Abstract:
A flexible stabilization device for dynamic stabilization of bones or vertebrae is provided comprising a rod construct including a rod made of an elastomeric material the rod having a first connection section, a second connection section and a third section therebetween, the first and second connection sections being connectable with a bone anchoring device, respectively, and a sleeve provided on at least a portion of the third section of the rod such that at least the first and second connection sections are exposed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a continuation of allowed U.S. patent application Ser. No. 11/642,566, filed Dec. 19, 2006, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/753,620, filed Dec. 23, 2005, and claims priority from European Patent Application EP05028283, filed Dec. 23, 2005, the entire disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to a flexible stabilization device for the dynamic stabilization of bones or vertebrae. 
         [0003]    A flexible stabilization device for stabilizing adjacent vertebrae is known from EP 0 669 109 B1. In this stabilization device monoaxial bone screws placed in adjacent vertebrae are connected by an elastic strap. The strap is fastened to the bone screws in a pre-stressed manner. A support body which is resistant to compression surrounds the strap between the bone screws to transmit compressive forces. The support body, the heads of the bone screws and the elastic strap form a kind of joint allowing a limited motion of the vertebrae. 
         [0004]    US 2003/0220643 A1 discloses a device for connecting adjacent vertebral bodies in which monoaxial pedicle screws are interconnected by a spring. The spring allows spinal flexion and a limited degree of lateral bending and axial rotation while preventing spinal extension without the need of a transverse member. A sleeve is placed over the spring. Impingement between the sleeve and the pedicle screws assists the spring in preventing spinal extension. The length of the spring is predetermined. An adaptation in length by the surgeon is not possible. 
         [0005]    WO 2004/105577 A2 discloses a spine stabilization system with one or more flexible elements having an opening or slit in form of a helical pattern. Adjustments of the system with regard to its flexible characteristics are not possible during surgery. 
         [0006]    A bone anchoring device comprising a monoaxial bone screw and a flexible rod which is made of an elastic material is known from EP 1 364 622 A2. The elastic characteristics of the bone anchoring device are determined by material and the shape of the rod which cannot be modified by the surgeon. Furthermore, the use of monoaxial bone screws limits the possibility of adjustment of the position of the shaft relative to the rod. 
         [0007]    Based on the above, there is a need for a flexible stabilization device for dynamic stabilization of bones or vertebrae which allows modification of the elastic characteristics of the device and at the same time the adaptation of the length of the rod construct during the surgical operation. 
       SUMMARY OF THE INVENTION 
       [0008]    A flexible rod assembly including an inner rod and outer rod or sleeve made of an elastomeric material allows an adjustment of the elastic characteristics of the stabilization device to a large extent. By means of selection of a rod and a sleeve with appropriate elastic properties which can be different from each other an adaptation of the elastic properties of the rod construct to the motion of a specific spinal segment is possible. In particular, flexion and compression of the spine can be controlled by means of the elastic properties of the inner rod, whereas extension of the spine can be controlled by selection of an appropriate sleeve. The separation of the damping with regards to flexion/compression and extension movements results in a harmonic behaviour of the vertebral segments under motion control of the construct. As a consequence thereof loosening of the bone screws can be prevented. Additionally, adjustment of the length of the inner rod and of the sleeve is possible. Hence, a modular system is provided which is allows adaptation at the time of surgery. In combination with polyaxial screws the possibilities of adjustment are further increased. 
         [0009]    Further features and advantages of the disclosure will become apparent and will be best understood by reference to the following detailed description of embodiments taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1   a  shows a perspective exploded view of a rod construct according to an embodiment of the disclosure. 
           [0011]      FIG. 1   b  shows the rod construct of  FIG. 1   a  in an assembled state. 
           [0012]      FIG. 2  shows an exploded view of a stabilization device comprising the rod construct of  FIG. 1   a.    
           [0013]      FIG. 3  schematically shows in an exploded view the accommodation of the rod of  FIG. 1   a  in the receiving part of a polyaxial bone screw. 
           [0014]      FIG. 4  shows a sectional view of the assembled parts of  FIG. 3 . 
           [0015]      FIG. 5  schematically shows the assembled stabilization device of  FIG. 2  applied to adjacent vertebrae of the spinal column. 
           [0016]      FIG. 6   a  schematically shows the stabilization device of  FIG. 5  with the spinal column in flexion. 
           [0017]      FIG. 6   b  schematically shows the stabilization device of  FIG. 5  with the spinal column in extension. 
           [0018]      FIG. 7  schematically illustrates the directions of displacement of the rod construct of  FIG. 1   b.    
           [0019]      FIG. 8   a  schematically shows the rod construct in a state of flexion according to  FIG. 6   a.    
           [0020]      FIG. 8   b  schematically shows the rod construct in a state of extension according to  FIG. 6   b.    
           [0021]      FIGS. 9   a  and  9   b  show a further example of application of the stabilization device in a top view. 
           [0022]      FIG. 10  shows modification of the rod construct in section. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    As shown in  FIGS. 1   a  and  1   b , the flexible stabilization device includes a rod assembly including rod  20  made of an elastomeric material and a sleeve  40  which is also made of an elastomer material. In the embodiment shown, the rod  20  has a cylindrical shape with a smooth surface. Due to the elastomer material, the rod is partially or fully flexible. For example, the rod  20  can be made of a biocompatible plastic material such as a polymer on the basis of polyurethane, polysilicone or PEEK. A particularly suitable material is Polycarbonate Urethane. The material of the rod includes well-defined elastic properties and the rod shows bending elasticity, compressive elasticity and tensile elasticity. 
         [0024]    The elastomeric material of the sleeve  40  can also be a biocompatible plastic material such as a polymer on the basis of polyurethane, polysilicone or PEEK which includes elastic properties which can be selected independently of the elastic properties of the rod  20 . Also for the sleeve  40 , Polycarbonate Urethane is particularly suitable. The sleeve  40  has a tube-like shape including a channel  41  the diameter of which is slightly larger than the outer diameter of the rod  20  so that the rod  20  can be inserted into the channel  41  as shown in  FIG. 1   b . The length of the sleeve  40  is selected to be smaller than the length of the rod  20  such that a first connection section  20   a  and a second connection section  20   b  of the rod  20  protrude from the channel  41  in the assembled state as shown in  FIG. 1   b . A section  20   c  between the first connection section  20   a  and the second connection section  20   b  of the rod  20  is accommodated in the channel  41  of the sleeve  40 . Preferably the length of the sleeve  40  corresponds approximately to the distance between the receiving parts of the bone anchoring devices, or can be slightly larger. 
         [0025]    With reference to  FIGS. 2 to 4  the connection of the rod  20  with the receiving part  6  of a bone anchoring element  1  is explained. Although the sleeve  40  is omitted in the illustration of  FIG. 3 , for the purpose of describing the connection of the rod  20  with the receiving part  6 , the sleeve  40  is placed on the rod  20  before the latter is secured to the respective receiving parts  6  of the bone anchoring elements  1 . 
         [0026]    The bone anchoring element  1  in this embodiment is a polyaxial bone screw having a shank  2  with a bone thread, a tip  3  at one end and a spherical head  4  at the opposite end. A recess  5  for engagement with a screwing-in tool is provided at the side of the head  4  which is opposite to the shank. The receiving part  6  has a first end  7  and a second end  8  opposite to the first end and a longitudinal axis  9  intersecting the plane of the first end and the second end. Coaxially with the longitudinal axis  9  a bore  10  is provided which extends from the first end  7  to a predetermined distance from the second end  8 . At the second end  8  an opening  11  is provided the diameter of which is smaller than the diameter of the bore  10 . A spherical or otherwise tapering section  12  is provided adjacent of the opening  11  which forms a seat for the spherical head  4 . 
         [0027]    The receiving part  6  further has a U-shaped recess  13  which starts at the first end  7  and extends in the direction of the second end  8  to a predetermined distance from said second end  8 . By means of the U-shaped recess  13  two free legs  14 ,  15  are formed extending towards the first end  7 . Adjacent to the first end  7  the receiving part  6  comprises an internal thread  16  on said legs  14 ,  15 . 
         [0028]    As can be seen in  FIG. 3 , a first pressure element  17  is provided which has a cylindrical construction with an outer diameter which is only slightly smaller than the inner diameter of the bore  10  to allow the first pressure element  17  to be introduced into the bore  10  of the receiving part  6  and to be moved in the axial direction. On its lower side facing towards the second end  8 , the pressure element  17  includes a spherical recess  18  the radius of which corresponds to the radius of the spherical head  4  of the bone screw. On the opposite side, the first pressure element  17  has a cylindrical recess  19  which extends transversely to the longitudinal axis  9 . The lateral diameter of this recess is selected such that the connection section  20   a  or  20   b  with a circular cross section, respectively, of the rod  20  which is to be received in the receiving part  6  can be inserted into the recess  19  and guided laterally therein. The depth of the cylindrical recess  19  is selected such that in an assembled state when the connection section  20   a  or  20   b  of the rod  20  is inserted and pressed against the bottom of the U-shaped recess  13 , the first pressure element  17  exerts a pressure on the head  5 . Further the depth is preferably about half of the diameter of the connection section  20   a  or  20   b  of the rod  20 . As can be seen in  FIG. 3 , the first pressure element  17  has a coaxial bore  21  for guiding a screwing-in tool therethrough. 
         [0029]    As shown in  FIGS. 3 and 4 , the bone anchoring element  1  further comprises a second pressure element  23  with a first end  24  and a second end  25 . The width of the second pressure element  23  is such that the second pressure element  23  can be inserted into the U-shaped recess  13  of the receiving part  6 . On opposite sides of the second pressure element  23  two cylindrical projections  26  are provided which fit into the space limited by the internal thread  16  to slide along the internal thread  16  when the second pressure element  23  is inserted. 
         [0030]    As can be seen in  FIG. 2 , the second pressure element  23  further includes a cylindrical recess  27  extending from the second end  25  in the direction towards the first end  24  the cylinder axis of which is perpendicular to that of the cylindrical projections  26 . On the side of the second end  25 , the cylindrical projections  26  include lower edges  26   a . The diameter of the cylindrical recess  27  corresponds to the diameter of the connection section  20   a  or  20   b  of the rod  20  and its depth to half or less than half of the diameter of the connection section  20   a  or  20   b.    
         [0031]    The bone anchoring element  1  further includes an inner screw  30  which can be screwed-in between the legs  14 ,  15 . The internal thread  16  and the cooperating thread of the inner screw  30  can have any known thread shape. Using a flat thread or a negative angle thread can prevent splaying of the legs  14 ,  15 . 
         [0032]    The receiving part  6  and the first pressure element  17  can have corresponding crimp bores  32 ,  33  on opposite sides by means of which the screw  1 , the receiving part  6  and the first pressure element  17  can be loosely pre-assembled. As shown in  FIGS. 3 and 4  the first pressure element  17  and the second pressure element  23  can have projections  22 ,  28 , respectively, which can contribute to the fixation of the elastic rod  20 . 
         [0033]    The other parts of the flexible stabilization device except the rod  20  and the sleeve  40  can be made of the commonly used biocompatible materials, such as stainless steel or titanium or any other material suitable for a bone screw. 
         [0034]    In use, at least two bone anchoring devices are anchored into the bone. Next, the rod  20  and the sleeve  40  are selected and combined to achieve the desired elastic properties of the flexible stabilization device. If, for example, more than two vertebrae are to be connected, different sleeves  40  having different elastic properties can be selected and provided between different vertebrae. In this way, the elastic properties of the stabilization device can be adapted at the time of surgery. Preferably, the sleeve  40  is selected to have a length corresponding to the distance of the two receiving parts when the pedicle screws are screwed into adjacent vertebrae. Since the rod  20  and the sleeve  40  are made of elastomeric material, shortening during surgery is possible. Then, rod  20  with the sleeve  40  or, if more that one motion segment shall be stabilized via a single rod  20 , with a plurality of sleeves  40  is inserted into the receiving parts  6  of the bone anchoring elements. Preferably, in the balanced position of the two adjacent vertebrae, the sleeve  40  is in contact with the receiving parts  6 . 
         [0035]    Thereafter, the second pressure element  23  is inserted in the receiving part  6  and the inner screw  30  is screwed-in between the legs  14 ,  15 . After adjusting the angular position of the bone screw, the inner screw  30  is tightened. By the pressure exerted by the inner screw  30  onto the second pressure element  23 , the rod  20  is clamped between the first and the second pressure element  17 ,  23  and simultaneously the head  4  of the bone screw is locked in its angular position. 
         [0036]    Next, with reference to  FIGS. 5 to 8   b  the elastic properties of the flexible stabilization device are described. In  FIG. 5  the assembled stabilization device is shown with the rod  20  and the sleeve  40  arranged to connect two polyaxial pedicle screws which are placed in adjacent vertebrae W Ruining a motion segment. The positions of the shanks of the pedicle screws are indicated by dash-dotted lines. As can be seen in  FIG. 5 , the receiving parts  6  of the bone anchoring elements  1  have a distance x in the balanced position in which the rod  20  and the sleeve  40  are in an unstressed state. 
         [0037]      FIG. 6   a  shows the stabilization device when flexion takes place. During flexion, tensile stress is applied to the rod  20  resulting in an elongation of the rod  20 . The distance between the bone anchoring elements is increased to x+ΔX 1 . The increase ΔX 1  in the distance is limited by the restoring force produced by the rod  20  due to its elastic properties. The increase in the distance can be, for example, in the range of approximately 1.5 mm. Hence, flexion/compression is controlled mainly by the inner rod  20 . 
         [0038]      FIG. 6   b  shows the stabilization device when extension takes place. During extension, a compressive force is applied to the rod  20  and the sleeve  40  by the receiving parts  6  of the bone anchoring elements  1 . The elasticity of the rod  20  and the sleeve  40  allows the distance between the receiving parts  6  to decrease to a distance x−Δx 2 . Due to the elastic properties of the rod  20  and the sleeve  40 , a restoring force acts on the receiving parts  6  which limits the decrease of the distance. The distance can, for example, decrease by approximately 0.5 mm. Hence, extension is controlled by the compressibility of the inner rod  20  and is limited by the sleeve. 
         [0039]    In an alternative manner of application, the sleeve  40  can be pre-compressed in the balanced state and/or the rod  20  can be pre-stressed in the balanced state. 
         [0040]      FIG. 7  illustrates the possible deformations which the rod  20  and the sleeve  40  can undergo.  FIGS. 8   a  and  8   b  show the deformation of the rod  20  and the sleeve  40  in flexion ( FIG. 8   a ) and in extension ( FIG. 8   b ). 
         [0041]      FIGS. 9   a  and  9   b  show an example of application of the stabilization device.  FIG. 9   a  shows two adjacent vertebrae V, V′ which are medio-laterally inclined with respect to each other in the case of the presence of scoliosis. To dynamically stabilize and correct such a motion, segment rods  200 ,  200 ′ with different sleeves  400 ,  400 ′ can be used on the left side and on the right side. The sleeve  400  used on the left side rod  200  has a length which is greater than the length of sleeve  400 ′ used on the right side rod  200 ′. In this manner, it is possible to eliminate the inclination of two vertebrae on the left side. In addition, the outer diameter of the sleeve  400  can be different from that of the sleeve  400 ′ in order to have a different motion control with respect to the left side and the right side. 
         [0042]    Further modifications of the above described embodiments are possible. In the embodiment described before, the sleeve  40  has the shape of a hollow cylinder; however, different shapes of the sleeve are possible. For example, a barrel-shape is possible. The length of the sleeve can differ from the embodiment shown. The rod  20  may also have a rectangular, square, oval or triangular cross-section or any other appropriate shape of the cross-section. In this case, the shape of the sleeve  40  is appropriately adapted. In particular, it is possible to form the rod  20  and/or the sleeve  40  with the shape varying in the longitudinal direction. The rod  20  and the sleeve  40  can be formed to be highly flexible or hardly flexible. 
         [0043]    The surface of the rod  20  and/or the sleeve  40  can be textured or structured.  FIG. 10  shows an example of an inner rod  201  having a corrugated surface, with corrugations  300  provided in the circumferential direction. The inner wall of the sleeve  401  has corresponding corrugations cooperating with that of the rod. This prevents or reduces a displacement of the sleeve relative to the rod. 
         [0044]    In the example of the bone anchoring element described above, the connection of the shanks  2  of the bone anchoring elements  1  to the respective receiving parts  6  is polyaxial. However, it is also possible to provide a monoaxial connection. 
         [0045]    For the inner screw  30 , all known modifications can be used. This includes also the use of an outer ring or nut. 
         [0046]    In the embodiments described, the bone anchoring element  1  is introduced from the top into the receiving part  6 . However, the bone anchoring element  1  can also be introduced from the bottom of the receiving part  6  if the receiving part  6  is constructed to allow this. 
         [0047]    The head of the bone anchoring element and the shaft can be constructed as separate parts which can be connected. 
         [0048]    The present disclosure is not limited to screws as bone anchoring elements but can be realized with bone hooks or any other bone anchoring element.