Abstract:
A dynamic rod assembly, such as that used for spinal stabilization, made of a number of interlocking segments whereby a limited amount of relative motion is permitted between each pair of adjacent segments. The dynamic rod assembly may also incorporate a separate central element that extends at least partially through a central channel within the interlocking segments to prevent the interlocking segments from disengaging while adding to the desired bending properties of the dynamic rod assembly.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to prostheses for treating spinal pathologies, and more specifically to dynamic stabilization rods for use with spinal fixation assemblies. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various methods of spinal immobilization have been used in the treatment of spinal instability and displacement. The most common treatment for spinal stabilization is immobilization of the joint by surgical fusion, or arthrodesis. This has been known for almost a century. Early in the century, post operative external immobilization, such as through the use of splints and casts, was the favored method of spinal fixation. As surgical techniques became more sophisticated, various new methods of internal and external fixation were developed. 
         [0003]    Internal fixation refers to therapeutic methods of stabilization that are wholly internal to the patient and include commonly known devices such as bone plates, screws, rods and pins. External fixation, in contrast, involves at least some portion of the stabilization device being located external to the patients&#39; body. As surgical technologies and procedures became more advanced and the likelihood of infection decreased, internal fixation eventually became the favored method of immobilization since it is less restrictive on the patient. 
         [0004]    Internal fixation of the spine may be used to treat a variety of disorders including degenerative spondylolisthesis, fracture, dislocation, scoliosis, kyphosis, spinal tumor, and failed previous fusion (pseudarthrosis). One of the main challenges associated with internal spinal fixation is securing the fixation device to the spine without damaging the spinal cord. The pedicles of a vertebra are commonly used for fixation as they generally offer an area that is strong enough to hold the fixation device in place even when the patient suffers from degenerative instability such as osteoporosis. 
         [0005]    Current fixation devices and hardware systems generally include a fixation device, such as a screw, a rod, and a body for fixing the position of the rod with respect to the screw, which in turn fixes the rod with respect to the spine. However, because traditional metal rods are far less compliant than bone, these rigid rods can cause significantly more stress on the neighboring levels of the spine and can contribute to premature degeneration of nearby levels. The present invention provides a novel dynamic rod assembly that allows the affected spinal levels to be stabilized by limiting excessive motion while allowing a degree of mobility without transmitting excessive forces. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    Disclosed is a rod for use with spinal fixation assemblies. The rod comprising a number of interlocking metal segments; at least one lateral channel extending there through; and at least one central element substantially filling at least one lateral channel extending through the interlocking metal segments. 
         [0007]    Also disclosed is a dynamic rod assembly for use with spinal fixation assemblies that comprising a number of interlocking metal segments; at least one lateral channel extending there through; and at least one central element substantially filling at least one lateral channel extending through the interlocking metal segments such that the interlocking metal segments together form a dynamic rod assembly at least a portion of which is generally cylindrical. 
         [0008]    The features of the present invention will be apparent with reference to the following description and attached drawings. In the description and drawings, particular embodiments of the invention have been disclosed in detail as being indicative of some of the ways in which the principles of the invention may be employed, but it is understood that the invention is not limited correspondingly in scope. Rather, the invention includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto. 
         [0009]    Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1A  is a perspective view of a dynamic rod assembly; 
           [0011]      FIG. 1B  is a front view of the dynamic rod assembly of  FIG. 1A ; 
           [0012]      FIG. 2A  is a perspective view of a single interlocking middle segment of the dynamic rod assembly of  FIG. 1A ; 
           [0013]      FIG. 2B  is a perspective view of an interlocking female end segment of the dynamic rod assembly of  FIG. 1A ; 
           [0014]      FIG. 2C  is a perspective view of an interlocking male end segment of the dynamic rod assembly of  FIG. 1A ; 
           [0015]      FIG. 2D  is a perspective view of a central element of the dynamic rod assembly of  FIG. 1A ; 
           [0016]      FIG. 2E  is a perspective view of a threaded element of the dynamic rod assembly of  FIG. 1A ; 
           [0017]      FIG. 3  is an exploded perspective view of the dynamic rod assembly of  FIG. 1A ; 
           [0018]      FIG. 4A  is a front view of a partial section of a dynamic rod assembly in its free state; 
           [0019]      FIG. 4B  is a front view of the partial section of a dynamic rod assembly of  FIG. 4A  with an applied load; 
           [0020]      FIG. 4C  is a side view of the partial section of a dynamic rod assembly of  FIG. 4B  with an applied load; 
           [0021]      FIG. 5  is a front view of a partial section of a dynamic rod assembly showing different segment lengths; and 
           [0022]      FIGS. 6A  &amp; B are front views of a partial section of a dynamic rod assembly showing alternate interlocking geometries. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The invention relates to novel spinal dynamic rod assemblies for use with spinal fixation assemblies. The dynamic rod assemblies preferably permit the affected spinal levels to be stabilized by limiting excessive motion while allowing a degree of mobility without transmitting excessive forces. This may be accomplished through a variety of designs, each of which includes a dynamic rod assembly made of a number of interlocking segments whereby a limited amount of relative motion is permitted between each pair of adjacent segments. 
         [0024]    Turning initially to  FIGS. 1A-B , perspective and front views of an exemplary dynamic rod assembly are illustrated. The rod  100  is made up of a number of interlocking components starting with a female end segment  102 , followed by a number of middle segments  101  and lastly by a male end segment  103 . A threaded element  104   a  is engaged within the male end segment  103  and a second threaded element  104   b  is engaged within the female end segment  102 . The threaded elements  104   a  &amp;  104   b  secure a captive central element  105  within the dynamic rod assembly  100  to prevent the lateral separation of adjacent segments. 
         [0025]    The metal material  106  may include, but is not limited to, titanium, titanium alloys (e.g., titanium/aluminum/vanadium (Ti/Al/V) alloys), cobalt-chromium alloys, stainless steel, and combinations thereof, which may include mechanically compatible mixtures of the above materials, or other similar metal material(s). In the presently preferred embodiment, the metal material  106  is a Ti/AlN alloy, such as Ti/6Al/4V ELI. 
         [0026]    Turning next to  FIGS. 2A-E  each component of the dynamic rod assembly is shown in perspective view. In  FIG. 2A  the middle segment  200  has one female interlocking end  201  and one male interlocking end  204  with can engage the female interlocking end of a second similar segment. Although the segment shown  200  shows one male interlocking end  204  and one female interlocking end  201 , those skilled in the art can appreciate that a single segment with two male interlocking ends or two female interlocking ends would function in a similar fashion. 
         [0027]    In the preferred embodiment, the male interlock end  204  is designed to pivot around its central axis  207  when it is engaged within a female interlocking end  201 . The range of motion of the pivoting is limited by coincidence of the surfaces on the male end  206   a  &amp;  206   b  and the surfaces on the female end  202   a  &amp;  202   b.    
         [0028]    The middle segment  200  of  FIG. 2A  has a generally cylindrical internal surface  203  along the same axis as the generally cylindrical outer surface  205  to allow engagement with a central element (shown in  FIG. 2D ). 
         [0029]    Turning to  FIG. 2B , the female end segment  210  has similar interlocking features  211  and similar motion limiting features  212   a  &amp;  212   b  as the middle segment  200  of FIG  2 A. Moreover, the female end segment also has a generally cylindrical internal surface  213  along the same axis as the generally cylindrical outer surface  215  to allow engagement with a central element (shown in  FIG. 2D ). 
         [0030]    In this preferred embodiment, the female end segment  210  and the male end segment  220  each has a threaded internal surface ( 214  &amp;  224  respectively) along its axis ( 213  &amp;  223  respectively) in the end opposite that which has the interlocking feature. This threaded internal surfaces  214  &amp;  224  allow for the engagement of a threaded element as shown in  FIG. 2E . The purpose of the threaded element is to hold the central element shown in  FIG. 2D  captive within the dynamic rod assembly  100 . However, those skilled in the art can appreciate that other means of holding the central element captive are possible such as a blind hole, welding, snap fit, or other means not herein defined. 
         [0031]    Turning to  FIG. 2C , the male end segment  220  has similar interlocking features  221  and similar motion limiting features  222   a  &amp;  222   b  as the middle segment  200  of  FIG. 2A . Moreover, the male end segment also has a generally cylindrical internal surface  223  along the same axis as the generally cylindrical outer surface  225  to allow engagement with a central element (shown in  FIG. 2D ). 
         [0032]    Turning to  FIG. 2D  the central element  230  has a generally cylindrical outer surface  231  to engage within the generally cylindrical internal surfaces  203 ,  213 , &amp;  223  of the middle segment  200 , the female end segment  210  and the male end segment  220  respectively. 
         [0033]    The overall length of the central element  230  should be such that when the dynamic rod  100  is fully assembled, the surfaces  232   a  &amp;  232   b  each are coincident with each surface  242  of the two thread elements  240  shown in  FIG. 2E  that are included within the dynamic rod assembly  100 . 
         [0034]    Turning to  FIG. 2E  the threaded element is configured in a way such that the threaded surface  241  can be engaged with the threaded internal surfaces  214  &amp;  224  of the female end segment  210  and the male end segment  220  respectively. Furthermore, the overall length of the threaded element  240  should be configured in conjunction with the overall length of the central element  230  such that when the dynamic rod  100  is assembled the surface  243  of one threaded element  240  is generally coincident with the surface  216  of the female end segment  210  and the surface  242  of the same threaded element  240  is generally coincident with the surface  232   a  of the central element  230 . Additionally, the surface  243  of a second threaded element  240  is generally coincident with the surface  226  of the male end segment  220  and the surface  242  of the same threaded element  240  is generally coincident with the surface  232   b  of the central element  230 . 
         [0035]    Turning now to  FIG. 3 , the exploded view of the dynamic rod assembly shows the assembly sequence of the preferred embodiment. Each assembly requires the following components: 1 female end segment  210 ; 1 male end segment  220 ; 1 central element  230 ; 2 threaded elements  240   a  &amp;  240   b;  and multiple middle segments  220 . The number of middle segments is determined by the overall length of the assembly  100  required for a specific spinal surgical procedure. 
         [0036]    To assembly the dynamic rod  100  the male end  204  of the first middle segment  200   a  is slid in laterally to the interlocking feature  211  of the female end segment  210 , following that the male end  204  of each subsequent middle segment  200   b  is slid in laterally to the female interlocking feature  201  of the previous middle segment  200 . This process continues for all the middle segments  200 . Then, the male interlocking feature  221  of the male end segment  220  is slid in laterally to the female end  201  of the final middle segment  200 . Following this, the central element  230  is inserted into the generally cylindrical internal surfaces  213 ,  203 , &amp;  223  of the female end segment  210 , the middle segments  200  and the male end segment  220  respectively. Finally, the two threaded elements  240   a  &amp;  240   b  are engaged with the internal threaded surfaces  214  &amp;  224  of the female end segment  210  and the male end segment  220  respectively. 
         [0037]    Turning now to  FIGS. 4A-C  the dynamic bending characteristics of the rod assembly  100  are illustrated. These figures are meant to show the bending and motion between two adjacent segments. The overall bending properties of the dynamic rod assembly  100  will be an accumulation of the bending properties between all adjacent segments within a complete assembly. 
         [0038]      FIG. 4A  shows a representative sample of middle segments  401   a,    401   b,  &amp;  401   c  in their free state without an applied load. Each male interlocking feature is concentric to its respective female interlocking feature (see  404   a  &amp;  404   b ). In this state there is a gap at the top of each joint  402   a  &amp;  402   b  and at the bottom of each joint  403   a  &amp;  403   b.  When the gaps  402  &amp;  403  are present, the bending properties of the rod result from the bending properties of the central element. 
         [0039]      FIG. 4B  shows the same representative sample of middle segments  411   a,    411   b,  &amp;  411   c  but with an applied load. The upward force  415  is applied to the center of segment  411   b  and is perfectly balanced with the downward force  416   a  &amp;  416   b  applied to the ends of segments  411   c  &amp;  411   a  respectively. The balanced forces result in an equilibrium and a steady state condition. The relative motion between the adjacent segments is limited by the elimination of the gap on the bottom side of the assembly  413   a  &amp;  413   b.  A corresponding increase in the gap on the top side of the assembly  412   a  &amp;  412   b  also occurs. 
         [0040]    Once a sufficient load is applied that the gaps  413   a  &amp;  413   b  are closed then the bending properties of the dynamic rod assembly are determined by both the interlocking segment and the central element. It is in this way that the dynamic rod assembly exhibits non-linear bending characteristics. Thus the dynamic rod assembly allows limited initial range of motion but greatly restricts excessive range of motion of the spine. 
         [0041]    Furthermore,  FIG. 4C  shows the same representative sample of middle segments  411   a,    411   b,  &amp;  411   c  with an applied load but from a side view. It can be seen that due to the geometry of the interlocking features, there is no relative motion between adjacent segments in this plane. It is this feature that results in different bending properties from side to side than from front to back. 
         [0042]    Turning now  FIG. 5 , the non-linear bending properties can be further enhanced by varying the length of the middle segments  501 ,  502 , &amp;  503  when assembling the rod. For a given length dynamic rod assembly, the sum of the gap distances  412   a  &amp;  412   b  determines the initial bending properties of the rod. If the dynamic rod assembly is made up of fewer longer middle segments such as  503  the sum of the gap distances will be less than a dynamic rod assembly made up of shorter middle segments such as  501 . 
         [0043]    Longer middle segments, such as  503 , will result in an increased resistance to bending and a decreased initial range of motion, while shorter middle segments, such as  501 , will have the opposite effect, decreased resistance to bending and an increased initial range of motion. One embodiment could include middle segments of various sizes within the same rod to provide bending properties specific to a surgical need. 
         [0044]      FIG. 6   a  shows an alternative embodiment of the invention, in which the geometry of the interlocking mechanism has a more triangular shape for both the male and female ends  602  and  603  than circular (see  FIGS. 404   a  &amp;  404   b ). With alternative geometries, gap distances  604   a  &amp;  604   b  are still present to provide non-linear bending characteristics as in the preferred embodiment. As in the preferred embodiment, the middle segments  601   a,    601   b  and  601   c  can vary in length to provide various bending properties. 
         [0045]    A second alternative embodiment is shown in  FIG. 6   b . In this figure, the gaps  605   a  &amp;  605   b  are greater than the gaps  606   a  &amp;  606   b,  resulting in less resistance to bending in direction  607  as compared to direction  608 . As in other described embodiments, these non-symmetrical segments shown in  FIG. 6   b  can vary in length within the same dynamic rod assembly and can be combined with segments such as those shown in  FIG. 5  and  FIG. 4   a  to provide bending properties specific to a surgical need. 
         [0046]    While the present invention has been described in association with exemplary embodiments, the described embodiments are to be considered in all respects as illustrative and not restrictive. Such other features, aspects, variations, modifications, and substitution of equivalents may be made without departing from the spirit and scope of this invention, which is intended to be limited only by the scope of the following claims. Also, it will be appreciated that features and parts illustrated in one embodiment may be used, or may be applicable, in the same or in a similar way in other embodiments. 
         [0047]    Although the invention has been shown and described with respect to certain embodiments, it is obvious that certain equivalents and modifications may be apparent to those skilled in the art upon the reading and understanding of the specification. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.