Calcium phosphatesCalcium phosphates
Calcium phosphate-based ceramics constitute, at present, the preferred bone
substitute in orthopedic and maxillofacial surgery. They are very similar to the
mineral phase of the bone, by their structure and/or their chemical composition.
Calcium phosphates usually found in ceramics are:
– hydroxyapatite (HAP): Ca10(PO4)6(OH)2;
– tricalcium phosphate ? (? TCP): Ca3 (PO4)2;
– mixtures of HAP and ? TCP.
500 Ceramic Materials
Name and chemical formula Abbreviation Type of materials and utilization
Hydroxyapatite Ca10(PO4)6(OH)2 HAP ceramics,
plasma sprayed coatings,
composites,
drug carrier
Calcium deficient apatites ns-HAP Low temperature coating
composites, drug carrier
? and ? tricalcium phosphates
Ca3(PO4)2
? and ? TCP ceramics, composites,
plasma coating cements,
drug carrier
Dicalcium phosphate dihydrate
CaHPO4, 2H2O
DCPD cements
Anhydrous dicalcium phosphate
CaHPO4
DCPA cements
Octocalcium phosphate Ca8(PO4)4
(HPO4)2, 5H2O
OCP cements
Tetracalcium phosphate
Ca4(PO4)2O
TTCP cements
Amorphous calcium phosphate ACP cements, drug carrier
low temperature coating
Table 12.1. Calcium phosphates used as biomaterials
These ceramics are bioactive and can be degraded to various degrees.
The stoichiometric HAP, characterized by an atomic ratio Ca/P = 1.67 and a
hexagonal structure, is the nearest relative of biologic apatite crystals. Moreover, the
HAP is the least soluble and the least resorbable calcium phosphate. When an HAP
ceramic is implanted in a bone site, the bone tissue formation is observed on its
contact (osteoconduction) (see Figure 12.1). Besides, in certain conditions, calcium
phosphate ceramics can induce the formation of bone tissue in ectopic sites. HAP
implants appear in the form of dense ceramics or with variable porosity or again, in
the case of prostheses, as thin coatings deposited by plasma sprayed on a metal.
Bioceramics 501
Figure 12.1. Micro-radiography of a section of an HAP-DCPD composite
showing the invasion of DCPD by bone tissue
The ? TCP, characterized by an atomic ratio Ca/P = 1.5, is perfectly
biocompatible and bioresorbable. Like HAP, it is capable of developing a chemical
bond with the bone and to stimulate its growth, but its resorption is more rapid.
It is difficult to make pure HAP or ? TCP and biphasic materials HAP-? TCP
have been developed initially by accident and later deliberately; they combine the
physicochemical properties of each of the compounds. These can be advantageously
used to prepare materials with controlled resorption and bone substitution [DAC 90].
The presence of pores in materials provides anchor points for the bone and
improves the mechanical quality of the bone/implant interface; the increase in the
specific surface further encourages cell colonization and the revascularization.
While calcium phosphate-based bioceramics are excellent materials for bone
reconstruction, they have a low mechanical strength (less than that of the bone), not
lending themselves to machining. This resistance diminishes while porosity
increases, making the utilization of very porous implants very delicate.
Even if the HAP remains the most important calcium phosphate, from an
industrial point of view, the development of low temperature processes, particularly
those concerning mineral cements and coating on metal, have led to the utilization
and emergence of other calcium phosphates which are more reactive. Table 12.1
lists the different calcium phosphates used as biomaterials.
502 Ceramic Materials
The development of calcium phosphate-based ceramics at high temperature
requires taking into account the thermal stability of these compounds. We can
distinguish two schemes of decomposition according to the temperature: irreversible
decompositions (condensation of hydrogenophosphate ions, decomposition of
carbonate ions, of hydroxide ions, etc.) at low temperature (150–1,000°C) and
reversible decomposition (decomposition of the apatite into TCP, TTCP and lime) at
high temperatures (T > 1,000°C).
12.2.3.4. Oxides and hydroxides
As discussed in the previous section, it would seem that the layer of hydrated
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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