Abstract:
A bone screw for lateral suture stabilization includes a post around which the suture winds. An attachment hole is located in the head of the bone screw above the post for connecting an end of the suture. Various suture configurations can be used around multiple bone screws in the lateral suture stabilization process to control the flexion of the suture upon movement of the joint. The suture is formed from a plurality of high strength high modulus polymeric fibers. The fibers are independent and free from intrinsic inter-fiber shear coupling found in braided or bonded fibers.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 60/913,276, filed Apr. 21, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a bone screw for repair of cruciate tear or failure. More particularly, it relates to a bone screw and suture for performing a lateral suture stabilization without premature wear or failure. 
         [0004]    2. Discussion of Related Art 
         [0005]    Anterior Cruciate Ligament (ACL) in the human knee joint, commonly called Cranial Cruciate Ligament in the canine stifle, is frequently torn in trauma, or, as it happens in dogs, it fails after a degenerative process of still unknown etiology. Direct repair is usually not possible—when attempted, it predictably fails. 
         [0006]    In human orthopedics, the standard procedure calls for replacement by an allograft, a part of the patellar ligament, or a part of the tendon removed from hamstring muscles. The procedure results in a stable knee, but the long term performance is often unsatisfactory with over a half of cases resulting in arthrosis of the joint. 
         [0007]    In dogs the standard procedure is either an extracapsular suture (usually placed on the lateral side of the joint) approximating the function of the ligament, or one of the geometry modifying surgical techniques, e.g. Tibial Plateau Leveling Osteotomy (TPLO), Cranial Closing Wedge Osteotomy (CWO), or Tibial Tuberosity Advancement (TTA). Intra-articular prostheses are also occasionally used, but those generally end up in failure. Extracapsular sutures also fail. These are intended to provide stability of the joint for several weeks while waiting for fibrosis to form around the joint, which then should provide for long term stability. Arthrosis of the joint at longer term, at a year or so, is the rule rather exception. 
         [0008]    A long standing technique for treating cruciate tear in dogs is lateral suture stabilization. This procedure involves using a monofilament nylon suture, typically 20-80 pound test passed around the lateral fabella and through the tibal tuberosity. The suture is tightened and secured with a crimp clamp system. The goal of the procedure is to stabilize the joint in all ranges of motion and eliminate both cranial drawer and cranial tibial thrust. The patient forms periaticular fibrosis to provide permanent stability. 
         [0009]    A number of problems exist with lateral suture stabilization. One of the principal ones relates to movement of the suture and the use of the crimp clamp system to secure the suture. Often, the suture is subject to wear and failure at the locations where it contacts the bones. Furthermore, the suture is subjected to rotational and flexion stresses from movement of the joint. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a bone screw for use in lateral suture stabilization. According to one aspect of the invention, the bone screw includes a post which extends above the bone around which a suture is wound. According to another aspect of the invention, the bone screw includes an attachment hole in a head of the screw for attaching an end of the suture. According to another aspect of the invention, a bead attached to an end of the suture is positioned within the attachment hole of the bone screw. 
         [0011]    According to another aspect of the invention, a suture is used which is formed of a plurality of independent filaments or fibers. Such fibers may include oriented, high modulus, ultra high molecular weight polyethylene. According to another aspect of the invention, the post of the bone screw has a diameter which exceeds a given ratio to the fiber diameter. According to another aspect of the invention, the ratio has an order of magnitude of about 100. According to another aspect of the invention, the fibers of the suture have a diameter on the order of 10 micrometers and the post diameter is on the order of 1 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a side view of bone screw according to an embodiment of the present invention. 
           [0013]      FIG. 2  is a top view of the bone screw of  FIG. 1 . 
           [0014]      FIG. 3  is a cross sectional view of a bead for holding a suture according to an embodiment of the present invention. 
           [0015]      FIG. 4  illustrates operation of a suture with respect to a bone screw according to an embodiment of the present invention. 
           [0016]      FIGS. 5(   a )- 5 ( d ) represent suture configurations for lateral suture stabilization. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The present invention provides a bone screw having improved performance and reduced wear and failure of sutures in lateral suture stabilization. The bone screw includes a post which extends above the surface of the bone around which the suture winds. The curvature of the post allows rotational movement of the suture. 
         [0018]      FIG. 1  is a side view of a bone screw  10  according to an embodiment of the invention. The body  16  of the screw  10  is threaded, the threads being preferably self-tapping. Attached to the body  16  is a top structure including a head  11 , base  15  and post  14  between the head  11  and base  15 .  FIG. 2  shows the top of the head  11 . Perpendicular slots  13  are formed in the top of the head. To install the screw, a hole is drilled in the bone. A screwdriver mates with the slots  13  to drive the screw into the bone until a lower surface of the base  15  abuts the bone. 
         [0019]    Rounded slots  12  are formed on opposing sides through the head  11  for attaching the suture. The slots  12  include a circular portion  32  and an opening  31 . The circular portion  32  is conical in shape, wider at the top of the head  11  and narrower towards the post  14 . A tubular bead  20 , illustrated in  FIG. 3 , has a conical outer surface  21  which matches the conical surface of the circular portion  32  of the slots  12 . The inner diameter  22  of the bead  20  is sized to accommodate the suture  23  and increases in diameter towards the ends. A knot  24  in the suture  23  cooperates with the decreasing diameter of the bead to prevent pullout of the suture through the bead  20 . The exit of the inner diameter  22  of the bead  20  is well rounded so as to prevent damage to the suture  23  as it bends from the bead  20 . To attach the suture  23  to the screw  10 , the suture  23  is passed through the opening  31  of the slot  12  into the circular portion  32 . The bead  20  is positioned on the suture  23  and a knot  24  is tied. The bead  20  is held in the circular potion  32  of the slot  12  and the suture  23  extends from the bead  20  to the space between the head  11  and base  15 . The knot  24  can be secured by melting over it a polymer sleeve (not shown) of melting temperature lower than the suture itself. For example if the suture is made from oriented, high modulus, ultra high molecular weight polyethylene with melting temperature of about 150 deg C., the sleeve can be made from a low molecular weight polyethylene melting at about 110 deg C. 
         [0020]    Alternatively, the circular portion  32  of the slot  12  could include an inner surface which decreases in diameter towards the ends, as with the bead  20 . The bead  20  could then be eliminated and the suture  23  directly connected to the slot  12 . 
         [0021]    Once the suture  23  is attached to one of the slots  12 , it is wrapped approximately one half way around the post  14 , as illustrated in  FIG. 4 . When in use, as the pull  26  on the suture  23  sweeps  25 , the suture  23  simply bends around the post  14 . 
         [0022]    The suture is preferably of a multifilament type without braiding or bonding of filaments.  FIG. 4  represents bending of individual filaments of the suture  25  about the post  14 . An individual filament has a diameter d and the post  14  has a diameter D. Maximum strain experienced by the filament bending over the post is approximately equal to d/D. If the fatigue limit on the strain in an individual fiber is ε max , the relative diameters of the post  14  and filaments  23  can be determined. The diameter of the post should be: 
         [0000]        D&gt;d/ε   max . 
         [0000]    With the best expectations of high performance polymeric fibers, the fatigue limit on the strain in the fiber is on the order of 0.015, thus the diameter of the post D should be at least 60 times larger than the diameter of the filament d. The strongest filaments of e.g. highly oriented polyethylene are on the order of 0.015 mm in diameter. For those the post should have at the least the diameter of about 1 mm. However, this would leave no capacity to resist any tension. A factor of 2 in the diameter, i.e. a post of 2 mm diameter would allow maximum tension in fatigue to be about one half of its nominal value—a reasonable compromise. 
         [0023]    Alternatively, if the fatigue strain at expected number of cycles in use be ε max , the fiber diameter d, and the factor for allowed functional tension k. Then the diameter of the post,  14 , of the anchor should be D&gt;k(d/ε max ). Expressed in terms of the radius of curvature, R, of the edges: R&gt;(k/2)(d/ε max ). Conversely, if the diameter of the anchor post D is given, one can determine that the fiber diameter d should be: d&lt;(D/ε max )/k. 
         [0024]    State of the art sutures are either monofilament or multifilament, braided in one or the other way. Neither type can offer satisfying performance at the suture anchor. For the monofilament fibers the radius of the post required is simply not possible in most situations, i.e. those sutures will predictably fail in use. Braiding as conventionally done will effectively increase the diameter of the fiber and will also lead to failure. Another serious drawback of braiding is the increased risk of infection—bacteria within a braided suture are not accessible to immune system cells and can thus remain a threat as long as the suture is in the tissue. Technical reasons for braiding are mostly related to the ease and reliability of the knots, which need to be tied to complete the repair. All or a portion of the multifilament suture can be held together with a gelatin or other substance which will dissolve within the body. This makes the suture easier to use and to tie, yet allows the filaments to separate in order to achieve improved wear and failure resistance. 
         [0025]    Preferred fiber for use with the anchor of the invention is that of oriented, high modulus, ultra high molecular weight polyethylene, such as DYNEEMA® from DSM, Netherlands, or SPECTRA® from Honeywell, USA. Preferred diameter of the fiber is between 10 and 20 micrometers, more preferably about 15 micrometers. Fibers are left free from each other, as in yarn; i.e. no diffusion bonding nor braiding is used in production. The suture is supplied with preferably two beads, a knot tied behind each of them and secured/overmolded with low molecular polyethylene. 
         [0026]    Other suitable polymeric fibers are polyethylene teraphthalate (polyester), polyamid (NYLON ®), aramid (KEVLAR®), or silk. Resorbable fibers can also be used, e.g. those based on polylactic acid, polyglycolic acid or polydioxanone. 
         [0027]    For lateral suture stabilization, two screws are positioned in the bones. A suture is connected around both of the screws to provide the desired stabilization.  FIGS. 5(   a )- 5 ( d ) represent possible configurations for a suture connection between posts  14 ,  14   a  of the two screws. In one embodiment, illustrated in  FIG. 5(   a ), one end of the suture  23  is connected to a slot  27  in the first screw and wrapped partly around the post  14 . It extends to and around the post  14   a  of the other screw. The suture  23  wraps around the first post  14  again and attaches to the other slot  28  of the first screw. The suture  23  could be wrapped multiple times around either post  14 ,  14   a.    
         [0028]    In another configuration, illustrated in  FIG. 5(   b ), the suture  23  is positioned similar to the first configuration and attached to one of the slots  29  of the second screw. In another configuration, illustrated in  FIG. 5(   c ), the suture  23  wraps in a first direction around the first post  14  and in the opposite direction around the second post  14   a  to attach to slot  30  in the second screw. In another configuration  5 ( d ), the suture  23  wraps in opposite directions around the posts, with both ends attached to the slots  27 ,  28  of the first screw. Different configurations results in variations of tightening and loosening of the suture as the bones move. 
         [0029]    Having disclosed at least one embodiment of the present invention, various adaptations, modifications, additions, and improvements will be readily apparent to those of ordinary skill in the art. Such adaptations, modifications, additions and improvements are considered part of the invention which is only limited by the several claims attached hereto.