Abstract:
Disclosed herein are distraction implants for treating, for example, spinal disorders. The implants can be comprised of a U-shaped elastic member, two bone anchor members and two tension members to couple the components together. Use of the tension members allows for transfer of distraction force without unintended forces in the perpendicular plane and parasitic moments. As there are several options for anchoring to bone, the present implants can be applied to all segments of the spine from the cervical spine to the lumbar regions. The implant can be temporary or permanent, covering indications from fracture treatment to management of chronic disc disorders, including disc protrusion. Use of the present implant and methods of use are also within the scope of the invention.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Application No. 61/984,383, filed Apr. 25, 2014. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to implants and methods for treating a spine or another skeletal structure in need of flexible distraction. 
       BACKGROUND OF THE INVENTION 
       [0003]    The spinal column is prone to disc injuries that are usually chronic in nature, but which can frequently culminate in disc protrusions, or extrusions, with potentially serious neurological consequences, including paralysis. Traumatic compression fractures of the vertebral bodies are also frequently seen and difficult to treat with the conventional means for fracture fixation of long bones. When used, plates and screws, for example, inevitably lead to bony fusion over one or more of the spinal discs because several vertebral bodies on each side of the fractured body must be used for fixation to get sufficient mechanical grip. There are numerous other specialized devices for these indications, but they all tend to involve a plurality of spinal column elements. 
         [0004]    There are many different devices for treating disc extrusions, including fusion cages and disc prostheses. These devices are associated with high morbidity and require costly, complex surgical procedures. 
         [0005]    Disc injuries are prevalent in both human patients and veterinary fields, although usually for somewhat different reasons. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention provides devices and methods that provide for a less invasive yet more robust stabilization of defective skeletal bodies (e.g., spinal column segments). The device comprises: (i) an elastic member; (ii) two anchor members for transferring distraction force to the spinal column utilizing bone screws; and (iii) two flexible tension members for connecting the elastic member to the anchor members. 
         [0007]    Flexibility of the tension member(s) allows application of a distraction force to the anchor member(s) without a coupled bending or torsion moment. This is important since it greatly reduces the stresses on the device/bone interface while also allowing some movement between the points of anchorage in all other degrees of freedom. 
         [0008]    All components of the device can be manufactured from metal, preferably from a titanium alloy with high strength, low modulus of elasticity and superb biocompatibility. 
         [0009]    The tension members can also be manufactured using non-metallic materials such as high strength polymeric fibers and in particular UHMWPE (ultra high molecular weight polyethylene) fibers. 
         [0010]    For improved strength and reliability, one can use locking screws such as those described in Perren et al. (U.S. Pat. No. 5,053,03; see also U.S. Pat. No. 5,180,382) to couple the anchor members to the bones. In many cases, a single screw may suffice; in most cases, two screws will provide an increased margin of safety, anatomical space permitting. The bone screw can include a head end, a tip end, and a shaft extending between the head end and the tip end. The shaft has a first threaded segment toward the head end, with a minor diameter d 1  and a second threaded segment towards the tip end with a minor diameter d 2 , d 2  being less than d 1 , and the thread in said segments having the same pitch. In some embodiments, a third transition segment is provided between the first and second segments. The screw tip can be designed to be self-cutting (e.g., in the form of a trocar or one or more cutting grooves radially distributed over the circumference. 
         [0011]    Accordingly, in a first aspect, the present invention features an implant for flexible distraction between two anchor points on the skeleton of a human or veterinary patient. More specifically, the implant comprises an elastic member, two anchor members, and two flexible tension members configured such that the elastic member straddles the two anchor members and applies a distraction force to the anchor members via the two flexible tension members situated between the elastic member and the two anchor members. The elastic member includes two terminal regions, with a flexible tension member connecting each terminal region to an anchor member. The implants in general and the component parts thereof (e.g., the two anchor member(s)) can be configured in size and shape for attachment to a spine of a human or veterinary patient. For example, the first anchor point can be located on a first vertebrae and the second anchor point can be located on a second (e.g., adjacent) vertebrae. The flexible tension member(s) can include a conical coupling element affixed (permanently or temporarily) to a flexible cable or bundle of filaments). The implant and component parts thereof (e.g., the flexible cable or bundle of filaments) can be or can include a titanium alloy (e.g., titanium-aluminium-vanadium, titanium-aluminium-niobium or a super-elastic nickel-titanium). The flexible cable or bundle of filaments can also, or alternatively, include high strength polymeric fibers (e.g., ultra high molecular weight polyethylene). In particular embodiments, the present implants can further include one or more (e.g., two) two bone screws (e.g., locking bone screws) for attaching the anchor member(s) to bone. 
         [0012]    In another aspect, the invention features an implant for flexible distraction between two anchor points on the skeleton of a human or veterinary patient, with the implant comprising an elastic member having one terminal region that attaches to the anchor point by a conventional means (e.g., by a bone screw or nail, by a glue, cement, or like substance, or by a suture). The implant further comprises a terminal region configured as described above (i.e., configured to interact with the anchor member via a flexible tension member. Thus, the implant can include first and second terminal regions, an anchor member, and a flexible tension member, wherein the first terminal region comprises means for attaching the elastic member to a skeletal element by conventional mechanisms and the second terminal region interacts with the anchor member via the unique flexible tension member described herein. In this aspect, the implant can include a first terminal region comprising an opening configured to receive a bone anchor, a surface compatible with bone cement (e.g., a flat, roughened surface), or a feature that can be affixed with a surgical suture (e.g., an opening or eyelet). 
         [0013]    In another aspect, the invention features methods of treating a patient who is suffering from an injury to the spine or a disease that causes damage to a spinal disc. The methods can include the step of affixing an implant described herein to vertebrae in the injured or diseased area of the spine. Other areas of the body in which distraction is indicated may also be treated. 
         [0014]    The injury to the spine can be a fracture, and in any event, the patient can be a human or a domesticated animal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a side view of an implant according to the present invention. 
           [0016]      FIG. 2  is a partial cross-sectional view of the flexible tension member. 
           [0017]      FIG. 3  shows orthogonal views and a perspective view of the anchor member with two screw-fixation and transverse groove for coupling with the flexible tension member. 
           [0018]      FIG. 4  shows orthogonal views of the anchor member with two screw-fixation and longitudinal groove for coupling with the flexible tension member. 
           [0019]      FIG. 5  shows orthogonal views and a perspective view of the anchor member with one screw-fixation. 
           [0020]      FIG. 6  shows orthogonal views of the elastic member. 
           [0021]      FIG. 7  shows the implant according to the invention applied to a spine that needs distraction. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The present invention provides a fixation system for distraction between two anchor points on a skeleton of a human or veterinary patient. Most frequently this need arises in connection with the spine, either in disc pathologies or in fractures of the vertebral bodies. Accordingly, the methods of the invention encompass treating a patient having a disc pathology or a fracture of a vertebral body. In any embodiment, the methods can include a step of identifying a patient in need of treatment. 
         [0023]    The defining feature of the system is its force controlling U-shaped spring, herein referred to as the elastic member. This elastic member straddles the two anchor members and applies tensile force to them via flexible tension members, essentially short segments of a cable provided with conical ends that engage conical recesses in the elastic member and in the anchor members. Thus, the elastic member can be U-shaped, flexible, dimensioned to straddle two anchor members, and designed to engage the flexible tension members at its free ends (i.e., the termination of the arms of the U). 
         [0024]    The implants can be designed in a very broad range of sizes to accommodate a variety of anatomical structures and can be manufactured with varying stiffnesses controlled by the geometry and the material employed. The materials of choice for most of these components are titanium alloys, e.g. titanium-aluminium-vanadium or titanium-aluminium-niobium. However, various beta alloys of titanium, as well as super-elastic Nitinol® (nickeltitanium), can also be used. 
         [0025]      FIG. 1  is a schematic representation of a device according to the present invention showing the elastic member  1  arching over two anchor members  2 , affixed to the skeletal bodies  5  and  6  with screws  4 . The elastic member  1  is connected to anchor members  2  via tension members  3 . Called for distraction force  7  is generated by elastically deforming the member  1  and then coupling it to the anchor members  2  via tension members  3 . The device produces distraction force  8  approximately of the same magnitude as  7 , but offset by the distance  9 . In most cases, the distance  9  should be kept to a minimum, but by strategic placement of the anchors members, or using them in pairs, the effects of the bending moment generated by the offset  9  can be minimized or put to good use. Flexibility of the tension members  3  allows transfer of the force  8  to the anchor members with only minimal forces in the other two directions or any of the three moments. 
         [0026]      FIG. 2  shows the tension member  3  in a partial cross-section and the conical coupling element  10  in a perspective view. A bundle of fibers  11  is fixed within the coupling element  10 , by welding or brazing  12 , or other mechanical means if, for example, polymeric fibers are used in the bundle  11 . The exit end of the coupling element  10  is conical in shape, as shown by arrow  14 . Once the ends of the bundle  11  are welded, brazed or otherwise affixed to the element  10 , the conical ends of the hole provide for higher resistance against the pullout of the bundle from the connecting element  10 . Transition from the exit section of the element  10  to the entry section is formed with a radius of curvature  13 , which provides the surface over which the fibers of the bundle  11  are bending. In preferred embodiments, the ratio of the radius of curvature  13  to the diameter of the individual fibers extending through the conical coupling element  10  should be maximized within technical limits. Tangential entry conical section allows the bundle to bend over the radius  13  by an angle  16  in each direction, without contacting the end face of the connecting element  10 . The connecting element&#39;s outside shape can also be conical, as shown by the arrow  15 , so that the connections to both the anchor member  2  ( FIG. 1 ) and the elastic member  1  ( FIG. 1 ) are strong and protected from fretting. The angle of this conical section can be as small as needed for self-locking, or somewhat larger to facilitate removal, if needed. The range considered best suited, expressed in the usual way of defining the conical section by the ratio of the change in the diameter to the length of the cone, is 1:10 (which leads to safe self-locking) to 1:5 (which does not result in self-locking, but provides secure contact conditions). The final choice is affected by the materials used and the surface finish of the interfaced components. 
         [0027]    For optimal distribution of forces between the fibers in the bundle  11  when bending over the radius  13 , there should preferably by a twist (e.g., of about 180 degrees) in the bundle over its length (not shown). By “about” we mean plus-or-minus 10%. 
         [0028]      FIG. 3  shows orthogonal views and a perspective view of the anchor member  20  provided with two holes  23  for insertion of the bone screws  24 . The axes of the screws can be somewhat divergent, as shown here, for a safer insertion on the dorsal side of the spinal vertebra. The conical hole  21 , oriented in the transverse direction to the axis of the member  20 , is matched to receive the conical elements  10  of the tension member  3  ( FIG. 2 ). The opening  22  into the hole  21 , allows the bundle  11  ( FIG. 2 ) to be slipped down into the hole  21 , before pulling on the member  3  to get the element  10  to engage with hole  21 . 
         [0029]      FIG. 4  shows an alternative orientation of the receiving hole  31  in the anchor element  30 . The axis of the hole  31  and the opening  32  is now along the long axis of  30 , in line with the screw holes  33 . 
         [0030]      FIG. 5  shows another embodiment  40  of the anchor member. Only one screw hole  43  is provided for fixation to the bone. The hole  41  for receiving the tension member is set above the screw hole, as close as possible. The opening  42  into the hole  41  is also shown here. 
         [0031]      FIG. 6  shows orthogonal views of the elastic member  1 , generally U-shaped (with the base of the “U” being longer than the arms of the “U” in this illustrated embodiment). Distance  54  from the receiving holes  51  for the tension members to the spine of the member  1 , is one of the factors controlling the elasticity and strength of the device. The other defining parameters are the length  53  and the cross-sectional dimensions  55  (the width) and  56  (the thickness). A rectangular section has been chosen to facilitate production from a plate by cutting the U-shape. In general terms, the length  53  will be dictated by the anatomy of the location where the implant is being deployed. Anatomy also plays a major role in defining the limits on the height  54  of the device. A large range of desired mechanical properties can be achieved by choosing the width  55  and the thickness  56 . 
         [0032]      FIG. 7  is a schematic representation of a device of the invention in use on the spinal column. A compression fracture  106  of a vertebral body can be treated by distracting the adjacent vertebral bodies  101  and  102  using the elastic member  1 , coupled to them by bone screws  103  and  104 , respectively, each affixing an anchor member  40  to the bone. Tension members  3  provide the links between  1  and  40 . Distraction force  105  is generated by selecting the member  1  and the lengths of the members  3 , so that the distance between the tension elements is larger than the distance between the receiving anchors. For installation, the elastic member  1  is compressed by an instrument (for example an adapted forceps) to allow for the conical elements  10  ( FIG. 2 ) to engage into receiving holes of the members  40 . If the distraction is maintained long enough to allow for healing of the fracture  106 , but without a strong impediment of the other movements in the effected segment of the spinal column, once the device is removed, the column may retain most of its natural range of motion. 
         [0033]    Another use of the device according to the present invention is for treating disc pathologies, e.g. extrusion of the disc  107 . If the intended treatment is fusion of the spine at this location, the device, deployed as shown, can restrict the range of movement sufficiently for the consolidation of bone graft applied over the affected disc  107 . 
         [0034]    While the main clinical indications are expected to be for distraction, the same principles can be used to produce compression, by simply machining the receiving holes with the conical holes in the other direction. In fact, it is a simple design and production matter to produce a receiving hole tapered from both ends, which then can be used in either direction. 
         [0035]    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.