Forming method for apatite prostheses

Abstract

Biological hydroxyapatites and closely related compounds form the inorganic or mineral phase of tooth enamel, dentine and bone.' The chemical composition of hydroxyapatite is Ca ,O(POa),(OH)2. The chemistry, structure and physical properties of biological, geological and synthetic hydroxyapatitites and related apatite compounds have undergone intensive investigation for a number of However, the potential uses of polycrystalline apatites for orthopedic prosthetic devices have not been reported. This is, in part, a result of the lack of an effective method of forming apatite powder into polycrystalline shapes. This difficulty has now been overcome by the application of a high temperaturehigh pressure forming method called hot pressing.The starting material in this investigation was a large single crystal of fluorapatite, Ca 10(P04)6F2. Grinding to -200 mesh powder was accomplished using an agate mortar and pestle. The powder was then placed in an inductively heated graphite mold and subjected to a pressure of 5000 p.s.i.Reaction and densification occurred over the temperature range 920 to 1230°C during a 30 minute period. The resulting polycrystalline body was in the form of a ring exhibiting desirable mechanical properties. Both X-ray and infrared spectrographic patterns were characteristic of highly crystalline apatite.The microstructure displayed (Fig. 1) is that of a typical sintered compact with grains forming equilibrium boundaries of 120" with one another. The temperature-pressure-time relationships as well as initial particle size distribution, atmosphere and doping agents can all be modified. This shouId allow adequate control over the porosity and other microstructural aspects of the final form, and thus of the resulting mechanical porperties.Calcified tissue ingrowth into a hot pressed apatite prosthetic device would serve to anchor it to adjacent hard tissues. Based on recent studies,4 it is expected that pore sizes of I50 to 200p would be necessary for this to occur. Whether apatite shapes would best serve as permanent orthopedic prostheses or as templates for directed or catalyzed bone growth with subsequet dissolution of the apatite, remains to be determined. Long term compatability studies must also be undertaken.Recent experiments indicate that apatite forms may be produced by standard sintering techniques. The extension of forming methods to synthetic hydroxyapatites is of immediate interest. In addition, the extremely large capacity for isomorphic substitution into the apatite lattice (ex. CO,, Na+, K+, F-) provides interesting possibilities for altering the physical and chemical properties of apatite prostheses.

History of Calcium Phosphates in Regenerative Medicine

“…In 1965, thorough investigations of carbonated apatites were started [293]. In 1969, an important paper on a forming method of HA prostheses was published [294], in which the authors hot-pressed HA power into dense and useful shapes. That study might be considered as the earliest one on the fabrication of calcium orthophosphate implants.…”

Aqueous impregnation of porous β-tricalcium phosphate scaffolds

“…Calcium phosphate scaffolds have proved to be excellent bone substitute material since their introduction in the late 1960s [1][2][3]. Typical applications include knee osteotomy [4,5] or periodontal defects [6].…”

Processing and mechanical properties of hydroxyapatite pieces obtained by the gelcasting method

“…1,2 Many efforts have been made in order to improve the mechanical properties of HA and several forming techniques have been used. In addition to common pressing method (uniaxial, 3 hot pressing, 4 hot isostatic pressing 5 ), other methods have been proposed, such as slip-casting, 6,7 tape-casting, 8,9 injection moulding 10 and viscous plastic processing. 11 Colloidal processing offers the potential to produce reliably ceramic films and bulk forms through careful control of the initial suspension "structure" and its evolution during fabrication.…”