Stimulation of Biological Function With Bioactive Glass
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MRS BULLETIN/NOVEMBER 1998
cium phosphate ceramics and glasses, either by themselves or as carriers for bone (or "osteogenic") cells or various bone-growth factors, are also prime candidates for these applications. In this article, we will discuss key properties of these reactive materials with an emphasis on the properties of the glasses. We will also illustrate unique opportunities in orthopedic care as well as applications in tissue engineering that have been brought about by progress in the understanding of the mechanisms by which these materials affect cells and biological function. Lack of space prevents us from discussing other interesting medical uses of glasses such as the use of glasses incorporating radioactive isotopes for 3 in situ radiation treatment of cancer. Bone Bioactivity Per definition, bone bioactive materials promote bone-tissue formation at their surface and bond to surrounding bone tissue. Using transmission electron microscopy, Beckham et al. documented the 4 bonding between BG and bone tissue. This glass was formulated with a composition to be reactive. Among the first BGs that were synthesized, the so-called 45S5 glass with a composition (in wt%) of 45% SiO 2 , 24.5% CaO, 24.5% Na 2 O, and 6% P2O5 stands out.5 As far as the enhancement of bone-tissue formation is concerned, we first presented data in 1977 that underscored the beneficial effect of the bioactive ceramic lining on the rate of bonding of porous coated mate6 rials. We used a commercially available hydroxyapatite (HA). This is a ceramic that in its perfect form corresponds to
the chemical formula When we performed our experiments, data started to appear supporting a bonding phenomenon to this material following a similar bonding phenomenological pathway for the bonding between BG and bone tissue.7"9 When a porous-metal fiber network was coated with a slip-cast HA lining, bone ingrowth was significantly larger than in the case of the same porous metal without the HA lining. This effect was pronounced at two and four weeks, creating the distinct therapeutic benefit of allowing for faster rehabilita10 tion of the patient. This work and stud15 ies by others since"" have led to the clinical use of plasma-sprayed coatings on hip and knee prostheses. BG Granules as Synthetic Bone Grafts Among all materials that affect bonetissue formation, the effect of the previously mentioned 45S5 BG is greatest, as was shown by Hench who reviewed literature data of comparable experiments ,6 with different materials. This extensive effect on bone-tissue formation renders BG an attractive synthetic graft to treat the numerous conditions of bone and bone-growth deficit. Because the effect of the glass is mediated by a reactive surface, it would appear that a granular material is most useful because the surface area affecting bone growth is much greater than that with blocks of the glass. In one approach with a granular-glass preparation, it was found that glass granules with uniform size distribution between 90 μ-m and 710 μm led to the repair of t
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