Abstract:
A multi-composite disc prosthesis is adapted to be implanted within the annulus of an evacuated disc nucleus space in a human spine. The disc prosthesis has a generally solid unitary body with a size and a shape adapted to be positioned within the annulus of the evacuated disc nucleus space. The body has an outer portion comprised of a first biomaterial and an inner portion comprised of a second biomaterial. The second biomaterial has a compressive modulus that is harder than a compressive modulus of the first biomaterial and the first and second biomaterials are chemically or physically bonded to form a multi-composite material that forms the solid body.

Description:
RELATED APPLICATIONS 
     The present invention claims priority to U.S. Provisional Patent Application No. 60/700,459, entitled “SPINE POLYMER PATENT,” filed Jul. 19, 2005, the disclosure of which is hereby incorporated by reference. The present application is also related to co-pending application Ser. No. 11/372,357, filed on Mar. 9, 2006 entitled, “INTERLOCKED MODULAR DISC PROSTHESIS,” to U.S. Provisional Patent Application No. 60/685,332, entitled “SPINE DISC NUCLEUS II,” filed May 24, 2005, and to U.S. Provisional Patent Application No. 60/660,107, entitled “MODULAR DISC PROSTHESIS,” filed Mar. 9, 2005, the disclosures of all of which are hereby incorporated by reference. The present invention is also related to co-pending application Ser. No. 11/372,477, filed on Mar. 9, 2006, entitled, “RAIL-BASED MODULAR DISC NUCLEUS PROSTHESIS,” the disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an implantable prosthesis for repairing damaged intervertebral discs. More particularly, the present invention relates to an artificial nucleus replacement prosthesis comprising a multi-part composite disc prosthesis that includes a first softer outer biomaterial and a second harder inner biomaterial. 
     BACKGROUND OF THE INVENTION 
     The spinal motion segment consists of a unit of spinal anatomy bounded by two vertebral bodies, including the two vertebral bodies, the interposed intervertebral disc, as well as the attached ligaments, muscles, and the facet joints. The disc consists of the end plates at the top and bottom of the vertebral bones, the soft inner core, called the nucleus and the annulus fibrosis running circumferentially around the nucleus. In normal discs, the nucleus cushions applied loads, thus protecting the other elements of the spinal motion segment. A normal disc responds to compression forces by bulging outward against the vertebral end plates and the annulus fibrosis. The annulus consists of collagen fibers and a smaller amount of elastic fibers, both of which are effective in resisting tension forces. However, the annulus on its own is not very effective in withstanding compression and shear forces. 
     As people age the intervertebral discs often degenerate naturally. Degeneration of the intervertebral discs may also occur in people as a result of degenerative disc disease. Degenerative disc disease of the spine is one of the most common conditions causing pain and disability in our population. When a disc degenerates, the nucleus dehydrates. When a nucleus dehydrates, its ability to act as a cushion is reduced. Because the dehydrated nucleus is no longer able to bear loads, the loads are transferred to the annulus and to the facet joints. The annulus and facet joints are not capable of withstanding their increased share of the applied compression and torsional loads, and as such, they gradually deteriorate. As the annulus and facet joints deteriorate, many other effects ensue, including the narrowing of the interspace, bony spur formation, fragmentation of the annulus, fracture and deterioration of the cartilaginous end plates, and deterioration of the cartilage of the facet joints. The annulus and facet joints lose their structural stability and subtle but pathologic motions occur between the spinal bones. 
     As the annulus loses stability it tends to bulge outward and may develop a tear allowing nucleus material to extrude. Breakdown products of the disc, including macroscopic debris, microscopic particles, and noxious biochemical substances build up. These breakdown products stimulate sensitive nerve endings in and around the disc, producing low back pain and sometimes, sciatica. Affected individuals experience muscle spasms, reduced flexibility of the low back, and pain when ordinary movements of the trunk are attempted. 
     Degeneration of a disc is irreversible. In some cases, the body will eventually stiffen the joints of the motion segment, effectively re-stabilizing the discs. Even in the cases where re-stabilization occurs, the process can take many years and patients often continue to experience disabling pain. Extended painful episodes of longer than three months often leads patients to seek a surgical solution for their pain. 
     Several methods have been devised to attempt to stabilize the spinal motion segment. Some of these methods include: heating the annular region to destroy nerve endings and strengthen the annulus; applying rigid or semi-rigid support members on the sides of the motion segment or within the disc space; removing and replacing the entire disc with a generally rigid plastic, articulating artificial device; removing and replacing the nucleus; and spinal fusion involving permanently fusing the vertebrae adjacent the affected disc. 
     Until recently, spinal fusion has generally been regarded as the most effective surgical treatment to alleviate back pain due to degeneration of a disc. While this treatment is often effective at relieving back pain, all discal motion is lost in the fused spinal motion segment. The loss of motion in the affected spinal segment necessarily limits the overall spinal mobility of the patient. Ultimately, the spinal fusion places greater stress on the discs adjacent the fused segment as these segments attempt to compensate for lack of motion in the fused segment, often leading to early degeneration of these adjacent spinal segments. 
     Current developments are focusing on treatments that can preserve some or all of the motion of the affected spinal segment. One of these methods to stabilize the spinal motion segment without the disadvantages of spinal fusion is total disc replacement. Total disc replacement is a highly invasive and technically demanding procedure which accesses the disc from an anterior or frontal approach and includes dividing the anterior longitudinal ligament, removing the cartilaginous end plates between the vertebral bone and the disc, large portions of the outer annulus and the complete inner nucleus. Then an artificial total disc prosthesis is carefully placed in the evacuated disc space. Many of the artificial total disc prosthesis currently available consist of a generally rigid plastic such as ultra high molecular weight polyethelyene (“UHMWPE) as the nucleus that is interposed between two metal plates that are anchored or attached to the vertebral endplates. A summary of the history of early development and designs of artificial discs is set forth in Ray, “The Artificial Disc: Introduction, History and Socioeconomics,” Chpt. 21,  Clinical Efficacy and Outcome in the Diagnosis of Low Back Pain , pgs. 205-225, Raven Press (1992). Examples of these layered total disc replacement devices are shown, for example, in U.S. Pat. Nos. 4,911,718, 5,458,643, 5,545,229 and 6,533,818. 
     These types of artificial total discs have several disadvantages. First, because the artificial disc prostheses are relatively large, they require relatively large surgical exposures to accommodate their insertion. The larger the surgical exposure, the higher the chance of infection, hemorrhage or even morbidity. Also, in order to implant the prosthesis, a large portion of the annulus must be removed. Removing a large portion of the annulus reduces the stability of the motion segment, at least until healing occurs around the artificial disc. Further, because the devices are constructed from rigid materials, they can cause serious damage if they were to displace from the disc space and contact local nerve or vascular tissues. Another disadvantage is that rigid artificial disc replacements do not reproduce natural disc mechanics. 
     An alternative to total disc replacement is nucleus replacement. Like an artificial disc prosthesis, these nucleus replacements are also inert, non-biological prostheses. The procedure for implanting a nucleus replacement is less invasive than the procedure for a total disc replacement and generally includes the removal of only the nucleus and replacement of the nucleus with a prosthesis that may be malleable and provide cushioning that mimics a natural disc nucleus. Examples of the prostheses used for nucleus replacement include: U.S. Pat. Nos. 4,772,287, 4,904,260, 5,192,326, 5,919,236 and 6,726,721. 
     Nucleus replacements are intended to more closely mimic natural disc mechanics. To that end, some nucleus replacements utilize hydrogels because of their water imbibing properties that enable these replacements to expand in situ to permit a more complete filling of the evacuated nucleus cavity. However, there is usually a trade-off in that the more expansion the hydrogel achieves, the less structural support the end product can provide. As a result, many hydrogel nucleus disc replacements have generally adopted the use of some form of a jacket or fabric to constrain the hydrogel material. For example, the implant described in U.S. Pat. Nos. 4,772,287 and 4,904,260 consists of a block of hydrogel encased in a plastic fabric casing. The implant described in U.S. Pat. No. 5,192,326 consists of hydrogel beads enclosed by a fabric shell. Without the jacket or other form of constraint, the hydrogel is susceptible to displacement because of the slippery nature of the hydrogel. Unfortunately, the jacket or fabric shell will be subject to long term abrasive wear issues that could result in failure of jacket or shell&#39;s ability to constrain the hydrogel and thus the hydrogel may be subject to displacement. 
     Another approach to nucleus replacement involves implantation of a balloon or other container into the nucleus, which is then filled with a biocompatible material that hardens in situ. Examples of this in situ approach to nucleus replacement include U.S. Pat. Nos. 6,443,988 and 7,001,431. One of the problems with this approach is that the chemical hardening process is exothermic and can generate significant amounts of heat that may cause tissue damage. In addition, there is a possibility that the balloon may rupture during expansion, causing leakage of material into the disc cavity and surrounding tissues, which may cause undesirable complications. 
     Yet another approach is disclosed in U.S. Pat. No. 5,865,846 to Bryan et al. in which a softer inner material is contained within a harder outer shell. The Bryan patent discloses the use of a resilient body made of two or more biocompatible materials wherein the soft inner portion of the body has a Shore D hardness of 30 and the harder outer portion has a Shore D hardness of 90. A similar approach using elastomers is described in U.S. Patent Publ. Appl. No. 2005/0119752A1 to Williams et al that discloses an artificial intervertebral disc fabricated of hydrogel, polyurethane, thermoplastic elastomers or other biocompatible materials wherein the softer inner nucleus portion has a Shore A hardness in the range of 20-70 and the harder outer portion has a Shore A hardness in the range of 35-90. While seemingly similar to the natural soft-hard combination of the disc nucleus and annulus, these implants still have the same approach and problems as the sandwiched metal and polymer implants. 
     Accordingly, there is a need for a nucleus disc replacement that addresses the shortcomings of the current approaches. 
     SUMMARY OF THE INVENTION 
     The present invention is a multi-composite disc prosthesis that is adapted to be implanted within the annulus of an evacuated disc nucleus space in a human spine. The disc prosthesis has a generally solid unitary body with a size and a shape adapted to be positioned within the annulus of the evacuated disc nucleus space. The body has an outer portion comprised of a first biomaterial and an inner portion comprised of a second biomaterial. The second biomaterial has a compressive modulus that is harder than the compressive modulus of the first biomaterial and the first and second biomaterials may be bonded together to form a multi-composite material that forms the solid body. 
     The present invention is an alternative to total disc replacement. The device of the present invention uses biocompatible materials to replace the disc nucleus. The present invention has many advantages over currently available nucleus replacements. One such advantage is that in certain embodiments the nucleus replacement of the present invention may be inserted through a minimally invasive procedure through a small hole in the posterior annulus, leaving much of the annulus and surrounding vertebral cartilage intact. 
     Further, the device of the present invention will offer pain relief by retensioning the annulus, providing a cushioning effect and restoring a more normal distribution of pressure between the annulus and the nucleus. In one embodiment of the device of the present invention, the nucleus replacement consists of at least two biocompatible materials including an inner layer of a hard modulus biomaterial and an outer surrounding coat of a softer modulus biomaterial. 
     In one aspect of the present invention, the implant may include an outer soft modulus biomaterial and inner hard modulus biomaterial. In another aspect of the present invention, the outer soft modulus biomaterial and the inner hard modulus biomaterial may be chemically bonded. 
     In another aspect of the present invention the implant comprises a composite system of two biomaterials, wherein the biomaterials may consist of a biocompatible polyurethane based on a diisocyanate and a polyol. 
     In one aspect of the invention, the implant may consist of several interconnected segments that slide along a track formed of the hard modulus material such that each segment is sequentially inserted into the disc space and connected to the other segments forming a unitary device. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a cross sectional view of a modular disc prosthesis according to the present invention. 
         FIG. 2  is a view of a modular disc prosthesis according to the present invention at a first stage of insertion. 
         FIG. 3  is a view of a modular disc prosthesis according to the present invention at the final stage of insertion. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In contrast to conventional disc or nucleus replacements, the present invention comprises a composite system wherein the outer portion consists of soft modulus material mimicking the natural disc and the inner portion consists of harder modulus material which provides support and stability. Two biocompatible polymers may be chemically bonded to form the composite system of the present invention. Many conventional total disc replacements include upper and lower rigid plates and a non-rigid material disposed therebetween, while other existing nuclear replacements consist of a mass of soft material without a stabilizing hard inner core. The composite system of the present invention offers advantages over the existing devices in that the soft outer portion provides cushioning while not eroding the endplates as may happen with harder materials of other disc nucleus replacements. Further, the soft outer portion is deformable to correspond to the desired modulus in response to normal physiologic forces of about 30 to 300 pounds. Because of this deformability, the prosthesis produces a physiologically appropriate amount of loading on the end plates of the intervertebral disc. As a result, the end plates will not excessively deform over time and ultimately conform to the contours of the implant as is the case with more rigid disc nucleus replacement implants. Further, the harder inner core of the present invention provides support and stability lacking in the implants made of hydrogel blocks or chunks. 
     In an embodiment of the present invention, the nucleus replacement  10  may include several components that are sequentially inserted into the evacuated disc nucleus space. This sequential insertion allows for a small surgical exposure because the device is inserted one component at a time as opposed to some problematic devices that are inserted in their entirety requiring a larger surgical exposure. As shown in  FIG. 1 , each component  20  may be composed of an inner connecting track of hard modulus biomaterial  22  and an outer surrounding coat of a softer modulus biomaterial  24 . During insertion, the first component may slide along a track consisting of a high modulus biomaterial and into place within the disc annulus. In one embodiment, the device is inserted in a minimally invasive procedure through a small opening in the posterior annulus. Each component may mechanically interlock with the adjacent component such that when all components are fully inserted, the interlocked components comprise a single unit. 
     In one embodiment, the device of the present invention may consist of two biocompatible materials of different hardness. In an embodiment of the device, the biomaterials may consist of a biocompatible polyurethane based on a diisocyanate and a polyol. In one embodiment, the isocyanate component may be 4,4′-diphenylmethane diisocyanate (‘MDI”) and the polyol component may be a combination of polytetramethyleneoxide (“PTMO”) 1000 and PTMO 2000. The polymers may also contain a chain extender, a cross linking agent and a catalyst. In one embodiment, the chain extender may be 1,4-butanediol (“BDO”); the cross linking agent may be trimethylpropane (“TMP”) and the catalyst may be bis-(dodecylthio)-dimethylstannane (“Fomrez catalyst UL22”). The two biomaterials may be bonded together forming a composite system. For example, such bonding may be chemical or physical. In one aspect of the present invention, such bonding may include a urethane bond. 
     One of ordinary skill in the art will recognize that additional biomaterials and constituents of the biomaterials suitable for the composition of the present nucleus prosthesis are contemplated and are within the scope of the present disclosure. Other biomaterials that may be used within the scope of the present invention include, but are not limited to: hydrogels, rubbers, silicones, thermoplastic elastomers, acrylate monomers, curable epoxies, curable monomers and any combination thereof. 
     In one embodiment of the device, the outer surrounding coat of the device may be comprised of a first biomaterial consisting of a softer polymer that provides cushioning and support, mimicking the characteristics of a natural disc nucleus. In an embodiment of the device, the outer polymer may be modified to provide for elution of medicants such as analgesics, antibiotics, antineoplastics, or bioosteologics such as bone growth agents or any other desired material. While motion preservation is generally a principle goal in nucleus replacement, in certain indications it may be desirable to promote some bony fusion. Such indications may include nuclear replacements in the cervical spine. 
     The solid polymer outer shell of the modular disc nucleus prosthesis may provide for better and more controllable elution rates than some hydrogel materials. In an alternate embodiment, the modular disc nucleus prosthesis may include different elution rates for each polymer material. This would allow for varying elution rates for different medicants. 
     The softer biomaterial may consist of a harder segment content in the range of about 15 to 25 weight percent. One of ordinary skill in the art will recognize that additional ranges of hard segment weight percent within this explicit range are contemplated and are within the scope of the present disclosure. The softer biomaterial may have a compressive modulus in the range of about 10-20 MPa. For example, the softer biomaterial may have a Shore A hardness no greater than 80 and a Shore D hardness no greater than 40. The tensile strength of the softer biomaterial may be in the range of 10-30 MPa. In an embodiment of the present device, the softer biomaterial may have a yield strength of 1-1.5 MPa. The modulus of elasticity of the softer biomaterial may be in the range of 6-8 MPa. One of ordinary skill in the art will recognize that additional ranges within the explicit ranges set forth hereinabove are contemplated and are within the scope of the present disclosure. 
     One embodiment of device may further include a second biomaterial. The second biomaterial may consist of a harder polymer of high durometer, preferably of at least a Shore D hardness of 55. The hardness of the second biomaterial provides structural support for the insertion track and the interlocking mechanism. In an alternative embodiment, the first or second biomaterial may consist of a thermoplastic polyether-urethane or polycarbonate-urethane, such as Pellethane®, Tecothane® or Bionate®. In an embodiment, the first or second biomaterial may consist of poly-ether-ether-ketone (PEEK) or another polymer of similar stiffness. In another alternative embodiment, the second biomaterial may consist of a MDI, PTMO based polyurethane processed to have a hard segment weight content in the range of about 50 to 70 percent, smaller homogenous molecular weight chain lengths in the prepolymer and an optimal micro-phase separation of the hard and soft segment components to provide a macroscopically homogenous distribution in the cured polymer. One of ordinary skill in the art will recognize that additional ranges of hard segment weight percent within this explicit range are contemplated and are within the scope of the present disclosure. 
     The harder biomaterial may have a tensile strength in the range of 40-75 MPa. The yield strength of the harder biomaterial may be in the range of 20-45 MPa. The harder biomaterial may have a modulus of elasticity in the range of 400-700 MPa. The compressive modulus of the harder biomaterial may be in the range of 200-400 MPa. One of ordinary skill in the art will recognize that additional ranges within the explicit ranges set forth hereinabove are contemplated and are within the scope of the present disclosure. 
     In an aspect of the first softer biomaterial, the weight percent of the MDI may be in a range of 5 to 35 weight percent of the total cured polymer. In an alternate embodiment of the first softer biomaterial, the weight percent of the MDI may be in a range of 15 to 25 weight percent of the total cured polymer. In one embodiment, the weight percent of the MDI may be in a range of about 18 to 20 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of MDI weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the PTMO 1000 of the first softer biomaterial may be in a range of 0 to 40 weight percent of the total cured polymer. In an alternate embodiment, the weight percent of the PTMO 1000 may be in a range of 10 to 30 weight percent of the total cured polymer. In one embodiment, the weight percent of the PTMO 1000 may be in a range of 25 to 27 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of PTMO 1000 weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the PTMO 2000 of the first softer biomaterial may be in a range of 0 to 80 weight percent of the total cured polymer. In an alternate embodiment, the weight percent of the PTMO 2000 may be in a range of 40 to 60 weight percent of the total cured polymer. In one embodiment, the weight percent of the PTMO 2000 may be in a range of 52 to 54 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of PTMO 2000 weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the BDO of the first softer biomaterial may be in a range of 0 to 10 weight percent of the total cured polymer. In an alternate embodiment, the weight percent of the BDO may be in a range of 0 to 5 weight percent of the total cured polymer. In one embodiment, the weight percent of the BDO may be in a range of 1 to 2 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of BDO weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the TMP of the first softer biomaterial may be in a range of 0 to 5 weight percent of the total cured polymer. In an alternate embodiment, the weight percent of the TMP may be in a range of 0 to 0.1 weight percent of the total cured polymer. In one embodiment, the weight percent of the TMP may be in a range of 0.06 to 0.08 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of TMP weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the UL22 of the first softer biomaterial may be in a range of 0 to 2 weight percent of the total cured polymer. In an alternate embodiment, the weight percent of the UL22 may be in a range of 0 to 1 weight percent of the total cured polymer. In one embodiment, the weight percent of the UL22 may be in a range of 0.0001 to 0.0030 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of UL22 weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     In one aspect of the first softer biomaterial, the combined weights of the MDI and BDO generally correlate to the hard segment content and hardness of the cured polymer. In an embodiment of the first softer biomaterial, the combined weight percentage of the MDI and BDO may be in a range of about 15 to 25 weight percent of the total cured polymer. In one embodiment of the first softer biomaterial, the combined weight percentage of the MDI and BDO may be in a range of about 20 to 22 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of combined MDI and BDO weight percentages of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The first softer biomaterial may comprise two separate prepolymers, Part A and Part B, that are mixed together to form the cured polymer. In one embodiment, Part A is formed by processing MDI and PTMO 2000 together and Part B is formed by processing PTMO 1000, BDO, TMP and UL22 together. Any combination of MDI, PTMO 1000, PTMO 2000, BDO, TMP, UL22 and/or other suitable constituents may be processed to form the prepolymers, Part A and Part B. In an embodiment of the first softer biomaterial where Part A and Part B are mixed together to form the cured polymer, Part A and Part B may be mixed such that the total isocyanate to polyol ratio is in the range of about 0.96 to 1.04. In one embodiment, Part A and Part B may be mixed together such that the total isocyanate to polyol ratio is in the range of about 1.01 to 1.03. One of ordinary skill in the art will recognize that additional ranges of total isocyantate to polyol ratios within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the MDI of the second harder biomaterial may be in a range of 30 to 70 weight percent of the total cured polymer. In an alternate embodiment of the second harder biomaterial, the weight percent of the MDI may be in a range of 40 to 60 weight percent of the total cured polymer. In one embodiment of the second harder biomaterial, the weight percent of the MDI may be in a range of about 47 to 49 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of MDI weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the PTMO 1000 of the second harder biomaterial may be in a range of 0 to 40 weight percent of the total cured polymer. In an alternate embodiment of the second harder biomaterial, the weight percent of the PTMO 1000 may be in a range of 10 to 30 weight percent of the total cured polymer. In one embodiment of the second harder biomaterial, the weight percent of the PTMO 1000 may be in a range of about 20 to 22 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of PTMO 1000 weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the PTMO 2000 of the second harder biomaterial may be in a range of 0 to 40 weight percent of the total cured polymer. In an alternate embodiment of the second harder biomaterial, the weight percent of the PTMO 2000 may be in a range of 10 to 30 weight percent of the total cured polymer. In one embodiment of the second harder biomaterial, the weight percent of the PTMO 2000 may be in a range of about 15 to 17 weight percent of the total cured polymer. One of ordinary skill in the art will recognize that additional ranges of PTMO 2000 weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the BDO of the second harder biomaterial may be in a range of 0 to 35 weight percent of the total cured polymer. In an alternate embodiment of the second harder biomaterial, the weight percent of the BDO may be in a range of 5 to 25 weight percent of the total cured polymer. In one embodiment of the second harder biomaterial, the weight percent of the BDO may be in a range of about 14 to 16 weight percent of the total cured polyurethane. One of ordinary skill in the art will recognize that additional ranges of BDO weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the TMP of the second harder biomaterial may be in a range of 0 to 5 weight percent of the total cured polyurethane. In an alternate embodiment of the second harder biomaterial, the weight percent of the TMP may be in a range of 0 to 1 weight percent of the total cured polyurethane. In one embodiment of the second harder biomaterial, the weight percent of the TMP may be in a range of about 0.1 to 0.3 weight percent of the total cured polyurethane. One of ordinary skill in the art will recognize that additional ranges of TMP weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     The weight percent of the UL22 of the second harder biomaterial may be in a range of 0 to 2 weight percent of the total cured polyurethane. In an alternate embodiment of the second harder biomaterial, the weight percent of the UL22 may be in a range of 0 to 1 weight percent of the total cured polyurethane. In one embodiment of the second harder biomaterial, the weight percent of the UL22 may be in a range of about 0.0001 to 0.002 weight percent of the total cured polyurethane. One of ordinary skill in the art will recognize that additional ranges of UL22 weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     In one embodiment of the second harder biomaterial, the combined weights of the MDI and the BDO generally correlate to the hard segment content and hardness of the cured polymer. The combined weight of the MDI and BDO may be in the range of about 50 to 70 weight percent of the total weight of the cured polymer. In one embodiment, the combined weight of the MDI and BDO may be in the range of about 62 to 64 weight percent of the total weight of the cured polymer. One of ordinary skill in the art will recognize that additional ranges of combined MDI and BDO weight percent of the total cured polymer within the above described explicit ranges are contemplated and are within the scope of the present disclosure. 
     For a more detailed description of one tracked embodiment of  FIGS. 2 and 3  of the present invention, reference is made to the previously identified co-pending application entitled, “RAIL-BASED MODULAR DISC PROTHESIS,” the disclosure of which is hereby incorporated by reference. 
     In one aspect of the implant of the present invention, the second harder biomaterial may be comprised of two separate prepolymers, Part A and Part B. Part A and Part B may be selected from the group consisting of MDI, TDI, PTMO 1000, PTMO 2000, BDO, TMP, UL22 or any other combination of suitable constituents. Further, Part A may be processed such that the prepolymer contains smaller molecular weight chain lengths of one or two polymer populations than that of Part B. In one embodiment, the MDI, PTMO 1000 and PTMO 2000 are processed together to form the Part A. Preferably, the BDO, TMP and UL22 are processed together to form the Part B. Part A and Part B may be mixed such that the total isocyanate to polyol ratio is in the range of about 0.96 to 1.04. In one embodiment, the isocyanate to polyol ratio is in the range 1.01 to 1.03. One of ordinary skill in the art will recognize that additional ranges of isocyanate to polyol ratios within the above described explicit ranges are contemplated and are within the scope of the present disclosure. Various modifications to the disclosed apparatuses and methods may be apparent to one of skill in the art upon reading this disclosure. The above is not contemplated to limit the scope of the present invention, which is limited only by the claims below.