Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2000-06-30
2003-01-28
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S292100, C428S375000, C428S361000, C428S374000
Reexamination Certificate
active
06511748
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
FIELD OF THE INVENTION
This invention relates to synthetic bioabsorbable fibers. The present invention also relates to methods of making bioabsorbable fibers from at least two different polymers by separately melt extruding the at least two different polymers and bonding the extruded polymers together to form a fiber with a semicrystalline polymer core and an amorphous polymer sheath. The invention also relates to reinforced composites, made at least in part from synthetic bioabsorbable fibers. Finally, the present invention relates to devices comprised of such reinforced composites, wherein the devices are designed for use as in vivo implants, including implants which can support high loads, such as for use in fracture fixation and spinal fusion.
BACKGROUND OF THE INVENTION
Metal implants have a long history of successful use in orthopedic surgery but also carry many risks for complications. In the case of metal rods and plates for fracture fixation, a second surgery for device removal is recommended about one year after confirmation of osseous union. If the device is not removed the bone can remodel into a weakened condition due to stress shielding. There is also the potential for an increased risk of infection. In the case of metal cages for spinal fusion, complications due to migration, infection, corrosion, reduced bone density, non-union, and fracture are especially serious since major surgery is required for device removal.
Poly(lactic acid) has been the subject of continuous research as a material for use in surgical devices since it was first proposed for this purpose in the mid 1960s. Since poly(lactic acid) is ultimately hydrolyzed into lactic acid, a normal intermediate carbohydrate metabolism in man, it continues to be viewed as the ideal implantable material from the standpoint of toxicological safety.
High strength and high modulus fibers produced from semicrystalline poly(L-lactic acid), also known as poly(lactide), hereinafter referred to as PLA, have been studied as braided implants for use as a ligament augmentation device. PLA fibers are known to be capable of retaining about 70% of their initial tensile strength after 10 months in vivo.
In spite of the excellent strength retention of PLA fibers in vivo, molded articles made from PLA have generally failed to achieve commercial success as orthopedic implants. The physical properties of a polymer in fiber form resulting from optimum drawing and annealing of the fiber cannot be duplicated in the same polymer processed by injection molding. Thus injection molded PLA typically may have a tensile strength of 60 MPa. This value may be increased up to about 300 MPa by stressing the injection molded parts to achieve orientation prior to crystallization. Highly drawn PLA fibers, on the other hand, can give tensile strength in excess of 2,000 MPa.
One possibility for obtaining fiber strength in a molded part would be to incorporate PLA fibers into a matrix of PLA or a similar polymer such as poly(dl-lacitc acid) which is totally amorphous. The problem with using poly(dl-lacitc acid) is that it degrades too rapidly for orthopedic applications. Pure self-reinforced PLA fiber composites have been made by sintering together bundles of PLA fibers thereby sacrificing some of the fibers to produce a molten matrix for embedding the remaining fibers. This process is difficult to control and yields unreliable results. It also tends to produce a substantial amorphous phase that can slowly recrystallize upon prolonged storage to give a brittle, non-reinforcing structure. Moreover, even if recrystallization is suppressed by copolymerization of L-lactide with small amounts of dl-lactide, degradation of the amorphous PLA tends to result in the build-up of acidic degradation products in the interior of the molded device resulting in an autocatalytic acceleration of the hydrolytic degradation process.
Fiber reinforced composites of PLA with the use of other bioabsorbable polymers as a matrix have generally failed to achieve adequate in vivo performance due to moisture penetration into the interface between fiber and matrix. This typical mode of failure has been the principal problem with all approaches to fully bioabsorbable composites of the prior art.
BRIEF SUMMARY OF THE INVENTION
In one aspect, the present invention is a bioabsorbable fiber comprising a core of a semicrystalline fiber-forming bioabsorbable core polymer with a crystalline core melting temperature, and a sheath of an amorphous bioabsorbable sheath polymer with a softening point below the crystalline core melting temperature, wherein the core polymer and sheath polymer are separately melt extruded, and the sheath is connected to the core through an adhesive bond.
In another aspect, the present invention is a reinforced composite, comprising a plurality of filaments of the bioabsorbable fiber and a molding resin reinforced therewith.
In yet another aspect, the present invention is a device designed for in vivo implantation or insertion, fabricated from the reinforced composite.
In a further aspect, the present invention is a method of making the bioabsorbable fiber, comprising the steps of:
a. selecting a core polymer which is semicrystalline, fiber-forming, and bioabsorbable, with a crystalline core melting temperature;
b. selecting a sheath polymer which is bioabsorbable, and which forms an amorphous phase on polymerization, with a softening point below the crystalline core melting temperature;
c. separately melt extruding the core polymer and sheath polymer; and
d. forming an adhesive bond between the core polymer and sheath polymer, such that the resulting bioabsorbable fiber comprises a core of the core polymer and a sheath of the sheath polymer.
Finally, in yet another aspect, the present invention is a method of making a surgical device of a reinforced composite of bioabsorbable fibers, comprising the steps of:
a. providing a plurality of the bioabsorbable fibers;
b. providing an injection mold having interior walls which define an interior cavity;
c. inserting the plurality of bioabsorbable fibers into the interior cavity of the injection mold; and
d. adding a bioabsorbable injection molding resin polymer to the injection mold at an injection temperature which is lower than the crystalline core melting temperature.
DETAILED DESCRIPTION OF THE INVENTION
A. Definitions
The following terms used herein shall have the following definitions:
“Poly(ester-amide)” shall mean to include any of the polymers described in U.S. Pat. No. 4,343,931, “Synthetic Absorbable Surgical Devices of Poly(esteramides)”, T. H. Barrows, Aug. 19, 1982, the teachings of which are incorporated herein by reference, and to include any of the polymers described in Provisional Patent Application Ser. No. 60/062,064, “Bioabsorbable Triglycolic Acid Poly(ester-amide)s”, T. H. Barrows, filed Oct. 16, 1997, the teachings of which are incorporated herein by reference.
“Tryosine-derived polycarbonates” shall mean to include any of the polymers described in U.S. Pat. No. 5,198,507, “Synthesis of Amino Acid-derived Bioerodible Polymers”, J. B. Kohn and S. K. K. Pulapura, Mar. 30, 1993, the teachings of which are incorporated herein by reference.
“PLA” shall mean poly(L-lactide).
“PGA” shall mean polyglycolide.
“PEA” shall mean poly(ester-amide).
“TMC” shall mean trimethylene carbonate.
“Softening point” shall mean the temperature range below which a polymer is non-tacky and non-self-adherent and above which the polymer is tacky and self-adherent.
“Melting temperature” shall mean the crystalline core melting transition temperature (Tm) of a semi-crystalline polymer.
“Injection temperature” shall mean the minimum temperature of a molten polymer that allows it to have adequately low viscosity under pressure to flow into an injection mold cavity containing multifilament fibers such that the spaces between the fibers are completely filled with the injected molten polymer.
“Bioabsorbable” shall mean the property of a composition,
Aderans Research Institute, Inc.
Dixon Merrick
Frenchick, Esq. Grady J.
Michael & Best & Friedrich LLP
Yager, Esq. Charlene L.
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