Patent Publication Number: US-2009240284-A1

Title: Stabilization rods

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to prostheses for treating spinal pathologies, and more specifically to stabilization rods for use with spinal fixation assemblies having an anchor for holding a fixation device and a stabilization rod. 
     BACKGROUND OF THE INVENTION 
     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. In many cases, however, pseudoarthrosis occurs, particularly in cases involving fusion across the lumbosacral articulation and when more than two vertebrae are fused together. 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. 
     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. 
     Internal fixation of the spine may be used to treat a variety of disorders including kyphosis, spondylolisthesis and rotation, segmental instability, such as disc degeneration and/or fracture caused by disease, trauma, congenital defects and tumor diseases. 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. 
     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. The present invention provides a novel rod and fixation device. 
     BRIEF SUMMARY OF THE INVENTION 
     Disclosed is a rod for use with spinal fixation assemblies. The rod comprises a metal infrastructure comprising: a top portion, a bottom portion, and at least one lateral channel extending therethrough. The rod also comprises an elastomeric material at least partially encapsulating the metal infrastructure and substantially filling at least one lateral channel extending through the metal infrastructure. 
     Also disclosed is a rod for use with spinal fixation assemblies that comprises a metal infrastructure having a top portion and a bottom portion. The rod also comprises an elastomeric material partially circumferentially encapsulating the metal infrastructure such that the metal infrastructure and elastomeric material together form a rod at least a portion of which is generally cylindrical wherein part of the generally cylindrical portion of the rod has a surface formed by the elastomeric material and part of the generally cylindrical portion has a surface that is formed by the bottom portion of the metal infrastructure that is not encapsulated by the elastomeric material. 
     Also disclosed is a rod for use with spinal fixation assemblies that comprises a non-cylindrical metal infrastructure comprising a top surface, a bottom surface and an end. The rod also comprises an elastomeric material at least partially encapsulating the metal infrastructure to form a rod that has having a generally cylindrical portion. In addition, the center of gravity along at least a portion of the length of the metal infrastructure is not equidistant from the top surface and the bottom surface of the metal infrastructure. 
     Further disclosed is a spinal fixation assembly comprising a rod and a locking mechanism. The rod has a top portion and a bottom portion and comprises metal material and elastomeric material. The elastomeric material partially surrounds the metal material such that at least part of the surface of the bottom portion of the rod is formed by the metal material. The locking mechanism is configured to receive the rod and engage part of the surface of the bottom portion of the rod that is formed by the metal material. 
     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. 
     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 
         FIGS. 1A-B  are top perspective views of a rod and metal infrastructure; 
         FIGS. 1C-D  are bottom perspective views of the rod and metal infrastructure of  FIGS. 1A-B ; 
         FIGS. 1E-F  are side views of the rod and metal infrastructure of  FIGS. 1A-B ; 
         FIGS. 1G-H  are views of a cross section of part of the rod and metal infrastructure of  FIGS. 1A-B ; 
         FIG. 1I  is a front side view of the rod of  FIG. 1A  showing illustrating the curvature of the rod; 
         FIG. 2  is a perspective view of the rod of  FIGS. 1A-I  and two locking mechanisms; 
         FIG. 3  is a cross-sectional view of the rod of  FIGS. 1A-I  and a locking mechanism; 
         FIG. 4  is a perspective view of an alternate metal infrastructure having an I-beam shaped cross section; 
         FIG. 5  is a perspective view of an alternate metal infrastructure having a star shaped cross section; 
         FIG. 6  is a perspective view of alternate metal infrastructure having a generally circular cross section; 
         FIG. 7  is a view of an exemplary testing procedure for measuring the dynamic properties of a rod; and 
         FIG. 8  is a chart showing finite element analysis testing results of various types of rods. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to novel stabilization rods for use with locking mechanisms for spine stabilization. The stabilization rods preferably permit a metal to metal contact surface between the rod and locking mechanism while exhibiting bending properties that are different than a metal rod of the same size. This may be accomplished through a variety of designs, each of which includes a rod made of both metal and elastomeric material. 
     Turning initially to  FIGS. 1A-F , perspective and side views of an exemplary stabilization rod are illustrated. The rod  100  has a top portion  102  and a bottom portion  104 , as well as sides  106   a  and  106   b.  The rod  100  includes metal material  108  and elastomeric material  110 . The metal material  108  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  108  is a Ti/Al/V alloy, such as Ti/6Al/4V ELI. The elastomeric material  110  is preferably one or more high strength polymers, such as Poly Ether Ether Keytone (PEEK), Poly Ether Ketone Ketone (PEKK), Poly Ether Ketone Ketone Ether Keyton (PEKKEK), Self-Reinforced Polyplenylene (SRP), Polyphenylsulfone (PPSU), Polysulfone (PSU), Ultra-High Molecular Weight Polyethylene (UHMWPE), or combinations thereof, or other similar material(s). In the presently preferred embodiment, the elastomeric material is PEKK. 
     As shown in  FIG. 1A , part of the top portion  102  may comprise metal material  108 , and other parts of the top portion  102  may comprise elastomeric material  110 . Similarly, as shown in  FIG. 1C , part of the bottom portion  104  may comprise metal material  108 , and other parts of the bottom portion  104  may comprise elastomeric material  110 . Moreover, the rod  100  may be formed such that the elastomeric material  110  partially encapsulates the metal material  108 , which may be accomplished by injection molding as will be understood by one of ordinary skill in the art. In addition, the top portion  102  and bottom portion  104  may each have zones of metal material  108  or elastomeric material  110 . For example,  FIG. 1C  illustrates multiple metal material  108  zones in the bottom portion  104 . Moreover, it may be preferable to configure the rod  100  such that the metal material  108  portions are located along the rod  100  to facilitate metal on metal interaction with a locking mechanism, such as the locking mechanism  144  of  FIGS. 2 and 3 . 
     The metal material  108  may be in the form of a metal infrastructure  108 , such as that illustrated in more detail in  FIGS. 1B ,  1 D,  1 F and  1 H, or metal infrastructures  408 ,  508  or  608  as illustrated in  FIGS. 4 ,  5  and  6 , respectively. As used herein, the term “infrastructure” is not limited to an internal structure and is meant to also include the infrastructure  608  of  FIG. 6 , even though the infrastructure  608  forms the exterior of the rod  600 . 
     Turning first to  FIGS. 1B ,  1 D,  1 F and  1 H, the metal infrastructure  108  includes a top portion  120  and a bottom portion  122  and has at least one lateral channel extending  116   a - d  therethrough. The metal infrastructure  108  is generally u-shaped. More specifically, a cross section of at least part of the metal infrastructure  108  resembles an upside down letter “U”. The sides of the “U” may be flat or have curvature, such as the generally concave side surfaces of the metal infrastructure  108 . In addition, the bottom portion  122  may have an axial channel  118  extending toward the center of the metal infrastructure and running along at least part of the length of the bottom portion  122 . The axial channel  118  may intersect with one or more of the lateral channels  116   a - d.    
     Thus, as shown in  FIGS. 1A-1I , the rod  100  includes both the metal material  108  in the form of a U-shaped metal infrastructure  108  and elastomeric material  110 . The elastomeric material  110  at least partially fills at least one of the lateral channels  116   a - d  of the metal infrastructure  108 . In addition, the elastomeric material may substantially fill one or all of the lateral channels  116   a - d.    
     At least one of the lateral channels  116   a - d  may be positioned such that it does not extend to the top portion  120  or bottom portion  122  of the metal infrastructure  108 . In other words, the lateral channels  116   a - d  may entirely reside within the metal infrastructure  108 . As shown, all of the lateral channels  116   a - d  are positioned such that they do not extend to the top portion  120  or bottom portion  122  of the metal infrastructure  108 . In addition, the lateral channels  116   a - d  may have varying dimensions. For example, the lateral channel  116   d  may be larger than the lateral channel  116   a.  Varying the size and location of the lateral channels  116   a - d  may change the bending properties of the metal infrastructure  108 , and thus, the rod  100 . 
     Turning next to  FIGS. 1G-H , a cross section of part of the rod  100  ( FIG. 1G ) and a cross section of part of the metal infrastructure  108  ( FIG. 1H ). In addition metal infrastructure  108  and elastomeric material  110 ,  FIG. 1G  illustrates the geometric center  130  as well as the center of gravity  132  of the cross section. While the center of gravity  132  may be located at the geometric center  130 , it is also possible for the center of gravity  132  and the geometric center  130  to not share the same location. For example, the geometric center  130  and the center of gravity  132  may separated by distance D 1 . In the example illustrated in  FIG. 1G , D 1  is approximately 0.2 mm. 
     Similarly,  FIG. 1H  illustrates the geometric center  134  and the center of gravity  136  of a cross section of the metal infrastructure  108 . While the center of gravity  132  may be located at the geometric center  134 , it is also possible for the center of gravity to be located such that it is not equidistant from the top portion  120  and the bottom portion  122  of the metal infrastructure  108 . Specifically, D 2  is the distance between the geometric center  134  and the center of gravity  136 . D 2  may be, for example, approximately 0.5 mm. In addition, the center of gravity  136  may be in the axial channel  118 . 
     Turning next to  FIG. 1I , a side view of the rod  100  is illustrated. As will be understood by those skilled in the art, the rod  100  may be any size appropriate for the intended use of the rod  100 . For example, when used for spinal stabilization, the rod  100  may have a diameter ranging from about 3 mm to about 7 mm. In addition, the rod may have a length L ranging from about 40 mm to about 180 mm. As can be seen, the top portion  102  of the rod  100  has a generally concave shape and the bottom portion  104  of the rod  100  has a generally convex shape due to the slight curvature of the rod  100 . For example, the rod may be generally curved such that it forms an arc of approximately 10 degrees to approximately 30 degrees, as represented by angle A 1 . For the rod  100  illustrated in  FIG. 1I , angle A 1  is 20 degrees. It will be understood by those of skill in the art, however, that the rod may be straight, and that the arc illustrated in  FIG. 1I  may be any generally curved shape and is not limited to a circular arc. 
     Turning next to  FIG. 2 , the rod  100  of  FIGS. 1A-I  is illustrated with two locking mechanisms  140   a  and  140   b.  The locking mechanisms  140   a  and  140   b  of  FIG. 2  may be and locking mechanism, such as any of the locking mechanisms described in U.S. patent application Ser. No. 11/816,802, the entirety of which is incorporated herein by reference. Moreover, for proper anatomical reference, “above” or “top” means posterior with respect to the patient and “below” or “bottom” means anterior with respect to the patient when the system of  FIG. 2  is used for spinal fixation. Thus, the bottom portion  104  of the rod  100  is anterior with respect to the patient and the top portion  102  of the rod  100  is posterior. Thus, the rod  100  is received by the locking mechanisms  140   a  and  140   b  as the rod  100  is moved in a posterior to anterior direction. 
     Each locking mechanism  140   x  includes a body  142   x  a fixation device  150   x  and a locking element  112 . The rod  100  is received by the body  142   x  and locked into position with respect to the fixation device  150   x.  The locking element  112  prevents the rod  100  from moving upward in the locking mechanism  140   x.    
     Turning next to  FIG. 3 , a cross section of a rod  100  and an exemplary locking mechanism  140  is illustrated in greater detail. It will be understood by those skilled in the art that the rods of the present invention may be used with any of the variety of types of locking mechanisms known in the art. The exemplary locking mechanism  140  includes a body  142  that is configured to receive a fixation device  150 , which may be, for example, a screw. The particular locking mechanism  140  illustrated also includes an insert  148  that engages the head of the fixation device  150 . The insert  148  may be compressible to enable insertion into the body  140 . Once inserted into the body  140 , however, the insert  108  may expand to have a greater width. 
     When the fixation device  150  is inserted into body  140 , the head of the fixation device  150  preferably engages the insert  108  in a snap-fit manner such that the insert  150  expands to accommodate the head. When the insert  148  and fixation device  150  combination is forced toward the bottom portion of the body  140 , the body  140  engages the sides of the insert  148 , causing the insert  148  to more tightly engage the head of the fixation device  150  and preventing the insert  148  and the fixation device  150  from exiting the body  142 . 
     The body  142  also includes a side portion that is configured to receive the rod  100 , such as by way of a channel that enables placement of the rod  100  by either sliding the rod  100  through the side portion of the body  142  or by inserting the rod  100  into the channel through the top portion of the body  142 . The body  142  is also configured to receive a rod seat  146 , for example, through a hole in the bottom or top of the body  142 . The rod seat  146  is preferably inserted into the body  142  prior to insertion of the rod  100  such that the rod seat  146  is eventually positioned between the rod  100  and the insert  148 . 
     The rod seat  146  may have a tapered portion for receiving the rod  100 . The tapered portion of the rod seat  146  may be configured to engage rods of varying diameters. For example, the tapered portion of the rod seat  146  may have multiple curvatures on each side of the rod seat  146  that provide varying surfaces for contacting rods of varying diameters. As can be seen in  FIG. 3 , the bottom portion  104  of the rod  100  contacts the rod seat  146 . Preferably, the metal material, which may be in the form of a metal infrastructure  108 , contacts the rod seat  146 , which is also preferably metal. The metal-on-metal interface may be more stable than a metal-on-elastomeric material interface. As shown, the bottom portion  122  of the metal infrastructure  108  engages the taper of the rod seat  146 . It will be understood by those of skill in the art that the contact surface between the rod  100  and the locking mechanism  140  may be achieved by other means and may be flat as opposed to tapered. Accordingly, it may be desirable in such circumstances to use a rod having a metal material  108  positioned differently along the bottom portion  104  of the rod in order to provide metal-on-metal contact with a flat surface. 
     The locking mechanism  140  may also include a locking element  144  that is configured to engage the body  142  and the rod  100  so as to force the rod  100  toward the fixation device  150 . Like the interface between the rod  100  and the rod seat  146 , the interface between the rod  100  and the locking element  144  may be metal-on-metal. For example, the top portion  102  of the rod  100  may be formed by the top portion  120  of the metal infrastructure  108 . 
     Turning next to  FIG. 4 , an I-beam shaped metal infrastructure  408  is illustrated. The metal infrastructure  408  includes a top portion  420 , bottom portion  422  and sides  414   a  and  414   b,  and at least one lateral channel extending  416   a - d  therethrough. At least one of the lateral channels  416   a - e  may be positioned such that it does not extend to the top portion  420  or bottom portion  422  of the metal infrastructure  408 . In other words, the lateral channels  416   a - e  may entirely reside within the metal infrastructure  408 . As shown, all of the lateral channels  416   a - d  are positioned such that they do not extend to the top portion  420  or bottom portion  422  of the metal infrastructure  408 . In addition, the lateral channels  416   a - e  may have varying dimensions. For example, the lateral channel  416   d  may be larger than the lateral channel  416   a.  Varying the size and location of the lateral channels  416   a - e  may change the bending properties of the metal infrastructure  408 , and thus, the rod. The metal infrastructure  408  is preferably similar in dimension and shape to the metal infrastructure  108 . As such, the top portion  420  of the metal infrastructure  408  may have a generally concave shape and the bottom portion  422  of the metal infrastructure  408  may have a generally convex shape. 
     Turning next to  FIG. 5 , a star-shaped metal infrastructure  508  is illustrated. The metal infrastructure  508  is similar in design and shape to the metal infrastructures  108  and  408  and includes a top portion  520 , bottom portion  522  and sides  514   a  and  514   b,  and at least one lateral channel extending  516   a - e  therethrough. 
     Turning next to  FIG. 6 , a generally cylindrical metal infrastructure  608  is illustrated. The metal infrastructure  608  is similar in design and shape to the metal infrastructures  108 ,  408  and  508  and includes a top portion  620 , bottom portion  622  and sides  614   a  and  614   b,  and at least one lateral channel extending  616   a - e  therethrough. In addition, the metal infrastructure  608  includes an internal axial channel  618 , which may intersect with one or more of the lateral channels  616   a - e.    
     At least one of the lateral channels  416   a - d  may be positioned such that it does not extend to the top portion  420  or bottom portion  422  of the metal infrastructure  408 . In other words, the lateral channels  416   a - d  may entirely reside within the metal infrastructure  408 . As shown, all of the lateral channels  416   a - d  are positioned such that they do not extend to the top portion  420  or bottom portion  422  of the metal infrastructure  408 . In addition, the lateral channels  416   a - d  may have varying dimensions. For example, the lateral channel  116   d  may be larger than the lateral channel  416   a.  Varying the size and location of the lateral channels  416   a - d  may change the bending properties of the metal infrastructure  408 , and thus, the rod. The metal infrastructure  408  is preferably similar in dimension and shape to the metal infrastructure  108 . As such, the top portion  420  of the metal infrastructure  408  may have a generally concave shape and the bottom portion  422  of the metal infrastructure  408  may have a generally convex shape. 
     Turning next to  FIGS. 7 and 8 , the bending properties of an exemplary rod according to the present invention are illustrated.  FIG. 7  illustrates a testing procedure for determining bending properties of a rod. Rod  100  is shown to illustrate the testing procedure. As shown, the rod  100  is placed upside down on a central support  702  such that top portion  102  of the rod  100  rests on the support  702 . The rod  100  has a known length, which is approximately 4 inches, and a known diameter, which is 6 mm, for the analysis illustrated in  FIG. 8 . A downward force F is then applied to bottom portion  104  of the rod  100  near the sides  106   a  and  106   b  and the angular displacement of the rod  100  is measured. 
     Preferably, the rod  100  is less resistant to bending when is applied as illustrated in  FIG. 7  than it is when the rod  100  is flipped over and a downward force is applied to top portion  102  of the rod  100  near the sides  106   a  and  106   b.  In other words, the same force F causes less angular displacement when applied to the top portion  102  than when applied to the bottom portion  104 . In addition, the rod  100  may have a different bending moment from side to side than from top to bottom. The rod  100  may also have a different bending moment from side to side than from bottom to top. 
     As can be seen in  FIG. 8 , the rod  100  shows more flexibility than the titanium and carbon filled PEEK rods tested. Indeed, the rod  100  preferably exhibits bending properties, namely angular displacement resulting from an applied load, that are similar to that of a 6 mm rod formed from material having an elastic modulus of greater than or equal to about 10 GPa and less than or equal to about 70 GPa. For example, the rod  100  may exhibit bending properties that are similar to that of a 6 mm rod formed from material having an elastic modulus of greater than or equal to about 15 GPa and less than or equal to about 40 GPa. In contrast, a solid 6 mm and a solid 5.5 mm Ti/6Al/4V rod having the same general shape as the rod  100  are much less flexible due to stiffness of Ti/6Al/4V, which has an elastic modulus of approximately 105 GPa. 
     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. 
     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.