Large bone defects can be defined as centimeter-scale gaps in the skeletal system . Historically, such defects have been repaired by harvesting bone from another part of the body (autogenous bone grafting or ?autografts? ) or using cadaver bone (?allografts?) . The harvest of an autogenous bone graft carries significant morbidity and cost which makes an off-the-shelf synthetic bone attractive (Younger and Chapman 1989). Allografts have problems with immunologie reaction and the risk of acquiring diseases transmissible by tissues and fluids. These limitations and concerns created substantial interest in the development of materials as bone graft substitutes or extenders. A pioneering example of hydroxyapatite and tricalcium phosphate for the repair of large bone defects was outlined in and is described in greater detail by Mclntyre, et al. (1991). In Section 4.1 an alternative material was introduced, viz., a so-called “coralline hydroxyapatite.” It was manufactured from coral by a thermo chemical process which converts the calcium carbonate manufactured by the marine organism to a calcium phosphate (hydroxyapatite). As seen in Figures 2.2 and 4.1, an attractive feature of using the coral-route is that it has an open porous structure which is ideal for accommodating bone ingrowth. Another recent development for bone repair utilizes a novel in situ ceramic processing technique (Constantz et al. 1995). The hydroxyapatite (HA) formed by conventional ceramic processing techniques tends to be more dense, coarse grained, and less fatigue-resistant than HA formed in vivo. A more natural HA can be produced by the surgical implantation of a paste that hardens in minutes under physiological conditions. The paste is produced by adding a sodium phosphate solution to a mixture of monocalcium phosphate monohydrate
[MCPM, CaH2PO4)2 • H2O], -tricalcium phosphate [TCP, Ca3(PO4)2] and calcium carbonate (CC, CaCO3). The paste is inject able for about 5 minutes and maintains physiologic temperature and pH. The injected paste hardens due to the crystallization of dahllite within about 10 minutes. Dahllite is a carbonated hydroxyapatite. The specific formula for the crystallized paste in this system is Ca8. 8(HPO4)0.7 (PO4)4.5 (CO3)0.7 (OH) 1.3, which would indicate some substitution by acidic phosphate, . The paste’s initial compressive strength is 10 MPa, and, within 12 hours, the conversion to dahllite is a
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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