Abstract:
A spinal stabilization system includes at least two anchors having a bone attachment portion and a head portion and a flexible assembly coupled to the anchors. The flexible assembly includes a flexible cord, at least two connectors slidably mounted to the flexible cord, and at least one spacer slidably mounted to the cord between adjacent connectors. Each connector couples with a head portion of an anchor. The flexible assembly may be assembled and appropriately adjusted outside the body prior to it being coupled to the anchors. In addition, the connectors may include angled outer surfaces that provide enhanced engagement with the ends of the spacer. A method of stabilizing the spine includes securing anchors to the spine, assembling a flexible assembly outside the body, and coupling the flexible assembly to the anchors. The method may further include providing connectors with angled surfaces to provide enhanced engagement with the spacer.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention generally relates to spinal support devices, and more particularly to an apparatus and method for dynamically stabilizing the spine. 
       BACKGROUND OF THE INVENTION  
       [0002]    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 and nerves. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including a portion of relatively weak cancellous bone and a portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal cord and nerves is located posterior to the vertebral bodies. 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. 
         [0003]    There are many types of spinal column disorders including 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), spondylolisthesis (forward displacement of one vertebra over another, usually in the lumbar or cervical spine), and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain, as well as diminished nerve function. These spinal disorders, pathologies, and injuries limit the spine&#39;s range of motion, or threaten the critical elements of the nervous system housed within the spinal column. 
         [0004]    The treatment of acute and chronic spinal instabilities or deformities of the thoracic, lumbar, and sacral spine has traditionally involved rigid stabilization. For example, arthrodesis, or spine fusion, is one of the most common surgical interventions today. The purpose of fusion or rigid stabilization is the immobilization of a portion of the spine to affect treatment. Rigid stabilization typically includes implantation of a rigid assembly having metallic rods, plates and the like that secure selective vertebrae relative to each other. Spinal treatment using rigid stabilization, however, does have some disadvantages. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and therefore, accelerated degeneration of adjacent non-fused segments. Another disadvantage of fusion is that it is an irreversible procedure. 
         [0005]    More recently, dynamic stabilization has been used in spinal treatment procedures. Dynamic stabilization does not result in complete spinal fusion 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 Spine, Inc. of Edina, Minn. 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 eyelets formed in the pedicle screws and a channel through the spacer. The flexible cord retains the spacer between the pedicle screws while cooperating with the spacer to permit mobility of the spine. 
         [0006]    Many current dynamic stabilization systems are typically assembled in situ. In these systems, a surgical site is established in the patient and a pair of bone anchors is coupled to adjacent vertebrae. Spacers are then inserted into the surgical site and positioned between the anchors while a flexible cord is threaded through the anchors and spacers, generally in a direction parallel to the axis of the spine, to assemble the device in the body. Once the stabilization system is assembled, the appropriate amount of pre-tensioning must be applied to the cord and other post-assembly adjustments must be made to effect spinal treatment. 
         [0007]    While dynamic stabilization systems are generally successful for treating various spinal conditions, manufacturers or providers of such stabilization systems continually strive to improve these stabilization systems. By way of example, manufacturers or providers strive to provide relatively quick and convenient assembly and installation of the stabilization system. For example, minimally invasive surgical techniques often utilize much smaller incisions and provide the benefits of less tissue and muscle displacement and quicker recovery. 
         [0008]    Manufacturers or providers of stabilization systems also strive to provide systems that transmit imposed loads on the spine through the system and to the underlying bone structure in an efficient and effective manner. In such applications, for example, engagement of the spacers with the connectors on the anchors should provide for optimal transmission of the loads imposed on the stabilization systems to the underlying bone structure. Ideally, when the stabilization system is assembled, the end faces of the spacer will substantially mate with the surfaces of the eyelet so as to maximize the contact area between the spacer and the anchor. 
         [0009]    If the contact between the ends of the spacer and the surfaces of the eyelets occurs at less than the full contact area results, then such a reduction in contact area between the components localizes the load transfer. This may be due to the specific vertebral physiology to which the stabilization system is being applied, the geometry of the components or the non-idealized placement of the anchors in the vertebrae. In any event, the resulting reduction in contact area between the spacer and anchors may diminish the capacity of the stabilization system to efficiently transmit applied loads to the vertebrae to which the anchors are attached. This may result in a reduction in the support provided by the stabilization system, a loss of the pre-tensioning of the system, or otherwise affect the stabilization system in a manner that impacts treatment of the spine. 
         [0010]    Accordingly, there is a need for an improved dynamic stabilization system and method of using the same that addresses these objectives. 
       SUMMARY OF THE INVENTION  
       [0011]    A dynamic stabilization system that provides improvements over existing stabilization systems includes at least two vertebral anchors having a bone attachment portion and a head portion. The vertebral anchors might be, for example, bone screws. Each of the vertebral anchors is adapted to be coupled to different vertebrae of the spine. A flexible assembly is removably coupled to the vertebral anchors and includes a flexible cord, at least two connectors slidably mounted to the flexible cord, and at least one spacer slidably mounted to the cord. Each of the connectors is adapted to be coupled with a head portion of a respective anchor. Moreover, the flexible assembly is configured such that each spacer is disposed between adjacent connectors. In one aspect of the invention, the spatial relationship of the connectors on the flexible assembly are capable of being fixed relative to the cord prior to the flexible assembly being coupled to the vertebral anchors. This aspect may allow, for example, the flexible assembly to be assembled outside the body of a patient and then subsequently coupled to the anchors in situ. 
         [0012]    In one embodiment, for example, each of the connectors includes a channel extending through the connector for receiving the cord therein. Each connector further includes a threaded bore extending from a surface of the connector and intersecting the channel. A set screw is threadably engaged with the bore and cooperates therewith so as to prevent relative movement between the cord and connector when the connector is mounted on the cord. 
         [0013]    In another embodiment, the stabilization system includes at least two vertebral anchors having a bone attachment portion generally defining an axis, wherein each of the anchors is adapted to be coupled to different vertebrae of the spine. A flexible cord extends between the anchors. The system further includes at least two connectors, each connector adapted to be coupled to the bone attachment portion of one of the vertebral anchors and to the flexible cord. A first spacer is disposed between adjacent connectors and includes a channel for receiving the cord theretherough. The first spacer includes first and second opposed end faces. At least one of the connectors includes a first outer surface that confronts the first end face of the first spacer. In another aspect of the invention, the first outer surface of the at least one connector forms an angle with respect to the axis of the bone attachment portion of the vertebral anchor. Angulation of the first outer surface enhances the contact area between the first outer surface of the at least one connector and the first end face of the first spacer. This aspect may allow, for example, improved transmission of loads through the stabilization system and to the vertebrae to which the system is attached. 
         [0014]    By way of example, in one embodiment the at least one connector includes a lower surface and an upper surface, and the first outer surface is angled inwardly toward the axis of the bone attachment portion in a direction from the lower surface toward the upper surface. In another embodiment, however, the first outer surface is angled outwardly away from the axis of the bone attachment portion in a direction from the lower surface toward the upper surface. In still another embodiment, the at least one connector includes a second outer surface that confronts an end face of a second spacer. The second outer surface likewise forms an angle with respect to the axis of the bone attachment portion so as to enhance the contact area between the second outer surface and the end face of the second spacer. Depending on the specific application, the angulation of the first and second outer surfaces may be the same or may be different from each other. 
         [0015]    In yet another embodiment, a spinal stabilization system includes a vertebral anchor having a bone attachment portion and a head portion. The anchor is adapted to be coupled to a vertebra of the spine. The head portion includes a base member coupled to the bone attachment portion at one end thereof and a pair of spaced apart legs extending from the base member. The base member and legs collectively define an open channel in the head portion. The stabilization system further includes an assembly having a connector for removably coupling the assembly to the anchor. The connector includes first and second body portions and a narrowed intermediate body portion extending between the first and second body portions. The first, second and intermediate body portions collectively define a pair of opposed cutouts that receive the legs of the head portion therein when the connector is coupled to the anchor. When the connector is so coupled to the anchor, the intermediate body portion is received in the open channel of the head portion. 
         [0016]    In one embodiment, each of the legs projects from the base member at an angle of approximately 90 degrees. In such an embodiment, the intermediate body portion may likewise be configured so as to be closely received in the open channel. Accordingly, a pair of side surfaces of the intermediate body portion may project from a lower surface thereof at an angle of approximately 90 degrees. In another embodiment, however, the open channel and intermediate body portion may have a converging configuration to provided a snap-fit feature between the two. Thus, each of the legs projects from the base member at an angle between approximately 85 degrees and 90 degrees. Similarly, the side surfaces of the intermediate body portion may project from the lower surface thereof at an angle between approximately 85 degrees and 90 degrees. Furthermore, the stabilization system may include a retaining mechanism for selectively retaining the connector with the anchor. To this end, the system may include a retaining clip that is applied to the ends of the legs opposite the base member. Each of the legs, for example, may include a retaining notch that receives a portion of the retaining clip therein. When the retaining clip is positioned in the retaining notches on the legs, the legs are prevented from moving away from each other and the connector is prevented from moving away from the anchor. 
         [0017]    A method of stabilizing a spine within a body of a patient includes securing at least first and second anchors to respective first and second vertebrae of the spine, assembling a flexible assembly outside the body, adjusting the flexible assembly outside the body such that the flexible assembly is capable of stabilizing the spine once disposed inside the body, and removably coupling the flexible assembly to the anchors to stabilize the spine. In such a method, assembling the flexible assembly may include slidably mounting at least two connectors onto a flexible cord and/or slidably mounting at least one spacer on the cord so as to be positioned between the connectors. Moreover, adjusting the flexible assembly may include pre-tensioning the flexible cord, spatially fixing the connectors relative to the cord, and/or adjusting the length of the spacer. 
         [0018]    A method of stabilizing the spine in accordance with an alternate embodiment of the invention includes securing at least first and second anchors to respective first and second vertebrae of the spine, and providing a plethora of connectors for coupling a flexible assembly to the anchors. The flexible assembly includes at least one spacer with first and second opposed end faces. The connector has at least a first outer surface that forms an angle with respect to an axis of the anchor when coupled thereto. The method further includes determining the angle of the first outer surface so that the first outer surface engages substantially all of one of the end faces of the spacer when the flexible assembly is coupled to the spine; constructing the flexible assembly using a connector having a first outer surface with the determined angle; and then coupling the flexible assembly to the anchors to stabilize the spine. 
         [0019]    These and other objects, advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate 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 invention. 
           [0021]      FIG. 1  is a side elevation view of an exemplary stabilization system in accordance with an embodiment of the invention implanted on the spine; 
           [0022]      FIG. 2  is a front view of a bone anchor in accordance with an embodiment of the invention; 
           [0023]      FIG. 3  is a perspective view of a flexible assembly in accordance with an embodiment of the invention; 
           [0024]      FIG. 4A  is a side view of a connector used in the flexible assembly shown in  FIG. 3 ; 
           [0025]      FIG. 4B  is a cross-sectional view of the connector shown in  FIG. 4A  taken generally along line  4 B- 4 B; 
           [0026]      FIG. 4C  is a top view of the connector shown in  FIG. 4A ; 
           [0027]      FIG. 5  is a perspective view of the flexible assembly of  FIG. 3  ready for insertion into the body; and 
           [0028]      FIG. 6  is a perspective view illustrating the coupling between the flexible assembly and the anchors. 
       
    
    
     DETAILED DESCRIPTION  
       [0029]    Referring now to the drawings, and to  FIG. 1  in particular, a spinal stabilization system  10  is shown implanted into a segment of a spine  12  defined by serially positioned spinal elements in the form of adjacent vertebrae  14 ,  16 ,  18  that are separated by discs  20 . The stabilization system  10  includes anchors  22  installed in vertebrae  14 ,  16 ,  18  and a flexible assembly  24  coupled to and extending between the anchors  22  to control abnormal motion of the spine  12 , while otherwise leaving the spinal segment mobile. 
         [0030]      FIG. 2  illustrates an exemplary embodiment of an anchor used in the spinal stabilization system  10  in more detail. As shown in this figure, each anchor  22  may be configured as a pedicle bone screw having a threaded portion  26  adapted to facilitate coupling between the anchor  22  and the pedicle  28  ( FIG. 1 ) of the vertebrae  14 ,  16 ,  18  and a head portion  30  adapted to couple to the flexible assembly  24 . While pedicle screws are shown and described herein, those of ordinary skill in the art will appreciate that the spinal anchors  22  may take the form of hooks or other devices coupled to the spine  12 . 
         [0031]    As best illustrated in  FIG. 2 , the head portion  30  of the anchors  22  define a U-shaped receiving portion having a base member  32  from which threaded portion  26  projects, and a pair of legs  34 ,  36  each having a first end coupled to the base member  32  and a second end  37  spaced from base member  32 . The base member  32  and legs  34 ,  36  collectively define a U-shaped channel  38  defined by side surfaces  40 ,  42 , and base surface  44 . The channel  38  is open along a top or posterior end (relative to the pedicle  28 ) so as to receive a portion of the flexible assembly  24  in a top-load manner. The anchors  22  may be formed from any suitable material including, for example and without limitation, titanium, stainless steel, or other materials recognized by those of ordinary skill in the art. 
         [0032]    The legs  34 ,  36  may project from base member  32  such that the side surfaces  40 ,  42  form an angle α with base surface  44  of approximately 90 degrees, i.e., the base surface  44  and side surfaces  40 ,  42  are orthogonal to each other. Alternately, the angle α may be less than 90 degrees, such as between approximately 80 and 90 degrees, and more preferably between 85 and 90 degrees, so that the legs  34 ,  36  converge toward an axis  46  of threaded portion  26  in a direction away from base surface  44 . As explained in more detail below, such a converging configuration of the receiving portion facilitates coupling of the flexible assembly  24  to the anchors  22 . In such an embodiment, the legs  34 ,  36  may be formed of a suitable material that provides some flexibility or resiliency to the legs  34 ,  36  toward and away from each other. For example, titanium would provide sufficient flexibility to legs  34 ,  36 . 
         [0033]      FIG. 3  illustrates an embodiment of a flexible assembly  24  in accordance with the invention. The flexible assembly  24  includes a generally flexible cord  48  capable of flexing in substantially all directions and is further capable of having one portion of the cord rotated relative to another portion of the cord, i.e., the cord  48  is capable of being twisted. Moreover, the cord  48  is further capable of withstanding and maintaining tension within the cord  48 . Such a cord  48  may be formed, for example and without limitation, from polyethylene terephthalate (PET), titanium or other metal materials, or other suitable materials recognized by those of ordinary skill in the art. The cord  48  may also have any desirable cross section, such as and without limitation, circular, rectangular, triangular, etc. The flexible assembly  24  further includes at least two connectors  50  and at least one spacer  52  mounted on the cord  48 . 
         [0034]    Those of ordinary skill in the art will recognize that the number of connectors  50  typically corresponds to the number of anchors  22  coupled to the vertebrae. Moreover, spacers  52  are typically positioned between adjacent connectors  50 . Thus, while  FIGS. 1 and 3  illustrate a stabilization system  10  having three anchors  22 , three connectors  50 , and two spacers  52 , the invention is not so limited as fewer or more anchors  18 , connectors  50  and spacers  52  may be used to construct the stabilization system  10 , as dictated by the specific application. The connectors  50  and spacers  52  will now be described in detail. 
         [0035]    As shown in  FIGS. 3 ,  4 A- 4 C, in one embodiment, each connector  50  includes a generally cylindrical body  54  including a first body portion  56 , a second body portion  58 , and a narrowed intermediate body portion  60  that connects the first and second body portions  54 ,  56 . The body  54  may be formed out of suitable materials, such as, for example and without limitation, titanium, stainless steel, a polymeric material, or other suitable materials known to those of ordinary skill in the art. The first body portion  56  includes an outer surface  62  and an inner surface  64 , and second body portion  58  similarly includes an outer surface  66  and an inner surface  68 . The body portions  56 ,  58  are configured such that the inner surfaces  64 ,  68  face each other and are each coupled to an end of intermediate portion  60 . The body  54  includes a longitudinal channel  70  formed through the first, second and intermediate body portions  56 ,  58 ,  60  so as to closely, but yet slidably, receive cord  48  therethrough. Additionally, channel  70  may have a cross section that corresponds to the cross section of cord  48 . Thus, while a circular cross section is shown in  FIG. 4B , those of ordinary skill in the art will recognize other cross sections, such as rectangular, triangular, etc., are within the scope of the invention. 
         [0036]    The intermediate body portion  70  has a maximum cross dimension  72  in a lateral direction ( FIG. 4B ) that is less than or equal to the cross dimension  74  of the inner surfaces  64 ,  68  of the first and second body portions  56 ,  58  to define a pair of U-shaped cutouts  76 ,  78  on opposed sides of the intermediate body portion  60  ( FIG. 4C ). The cutouts  76 ,  78  are each defined by portions of the inner surfaces  64 ,  68  and respective side surfaces  80 ,  82  on the intermediate body portion  60 . The cutouts  76 ,  78  are configured to receive the legs  34 ,  36  of head portion  30  of anchors  22  therein. In addition, the intermediate body portion  60  includes a lower surface  84  that is spaced from the lower surface  86  of the first and second body portions  56 ,  58  and toward an upper surface  88  of the body portions  56 ,  58 . An upper surface  90  of the intermediate body portion  60  may be generally flush with the upper surface  88  of the first and second body portions  56 ,  58 , although not so limited. 
         [0037]    The intermediate body portion  60  on each of the connectors  50  is configured to fit within the U-shaped channel  38  in the head portion  30  of a corresponding anchor  22 . In this regard, the lower surface  84  of the intermediate body portion  60  engages the base surface  44  of base member  32  and the side surfaces  80 ,  82  of intermediate body portion  60  are closely received within the side surfaces  40 ,  42  of the legs  34 ,  36 . The surfaces  80 ,  82 , and  84  of intermediate body portion  60  define a shape generally corresponding to the shape of channel  34 . For example, the side surfaces  80 ,  82  may form an angle β with the lower surface  84  of approximately 90 degrees when the side surfaces  40 ,  42  are generally orthogonal to base surface  44 . 
         [0038]    In an alternate embodiment, however, the side surfaces  80 ,  82  may have a converging relationship such that the angle β is less than 90 degrees, such as between approximately 80 and 90 degrees, and more preferably between 85 and 90 degrees, so that the surfaces  80 ,  82  converge toward one another in a direction from lower surface  84  to upper surface  90 . The angle β is typically equal to the angle α so that the side surfaces  40 ,  42  of the legs  34 ,  36  mate with the side surfaces  80 ,  82  of intermediate body portion  60  over a substantial portion of the contact area between the two. As discussed in more detail below, the converging feature to the mating side surfaces of the legs  34 ,  36  and the intermediate body portion  60  provide a snap-fit type of feature between the flexible assembly  24  and the anchors  22 . 
         [0039]    As noted above, in some prior applications the contact area between the connectors and spacers may be reduced which in turn reduces the efficiency that loads are transmitted through the stabilization system and to the vertebrae. To improve load transmission in these cases, the outer surfaces  62 ,  66  of the first and second body portions  56 ,  58  of the connectors  50  may be angled. For example, as best shown in  FIG. 4A , the outer surfaces  62 ,  66 , form angles γ, θ with respect to planes  92  that are generally orthogonal to an axis  94  extending along channel  70 . Stated in an alternate way, the outer surfaces  62 ,  66  may form angles γ, θ with respect to the axis  46  of the threaded portion  26  of anchors  22  when the connectors  50  are coupled to the anchors  22 . While the angles γ, θ are shown as being substantially equal, the invention is not so limited as the angles may be different from each other. 
         [0040]    Moreover, while the connector  50  in  FIG. 4A  shows the outer surfaces  62 ,  66  as being angled inwardly, i.e., toward the opposed outer surface, in a direction from the lower surface  86  toward the upper surface  88 , one or both of the outer surfaces  62 ,  66  may be angled outwardly, i.e., away from the opposed outer surface, in a direction from the lower surface  86  toward the upper surface  88 . Thus, a plurality of connectors  50  with various angular configurations of the outer surfaces  62 ,  66  may be provided to accommodate the construction of a stabilization system that meets a specific application so as to provide excellent load transmission to the vertebrae. 
         [0041]    As shown in  FIG. 3 , the spacers  52  include a generally cylindrical body  96  having a first end defining a first end face  98 , a second opposed end defining a second end face  100 , and a longitudinal channel  102  extending between and through the end faces  98 ,  100 , as is conventional. The channel  102  is configured to closely, but yet slidably, receive cord  48  therethrough. Moreover, channel  102  may have a cross section that corresponds to the cross section of cord  48  and may be circular, rectangular, triangular, etc. The spacers  52  maintain the distraction between adjacent vertebrae, such as vertebrae  14 ,  16 ,  18 , while also providing some flexibility to the stabilization system  10  for enhanced mobility of the spine  12 . For example, the spacers  52  may be formed from polycarbonate urethane (PCU) or other suitable materials recognized by those of ordinary skill in the art. Furthermore, and as is conventional, the end faces  98 ,  100  may be generally orthogonal to a longitudinal axis  104  of the spacer  52 . 
         [0042]    Use of the stabilization system  10  in accordance with the invention will now be described in detail in reference to  FIGS. 3 ,  5  and  6 . To install the stabilization system  10  to the spine  12 , the anchors  22  are secured to the selected vertebrae  14 ,  16 ,  18  of the spine  12 . For example, the threaded portion  26  of the bone screw may be secured within the vertebrae as is known in the art. As noted above, due to vertebral physiology, non-idealized placement of the anchors  22  and/or other reasons, the outer surfaces  62 ,  66  of the connectors  50  may require some angulation to ensure improved contact between the spacers  52  and connectors  50 . Once the anchors  22  have been secured to the vertebrae  14 ,  16 ,  18 , the angles γ, θ of the outer surfaces  62 ,  66  of each of the connectors  50  may be calculated or determined, in a manner generally known in the art, that will provide increased contact between the end faces  98 ,  100  of spacers  52  and the outer surfaces  62 ,  66  of the connectors  50 . 
         [0043]    Once the angles for the outer surfaces  62 ,  66  of each of the connectors  50  have been determined, the flexible assembly  24  may be constructed. In one aspect of the invention, because the connectors  50  are separate elements or components from the anchors  22 , the flexible assembly  24  may be constructed prior to being inserted into the patient through the surgical site. Thus, as shown in  FIG. 4 , the connectors  50  (having the pre-determined angulation of their outer surfaces), and spacers  52  may be slidably mounted on the cord  48 . In addition, the various adjustments to the flexible assembly  24  to effect treatment of the spine  12  may be made thereto prior to the insertion of the flexible assembly into the patient. Thus, for example, the length of the spacers  52 , the relative positions of the connectors  50 , the tension in the cord  48 , and/or other design features may all be set while the flexible assembly  24  is outside the body of the patient. 
         [0044]    In this regard, the connectors  50  may be secured relative to the cord  48  so as to spatially fix the components of the flexible assembly  24 . To this end, and as best shown in  FIG. 4B , each of the connectors  50  include a threaded bore  106  that extends from the upper surface  90  of the connector body  54  to the channel  70  that receives the cord  48  therethrough. As shown in  FIG. 3 , a set screw  108  is received in the threaded bore  106  and may be rotated in a conventional manner so that an end of the set screw  108  engages the cord  48  to secure the connector  50  thereto and prevent relative movement therebetween. Accordingly, as illustrated in  FIG. 5 , the design configuration of the flexible construct  24  may be completed outside the body of the patient. Moreover,  FIG. 5  also illustrates that in the design configuration, i.e., the pre-tensioning of the cord  48 , length of spacers  52 , etc. have all been completed, the end faces  98 ,  100  of the spacers  52  mate with the outer surfaces  62 ,  66  of the connectors  50  over a relatively large contact area. For example, the end faces  98 ,  100  mate with the outer surfaces  62 ,  66  of the connectors  50  over substantially the entire surface area of the end faces  98 ,  100 . The enhanced contact area provides improved load transmission through the stabilization system  10  and to the vertebrae  14 ,  16 ,  18  to which the system is attached. 
         [0045]    Once the flexible assembly  24  has been constructed and configured for operation with the stabilization system  10 , the flexible assembly  24  may be removably coupled to the anchors  22 , which have already been coupled to the vertebrae, to complete the construction of the stabilization system  10 . To this end, and in another aspect of the invention, the flexible construct  24  may be coupled to the anchors  22  in a top load manner. In reference to  FIGS. 1 and 6 , the connectors  50  on the flexible assembly  24  are aligned with the U-shaped head portions  30  of the anchors  22  and moved downward in a generally anterior direction relative to the spine  12 . As the flexible assembly  24  is moved in the anterior direction, the legs  34 ,  36  of the head portions  30  engage the cutouts  76 ,  78  so that the intermediate body portion  60  of the connectors  50  is seated within the channel  38  of the head portions  30 . 
         [0046]    When the side surfaces  40 ,  42  of the legs  34 ,  36  are orthogonal to the base surface  44  and the side surfaces  80 ,  82  of the intermediate body portion  60  are orthogonal to lower surface  84 , the channel  38  closely receives the intermediate body portion  60 . However, when the channel  38  and intermediate body portion  60  have the converging configuration as discussed above, the legs  34 ,  36  initially flex outward as the connectors  50  are moved into the channels  38 . As the connectors  50  are further moved in the anterior direction, however, the legs  34 ,  36  spring back to essentially pull the connectors  50  into the channels  38  in a snap-fit manner. Moreover, because the legs  34 ,  36  converge, the legs  34 ,  36  at least provisionally secure the connectors  50  with the anchors  22  to provide some level of resistance to the movement of the connectors  50  in a posterior direction and away from the anchors  22 . 
         [0047]    As illustrated in  FIGS. 2 and 6 , when the connectors  50  of the flexible assembly  24  are positioned within the channels  38  of the head portions  30  of the anchors  22 , the connectors  50  may be secured or further secured with the anchors  22  to prevent any relative movement of the connectors  50  relative to the anchors  22 . To this end, a retainer in the form of a retaining clip  110  may be used to achieve the securement of the connectors  50  to the anchors  22 . The retaining clip  110  may be generally rectangular having a rectangular aperture  112  that receives the second ends  37  of the legs  34 ,  36  therethrough. To secure the retaining clip  110  to the ends  37  of the legs  34 ,  36 , the legs  34 ,  36  may include retaining notches  114  along outer side surfaces  116 . Other retainers include screws, bolts, pins, adhesives and the like. 
         [0048]    In this way, when a connector  50  is positioned in the channel  38  of head portion  30  of anchor  22 , the second ends  37  of legs  34 ,  36  may be squeezed or pushed together so as to be inserted through the aperture  112  in retaining clip  110 . Once through the aperture  112 , the legs  34 ,  36  may be released so that the side edges of the clip  110  are positioned in the retaining notches  114 . When so coupled, the retaining clip  110  is adjacent the upper surface  88  of the connector  50  and prevents movement of the connector  50  in a posterior direction away from the anchor  22 . 
         [0049]    Embodiments of the stabilization system as described herein and in accordance with the invention provide a number of improvements over current stabilization systems. For example, embodiments of the invention permit the flexible assembly  24  to be constructed outside the body of the patient and then subsequently coupled to the anchors in situ. Such an arrangement allows the pre-tensioning of the cord, spacer length, and other design aspects of the flexible assembly to be done prior to insertion into the body. This may facilitate the use of such dynamic stabilization systems with minimally invasive surgical techniques and therefore gain the benefits of those surgical techniques. 
         [0050]    Embodiments of the invention described herein also improve the load transmission efficiency of dynamic stabilization systems in those cases where ideal contact between the connectors and spacers may not be achieved. By selectively angling the surfaces of the connectors, the contact area between the spacers and connectors may be enhanced to improve the ability of the stabilization systems to transmit loads to the underlying vertebrae to which the systems are attached. 
         [0051]    While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the inventor 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. The various features of the invention may be used alone or in numerous combinations depending on the needs and preferences of the user.