Abstract:
A spinal stabilization system generally comprises first and second anchor members configured to be secured to first and second vertebrae within a patient&#39;s body, a flexible element secured to the first anchor member, and a rigid element secured to the second anchor member. An end portion of the rigid element is coupled to an end portion of the flexible so that the system is able to provide both rigid and dynamic stabilization. The coupling is maintained even if the flexible element relaxes after a period of time within the patient&#39;s body.

Description:
FIELD OF THE INVENTION 
     This invention relates to spinal stabilization systems, and more particularly to such systems including both a rigid element and a flexible element. 
     BACKGROUND 
     The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. The vertebrae in the cervical, thoracic, and lumbar regions of the spine are separated by intervertebral discs, which serve as cushions between adjacent vertebrae to dampen compressive forces experienced by the spine. A vertebral canal containing the spinal cord is formed by the intervertebral foramen of the vertebrae. In spite of the complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. For example, the kinematics of the spine normally includes flexion, extension, rotation, and lateral bending. 
     There are many types of conditions that can lead to significant pain and affect movement of the spine, including spinal disorders such as scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), and spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine), as well as conditions caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. In addition to causing pain, these conditions may also threaten the critical elements of the nervous system housed within the spinal canal. 
     One of the most common methods for treating these conditions is to immobilize a portion of the spine to allow treatment. Traditionally, immobilization has been accomplished by rigid stabilization. For example, in a conventional spinal fusion procedure, a surgeon restores the alignment of the spine or the disc space between vertebrae by installing a rigid fixation rod between pedicle screws secured to adjacent vertebrae. Bone graft is placed between the vertebrae, and the fixation rod cooperates with the screws to immobilize the two vertebrae relative to each other so that the bone graft may fuse with the vertebrae. 
     Dynamic stabilization has also been used in spinal treatment procedures. Dynamic stabilization does not result in complete immobilization, but instead permits enhanced mobility of the spine while also providing sufficient stabilization to effect treatment. One example of a dynamic stabilization system is the Dynesys® system available from Zimmer, Inc. of Warsaw, Ind. Such dynamic stabilization systems typically include a flexible spacer positioned between pedicle screws installed in adjacent vertebrae of the spine. Once the spacer is positioned between the pedicle screws, a flexible cord is threaded through a channel in the spacer. The flexible cord is also secured to the pedicle screws by a housing and set screw, thereby retaining the spacer between the pedicle screws while cooperating with the spacer to permit mobility of the spine. 
     In some instances, it is desirable to immobilize a portion of the spine using a rigid stabilization system without significantly limiting the mobility or increasing the stress on nearby areas of the spine. Although combining the rigid stabilization system with a dynamic stabilization system would help achieve this objective, there are several challenges associated with doing so. Specifically, there are several challenges associated with combining a flexible element, such as a braided polymer cord, with a rigid element, such as a rigid fixation rod, in a single construct. The cord and rod are ideally connected or coupled to each other before or during a surgical procedure. But the stiffness of the flexible element is often designed to decrease after placement into a patient&#39;s body and as treatment occurs to provide increased range of motion. Therefore, a spinal stabilization system in which the rigid element remains sufficiently coupled to the flexible element after this “relaxation” is highly desirable. 
     SUMMARY OF THE INVENTION 
     This invention provides a system or construct incorporating both a rigid element and flexible element to stabilize a portion of the spine. The system generally includes first and second anchor members, which may be pedicle screw assemblies, configured to be secured to first and second vertebrae within a patient&#39;s body. The rigid element is secured to the first anchor member, while the flexible element secured to the second anchor member. Respective end portions of the rigid and flexible elements are coupled to each other in a manner that securely retains their connection, even after the system has been positioned within the patient&#39;s body for an extended period of time. 
     In some embodiments, the end portion of the flexible element is received over the end portion of the rigid element. For example, the flexible element may be a cord constructed from polymer fibers braided over the end portion of the rigid element. To further facilitate retaining the cord on the rigid element, the fibers may be ultrasonically cut and/or ultrasonically welded to an enlarged ball tip of the rigid element. Such an arrangement increases the amount of surface area in contact between the cord and the rigid element and makes it difficult to pull the cord off the rigid element. A compression-fit collar may also be received over the end portion of the cord so that the fibers are gripped between the ball tip of the rigid element and the collar. 
     In other embodiments, the end portion of the rigid element includes an axial bore that receives the end portion of the flexible element. The axial bore extends at least partially into the rigid element from an end surface and is shaped to retain an end portion of the flexible element therein. For example, the end portion of the flexible element may include an enlarged section having a first diameter and the axial bore may include a restricted or tapered portion having a second diameter less than the first diameter. The enlarged section of the flexible element may be formed by positioning an insert or plug into the end portion of the flexible element. The restricted portion of the axial bore may be incorporated into the shape of the bore at the time of manufacture or may be formed by swaging a portion of the rigid element. 
     If desired, the end portion of the rigid element with the axial bore may be received in a housing of a vertebral anchor, such as a pedicle screw assembly. One or more openings extend through an outer surface of the rigid element and into the axial bore. A pin is press-fit into the opening by means of a hand press or by tightening a set screw that secures the rigid element within the housing of the pedicle screw assembly. Because the pin extends into the axial bore, it applies a compression force to the end portion of the flexible element received by the bore. This compression force retains the end portion of the flexible element within the bore. 
     By virtue of the foregoing, a spinal stabilization system that effectively incorporates aspects of both rigid and dynamic stabilization is provided. The different manners of coupling the rigid element to the flexible element are each designed so that the coupling is maintained even after relaxation of the flexible element over time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
         FIG. 1  is a partial side elevational view showing a spinal stabilization system including both a rigid element and a flexible element secured within a patient&#39;s body; 
         FIG. 2  is a schematic view showing how the rigid and flexible elements of  FIG. 1  may be coupled together according to one embodiment of the invention; 
         FIG. 3  is a side elevational view, partially in cross-section, showing how the rigid and flexible elements of  FIG. 1  may be coupled together according to another embodiment of the invention; 
         FIG. 4  is a side elevational view, partially in cross-section, showing how the rigid and flexible elements of  FIG. 1  may be coupled together according to another embodiment of the invention; 
         FIG. 5  is cross-sectional view showing how the rigid and flexible elements of  FIG. 1  may be coupled together according to another embodiment of the invention; 
         FIG. 6  is a cross-sectional view showing how the rigid and flexible elements of  FIG. 1  may be coupled together according to another embodiment of the invention; 
         FIG. 6A  is a perspective view of a portion of the rigid element shown in  FIG. 6 ; 
         FIG. 7  is a perspective view showing a portion of the flexible element of  FIG. 1  according to another embodiment of the invention; 
         FIG. 8  is a cross-sectional perspective view showing how the flexible element of  FIG. 6  may be coupled to the rigid element of  FIG. 1 ; 
         FIG. 9  is a cross-sectional side view showing the rigid and flexible elements of  FIG. 1  according to another embodiment of the invention; 
         FIG. 10  is a perspective view showing how the rigid and flexible elements of  FIG. 9  may be coupled together; and 
         FIG. 11  is a partial side elevational view, partially in cross-section, showing how the rigid and flexible elements of  FIG. 1  may be coupled together according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows one embodiment of a spinal stabilization system or construct  10  according the invention within a patient&#39;s body. The stabilization system  10  includes first, second, and third anchor members  12 ,  14 ,  16  secured to respective first, second, and third vertebrae  18 ,  20 ,  22  within the patient&#39;s body. The anchor members  12 ,  14 ,  16  may be any type of anchor such as a screw or hook designed to cooperate with a rigid element  24  or a flexible element  26  to stabilize a portion of the spine. For example, in the embodiment shown in  FIG. 1 , the anchor members  12 ,  14 ,  16  are pedicle screw assemblies each having a screw body  30 , a housing or retainer  32  coupled to the screw body  30 , and a set screw  34 . Each housing  32  receives the rigid element  24  or the flexible element  26 , which are secured to the associated housing  32  by one of the set screws  34 . One example of this type of pedicle screw arrangement is the Optima® Spinal Stabilization System available from Zimmer, Inc. of Warsaw, Ind. 
     The rigid element  24  and the flexible element  26  each extend between two or more of the pedicle screw assemblies. The rigid element  24  may be a metal rod, such as those commonly used in rigid spinal fixation procedures, while the flexible element  26  may be a cord, such as those commonly used in dynamic stabilization procedures. For example, the flexible element  26  may be constructed from braided polyethylene-terephalate (PET) fibers or other braided polymer fibers. A flexible spacer  36  is received over the flexible element  26  to provide additional support during movement of the spine in some embodiments. 
     As shown in  FIG. 1 , the rigid element  24  is coupled to the flexible element  26  between the first and second anchor members  12 ,  14 . Such an arrangement enables the overall system or construct  10  to combine the features of both rigid stabilization and dynamic stabilization. In particular, the rigid element  24  enables the system  10  to rigidly immobilize a desired area of the spine to promote fusion or other treatment in a desired area, while the flexible element  26  provides additional stabilization without significantly increasing the stress on nearby vertebrae or compromising mobility. The rigid and flexible elements  24 ,  26  may be coupled to each other in a variety of different manners, examples of which will be described below with reference to  FIGS. 2-11 . 
     For example,  FIGS. 2 and 3  illustrate embodiments in which an end portion  50  of a braided cord  52  is received over an end portion  54  of a rigid fixation rod  56 . The end portion  50  of the cord  52  is retained on the end portion  54  of the rod  56 , which may be achieved by braiding or weaving the fibers of the cord  52  over the end portion  54 . Once this step is complete, the region where the cord  52  overlaps the rod  56  is heat treated in a manner that promotes intimate chemical and physical bonding of the cord  52  to the rod  56 . For example, the cord  52  may be ultrasonically welded to the rod  56 . Such an arrangement results in the cord  52  being coupled to the rod  56  prior to implantation without placing meaningful stresses on the cord  52  and without the system  10  requiring additional components. 
     Additionally, as shown in  FIG. 3 , in some embodiments the end portion  54  of the fixation rod  56  may further include an enlarged ball tip  58 . Once the cord  52  is braided over the ball tip  58 , the end portion  50  of the cord  52  is cut at location on the rod  56  spaced from the ball tip  58  (i.e., where the cord  52  has a smaller diameter than that of the ball tip) by ultrasonic cutting. The ultrasonic cutting and/or welding results in the fiber ends joining together so that the end portion  50  includes a permanent diameter smaller than the ball tip  58 , thereby preventing the cord  52  from fraying and further retaining it on the end portion  54  of the fixation rod  56 . The relatively large amount of contact area between the cord  52  and the rod  56  helps distribute any pre or post-operative loads on the cord  52 , which in turn minimizes the effects of changes in those loads resulting from post-operative relaxation of the cord  52  or other conditions. 
     If desired, a collar  60  may also be compression-fitted around the end portion  50  of the cord  52  to further retain the cord  52  on the fixation rod  56 . When tension is applied to the cord  52 , the collar  60  cooperates with the ball tip  58  to provide a gripping force. The collar  60  includes a polished end surface  62  configured to confront the spacer  36  ( FIG. 1 ), with the end surface  62  optionally defined by a radially extending flange  64 . Those skilled in the art will appreciate that the collar  60  may also be designed to interact with one of the pedicle screw assemblies. 
     Rather than being received over the end portion of the rigid element  24 , the flexible element  26  may be received and retained within a portion of the rigid element  24 . For example,  FIG. 4  illustrates an embodiment in which an end portion  70  of a fixation rod  72  includes an axial bore  74  extending from an end surface  76 . An end portion  78  of a flexible cord  80  is received in the axial bore  74 . To retain the cord  80  in the bore  74 , the end portion  70  of the fixation rod  72  is swaged (i.e., cold-worked) about its circumference at one or more locations designated by  82 . The 360 degree swages  82  place the cord  80  into high, radially-symmetric compression so that it cannot be easily pulled out of the axial bore  74 . The swages  82  also provide the end portion  70  of the fixation rod  72  with a rib-like appearance. 
       FIG. 5  also illustrates an embodiment of the spinal stabilization system  10  in which a fixation rod go is swaged to retain an end portion  92  of a flexible cord  94  within an axial bore  96 . Specifically, the fixation rod go terminates in an end surface  98  defined by a radially extending flange  100 . The end surface  98  may be configured to confront a spacer  36  ( FIG. 1 ) and further includes flange  102  extending distally therefrom around the opening of the axial bore  96 . After the end portion  92  of the cord  94  is inserted into the axial bore  96 , the distal flange  102  is swaged in a radially inward direction to define a restricted portion  104  of the axial bore  96 . The restricted portion  104  has a diameter less than that of the end portion  92  of the cord  94  so that the cord  94  is retained in the axial bore  96 . 
     For this purpose, the end portion  92  of the cord  94  may include an insert or plug  106  to define an enlarged diameter section  108 . The insert  106  may be constructed from metal or any other biocompatible material and is surrounded and retained by the end portion  92  of the cord  94 . For example, after weaving fibers of the cord  94  around the insert  106  or positioning the insert  106  in a predefined space within the end portion  92 , the cord  94  may be ultrasonically heated while being compressed around the insert  106  in a mold (not shown). This ultrasonic forming process promotes bonding of the cord fibers to the insert  106  and provides the cord  94  with a shape that retains the insert  106  in the end portion  92 . Thus, when the end portion  92  of the cord  94  is received in the axial bore  96  and the distal flange  102  is swaged inwardly to define the restricted portion  104 , pulling on the cord  94  results in the cord fibers being “wedged” between the insert  106  and the restricted portion  104 . This resistance to pull-out remains effective even after warming and relaxation of the cord  94  within a patient&#39;s body. 
     An embodiment that operates upon similar principles is shown in  FIGS. 6 and 6A . In this embodiment, an end portion  112  of a flexible cord  114  is provided with an insert  116  in the same manner as the previous embodiment to define an enlarged diameter section  118 . A fixation rod  120  having an enlarged end portion  122  includes an axial bore  124  extending from an end surface  126 . The axial bore  124  receives the end portion  112  of the cord  114 , but includes a restricted portion  128  having a smaller diameter than that of the enlarged section  118 . If desired, an interior surface  130  of the axial bore  124  may be tapered to help define the restricted portion  128  and to define a shape that more closely resembles that of the cord end portion  112 . 
     In this arrangement, the cord  114  cannot be end-loaded into the axial bore  124  through an opening  132  on the end surface  126  of the fixation rod  120 . Instead, the end portion  112  of the cord  114  is inserted through a slot  134  on the end portion  122  of the rod  120 . The slot  134  extends into the axial bore  124  and includes an enlarged opening  136  to accommodate the enlarged section  118  of the cord  114 , as shown in  FIG. 6A . Applying tension to the cord  114  after the end portion  112  is received in the axial bore  124  creates a wedge-like effect due to the interference between insert  116  and the restricted portion  128 . In other words, as with the embodiment shown in  FIG. 5 , the fibers of the cord  114  are “wedged” between the insert  116  and the restricted portion  128  to retain the cord  114  within the axial bore  124 . The more tension that is placed on the cord  114 , the stronger it is gripped between the insert  116  and the restricted portion  128 . 
     The cord  114  may be inserted through the slot  134  and into the axial bore  124  prior to or even during an operation because of the pre-formed shape of the fixation rod  120 . For example, during a surgical procedure, the rod  120  may first be secured to a top-loading pedicle screw  30  ( FIG. 1 ) using the housing  32  and set screw  34 . After inserting the end portion  112  of the cord  114  through the slot  134  and into the axial bore  124 , the cord  114  may then be secured to a different pedicle screw assembly to stabilize the entire construct  10 . Because the cord  114  is not put under any stress prior to insertion into the patient&#39;s body, concerns about stress relaxation during storage are avoided. Additionally, if further surgical procedures are later required to effect treatment, the cord  114  may be easily replaced without requiring removal of the fixation rod  120 . This is particularly advantageous when seeking to modify the amount of dynamic stabilization provided by the entire construct  10  by replacing the original cord  114  with a different one. 
       FIGS. 7 and 8  illustrate another embodiment in which a cord  140  may be coupled to a rigid element  142  between the first and second anchor members  12 ,  14  ( FIG. 1 ). In this embodiment, the cord  140  is provided with a preformed shape. For example, the cord  140  may be constructed from polymer fibers and may be ultrasonically heated while being compressed in a mold. This ultrasonic forming process in one embodiment results in an end portion  144  of the cord  140  having a reduced diameter and first and second recesses  146 ,  148 . 
     The rigid element  142  includes an end portion  150  with an outer surface  152  and an end surface  154 . The end surface  154  is defined by a radially extending flange  156  and configured to confront the spacer  36  ( FIG. 1 ). An axial bore  158  extends into the end portion  150  from the end surface  154 , and the outer surface  152  includes first and second openings or holes  160 ,  162  extending into the axial bore  158 . The axial bore  158  receives the end portion  144  of the cord  140 , with the first and second openings  160 ,  162  aligned with the respective first and second recesses  146 ,  148 . To retain the cord  140  within the axial bore  158 , first and second fasteners  164 ,  166  are inserted through the respective first and second openings  160 ,  162  until they are received in the respective first and second recesses  146 ,  148 . Because the first and second recesses  146 ,  148  are permanently formed in the end portion  144  of the cord  140 , relaxation of the cord  140  has minimal or no affect on the engagement between the first and second recesses  146 ,  148  and the first and second fasteners  164 ,  166 . 
     The fasteners  164 ,  166  shown in  FIG. 8  are pins that are press-fit into the first and second openings  160 ,  162 . It will be appreciated, however, that a wide variety of other types of fasteners (screws, rings, clips, etc.) may be secured within the first and/or second openings  160 ,  162  to retain the end portion  144  of the cord  140  within the axial bore  158 . It will also be appreciated that only one fastener may be used to retain the cord  140  and that the axial bore  158  of the rigid element  142  may be shaped with features adapted to cooperate with the preformed shaped of the cord  140 . For example, rather than including the second opening  162 , the rigid element  142  may be machined to define a protrusion (not shown) in the axial bore  158  at the same location. The protrusion would cooperate with the second recess  148  to retain the end portion  144  of the cord  140  in the axial bore  158 . The end portion  144  of the cord  140  and axial bore  158  of the rigid element  142  may therefore be shaped in a variety of different manners to achieve this type of relationship. 
       FIGS. 9-11  illustrate embodiments of the system  10  shown in  FIG. 1  in which the rigid member  24  is shaped to cooperate with one of the pedicle screw assemblies  12 ,  14 ,  16  to retain the flexible element  26  within a portion thereof. The housings  32  and set screws  34  shown in  FIG. 1  have a different configuration in the embodiments shown in  FIGS. 9-11  and will be indicated with prime marks (′) in the description below. 
     To this end,  FIGS. 9 and 10  illustrate a rigid element  172  having an end portion  174  received in the housing  32 ′ of a pedicle screw assembly. The end portion  174  includes an end surface  176  configured to confront a spacer  36  ( FIG. 1 ), an axial bore  178  extending from the end surface  176 , an outer surface  180 , and first and second openings  182 ,  184  on the outer surface  180  extending into the axial bore  178 . The first opening  182  has a relatively small diameter and receives a needle member  186 , while the second opening  184  has a larger diameter and receives a pin  188 . An interference fit may be provided between the needle member  186  and the first opening  182  and the pin  188  and the second opening  184 . 
     A flexible element  190 , such as a cord constructed from braided polymer fibers, includes an end portion  192  received in the axial bore  178 . The flexible element  190  is initially secured within the axial bore  178  by inserting the needle member  186  through the first opening  182 . The manufacturer typically accomplishes this step so that the construct is pre-assembled with the flexible element  190  coupled to the rigid element  172  prior to delivery to the customer. The needle member  186  engages the cord  190  proximate an end  194 , which serves little function in terms of ultimately providing stabilization once in a patient&#39;s body. 
     The pin  188  may also be partially inserted into the second opening  184  by the manufacturer, but is not advanced far enough to place any appreciable stresses on the cord  190 . Instead, the final pressing of the pin  188  is accomplished prior to use with a hand press (not shown) or other similar tool. The pin  188  is ideally advanced through the second opening  184  until a top surface  196  of the pin  188  becomes substantially flush with the outer surface  180  of the rigid element  172 . Such an arrangement prevents the pin  188  from interfering with the operation of the set screw  34 ′, which secures the rigid element  172  to the housing  32 ′ of the pedicle screw assembly. 
     The pin  188  compresses the flexible element  190  within the axial bore  178  to retain the flexible element  190  therein. A protrusion  198 , such as a bump or rib, may be provided in the axial bore  178  opposite the second opening  184  so that the flexible element  190  is gripped between the pin  188  and the protrusion  198 . The pin  188  applies sufficient force to securely retain the cord  190  even after relaxation once inserted into a patient&#39;s body. Although only a press-fit pin is shown, any type of fastener capable of applying forces to the cord  190  may be used instead. 
       FIG. 11  shows a similar embodiment having a pin  210  for retaining an end portion  212  of a flexible element  214  within an axial bore  216  of a rigid element  218 . As with the previous embodiment, the axial bore  216  is positioned within an end portion  220  of the rigid element  218  received in the housing  32 ′ of a pedicle screw assembly and has an end surface  222  configured to confront the spacer  36  ( FIG. 1 ). An opening  224  on an outer surface  226  of the end portion  220  extends into the axial bore  216  and is aligned with the set screw  34 ′ received in the housing  32 ′. The set screw  34 ′ normally engages internal threads  230  to secure the end portion  220  of the rigid element in a socket defined by the housing  32 ′. To accommodate for the pin  210 , the housing  32 ′ further includes first and second tabs  234 ,  236  extending upwardly. Each of the first and second tabs  234 ,  236  includes internal threads  238  as well. 
     In use, the end portion  212  of the flexible element  214  is inserted into the axial bore  216 . The pin  210  is then inserted into the opening  224  and the set screw  34 ′ is advanced along the internal threads  238  of the first and second tabs  234 ,  236  until it contacts a top surface  240  of the pin  210 . To secure the flexible element  214  within the axial bore  216 , the set screw  34 ′ is further advanced to engage the internal threads  230  of the housing  32 ′ and to push the pin  210  into the opening  224 . The set screw  34 ′ is advanced until the top surface  240  of the pin  210  is substantially flush with the outer surface  226  of the rigid element  218 . In this position, the pin  210  applies a sufficient compression force to retain the end portion  212  of the flexible element  214  within the axial bore. One or more protrusions  242  or the like may be provided within the axial bore  216  to help grip the flexible element  214 , much like the previous embodiment. 
     Thus, the flexible element  214  may be secured to the rigid element  218  without any additional tools. The same tool normally used to secure the set screw  34 ′ is used to advance the pin  210  into the axial bore  216 . Although the pin  210  and set screw  34 ′ are shown as separate components, they may alternatively be integrally formed as a single component. The first and second tabs  234 ,  236  may also be configured to be removed from the housing  32 ′ after the set screw  34 ′ is completely advanced. In particular, the first and second tabs  234 ,  236  serve to distribute the force applied to the housing  32 ′ while tightening the set screw  34 ′ with a screwdriver or other tool. The tabs  234 ,  236  may be frangibly connected or otherwise separable from the housing  32 ′ of the pedicle screw assembly. Once the set screw  34 ′ is advanced so that it only engages the internal threads  230  of the housing  32 ′, the first and second tabs  234 ,  236  may be broken off from the housing  32 ′ and removed. 
     While the invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, although the rigid element is primarily described above as a metal rod, those skilled in the art will appreciate that “rigid” is a relative term. To this end, the rigid element may be a metal cable and the flexible element may be a polymer cord. The cable and cord may be coupled using the techniques described above or may simply be spliced together. 
     Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of the general inventive concept.