Abstract:
An implant for stabilizing bones or vertebrae is provided, the implant including an implant body that can be fixed by means of at least a first screw and a second screw to the bone or a vertebra, wherein the implant body includes at least a first hole and a second hole. Each of the first screw and the second screw includes a screw thread. Each of the first hole and the second hole has a threaded portion with an internal thread for cooperating with the screw thread, wherein the threaded portion of at least the first hole is formed of a flexible material. A guiding member that is a separate part is insertable into the implant body such that it is arranged at least partially within the first hole. The guiding member is configured to guide the first screw when the firstscrew is screwed through the threaded portion of the first hole. The second screw is spaced apart from the guiding member when the second screw is screwed through the threaded portion of the second hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This Application claims priority to and the benefit of U.S. Provisional Application No. 61/308,940, filed Feb. 27, 2010, the entire contents of which are incorporated herein by reference. This Application also claims priority to and the benefit of EP 10 154 844.4, filed Feb. 26, 2010, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to an implant for stabilizing bones or vertebrae. The implant comprises an implant body that is to be fixed by means of at least one screw to the bone or vertebra. The implant body has at least one hole with a threaded portion for receiving the bone screw. A guiding member is provided in the hole, which guides the screw when it is screwed through the threaded portion. In particular, the implant is an intervertebral cage or a bone plate, however, it can be any other implant that is to be fixed to a bone part of the human or animal body by means of a screw extending through a threaded hole. 
         [0003]    An intervertebral implant in the form of a cage that is fixed to the adjacent vertebrae by means of screws is described, for example, in US 2009/0030520 A1. The cage can be made of a bio-compatible material such as Titanium or PEEK (polyetheretherketone). 
         [0004]    If the cage is made of a bio-compatible plastic material, such as, for example PEEK, a problem may occur in that due to the greater flexibility of such a material compared to a metal, the bore hole for the screw may yield to some extent when the screw is screwed through the threaded portion. This may result in that the torque necessary for screwing the screw through the bore hole is considerably lower as compared to a metallic interconnection. In some clinical applications, for example, in the case of weak and/or osteoporotic bone substance, with such a low torque, there is only a reduced feed-back for the surgeon, which indicates to him how far the screw has already been advanced and whether the screw has the correct orientation. 
         [0005]    U.S. Pat. No. 7,618,456 B2 describes an intervertebral implant in the form of a cage that can be made of any physiologically compatible material, preferably however, of an un-reinforced plastic. Un-reinforced plastic is said to be advantageous over fiber-reinforced plastics. Appropriate bone screws consisting of un-reinforced plastic of which the external threads exhibit load bevels of 11° to 14° may be used. Due to the relatively small slope of the load bevel high clamping forces can be achieved, as a result of which radial elongation and the danger of cracking of the plastic are reduced. Also, the bore hole may be in the form of a metal bush fitted with an inner thread for improving anchoring the bone screw in the plastic implant structure. The known implant may also consist partly of plastic and in the bore hole zones of metal. 
       SUMMARY 
       [0006]    It is the object of the invention to provide an implant for stabilizing bones or vertebrae, in particular an intervertebral implant or a bone plate that provides an improved handling for the surgeon while simultaneously ensuring high safety. 
         [0007]    The implant particularly can be made of a body-compatible plastic material, such as PEEK (polyetheretherketone), PEKK (polyetherketoneketone) or similar materials. Although these materials are more flexible than implants made of metal, the surgeon can use metal bone screws and obtain the necessary feed-back and locking force when screwing-in the bone screw. 
         [0008]    With the guiding member according to the invention, the risk of misplacement of the bone screw is considerably reduced. 
         [0009]    If the implant is made of a bio-compatible plastic material, such as, for example, PEEK or PEKK, which is not visible under irradiation with X-rays, the guiding member is visible under irradiation with X-rays, which is advantageous for examinations after surgery. 
         [0010]    The guiding member can be provided even in the case wherein the hole in the implant body for the bone screw is very small. 
         [0011]    Further features and advantages will become apparent from the description of embodiments of the invention by means of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective exploded view of an implant according to a first embodiment in the form of an intervertebral cage. 
           [0013]      FIG. 2  is a perspective view of the cage of  FIG. 1  with the bone screw passed through the respective hole. 
           [0014]      FIG. 3  is a schematic top view of a portion of the implant in the region of only the hole showing a portion of the pin hole for receiving the pin. 
           [0015]      FIG. 4  shows a sectional view of the hole with the pin, the section being taken in a plane containing the thread axis of the hole. 
           [0016]      FIG. 5  is a first step of inserting the screw in the hole wherein the hole is schematically shown as a straight hole. 
           [0017]      FIG. 6A  is a schematic view of a second step, in which the screw is screwed through the threaded portion of the hole. 
           [0018]      FIG. 6B  is an enlarged view of a portion of  FIG. 6A . 
           [0019]      FIG. 7A  shows a third step in which the bone screw is fully screwed through the hole. 
           [0020]      FIG. 7B  is an enlarged portion of  FIG. 7A . 
           [0021]      FIG. 8  is a perspective exploded view of a second embodiment of the implant in the form of a plate. 
           [0022]      FIG. 9  is a perspective view of the implant of  FIG. 8  with the bone screw screwed through the hole. 
           [0023]      FIG. 10  is a perspective view of a modification of the first and second embodiment. 
           [0024]      FIG. 11  is a perspective view of the pin according to a further modification. 
           [0025]      FIG. 12  is a perspective exploded view of a third embodiment of the implant in the form of a plate. 
           [0026]      FIG. 13  is a perspective view of the implant of  FIG. 12  with the bone screw screwed through the hole. 
           [0027]      FIG. 14  is a top view of the hole of the third embodiment with the guiding clip inserted. 
           [0028]      FIG. 15  is a sectional view of the hole without clip. 
           [0029]      FIG. 16  is a perspective view of a clip that is to be inserted into the hole. 
           [0030]      FIG. 17  is a sectional view of the clip along line B-B in  FIG. 16 . 
           [0031]      FIG. 18  is a bottom view of the clip. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The implant according to the first embodiment shown in  FIGS. 1 and 2  includes an implant body  1  that can be fixed via bone screws  2  to adjacent vertebral bodies. The implant body is in the form of an anterior lumbar interbody fusion cage (ALIF-cage) that is to be introduced between two adjacent vertebrae of the lumbar spinal column from the anterior direction in order to replace an intervertebral disc. However, the implant can also be in the form of any other cage—like element suitable for replacing an intervertebral disc or a vertebrae that has been removed. 
         [0033]    The implant body  1  has a front wall  3 , a back wall  4 , a right side wall  5  and a left side wall  6  that are integrally formed such that the right and the left side wall connect the front wall and the back wall. The front wall  3  represents an anterior wall and the back wall  4  represents a posterior wall of the implant body  1 . Further, an inner wall  7  extends from the back wall  4  to the front wall  3  and connects them substantially in the middle between the right side wall  5  and the left side wall  6 . The empty spaces between the side walls  5 ,  6  and the inner wall  7  may be filled with bone graft material. 
         [0034]    Teeth  8  are provided on the top and bottom surfaces of the walls for facilitating an engagement with the end plates of the adjacent vertebral bodies. The contour of the implant body  1  along the front wall  3 , the side walls  5 ,  6  and the back wall  4  is adapted to fill the space between the two adjacent vertebral bodies. Hence, in one embodiment, the width of the anterior front wall  3  is greater than the width of the posterior back wall  4  and the height of the implant body  1  increases from the back wall  4  in the direction of the front wall  3 . 
         [0035]    The implant body  1  further comprises at least one hole. In the embodiment shown, three holes  9   a ,  9   b ,  9   c  are provided in the front wall  3 . A center hole  9   a  is provided substantially at the center between the left side wall  6  and the right side wall  5  and is inclined upwards. The center hole  9   a  extends also through a part of the inner wall  7 . The side holes  9   b ,  9   c  are arranged at either side of the center hole  9   a  and are inclined downwards. The implant body is, however, not restricted to the implant body shown, it can have additional holes and/or holes with a different inclination. For example, four holes can also be provided in the front wall. 
         [0036]    As can been seen in particular in  FIGS. 3 and 4 , each of the holes  9   a ,  9   b ,  9   c  has a threaded portion  91  with an internal thread that cooperates with a screw thread. Further, a non-threaded portion  92  is provided between the threaded portion  91  and the surface of the front wall  3 . The inner diameter of the non-threaded portion  92  is slightly larger than the outer diameter of the screw thread. A pin hole  10  is provided, which extends in such a way that it cuts a portion on the circumference of the non-threaded portion  92  as can be seen in  FIGS. 3 and 4 . The longitudinal axis of the pin hole  10  extends perpendicular to the thread axis of the threaded portion  91 . A guiding member in the form of a pin  11  is inserted into the pin hole  10 . In the embodiment shown, the pin  11  is substantially cylindrical. The pin  11  may have a roughened surface. The diameters of the pin  11  and of the pin hole  10  are such that when the pin  11  is inserted, it projects into the non-threaded portion  92  of the hole  9   a  like a secant of a circle but only to such an extent that it does not block the insertion of the screw  2 . The pin  11  is configured to restrict the diameter of the non-threaded portion  92  in such a way so as to enhance the torque to be applied to the screw  2  when the screw  2  is screwed through the threaded portion  91 . The restriction of the diameter is asymmetric in a plane perpendicular to the thread axis of the threaded portion  91 . 
         [0037]    Preferably, the pin  11  is arranged at a position in the non-threaded portion  92  that is adjacent to or close to the threaded portion  91 . 
         [0038]    In  FIGS. 1 and 2  only the center hole  9   a  has the pin  11 . However, also the side holes  9   b ,  9   c  may have a pin  11  as a guiding member. 
         [0039]    The bone screw  2  has a threaded shaft  21  with a bone thread that cooperates with the internal thread of the threaded portion  91 . Adjacent the threaded shaft  21 , a non-threaded portion  22  is provided, which has an outer diameter that is slightly smaller than the inner diameter of the non-threaded portion  92  of the hole. Between the non-threaded portion  22  and an end portion  24  that has a recess  24   a  for engagement with a screw driver, a neck portion  23  having a reduced diameter is provided. The size of the neck portion  23  is such that it forms a groove for engagement with the pin  11 . 
         [0040]    Preferably, the material of the implant body is a bio-compatible plastic material, such as, for example, PEEK (polyetheretherketone) or PEKK (polyetherketoneketone). In particular, the threaded portion  91  is formed in the bio-compatible plastic material. The material of which the pin is made is preferably a bio-compatible metal such as Titanium, stainless steel or a suitable bio-compatible alloy, such as, for example, an Ni—Ti alloy, for example Nitinol; or a fiber re-inforced plastic material. Generally, the material of the implant body is more flexible than the material of the pin. For example, it has a lower modulus of elasticity and/or a lower tensile strength than the material of the pin. Any combinations of materials, not limited to the combination of plastic material and metal that have this relationship can be used. For example, the implant body can be made of a softer plastics and the pin can be made of a harder plastics. 
         [0041]    The screw is usually made of a bio-compatible metal or reinforced plastic material as described above. It can be made of the same as or a different material from that of the pin. It can also be made of the same material as that of the implant body. 
         [0042]    In use, once the implant body has been inserted between two adjacent vertebrae after removal of an intervertebral disc, the implant body is fixed by screwing the bone screws  2  through the respective holes into the end plates of the vertebral bodies. 
         [0043]    The cooperation of the bone screw and the hole will now be described with reference to  FIGS. 5 to 7 , which show in a simplified schematic manner the insertion of the bone screw into a straight hole as compared to the inclined holes of the implant body shown in  FIGS. 1 to 4 . In a first step as shown in  FIG. 5  a bone screw  2  of the appropriate length is selected and advanced into one of the holes  9   a ,  9   b ,  9   c  that has the pin  11  as a guiding member. Then, as shown in  FIGS. 6A and 6B , the screw is screwed into the threaded portion  91 . Thereby, the threaded portion  91  may widen to some extent due to the applied torque, if the implant body is made of a more flexible material. However, since the pin  11  restricts the diameter of the hole in such a way that the crests of the screw thread slide along the pin and snap behind it, the pin acts as a guiding and securing member. It prevents that the screw is screwed into the bone along the wrong trajectory if the material of the implant body yields during insertion of the screw. The pin also enhances the necessary torque to be applied during insertion. This provides a feed-back for the surgeon, in particular in a case in which the bone is weak and/or osteoporotic. The guiding function is also accomplished by a pre-stress or spring function exerted by the pin when the crests of the screw thread snap behind the pin. 
         [0044]    When the screw has been fully inserted the pin snaps into the neck portion  23  of the screw and the end portion  24  abuts against the pin with its flat surface facing the pin and acting as a stop. In this condition, shown in  FIGS. 7A and 7B , the abutment provides a feed-back to the surgeon that the screw has been fully inserted. Simultaneously, the screw is locked, since the pin rests in the neck portion  23 . 
         [0045]    Since the non-threaded portion  22  of the screw has a greater length than the threaded portion  91  of the hole, the implant body can be drawn against the bone. The threaded portion  91  prevents the screw from being pushed out backwards through the hole. 
         [0046]    A second embodiment of the implant is shown in  FIGS. 8 and 9 . The implant body  100  is in the form of a bone plate. Parts of the second embodiment that are identical to that of the first embodiment are designated with the same reference numerals and the detailed description thereof is not repeated. The bone plate  100  has a generally rectangular shape, it is, however, not limited to such a rectangular shape. It has a top surface  101 , a bottom surface  102  and a plurality of holes  9   a ′,  9   b ′, which extend from the upper surface  101  to the lower surface  102 . In the specific embodiment depicted in the figures there is an inclined hole  9   a ′ and a straight hole  9   b ′. The holes have the threaded portion  91 ′ and the non-threaded portion  92 ′ similar to the previous embodiment. Pin holes  10  are associated with each hole and pins  11  are inserted as in the previous embodiment. The guiding and feed-back function is the same as that of the previous embodiment. 
         [0047]    A modification of the first and second embodiment is shown in  FIGS. 10 and 11 . It differs in that the pin  11 ′ as shown in  FIG. 10 , has a thinned portion  11   a  at one of its sides that is substantially symmetrical with respect to the center of the pin. In a further modification, the pin  11 ″ as shown in  FIG. 11  has a thinned portion  11 ″ a  at one of its sides and only at one end. Other designs of thinned portions may also be contemplated. The pin  11 ′, 11 ″ with such a thinned portion has some resiliency and can compensate tolerances of the pin, the screw and the hole. 
         [0048]    A third embodiment will now be described with reference to  FIGS. 12 to 18 . The implant body of the third embodiment is a bone plate  100 ′ as in the second embodiment. It has a top surface  101  and a bottom surface  102  and holes  900   a  and  900   b . The third embodiment differs from the previous embodiments in that the guiding member is a circular member instead of a straight pin. The guiding member shown in  FIGS. 14 to 18  is a substantially circular clip  111  that has a slot and two open ends, like a snap ring. Further, the clip may have a recess  111   a  at the top side and a recess  111   b  at the bottom side, which are similar to a portion of a turn of an internal thread and which facilitate screwing-through of the bone screw  2 . The non-threaded portion  920  of the hole comprises a circular groove  920   a  with an enlarged diameter, which is adjacent to the threaded portion  910  in the embodiment shown in  FIG. 15 . The groove  920   a  serves to accommodate the clip  111 . The size of the groove  920   a  and of the clip  111  is such that when the clip is inserted under pre-tension and has expanded in the groove  920   a , it restricts the diameter of the non-threaded portion  920  in such a way that the torque necessary for inserting the screw  2  is increased. Thereby, the clip provides a guiding and feed-back function similar to the previous embodiments. The clip  111  is made of a less flexible material than the material of the implant body, such as of a metal. In use, the clip  111  is first inserted into the groove. Then, the screw is introduced and screwed through the clip and the threaded portion of the hole. The clip guides the screw such that the torque to be applied is enhanced. Finally, the clip acts as a stop for the end portion of the screw and also secures the screw from being pushed out. 
         [0049]    If the guiding member according to the embodiments is formed of a metal, it is visible under X-ray irradiation even if the implant body itself is not visible. 
         [0050]    The invention is not limited to the above described embodiments. Modifications can be contemplated. For example, many different types of plates and cages can be contemplated. Also, the shape of the bone screw is not limited to the bone screw shown in the embodiments. Many different types of bone screws can be used. The guiding member can also have another shape. The threaded portion of the hole may be provided with a double or higher lead thread and two or more pins may be provided.