Plastic and nonmetallic article shaping or treating: processes – Dental shaping type
Reexamination Certificate
2000-08-03
2002-01-15
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Dental shaping type
C264S019000, C264S234000, C264S667000, C427S002270
Reexamination Certificate
active
06338810
ABSTRACT:
DESCRIPTION
1. Field of the invention
The present invention involves a process for manufacturing an apatite ceramic, usable in particular for making apatites for biological use.
Apatites are very useful materials for many applications, for example in agriculture as a fertiliser, in orthopaedics as a biomaterial and in analytic chemistry as a chromatographic support.
The apatites have the general formula:
Me
10
(XO
4
)
6
Y
2
(I)
in which Me is one or several cations, XO
4
represents PO
4
, and/or other anionic groups, and Y represents one or several anions such as OH, Cl and F. Among these apatites, phosphocalcic hydroxyapatite:
Ca
10
(PO
4
)
6
(OH)
2
(II)
is the best known compound.
The apatites of formula (I) can have various substitutions, for the cationic sites (Me) as well as for the anionic sites (XO
4
and/or Y
2
).
Depending on the various uses planned, these substitution possibilities can be used to improve the physical and/or chemical properties of these apatites.
Apatites, particularly those of formula (II), can be non-stoichiometric, i.e. having a calcium/phosphorous atomic ratio different from that of the stoichiometric apatite of formula (II) which is 1.677.
In the non-stoichiometric apatites, this ratio is generally less than 1.677. This non-stoichiometry is explained in particular by the presence of flaws in the cationic/calcium sites, and/or by the presence of HPO
4
2−
ions substituting for phosphate ions. A general formula for the apatites could be given as:
Ca
10−x
V
x
(PO
4
)
6−y
(HPO
4
)
y
(OH)
2+y−2x
(III)
in which V represents a flaw and in which x and y are such that x<1, y<1, and y·x. When x=y=0, the apatite is stoichiometric.
Among the apatites, there are known non-stoichiometric biological apatites which form the mineral part of hard tissues, teeth and bones.
The use of apatites in biological areas, orthopaedics or dentistry is due to their perfect biocompatibility. This biocompatibility is attributed to the structure and composition of hydroxyapatite which are very close to those of the mineral part of calcified tissues. Most calcium orthophosphates including tricalcium phosphate, dicalcium phosphate and octocalcium phosphate also have excellent biocompatibility properties. Hydroxyapatites and more generally calcium orthophosphates are recognised as having osteoconduction properties.
With regard to solubility, hydroxyapatites have low solubility in biological media while other calcium orthophosphates are much more soluble, as can be seen in the appended table which gives the solubilities of these various phosphates.
There are many biological uses for apatites. They can be used for filling (in the form of powder or granulates), covering (in the form of powder projected by plasma projection) or in the form of massive pieces for resistant fillings or fixing: osteotomy wedges, screws, interstomatic frames, etc. For the latter uses, the apatite must be prepared in the form of a massive piece.
2. State of the Prior Art
Until now, massive pieces in apatite were prepared from powdered apatites subjected to sintering at high temperatures (greater than 1000° C.) with or without pressure.
The document Bioceramics, Vol. 10, 1997, pages 75 to 78, illustrates the densification of polycrystalline hydroxyapatite by hot compression at 1165° C. under 10 MPa.
The process currently used for preparation of hydroxyapatite biomaterials thus requires prior preparation of apatite power, putting it into granular form and sintering it at a high temperatures, according to various process such as natural sintering, pressure-assisted sintering, and sintering after using slip.
These techniques yield mass pieces with good mechanical properties, but they require high-temperature thermal treatments involving:
high costs of energy for preparation of the apatite
partial transformation of hydroxyapatite to oxyapatite, and
difficulties in enclosing species which are volatile or degradable at the temperature of the thermal treatment in the piece of apatite.
BRIEF DESCRIPTION OF THE INVENTION
This invention precisely involves a process for making apatite ceramics which produces pieces with good mechanical properties but without the need for thermal treatment at high temperatures.
According to the invention, the process for manufacturing of an apatite ceramic involves the following steps:
a) preparing a homogeneous mixture of powders including at least two calcium phosphates chosen from: Ca(H
2
PO
4
)
2
, Ca(H
2
PO
4
)
2
.H
2
O, Ca(HPO
4
), Ca(HPO
4
).2H
2
O, Ca
3
(PO
4
)
2
, • or • variety, and Ca
4
(PO
4
)
2
O, in quantities such that the mixture corresponds to a hydroxyapatite of formula:
Ca
10
(PO
4
)
6
(OH)
2
(II)
having a Ca/P atomic ratio equal to 1.667 or a non-stoichiometric hydroxyapatite of formula:
Ca
10−x
V
x
(PO
4
)
6−y
(HPO
4
)
y
(OH)
2+y−2x
(III)
in which V represents a flaw and in which x and y are such that x<1, y<1, and y·x, having a Ca/P atomic ratio less than 1.667.
b) compacting the mixture of powders obtained in step a) at room temperature, under a pressure of 100 to 500 MPa, to yield a compacted piece; and
c) subjecting the compacted piece to hydrothermal treatment in a sealed chamber containing an aqueous medium, at a temperature of 100 to 500° C., for a period of at least 8 hours.
The process described above yields a stoichiometric or non-stoichiometric hydroxyapatite, at low temperature, because the temperature used in the last step does not exceed 500° C., thus offering many advantages in terms of energy costs and the possibility of enclosing species which are volatile or unstable at temperatures greater than 500° C. in the hydroxyapatite. The massive pieces obtained have good mechanical properties and can be easily machined.
The invention process can also be used to make stoichiometric apatite ceramics with the formula:
Ca
10
(PO
4
)
6
(OH)
2
(II)
or non-stoichiometric apatite ceramics with the formula:
Ca
10−x
V
x
(PO
4
)
6−y
(HPO
4
)
y
(OH)
2+y−2x
(III)
in which V represents a flaw and in which x and y are such that x<1, y<1, and y·x,
in which Ca, PO
4
and/or OH are partly replaced by other metals in the case of Ca, and/or by other anions in the case of PO
4
and OH.
According to this embodiment of the invention, in step a), a mixture of powders is prepared including in addition at least one compound chosen from the salts, oxides and hydroxides of the alkaline metals, alkaline-earth metals, silver or other metals, and silicon oxide, the aforesaid mixture being able to form a stoichiometric apatite of formula:
Ca
10
(PO
4
)
6
(OH)
2
(II)
or non-stoichiometric apatite with the formula:
Ca
10−x
V
x
(PO
4
)
6−y
(HPO
4
)
y
(OH)
2+y−2x
(III)
in which V represents a flaw and in which x and y are such that x<1, y<1, and y·x,
in which Ca, PO
4
and/or OH are partly replaced respectively by other metals and/or by other anions.
For making this mixture, the salts used can be chosen from the phosphates, silicates, citrates, nitrates, carbonates, and halides.
This embodiment of the invention process allows for including cations and anions which have particular value for the planned application within the apatite structure.
If the process is used to produce an apatite ceramic for biological use, it may be useful to include strontium in this structure by preparing a mixture of powders including strontium and calcium phosphate of the formula:
Ca
2
Sr(PO
4
)
2
or by replacing one or several of the calcium phosphate compounds used by the analogous strontium phosphate compounds, or by mixed calcium-strontium compounds. The following compounds could be used: Sr(H
2
PO
4
)
2
, Sr(H
2
PO
4
)
2
.H
2
O, Sr(HPO
4
), Sr(PHO
4
). 2H
2
O, Sr
3
(PO
4
)
2
and Sr
4
(PO
4
)2O.
The presence of strontium in an apatite ceramic for biological use is valuable because it facilitates bone regeneration.
For biological applications, ca
Carpena Joëlle
Donazzon Benoît
Freche Michèle
Lacout Jean-Louis
Burns Doane , Swecker, Mathis LLP
Commissariat a l'Energie Atomique
Fiorilla Christopher A.
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