Abstract:
A bone screw is provided including a tubular body and a head as separate parts. The tubular body has a first end, a second end, a tubular wall defining a cavity and a plurality of openings extending through the tubular wall into the cavity. The tubular body also includes a continuous exterior bone thread on an exterior tubular surface of the tubular wall. The continuous exterior bone thread extends for at least one full revolution about the tubular surface. The head defines an inner bore and includes an engagement structure to engage with a driver to advance the bone screw in the bone when a rotating force from the driver is applied to the engagement structure in a first direction, and a connection structure to connect to the tubular body at the first end. The head is connected to the tubular body by the connection structure in such a way that the head is locked against disconnection from the tubular body when a rotating force from a driver is applied to the engagement structure in a second direction opposite to the first direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/293,433, filed Jan. 8, 2010, the entire contents of which are incorporated herein by reference. This application also claims priority to and the benefit of EP 10 150 373.8, filed in the European Patent Office on Jan. 8, 2010, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a bone screw. In particular, the invention relates to a bone screw that can be used as an anchor screw and a fusion screw that fuses with the surrounding bone material. 
         [0003]    A bone screw that can be used as a fusion screw is known from US 2004/0015172 A1. This bone screw has a tubular thread section with a bone thread and with a plurality of recesses in its wall, a head, and a tip that can be connected to the tubular thread section. In use, the tubular portion can be filled with bone material or other growth promoting material and then the tip and/or the head are connected to the tubular portion. Usually the screw is inserted into a core hole in the bone, which is prepared in advance. After insertion of the screw into the bone, fusion of the screw with the surrounding bone material takes place. The screw can act as a traction element to connect shattered or split-off parts of bones together by means of the screw. 
       SUMMARY 
       [0004]    It is the object of the invention to provide an improved bone screw of the aforementioned type, the position of which in the bone can be corrected after insertion and which can be easily removed later, if required. 
         [0005]    The bone screw according to the invention is fillable with a substance to support fusion and is precisely positionable. When the bone screw is inserted into a core hole that has been prepared in advance, the bone thread at an exterior wall surface engages the bone and the screw is advanced by screwing it deeper into the bone by means of a screw driver. To adjust the position of the bone screw, it might be necessary to screw it back out to reposition the screw. This is facilitated, since the head and the tubular body of the bone screw are firmly connected without a risk of loosening or disconnection. 
         [0006]    The bone screw can act as a bone anchor or can be used to connect broken elements of bones or as a support beam to strengthen weak bones. 
         [0007]    In certain situations, it might be clinically necessary to remove an implanted bone screw at a later stage, when the bone screw may have already fused with the surrounding bone material. Since the head and the tubular body are firmly connected, it is possible to remove a bone screw that has already fused with the surrounding bone material. 
         [0008]    Further features and advantages of the invention will become apparent from the description of embodiments of the invention by means of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows a perspective exploded view of a bone screw according to a first embodiment. 
           [0010]      FIG. 2  shows a perspective view of the bone screw of  FIG. 1  in an assembled state. 
           [0011]      FIG. 3  shows an enlarged exploded side view of a bone screw according to the first embodiment, a tubular portion being shown only in part. 
           [0012]      FIG. 4  shows a perspective exploded view of the bone screw of  FIG. 3 . 
           [0013]      FIG. 5  shows a sectional view of the bone screw of  FIG. 3  in an assembled state, the section being taken in a plane containing the screw axis. 
           [0014]      FIG. 6  shows an sectional view of an enlarged portion of the bone screw according to  FIG. 1  in the region of the tip, the section being taken in a plane containing the screw axis. 
           [0015]      FIG. 7  shows an enlarged perspective view of the tip. 
           [0016]      FIGS. 8   a  to  8   d  show steps of using the bone screw according to the first embodiment. 
           [0017]      FIG. 9  shows a perspective exploded view of a bone screw according to a second embodiment. 
           [0018]      FIG. 10  shows a perspective view of the bone screw of  FIG. 9  in an assembled state. 
           [0019]      FIG. 11  shows an enlarged portion of the bone screw of  FIG. 9  in an exploded view. 
           [0020]      FIG. 12  shows an enlarged sectional view of a portion of the tubular portion of the bone screw of  FIG. 9 . 
           [0021]      FIG. 13  shows an enlarged sectional view of the connection of the tubular body with the head of the bone screw of  FIG. 11 . 
           [0022]      FIG. 14  shows a modified example of the bone screw according to the previous embodiments in a side view. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The bone screw according to a first embodiment as shown in  FIGS. 1 to 7  comprises a tubular body  1  with an open first end  2  and a second end  3  and a screw axis L. In the example shown, the tubular body is cylindrical. The tubular body has a tubular wall defining a cavity. On the exterior surface of the tubular wall, a so-called bone thread  4  is provided. In the embodiment shown, the bone thread is provided along the whole length of the tubular body. The bone thread  4  is configured to cut into the bone when the bone screw is screwed into the bone. Further, a plurality of openings  5  are located in the tubular wall, which extend entirely through the tubular wall into the cavity. The plurality of openings  5  are shown as diamond-shaped and are located between the crests of the bone thread  4 . However, any other variations of the shapes and locations of the plurality of openings  5  are conceivable. The cavity provided by the tubular body  1  has a volume that is suitable for accommodating bone material. The wall thickness of the tubular body is preferably smaller than about 15% of the outer diameter of the tubular body. 
         [0024]    As shown in  FIG. 5 , the tubular body  1  has in the region of its first end  2  an internal thread  6  provided on its interior wall surface. At the first end  2  a plurality of recesses  7  are provided in the wall of the tubular body, which extend in the axial direction from the first end  2  to a certain distance in direction of the second end  3 . Seen in the radial direction, the recesses extend completely through the wall of the tubular body. In the embodiment shown, four triangular recesses  7  are provided at equidistant locations in the circumferential direction of the edge of the first end  2 . 
         [0025]    The bone screw further comprises a head  8  that is connectable to the first end  2 . The head  8  is substantially cylindrical and has an outer diameter that is greater than the outer diameter of the tubular body  1 . The head  8  has a first end  9  facing away from the tubular body  1  and a second end  10  facing the first end  2  of the tubular body. At its second end  10 , the head  8  has a plurality of projections  11 , the shape and location of which is such that the projections  11  engage the recesses  7  of the tubular body  1  so that a form-fit connection between the head  8  and the tubular body  1  is provided. In this embodiment, four equidistantly-located triangular projections extend from the second end  10  of the head  8 . Instead of recesses at the tubular body and projections at the head, recesses can be present at the head and projections can be present at the tubular body, or the head and the tubular body can have both recesses and projections. The head  8  in this embodiment is cannulated. It comprises a coaxial bore  12  extending from the first end  9  to the second end  10  completely through the head. 
         [0026]    At its first end  9  a recess  13  is provided, which is configured to engage with a screw driver used to screw the bone screw into the bone by rotating the driver in a first direction that is the screw-in direction. The recess  13  has an inner diameter that is larger than the inner diameter of the coaxial bore  12 . The shape of the recess is, for example, a hexagon shape. However, any other shape that is suitable for engagement with a screw driver can be used. Such other shapes can be, for example, square shape or any other polygon shape or star shape, for example a Torx® shape. Between the recess  13  and the second end  10  of the head  8  there is a transition portion  14  that is, for example, a tapered portion that can act as an abutment for a securing screw with tapered head to be described below. However, the transition portion can have another shape adapted to the shape of the head of the securing screw or can be omitted. The second end  10  of the head  8  of the bone screw has preferably a flat surface in order to provide an abutment for the bone surface. 
         [0027]    The bone screw also comprises a securing element  15  in the form of a screw, the screw thread of which cooperates with the internal thread  6  provided at the interior wall surface of the tubular body  1 . The screw has a tapered head  16  abutting against the tapered transition portion  14  of the head. Further, a recess  17 , for example, a polygon recess, is provided for engagement with a screw driver. 
         [0028]    By means of the above described design, the head  8 , which has the engagement structure for the screw driver to insert the screw into the bone, is connected by a form-fit connection to the tubular body, which prevents the head from being disconnected from the tubular body in the case that the screw driver is rotated in the reverse direction of the screw-in direction. Further, the connection between the head and the tubular body via the securing element is independent from the force acting onto the head when the screw is being screwed in. 
         [0029]    For the securing element, other screw types or other devices such as a bayonet locking device can be used. 
         [0030]    As shown in  FIGS. 1 ,  6  and  7 , the bone screw further has a separate tip  20  having a projection. The tip  20  in this embodiment is detachably connected to the tubular body  1 . For this purpose, the tubular body  1  has adjacent to its second end  3  an internal thread  31 , which can be engaged by an external thread  21  provided at the projection of the tip  20 . In this embodiment, the tip  20  is cannulated. It has a coaxial bore  22  extending completely through the tip  20 , which narrows towards the tip end opposite the projection. The tip  20  that is shown as an example also has a self-cutting structure  23 , which allows the tip  20  to cut the bone when the bone screw is rotated so that it is not necessary to prepare a core hole in advance. 
         [0031]    Other kinds of tips are conceivable. For example, the design of the tip can be such that it can be connected via a press-fit connection to the tubular body. Tips without self-cutting structures or without coaxial bores are also conceivable. Finally, it is also possible that the tip is integrally formed with the tubular body  1 . 
         [0032]    The coaxial bore  12  in the head and the coaxial bore  22  in the tip can serve as a guide structure for guiding a guide wire therethrough. They also can serve as a channel for introducing liquid bone cement or fluid drugs. 
         [0033]    All parts of the bone screw are made of a body-compatible material such as a body-compatible metal, for example stainless steel or titanium; a body-compatible metal alloy, for example Nitinol; a body-compatible plastic material, for example PEEK; or combinations thereof. 
         [0034]    In addition, the tubular body or the other parts of the bone screw can be coated with an in-growth promoting material or can be roughened to enhance in-growth of bone or vessels. 
         [0035]    The steps of using the bone screw are shown in  FIGS. 8   a  to  8   d . In a first step shown in  FIG. 8   a , the tip  20  is mounted onto the second end of the tubular body  1 . Then, bone chips  100  are filled into the cavity provided by the tubular body  1 . Then, as shown in  FIG. 8   b , the head  8  and the securing screw  15  are mounted onto the first end of the tubular body  1 . The head  8  is mounted such that the projections  11  at the second end  10  of the head engage the recesses  7  at the first end  2  of the tubular body. In this position, the securing screw  15  is introduced and tightened, so that the bone screw is closed, as shown in the side view of  FIG. 8   c  and the sectional view of  FIG. 8   d.    
         [0036]    The thus-prepared bone screw can be inserted into a core hole in the bone, which has been prepared in advance. The recess  13  is engaged with the screw driver (not shown) and the bone screw is advanced into the core hole by rotating the screwdriver in the screw-in direction. Slight corrections of the position of the bone screw in the core hole can be made by rotating the screwdriver in a reverse direction, so that the screw is screwed back out. Because of the shape-fit connection between the head and the tubular body, this is easily possible without the risk of loosening or disconnection of the head and the tubular body. After a certain time, fusion of the surrounding bone with the bone screw takes place. 
         [0037]    In another embodiment of use, the coaxial bores  18  and  22  of the securing screw and the tip are used for guiding through a guide wire (not shown). In this case the guide wire, which is guided through the bone screw, is introduced through the skin of the patient and advanced through the tissue until it reaches the position where the bone screw is to be placed. The guide wire is inserted into the bone to the appropriate direction and depth. The bone screw is then guided along the guide wire extending therethrough until it reaches the surface of the bone and then screwed into the bone guided by the guide wire. This is in particular used in minimally invasive surgery (“MIS”). 
         [0038]    A second embodiment is now described with reference to  FIGS. 9 to 13 . Identical parts are designated with the same reference numerals and the description thereof is not repeated. The tubular body  1 ′ of the second embodiment differs from the tubular body  1  of the first embodiment in that a recess in the form of a circumferential groove  60  is provided at the interior wall surface of the tubular body  1 ′ at a distance from the first end  2  and from the internal thread  6 . The groove  60  serves for engagement with a projection provided at a head  80 . 
         [0039]    The head  80  has a first end  81  facing away from the tubular body  1 ′ and a second end  82  facing the tubular body F. The surface of the first end  81  can be, for example, lens-shaped. The second end  82  comprises a flat surface providing an abutment for the bone surface. At its second end  82 , the head  80  has a cylindrical projection having a first portion  83  with an external thread that cooperates with the internal thread  6  provided at the interior wall surface of the tubular body  1 ′ adjacent the first end  2 . Following the threaded portion  83  a substantially cylindrical portion with flexible wall sections  84  is provided. At the free end of each wall section  84 , a projection  85  is provided, the shape of which is such that the projection  85  fits into the groove  60  of the tubular body  1 ′. 
         [0040]    The axial length of the threaded projection  83  and the flexible wall sections  84  is such that, as shown in  FIG. 13 , when the head  80  is inserted into the tubular body  1 ′ and the threaded connection between the portion  83  and the internal thread  6  is tightened, the projection  85  engages the groove  60 . 
         [0041]    Further, the head  80  has a recess  86  at its first end for engagement with a screw driver. The recess  86  is shown to be star-shaped but can have any polygon shape. A coaxial bore  87  extends through the head  80 , which has a portion within an internal thread  88  adjacent the recess  86  and which has in the region of the flexible wall sections  84  a section  89  with a smaller inner diameter that tapers in a portion  89   a  towards the free end. 
         [0042]    The securing element  50  according to the second embodiment is a set screw with a cylindrical projection  51  without a thread and with a taper  51   a  at its free end. The set screw can be screwed into the coaxial bore of the head  80  when the head  80  is inserted into the tubular body, and tightened until the tapered section  89   a  of the screw and the tapered section  51   a  of the head engage each other. By means of this, the flexible wall portions  84  are slightly pressed outwards, so that the projections  85  engage the groove  60 . When the projections  85  engage the groove  60 , the head  80  is secured to the tubular body  1 ′ and cannot be loosened or disconnected. A coaxial bore  52  can be provided for guiding a guide wire therethrough. 
         [0043]    Use of the bone screw according to the second embodiment is similar to the use of the bone screw according to the first embodiment. The description thereof will not be repeated. 
         [0044]      FIG. 14  shows a bone screw according to a modified embodiment of the previous embodiments. Head  8  and tip  20  can be that of the first embodiment, the second embodiment or the third embodiment. The tubular body  1 ″′ has a first section  101  adjacent to the tip  20  comprising a bone thread  4 , and a second section  102  adjacent to the head  8  with a smooth surface without a bone thread. The screw according to the modified embodiment can be used as a compression screw to press together two bone parts  201 ,  202 . The bone screw is screwed into one bone part  202 . In the other bone part  201  there is only a core hole, through which the first section  102  of the bone screw extends. By tightening the bone screw, the second end  10  of the head  8  abuts against the surface of the bone part  201  and presses the bone part  201  against the bone part  202 . After a certain time, fusion of the bone parts  201 ,  202  with the bone screw and with each other takes place. 
         [0045]    In a further modification, the second end  3  of the tubular body of all of the previous embodiments is free and the tip is omitted. In this case, the second end  3  can be provided with a plurality of cutting teeth that are configured to cut into the bone so that the cavity of the tubular body is automatically self-filled with cut bone material by screwing-in the bone screw into the bone. 
         [0046]    In a further modification the outer diameter of the head may be equal to or even smaller than the outer diameter of the tubular body. This allows the bone screw to immerse fully into the bone. 
         [0047]    In a further modification, the head and the tip are not cannulated.