Effects of Metal Ions on Apatite Formation and Bone Mineralization
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EFFECTS OF METAL IONS ON APATITE FORMATION AND BONE MINERALIZATION N. C. Blumenthal, Ph.D. V. Cosma, M.S. Department of Bioengineering Hospital for Joint Diseases Orthopaedic Institute 301 East 17th Street New York, NY 10003
INTRODUCTION Certain metals (such as Al) are known to cause bone pathologies in humans and animals. While little is known about the mechanism of action of metals on either the formation of bone or on the physical-chemical properties of the mineral phase, there is considerable evidence that (1) Al and Cd directly affect the formation and properties of HA and thus are a causative factor in metal-related defective bone formation-in addition to any cellular effects which they may induce; (2) that other transition metals (such as Cr, V, Ti, Ni, Co), which are used in joint prostheses, significantly affect HA formation; and (3) gallium, the most effective antihypercalcemic agent, affects apatite formation and solubility. In recent years the role of aluminum in producing a vitamin D-resistant osteomalacia in renal hemodialysis patients has received considerable attention [1,2]. The aluminum ions enter from the dialysate water into the blood, where they are presumed to bind initially to plasma proteins. Another source of aluminum is the antacids given to dialysis patients to help control hyperphosphatemia. Aluminum accumulates at the mineralization front in these osteomalacia patients, as has been demonstrated by histochemical staining, electron probe x-ray microanalysis, and secondary ion mass spectrophotometry. When osteomalacia patients with localized bone aluminum are treated with desferioxamine (DFO), a known chelator of iron and other cations, serum levels of free aluminum initially rise [3] as the bone aluminum is complexed and removed. In a considerable number of the DFO-treated patients, bone biopsies revealed that newly formed bone matrix mineralized normally, with an attendant decrease in bone pain and a later decrease in fracture rate. When rats are injected with aluminum chloride, their serum aluminum levels rise and they develop an osteomalacia resembling that seen in dialysis patients. Bone biopsies of the rats clearly show unmineralized osteoid and a localization of aluminum at the mineralization front [4]. Vanadium, titanium, nickel, cobalt, and chromium are metals that are widely used in alloys from which various prosthetic devices are manufactured. In light of the large number of joint replacement operations performed every year and the ever-increasing length of time that prosthetic devices remain in the body, determining the fate and consequences of wear particles from these prostheses becomes ever more urgent. One study, for example, Mat. Res. Soc. Symp. Proc. Vol. 252. '1992 Materials Research Society
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not only demonstrated phagocytosis of wear particles from hip joint prostheses by fibrous capsule cells, but also described an entire series of subsequent stages of inflammation as a response to the wear particles [5]. The net biochemical result of these tissue responses to sm
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