Bioactive, bioabsorbable surgical composite material

Details Bioactive, bioabsorbable surgical composite material Bioactive, bioabsorbable surgical composite material

1998-09-04

2002-06-18

Mancene, Gene (Department: 3732)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Having bio-absorbable component

C623S011110

Reexamination Certificate

active

06406498

ABSTRACT:

The invention relates to bioactive, biocompatible, bioabsorbable surgical composites and devices, such as pins, screws, plates, tacks, bolts, intramedullary nails, suture anchors, staples, or other devices which are applied in bone-to-bone, soft tissue-to-bone or soft tissue-to-soft tissue fixation or in fixation of bioabsorbable and/or biostable implants in, and/or on, bone or soft tissue, which composites and devices are fabricated of bioabsorbable polymers, copolymers or polymer alloys that are self-reinforced and contain ceramic particles or reinforcement fibers and porosity.
BACKGROUND OF THE INVENTION
Bioabsorbable surgical devices such as, e.g., pins, screws, plates, tacks, bolts, intramedullary nails, suture anchors, or staples, etc., made from bioabsorbable polymers are becoming more frequently used in the medical profession in bone-to-bone, soft tissue-to-bone or soft tissue-to-soft tissue fixation. Numerous publications describe the aforementioned and other bioabsorbable devices for such tissue fixation applications, e.g., U.S. Pat. No. 4,655,203, U.S. Pat. No. 4,743,257, U.S. Pat. No. 4,863,472, U.S. Pat. No. 5,084,051, U.S. Pat. No 4,968,317, EPO Pat. No. 449,867, U.S. Pat. No. 5,562,704, PCT/FI 96/00351, PCT/FI 96/00511, FI Pat. Appl. No. 965111, U.S. Pat. appl. Ser. No. 08/873,174, U.S. Pat. appl. Ser. No. 08/887,130, U.S. Pat. appl. Ser. No. 08/914,137, and U.S. Pat. appl. Ser. No. 08/921,533, the entire respective disclosures of which are incorporated herein by way of this reference.
Surgeons would prefer to use bioabsorbable devices that eventually resorb and disappear from the body after they have served their purpose during tissue fixation and healing and, accordingly, are not needed any more. However, a device made from bioabsorbable polymer must have sufficient strength and stiffness for effective tissue fixation and it must retain sufficient strength to perform its function during the tissue healing process, before it eventually is absorbed by the body. It is advantageous to mix different additives into bioabsorbable polymers to modify their properties and to yield devices having useful properties. Such typical additives include ceramic, which optionally can be bioactive, particle fillers and short fiber reinforcements (having fiber lengths typically between 1 &mgr;m-10 mm), each of which can promote osteoconductivity of bioabsorbable bone fracture fixation devices, such as pins, screws or plates or other fixation implants like suture anchors and tacks, which are in contact with bone tissue.
Bioactive, bioabsorbable ceramic fillers and fibers and/or their use in bioabsorbable devices as bioactive ceramic fillers and/or reinforcements have been described in several of the aforementioned publications, and also are describe in, e.g., EPO Pat. Appl. 0 146 398, U.S. Pat. No. 4,612,923, and PCT Pat. Appl. WO 96/21628, the entire disclosures of each of which are incorporated herein by way of this reference.
Ceramic particle fillers and/or short fiber reinforcements typically are first dry blended with bioabsorbable polymer powder, granulate or flakes, and the mixture is then melt blended in an extruder, injection molding machine or in a compression molding machine. The melt blended extrudate can be pelletized or cooled and crushed and sieved to the desired grain size. Such pellets or grains can be further melt processed, e.g., by extrusion, injection molding or compression molding, into bioabsorbable preforms or they can be used as masterbatches and mixed with nonblended bioabsorbable polymers and melt processed into bioabsorbable preforms which can be processed further mechanically and/or thermomechanically to make surgical devices. It also is possible to melt process many devices directly from pellets or grains or masterbatches of polymer mixtures, e.g., with extrusion, injection molding or compression molding.
Particles or short fibers of bioactive glass, such as are described in PCT Pat. Appl. WO 96/21628, the entire disclosure of which is incorporated herein by way of this reference, are especially advantageous ceramic fillers and/or reinforcements in bioabsorbable polymers because they slowly dissolve under tissue conditions and form hydroxyapatite precipitations, (see, e.g., M. Brink, “Bioactive glasses with a large working range”, Doctoral Thesis, Åbo Akademi University, Turku, Finland, 1997, the entire disclosure of which is incorporated herein by way of this reference), which enhances the bone growth in contact with the surface of the device.
However, the surface of melt-molded bioabsorbable polymer composites containing bioactive glass filler and/or fiber reinforcements is coated with a “skin” of bioabsorbable polymer which prevents the immediate direct contact of glass particles with the surrounding tissues and tissue fluids when the melt molded device has been implanted into living tissue. The advantageous direct contact of bioactive glass particles with the tissue environment can develop only weeks or months after implantation when biodegradation of the polymeric surface layer (skin) has proceeded so far that cracks or crazes have developed in the surface layer of composite. Therefore, it is necessary to machine the surfaces of such melt molded composites mechanically to remove the isolating skin layer if immediate contact between glass particles (filler or fibers) is desired. Such a surface machining is, however, time consuming process.
An additional general problem with ceramic particle filled thermoplastic polymer composites is their brittleness, because addition of ceramic fillers into the polymer matrix changes most thermoplastic polymers from tough and ductile to brittle in nature. This is evidenced by significant reduction of both elongation at break and impact strength (see, e.g., Modern Plastics, Guide to Plastics, 1987, McGraw-Hill, N.Y., pp. 152-153 and Modern Plastics Encyclopedia, Mid-October Issue 1989, McGraw-Hill, N.Y., 1989, pp. 600, 606-607, 608-609, 614, the entire disclosures of both of which are incorporated herein by way of this reference). Moreover, even non-filled bioabsorbable thermoplastic polymer devices, which are manufactured by melt molding, may be brittle in their mechanical behavior. That brittleness can be a severe limitation on bioabsorbable devices, leading to premature breaking or to other adverse behavior (see, e.g., D. McGuire, et al., American Academy of Orthopaedic Surgeons, New Orleans, 65th Annual Meeting, Mar. 19-23, 1998, Final Program, p. 261, the entire disclosure of which is incorporated herein by way of this reference). Just as in nonbioabsorbable thermoplastic polymers, ceramic fillers also increase the brittleness of bioabsorbable polymers (see, e.g., Example 1 of this application).
Additionally, the prior art bioabsorbable, particle filled or short fiber filled composites and devices made of them must have low porosities, because porosity weakens the composite and increases its brittleness. However, porosity also provides advantages to an implant which is in contact with bone or other tissue, because (bone) tissue can grow into the pores, accelerating new tissue (bone) formation and locking the implant into contact with the tissue (bone), thereby preventing implant migration. Such surface porosity also would facilitate the contact between the growing bone and ceramic particle or fiber fillers, if the ceramic particles or fibers are at least partially exposed into the pores.
It would, therefore, be advantageous to have a strong and tough (nonbrittle), bioabsorbable composite comprising: (a) a matrix of a bioabsorbable polymer, copolymer (consisting of two or more monomer components) or polymer blend, which matrix is oriented and/or self-reinforced; (b) bioabsorbable, bioactive ceramic particles and/or short fiber filler or reinforcement dispersed in the polymer matrix; (c) pores which are dispersed in the polymer matrix and isolated or at least partially connected with one another, and into which pores at least some free surfaces of the particles or fibers are exposed; and (d) an outer surfa

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