Abstract:
A device for attaching a tissue replacement scaffold to a bone has a platform positionable in substantially parallel relationship to the bone for retaining the tissue scaffold proximate to the bone. A post extends from the platform and is insertable into a hole formed in the bone. One or more ribs extend from a side surface of the post along a portion of its length. The ribs have an increasing cross-sectional area to establish an increasing interference fit relative to the hole in the bone tissue. The ribs have a sharp edge that grips the sides of the hole in the bone such that the ribs restrict rotation or withdrawal of the device.

Description:
FIELD OF THE INVENTION 
     The present invention relates to scaffold fixation devices useful in articular cartilage repair and more specifically to a device for fastening an articular cartilage scaffold to underlying bone. 
     BACKGROUND OF THE INVENTION 
     Articular cartilage is a tissue that covers the articulating surfaces between bones in joints, such as the knee or elbow, which is subject to catastrophic or repetitive stress injury. Various means have been proposed to address such injuries including repair via tissue engineering. Tissue engineering is defined as the application of engineering disciplines to either maintain existing tissue structures or to enable new tissue growth. This engineering approach generally includes the delivery of a tissue scaffold that serves as an architectural support onto which cells may attach, proliferate, and synthesize new tissue to repair a wound or defect. Surgical use of a tissue scaffold requires a fixation means to secure the scaffold to the bone beneath the wounded cartilage site. Secure fixation of the scaffold within the wound site is necessary for proper healing. 
     Frequently, scaffolds, prostheses and fasteners used in orthopedic applications are made from synthetic absorbable biocompatible polymers which are well known in the art. Such polymers typically are used to manufacture medical devices which are implanted in body tissue and absorb over time. Synthetic, absorbable, biocompatible aliphatic polyesters include homopolymers, copolymers (random, block, segmented and graft) of monomers such as glycolic acid, glycolide, lactic acid, lactide(d, I, meso and mixtures thereof), ε-caprolactone, trimethylene carbonate and p-dioxanone. Numerous U.S. Patents describe these polymers, including U.S. Pat. Nos. 5,431,679; 5,403,347; 5,314,989; and 5,502,159. Devices made of an absorbable material have the advantage that they are absorbed by the body after healing has occurred. 
     U.S. Pat. No. 5,067,964 describes an articular cartilage repair piece which includes a backing layer of non-woven, feted fibrous material which is either uncoated or covered by a coating of tough, pliable material. A number of means are disclosed for fastening the repair piece to the underlying bone. U.S. Pat. Nos. 5,306,311 and 5,624,463 describe a prosthetic, resorbable articular cartilage and methods of its fabrication and insertion. U.S. Pat. No. 5,713,374 describes an attachment method to hold a biomaterial in place until healing occurs. U.S. Pat. Nos. 5,632,745 and 5,749,874 and 5,769,899 describe a bioabsorbable cartilage repair system. 
     Articular joint loading is very complex, involving high compressive loads combined with high shear loads associated with sliding articulation of the opposing surfaces. A device implanted into the articular joint space must have sufficient strength to withstand these loads. Particularly important is that the device should be fixed in the underlying bone so that it cannot rotate or separate from the bone under the action of high shear loads in the joint space. U.S. Pat. No. 5,749,874 teaches that if vascular invasion and cellular migration is to be effected between the healthy tissue and the scaffold, means must be provided to preclude rotation of the scaffold relative to the fixation device, but does not describe a means of keeping the fixation device itself from rotating in relation to the surrounding tissues or from pulling out. 
     Accordingly, it would be advantageous to provide a scaffold fixation device which has a fixation means that engages the bone to prevent rotation and separation. 
     SUMMARY OF THE INVENTION 
     The limitations of prior art devices for attaching a tissue scaffold to bone tissue are overcome by the present invention which includes an attachment device having a platform positionable in substantially parallel relationship to the bone tissue for retaining the tissue scaffold proximate to the bone tissue. A post extends from the platform and is insertable into a hole formed in the bone tissue. At least one rib extends from a surface of the post along a portion of its length from a first point distal to the platform to a second point intermediate the first point and the platform. The rib has a cross-sectional area that increases along the length of the rib in the direction from the first point to the second point and establishes an interference fit relative to the hole in the bone tissue to prevent rotation of the device relative to the bone tissue. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a side elevation view of a scaffold fixation device in accordance with an exemplary embodiment of the present invention; 
     FIG. 2 is a perspective view of the device of FIG. 1; 
     FIG. 3 is a side elevation view of the device of FIG. 1 deployed in bone; 
     FIG. 4 is an exploded view of a second exemplary embodiment of the present invention; 
     FIG. 5 is a side elevation view of the device of FIG. 4, assembled; and 
     FIG. 6 is a side elevation view of the device of FIG. 4 deployed in bone. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a scaffold fixation device  10  for fastening an articular cartilage scaffold to underlying bone. The device  10  has a scaffold attachment platform  12  with a post  14  extending therefrom at approximately 90°. Depending upon the application, this angular relationship may be varied. Vertical ribs  16  extend along a portion of the length of the post  14  and taper downwards in width and height as they extend from edge  17  to chamfered distal tip  18 . The taper of vertical ribs  16  enhances the ability of the vertical ribs  16  to gradually cut into surrounding bone during insertion of scaffold fixation device  10  into an appropriately sized hole in a bone to which the device  10  is attached. While the ribs  16  shown are in the form of a longitudinally bisected, elongated cone, other tapering shapes could be employed, such as an elongated wedge with or without a knife-edge bevel. 
     FIG. 2 shows a perspective view of scaffold fixation device  10  showing perforations  20  in disk-shaped platform  12  that allow fluid and cells to travel to and from the scaffold promoting cell proliferation and ingrowth. While six triangular perforations  20  are shown in FIG. 2, the perforations  20  can be any number, size or shape, e.g., circular or trapezoidal and accordingly are not limited to the shape or arrangement shown in the figures. A guide wire channel  22  extends longitudinally through fixation device  10  along the axis of post  14 . As is known in the art, a guide wire may be utilized to assist in placing the device  10 , viz, by inserting an end of a guide wire into a hole bored in a bone and then threading the device  10  over the guide wire, i.e., via channel  22 , such that the post  14  enters the hole in the bone (See FIG.  3 ). 
     FIG. 3 shows a side elevation view of scaffold fixation device  10  which has been surgically positioned within a hole  40  drilled in bone tissue  42 . The diameter of the hole  40  is selected such that an interference fit is made between the hole  40  and post  14  with vertical ribs  16 . That is, hole  40  has diameter which is less than the outermost diameter of vertical ribs  16 . Preferably, hole  40  has a diameter that is the same as or slightly smaller than the outermost diameter (root diameter) of post  14  (not including ribs  16 ). The scaffold fixation device  10  is preferably fabricated from a material that is sufficiently unyielding such that post  14  and vertical ribs  16  have sufficient radial stiffness and strength to cause the vertical ribs  16  to cut into the bone tissue  42  surrounding the hole  40 . This intrusion into the bone  42  has the effect of rotationally fixing the scaffold fixation device  10  to the bone tissue  42 . In addition, axial fixation of the device  10  is achieved by vertical ribs  16 , the sharp edges  17  of which engage trabecular bone tissue  42  when subjected to an axial force which would otherwise pull the scaffold fixation device  10  out of the hole  40  in the bone  42 . A hole  44  is drilled in cartilage tissue  46  with a diameter at least as large as the outermost diameter of platform  12  to accommodate the platform  12  therein in a position permitting the scaffold  47  (shown diagrammatically in dotted lines and displaced slightly) to be attached to the device  10  by sutures or adhesives, in a known manner. The depths of hole  40  in the bone and the hole  44  in the cartilage are selected such that, when post  14  is inserted completely into hole  40 , upper surface  50  of platform  12  is in alignment with or slightly below upper surface  52  of the bone tissue  42 , i.e., the platform  12  may be countersunk into the bone  42 . The scaffold  47  is accommodated within hole  44  in the cartilage (between platform  12  and upper cartilage surface  54 ). Post  14  may also have a chamfered lower edge  18  which aids in guiding post  14  into the hole  40  in the bone tissue  42 . As noted above, a surgical guide wire may be passed through guide wire channel  22  during surgery to align scaffold fixation device  10  with bone hole  40 . The fixation device  10  may be made from a non-porous material or from materials that are partially or wholly porous to allow cell invasion into the device. 
     A two-piece embodiment of the invention is shown in FIGS. 4 through 6, which show a two-piece scaffold fixation device  130  similar to a device described in the copending patent application entitled, “Scaffold Fixation Device for Use in Articular Cartilage Repair”, U.S. application Ser. No. 09/517,602 filed Mar. 2, 2000 and assigned to Ethicon, Inc., hereby incorporated herein by reference, FIGS. 14 through 20 and the associated description thereof being particularly relevant in describing the interlocking relationship displayed by a two-piece scaffold fixation device. 
     FIG. 4 shows a two-piece scaffold fixation device  130  with top component  132  and fixation component  134 . The top component  132  has a scaffold attachment platform  112  from which extends a coupling pin  114  with a pair of latches  116 ,  118  projecting from corresponding resilient arms  120 ,  122 . The coupling pin  114  telescopes into a mating axial bore  124  in the fixation component  134 , with the latches  116 ,  118  clipping over an internal ledge  126  when the pin  114  is pressed fully home into the bore  124 . The fixation component  134  has vertical anchoring ribs  180  having a similar form and function as the vertical ribs  16  shown in FIGS. 1-3. The ribs  180  are disposed about the outer peripheral surface of cylindrically shaped anchor section  148  of the fixation component  134 . FIG. 5 shows the scaffold fixation device  130  with the top component  132  and fixation component  134  assembled. 
     FIG. 6 shows scaffold fixation device  130  after having been surgically inserted in bone tissue  162 , showing the vertical anchoring ribs  180  embedded in the bone tissue  162  surrounding hole  160  to prevent rotation of fixation component  134  within the hole  160 . The device  130  would be utilized for attaching a scaffold (see FIG. 3) to a bone  162  by boring a suitable hole  160  in the bone  162 . The fixation component  134  is inserted into the hole  160  and driven home. The coupling pin  114  of the top component  132  can then be inserted into bore  124  of the fixation component and pressed in until the latches  116 ,  118  latch over ledge  126  (See FIG.  4 ). 
     Although FIGS. 1-6 show a certain number and shape of vertical ribs  16  and vertical anchoring ribs  180 , those skilled in the mechanical arts will appreciate that various numbers and shapes of ribs  16 ,  180  protruding from post  14  or anchor section  148  will create a noncircular cross-section along at least a portion of post  14  or anchor section  148  and result in rotational and axial fixation in bone. Fixation device  130  may be either solid or partially or wholly porous to allow cell invasion into the device. 
     Suitable materials from which the scaffold fixation device  10 ,  130  may be formed include biocompatible polymers such as aliphatic polyesters, polyorthoesters, polyanhydrides, polycarbonates, polyurethanes, polyamides and polyalkylene oxides. The present invention also can be formed from absorbable polymers, glasses or ceramics comprising calcium phosphates and other biocompatible metal oxides (i.e., CaO), metals, combinations of metals, autograft, allograft, or xenograft bone tissues. 
     In the preferred embodiment, the scaffold fixation device  10 ,  130  is formed from aliphatic polymer and copolymer polyesters and blends thereof. The aliphatic polyesters are typically synthesized in a ring opening polymerization. Suitable monomers include but are not limited to lactic acid, lactide (including L-, D-, meso and D,L mixtures), glycolic acid, glycolide, ε-caprolactone, p-dioxanone (1,4-dioxan-2-one), trimethylene carbonate (1,3-dioxan-2-one), delta-valerolactone, beta-butyrolactone, epsilon-decalactone, 2,5-diketomorpholine, pivalolactone, alpha, alpha-diethylpropiolactone, ethylene carbonate, ethylene oxalate, 3-methyl-1,4-dioxane-2,5-dione, 3,3-diethyl-1,4-dioxan-2,5-dione, gamma-butyrolactone, 1,4-dioxepan-2-one, 1,5-dioxepan-2-one, 6,6-dimethyldioxepan-2-one, 6,8-dioxabicycloctane-7-one and combinations thereof. These monomers generally are polymerized in the presence of an organometallic catalyst and an initiator at elevated temperatures. The organometallic catalyst is preferably tin based, e.g., stannous octoate, and is present in the monomer mixture at a molar ratio of monomer to catalyst ranging from about 10,000/1 to about 100,000/1. The initiator is typically an alkanol (including diols and polyols), a glycol, a hydroxyacid, or an amine, and is present in the monomer mixture at a molar ratio of monomer to initiator ranging from about 100/1 to about 5000/1. The polymerization typically is carried out at a temperature range from about 80° C. to about 240° C., preferably from about 100° C. to about 220° C., until the desired molecular weight and viscosity are achieved. 
     In another embodiment of the present invention, the polymers and blends from which it is formed can be used as a therapeutic agent release matrix. Prior to forming the device  10 ,  130 , the polymer would be mixed with a therapeutic agent. The variety of different therapeutic agents that can be used in conjunction with the polymers of the present invention is vast. In general, therapeutic agents which may be administered via the pharmaceutical compositions of the invention include, without limitation: antiinfectives such as antibiotics and antiviral agents; chemotherapeutic agents (i.e. anticancer agents); anti-rejection agents; analgesics and analgesic combinations; anti-inflammatory agents; hormones such as steroids; growth factors, including bone morphogenic proteins (i.e. BMP&#39;s 1-7), bone morphogenic-like proteins (i.e. GFD-5, GFD-7 and GFD-8), epidermal growth factor (EGF), fibroblast growth factor (i.e. FGF 1-9), platelet derived growth factor (PDGF), insulin like growth factor (IGF-I and IGF-II), transforming growth factors (i.e. TGF-β I-III), vascular endothelial growth factor (VEGF); and other naturally derived or genetically engineered proteins, polysaccharides, glycoproteins, or lipoproteins. The foregoing growth factors are known to those with skill in the art and described in  The Cellular and Molecular Basis of Bone Formation and Repair  by Vicki Rosen and R. Scott Thies, published by R. G. Landes Company hereby incorporated herein by reference. 
     Matrix materials for the present invention may be formulated by mixing one or more therapeutic agents with the polymer. Alternatively, a therapeutic agent could be coated on to the polymer, preferably with a pharmaceutically acceptable carrier. Any pharmaceutical carrier can be used that does not dissolve the polymer. The therapeutic agent may be present as a liquid, a finely divided solid, or any other appropriate physical form. Typically, but optionally, the matrix will include one or more additives, such as diluents, carriers, excipients, stabilizers or the like. 
     The amount of therapeutic agent will depend on the particular drug being employed and medical condition being treated. Typically, the amount of drug represents about 0.001 percent to about 70 percent, more typically about 0.001 percent to about 50 percent, most typically about 0.001 percent to about 20 percent by weight of the matrix. The quantity and type of polymer incorporated into the drug delivery matrix will vary depending on the release profile desired and the amount of drug employed. 
     Upon contact with body fluids, the polymer undergoes gradual degradation (mainly through hydrolysis) with concomitant release of the dispersed drug for a sustained or extended period. This can result in prolonged delivery (over, say 1 to 5,000 hours, preferably 2 to 800 hours) of effective amounts (say, 0.0001 mg/kg/hour to 10 mg/kg/hour) of the drug. This dosage form can be administered as is necessary depending on the subject being treated, the severity of the affliction, the judgment of the prescribing physician, and the like. Following this or similar procedures, those skilled in the art will be able to prepare a variety of formulations.