Biodegradable implant material comprising bioactive ceramic

Details Biodegradable implant material comprising bioactive ceramic Biodegradable implant material comprising bioactive ceramic

1999-10-05

2002-02-05

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Processes of preparing a desired or intentional composition...

C523S115000, C523S124000, C523S128000, C524S414000, C524S417000, C524S494000, C525S415000, C525S450000, C623S016110, C623S016110

Reexamination Certificate

active

06344496

ABSTRACT:

BACKGROUND OF THE INVENTION
Bioactive ceramic materials are known to the art, and typically contain less than 60 mole percent SiO
2
, high sodium and CaO content (20-25% each), and a high molar ratio of calcium to phosphorus (ranging around five). Such materials are called “bioactive” because interfacial bonds form between the material and surrounding tissues. When such glasses are exposed to water or body fluids, several key reactions occur. The first is cation exchange wherein interstitial sodium and calcium ions from the glass are replaced by protons from solution, forming surface silanol groups and nonstoichiometric hydrogen-bonded complexes:
This cation exchange also increases the hydroxyl concentration of the solution, leading to attack of the fully dense silica glass network to produce additional silanol groups and controlled interfacial dissolution:
Si—O—Si+H
+
OH
−
→Si—OH+HO—Si
As the interfacial pH becomes more alkaline and the concentration of hydrolyzed surface silanol groups increases, the conformational dynamics attending high numbers of proximal silanol groups, combined with the absence of interstitial ions, cause these groups to repolymerize into a silica-rich surface layer:
Si—OH+HO—Si→Si—O—Si+H
2
O
Another consequence of alkaline pH at the glass-solution interface is crystallization into a mixed hydroxyapatite phase of the CaO and P
2
O
5
that were released into solution during the network dissolution. This takes place on the SiO
2
surface. This phase contains apatite crystallites which nucleate and interact with interfacial components such as glycosaminoglycans, collagen and glycoproteins. It is thought that incorporation of organic biological constituents within the growing hydroxyapatite- and SiO
2
-rich layers triggers close interactions with living tissues characteristic of bioactivity. See Greenspan et al. (1994), Bioceramics 7:55-60.
Use of bioactive ceramics in implant prosthetic devices and coatings for prosthetic devices has been described, e.g. in U.S. Pat. No. 4,775,646 to Hench et al. issued Oct. 4, 1988 for “Fluoride-Containing Bioglass® Compositions” which teaches a glass formulation containing 46.1 mole percent SiO
2
, 2.6 mole percent P
2
O
5
, 26.9 mole percent CaO and 24.4 mole percent Na
2
O, or 52.1 mole percent SiO
2
, 2.6 mole percent P
2
O
5
, 23.8 mole percent CaO and 21.5 mole percent Na
2
O, and compositions in which 40 to 60 mole percent of the CaO is substituted with CaF
2
. The patent states that implants made of this material are useful where optimization of durable chemical bonding with living tissue is desirable.
Alkali-free bioactive glass compositions based on SiO
2
, CaO and P
2
O
5
are disclosed in U.S. Pat. No. 5,074,916 to Hench et al. issued Dec. 24, 1991 for “Alkali-Free Bioactive Sol-Gel Compositions.”
Bioglass® is a registered trademark of the University of Florida licensed to USBiomaterials Corporation. Other issued patents related to this material include U.S. Pat. No. 5,486,598 issued Jan. 23, 1996 to West, et al. for “Silica Mediated Synthesis of Peptides,” U.S. Pat. No. 4,851,046 issued Jul. 25, 1989 to Low et al. for “Periodontal Osseous Defect Repair,” U.S. Pat. No. 4,676,796 issued Jun. 30, 1987 to Merwin et al. for “Middle Ear Prosthesis,” U.S. Pat. No. 4,478,904 issued Oct. 23, 1984 to Ducheyne et al. for “Metal Fiber Reinforced Bioglass® Compositions,” U.S. Pat. No. 4,234,972 issued Nov. 25, 1980 to Hench et al. for “Bioglass®-Coated Metal Substrate,” and U.S. Pat. No. 4,103,002 issued Jul. 25, 1978 to Hench et al. for “Bioglass® Coated A1203 Ceramics.” Pending applications include Patent Cooperation Treaty Publication WO 9117777 published Nov. 28, 1991, Walker, et al., inventors, for “Injectable Bioactive Glass Compositions and Methods for Tissue Reconstruction,” claiming a priority date of May 22, 1990 based on a U.S. application.
PerioGlas® is a registered trademark of USBiomaterials Corporation licensed to Block Drug Corporation. It refers to a synthetic bone graft particulate material containing Bioglass® ceramic. Product literature describes this material as bonding to both bone and soft tissue, and indicates it is packed directly into a bone defect. Hench, L. L. (1995) “Bioactive Implants,”
Chemistry and Industry
(July 17, n. 14, pp.547-550), reports that collagen fibrils in the surrounding tissue interact directly with the surface layer which forms on bioactive glass.
PCT Publication WO 96/00536, published Jan. 11, 1996, Ducheyne et al. inventors, discloses a method of forming osseous tissue comprising filling an osseous defect with a bioceramic material.
Composite materials comprising particulate bone replacement material have been described, e.g. in U.S. Pat. No. 4,192,021 issued Mar. 11, 1980 to Deibig et al. for “Bone Replacement or Prosthesis Anchoring Material.” This patent teaches bone replacement prosthesis anchoring materials which are mixtures of sintered calcium phosphates and biodegradable organic materials at a ratio of calcium phosphate to organic materials between 10:1 and 1:1.
U.S. Pat. No. 5,017,627, issued May 21, 1991, to Bonfield et al. for “Composite Material for Use in Orthopaedics” discloses an apparently non-biodegradable polyolefin material containing particulate inorganic solid particles for bone replacement materials.
U.S. Pat. No. 5,552,454 issued Sep. 3, 1996 to Kretschmann et al. for “New Materials for Bone Replacement and for Joining Bones or Prostheses” discloses compositions comprising biodegradable waxes or polymeric resins (molecular weight 200 to 10,000) and a body-compatible ceramic material, wherein the polymer is substantially free from free carboxyl groups.
Other bioactive ceramics are disclosed in U.S. Pat. No. 4,189,325 issued Feb. 19, 1980 to Barrett et al. for “Glass-Ceramic Dental Restorations,” and U.S. Pat. No. 4,171,544 issued Oct. 23, 1979 to Hench et al. for “Bonding of Bone to Materials Presenting a High Specific Area, Porous, Silica-Rich Surface.”
PCT Publication WO 96/19248 published Jun. 27, 1996 for “Method of Controlling pH in the Vicinity of Biodegradable Implants and Method of Increasing Surface Porosity,” discloses the use of bioactive ceramics as pH-controlling agents in biodegradable polymeric implants.
Composite materials for bone and tissue-healing use made with biodegradable polymers having molecular weights above about 10,000 do not appear to have been disclosed in the literature, nor have the advantageous properties of these materials been reported.
All publications referred to herein are hereby incorporated by reference in their entirety.
BRIEF SUMMARY OF THE INVENTION
This invention provides both porous and nonporous therapeutic implant materials comprising a biodegradable polymer and a bioactive ceramic. Such implants comprising biodegradable polymer and bioactive ceramic are termed “composite implants” herein. Incorporation of bioactive ceramic into the polymer has a number of advantages.
An important advantage is that the material has better mechanical properties (e.g. Young's modulus is higher) than in polymeric materials without the bioactive ceramic. Porous implant materials of this invention are preferably used as cell scaffolds, for placing in defects of cancellous (spongy, trabecular) bone. The Young's modulus of such bone ranges from approximately 10 MPa to 3000 MPa. The porous implant material preferably has a Young's modulus similar to that of the bone in which it is to be placed. These porous implant materials may also be used for cell scaffolds for placing in other types of bone. They may also be used as bone graft substitutes, bone on lays and for spinal fusion. Porous implant materials of this invention preferably have a Young's modulus measured under physiological conditions (37° C. in an aqueous environment) by the three point bending dynamic mechanical analysis described herein between about 0.1 MPa and about 100 MPa, and preferably between about 0.5 MPa and about 50 MPa, and a porosity greater than about 50%, and preferably between about 65% and

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