We noted that the successful use of tricalcium phosphate (TCP), with the chemical formula Ca3(PO4)2, as a bioceramics as early as 1920 (Albee and Morrison 1920). It was also noted that in numerous studies on calcium hydroxide, this ceramic salt tended to stimulate the formation of immature bone and was an example of an unsuccessful bioceramics (Hulbert et al. 1982–83) Today, TCP remains as a useful bioceramic and a good example of the “resorbable” category of bioceramics. The use of TCP in conjunction with hydroxyapatite (HA) is described in the previous section .
5.6
SILICATE CERAMICS AND GLASSES
Silicates represent the dominant category of the traditional ceramics and glass industries (Shackelford 1996). These materials are economical due to the abundant availability of raw materials. Also, silicates provide adequate mechanical, thermal, and optical properties for a wide range of traditional and advanced materials applications. The specialized requirements of biomedical applications, however, make silicates less significant as bioceramics. One should recall that ceramics and glasses are distinguished primarily by the presence respectively of crystalline or noncrystalline structure on the atomic-scale. For crystalline silicate ceramics, biomedical applications have been relatively negligible. For noncrystalline silicate glass, biomedical applications have been more significant because of the development of Bioglass, the classic example of the “surface reactive” category of bioceramics .
Bioglass can be called a “bioactive” material. It has been shown to bond to bone, and specialized compositions can even bond to soft tissues. These bioactive materials typically undergo a surface modification upon implantation, forming a biologically active hydroxycarbonate apatite (HCA) layer which provides the bonding interface with the tissues. The comparable chemical structure in HCA and the mineral phase of bone accounts for the tendency for interfacial bonding in that case. The bonded interface provides substantial strength. Often, the interfacial strength is greater than the cohesive strength of either the adjacent ceramic or tissue. , many bioactive silica glasses have been based on a
composition labeled 45 S5 containing 45 wt% SiO2, 24.5 wt% CaO, 24.5 wt% Na2O, and 6 wt% P2O5, noticeably lower in silica and higher in lime and soda than conventional window and container glasses. Also distinctive in comparison to traditional silicate glasses is the significant phosphate component, P2O5, which plays a critical role in the bioactivity. Hench (1993) and co-workers have determined that a molar ratio of at least 5:1 CaO : P2O5 is desirable to ensure that the Bioglass surface bonds to bone. In general, biologically active glasses contain less than 60 mol% SiO2, relatively high Na2O and CaO contents, and a CaO:P2O5 ratio greater than 5:1. Bioglass implants based on the 45S5 composition have been successfully applied in a variety of dental and medical applications. For example, certain ear bones have been replaced, and, for denture wearers, the 45S5 material has been used t
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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