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 are mounted on opposing flexible members and establish an interference fit relative to the hole in the bone tissue. The ribs are urged radially outwardly by the flexible members and have a sharp edge that grips the sides of the hole in the bone such that the ribs restrict withdrawal of the device. Vertical ribs may also be included to prevent rotation of the device within the hole in the bone.

Description:
FIELD OF THE INVENTION  
         [0001]    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  
         [0002]    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.  
           [0003]    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, l, 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; 5,431,679; 5,403,347; and 5,502,159. Devices made of an absorbable material have the advantage that they are absorbed by the body after healing has occurred.  
           [0004]    U.S. Pat. No. 5,067,964 describes an articular cartilage repair piece which includes a backing layer of non-woven, felted 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.  
           [0005]    It is well know that there is wide variability in stiffness, strength, and other physical properties of human bone, and that the properties vary from site to site among humans. It is therefore challenging to design mechanical fasteners for fixing a prosthetic scaffold to bone because the mechanical function of the device must be able to accommodate a range of bone physical properties.  
           [0006]    Accordingly, it would be advantageous to provide a scaffold fixation device which has a fixation means that can perform in a variety of human bone.  
         SUMMARY OF THE INVENTION  
         [0007]    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 generally perpendicular to the axis of the post, the rib positioned intermediate the platform and a distal end of the post and establishing an interference fit relative to the hole in the bone tissue to prevent withdrawal of the device from a hole in the bone tissue into which it has been inserted. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    [0008]FIG. 1 is a side elevation view of a scaffold fixation device in accordance with an exemplary embodiment of the present invention;  
         [0009]    [0009]FIG. 2 is a perspective view of the device of FIG. 1;  
         [0010]    [0010]FIG. 3 is a front elevation view of the device of FIG. 1;  
         [0011]    [0011]FIG. 4 is a partially cross-sectional view of the device of FIG. 1 deployed in bone;  
         [0012]    [0012]FIG. 5 is a side elevation view of a second exemplary embodiment of the present invention;  
         [0013]    [0013]FIG. 6 is a partially cross-sectional view of the device of FIG. 5 deployed in bone. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIGS. 1, 2 and  3  show 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. The post  14  has a central slot  16  defining a pair of flexible members,  18 ,  20  which are conjoined at one end proximate the platform  12  and at the other end proximate a tapered tip  22 . One or more ribs  24  extend radially from the peripheral surface of the flexible members  18 ,  20 . Each rib  24  terminates in a sharp ledge or tip  25 . The flexibility of members  18 ,  20  can be selected by controlling the cross-sectional area thereof and the length of slot  16 . Slot  16  allows flexible members  18 ,  20  to deflect inwardly under radial compressive loading. A significant increase in radial stiffness of flexible members  18 ,  20  occurs when they are deflected inwardly to a degree such that mating surfaces  26 ,  28  come in contact, at which point further deflection of flexible members  18 ,  20  is limited. In the preferred embodiment, the geometry of slot  16  and flexible members  18 ,  20  is such that the outermost diameter of opposing ribs  24  is larger than the diameter of the post  14  proximate to the platform  12  when mating surfaces  26 ,  28  are initially in contact.  
         [0015]    Scaffold fixation device  10  has a centrally disposed guide wire channel  30  which extends longitudinally through fixation device  10  along the axis of post  14 . Perforations  28  in the platform  12  allow fluid and cells to travel to and from the scaffold and are not limited to the shape or arrangement shown in the figures. Platform  12  may also be solid.  
         [0016]    The cross-sectional area of flexible members  18 ,  20  can be controlled by selecting the width of the slot  16 , as well as by the incorporation of flats  34 ,  36  on one or more opposing sides of flexible members  18 ,  20 . The flats  34 ,  36  reduce the cross-sectional area of flexible members  18 ,  20  and increase their flexibility. In addition, the flats  34 ,  36  act as reliefs to allow the flexible members  18 ,  20  to flex inwardly while conforming to the confines of a hole  40  drilled in bone tissue  42  (See FIG. 4). More specifically, when a generally cylindrical object is compressed along one diameter, it expands along a diameter at 90° relative thereto. A cross-section of the compressed cylindrical object would therefore be elliptical. The flats  34 ,  36  truncate the ellipse (at the ends of the major axis) formed when the flexible members  18 ,  20  are compressed, allowing the flexible members  18 ,  20  to compress without the flats  34 ,  36  bearing against the bone tissue  42  proximate the hole  40 . (See FIG. 4)  
         [0017]    [0017]FIG. 4 shows the surgical placement of the scaffold fixation device  10  in a hole  40  drilled in bone tissue  42 . The hole  40  has a diameter establishing an interference fit between the bone tissue  42  surrounding hole  40  and the post  14 , most significantly, relative to the ribs  24 . Hole  40  preferably has a diameter which is less than the outermost diameter of opposing ribs  24 , i.e., the tip  25 -to-tip  25  distance, as would be measured by outside calipers. When the hole  40  has a diameter which is less than or equal to the diameter of post  14 , mating surfaces  26 ,  28  of flexible members  18 ,  20  will come into contact during the insertion of fixation device  10  into the hole  40  causing close engagement with bone tissue  42  proximate the hole  40  through the radial deflection and radial material strain of flexible members  18 ,  20 . When hole  40  has a diameter which is larger than the diameter of post  14  but less than the outermost diameter of ribs  24 , the device  10  engages with bone tissue  42  through radial force exerted in reaction to the inward deflection of flexible members  18 ,  20 , i.e., due to elastic memory. Flexible members  18 ,  20  allow the device  10  to accommodate variations in hole  40  diameter and material properties of the bone tissue  42 . In very hard bone, slot  16  allows flexible members  18 ,  20  to deflect inwardly to conform to the hole  40  without material yield or damage occurring to post  14  or flexible members  18 ,  20 . To install the device,  10  a hole  44  is drilled in the cartilage tissue  46  with a diameter at least as large as the outermost diameter of platform  12 . The depths of hole  40  in the bone tissue  42  and the hole  44  in the cartilage  46  are selected such that, when post  14  is inserted completely into hole  40  in the bone  42 , upper surface  50  of platform  12  is in alignment with or slightly below upper bone surface  52 . The scaffold  53  (shown diagrammatically in dotted lines) resides within the space available within hole  44  between platform  12  and upper cartilage surface  54 . Tapered tip  22  aids in introducing post  14  into hole  40 . The taper  22  is reproduced in part on each rib  24  distal to the tip  25  to aid in introducing each rib  24  into the hole  40 . As noted above, a surgical guide wire (not shown) may be passed through guide wire channel  30  during surgery to align scaffold fixation device  10  with the hole  40  in the bone tissue  42 . After the guide wire has been used to align the tapered tip  22  with the hole  40 , it is removed prior to full insertion of the post  14  to allow flexible members  18 ,  20  to deflect inwardly without contacting the guide wire.  
         [0018]    [0018]FIG. 5 and  6  show a device  110  in accordance with an alternative embodiment of the present invention, in which post  114  has one or more vertical ribs  160  extending radially from the outer surface thereof. The vertical ribs  160  have a tapered profile similar to the taper on the tapered tip  122 . When device  110  is deployed in bone tissue  142 , as shown in FIG. 6, the ribs  160  cut into the bone tissue  142  surrounding hole  140  to prevent rotation of device  110  within the hole  140 . Those skilled in the art will appreciate that a variety of shapes and sizes of protrusions from post  114  which make a noncircular shape will engage bone tissue  142  proximate hole  140  to prevent relative rotation of device  110  within the hole  140 .  
         [0019]    Fixation device  10 ,  110  may be formed from a non-porous or a partially or wholly porous material to allow cell invasion into the device  10 ,  110 . Suitable materials from which the scaffold fixation device  10 ,  110  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 glasses, ceramics including calcium phosphates and other biocompatible metal oxides (i.e., CaO), combinations of metals, absorbable polymers or autograft, allograft, or xenograft bone tissues.  
         [0020]    In the preferred embodiment, the scaffold fixation device  10 ,  110  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-dimethyl-dioxepan-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.  
         [0021]    In another embodiment of the present invention, the polymers and blends can be used as a therapeutic agent release matrix. To form this matrix, the polymer would be mixed with a therapeutic agent prior to forming the device. 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-βI-Ill), vascular endothelial growth factor (VEGF); and other naturally derived or genetically engineered proteins, polysaccharides, glycoproteins, or lipoproteins. These growth factors are known in the art and are 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.  
         [0022]    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.  
         [0023]    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.  
         [0024]    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 I 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.