Abstract:
A system for stabilizing the spine, according to which a first dampening member is compressed in response to compressive loads on the spine, and a second dampening member is compressed in response to tensile loads on the spine.

Description:
BACKGROUND  
       [0001]    The present invention relates to a system for stabilizing the human spine. 
         [0002]    Intervertebral discs that extend between adjacent vertebrae in vertebral columns of the human body provide critical support between the adjacent vertebrae while permitting multiple degrees of motion. These discs can rupture, degenerate, and/or protrude by injury, degradation, disease, or the like, to such a degree that the intervertebral space between adjacent vertebrae collapses as the disc loses at least a part of its support function, which can cause impingement of the nerve roots and severe pain. 
         [0003]    Some of the current procedures for treating this malady involve pedicular systems for dynamic stabilization of the vertebrae that include a viscoelastic dampening member to allow motion in compression. However, these systems are not flexible, or compliant, in tension, and therefore produce asymmetric flexion-extension biomechanics which is undesirable. 
         [0004]    The present invention is directed to an improved system of the above type that allows motion in compression and tension and produces symmetric flexion-extension biomechanics. Various embodiments of the invention may possess one or more of the above features and advantages, or provide one or more solutions to the above problems existing in the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0005]      FIG. 1  is a side elevational view of an adult human vertebral column. 
           [0006]      FIG. 2  is a posterior elevational view of the column of  FIG. 1  and depicting a system according to an embodiment of the invention. 
           [0007]      FIG. 3  is an elevational view of one of the vertebra of the column of  FIGS. 1 and 2 . 
           [0008]      FIG. 4  is an enlarged view of a portion of the column of  FIGS. 1 and 2  and the system of  FIG. 2 . 
           [0009]      FIG. 5  is an enlarged isometric view of a dampening mechanism of the system of  FIGS. 2 and 4 . 
           [0010]      FIG. 6  is a cross-sectional view of the mechanism of  FIG. 5 . 
           [0011]      FIGS. 6A and 6B  are views similar to  FIG. 6 , on a reduced scale, depicting the movements of the dampening mechanism. 
           [0012]      FIG. 7  is an exploded view of an alternate embodiment of the mechanism of  FIG. 6 . 
           [0013]      FIG. 8  is a cross-sectional view of the mechanism of  FIG. 7 . 
           [0014]      FIGS. 8A and 8B  are views similar to  FIG. 8 , on a reduced scale, depicting the movements of the dampening mechanism. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    With reference to  FIGS. 1 and 2 , the reference numeral  10  refers, in general, to the lower portion of a human vertebral column. The column  10  includes a lumbar region  12 , a sacrum  14 , and a coccyx  16 . The flexible, soft portion of the column  10 , which includes the thoracic region and the cervical region, is not shown. 
         [0016]    The lumbar region  12  of the vertebral column  10  includes five vertebrae V 1 , V 2 , V 3 , V 4  and V 5  separated by intervertebral discs D 1 , D 2 , D 3 , and D 4 , with the disc D 1  extending between the vertebrae V 1  and V 2 , the disc D 2  extending between the vertebrae V 2  and V 3 , and the disc D 3  extending between the vertebrae V 3  and V 4 , and the disc D 4  extending between the vertebrae V 4  and V 5 . 
         [0017]    The sacrum  14  includes five fused vertebrae, one of which is a superior vertebra V 6  separated from the vertebra V 5  by a disc D 5 . The other four fused vertebrae of the sacrum  14  are referred to collectively as V 7 . A disc D 6  separates the sacrum  14  from the coccyx  16 , which includes four fused vertebrae (not referenced). 
         [0018]    With reference to  FIG. 3 , the vertebra V 4  includes two laminae  20   a  and  20   b  extending to either side (as viewed in  FIG. 2 ) of a spinous process  22  that extends posteriorly from the juncture of the two laminae. Two transverse processes  24   a  and  24   b  extend laterally from the laminae  20   a  and  20   b,  respectively; and two articular processes  28   a  and  28   b  extend inferiorly from the laminae  20   a  and  20   b,  respectively. The inferior articular processes  28   a  and  28   b  rest in the superior articular process of the vertebra V 5  ( FIG. 5 ) to form a facet joint. Since the vertebra V 1 -V 3  and V 5  are similar to the vertebra V 4 , and since the vertebrae V 6  and V 7  are noninvolved in the present invention, they will not be described in detail. 
         [0019]    It will be assumed that, for one or more of the reasons set forth above, the vertebra V 4  and/or V 5  are not being adequately supported by the disc D 4  for one or more of the above reasons, and that it is therefore necessary to provide supplemental support and stabilization of these vertebrae. To this end, a system  30  is provided that is shown in  FIG. 2  and in greater detail in  FIG. 4 . 
         [0020]    Referring to  FIG. 4 , the system  30  includes a fixation device, in the form of a screw  32 , that is fastened to the vertebra V 4 ; and a fixation device, in the form of a screw  34 , that is fastened to the vertebra V 5 . It is understood that the screws  32  and  34  can be fastened to various areas of the vertebrae V 4  and V 5  including, but not limited to, the processes, the laminae, or the pedicles. 
         [0021]    The screw  32  has a head  32   a  extending from an externally threaded shank  32   b  that is screwed in the vertebra V 4 , and the screw  34  has a head  34   a  extending from an externally threaded shank  34   b  that is screwed in the vertebra V 5 . Each head has a bore, or through opening, extending therethrough, and two set screws  32   c  and  34   c  are provided in the heads  32   b  and  34   b,  respectively, that can be torqued to secure a member in each opening, as will be described. 
         [0022]    Referring to  FIGS. 4 and 5 , a dampening mechanism  40  is provided that is mounted to the screws  32  and  34 . The mechanism  40  has a slight overall curvature and includes a rod  42 , and end portion of which extends in the above opening in the screw  32 . The set screw  32   c  is torqued over the rod  42  as necessary to secure the rod  42  to the screw  32 . 
         [0023]    A tubular member  44  is also provided, and as shown in  FIG. 6 , a portion of the rod  42  extends through the bore of the tubular member  44 , with the corresponding end portion of the rod projecting from the tubular member. An annular flange  42   a  projects radially outwardly from the rod  42  between its respective ends, and an annular flange  44   a  projects radially outwardly from one end of the tubular member  44 . The flange  44   a  projects radially outwardly from one end of the tubular member  44 . The flange  44   a  extends in a spaced relation to the flange  42   a.    
         [0024]    A ring-shaped dampening member  46  extends around the rod  42  and between the flanges  42   a  and  44   a  and approximately mid-way between the screws  32  and  34 . The dampening member  46  is fabricated from a material having appreciable and conjoint viscous and elastic properties. The axial length of the damping member  46  is greater than that of the damping member  50  so as to have different dampening properties. 
         [0025]    A cap  48  has an externally threaded shank  48   a  that is threadedly engaged with a corresponding internally threaded bore in the other end portion of the rod  42 . The diameter of the cap  48  is greater than that of the rod  42  so as to define, with the corresponding end of the rod, an annular space. A ring-shaped dampening member  50  extends around the rod  42  and in the latter space. The dampening member  50  is fabricated from a material having appreciable and conjoint viscous and elastic properties. 
         [0026]    A portion of the member  44  extends in the opening in the screw  32 , and the length of the member  44  is greater than the diameter of the screw  32  so that the cap  48  and the dampening member  50  extend outside of the opening in the screw. The set screw  34   c  is torqued over the latter portion of the member  44  as necessary to secure the tubular member  44  to the screw  32 . 
         [0027]    The mechanism  40  is shown in  FIG. 6  in its unloaded state, i.e., when there is no appreciably tensile or compression loads on the vertebrae V 4  and/or V 5 . However when there is flexion or extension of the column  10  caused by corresponding movements of the patient, the mechanism  40  will respond to the resulting compressive and tensile loads on the vertebrae V 4  and V 5  as follows. 
         [0028]    Compressive loads on the vertebrae V 4  and V 5  causes relative movement of the screws  32  and  36  ( FIG. 4 ) towards each other. This causes relative movement of the rod  42  and the member  44 , and therefore the flanges  42   a  and  44   a,  towards each other and compresses the dampening member  46 , as shown in  FIG. 6A , to dampen the movement. After the compressive load and the above relative movements of the screws  32  and  34  towards each other cease, the dampening member  46  will tend to return to its original, non-compressed state, causing relative movement of the flanges  42   a  and  44   a,  and therefore the rod  42  and the member  44 , away from each other so that the system  30  returns to the unloaded position of  FIG. 6 . 
         [0029]    Relative movement of the screws  32  and  34  away from each other in response to tensile loads on the vertebrae V 4  and V 5  causes relative movement of the rod  42  and the tubular member  44  away from each other. This causes relative movement of the cap  48  and the member  44  towards each other and thus compresses the dampening member  50  to dampen the movements, as shown in  FIG. 6B . After the tensile load and the above relative movements of the screws  32  and  34  away from each other cease, the dampening member  50  will tend to return to its original, non-compressed state and move the cap  48  and the member  44  away from each other so that the system  30  takes the unloaded position of  FIG. 6 . 
         [0030]    According to the embodiment of  FIGS. 7 and 8 , a system is provided that includes the screws  32  and  36  ( FIG. 4 ) of the previous embodiment along with a dampening mechanism  60  that is mounted to the screws. In particular, the mechanism  60  includes two axially aligned and spaced rods  62  and  64 , with an end portion of the rod  62  extending in the screw  32  and an end portion of the rod extending in the screw  34 . The set screws  32   c  and  34   c  can be torqued as necessary to secure the rod  62  and the tubular member  64  to the screws  32  and  34 , respectively. 
         [0031]    A stem  66  extends through a bore formed through the rod  62  and is secured in the bore in any conventional manner. One end of the stem  66  extends flush with the corresponding end of the rod  62 , and a portion of the stem  66  projects from the latter rod. A bore is formed in the corresponding end of the rod  64  into which the other end portion of the stem extends. 
         [0032]    An annular flange  62   a  projects radially outwardly from the other end of the rod  62 , and an annular flange  64   b  projects radially outwardly from the other end of the rod  64  and extends in a spaced relation to the flange  62   a.  A ring-shaped dampening member  70  extends around the stem  66  and between the flanges  62   a  and  64   b.  The dampening member  70  is fabricated from a material having appreciable and conjoint viscous and elastic properties. 
         [0033]    Two substantially semi-circular plates  72  and  74  are provided with interlocking ring portions  72   a  and  74   a,  that are interlocked in the notch  64   a  and are connected to the corresponding end portion of the stem  66  in any conventional manner. A ring-shaped dampening member  76  extends around the corresponding portion of the rod  64  and in the space between the flange  64   b  and the interlocked plates  72  and  74 . The dampening member  76  is fabricated from a material having appreciable and conjoint viscous and elastic properties. 
         [0034]    The mechanism  60  is shown in  FIG. 8  in its unloaded state, i.e., when there is no appreciable tensile or compression loads on the vertebrae V 4  and/or V 5 . However, when there is flexion or extension of the column  10  caused by corresponding movements of the patient, the mechanism  60  will respond to the resulting compressive and tensile loads on the vertebrae V 4  and V 5  as follows. 
         [0035]    Compressive loads on the vertebrae V 4  and V 5  causes relative movement of the screws  32  and  36  ( FIG. 4 ) towards each other. This causes relative movement of the rods  62  and  64 , and therefore the flanges  62   a  and  64   b,  towards each other and compresses the dampening member  70 , as shown in  FIG. 8A , to dampen the movement. After the compressive load and the above relative movement of the screws  32  and  36  towards each other cease, the dampening member  70  will tend to return to its original, non-compressed state and cause relative movement of the flanges  62   a  and  64   b,  and therefore the rods  62  and  64 , away from each other so that the system  30  returns to the unloaded position of  FIG. 8 . 
         [0036]    Relative movement of the screws  32  and  36  away from each other in response to tensile loads on the vertebrae V 4  and V 5  causes relative movement of the rods  62  and  64 , away from each other. This causes movement of the stem  66 , and therefore the interlocked plates  72  and  74 , relative to the flange  64   b  in a direction towards each other, thus compressing the dampening member  76  to dampen the movements, as shown in  FIG. 8B . After the tensile load and the above relative movement of the screws  32  and  36  away from each other cease, the dampening member  76  will tend to return to its original, non-compressed state and cause relative movement of the stem  66  and therefore the interlocked plates  72  and  74  away from the flange  64   b,  so the system  30  takes the unloaded position of  FIG. 8 . 
         [0037]    In both of the above embodiments it is understood that as the dampening members  46 ,  50 ,  70  and  76  compress in response to the loads on the vertebrae V 4  and V 5  discussed above, the resistance of the dampening members to the loads will increase with increases in the loads. 
       Variations 
       [0038]    It is understood that variations may be made in the foregoing without departing for the invention and examples of some variations are as follows: 
         [0039]    (1) The systems in each of the above embodiments can be connected to anatomical structures other than vertebrae. 
         [0040]    (2) Fixating devices other than the screws described above can be used to connect the dampening mechanisms to the anatomical structures. 
         [0041]    (3) The dampening mechanisms in each of the previous embodiments can be rigidly connected at different locations of the vertebrae. 
         [0042]    (4) Extra fixation devices, or screws, can be attached to two adjacent vertebrae as shown in the above examples, or to a third vertebrae adjacent to one of the two vertebrae. In each case the rods and/or tubular members described above would be long enough to extend to the extra screws. 
         [0043]    (5) In the event that one or more extra fixation devices, or screws, are attached to the vertebrae, an extra dampening mechanism can be attached between the extra fixation device and its adjacent screw. 
         [0044]    (6) The dampening members disclosed above can be fabricated from materials other than those described above and many include a combination of soft and rigid materials other than those described above and may include a combination of soft and rigid materials. 
         [0045]    (7) The dampening properties of the dampening member  46  and  50  can be varied in manners other than providing them with different axial lengths, such as fabricating them from different materials, etc. 
         [0046]    (8) One or more of the components disclosed above may have through-holes formed therein ti improve integration of the bone growth. 
         [0047]    (9) The components of one or more of the above embodiments may vary in shape, size, composition, and physical properties. 
         [0048]    (10) Through-openings can be provided through one or more components of each of the above embodiments to receive tethers for attaching the devices to a vertebra. 
         [0049]    (11) The systems of each of the above embodiments can be placed between two vertebrae in the vertebral column other than the ones described above. 
         [0050]    (12) The systems of the above embodiments can be inserted between two vertebrae following a discectemy in which a disc between the a adjacent vertebrae is removed, or corpectomy in which at least one vertebrae is removed. 
         [0051]    (13) The spatial references made above, such as “under”, “over”, “between”, “flexible, soft”, “lower”, “top”, “bottom”, “axial”, “transverse”, etc., are for the purpose of illustration only and do not limit the specific orientation or location of the surface described above. 
         [0052]    The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, therefore, that other expedients known to those skilled in the art or disclosed herein, may be employed without departing from the invention or the scope of the appended claims, as detailed above. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts a nail and a screw are equivalent structures.