Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-11-12
2001-11-13
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C264S043000, C264S044000, C264S321000, C264S327000, C264S333000, C264S337000, C428S312200, C501S081000, C623S923000, C623S926000
Reexamination Certificate
active
06316091
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for preparing a macroporous synthetic ceramic intended especially as a bone replacement, the ceramic having a controlled interconnection dimension between the pores, as well as a controlled porosity and a controlled pore size.
The processes of the aforementioned type, known from the prior art, have a number of drawbacks.
This is because these processes do not allow the porous architecture of the ceramic obtained to be completely controlled, namely especially to control not only the size and shape of the macropores and their distribution within the ceramic matrix but also the size of the interconnections between macropores.
Now, this lack of control reduces the biological effectiveness of the ceramics, which is characterized, in the case of an application as a bone replacement, by poor bone rehabilitation or, at the very least, partial rehabilitation of the bone replacement.
In addition, heterogeneities in the mechanical behaviour, especially in compression, are often found because of the imperfect reproducibility of the architectures.
Certain processes recommend exerting pressure on the compacted particles intended to form the pores, so as to control the interconnection diameter.
However, this type of process does not allow effective control of the interconnection, which is homogeneous, easily reproducible and modifiable.
Moreover, the structural heterogeneities in the ceramic bone replacements of the prior art often cause variations in the mechanical behaviour.
Because the architectures are not completely controlled, the mechanical strength is often low, particularly in compression. It is necessary to limit the mechanical stresses on the implant and, consequently, to reduce the size of the manufactured components for the purpose of limiting the risk of mechanical failure of the implant/receiving bone system.
The prior art is represented, in particular, by documents WO-A-92/06653, DE-A-4,403,509, DE-A-3,123,460 and WO-A-95/32008.
SUMMARY OF THE INVENTION
The object of the invention is therefore to alleviate the drawbacks of the aforementioned prior art.
Thus, one objective of the process of the invention is especially:
to control the interconnection between the macropores of a synthetic ceramic in a reproducible and modifiable manner so as, in particular, to allow the passage of bone cells and thus to ensure bone neoformation right into the core of the biomaterial in the case of an application as a bone replacement, this control of the interconnection having to be implemented in a homogeneous manner and right into the core of the replacement, whatever its size;
to control the porosity of the ceramic and the dimensions of the pores;
to produce biocompatible ceramics, “on request”, having dimensions and a predefined shape.
For this purpose the invention proposes a process of the aforementioned type, characterized by the following successive steps:
a) construction of an edifice from pore-forming elements;
b) thermoforming of the edifice so as to ensure controlled coalescence between the pore-forming elements;
c) impregnation of the edifice with a suspension so as to fill the spaces between the pore-forming elements;
d) removal of the pore-forming elements so as to generate the macroporosity with a controlled interconnection diameter.
More specifically, according to the process of the invention, particles of a pore-forming organic compound of low thermal expansion are packed into a container, the particles having a predetermined shape.
In order for there to be intimate contact between the particles, and therefore for the interconnection between macropores to be generated, the particles are subjected to a thermoforming treatment.
The purpose of this operation is to reach the working temperature, greater than the glass transition temperature of the organic compound, so as to place it in its rubbery plateau, so as to produce controlled welding between these particles.
The generation of the controlled bridging between particles, and therefore of the future controlled interconnection between macropores, is achieved by regulating the thermoforming treatment time parameter and the thermoforming treatment temperature parameter.
Other approaches are possible if it is desired to reduce the treatment time. It is possible, for example, to increase the working temperature (without however reaching the decomposition temperature of the polymer) or else to apply pressure to the polymeric edifice (at a temperature above the glass temperature) so as to speed up the development of the welds which form.
Once the connections have been made between the particles, the monobloc formed by these interconnected particles is extracted, after the structure has cooled, in order to place it in a porous mold.
The spaces between the particles are then filled up with a calcium phosphate powder in suspension in an aqueous medium so as to form the ceramic reinforcement of the material.
After removing the water via the porous structure of the mold, the product obtained is demoulded and then heat-treated so as, in a first step, to remove the organic compound, and therefore to generate the porosity of the product, and then, in a second step, to densify the walls of the ceramic.
According to the invention, the organic compound is chosen especially from acrylic resins, such as, in particular, polymethyl methacrylate (PMMA) and polymethacrylate (PMA), polystyrene, polyethylene or similar materials.
Furthermore, in one embodiment of the invention, the particles have an approximately spherical general shape.
According to the invention, the calcium phosphate is chosen especially from hydroxyapatite (HA) or tricalcium phosphate (&bgr; TCP), or the like, or a mixture of them.
The process of the invention thus makes it possible to obtain a macroporous synthetic ceramic whose interconnection between macropores is perfectly controlled and whose pores have controlled dimensions and are distributed, in terms of number and area, in a predetermined manner.
More specifically, the process of the invention allows perfect control of the diameter of the spherical pores, especially between 100 &mgr;m and 800 &mgr;m, with perfectly controlled interconnections, especially between 0.1 and 0.8 times the diameter of the macropore involved, and more particularly between 40 and 640 &mgr;m.
Moreover, control of the parameters, such as the treatment temperature, the treatment pressure and the treatment time, allows the interconnection diameter to be controlled.
A polymer redistribution law at a temperature above the glass transition temperature, of the viscous-flow type, for the formation of interparticle necks, governs the coalescence of the particles and thus allows the final interconnection diameter to be perfectly controlled.
For experimental implementation reasons, ceramics are produced in which the interconnection diameter between the macropores is greater than 30 &mgr;m. Consequently, for each given particle-size class of beads, the coalescence, and therefore the final interconnection of the product, can be precisely adjusted. It should be understood that these two parameters are distinct from each other and can be adjusted independently.
The invention also makes it possible to obtain ceramics of the aforementioned type in any size, whether these are small or large (several cm
3
), and having a high mechanical strength.
Furthermore, this process also makes it possible to obtain ceramics that can be bone replacements of complex shape and of constant or variable porosity.
Finally, according to another aspect, the invention relates to the use of such a ceramic as a bone replacement.
REFERENCES:
patent: 4371484 (1983-02-01), Inukai et al.
patent: 5298205 (1994-03-01), Hayes et al.
patent: 5958314 (1999-09-01), Draenert
patent: 31 23 460 A1 (1982-02-01), None
patent: 4403 509 (1995-08-01), None
patent: WO92/0665A1 (1992-04-01), None
patent: WO95/32008 (1995-11-01), None
Descamps Michel
Gallur-Greme Amparo
Hardouin Pierre
Richart Olivier
Szarzynski Stephan
Copenheaver Blaine
Roché Leanna
SDGI Holdings Inc.
Woodard Emhardt Naughton Moriarty & McNett
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