Composite Materials Using Bone Bioactive Glass and Ceramic Fibers

The present invention relates to composites made from fibers comprised of bioactive glass and the use of such composites to form implantable surfaces. In particular, the present invention relates to composites comprised of bone bioactive glass or ceramic fibers intermingled with structural fibers such as carbon fibers in a matrix of a polymeric material.

Low modulus composite materials have been employed as femoral components of hip implants to reduce stress shielding of the bone and consequently reduce bone tissue resorption. Currently, composite implants are stabilized in their bony bed by a press fit. With this method of stabilization, however, optimum stress distribution effects are not fully realized.

Several attempts have been made to improve the fixation of composite femoral implants to bone. These include porous polymer coatings and particulate bioactive coatings. Implants using porous polymer coatings seek to achieve fixation through mechanical interlocking between the implant and surrounding bone tissue, while the bioactive coatings are designed to attain fixation through a chemical bond between the implant and bone.

Implant surfaces coated with polysulfone particles in an effort to create a porous coating which would behave similarly to a porous metal coating are disclosed in M. Spector, et al., "Porous Polymers for Biological Fixation," Clin. Ortho. Rel. Res., 235:207-218 (1988). Although it is disclosed that some bone growth was evident, the majority of the tissue about the implant surface was fibrous. The porous polymer did not enhance the bone tissue growth in any way.

Composite system of calcium phosphate ceramic powder pressed onto a polymer surface and then cured are also known. See P. Boone, et al., "Bone Attachment of HA Coated Polymers," J. Biomed. Mater. Res. 23, No. A2:183-199 (1989); and M. Zimmerman, et al., "The Attachment of Hydroxyapatite Coated Polysulfone to Bone," J. Appl. Biomat., 1:295-305 (1990). These systems are provided in two fashions. First, the ceramic is flush with the polymer surface, hence, only bonding occurs. Second, the calcium phosphate particles extend from the polymer surface. When interfacial bonding is tested, the failure is between the polymer and the calcium phosphate particles. Hence, the interface between the calcium phosphate particles and the polymer is the weak link in the system. These references disclose the use of polyurethane thermoset and polysulfone thermoplastic polymers, a number of other polymers are similarly used as a matrix for a filler of calcium phosphate ceramic powder in U.S. Pat. No. 4,202,055--Reiner et al. The ceramic particles at the surface of this implant resorb and are replaced by bone tissue. There are no structural fibers and the polymer alone is intended to bear the load. This limits the load-bearing applications of this material to those of the polymer. An implantable bone fixation device comprised of an absorbable polymer and a calcium phosphate ceramic powder filler material is disclosed in U.S. Pat. No. 4,781,183 Casey et al. The device disclosed is a temporary load bearing device which resorbs upon implantation. The calcium phosphate particles are added for strength and also resorb, therefore this device is not fixed to bone tissue through the chemical bonding of bioactive material or porous ingrowth.

Structural fibers will improve certain mechanical properties of composite materials. For example, U.S. Pat. No. 4,239,113--Gross et al. discloses a composition of methylmethacrylate polymers and a bioactive ceramic powder combined with vitreous mineral fibers less than 20 millimeters long. This device is used as a grouting material to bond implants to bone tissue. The chopped fibers are not specifically tailored or designed for mechanical property optimization. A similar composition is disclosed in U.S. Pat. No. 4,131,597--Bluethgen et al., which mentions the use of glass or carbon fibers to add strength to the composite. This patent, however, does not specifically discuss placing fibers to achieve bone bonding regionally. Also, no method of optimization of material properties through arrangement of the structural fibers is suggested. Finally, the method of fixation to be achieved by the disclosed material is not explained.

A similar approach using a textured device of carbon fiber/triazin, coated or non-coated with calcium phosphate particles is discussed in G. Maistrelli, et al., "Hydroxyapatite Coating on Carbon Composite Hip Implants in Dogs," J. Bone Jt. Surg., 74-B:452-456 (1992). The results reported show a higher degree of bone contact for the coated devices after six months. However, longer studies are needed to evaluate the long term fatigue effects on the triazin/calcium phosphate interface.

In all these prior art systems, however, it has been found that although a bond between the substrate polymer and bone may be achieved through the use of a bioactive material at the interface, the resulting implant is still unsatisfactory. As discussed above, the significant limitation remains the interfacial bond between the bioactive material and the polymer.

Much of the prior art discussed immediately above utilized calcium phosphate ceramic powders as the bioactive component of the composite. Bioactive glass materials were developed by Hench in 1969. See L. Hench, et al., "Bonding Mechanisms at the Interface of Ceramic Prosthetic Materials," J. Biomed. Mater. Res., 2:117-141 (1971). More recently, elongated, continuous bioactive glass fibers have been fabricated. See U. Pazzaglia, et al., "Study of the Osteoconductive Properties of Bioactive Glass Fibers," J. Biomed. Mater. Res., 23:1289-1297 (1989); and H. Tagai, et al., "Preparation of Apatite Glass Fiber for Application as Biomaterials," Ceramics in Surgery, Vincenzini, P. (Ed.), Amsterdam, Esevier Sci. Pub. Co. (1983), p. 387-393. The latter reference discloses bioactive glass fibers in resorbable bone plates.

As seen from the foregoing, it would be desirable to provide a composite material for use as a prosthetic device that could be designed to provide a structural modulus that closely matched bone. It is thus an object of the present invention to provide composite structures that incorporate a bioactive material in a polymer matrix along with a structural fiber to provide adequate strength. Additionally, it is a further object of the present invention to provide three dimensional and hybrid composite materials that overcome the deficiencies of the prior art, and in particular that provide an adequate interfacial bond between the bioactive material and the polymer.

It has now been found that bone bioactive glass or ceramic fibers are useful as a chemical bonding vehicle in combination with a structural three-dimensional braided fiber substrate. The bioactive fibers enhance bone growth and bond to surrounding bone tissue. These bone bioactive glass or ceramic fibers are interwoven in the three dimensional braid with carbon fibers and infiltrated with a thermoplastic polymer to form a three-dimensional bioactive composite material. The glass fibers are preferably concentrated on the outer surface of the composite so as to be exposed to physiological fluids upon implantation. This leaves the carbon fibers concentrated in the center region of the implant material to bear the majority of the load.

The stress transfer achieved by interfacial bonding between the implant and bone, combined with the "matched modulus" of the composite implant, provides near optimal stress distribution in the bone, thereby improving long term stability and fixation. In addition, with adequate fixation, there is decreased micromotion between the implant and bone, hence the potential for abrasion of the composite material surface is greatly reduced. Consequently, the chances of particulate debris from the implant causing an inflammatory response, which often leads to loss of implant stability, are also greatly reduced.

FIG. 1 is an illustration of the apparatus used to draw bone bioactive glass or ceramic fibers used in the present invention.

FIG. 2 is a photomicrograph of a section of bone showing ingrowth achieved in an implant.

FIG. 3 is a photomicrograph of a section of bone similar to FIG. 2, but taken at higher magnification.

FIG. 4 is a cross-section of a composite fiber braid made in accordance with the present invention.

FIG. 5 is a schematic illustrating the orientation and placement of fibers in a textile woven from bone bioactive glass or ceramic fibers and structural fibers in accordance with the present invention.

FIG. 6 is an elevation view of a hip prosthesis made in accordance with the present invention.

The uniquely constructed composite material of the present invention is able to maintain continuity from the interface of the structural substrate and the carbon fiber/polymer interface, and to the interface between the bioactive surface and the bioactive glass fiber/polymer interface. As explained below, the bioactive section of the implant material is integrally incorporated into the substrate through a braided interface. Because of this construction there is an increased bond surface between the bioactive material and the polymer that imparts a higher degree of integrity to the bioactive composite material as compared to a particulate coating on the surface of a polymeric composite, such as that found in the prior art. Thus, the likelihood of delamination of the bioactive material from the polymer is greatly reduced. Thus, the present invention provides interfacial bonding between the polymer and bioactive coating, overcoming the main limitation of the prior art.

The configuration of the fiber architecture results in the load being applied to a central portion of the composite, which is preferably comprised of strong, inert fibers such as carbon fibers or other biocompatible fibers integrated with the bioactive fiber surface. Stress is therefore transferred from the inert structural fibers to the bioactive fibers at the implant surface. The integration also serves to increase the mechanical integrity of the material system and prevent delamination within the composite structure.

In the local environment of the bioactive glass fiber, a partial degradation occurs. As the bioactive glass fiber is resorbed it is replaced by bone tissue; the bone tissue is chemically bonded to the glass fiber and also interlocked with these fibers. Furthermore, the bioactivity reactions occurring at the glass surface lead to a precipitation layer on the polymer. This layer, in turn, promotes bone tissue formation and bonding. The triple means of interfacial bonding leads to an interface which stabilizes the implant in its bony bed and provides stress transfer from the implant across the bonded interface into bone tissue. The bone is stressed, thus limiting bone tissue resorption due to stress shielding. This significant occurrence will increase the life of an implant because fixation and stability will not be lost due to bone tissue resorption, which is an initiator in the cascade of events leading to prosthesis loosening.

In the present invention, a composition of bone bioactive glass or ceramic fibers is preferred. In the case of glass the preferred composition leads to a slowly reacting glass while maintaining the ability to be fabricated. A slow reaction rate is desired because a large surface area of glass is exposed to physiological solutions during implantation with glass in a fibrous configuration. A bioactive glass that quickly degrades may lead to an adverse inflammatory response, impeding bone growth and bonding. The tradeoff is that since the glass must be drawn into continuous fibers it cannot be too viscous or too fluid, or the fibers would break upon drawing. Describing the compositional range for materials capable of being drawn into bioactive glass or ceramic fibers thus involves bioactivity versus manufacturability. A most preferred composition that can be successfully drawn into fibers while maintaining bioactivity is: 52% SiO.sub.2 ; 30% Na.sub.2 O; 15% CaO; 3% P.sub.2 O.sub.5. In developing this range, experimental trials showed that a composition of 52% SiO.sub.2 ; 32% CaO; 3% P.sub.2 O.sub.5 ; 13% Na.sub.2 O would be bioactive, however, it is difficult to draw this composition of glass into fibers. This is because the CaO and the P.sub.2 O.sub.5 work against fiberization, while the Na.sub.2 O and SiO.sub.2 work for it. It was also found that a composition of 52% SiO.sub.2 ; 27% CaO; 2% P.sub.2 O.sub.5 ; 19% Na.sub.2 O led to the same difficulties relating to fiberization. The following trends were seen among experimental batches:

Thus, in preferred embodiments, glass compositions used with the present invention will be comprised of 40-60% SiO.sub.2 ; 10-21% CaO; 0-4% P.sub.2 O.sub.5 ; and 19-30% NaO. A more preferred range will be comprised of 45-55% SiO.sub.2 15-20% CaO; 25-35% Na.sub.2 O; and 0-3% P.sub.2 O.sub.5 by mole. As noted above, the most preferred composition on the criteria of slow reaction rate and the ability to be manufactured is 52% SiO.sub.2, 30% Na.sub.2 O, 15% CaO, and 3% P.sub.2 O.sub.5 by mole. Modifiers which may be added to the base composition (by mole) include: 0-3% K.sub.2 O; 0-2% MgO; 0-0.2% Al.sub.2 O.sub.3 ; and 0-3% F.sub.2. As known in the art, such modifiers may be added in small quantities to vary the properties and process parameters without severely affecting the bioactivity or ability of the material to be manufactured.

The following Examples will discuss and explain the formation of a continuous fiber of a bioactive glass for use in a braided fiber and a woven fabric, both of which are impregnated with a polymeric material such as polysulfone to create a three dimensional composite material.

The most preferred glass fiber composition disclosed immediately above (52% SiO.sub.2, 30% Na.sub.2 O, 15% CaO, and 3% P.sub.2 O.sub.5 (in mole %)) was prepared from powders. The powders were weighed, mixed, and melted at 1350.degree. C. for two hours in a silica crucible. The glass drawing apparatus used for this Example is shown in FIG. 1 and includes a resistance heated platinum crucible 50 with an orifice at the bottom. Glass shards were placed into the crucible 50 and melted at approximately 1150.degree. C. The viscous melt formed a meniscus at the crucible orifice 52. To form the fiber 100, the glass meniscus was gently touched with a glass rod and the glass rod was quickly, yet smoothly, pulled from the crucible orifice 52 to form a glass fiber 100 from the melt. The fiber 100 was manually pulled and attached to the take-up wheel 60 spinning at 300-500 rpm, as determined by the speed control 72 attached to the motor 70 that rotates the take-up wheel 60. A smooth, continuous glass fiber 10-13 microns in diameter was obtained.

Polymer plates were manufactured using a closed die and a hot press. The polymer (polysulfone) was weighed and dried in an oven at 163.degree. C. for two hours to drive out excess moisture. The mold was cleaned with ethanol and sprayed with teflon mold release. The thermoplastic powder was poured into the mold and the mold was placed in the hot press. The press was heated to 260.degree. C. and pressure was then applied to 14,000 lbs and released. This was repeated twice. The mold was then heated to 300.degree. C. and a pressure of 620 psi was maintained for thirty minutes. At this time the pressure was released and the mold was air cooled.

The same processing parameters were followed to make plates of a composite material. The polymer was first mixed with chopped glass fibers and then processed with the closed die in the hot press, as described above.

Plugs 4 mm in diameter and 3 mm thick, were machined out of both the polymer plates and the plates that included the chopped bioactive fibers using a core drill tip. The samples were then cleaned with soap and water to remove cutting fluids, and ultrasonically cleaned in ethanol and deionized water, being dried after each cleaning. The implants were sterilized with ethylene oxide.

One bioactive glass fiber/polysulfone and one control polysulfone plug were implanted bilaterally in the medial proximal aspect of the tibia using aseptic techniques. Each rabbit served as its own control. Five rabbits were euthanized at three weeks and five at six weeks.

The retrieved tibiae were immersed in formalin fixative for two weeks. They were rinsed in deionized water and gross sectioned with a low speed blade saw using 70% ethanol as cutting fluid. The sections were dehydrated according to a graded alcohol immersion plan from 70% ethanol to 100% absolute ethanol over a two week period. Following dehydration, the specimens were sectioned perpendicular to the implant long axis into approximately 1 mm thick sections. The sections were infiltrated with Spurr's embedding media according to a graded infiltration sequence in a vacuum desiccator using polyethylene embedding molds. The Spurr's infiltration cycle was as follows:

The specimens were then cured for 2 days in an oven at 21.degree. C. Following embedding, the specimens were sectioned to approximately 0.5 mm thick sections using a low speed diamond wafered rotating blade saw. These sections were ground and polished using 800 and 1200 grit paper to a final section thickness of about 50 .mu.m. The sections were stained using Villanueva Mineralized bone Stain (Polyscientific, New York).

As seen in FIG. 2, taken at 400X. The composite material shows very close apposition to bone in areas of high fiber concentration. In these areas, bone bioactive glass or ceramic fibers are partially resorbed, more clearly seen in FIG. 3, taken at 1000X. In addition, in regions where fibers are close together and bone apposition is achieved, there is also bone apposition to the adjacent polysulfone matrix. In contrast, the polysulfone implants show bone tissue surrounding the plug, but with an interposing layer between the implant and bone tissue.

Thus, the foregoing Example shows that the bioactive glass fiber/polysulfone plugs made in accordance with the present invention achieve a bond between surrounding bone tissue and the glass fibers at the implant surface. The bonded fibers are partially resorbed with bone tissue replacing the glass. Consequently, the method of glass fiber fixation to bone is not only by chemical bonding, but also by micromechanical interlocking. Additionally, there appears to be a bond between the adjacent polymer and surrounding bone tissue. This would lead to increased areas of fixation between the composite and bone beyond that of the fiber itself.

The bond between polysulfone and bone may be due to a calcium phosphate layer being precipitated onto the adjacent polymer surface as it was being precipitated onto the glass fiber. Once this calcium phosphate layer is formed, the polymer itself may act as a substrate for bone growth. Similar findings after implantation of a titanium fiber/bioactive glass composite in dogs will be recently reported. L. Van Hove, E. Schepers and P. Ducheyne, "Titanium Fiber Reinforced Bioactive Glass composite Implants," Bioceramics, Vol. 6; (P. Ducheyne and D. Christiansen, Eds.), Butterworth-Heinemann Ltd. (in press). This study shows bone growth over a titanium fiber which was between two islands of bioactive glass. If the separation between the glass was less than 50 microns, the titanium was covered with bone, but if it was greater than 100 microns (two fiber diameters) there was incomplete bone coverage. An in vitro study has concluded that when a polymer is faced 1 mm or less away from a bioactive glass in simulated body fluid, a calcium phosphate layer is precipitated onto the polymer surface. See T. Kokubo, et al, "International Symposium on Ceramics in Medicine," Butterworth-Heinemann Ltd., London (1991).

The histological observations of the foregoing Example indicate that bone bioactive glass or ceramic fibers in combination with polysulfone polymer will bond to bone tissue. This finding indicates that bone bioactive glass or ceramic fibers on the surface region of low modulus composite implants, such as hip stems and bone plates will achieve improved results.

Another aspect of the present invention is the optimization of the amount of fiber used in the composite. As explained above, previous bioactive polymeric composites had used continuous particle coatings on the surface of polymers or polymeric composites with a bioactive powder dispersed through the polymer matrix. It has been determined having surface area partially covered by bioactive glass in a composite form leads to bone bonding in vivo. A calcium phosphate layer is the substrate for bone growth. As explained above, the desirable development of a calcium phosphate layer on a non-bioactive material is possible if the material is in close apposition to the bioactive glass. Consequently, it has been found that a composite with only a partial bioactive surface would still achieve bonding. Preferably, the proportion of bioactive surface area exposed should be greater than 30% of the total surface area and the bioactive material should be homogeneously distributed over the surface of the composite to maintain the 30% surface area of bioactive material over the entire surface desired for fixation.

Based upon the foregoing, it has also been discovered that fibers made in accordance with Example I and similar fibers can be advantageously used in composite materials that incorporate a structural fiber along with the polymer and the bioactive fiber. Thus, the present invention also relates to composite materials formed of woven, intermingled or juxtaposed elongated fibers of both a bioactive material and another material chosen for its structural properties. These two fibers are combined in a polymeric matrix. The following Examples will illustrate embodiments of this aspect of the present invention.

One manner by which the location and density of fibers within a composite can be controlled is by forming a braid of one or more types of fibers and impregnating the braid with a filler material, such as a polymer. In preferred embodiments of the present invention, continuous bone bioactive glass or ceramic fibers are grouped into 5000 filament fiber bundles. The fiber bundles (or "tow") are interwoven with carbon fibers into a braided textile preform. Most preferably, the bone bioactive glass or ceramic fibers are made in accordance with the composition formulation set forth above. As seen in FIG. 4, a preferred construction has glass fibers 100 woven into a three dimensional tube about a central, but separate, carbon fiber core 110. The two braids are woven simultaneously while the carbon fibers in the core 110 and glass fibers 100 at the carbon/glass interface are interwoven, overlaid or otherwise intermingled. This results in structural interlocking and brings continuity to the structure, even before the polymer is infiltrated.

To create a composite in accordance with this embodiment of the invention, the carbon fibers in the core 110 are commingled with polymer and unidirectional thick polymer fibers are intermingled with the glass fibers 100 in the outer region of the preform. The hybrid preform is then processed in a closed die using a hot press, as described above. The amount of polymer is calculated to give the final total volume fraction desired, thus no additional polymer is added before processing. Also, the resulting composite does not need to be injection molded due to the placement of the polymer fibers so as to achieve uniform polymer distribution throughout the fibrous preform. The final composite is machined to expose bioactive glass 100 fibers at the surface.

The present invention is also directed to the integration of a bioactive phase in fibrous form into a carbon fiber, three dimensional structural reinforcement network. This results in a delamination resistant, interpenetrating fibrous network which allows bone tissue ingrowth.

To facilitate composite processing, a thermoplastic matrix in filamentous form is co-mingled with the reinforcement fibers. As a result, the thermoplastic fibers are uniformly distributed through out the structure. A composite can be formed with bioactive fibers and by the application of heat and pressure to melt the thermoplastic according to well-established regimens known in the art.

By proper selection of fiber architecture and textile processing technique, the quantity and distribution of the bioactive phase can be controlled so that a preferred concentration of the bioactive fibers it disposed near the surface of the structure. The thermal and mechanical properties of the composite system can be further tailored by changing the fiber volume fraction and fiber orientation distribution. Depending on the type of implant, two or three dimensional fiber architectures can be selected and fabricated into net shape or near-net shape fibrous assemblies by weaving, knitting or braiding techniques, such as those disclosed in F. K. Ko, "Preform Fiber Architecture for Ceramic Matrix Composites," Bull. Am. Cer. Soc. (Feb. 1989).

For illustrational purposes, a three dimensional hybrid mesh will be provided as a specific example. It should be noted, however, that the same principles can be applied to cylindrical shapes and other complex structural shapes as seen in the three dimensional braiding loom design diagram illustrated in FIG. 5.

In FIG. 5, the X's represent the bone bioactive glass or ceramic fibers, the number and distribution of which can vary, and the O's are a structural fiber, preferably carbon fibers, which also may be provided in large and small bundles. The vertical rows of the loom are called "tracks" whereas the horizontal rows of the loom are "columns." As known in the art, a three dimensional braided structure is fabricated on the alternate motions of tracks and columns of bundles of fibers attached to a carrier based on the movement instructions indicated in the track and column direction in an alternate manner. An "0" means no movement, and a "1" means moving in the positive direction by one carrier position and a "-1" means moving the carrier in the opposite direction of the other half of the carrier in the same direction. Naturally, a "2" means moving two carrier positions.

In a preferred embodiment, the integration of the bone bioactive glass or ceramic fibers into the carbon fiber in an interfacial region is accommodate by the position of the carrier in track/column coordinates 6/6, 6/7; 7/7, 7/8; 8/6, 8/7; 9/7, 9/8; 10/6, 10/7; 11/7, 11/S; 12/6, 12/7; 13/7, 13/8 being exchanged after each cycle of track/column movement.

From the foregoing, it can be seen that for a given yarn bundle size, the fiber orientation and fiber volume fraction can be designed. Knowing the fiber and matrix material properties, the elastic properties in the form of a stiffness matrix [C] can be established for the composite. Finite element analysis can also be performed to assess the stress-strain response of the implant under a set of boundary conditions. This preform design, micromechanics analysis and structural mechanics analysis can be performed in an iterative manner to optimize the design of the implant and predict the performance capability of the structure.

Additional aspects of the present invention will best be understood with reference to FIG. 6, which illustrates a hip implant prosthesis 200 as an example of a composite structure that can be constructed using the present invention. First, it will be understood by those of skill in the art that the surface, or part of the surface of the prosthesis 200 can be covered with grooves 210 or other surface irregularities. For example, as seen in FIG. 6, it is preferred that the proximal circumferential third of the prosthesis 220 have grooves. These features aid in the macroscopic aspects of bone fixation and have been shown to be beneficial. In accordance with the present invention, the grooves 210 are most preferably formed by molding the preform to the desired texture, rather than machining a smooth surface.

Additionally, the present invention also permits bioactive and structural fibers to be localized to achieve a local fixation using a bioactive surface. In other words, the fibers can be varied so that the bioactivity is concentrated at a particular section or portion of an implant, device or prosthesis. Referring still to FIG. 6, the hip prosthesis 200 would most preferably have the bioactive fibers concentrated in the proximal one third 220 of the implant device. It has been shown that proximal stress transfer in a total hip arthroplasty is better achieved by using a material with fixation to bone in this region.

Thus, it will be appreciated that the present invention is very versatile in many of its parameters. The bone bioactive glass or ceramic fiber can be selected from bioactive glass or glass-ceramic materials, including calcium phosphate ceramic fibers. The polymer system used may be any polymer which bonds to the bone bioactive glass or ceramic fibers, is biocompatible, and does not inhibit the bioactivity of the fibers in vivio. Examples of such polymer systems are polysulfone, polyetheretherketone (PEEK), and polyetherketoneketone (PEKK). The structural fiber can be any inert fiber that fits the collstraints of biocompatibility and exhibits the ability to bond to the chosen polymer. In addition to the carbon fibers disclosed above, such fibers include inert high strength glass, aramid fibers, and inert ceramic fibers, such as alumina. The fiber orientation and type of weave may be varied for different applications and can be pre-selected and optimized using well known analysis techniques. Moreover, the disclosed hybrid woven bioactive composite can be constructed not only as the three dimensional braided textile structure discussed above, but any woven textile structure such as a two-dimensional braid, fiber interlock weave, or laminated composite, among others. Finally, those of skill in the art will understand that the present invention may be adapted to many applications where material shape, strength, stiffness, and fixation to bone are among the design parameters. In accordance with the present invention fibrous composites of biocompatible materials can be made into bioactive composites by incorporating bone bioactive glass or ceramic fibers into the weave at the bone contact surface.

Based upon the foregoing, it will be realized that a number of embodiments of the present invention beyond those discussed in detail above are possible. Such embodiments, however, will still employ the spirit of the present invention and, accordingly, reference should be made to the appended claims in order to determine the full scope of the present invention.