory reaction known as “the foreign-body response” occurs when conventional implant materials are placed within the body.4 The foreign-body response is characterized by several vascular and cellular processes, including fibroblast proliferation, collagen synthesis, and blood-vessel proliferation. These events lead to
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0284 2288
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 18 Mar 2015
the formation of an avascular connective tissue periimplant capsule that consists of several different cellular layers, including an inner layer of macrophages, a concentric layer of fibrous tissue and fibroblasts (30–100 m), and an outer layer of vascularized tissue.5 In addition, micromotion of the implant materials produces a large number of wear particles. Polyethylene wear is estimated at 0.10–0.20 mm/year; in fact, some investigators have suggested that 100,000 polyethylene particles are released with each step.6 Wear of the metal component of the joint prosthesis also occurs; for example, cobalt–chromium–molybdenum alloy degrades at an average rate of 0.02–0.06 mm in 10 years. The meta
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