Forming Method .for Apatite Prostheses
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 1230ยฐC 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.