Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C39/00—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor
  • B29C39/003—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor characterised by the choice of material
  • B29C39/006—Monomers or prepolymers
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L31/00—Materials for other surgical articles, e.g. stents, stent-grafts, shunts, surgical drapes, guide wires, materials for adhesion prevention, occluding devices, surgical gloves, tissue fixation devices
  • A61L31/12—Composite materials, i.e. containing one material dispersed in a matrix of the same or different material
  • A61L31/125—Composite materials, i.e. containing one material dispersed in a matrix of the same or different material having a macromolecular matrix
  • A61L31/129—Composite materials, i.e. containing one material dispersed in a matrix of the same or different material having a macromolecular matrix containing macromolecular fillers
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L31/00—Materials for other surgical articles, e.g. stents, stent-grafts, shunts, surgical drapes, guide wires, materials for adhesion prevention, occluding devices, surgical gloves, tissue fixation devices
  • A61L31/14—Materials characterised by their function or physical properties, e.g. injectable or lubricating compositions, shape-memory materials, surface modified materials
  • A61L31/146—Porous materials, e.g. foams or sponges
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C39/00—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor
  • B29C39/02—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor for making articles of definite length, i.e. discrete articles
  • B29C39/021—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor for making articles of definite length, i.e. discrete articles by casting in several steps
  • B29C39/025—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor for making articles of definite length, i.e. discrete articles by casting in several steps for making multilayered articles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C39/00—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor
  • B29C39/02—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor for making articles of definite length, i.e. discrete articles
  • B29C39/10—Shaping by casting, i.e. introducing the moulding material into a mould or between confining surfaces without significant moulding pressure; Apparatus therefor for making articles of definite length, i.e. discrete articles incorporating preformed parts or layers, e.g. casting around inserts or for coating articles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
  • B29L2031/00—Other particular articles
  • B29L2031/753—Medical equipment; Accessories therefor
  • B29L2031/7532—Artificial members, protheses

Definitions

• the present invention relates to implants for use in interbody fusion and methods of manufacturing such implants and, more particularly, to implants formed from synthetic bone polymers.
• a discectomy or corpectomy may be performed to remove an intervertebral disc or a vertebral body or portion thereof. It is known to implant interbody spacers to replace the removed intervertebral disc or vertebral body to restore height and spinal stability.
• interbody spacer used for spinal fusion is a machined allograft interbody spacer, which is formed from bone transplanted from another person, typically a cadaver.
• machined allograft interbody spacers are advantageous because they eliminate the need for the second operative site.
• machined allograft interbody spacers have other drawbacks that make them undesirable for spinal fusion applications. For example, there is a limited supply of qualified bone that can be formed into machined allograft interbody spacers, which results in increased cost and product backorder. Also, the size and shape of available qualified bone limits the size of machined allograft spacers.
• the transplanted bone must be tested for disease and undergo expensive sterilization to reduce the risk of disease transmission.
• the cadaver bone must also be manufactured into the proper spacer geometry for the machined allograft interbody spacer since the transplanted cadaver bone cannot exactly match the disk being removed from a patient.
• the varied quality of source bone also makes it challenging to maintain uniform mechanical properties of allograft interbody spacers.
• Some allograft multiple bone density spacers may be cut as a single piece from cadaver bone, for example, from the femur bone.
• allograft interbody spacers are typically assembled from multiple bone density regions, which requires the additional manufacturing of a mechanical interlock, such as a pin feature or a dovetail feature, between the parts of the multipart spacer, thereby increasing cost of manufacturing.
• Interbody spacers have also been formed from non-bone material as hollow rigid structures, for example, from metal or polyaryletheretherketone (PEEK). These hollow rigid spacers have many deficiencies. For example, metal spacers are too stiff to share the load across the vertebrae and PEEK is very brittle. Rigid spacers formed from metal or PEEK also fail to provide a structure for osteoconduction. Thus, if osteoconduction is desired, a secondary material is required to act as an osteoconductive scaffold. Additionally, hollow rigid spacers may result in vertebrae getting crushed due to their stiffness. Hollow rigid spacers formed from metal also require a relatively significant amount of machining, increasing manufacturing complexity. [0007] Interbody spacers have also been formed from composite synthetic structures using heat to expand and contract metal tube over porous ceramic structure. These have the same disadvantage of hollow rigid structures formed of metal in that they are too stiff to share the load.
• Single density interbody spacers formed from polyurethanes have also been manufactured for spinal fusion applications.
• Polyurethanes are advantageous for orthopedic applications because fillers, such as calcium phosphate or calcium carbonate, can be incorporated into the polyurethane to form a more porous structure through resorption, which allows a targeted porosity for osteoconduction to be achieved.
• fillers such as calcium phosphate or calcium carbonate
• polyurethane interbody spacers formed with a porous structure lack the strength to withstand the forces seen after spinal fusion.
• a multi-density polymeric interbody spacer is a synthetic spacer that may be implanted to restore height and promote bone fusion after discectomy or corpectomy.
• the multi-density polymeric interbody spacer is formed from biocompatible polymeric foam for
• the multi-density structure provides for combined strength and porosity.
• the multi-density spacer includes direct adhesion and mechanical interlocking between different density regions to increase the strength of the interbody spacer.
• the multi-density spacer may also include geometric surface features to enhance positioning and fit of the spacer.
• the multi- density polymeric interbody spacer includes a central first density region of lower density and two lateral second density regions of greater density adjacent to the central first density region.
• a method for forming a multi- density polymeric interbody spacer includes curing the first density region of lower density in a vacuum to achieve a target porosity.
• the cured first density region may be machined to achieve a desired shape, for example a cylinder or a rectangular shape.
• the second density region or regions of greater density may then be molded, under pressure, to the first density region of lower density.
• a portion of the region of greater density partially flows into the pores of the first density region of lower density, to form an interface region providing direct adhesion and porous interlocking between the first density region of lower density and the second density region or regions of greater density.
• the multi-density polymeric interbody spacer may then be machined to achieve a desired final shape or to add geometric features to enhance positioning and fit of the spacer.
• multiple multi-density polymeric interbody spacers may be molded as a single multi-density polymeric volume.
• the multi-density polymeric interbody spacers are then cut from the multi-density polymeric volume.
• the second density region may be formed in a closed mold to achieve the second pressure.
• the multi-density polymeric interbody spacer is molded between first and second platens.
• the orientation of the first and second platens is changed during the curing process to impart the multi-density polymeric interbody spacer with anisotropic material properties.
• FIG. 2 is a perspective view of another embodiment of the multi- density polymeric interbody spacer
• FIG. 4 is a cross-sectional view of the multi-density polymeric interbody spacer according to FIG. 1 implanted between vertebrae;
• FIG. 5 is a perspective view of another embodiment of the multi- density polymeric interbody spacer of FIG. 2;
• FIG. 6 is an enlarged cross-sectional view of a portion of an interface region of the multi-density polymeric interbody spacer of FIG. 4;
• FIG. 8 is a cross-sectional view of a multi-density polymeric interbody spacer according to another embodiment of the present invention.
• FIG. 9 is a process diagram showing a method of making the multi- density polymeric interbody spacer of FIG. 1 ;
• FIG. 10 is a perspective view of a second density region of another embodiment of the multi-density polymeric interbody spacer;
• FIG. 11 is a perspective view of a first density region of another embodiment of the multi-density polymeric interbody spacer
• FIG. 12 is a perspective view of another embodiment of the multi- density polymeric interbody spacer
• FIG. 13 is a process step for fabricating a plurality of the multi- density polymeric interbody spacers of FIG. 1 ;
• FIG. 15 is a cross-sectional view of the multi-density polymeric interbody spacer of FIG. 14;
• FIG. 16 is a perspective view of another embodiment of the multi- density polymeric interbody spacer
• FIG. 17 is a process step for fabricating a plurality of the multi- density polymeric interbody spacers of FIG. 16;
• FIG. 18 is a perspective view of another embodiment of the multi- density polymeric interbody spacer.
• FIG. 19 is a perspective view of another embodiment of the multi- density polymeric interbody spacer.
• FIG. 20 is a process diagram showing a method of implanting the multi-density polymeric interbody spacer of FIG. 19;
• FIG. 21 is a perspective view of another embodiment of the multi- density polymeric interbody spacer;
• FIG. 22 is a process diagram showing another method of making the multi-density polymeric interbody spacer of FIG. 1 ;
• FIG. 23 is a process diagram showing another method of forming a first density region of the multi-density polymeric interbody spacer of FIG. 1 ;
• FIG. 24 is a cross-sectional view of a biocompatible polymeric material of FIG. 23;
• FIG. 25 is a process diagram showing another method of forming the multi-density polymeric interbody spacer of FIG. 1 ;
• FIG. 26 is a process diagram showing another method of forming the multi-density polymeric interbody spacer of FIG. 1 ;
• FIG. 27 is a cross-sectional view of another embodiment of the multi-density polymeric interbody spacer
• FIG. 28 is a process step of another embodiment for fabricating the the multi-density polymeric interbody spacer
• FIG. 29 is a cut-away perspective view of another embodiment of the multi-density polymeric interbody spacer.
• FIG. 30 is a perspective view of another embodiment of the multi- density polymeric interbody spacer. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
• the multi-density polymeric interbody spacer 10 includes an interface region 36 connecting the first density region 12 to the second density region 14.
• the first density region 12 and the second density region 14 may be connected to one another through direct adhesion in the interface region 36; for example, by adhesive properties of the first density region 12, the second density region 14 or both.
• Direct adhesion as used herein includes adsorption, chemical bonding and/or diffusion or any other method of adhesion know to one skilled in the art.
• the interface region 36 may include mechanical interlocking in the form of a porous interlocking 38, which forms a mechanical connection between the first density region 12 and the second density region 14.
• the porous interlocking 38 is formed by a portion of the second density region 14 that occupies pores 16 located around the first region perimeter surface 26 of the first density region 12.
• the interface region 36 is less than one millimeter (1 mm) in thickness.
• the first density region 312 may instead, more preferably, have an open pore cell structure as shown in FIG. 7 by providing more interconnectivity between pores 316.
• the open pore cell structure with increased pore interconnectivity provides for strong mechanical interlocking through porous interlocking 338.
• the multi-density polymeric interbody spacer 410 may also include macro features 440, for example, a dovetail feature or a pin feature, to provide an additional or alternative mechanical interlock between the first density region 412 and the second density region 414.
• the multi-density polymeric interbody spacer 410 may include the porous interlocking, the mechanical interlock or a combination of both the porous interlocking and the mechanical interlock.
• the macro features 440 may be formed to extend from a perimeter surface 426 of the first density region 412, as shown.
• the macro features 440 may be formed as cavities in the perimeter surface 426 of the first density region 412, into which a portion of the second density region 414 is able to penetrate.
• a method of forming the multi-density polymeric interbody spacer 10 using varying pressures to affect porosity is shown.
• a biocompatible polymeric material 42 in a liquid state, is poured into a first mold 44 at a first pressure 46.
• the biocompatible polymeric material 42 is preferably the KRYPTONITETM bone matrix product, available from DOCTORS RESEARCH GROUP, INC. of Southbury, Connecticut.
• the first pressure 46 may be established, for example, by placing the first mold 44 in a vacuum chamber 48 to create a low-pressure environment.
• biocompatible polymeric material 42 may instead be poured into the first mold 44 and then subjected to the first pressure 46, for example, by placing the filled first mold 44 in a vacuum chamber 48.
• step S4 the biocompatible polymeric material 42 is maintained at the first pressure 46 and allowed to polymerize, which results in off-gassing of carbon dioxide byproducts, to form the first density region 12.
• the carbon dioxide byproducts of the polymerization process expand and form large pores 16 with a high degree of pore interconnectivity in the first density region 12.
• the first pressure 46 is in the range of approximately ten inches of mercury to thirty inches of mercury (10" Hg to 30" Hg) to produce the first density region 12 having approximately sixty percent to ninety percent (60%-90%) porosity. All pressures are gauge pressures relative to atmospheric pressure.
• the first pressure 46 is preferably selected to provide high pore interconnectivity by allowing for a high degree of carbon dioxide cell rupture during polymerization, resulting in pores 16 that are interconnected.
• the low first pressure 46 makes it possible to form an open cell structure within a biocompatible polymeric material 42 that would have a substantially closed pore structure at ambient pressure.
• the first pressure 46 is preferably a vacuum, the first pressure 46 may be any other pressure capable of forming the desired porosity of the first density region 12, including ambient pressure.
• step S6 the fully polymerized biocompatible polymeric material
• the fully polymerized biocompatible polymeric material 42 may include a skim coat 50 around its perimeter surface, which may result from the molding process.
• the skim coat 50 is a smooth layer of biocompatible polymeric material 42, formed on the perimeter surface, with substantially no pores and is typically less than one millimeter (1 mm) thick.
• the skim coat 50 if present, is removed from the molded biocompatible polymeric material 42, for example by cutting or blasting, from the first density region 12 and to expose pores 16 around the first region perimeter surface 26 of the first density region 12.
• the molding process results in near net production of the first density region 12, thereby obviating or minimizing post molding machining.
• the first density region 12 may be machined to the proper and/or desired final shape, for example, from a larger block of the molded biocompatible polymeric material 42.
• Other known methods of increasing porosity in a primarily closed cell porous structure to form a relatively open cell porous structure may also be implemented to produce the first density region 12. For example, as an
• step S10 the first density region 12 is positioned in a second mold 52 that provides space 54 for molding the second density region 14.
• biocompatible polymeric material 42 in liquid state, is added to the second mold 52 at a second pressure 56 to fill space 54.
• the liquid state biocompatible polymeric material 42 is able to flow and expand into the pores 16 formed on the first region perimeter surface 26 of the first density region 12.
• the biocompatible polymeric material 42 may instead be added to the second mold 52 and then subjected to the second pressure 56.
• step S14 the biocompatible polymeric material 42 is maintained at the second pressure 56 and allowed to polymerize to form the second density region 14.
• step S16 the fully polymerized biocompatible polymeric material
• the present invention has been described as implementing the lower first pressure 46 to fabricate the high porosity first density region 12 in the form of a core and implementing the relatively high second pressure 56 to fabricated the low porosity second density region 14 to surround the high porosity first density region 12.
• the lower first pressure 46 may instead be used to fabricate a high porosity outer first density region 12 and the second pressure 56 used to form the core low porosity second density region 14.
• Forming the first density region 12 with a higher porosity prior to forming the second density region 14 with a lower porosity is advantageous because larger and more numerous pores 16 are formed on the first region perimeter surface 26, providing for a strong porous interlocking 38.
• the user may be provided with only the high porosity first density region 12, i.e. in the form of a cylindrical core.
• the user adds the biocompatible polymeric material 42, in taffy- like form as discussed above, around the first density region 12 to form the second density region 14. This allows the user to shape a customized spacer perimeter surface 30 and customized geometry for the multi-density polymeric interbody spacer 10.
• Additional density regions may be formed in the same manner to provide the multi-density polymeric interbody spacer 510 with any desired number of regions of alternating or differing porosities. Forming the multi-density polymeric interbody spacer 510 with a relatively low density external third density region 558 may speed bone ingrowth in applications where osteoclasts and osteoblasts migrate from the exterior surfaces of the host bone.
• the relatively porous exterior also provides a structure to which secondary osteoconductive or osteoinductive agents can be more readily added and retained.
• a plurality of multi-density polymeric interbody spacers 10 may be fabricated according to the process of FIG. 9 by forming elongated first and second molds 44, 52.
• the first and second molds 44, 52 may be ten (10) times the desired length of a single multi- density polymeric interbody spacer 10.
• the elongated first and second molds 44, 52 produce a multi-density polymeric volume 60.
• the process of FIG. 9 includes an additional final step S18, shown in FIG. 13, to cut the multi-density polymeric interbody spacers 10 from the multi-density polymeric volume 60 using a cutting tool 62.
• the multi-density polymeric interbody spacer 10 of FIG. 1 is shown with first region perimeter surface 26 and second region perimeter surface 28 as substantially cylindrical, the shape and geometry of the multi-density polymeric interbody spacer 10 may be varied to balance a variety of factors including implant strength, osteoconductive potential, ease of implantation, anatomic fit and user familiarity to currently available products.
• the multi-density polymeric interbody spacer 610 may include lateral slots 663 for mating with an insertion tool (not shown) to ease implantation and handling of the multi-density polymeric interbody spacer 610.
• the multi-density polymeric interbody spacer 710 includes two second density regions 714 disposed laterally on either side of the medial first density region 712.
• the second density regions 714 include lateral edges 768 that form the remainder of the spacer perimeter surface 730.
• the multi-density polymeric interbody spacer 710 has two interface regions 736, each formed between one of the second density regions 714 and the adjacent edge of the first density region 712.
• the multi-density polymeric interbody spacer [0078] Referring to FIG 19, the multi-density polymeric interbody spacer
• step S920 the multi-density polymeric interbody spacer 910 is placed between the first vertebral end plates 932 and the second vertebral end plate 934.
• step S922 the biocompatible polymeric material 942 is injected into radial channel 976.
• the biocompatible polymeric material 942 passes through the radial channel 976, into the axial channel 974 and extrudes from the multi-density polymeric interbody spacer 910 onto and into the first vertebral end plate 932 and the second vertebral end plate 934.
• step S924 the biocompatible polymeric material 942 cures to form an adhesive bond region 978.
• the adhesive bond region 978 may have the same density as the first density region 912 or the second density region 914. Alternatively, the adhesive bond region 978 may have a density that differs from both the first density region 912 and the second density region 914.
• step S1026 the second mold 1052 is closed with the mold closure member 1080 and biocompatible polymeric material 1042, in liquid state, is injected into the closed second mold 1052 through an injector 1082.
• the biocompatible polymeric material 1042 may instead be poured into the second mold 1052, as shown in step S12 of FIG. 9, prior to closing the second mold 1052 with the mold closure member 1080.
• the liquid state biocompatible polymeric material 1042 fills the space 1054 and flows into the pores 1016 formed on the first region perimeter surface 1026 of the first density region 1012.
• step S1028 the biocompatible polymeric material 1042 is allowed to polymerize to form the second density region 1014.
• the closed second mold 1052 restricts expansion of the biocompatible polymeric material 1042 during polymerization, which produces the high-pressure environment and provides the second pressure 1056. Since the expansion of the carbon dioxide byproducts of the polymerization process is restricted, the carbon dioxide produces smaller pores, resulting in a second density region 1014 with a lower porosity and, conversely, a higher density than the first density region 1012. Since the liquid biocompatible polymeric material 1042 is able to flow into the pores 1016 of the first density region 1012 during step S1026, the biocompatible polymeric material 1042 cures in the pores 1016 during step S1028 to form the porous interlocking shown in FIGS. 3 and 4.
• the first density region 1112 of the multi-density polymeric interbody spacer may be formed with anisotropic material properties to improve
• step S1130 the biocompatible polymeric material 1142, in liquid state, is poured into a first platen 1184 at the first pressure 1146.
• the biocompatible polymeric material 1142 has isotropic material properties.
• the first platen 1184 is designed to mold the biocompatible polymeric material 1142 into the first density region 1112.
• the first pressure 1146 may be established, for example, by placing the first platen 1184 in a vacuum chamber 1148 to create a low-pressure environment.
• a second platen 1186 is lowered onto the liquid biocompatible polymeric material 1142 and held in a fixed position while the biocompatible polymeric material 1142 is allowed to partially cure.
• the first and second platens 1184, 1186 may be held in the fixed position for approximately five minutes to fifteen minutes (5 minutes - 15 minutes) to allow for partial curing.
• the time necessary for partial curing of the biocompatible polymeric material 1142 will largely depend upon the material formulation and may, therefore, vary.
• the curing temperature may also be employed to affect the curing rate and alter the time necessary to fix the first and second platens 1184, 1186. Off-gassing of carbon dioxide byproducts forms large pores 1116 in the same manner discussed in connection with FIG. 9.
• step S1134 when the biocompatible polymeric material 1142 is in the taffy-like stage of the curing process, the first platen 1184 and the second platen 1186 are pulled apart from one another in a displacement direction 1188, thereby pulling the partially cured biocompatible polymeric material 1142, which, therefore, elongates in the displacement direction 1188.
• the biocompatible polymeric material 1142 may elongate in thickness in the range of approximately fifty percent to three hundred percent (50%- 300%) after material expansion due to carbon dioxide release during polymerization. The elongation of the biocompatible polymeric material 1142 results in an anisotropic orientation of the partially cured biocompatible polymeric material 1142.
• step S1136 the displacement of the first and second platens 1184, 1186 stretches the pores 1116, formed in the taffy-like biocompatible polymeric material 1142, in the displacement direction 1188.
• step S1136 the first platen 1184 and the second platen 1186 are held in the displaced position while the biocompatible polymeric material 1142 is maintained at the first pressure 1146 and allowed to fully cure.
• the taffy-like biocompatible polymeric material 1142 retains its anisotropic orientation while the curing process is completed, which results in anisotropic properties for the fully cured biocompatible polymeric material 1142.
• curing As noted above, curing
• step S11308 the biocompatible polymeric material 1142 is removed from the first platen 1184 and the second platen 1186. Additionally, the cured biocompatible polymeric material 1142 may be removed from the first pressure 1146. The cured biocompatible polymeric material 1142 may then undergo the remainder of the process of FIG. 9 to remove the skim coat 1150 and/or be shaped to form the first density region 1112. [0085] Referring to FIG. 24, the fully cured anisotropic biocompatible polymeric material 1142 may also have a middle portion 1190 sectioned to form the first density region 1112 if necking, i.e. a localized decrease in cross section of a portion of the biocompatible polymeric material 1142, results from the first and second platens 1184 and 1186 being pulled apart. Even if necking occurs, the middle portion 1190 will retain substantially uniform anisotropic properties.
• biocompatible polymeric material 1142 will be oriented longitudinally, providing increased passageways for cell and nutrient migration through the multi-density polymeric interbody spacer (not shown).
• the desired porosity of the first density region 1112 may also be formed by
• step S1240 the biocompatible polymeric material 1242, in liquid state, is poured into the first mold 1244 at the first pressure 1246.
• a temperature control unit 1294 for example a heater, elevates the temperature of the first mold 1244.
• step S1244 the biocompatible polymeric material 1242 polymerizes while the elevated temperature and the first pressure 1246 are maintained.
• a density gradient from the center to the edge of the biocompatible polymeric material 1242 is produced during the polymerization process because the elevated temperature accelerates polymerization near the external surface of the biocompatible polymeric material 1242.
• the accelerated polymerization results in minimal off-gassing of carbon dioxide, which produces the low porosity second density region 1214.
• biocompatible polymeric material 1242 polymerizes more gradually and off-gases carbon dioxide, producing the higher porosity first density region at the core.
• biocompatible polymeric material 1242 is removed from the first mold 1244 to produce the multi-density polymeric interbody spacer 1210.
• the molding process results in near net production; however, if necessary the multi-density polymeric interbody spacer 1210 may be machined to the proper shape.
• step S1240 the biocompatible polymeric material 1242, in liquid state, is poured into the first mold 1244 at the first pressure 1246.
• the biocompatible material 1242 may be poured into the first mold 1244 and then subjected to the first pressure 1246.
• step S1242 the first mold 1244 is placed in a mold rotation device 1295 and spun at an angular velocity 1297.
• step S1244 the biocompatible polymeric material 1242 polymerizes while the angular velocity 1297 is maintained.
• the angular velocity 1297 produces centrifugal forces that drive carbon dioxide produced during polymerization to the center of the first mold 1244.
• a density gradient of pores 1216, from the center to the edge of the biocompatible polymeric material 1242, is produced during the polymerization process.
• the fully polymerized biocompatible polymeric material 1242 is removed from the first mold 1244 to produce the multi-density polymeric interbody spacer 1210.
• the molding process results in near net production; however, if necessary the multi-density polymeric interbody spacer 1210 may be machined to the proper shape.
• another embodiment of the multi-density polymeric interbody spacer 1310 having first and second density regions 1312, 1314, includes a superior porous surface 1396 and an inferior porous surface 1398.
• superior surface 1396 and inferior surface 1398 are relatively highly porous, they will partially crush upon implantation between first and second vertebrae (not shown) under the load from the first and second end plates (not shown). This partial crushing forms a custom fit for the multi-density polymeric interbody spacer 1310 between the first and second end plates (not shown).
• first density region and second density region may instead be formed using biocompatible materials of different formulation.
• the water concentration of the liquid biocompatible material 42, 942, 1042, 1142 and 1242 used to form the second density region may be decreased from that used to form the first density region.
• the water in the liquid biocompatible material reacts to produce the carbon dioxide.
• a thin dividing member 99 may be used to initially separate the first and second formulations of biocompatible polymeric material 42 within the mold 44 during the pouring step S2. After pouring, the dividing member 99 may be removed once the formulations have reached a desirable viscosity, allowing the formulations of biocompatible polymeric material 42 to flow against one another and mix at the interface.
• the porosity of the first density region and the second density region may also be controlled by mixing technique for preparing the liquid biocompatible polymeric material. For example, mechanical speed mixing, e.g. using a blender, typically results in a uniform pore structure with a small average pore size, while hand mixing typically results in a more random distribution of pore sizes.
• the multi-density polymeric interbody spacer of the present invention may also be coated and/or treated with antibiotics and/or an
• osteoinductive agent to assist in healing and accelerate bone growth after spinal fusion surgery.
• a further advantage of the present invention is that the method for forming the multi-density polymeric interbody spacer provides for highly
• cadaver bone varies from sample to sample
• spacers of the present invention are fabricated with known and reproducible properties. Additionally, the present invention does not have the storage limitations that accompany cadaver bone spacers. Also, supply of spacers according to the present invention is not limited by available cadaver specimens. Additionally, the size and shape of the multi-density polymeric interbody spacer of the present invention is not restricted by the size and shape of human bone. The multi-density polymeric interbody spacer also eliminates the risk of disease transfer associated with many prior art interbody spacers.
• multi-density polymeric interbody spacer may be formed to customized shapes and geometries for different bone fusion applications. Additionally, the multi-density polymeric interbody spacer of the present invention may incorporate a variety of surface features to improve fit between and contact with first and second vertebrae.

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