Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1997-04-04
2001-02-06
Fiorilla, Christopher A. (Department: 1731)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C264S497000, C264S628000, C264S632000
Reexamination Certificate
active
06183515
ABSTRACT:
BACKGROUND OF THE INVENTION
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
A bibliography of references cited herein is included at the id end of this specification. The references listed in the bibliography, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to manufacturing methods and systems (collectively referred to as “processes”) for the freeform shaping of calcium-containing powders. This invention more specifically relates to processes for shaping bone implants from various calcium phosphate powders and polymer-emulsion binders. Certain embodiments of these processes focus on the use of a Selective Laser Sintering™ (“SLS™”) device to automatically and selectively fuse the polymer binder. In such processes, complex three-dimensional objects may be built by selectively fusing successive thin layers of the powdered material.
2. Description of the Related Art
Much attention has been given in the art to the development of materials to assist in the regeneration of bone defects and injuries. In 1926, DeJong observed the similarities between the powder X-ray diffraction pattern of the in vivo mineral and the hydroxyapatite (Cas(OH) (PO
4
)
3
, “CHA”). Calcium compounds, including calcium sulfate (Nielson, 1944), calcium hydroxide (Peltier, 1957), and tricalcium phosphate (“TCP”) (Albee et al., 1920), have been observed to stimulate new bone growth when implanted or injected into bone cavities (Hulbert et al., 1983). These materials also exhibit good biocompatibility and compositional similarities to human bone and tooth and can serve as resorbable or non-resorbable implants depending on their degree of microporosity.
Some TCP implants are known to be readily resorbable. For example, sintered TCP plugs with pore sizes between 100-200 microns have been implanted in rats (Bhashar et al., 1971). Very rapid bone formation was reportedly observed at three days after implantation, and highly cellular tissue, consisting of osteoblastic and fibroblastic proliferation, was found within the pores. At one week, the size of the implant was reduced, and new bone formation was extensive. After two weeks, connective tissue had infiltrated throughout the ceramic. During the next four weeks, the boney material within the ceramic continued to mature. Electron micrographs indicated that within clastlike cells, ceramic could be depicted in membrane-bound vesicles. The authors concluded that TCP implants were biodegradable, via phagocytosis, the ceramic did not elicit a marked inflammatory response, and connective tissue grew rapidly within the pores.
Similar results have also been reported by Cutright et al. (1972) who also implanted TCP in rat tibiae. In this study, the ceramic cavities were filled with osteoid and bone after 21 days and the TCP implant was no longer detectable after 48 days.
Larger implants in dogs are reported to elicit slower responses. Cameron et al. (1977) found that TCP implants in dog femurs were completely infiltrated with new bone by four weeks. However, after six weeks, the rate of new bone growth had slowed as the TCP was resorbed. Additionally, only 15% of a 2 cm×2 cm iliac TCP implant in dogs was resorbed after 18 monthsa (Ferraro et al., 1979).
Koster et al. (1976) reported the testing of the calcium phosphate formulations monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, tetracalcium phosphate, and combinations consisting of 20% monocalcium phosphate and 80% of either di-, tri- or tetracalcium phosphate as implant materials in dog tibiae. These investigators tested both dense ceramics and porous ceramics with pore sizes between 800-1000 microns. They reported that tissue compatibility is dependent on the CaO/P
2
O
5
ratio. All materials with ratios between 2/1 and 4/1 are compatible with the optimum ratio being about 3/1 for TCP. After 10 months, Koster et al. (1977) found that tetracalcium phosphate was resorbed only to a minor extent, but that TCP demonstrated lamellar bone growth throughout its pores. Both were found to be tissue compatible. The authors stated that the 3/1 material was not as strong as the 4/1 material and suggested that TCP should be used only in low stress areas while tetracalcium phosphate could be used in high stress environments.
Jarcho et al. (1976, 1977) reported the development of a process for preparing dense, polycrystalline, calcium hydroxyapatite (CHA), with the empirical formula 2(Ca
5
(PO
4
)
3
OH) or (3Ca
3
(PO
4
)
2
) Ca(OH)
2
. In this study, plugs were fabricated at 100% density and implanted in dogs. No evidence of tissue inflammation occurred, and in contrast to the porous TCP implants described above, little resorption or biodegradation was observed after six months.
Holmes (1979) reported that resorption did occur in porous CHA structures. These results led deGroot (1980) to suggest that all calcium phosphates are degradable (resorbable), but the rate is determined by the degree of microporosity. A dense calcium phosphate with negligible porosity would thus degrade only nominally. These results seem to be verified by Farris et al. (U.S. Pat. No. 4,673,355), who claim biocompatible materials with good properties over the range of Ca/P atomic, or molar, ratios from 0.1 to 1.34. (All patents and patent applications cited herein are incorporated by reference.) These ratios convert to CaO/P
2
O
5
ratios between 0.2 and 2.68, lower than the 3.0 ratio suggested above. They suggest that the Ca/P or CaO/P
2
O
5
ratio is not critical for implant applications. Ca/P ratios in the range 0.1 to 2.0 probably show satisfactory biocompatibility. Capano (1987) found that a Ca/P ratio of 0.5, which corresponds to calcium metaphosphate (“CMP”), has the best biocompatibility when implanted in small animals.
As the apatites are nearly identical in properties and chemical compositions to bone and tooth enamel, a considerable amount of synthetic effort has been done in this area. Patents in this area include: U.S. Pat. No. 4,046,858; U.S. Pat. No. 4,274,879; U.S. Pat. No. 4,330,514; U.S. Pat. No. 4,324,772; U.S. Pat. No. 4,048,300; U.S. Pat. No. 4,097,935; U.S. Pat. No. 4,207,306; and U.S. Pat. No. 3,379,541.
Several patents describe methods for treating apatite materials to render implantable shapes. These methods of heating and compaction under pressure in molds produce solid porous articles in various shapes. These patents include: U.S. Pat. No. 4,673,355; U.S. Pat. No. 4,308,064; U.S. Pat. No. 4,113,500; U.S. Pat. No. 4,222,128; U.S. Pat. No. 4,135,935; U.S. Pat. No. 4,149,893; and U.S. Pat. No. 3,913,229.
Several patents speak to the use of laser radiation to bond apatite materials to tooth and other surfaces, for example, U.S. Pat. No. 4,673,355 and U.S. Pat. No. 4,224,072.
Other patents describe the use of particulate or compacted apatite in conjunction with various compounds, filler, and cements, for example, U.S. Pat. No. 4,673,355; U.S. Pat. No. 4,230,455; U.S. Pat. No. 4,223,412; and U.S. Pat. No. 4,131,597.
The above discussion indicates that calcium phosphates or compounds, such as CHA that are substantially TCP (Monsanto, for example, markets CHA as TCP), are useful for a variety of bioceramic applications because they are biocompatible and can be fabricated into shapes that have a desirable combination of strength, porosity, and longevity for particular sorbable and non-sorbable needs.
Virtually any calcium and phosphate source can be used to prepare materials of interest. An important issue is the ratio of Ca to P or, as it is usually expressed, CaO to P
2
O
5
molar ratio in the reactant mixture. For example, one
Barlow Joel W.
Beaman Joseph J.
Crawford Richard H.
Lagow Richard J.
Lee Goonhee
Akin Gump Strauss Hauer & Feld & LLP
Board of Regents , The University of Texas System
Fiorilla Christopher A.
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