Patent Publication Number: US-2010114167-A1

Title: Transition rod

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to systems and methods for spinal treatment using spinal rods. 
     Elongated rigid plates and rods can aid in the stabilization and fixation of a spinal motion segment, in correcting abnormal curvatures and alignments of the spinal column, and for treatment of other conditions. While external rod systems have been employed along the vertebrae, the geometric and dimensional features of these rod systems and patient anatomy constrain the surgeon during surgery and can prevent optimal placement and attachment along the spinal column. For example, elongated, one-piece rods can be difficult to maneuver into position along the spinal column, and also provide the surgeon with only limited options in sizing and selection of the rod system to be placed during surgery. 
     One known system disclosed in U.S. Publication No. 2005/0277926 to Farris (incorporated herein by reference) includes two rod portions connected together with threaded ends on the rod to provide a modular solution for surgeons. While suitable for many applications, the system can be improved upon to aid in revision surgery and to secure the devices when rods are formed of alternative materials, among other things. 
     Spinal rod revision surgeries to treat vertebrae adjacent those previously stabilized typically require removal and replacement of the old spinal rod with a new, longer spinal rod. The new rod is typically aligned or bent to provide the same benefits as the old rod. Accordingly, revision surgery requires the same incisions as the original surgery and in some respects, is re-performing a new implantation. 
     Further, in order to vary rod stiffness, the rod material may be varied at certain spinal levels. Some materials are better suited for a non-threaded solution than others. 
     A need therefore exists for a spinal rod usable to elongate a previously implanted spinal rod without removing the implanted spinal rod and that may be suitable for securely attaching rods of various materials. The present disclosure overcomes one or more disadvantages of prior spinal rod systems. 
     SUMMARY OF THE INVENTION 
     In one exemplary aspect, the present disclosure is directed to a device for supporting vertebral components of a spinal column. The device includes a first spinal rod, a second spinal rod, and a connection element disposed between the first and second rods. The connection element has a front surface with a rod-receiving opening having a first region sized greater than a cross-section of the second rod, and has a second region sized smaller than the cross-section of the second rod, wherein the first and second rods align in series in an end-to-end manner. A locking member is associated with the connection element that urges the second rod toward the second region in a manner that the second rod frictionally engages with the connection element to restrict removal of the second rod from the connection element. 
     In some examples, the first spinal rod and the connection element are integrally formed together. In some examples, the first spinal rod is more rigid than the second spinal rod, or the first spinal rod is a metal material and the second spinal rod is a polymeric material. In other examples, the first and second spinal rods have different diameters. 
     In some examples, the first and second regions form a generally tapering shape, and sides of the taper engage the second rod such that a gap is disposed between the second rod and rod-receiving bore wall directly opposite the locking member. 
     In another exemplary aspect, the present disclosure is directed to a connection element for receiving and securing a spinal rod. The connection element includes a main body having an upper facing outer surface, a lower facing lower surface, and a front surface. The main body also includes a rod-receiving bore extending inwardly from the front surface to a bore end. The rod-receiving bore has a first region sized greater than a cross-section of the spinal rod, and a second region sized smaller than a cross-section of the spinal rod, such that the first and second regions generally form a tapering shaped opening. The rod receiving bore has a substantially constant inner wall profile extending longitudinally inwardly from the front surface to the bore end. The main body of the connection element also includes a fastener receiving bore formed through the upper facing outer surface in a direction transverse to the direction of the rod-receiving bore. The fastener receiving bore intersects with the rod-receiving bore in an interior region of the connection element. The connection element also includes an integral rod extending rearwardly in a direction opposite the front surface. 
     In other examples, the connection element includes a rear surface opposite the front surface and includes a second rod-receiving bore extending inwardly from the rear surface in the direction of the front surface to a second bore end. In some examples, the rod-receiving bore is shaped as two overlapping circular bores with the upper circular bore being larger than the lower circular bore. 
     In yet another exemplary aspect, the present disclosure is directed to a method of implanting a spinal rod. The method includes introducing a first spinal rod in an axial direction to a rod receiving bore formed in a front side of a connection element in a manner that aligns the rod in series with a second spinal rod. The introducing is substantially translational without significant amounts of rotation about a first rod longitudinal axis. A locking element is driven from an adjacent side of the connection element into the rod-receiving bore in a direction transverse to the direction of the rod-receiving bore until the locking element engages the first spinal rod. The first spinal rod is taper-locked in the connection element by using the locking element to force the first spinal rod against at least two surfaces of the rod-receiving bore such that the first spinal rod is engaged between the locking element and the at least two surfaces. 
     In some examples, the method includes creating an incision and introducing the connection element into the incision, wherein the introducing, driving, and taper locking steps are performed in situ. Accordingly, in some examples, the spinal rod is a previously implanted spinal rod and the surgery is a revision surgery. 
     These and other advantages of the present invention will be apparent from the descriptions herein. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is an illustration of a perspective view of first and second assemblies comprising fixation rods attached to vertebral members according to one or more embodiments; 
         FIG. 2  is an illustration of a perspective view of a device for anchoring a spinal implant in an intervertebral disc space. 
         FIG. 3  is an illustration of an exploded configuration of the device of  FIG. 2 . 
         FIG. 4  is an illustration of an end view of the device of  FIG. 2 . 
         FIG. 5  is an illustration of the spinal rod of  FIG. 2 . 
         FIG. 6  is an illustration of a top view of the spinal rod of  FIG. 2 . 
         FIG. 7  is an illustration of an exploded cross-sectional side view of a portion of the spinal rod in  FIG. 2 . 
         FIG. 8  is an illustration of a cross-sectional side view of a portion of the spinal rod in  FIG. 2 . 
         FIGS. 9 and 10  are illustrations of alternative embodiments of the spinal rod shown in  FIG. 2 . 
         FIGS. 11 and 12  are illustrations of an alternative embodiment showing a connection element that couples to two spinal rods. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to preferred embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications of the invention, and such further applications of the principles of the invention as illustrated herein, being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     The present disclosure is directed to a spinal rod for stabilization of one or more vertebra of a spinal column. In some embodiments, the spinal rod includes a transition rod having a connection element connectable to at least one additional spinal rod. The connection element employs a taper-lock system that allows spinal rods of different materials and/or different sizes to be connected in series with one another. Accordingly, while treatment of one vertebral level may require a stiff or rigid rod, an adjacent level may be treated with a rod that is relatively less stiff or rigid. With this connection element, a surgeon may choose any suitable rod size with a suitable material and stiffness and connect it to the transition rod. Therefore, the transition rod addresses both modularity and the attachment of rods of any rigidity. Furthermore, the transition rod curtails adjacent level syndrome and grants surgeons better control over vertebral skipping due to rod design, regardless of surgeon preference. With the transition rod disclosed herein, a surgeon may more easily customize the rod to a patient in the manner desired. 
     The transition rod&#39;s design and structural arrangement also permit a surgeon to effectively elongate a previously implanted spinal rod in a revision surgery. The coupling element on the transition rod is designed to connect to an end of a previously implanted rod, thereby elongating the total rod in situ. The coupling element can receive a rod of any reasonable size because the connection is not dependent on structural features at the end of the rod. Therefore, a surgeon can connect the transition rod to any rod already implanted. 
     As a further advantage, the transition rod connection elements can connect with a rod irrespective of features at its ends, including rods having smooth outer surfaces. Accordingly, rods made of notch sensitive materials, such as some polymers, may be connected to the transition rod without introducing potentially weakening grooves, notches, threads or other similar characteristics. Accordingly, a broader range of material types are connectable to the transition rod than in conventional systems. 
       FIG. 1  shows a perspective view of first and second spinal rod assemblies  20  in which spinal rods  10  are attached to vertebral members V 1  and V 2 . In the example assembly  20  shown, the spinal rods  10  are positioned at a posterior side of the spine, on opposite sides of the spinous processes S. Spinal rods  10  may be attached to a spine at other locations, including lateral and anterior locations. The spinal rods  10  may also be attached at various sections of the spine, including the base of the skull and to vertebrae in the cervical, thoracic, lumbar, and sacral regions. Thus, the illustration in  FIG. 1  is provided merely as a representative example of one application of a spinal rod  10 . 
     In the exemplary assembly  20 , the spinal rods  10  are secured to vertebral members V 1 , V 2  by pedicle assemblies  12  comprising a pedicle screw  14  and a retaining cap  16 . The outer surface of the spinal rod  10  is grasped, clamped, or otherwise secured between the pedicle screw  14  and retaining cap  16 . In some embodiments, these are multi-axial pedicle screws. Other mechanisms for securing spinal rods  10  to vertebral members V 1 , V 2  include hooks, cables, and other such devices. Further, examples of other types of retaining hardware include threaded caps, screws, and pins. The spinal rods  10  are also attached to plates in other configurations. In some examples, interbody devices or implants, fusion or dynamic, may be disposed between the adjacent vertebrae. Thus, the exemplary assemblies  20  shown in  FIG. 1  are merely representative of one type of attachment mechanism. 
       FIG. 2  shows one example of the spinal rod assembly  20  in greater detail. The spinal rod  10  includes a first transition rod  100  and a second rod  102 . The transition rod  100  includes a rod portion  104  and a connection element  106  that receives the second rod  102  such that the rod portion  104  and the second rod  102  are releasably coupled to one another in a serial end-to-end fashion. This end-to-end rod coupling arrangement minimizes the footprint or intrusiveness of the coupling mechanism  106  into the tissue surrounding the spinal rod  10 , and maximizes the length of the rod portion  104  and the second rod  102  available for positioning and/or attachment along the spinal column. A locking member  108 , shown as a set screw, cooperates with the connection element  106  to secure the second rod  102  into the connection element  106 . 
       FIG. 3  shows an exploded view of the spinal rod  10 . Here, the transition rod  100  includes the rod portion  104  and the connection element  106 , with the rod portion  104  extending from the connection element  106 . Although not shown in  FIGS. 2  or  3 , the ends of the first rod portion  104  and the end of the second rod  102  may include another connection element  106  for attachment to additional rods, or may simply terminally end as shown in  FIG. 2 . Accordingly, in the embodiments illustrated herein, although only one connection element  106  is shown in each rod system  20 , one or more of the first and second rods may include an additional connection element or be received into an additional connection element so that three or more rods may comprise the rod system. 
     An advantage arising from the transition rod  100  disclosed herein is that the first rod portion  104  can be provided with a characteristic that differs from a characteristic of the second rod  102 . The connection element  106  allows rods of differing characteristics and rods having the same characteristics to be secured to one another in end-to-end fashion to provide a rod system that is particularly adapted for the anatomy, surgical condition, or surgical procedure. In one embodiment, the characteristic includes a cross-sectional dimension of the rod portions. Other embodiments contemplate selection criteria for selection and assembly of the rod portion to include any one or combination of characteristics, including length, contouring, flexibility, surface features, shape, section modulus, elasticity, materials and material properties, and coatings, for example. For example, in one embodiment a first rod provides a rigid support between a first set of anchors, while the second rod is flexible to provide dynamic stabilization between a second set of anchors. The second rod can be in the form of a tether, cable wire, spring, bumper, or other motion permitting construct. 
     As indicated above, the second rod  102  may be formed of a material different than the material of the transition rod  100 . Either rod may be formed of any of a variety of materials, preferably a biocompatible material. Some examples of materials that can be used include cobalt-chromium alloys, titanium alloys, nickel titanium alloys, and/or stainless steel alloys, any member of the polyaryletherketone (PAEK) family such as polyetheretherketone (PEEK), carbon-reinforced PEEK, or polyetherketoneketone (PEKK); polysulfone; polyetherimide; polyimide; ultra-high molecular weight polyethylene (UHMWPE); and/or cross-linked UHMWPE. Any combination of these materials may also be suitable. For example, a suitable material may include a layer of carbon fiber reinforced PEEK inside an otherwise uniform PEEK material. In one example, the transition rod  100  is formed of a cobalt-chromium material and the second rod  102  is formed of a PEEK material. In another example, both rods are formed of cobalt-chromium material. All suitable combinations are contemplated. 
     In some embodiments, the second rod  102  is a previously implanted spinal rod and the transition rod  100  is introduced to the previously implanted spinal rod to elongate the rod, or alternatively to replace a certain portion of the previously implanted rod. Accordingly, during a revision surgery, the transition rod  100  may be attached to the implanted rod to provide either additional length, support, or to provide different rod characteristics, such as rigidity. 
     Referring to  FIGS. 3-5 , the second rod  102  includes a first cross-sectional dimension  112  between opposite sides thereof and the rod portion  104  includes a second cross-sectional dimension  114  between opposite sides thereof. In some embodiment, these dimensions are similar, while in others, they are different values. In the illustrated embodiment, the cross-sectional dimension  112  corresponds to a diameter of a cylindrical rod portion  102  that is smaller than a diameter corresponding to cross-sectional dimension  114  of the cylindrical rod portion  104 . In one specific application, the diameter of second rod portion  102  is sized to extend along a first portion of the spine, such as the cervical region, and the diameter of first rod portion  104  is sized to extend along a second portion of the spine, such as the thoracic region. Other systems contemplate multiple rod portions coupled to one another in end-to-end fashion with characteristics adapted for positioning along any one or combination of the sacral, lumbar, thoracic and cervical regions of the spinal column. 
     In the example shown, the connection element  106  appears as a flange or hub on the first rod portion  104  that receives, and it couples with the second rod  102  to connect the rod portion  104  and the second rod  102 . In the illustrated embodiment, the connection element  106  is generally a rectangular block, although other shapes are also contemplated, such as square, cylindrical, and non-uniform shapes. The connection element  106  includes a rod-receiving first bore  116  formed internally therein that extends inwardly from an end surface  117  of the connection element  106  opposite the rod portion  104 . The connection element  106  further includes a fastener receiving second bore  118  extending therein transversely to the rod-receiving first bore  116 . As shown in  FIG. 3 , the second bore  118  can be internally threaded for receipt of the locking member  108 . The second bore  118  can also be orthogonal to the first bore  116 , although other orientations are also contemplated. For reference, we refer to the surface having the fastener receiving bore  118  as an upper surface  120 , and the surface opposite the upper surface  120  as the lower surface  122 . 
     Referring now to  FIGS. 3 and 4 , the rod-receiving first bore  116  is distinctly shaped to include a greater area at the upper portion and a smaller area the lower portion, such that the width of the rod-receiving bore  116  decreases from the upper portion down, generally forming a tapered bore. Here, the rod-receiving first bore  116  includes a larger bore region  124  and a smaller bore region  126 . In the embodiment shown, the larger bore region  124  has an area greater than the smaller bore region and is disposed relatively closer to the upper surface  120  than the smaller bore region  126 . The smaller bore region  126  has an area smaller than the larger bore region  124  and is disposed relatively closer to the lower surface  122 . The bore regions  124 ,  126  cooperate to have a width that generally tapers as the distance from the upper surface  120  of the connection element  106  increases. As can be understood with reference to  FIG. 4 , this greater width at the upper bore region  126  permits a surgeon to easily introduce the second rod  102  into the rod-receiving bore  116 , but the narrowing width resulting from the smaller bore region  126  also aids in securing the rod  102  in place in the receiving bore  116  with a taper lock. Accordingly, the upper or larger bore region  124  is sized to be greater than typical conventional spinal rods, while the lower or smaller bore region  126  is sized to be less than typical conventional spinal rods. 
     In the example shown, the upper bore region  124  and the lower bore region  126  are each formed as overlapping cylindrical bores. The resulting taper creates sides that interface with the second rod  102  at contact points  130   a ,  130   b . The exact location of the contact points  130   a ,  130   b  will vary depending on the size of the second rod  102 . 
     In addition, the edge  128  between the inner surface of the bore  116  and the end surface  117  is rounded, chamfered, or both. This not only serves to funnel a rod when its introduced into the connection element  106 , but it also protects the rod from nicks or high stress concentrations that may occur if the edge were square. This can be more important when the second rod is formed of polymer materials, such as PEEK, that are notch sensitive. It should be noted that square edges are contemplated for non-notch sensitive materials. 
     Referring to  FIGS. 4 and 5 , when a rod is inserted into the larger region of the rod-receiving bore  116 , driving the set screw locking member  108  into the connection element  106  against the rod forces the rod  102  downward in the taper until the rod contacts the contact points  130   a ,  130   b . This centralizes the second rod  102  within the rod-receiving bore  116  and, with the locking member  108 , taper locks the rod within the connection element  106  with a three-point contact lock. It should be noted that although referred to herein as “three-point,” it is contemplated that the contact need not be a “point” contact in that each contact location may be over an area greater than a single point, and may include an area. One example of this is where the locking member  108  includes a saddle shaped to interface with the rod surface across an area. 
     In some examples, when the second rod  102  is inserted and secured into the connection element  106 , due to the tapering rod-receiving bore  116 , the second rod  104  is locked in place and lies above and spaced from the lowermost inner surface portion of the rod receiving bore  116  and below and spaced from the uppermost inner surface of the rod receiving bore  116 . Accordingly, gaps are formed both above and below the second rod  102 . In the example shown in  FIG. 3 , the inner surface of the rod-receiving bore  116  is un-threaded, and may include the same inner-wall profile, or a constant inner-wall profile through all or substantially all of the bore axial depth. Accordingly, the contact points  130   a ,  130   b  extend all or substantially all the way to the bottom or end of the rod receiving first bore  116 . Other arrangements also are contemplated. 
     Referring now to  FIG. 3 , the locking member  108  is movably engageable with the connection element  106  to secure the second rod  102  in engagement with the tapering rod-receiving first bore  116 . In the example shown, the locking member  108  includes a distal threaded portion  134  and a proximal portion  136 . The distal threaded portion  134  is illustrated as an externally threaded set screw that engages the internal thread profile in the screw-receiving second bore  118 , although other configurations are contemplated. The proximal portion  136  may include a recess for receive a driving tool, or can be configured to engage a driving tool disposed about its perimeter. In the embodiments shown, the proximal portion  136  can be configured to sever or break-off upon application of a threshold torque. In some embodiments, the distal portion  134  includes an internal bore that can receive a driving tool to facilitate removal or tightening of distal portion  134  after the proximal portion  136  has been broken off or otherwise been removed.  FIG. 5  shows the locking member  108  with the proximal portion  136  severed and the rod  102  in place in the connection element  106 . 
     Referring now to  FIG. 6 , as well as  FIG. 3 , the screw-receiving second bore  118  includes a view slot  138 . This slot enables a surgeon to view the bottom portion of the rod receiving bore  116 . In use, through the view slot  138 , and with the locking member  108  in place, the surgeon may observe whether the second rod is completely inserted into the rod receiving bore  116 . Thus, the surgeon makes a visual determination that the rod is fully seated in the bore  116 . Additional ports may be provided in the connection element  106  that permit a surgeon to shine a light into a side of the bore  116  so the surgeon can more easily see whether the second rod is seated. 
       FIGS. 7 and 8  show the spinal rod  10  in cross-section in an exploded configuration and in an assembled configuration, respectively. As can be seen in these and other figures, the spinal rod  10  can be assembled with the transition rod  100  and the second rod  102  in end-to-end fashion. Here, the rod portion  104  includes a first longitudinal axis  140  and the second rod  102  includes a second longitudinal axis  142 . The first longitudinal axis  140  may also align with a longitudinal axis or centerline of the rod-receiving bore  116 . Because of the tapering cross-section of the rod-receiving bore  116 , different rods with different diameters may have respective longitudinal axes that do not align with the longitudinal axis  140  of the rod portion  104 . For example, because of the three-point contact, a rod with a relatively smaller diameter sits lower in the tapered rod-receiving first bore  116  than a rod with a relatively larger diameter. Accordingly, in some embodiments, including the example shown in  FIG. 8 , the axes  140  and  142  are not aligned. In some embodiments, the taper of the rod-receiving first bore is arranged so that when the first rod portion  104  and the second rod  102  have a similar diameter, the axes  140  and  142  align, but when the diameter of the second rod  102  is smaller than the diameter of the rod portion  104 , the axes do not align. Accordingly, when the diameter of the second rod  102  is smaller than the diameter of the rod portion  104 , the second rod  102  sits lower in the taper, rendering the bottom of the outer surface of the second rod more closely aligned with the bottom surface of the first rod portion. Because the depth of the rods is similar, even when the diameters differ, the spinal rod  10  may be more easily implanted into fasteners, such as screws, that may be driven to the same depth. 
     In one embodiment, the angle of the taper of the rod-receiving bore  116  is selected so that as the diameter of the second rod decreases, the bottom portion of the second rod substantially aligns with the bottom portion of the rod portion  104 . 
     One advantage of the system disclosed herein is that it is arranged to receive standard cylindrical rod ends, without special treatment or engaging features on the rod ends. Accordingly, it can receive off-the-shelf rods manufactured by any number of manufacturers, including previously implanted rods. Further, because the device employs a taper lock, the device does not rely upon surface features to secure the rod in place. In the example shown, the second rod  102  includes a smooth, cylindrical outer surface. Yet the second rod can securely be locked into the connection element  106 . This fixation occurs at least in part because the contact points are not directly opposite each other across the rod. Instead, they are offset from each other, creating the point lock. In addition, tightening the locking member  108  applies high force levels at the contact points due to the point loading. Localizing these force levels at contact points, instead of distributing them throughout the circumference of the smooth rod, creates longitudinal lines of high loading that frictionally resist removal of the rod. In some embodiments, because of the point loading, the rod or the connection element may deform slightly to further frictionally engage and secure the rod in place. 
     Although the rod is described as smooth and cylindrical, in some embodiments, the rod may have a cross-section other than circular, and may be, for example, oval, rectangular, star-shaped, or other shape. In addition, in some embodiments, the rod end of the inner surface of the rod receiving first bore  116  may be roughened, knurled, peened, or may include other surface imperfections that roughen and increase the frictional engagement of the rod and connection element  116  while still maintaining the rod as substantially smooth. 
     Still referring to  FIG. 8 , the round or chamfered edge  128  between the inner surface of the bore  116  and the end surface  118  also assists in distributing the loading applied by the locking member  108 . For example, because the point loads are not exactly opposite each other the rod, some rods formed of relatively softer materials, such as polymeric materials, deflect due to the loading applied by the locking member  108 . The edge  128  aids in limiting or distributing that deflection by controlling the depth of rod contact within the bore  116 . 
     In use a surgeon may introduce fasteners, such as for example, pedicle screws into vertebrae. The vertebrae may be adjacent or spaced by additional vertebrae. If the surgery is a revision surgery, the spinal rod  10  may be assembled with the previously implanted rod in situ during the process. Likewise, even during a new surgery, the surgeon may choose to assemble the spinal rod  10  in situ. 
     In a revision surgery, the surgeon may access an end of prior placed spinal rod through an incision. If the revision surgery is to extend the previously placed spinal rod, one or more suitable fasteners, such as the pedicle screws, are inserted into the proper vertebra. 
     The transition rod  100  may be selected by the surgeon to have difference characteristics than the previously placed rod. It may be formed of a different material, may have a different diameter or surface profile, and/or may have different stiffness. The transition rod  100  may be slid over the end of the previously placed rod so that the previously placed rod is in the connection element  106 . Because of the non-circular shape of the rod-receiving bore  116 , and because the rod receiving bore has a width that decreases as the distance from the upper surface increases, the rod may be axially slid into the connection element  106  by axial translation, without significant rotation, or any rotation, of the rod about its longitudinal axis. This enables the surgeon to connect the rod, which may be bent to provide desired spinal reinforcement without turning the rod as might occur if the rod were to be threaded into the rod-receiving first bore  116 . As indicated above, the relatively smooth, unthreaded interior surface of the connection element permits the rod to also be smooth. 
     Once the second rod is inserted into the connection element  106 , the surgeon may then tighten the locking member  108  to secure the previously implanted rod in place. As the locking member  108  is tightened into the connection element  106 , the locking member presses the rod downward toward the region of the bore  116  having a smaller width. Once the rod engages the taper on the connection element  106 , any additional tightening increases the frictional forces at the contact points. In the example in  FIG. 3 , this results in a three-point loading scenario that secures the rod in place in the connection element  106 . 
     When the locking member  108  is a break-off set screw, tightening continues until the break-off portion severs from the threaded distal portion indicating that the proper amount of torque has been achieved. If additional levels are to be connected using additional connection elements, those may be assembled as described above. With the spinal rod assembled, the fasteners may be fully tightened, securing the spinal rod in place. Thus, the rod disclosed herein may be assembled in situ. 
     When the surgeon assembles the spinal rod  10  prior to implantation, the rod may be assembled as described above, and the rod may be introduced to the fasteners after being fully assembled. Other implantation processes also are contemplated. 
       FIGS. 9 and 10  show some examples of alternative transition rods, reference herein as  200  and  300 , respectively. These transition rods respectively include rod portions  202 ,  302  and connection elements  204 ,  304 , and each has a rod-receiving bore with a larger first bore region and smaller second bore region. The larger bore region has an area greater than the smaller bore region and is disposed relatively closer to the upper surface than the smaller bore region. The smaller bore region has an area smaller than the larger bore region and is disposed relatively closer to the lower surface. The bore regions cooperate to have a width that generally tapers as the distance from the upper surface of the connection element increases. 
     The transition rod  200  in  FIG. 9  includes a rod-receiving bore  206  that tapers from the top to the bottom, with the bore  206  shaped to be generally triangular, but having an arc forming an upper end that facilitates entry of a second rod. The transition rod  300  in  FIG. 10  includes a rod-receiving bore  306  that generally tapers from the top to the bottom, with the bore  306  shaped to be generally triangular, but having stepped surfaces that engage a second rod. These stepped surfaces create point contacts that extend longitudinally along the rod. As with the connection element  106  discussed above, the inner surface features, such as the steps, may extend the length of the bore providing a constant inner wall profile that extends to the end of the bore. It should be noted that these are exemplary only, and other designs also are contemplated. Further, the transition rods  200 ,  300  may include any feature or benefit discussed with respect to any other embodiment disclosed herein. 
       FIGS. 11 and 12  show an alternative embodiment of a connection element, referenced herein by the numeral  400 . The connection element  400  is similar to the connection element  106  described above in some respects, but is not integral with a rod portion and instead is configured to receive a separate rod  402  at a first end  404  and receive a separate rod  406  at an opposing second end  408 . To this end, each end  404 ,  408  of the connection element  400  includes rod-receiving bores  410   a ,  410   b , and includes a top surface  412  having two fastener receiving second bores  414   a ,  414   b  extending therein transversely to the rod-receiving bores  410 . The second bores  414  can be internally threaded for receipt of a locking member  416 . Here, the connection element  400  may be considered to be two integral back-to-back connection elements  106  described above. 
     In this case, the bores  410  do not intersect within the interior of the connection element  400 , but each extends inwardly to a bottom wall or bore end (not shown). In other embodiments, the bores  410  intersect each other such that rods inserted in each ends may abut one another within the connection element  400 . The connection element  400  may include any feature or benefit discussed with respect to any other embodiment disclosed herein. 
     The connection element  400  permits a surgeon to select two independent rods for connection via the taper lock. As shown in  FIG. 12 , the rods  402 ,  406  are aligned and coupled end-to-end. The rods may include different characteristics as discussed above. Each rod is independently secured using the taper lock formed by the shape of the respective bore  410 . Accordingly, longitudinal axes of the rods may be aligned or may be offset in the manner discussed above. 
     The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being determined solely by the appended claims.