A multicrystalline porous form of ?-tricalcium phosphate [?-Ca3(PO4)2] has been used successfully to correct periodontal defects and augment bony contours. X-ray diffraction of ?-tricalcium phosphate shows an average interconnected porosity of over 100 ?m.
?-Tricalcium phosphate is prepared by a wet precipitation procedure from an aqueous solution of Ca(NO3)2 and NaH2PO4. The precipitate is calcined at 1150°C for 1 h, ground, and sieved to obtain the desired size particles for use as bone substitutes and for making ceramic matrix drug delivery systems. These particles are used as such or pressed into cylindrical shapes and sintered at 1150 to 1200°C for 36 h to achieve the appropriate mechanical strength for use as drug delivery devices. A scanning electron micrograph (500×) of a set and hardened TCP-cysteine composite is shown in Fig. 2.9. TCP is usually more soluble than synthetic hydroxyapatite and, on implantation, allows good bone ingrowth and eventually is replaced by endogenous bone.
FIGURE 2.9 Scanning electron micrograph (500×) of a set and hardened TCP–cysteine composite. The small white
cysteine particles can be seen on the larger TCP particles.
2.4 Bioactive or Surface-Reactive Ceramics:
Upon implantation in the host, surface-reactive ceramics form strong bonds with adjacent tissue. Examples of surface-reactive ceramics are dense nonporous glasses, Bioglass and Ceravital, and hydroxyapatites. One of their many uses is the coating of metal prostheses. This coating provides a stronger bonding to the adjacent tissues, which is very important for prostheses.
Glass Ceramics:
Glass ceramics used for implantation are silicon-oxide-based systems with or without phosphorous pentoxide. Glass ceramics are polycrystalline ceramics made by controlled crystallization of glasses. Glass ceramics were first utilized in photosensitive glasses, in which small amounts of copper, silver, and gold are precipitated by ultraviolet light irradiation. These metallic precipitates help to nucleate and crystallize the glass into a fine-grained ceramic that possesses excellent mechanical and thermal properties.
The formation of glass ceramics is influenced by the nucleation and growth of small crystals as well as the size distribution of these crystals. It is estimated that about 1012 to 1015 nuclei per cubic centimeter are required to achieve such small crystals. In addition to the metallic agents already mentioned, Pt groups, TiO2, ZrO2, and P2O5 are widely used for nucleation and crystallization. The nucleation of glass is carried out at temperatures much lower than the melting temperature. Deformation of the product, phase transformation within the crystalline phases, or redissolution of some of the phases should be avoided. The crystallization is usually more than 90% complete with grain sizes 0.1 to 1 ?m. These grains are much smaller than those of conventional ceramics. The bonding to bone is related to the simultaneous formation of a calcium phosphate and SiO2-rich film layer on the surface.
Ceravital:
The composition of Ceravital is similar to that of Bioglass in SiO2 content but differs somewhat in other components (see Table 2.13). In order to control the dissolution rate, Al2O3, TiO2, and Ta2O5 are added in Ceravital glass ceramic. The mixtures, after melting in a platinum crucible at 1500°C for 3 h, are annealed and cooled. The nucleation and crystallization temperatures are 680°C and 750°C, respectively, each for 24 h. When the size of crystallites reaches approximately 4 ? and the characteristic needle structure is not formed, the process is stopped to obtain a fine-grain-structured glass ceramic.
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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