Abstract:
A spinal cross-connector for connecting two stabilization rods installed in a patient&#39;s spine is provided. The cross-connector includes novel rod attachment elements dynamically connected together by connector elements. The cross-connector provides multi-dimensional adjustability for easy and accurate installation with full lock-down.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 11/427,738, filed on Jun. 29, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 11/436,407, filed on May 17, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 11/033,452, filed on Jan. 10, 2005, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 11/006,495, filed on Dec. 6, 2004, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 10/970,366, filed on Oct. 20, 2004, entitled “Systems and methods for posterior dynamic stabilization of the spine”. U.S. patent application Ser. No. 11/427,738, filed on Jun. 29, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine” is a continuation-in-part of U.S. patent application Ser. No. 11/362,366, filed on Feb. 23, 2006, entitled “Systems and methods for stabilization of bone structures”, which claims priority to U.S. Provisional Patent Application Ser. No. 60/701,660, filed on Jul. 22, 2005, entitled “Systems and methods for stabilization of bone structures”. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/726,093, filed on Mar. 20, 2007, entitled “Screw systems and methods for use in stabilization of bone structures”, which is a continuation-in-part of U.S. patent application Ser. No. 11/586,849, filed on Oct. 25, 2006, entitled “Systems and methods for stabilization of bone structures”, which is a continuation-in-part of U.S. patent application Ser. No. 11/362,366, filed on Feb. 23, 2006, entitled “Systems and methods for stabilization of bone structures”, which claims priority to U.S. Provisional Patent Application Ser. No. 60/701,660, filed on Jul. 22, 2005, entitled “Systems and methods for stabilization of bone structures”. U.S. patent application Ser. No. 11/726,093, filed on Mar. 20, 2007, entitled “Screw systems and methods for use in stabilization of bone structures” is also a continuation-in-part of U.S. patent application Ser. No. 11/427,738, filed on Jun. 29, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 11/436,407, filed on May 17, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 11/033,452, filed on Jan. 10, 2005, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 11/006,495, filed on Dec. 6, 2004, entitled “Systems and methods for posterior dynamic stabilization of the spine”, which is a continuation-in-part of U.S. patent application Ser. No. 10/970,366, filed on Oct. 20, 2004, entitled “Systems and methods for posterior dynamic stabilization of the spine”. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/726,093, filed on Mar. 20, 2007, entitled “Screw systems and methods for use in stabilization of bone structures”, which is a continuation-in-part of U.S. patent application Ser. No. 11/586,849, filed on Oct. 25, 2006, entitled “Systems and methods for stabilization of bone structures”, which is a continuation-in-part of U.S. patent application Ser. No. 11/362,366, filed on Feb. 23, 2006, entitled “Systems and methods for stabilization of bone structures”, which claims priority to U.S. Provisional Patent Application Ser. No. 60/701,660, filed on Jul. 22, 2005, entitled “Systems and methods for stabilization of bone structures”. All of the above applications are claimed for their benefit of priority and are further incorporated herein by reference in their entirety. 
     
    
     FIELD  
       [0002]     The present invention is directed towards the treatment of spinal disorders and pain. More particularly, the present invention is directed to systems and methods of treating the spine which reduce pain and enable spinal motion, and which effectively mimic that of a normally functioning spine.  
       BACKGROUND  
       [0003]      FIGS. 1A and 1B  illustrate a portion of the human spine having a superior vertebra  2  and an inferior vertebra  4 , with an intervertebral disc  6  located in between the two vertebral bodies. The superior vertebra  2  has superior facet joints  8   a  and  8   b , inferior facet joints  10   a  and  10   b , posterior arch  16  and spinous process  18 . Pedicles  3   a  and  3   b  interconnect the respective superior facet joints  8   a ,  8   b  to the vertebral body  2 . Extending laterally from superior facet joints  8   a ,  8   b  are transverse processes  7   a  and  7   b , respectively. Extending between each inferior facet joint  10   a  and  10   b  and the spinous process  18  are lamina  5   a  and  5   b , respectively. Similarly, inferior vertebra  4  has superior facet joints  12   a  and  12   b , superior pedicles  9   a  and  9   b , transverse processes  11   a  and  11   b , inferior facet joints  14   a  and  14   b , lamina  15   a  and  15   b , posterior arch  20 , spinous process  22 .  
         [0004]     The superior vertebra with its inferior facets, the inferior vertebra with its superior facets, the intervertebral disc, and seven spinal ligaments (not shown) extending between the superior and inferior vertebrae together comprise a spinal motion segment or functional spine unit. Each spinal motion segment enables motion along three orthogonal axes, both in rotation and in translation. The various spinal motions are illustrated in  FIGS. 1C-1E . In particular,  FIG. 1C  illustrates flexion and extension motions and axial loading,  FIG. 1D  illustrates lateral bending motion and translation, and  FIG. 1E  illustrates axial rotational motion. A normally functioning spinal motion segment provides physiological limits and stiffness in each rotational and translational direction to create a stable and strong column structure to support physiological loads.  
         [0005]     Traumatic, inflammatory, metabolic, synovial, neoplastic and degenerative disorders of the spine can produce debilitating pain that can affect a spinal motion segment&#39;s ability to properly function. The specific location or source of spinal pain is most often an affected intervertebral disc or facet joint, and in particular the nerves in and around the intervertebral disc or facet joint. Often, a disorder in one location or spinal component can lead to eventual deterioration or disorder, and ultimately, pain in another.  
         [0006]     Spine fusion (arthrodesis) is a procedure in which two or more adjacent vertebral bodies are fused together once the natural height of the degenerated disc has been restored. It is one of the most common approaches to alleviating various types of spinal pain, particularly pain associated with one or more affected intervertebral discs. However, fusion is only as good as the ability to restore disc height to relieve the pain by taking pressure off the nerves, nerve roots, and/or articulating surfaces—i.e., facet joints and end plates of the vertebral bodies.  
         [0007]     One way of accomplishing fusion is to install pedicles screws in adjacent vertebral bodies, followed by installation of fusion rods between the screws. This type of system can be strengthened by attaching a cross-connector between the fusion rods. In many current systems, however, attachment and deployment of such a cross-connector is difficult.  
         [0008]     With the limitations of current spine stabilization technologies, there is clearly a need for an improved means and methods for stabilization of the spine which addresses the drawbacks of prior devices. In particular, it would be highly beneficial to have a fusion stabilization system that has high strength and that enables the spine to mimic the motion of one or more healthier, uncompromised vertebral segments, especially with regard to torsional motions. It would be additionally beneficial if such a system could be conveniently installed and used to treat various spinal indications regardless of pain source, prevent or slow the deterioration of the intervertebral discs, or even restore disc height, and be used in conjunction with prosthetic intervertebral discs.  
       SUMMARY OF THE INVENTION  
       [0009]     According to one aspect of the invention, a spinal stabilization system is provided. The system includes a first rod attachment element configured to connect to a first vertebral stabilization rod. A second rod attachment element configured to connect to a second vertebral stabilization rod. The system includes a first bar attached to the first rod attachment element and a second bar attached to the second rod attachment element. The system includes a connector connecting the first and second bars. At least one rod attachment element has a two-part design such that one part of the two-part design contacts one portion of a corresponding rod and the other part of the two-part design contacts another portion of the corresponding rod to capture the rod.  
         [0010]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to connect to a first vertebral stabilization rod. The first rod attachment element includes a first biasing section. The system includes a second rod attachment element configured to connect to a second vertebral stabilization rod. The second rod attachment element includes a second biasing section. The system includes a first bar connected to the first rod attachment element and a second bar connected to the second rod attachment element. A connector is provided that connects the first and second bars. At least the first rod attachment element further comprises a rod-contacting surface and a corresponding screw having a head with a cam section such that rotation of the screw having a head with a cam section into the rod attachment element forces the cam section towards the rod-contacting surface capturing the rod between the rod-contacting surface and the cam section.  
         [0011]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to connect to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector configured to connect the first and second rod attachment elements is also provided. At least one rod attachment element has a two-part design such that one part of the two-part design contacts one portion of a corresponding rod and the other part of the two-part design contacts an other portion of the corresponding rod to capture the rod.  
         [0012]     According to another aspect of the invention, a method for stabilizing a patient&#39;s spine is provided in which a first set of two pedicle screw systems is installed into a superior vertebral segment. A second set of two pedicle screw systems is installed into an inferior vertebral segment. A first rod is connected between one of the pedicle screw systems in the first set and one of the pedicle screw systems in the second set. A second rod is connected between the other of the pedicle screw systems in the first set and the other of the pedicle screw systems in the second set. A first rod attachment element is connected to the first rod and a second rod attachment element is connected to the second rod. A first bar is connected to the first rod attachment element and a second bar is connected to the second rod attachment element. A cross connector is connected to both the first bar and the second bar. At least one rod attachment element has a two-part design such that one part of the two-part design contacts one portion of a corresponding rod and the other part of the two-part design contacts an other portion of the corresponding rod to capture the rod.  
         [0013]     According to another aspect of the invention, a method for stabilizing a patient&#39;s spine is provided in which a first set of two pedicle screw systems is installed into a superior vertebral segment. A second set of two pedicle screw systems is installed into an inferior vertebral segment. A first rod is connected between one of the pedicle screw systems in the first set and one of the pedicle screw systems in the second set. A second rod is connected between the other of the pedicle screw systems in the first set and the other of the pedicle screw systems in the second set. A first rod attachment element is connected to the first rod and a second rod attachment element is connected to the second rod. A cross-connector is connected between the first and second rod attachment elements. At least one rod attachment element has a two-part design such that one part of the two-part design contacts one portion of a corresponding rod and the other part of the two-part design contacts another portion of the corresponding rod.  
         [0014]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. The system includes a cross-connector configured to connect the first and second rod attachment elements. At least one rod attachment element has a two-part design such that one part of the two-part design contacts one portion of a corresponding rod and the other part of the two-part design contacts an other portion of the corresponding rod to capture the rod. The one and the other parts of the two-part design move relative to each other upon the tightening of a screw.  
         [0015]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector configured to connect the first and second rod attachment elements is provided. At least one rod attachment element has a two-part design such that a first part of the two-part design contacts a circumferential portion of a corresponding rod along a portion thereof, and the other part of the two-part design contacts a portion of the first part. The one and the other parts of the two-part design move relative to each other upon the tightening of a screw, and movement of the other part causes the first part to tighten around the circumferential portion of the rod.  
         [0016]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector coupled between the first and second rod attachment elements is provided. The first and second rod attachment elements define openings for capturing first and second vertebral stabilization rods, and the openings face in a substantially anterior direction when the rods are being captured.  
         [0017]     According to another aspect of the invention, a method for stabilizing a patient&#39;s spine is provided. The method includes the step of installing a first set of two pedicle screw systems into a superior vertebral segment. A second set of two pedicle screw systems is installed into an inferior vertebral segment. A first rod is connected between one of the pedicle screw systems in the first set and one of the pedicle screw systems in the second set. A second rod is connected between the other of the pedicle screw systems in the first set and the other of the pedicle screw systems in the second set. A first rod attachment element is connected to the first rod, and a second rod attachment element is connected to the second rod. At least one of the first and second rod attachment elements is connected by moving the rod attachment element, having an anteriorly-facing opening, towards the rod, such that the rod enters the opening in the rod attachment element in a posterior direction.  
         [0018]     According to another aspect of the invention, a method for stabilizing a patient&#39;s spine is provided. In the method, a first set of two pedicle screw systems is installed into a superior vertebral segment. A second set of two pedicle screw systems is installed into an inferior vertebral segment. A first rod is connected between one of the pedicle screw systems in the first set and one of the pedicle screw systems in the second set. A second rod is connected between the other of the pedicle screw systems in the first set and the other of the pedicle screw systems in the second set. A first rod attachment element is connected to the first rod, and a second rod attachment element is connected to the second rod. A cross-connector is connected between the first and second rod attachment elements. A screw is provided in the cross-connector. The method includes the step of and the cross-connector is configured such that tightening one screw in the cross-connector such that the one tightening prevents all polyaxial and/or translational movement of the cross-connector relative to the first and second rod attachment elements.  
         [0019]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector attached between the first and second rod attachment elements is provided. The system is configured such that the cross-connector is displaced by a predetermined distance in a posterior direction relative to a point where at least one rod attachment element attaches to a bar. The displacement accommodates the shape of the anatomy and bridges anatomy located anterior of the cross-connector.  
         [0020]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector attached between the first and second rod attachment elements is provided. The first rod attachment element defines an interior surface that is configured to encompass a rod along as great a percentage of a circumference of the rod as possible while allowing the rod to be snap-fit into the rod attachment element.  
         [0021]     According to another aspect of the invention, a method for stabilizing a patient&#39;s spine is provided. In the method, a first set of two pedicle screw systems is installed into a superior vertebral segment and a second set of two pedicle screw systems is installed into an inferior vertebral segment. A first rod is connected between one of the pedicle screw systems in the first set and one of the pedicle screw systems in the second set. A second rod is connected between the other of the pedicle screw systems in the first set and the other of the pedicle screw systems in the second set. A first rod locking procedure is performed by connecting a first rod attachment element to the first rod in a snap-fit manner. A second rod locking procedure is performed by connecting a second rod attachment element to the second rod in a snap-fit manner.  
         [0022]     According to another aspect of the invention, a centered spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector attached between the first and second rod attachment elements is provided. The first rod attachment element defines a channel having a rod-contacting surface that is configured to engage a rod and to center the rod in the channel when the rod is fully engaged.  
         [0023]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element to attach to a first vertebral stabilization rod and a second rod attachment element to attach to a second vertebral stabilization rod. A cross-connector coupled between the first and second rod attachment elements is provided. The first and second rod attachment elements define a first and second channel having first and second rod-contacting surfaces for capturing first and second vertebral stabilization rods. At least one of the first rod-contacting surface and second rod-contacting surface includes a gripping surface along a portion thereof.  
         [0024]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector coupled between the first and second rod attachment elements is provided. The cross-connector is coupled to the first rod attachment element by a first screw and to the second rod attachment element by a second screw. At least one of these couplings includes a slot such that the corresponding screw can slide a distance along the slot prior to tightening of the screw to allow for variations in patient anatomy.  
         [0025]     According to another aspect of the invention, a method for stabilizing a patient&#39;s spine is provided. In the method, a first set of two pedicle screw systems is installed into a superior vertebral segment and a second set of two pedicle screw systems is installed into an inferior vertebral segment. A first rod is connected between one of the pedicle screw systems in the first set and one of the pedicle screw systems in the second set and a second rod is connected between the other of the pedicle screw systems in the first set and the other of the pedicle screw systems in the second set. A first rod attachment element is connected to the first rod and a second rod attachment element is connected to the second rod. A cross-connector is connected between the first and second rod attachment elements such that the cross-connector is coupled to the first rod attachment element by a first screw and to the second rod attachment element by a second screw, and at least one of the couplings includes a slot wherein the corresponding screw can slide a distance along the slot prior to tightening to allow for variations in patient anatomy.  
         [0026]     According to another aspect of the invention a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A cross-connector is configured to connect the first and second rod attachment elements and has a locked configuration and an unlocked configuration such that the cross-connector moves relative to at least one of the first and second rod attachment elements while in the unlocked configuration.  
         [0027]     According to another aspect of the invention, a spinal stabilization system for a patient is provided. The system includes a first rod attachment element configured to attach to a first vertebral stabilization rod and a second rod attachment element configured to attach to a second vertebral stabilization rod. A first bar is provided and connected to the first rod attachment element. The first bar extends towards the second rod attachment element. A second bar is provided and connected to the second rod attachment element and extends towards the first rod attachment element. One of the first or second bars overlaps at least a portion of the other of the first or second bars and a cross-connector is provided to connect the first and second bars together at the overlapping portion.  
         [0028]     Advantages of the invention may include one or more of the following. Devices according to embodiments of the invention may be easily installed once other spinal components are installed, such as screws, rods, dynamic elements, facet constructs, and so on. The cross-connector system allows ease of operator assembly and surgical placement, and allows multi-degree-of-freedom adjustability prior to final stabilization. The cross-connector system further allows repositioning in subsequently-performed procedures. Devices according to embodiments of the invention may have a low profile and be minimally invasive.  
         [0029]     Systems according to the invention may be employed to treat various spinal disorders and pain, including those involving degenerative disc disease, spinal stenosis, spondylolisthesis, spinal deformities, fractures, pseudarthrosis, tumors, failed fusions, arthritic facet joints, severe facet joint tropism, facet joint injuries, deformed facet joints, scoliosis, and other vertebral segment traumas and diseases.  
         [0030]     These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]     The invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to-scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures:  
         [0032]     FIGS.  1 (A)-(B) illustrate certain aspects of the anatomy of spinal segments.  
         [0033]     FIGS.  1 (C)-(E) illustrate various spinal movements that may be performed by the spinal segments of FIGS.  1 (A)-(B).  
         [0034]      FIG. 2 (A) illustrates a side schematic view of a cross-connector with accompanying rod attachment elements and rods according to a first embodiment of the invention.  
         [0035]      FIG. 2 (B) illustrates an exploded perspective view of a cross-connector according to the first embodiment of the invention.  
         [0036]      FIG. 3  illustrates a side view of a cross-connector according to the first embodiment of the invention.  
         [0037]      FIG. 4  illustrates a side schematic view, in partial cross-section, of a cross-connector system according to a second embodiment of the invention.  
         [0038]     FIGS.  5 (A)-(D) illustrate exploded and non-exploded perspective and side cross-sectional views of a cross-connector according to a third embodiment of the invention.  
         [0039]      FIG. 6  illustrates a perspective view of a cross-connector according to a fourth embodiment of the invention.  
         [0040]     FIGS.  7 (A)-(B) illustrate perspective and exploded views of a cross-connector according to a fifth embodiment of the invention.  
         [0041]      FIG. 8  illustrates a perspective exploded view of a cross-connector according to a sixth embodiment of the invention.  
         [0042]     FIGS.  9 (A)-(E) illustrate various views of a cross-connector system according to a seventh embodiment of the invention.  
         [0043]     FIGS.  10 (A)-(C) illustrate sectional perspective, top, and side views of the cross-connector system according to the seventh embodiment of the invention.  
         [0044]     FIGS.  11 (A)-(B) illustrate side sectional and perspective views of the cross-connector system according to the seventh embodiment of the invention.  
         [0045]      FIG. 12  illustrates an exploded perspective view of a cross-connector system according to an eighth embodiment of the invention.  
         [0046]     FIGS.  13 (A)-(C) illustrate sectional perspective, top, and side views of the cross-connector system according to the eighth embodiment of the invention.  
         [0047]     FIGS.  14 (A)-(C) illustrate top, bottom, and detailed views of the cross-connector system according to the eighth embodiment of the invention.  
         [0048]     FIGS.  15 (A)-(C) illustrate perspective and side views, in partial cross-section, of a cross-connector system according to a ninth embodiment of the invention.  
         [0049]     FIGS.  16 (A)-(C) illustrate more detailed views of the cross-connector system according to the ninth embodiment of the invention.  
         [0050]     FIGS.  17 (A)-(C) illustrate more detailed views of the cross-connector system according to the ninth embodiment of the invention.  
         [0051]     FIGS.  18 (A)-(D) illustrate more detailed views of the cross-connector system according to the ninth embodiment of the invention.  
         [0052]     FIGS.  19 (A)-(B) illustrate side and perspective exploded views of a cross-connector system according to a tenth embodiment of the invention.  
         [0053]     FIGS.  20 (A)-(C) illustrate more detailed views of the cross-connector system according to the tenth embodiment of the invention.  
         [0054]     FIGS.  21 (A)-(B) illustrate more detailed views of the cross-connector system according to the tenth embodiment of the invention.  
         [0055]      FIG. 22  illustrates a detailed view of an alternative cross-connector system related to the tenth embodiment of the invention.  
         [0056]     FIGS.  23 (A)-(B) illustrate side and perspective exploded views of a cross-connector system according to an eleventh embodiment of the invention.  
         [0057]      FIG. 24  illustrates a more detailed view of the cross-connector system according to the eleventh embodiment of the invention.  
         [0058]     FIGS.  25 (A)-(B) illustrate side and perspective exploded views of a cross-connector system according to a twelfth embodiment of the invention.  
         [0059]     FIGS.  26 (A)-(B) illustrate more detailed views of the cross-connector system according to the twelfth embodiment of the invention.  
         [0060]     FIGS.  27 (A)-(B) illustrate side and perspective exploded views of a cross-connector system according to a thirteenth embodiment of the invention.  
         [0061]     FIGS.  28 (A)-(C) illustrate more detailed views of the cross-connector system according to the thirteenth embodiment of the invention.  
         [0062]     FIGS.  29 (A)-(B) illustrate more detailed views of the cross-connector system according to the thirteenth embodiment of the invention.  
         [0063]      FIG. 30  illustrates a detailed view of an alternative cross-connector system related to the thirteenth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0064]     Before the subject devices, systems and methods are described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.  
         [0065]     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.  
         [0066]     It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a spinal segment” may include a plurality of such spinal segments and reference to “the screw” includes reference to one or more screws and equivalents thereof known to those skilled in the art, and so forth.  
         [0067]     Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.  
         [0068]     All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.  
         [0069]     The present invention will now be described in greater detail by way of the following description of exemplary embodiments and variations of the systems and methods of the present invention. While more fully described in the context of the description of the subject methods of implanting the subject systems, it should be initially noted that in certain applications where the natural facet joints are compromised, as illustrated in  FIG. 1 (A), inferior facets  10   a  and  10   b , lamina  5   a  and  5   b , posterior arch  16  and spinous process  18  of superior vertebra  2  may be resected for purposes of implantation of certain of the dynamic stabilization systems of the present invention. In other applications, where possible, the natural facet joints, lamina and/or spinous processes are spared and left intact for implantation of other dynamic stabilization systems of the present invention.  
         [0070]     It should also be understood that the term “system”, when referring to a system of the present invention, most typically refers to a set of components which includes a superior, cephalad or rostral (towards the head) component configured for implantation into a superior vertebra of a vertebral motion segment and an inferior or caudal (towards the feet) component configured for implantation into an inferior vertebra of a vertebral motion segment. A pair of such component sets includes one set of components configured for implantation into and stabilization of the left side of a vertebral segment and another set configured for the implantation into and stabilization of the right side of a vertebral segment. The left set of components may move independently of the right set of components or their motions may be coordinated via an attachment between the two. In other words, they may move in conjunction with one another, with both moving relative to the more fixed attachment between the two. Many of the systems disclosed here concern such an attachment between the two.  
         [0071]     Where multiple spinal segments or units are being treated, the term “system” may refer to two or more pairs of component sets, i.e., two or more left sets and/or two or more right sets of components. Such a multilevel system involves stacking of component sets in which each set includes a superior component, an inferior component, and one or more medial components therebetween. These multilevel systems may include cross member or cross connector components or strut systems having differing properties, e.g., lengths, limits on travel or other limited ranges of motion; resistance to motion or other forces, attachment locations, etc.  
         [0072]     The superior and inferior components (and any medial components therebetween), when operatively implanted, are engaged or interface with each other in a manner that enables the treated spinal motion segment to mimic the function and movement of a natural healthy segment. The disclosed systems include one or more structures or members which enable, limit and/or otherwise selectively control spinal motion. The structures may perform such functions by exerting various forces on the system components, and thus on the target vertebrae. The manner of coupling, interfacing, engagement or interconnection between the subject system components may involve compression, distraction, rotation or torsion, or a combination thereof. In certain embodiments, the extent or degree of these forces or motions between the components may be intraoperatively selected and/or adjusted to address the condition being treated, to accommodate the particular spinal anatomy into which the system is implanted, and to achieve the desired therapeutic result, such as to restore disc height and offset the facet joints.  
         [0073]     In certain embodiments, the superior and inferior components are mechanically coupled to each other by one or more interconnection or interfacing means. In other embodiments, the superior and inferior components interface in an engaging manner which does not necessarily mechanically couple or fix the components together but rather constrains their relative movement and also enables the treated spinal motion segment to mimic the natural function and movement of a healthy segment. Typically, the interconnecting means is a posteriorly-positioned component, i.e., one positioned posteriorly of the superior and inferior components, or it may be a laterally-positioned component, i.e., one positioned to the outer side of the posterior and inferior components. The structures may involve one or more strut systems and/or joints which provide for dynamic movement of a stabilized spinal motion segment.  
         [0074]     In this description, the following terms are used throughout, and are defined here. A “cross-connector system” is a device that extends between and attaches to two fixation or stabilization rods. A “rod attachment element” forms a portion of a cross-connector system, and is the portion of the cross-connector system that attaches to the rod. The portion of the cross-connector system, that is not the rod attachment element, is the cross connector itself.  
         [0075]     It is noted that the following patent applications, owned by the assignee of the present invention and incorporated herein by reference in their entirety for all purposes, disclose various dynamic rod systems, pedicle screw systems, and facet augmentation systems that may be employed in conjunction with the current invention: U.S. patent application Ser. No. 11/427,738, filed on Jun. 29, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine”; U.S. patent application Ser. No. 11/436,407, filed on May 17, 2006, entitled “Systems and methods for posterior dynamic stabilization of the spine”; U.S. patent application Ser. No. 11/033,452, filed on Jan. 10, 2005, entitled “Systems and methods for posterior dynamic stabilization of the spine”; U.S. patent application Ser. No. 11/006,495, filed on Dec. 6, 2004, entitled “Systems and methods for posterior dynamic stabilization of the spine”; U.S. patent application Ser. No. 10/970,366, filed on Oct. 20, 2004, entitled “Systems and methods for posterior dynamic stabilization of the spine”; U.S. patent application Ser. No. 11/726,093, filed on Mar. 20, 2007, entitled “Screw systems and methods for use in stabilization of bone structures”; U.S. patent application Ser. No. 11/586,849, filed on Oct. 25, 2006, entitled “Systems and methods for stabilization of bone structures”; U.S. patent application Ser. No. 11/362,366, filed on Feb. 23, 2006, entitled “Systems and methods for stabilization of bone structures”; U.S. Provisional Patent Application Ser. No. 60/701,660, filed on Jul. 22, 2005, entitled “Systems and methods for stabilization of bone structures”; all of which are incorporated by reference herein in their entirety.  
         [0076]     In many of the systems described, the adjustability of the system may be used to prevent undesired stress on spine and system components. The adjustability also provides for a simplified installation process. To this end, the requirements for precision on drilling locations, angles, etc., may be reduced. In the same way, an installation kit may be provided with a lesser number of components, as the provided components can accommodate more varying anatomy. Thus, in many systems, the components are aligned to the anatomy and then tightened.  
         [0077]      FIG. 2 (A) shows a high-level design of a top-loading cross-connector system  51  in side view. The cross-connector system  51  includes a cross connector  50  and two rod attachment elements  52   a  and  52   b . Bars  68   a  and  68   b  extend from the rod attachment elements  52   a  and  52   b  toward a connector  56 . Two rods  70   a  and  70   b  are shown as well, with the cross-connector system  51  in a loading position above the rods. As may be seen in the figure, rod attachment elements  52   a  and  52   b  each have arch-shaped channels  53   a  and  53   b  defined therein to engage rods  70   a  and  70   b , respectively.  
         [0078]     Bars  68   a  and  68   b  may be individually or collectively formed integral with or pre-attached to either rod attachment elements  52   a  and  52   b , or with connector  56 , or both. This alternative is true for any of the embodiments described in this application, unless otherwise noted.  
         [0079]     In use, and in general for all of the systems, pedicle screw systems are installed in the pedicles of a patient, and rods  70   a  and  70   b , as well as additional rods or less rods as necessary, are installed between the pedicle screws. Once the rods are installed, one or more rod attachment elements are affixed to the rods. If the rod attachment elements include bars and/or connectors in a pre-attached or integral fashion, then the technique is completed following attachment of the rod attachment elements or bars to any unattached elements. If the rod attachment elements do not include bars and/or connectors in a pre-attached or integral fashion, then the bars are attached to the rod attachment elements. If at least one bar includes a connector in a pre-attached or integral fashion, then the technique is completed following any necessary affixation of the connector to either bar. If the bars do not include a connector in a pre-attached or integral fashion, then the connector is attached to the bars, and the technique is completed. For this system and for the other cross-connector systems described, once an implantation procedure is completed, all components may be fixed relative to one another, preventing relative motion. Alternatively, one or more components may be secured and yet relative motion may still be allowed, such as resiliently-biased motion via a dynamic element or otherwise. Motions may also be permitted such as simple rotation or sliding motions, such as via a spring-biased attachment. One example of this may be the sliding swivels described in embodiments below. When the cross-connector components are tightened, all swivel motion may be prevented or alternatively some limited motions may continue to be allowed, in one or more directions.  
         [0080]      FIG. 2 (B) shows a cross-connector system  51  in more detail. The cross-connector system  51  includes rod attachment elements  52   a  and  52   b , as well as a cross connector  50 . A two-part rod attachment element  52   a  includes (the rod attachment element  52   b  has similar corresponding elements, though each rod attachment element may be different if dictated by the application) a clamp assembly having a biasing section or hook  76   a  and a pivoting clamp  78   a.    
         [0081]     In this and in other embodiments, the “two-part rod attachment element” refers to a rod attachment element in which one element or a portion of one element is moved toward another element in order to secure a rod therebetween, but generally neither of these elements, in this embodiment, is a screw, though a screw may be used to move one element towards another element. Moving one element towards another may include either translation or rotational motion or both. Moving one element towards another may tend to provide a clamping action or the like. It should be noted that “moving one element towards another” refers to any action that can secure a rod, such as the above described clamping action. Indeed, the actual amount that elements are moved towards each other may be minimal (and parts of the elements may even move away from each other), and substantial movement is not required. In another embodiment, described below, one of the elements is a screw with a cam portion.  
         [0082]     A locking screw  82   a  serves to, upon installation, force the pivoting clamp  78   a  towards hook  76   a , thereby clamping the hook against the pivoting clamp and thus securing the rod attachment element to the rod  70   a . It should be noted that the hook need not clamp against the pivoting clamp in an extreme sense; it may be sufficient that the hook pivots to reduce the internal diameter of the arch-shaped channels such that the hook and pivoting clamp capture and apply force to the rod.  
         [0083]     In more detail, the hook  76   a  includes two side pieces  57   a  and  59   a  joined at a central piece  61   a . Hingedly attached to the hook  76   a  is the pivoting clamp  78   a . A hinge  86   a  is shown in  FIG. 3  to demonstrate this attachment. Also shown in  FIG. 3  is the hole  63   a  defined in pivoting clamp  78   a  into which the screw  82   a  is installed. As may be seen in that figure, a bottom surface  69   a  of the head of the screw  82   a  forms an angle  65   a  with the top surface  67   a  of the hook  76   a . Downward translation of the screw  82   a  forces surfaces  69   a  and  67   a  together (reducing angle  65   a ), and causes rotation of the hook  76   a  in the direction indicated by arrow  80 , the rotation being about the hinge  86   a . When the hook  76   a  rotates in this direction, a rod-contacting portion  88   a  of the rod attachment element clamps down on the rod  70   a , securing it against movement. The rod-contacting portion  88   a  of the rod attachment element preferably encircles the rod  70   a  by greater than 180°.  
         [0084]     As noted, bar  68   a  extends from a central section  55   a  of the rod attachment element  52   a , and may be integral with or pre-attached to the same. The central section is installed within the biasing or hook section  76   a , but may also be integral therewith.  
         [0085]     In an alternative embodiment, the bar  68   a  may be installed in (e.g. screwed into) rod attachment element  52   a  after rod attachment element  52   a  is attached to the rod  70   a . Bar  68   a  includes a swivel  72   a  that translates along a groove  84   a . The swivel  72   a , at its radial extreme, approximates a spherical shape. When disposed in a corresponding approximately-spherical cavity within opening  74   a  in cross connector  56 , the swivel&#39;s shape allows a degree of polyaxial movement or adjustment of each rod attachment element relative to the cross connector. For example, the rod attachment element may rotate about an axis  60  parallel to the longitudinal axis of the cross connector. This may be particularly important when accommodation is necessary for non-parallel rods. Of course, following such polyaxial adjustment, the system may be tightened down, prohibiting future movements, for most fusion procedures. As another adjustment mechanism, the groove  84   a  allows a constrained degree of translational movement and adjustment along axis  60 . That is, the bar may move in or out of the swivel, to accommodate various spacings between rods.  
         [0086]     The cross connector  56  includes a connector top  62  and a connector bottom  58 . The connector top  62  and the connector bottom  58  engagedly mate and are affixed via an bar clamping screw  64  which is installed through a hole  66   a  defined in connector top  62  and which is threaded into a threaded hole  66   b  defined in connector bottom  58 . The bar clamping screw or the threaded hole may be provided with an anti-rotation feature, such as a nylon insert or metal swage.  
         [0087]     Once the bar clamping screw is installed in holes  66   a  and  66   b , further movement of the swivels along the groove, as well as polyaxial motion, may be prohibited. Alternatively, installation of the bar clamping screw  64  may only serve to prevent removal, while allowing one or both of these motions.  
         [0088]     While the above embodiment has been described with respect to the rod attachment element with elements having “a” suffixes, a similar description applies to the rod attachment element with elements having “b” suffixes.  
         [0089]     In use, after the rods  70   a  and  70   b  are attached to the installed pedicle screws, the rod attachment elements above may be attached to the rods by placing the rods against rod-contacting portion  53   a  (a corresponding portion,  53   b , is not shown), and tightening screws  82   a  and  82   b  into holes  63   a  and  63   b  (hole  63   a  is indicated in  FIG. 3 ). The bars  68   a  and  68   b  along with swivels  72   a  and  72   b  may then be disposed in the voids  74   a  and  74   b  of the cross connector  56 , i.e., in the connector bottom  58 , with the swivels located along the grooves  84   a  and  84   b . The connector top  62  is then placed above the connector bottom  58  and the screw  64  is inserted through the hole  66   a  and is threaded into hole  66   b . Once tightened, the system is secured and the procedure concluded. In an alternative embodiment, the connector top and connector bottom are secured first, or partially secured first, and then the rod attachment elements are secured to the rods.  
         [0090]      FIG. 4  shows a second embodiment of the invention, with some elements in common with the embodiment of  FIGS. 2-3 . A cross-connector system  90  is shown with two-part rod attachment elements  92   a  and  92   b , and a cross connector  91 . In this embodiment, the cross connector  91  includes a dynamic element  114 . The dynamic element  114  may include any type of element that can provide a degree of motion to the cross connector  91 , including the types of dynamic elements disclosed in U.S. patent Ser. No. 11/427,738. For example, the dynamic element may provide a resilient bias, such as with a flexible portion or a spring. One or more characteristics of dynamic element  91  may be adjustable (adjustment means not shown but may be, e.g., a rotatable set screw), such as an adjustment to the range of motion and/or a force applied to resist motion.  
         [0091]     The cross connector  91  further includes depending cylindrical projections  112   a  and  112   b , these depending from opposite sides of the dynamic element  114 . Into each cylindrical projection  112   a  and  112   b  may be placed corresponding bars  106   a  and  106   b , respectively. As in the first embodiment, the bars  106   a  and  106   b  have disposed thereon swivels  108   a  and  108   b . The swivels  108   a  and  108   b  may slide along, and/or pivot within, a groove as in the first embodiment (not shown in  FIG. 4 ).  
         [0092]      FIG. 4  shows the bars  106   a  and  106   b  as threadingly engaging the rod attachment elements  92   a  and  92   b , though may also be constructed integral to the same. If threadingly engaged, they may be pre-attached before the surgical installation procedure or attachment may be contemporaneous, during the surgical installation procedure.  
         [0093]     In  FIG. 4 , the rod attachment element  92   a  is displayed as being of a different construction from the rod attachment element  92   b . In more detail, the rod attachment element  92   a  includes dual screws  94   a  and  96   a  that may be employed to grasp a rod at rod-contacting surface  116   a . While not indicated in  FIG. 4 , they may act in a way similarly to that of  FIG. 3 , in which the bottom surface of the screw head contacting the top surface of the rod attachment element causes a pivoting action, closing the rod attachment element around the rod. In an alternative embodiment (not shown), the two screws  94   a  and  96   a  may each contact an interior surface of the clamp assembly. This contact may then deflect the contacted surface in a way to clamp around the rod. The screw  94   a  deflects the left side and screw  96   a  deflects the right side. In any case, the two screws  94   a  and  96   a  may cause rod-contacting portion  116   a  to close in a complimentary fashion around the rod.  
         [0094]     The rod attachment element  92   a  is also shown with a drug delivery element  104 . The drug delivery element  104  may be appropriately configured to provide a time-release of, e.g., an antibiotic drug, and may be refillable via an injection port integral to drug delivery element  104  (injection port not shown). Such a drug delivery element may be provided or performed on any of the described embodiments.  
         [0095]     The rod attachment element  92   b  also has some similarities to the rod attachment element  52   b , with the following differences. First, either the screw  94   b  or the rod attachment element  92   b  may be provided with a nylon insert  98  to provide an anti-rotation function. The nylon insert  98  may be replaced with a metal swage or the like to perform a similar function. The rod attachment element  92   b  also incorporates a cover  102  to cover the head of the screw  94   b . Such a contamination cover may be provided on any of the described embodiments. The cover  102  may be replaced with a degree of filling of the hole, such as by an elastomer. Either will serve to help prevent tissue in-growth, or the ingress of other forms of contamination. Keeping this area free of contamination may provide significant assistance in post-procedural adjustment or removal. Another difference between the rod attachment elements  92   b  and  52   b  is that  92   b  uses a single screw  94   b  to activate clamping function.  
         [0096]     In use, after the rods  70   a  and  70   b  are attached to the installed pedicle screws, the rod attachment elements above may be attached to the rods by placing the rods against rod-contacting portions  116   a  and  116   b , and tightening screws  94   a ,  94   b , and  96   a  into their respective holes. The bars  106   a  and  106   b  along with swivels  108   a  and  108   b  may then be disposed in the voids of the cross connector  91  in any of the manners disclosed above or below. The bars may be pre-installed in the cross connector during, e.g., the time of construction of the dynamic element. Once tightened, the system is secured and the procedure concluded. In an alternative embodiment, the bars are secured to the cross connector first, and then the rod attachment elements are secured to the rods. In another alternative embodiment, dynamic element  114  is adjusted such as at a time prior to, during and/or after implantation of cross-connector system  90 .  
         [0097]      FIG. 5 (A)-(D) illustrates a cross-connector system  120  according to a third embodiment of the invention, this embodiment incorporating certain features of the aforedescribed embodiments.  
         [0098]     In  FIG. 5 (A), two stabilizing rods  110   a  and  110   b  are engaged by two corresponding two-part rod attachment elements  118   a  and  118   b . The rod attachment elements  118   a  and  118   b  each have a biasing section or hook section  126   a  and  126   b , respectively, which operate in conjunction with sliding clamps  128   a  and  128   b  to grasp rods  110   a  and  110   b.    
         [0099]     In more detail, sliding clamps  128   a  and  128   b  each have corresponding hook-engaging elements  132   a  and  132   b  (see  FIG. 5 (D)) which are slidingly received by corresponding holes defined in the hook sections  126   a  and  126   b . At the opposite end of each of sliding clamps  128   a  and  128   b  is a section defining an upwardly-facing recess  134   a  and  134   b . Two rod-locking screws  138   a  and  138   b  are provided to tighten the sliding clamps  128   a  and  128   b  to the hook sections  126   a  and  126   b , and this tightening is accomplished by the rod-locking screws  138   a  and  138   b  each being installed in holes  142   a  and  142   b  and then respectively engaging the recesses  134   a  and  134   b  (note  134   b  is not shown in the figure). That is, once the hook-engaging elements  132   a  and  132   b  are slidingly received by the corresponding holes defined in the hook sections  126   a  and  126   b , they cannot be forced downward any further, and the downward pressure of the rod-locking screws  138   a  and  138   b  then serves to frictionally engage and make secure the connection between the hook sections and the clamps, as well as closing around the rods. The presence of the recesses tends to secure the clamps in a predetermined position relative to the hook sections, this predetermined position chosen to ensure sufficient force is applied against the rods  110   a  and  110   b  to secure the same against movement.  
         [0100]     The cross connector  121  as shown in  FIG. 5 (C) includes a body section  123  from which depends two clamp sections  122   a  and two clamp sections  122   b . The two clamp sections  122   a  and two clamp sections  122   b  each form a “C” clamp, and each has a hole defined therein through which screws  124   a  and  124   b  may be inserted to tighten the respective clamp sections. A swivel  144   b  is shown in  FIG. 5 (C) in a position in which the same may be inserted into the cross connector  121 . A bore of the swivel allows entry into the swivel of a bar. Following insertion, the swivel  144   b  may be rotated such that it can no longer be removed from the cross connector  121  under normal motions encountered by the cross-connector system  120  in normal patient use.  
         [0101]     In use, after the rods  110   a  and  110   b  are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by placing the rods between the hook and the clamp sections, and tightening screws  138   a  and  138   b  into holes  142   a  and  142   b . The bars  136   a  and  136   b  along with swivels  144   a  and  144   b  may then be disposed in the voids of the cross connector  121 . That is, the bars are inserted into the swivels, rotated, and then advanced into the voids of cross connector  121 &gt;. The screws  124   a  and  124   b  may then be installed and tightened, securing the bars and swivels against further movement. Once tightened, the system is secured and the procedure concluded. In an alternative embodiment, the cross connector  121  is constructed and secured first, and then the rod attachment elements are secured to the rods.  
         [0102]      FIG. 6  shows an embodiment of the invention, similar to that of  FIG. 5 (A)-(D), in which a single screw provides the compressive force. In particular, two rods  110   a  and  110   b  are attached to two two-part rod attachment elements  131   a  and  131   b , respectively, via two respective screws  138   a  and  138   b . Of course, other attachment mechanisms can also be employed. The rod attachment elements  131   a  and  131   b  each have a corresponding bar  127   a  and  127   b , which form a part of a cross-connector system  141 . The cross-connector system  141  also includes a cross connector portion  146 , formed of a wrap-around partial cylindrical portion  147  which is attached in its general mid-section to a top projecting portion  152   a  and a bottom projecting portion  152   b , which when forced together by a screw  148  tends to frictionally hold the bars  127   a  and  127   b  in a predetermined and desired relationship.  
         [0103]     In use, the system of  FIG. 6  is constructed in a manner similar to that of  FIG. 5 (A)-(D), except that only one bar clamping screw need be tightened.  
         [0104]     FIGS.  7 (A) and  7 (B) show a related embodiment, in which a two-piece cross connector  139  includes an upper housing  159  with a projecting mid-portion. While the reference numerals for common elements remain the same as in  FIG. 6 , changed elements include the upper housing  159  which engages a lower housing  154 . The upper housing  159  and lower housing  154  are held together via a screw  162 . The screw  162  is inserted through a downwardly-projecting section  158  that defines a hole therethrough. The upper housing  159  has a first portion  156   a  which engages a bar  127   a  and a second portion  156   b  which engages a bar  127   b . In both cases, the first and second portions primarily engage their respective corresponding bars via contacting the swivels that are slid onto the bars, though the first and second portion may in some cases also contact the bars themselves. The screw  162  is inserted through a hole  158  in the upper housing  159  and is threaded into a threaded hole  161  in the lower housing  154 .  
         [0105]     In use, after the rods  110   a  and  110   b  are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by placing the rods between the hook and the clamp sections, and tightening screws  138   a  and  138   b  into their respective holes. The bars  127   a  and  127   b  along with their corresponding swivels may then be disposed in the voids of the cross connector  139 , i.e., in the lower housing  154 . The upper housing  159  is then placed above the lower housing  154  and the screw  162  is inserted through the hole  158  and is threaded into hole  161 . Once tightened, the system is secured and the procedure concluded. In an alternative embodiment, the upper and lower housings are secured first, and then the rod attachment elements are secured to the rods.  
         [0106]      FIG. 8  shows a related embodiment. Whereas the embodiment of FIGS.  7 (A) and (B) included an upper housing with a downwardly-projecting mid-portion where the downwardly-projecting mid-portion stabilizes internal components, the embodiment of  FIG. 8  includes a lower housing with a upwardly-projecting mid-portion, this upwardly-projecting mid-portion similarly capable of stabilizing internal components. In particular, the cross-connector system  149  includes an upper housing  166  and a lower housing  164 , the lower housing having a raised mid-portion  153  in which is defined a threaded hole  151 . A screw  168  is inserted through a hole  172  in the upper housing  166  and is threaded into the threaded hole  151 .  
         [0107]     In use, the system of  FIG. 8  is constructed in a manner similar to that of  FIG. 7 (A)-(B).  
         [0108]     FIGS.  9 (A)-(E) illustrate another embodiment of the invention. In this embodiment, as will be described, the rod attachment element snaps over a rod and a screw insertion closes a clamp around the rod. In addition, the bars include upper and lower bars with portions that overlap, slide and mate with each other. In this embodiment, the cross connector may be formed of a single piece that surrounds both the upper and lower bars at a single cross-sectional location. A screw may directly contact the bars, compressing them together.  
         [0109]     In more detail, and referring initially to  FIG. 9 (A), a cross-connector system  170  includes first and second rod attachment elements  174   a  and  174   b , each having a respective rod-contacting portion  194   a  and  194   b  for contacting rods (a rod  110   a  is shown in  FIG. 9 (D)). The rod-contacting portions may be generally sized such that the same contact the rods around as great a percentage of the rods as possible. Of course, if the rod-contacting portions are sized to extend around too great a circumference, the rods would not be able to be installed within the rod-contacting portions in a snap-fit fashion; in this case, pre-installation or pre-engagement would be necessary. Moreover, the rod-contacting portions are generally circular in cross-section. The radius of the circle described may be chosen such that the rod is automatically centered when the rod is installed in the rod attachment element. That is, in most embodiments the rod should not move around within the rod-contacting portion of the rod attachment element. In many cases, this means that the center of the rod-contacting portion and the center of the rod are substantially coincident when the rod is installed. The above features of the rod-contacting portions may be extended to various other embodiments in this description.  
         [0110]     The rod attachment elements  174   a  and  174   b  have corresponding bars  198   a  and  198   b . The bars  198   a  and  198   b  are configured to attach to their respective rod attachment elements at different heights, so that one may be slid on top of another when the two are each inserted through a hole  188  in a unitary cross connector  176 . The hole  188  may have an appropriate shape to allow a substantial clamping effect when an bar clamping screw  186  is threadingly inserted through a hole  192  defined in the top of the cross connector  176 . A distal end  187  of the screw  186  may mate with a corresponding recess in the top of the bar  198   a  (see  FIG. 10 (A)). The tolerances of the bars in the hole  188  may be such as to allow a degree of rotation, as best seen in  FIG. 9 (E). In other words, the bars need not be exactly collinear. The allowed rotation may be about an axis defined by a nub  202  in the bar  198   a  which mates with a recess  204  in the bar  198   b  (see  FIG. 10 (A)).  
         [0111]     As may be seen in  FIG. 9 (D), the cross-connector is displaced a certain distance in a posterior direction from a position where the rod attachment element attaches to a rod. This displacement allows the system to accommodate the shape of and bridge the anatomy in the vertebral region. That is, the bars  198   a  and  198   b  are above the line defined by the cross-sectional midpoints of rods  110   a  and  110   b  at the point of attachment to the cross-connector  170 . FIGS.  15 (A)-(C) show a related embodiment, where the bridge-like accommodation is provided by the cross-connecting forming a domed shape.  
         [0112]     Referring back to FIGS.  9 (A)-(D), the rod-contacting surfaces  194   a  and  194   b  may surround the rod by greater than 180°, and may be provided with a roughened surface or coating so as to enhance the same&#39;s grip on the rod. The roughened surface may be accomplished via grit-blasting the surface, defining knurling, serrations, or splines thereon, or the like, and the same may be provided or performed on any of the described embodiments. Besides increasing the grip, various other advantages may inure to embodiments including serrations or the like. For example, the removal of material from the rod-contacting surface may allow the rod to slide more easily due to decreased friction. Similarly, the removal may allow the rod attachment element, or its biasing or hook section, to flex more easily.  
         [0113]     Referring to  FIG. 9 (C), at an inner extremity  179  of the rod-contacting surface  194   a , two opposite-facing projections may be provided. Of course, the same may be provided on the rod attachment element  174   b . A first projection  175  may project in a direction such that the first projection  175  further circumferentially surrounds a rod  110   a  disposed adjacent the rod-contacting surface  194   a . The presence of this first projection  175  may also be such that the rod  110   a , when placed adjacent the rod-contacting surface  194   a , in fact “snap-fits” into the volume defined by the same. This snap-fit may in some cases be sufficient attachment of the rod attachment element to the rod. In many cases, however, this snap-fit will not be sufficient but will serve to help the clinician to precisely position and adjust the connector allowing movement of the connector relative to the rod before completely locking down the device to the rod.  
         [0114]     One way of increasing the grip of the rod attachment element on the rod is via use of a second projection  177 . Two screws  178   a  and  178   b  are provided, each with respective threads  184   a  and  184   b  and respective tapering portions  182   a  and  182   b , for insertion into the rod attachment elements  174   a  and  174   b  in holes  196   a  and  196   b  As best seen in  FIG. 9 (C), as the screw  178   a  is inserted into the hole  196   a , the leading edge of the screw, adjacent the tapering portion  182   a , contacts the second projection  177  and deflects the same in a direction away from the screw  178   a , i.e., towards the rod  110   a . In particular, the second projection  177  is deflected under the rod  110   a , surrounding the rod a greater angular distance and increasing the level of contact and pressure between the rod attachment element and the rod, further frictionally securing the rod against the rod attachment element.  
         [0115]     As noted above, a certain degree of rotation is allowed in the system to accommodate situations where the bars are required to be non-collinear. The amount of allowed rotation can vary and can be predetermined based on various factors, especially the width of the bars, their width at their distal tips, and the width of the cross connector  176 . This type of alignment, which may be intraoperative, is indicated by arrows  190  and  190 ′ in  FIG. 10 (B). The tolerances of the bars and the hole  188  may further allow for a degree of rotation out of the plane defined by arrows  190  and  190 ′, i.e., in directions defined by arrows  180  and  180 ′ in  FIG. 10 (C). Generally, various movements, such as rotation, translation, etc., are usually prevented by further tightening of the associated screws prior to completion of the procedure.  
         [0116]     The cross connector  176  may include screw threads  192  which have an anti-rotation feature, or the screw  186  may have an anti-rotation feature, as has been described in connection with other screws above. The tip of the bar clamping screw  186  may engagingly mate with a recess  201  on the upper surface of the bar  198   a , i.e., the surface opposite that of nub  202 .  
         [0117]      FIG. 11 (A) indicates the embodiment in cross-section, as well as how the cross connector may be slid along the bars to accommodate various placement locations. To further assist the engagement of the bars as the cross connector is translated, the bars may have a number of nubs and recesses to accommodate various placement locations (just one nub and recess is shown in  FIG. 11 (A) for clarity).  
         [0118]      FIG. 11 (B) indicates an alternative embodiment of the rod attachment element  174   b . In  FIG. 11 (B), the rod attachment element  174   b  is composed in part of a slotted arrangement that makes up part of the rod-contacting surface. The central section  208  performs the functions described above in connection with  FIGS. 9 and 10 . In addition, as the central section  208  is separated from the remainder of the rod attachment element, at least in the region of the rod-contacting surface, the second projection may be easier and more convenient to deflect. The remainder of the rod attachment element, in the region of the rod-contacting surface, comprises a set of peripheral sections  206  and  212  which provide additional strength to the rod attachment element.  
         [0119]     Referring back to  FIG. 9 (A)-(E), after the rods  110   a  and  110   b  are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by placing the rods against rod-contacting surfaces  194   a  and  194   b , and tightening screws  178   a  and  178   b  into their respective holes. The tightening of screws  178   a  and  178   b  flexes undercuts  175   a  and  175   b  further under the rod, further securing the same against movement. The bars  198   a  and  198   b  are then inserted in an overlapping fashion into the void  188  of cross connector  176 , such that the nub  202  engages the recess  204 . A degree of orientation may be performed by the physician, to accomplish a particular treatment goal, following which the screw  186  is threadingly inserted into the hole  192 . Once tightened, the system is secured and the procedure concluded. In an alternative embodiment, the bars are engaged to the cross connector first, and then the rod attachment elements are secured to the rods.  
         [0120]     In many insertion procedures, the screws are inserted and tightened to a point where the same are not fully tightened. Following this, the system can be adjusted according to the preferences of the physician, and then the screws fully tightened to prevent undesired motion.  
         [0121]     Referring to  FIG. 12 , an alternative embodiment of a cross-connector system is shown. Certain features are in common with above-described embodiments. For example, two rod attachment elements  214   a  and  214   b  are shown, each with a corresponding bar  222   a  and  222   b . The bar  222   a  has a nub  226  on a lower portion of a distal end  224   a  while, on an upper portion, the same has a recess  225 . The bar  222   b  has a recess  228  along a portion of its length. When the two bars are inserted into a hole  232  in cross connector  230 , the same may be tightened into position by inserting a bar clamping screw  250  having tip  254  and threads  252  into threaded hole  234 .  
         [0122]     Each rod attachment element has a threaded hole  218   a  (or  218   b ) and a biasing section or hook section  216   a  (or  216   b ). The hook section has a concave surface for contacting a portion of a rod (not shown). Two rod attachment element screws  238   a  and  238   b  are provided, one each for threading engagement with corresponding holes  218   a  and  218   b . The two rod attachment element screws  238   a  and  238   b  have respective threads  242   a  and  242   b  and respective heads  244   a  and  244   b . The heads  244   a  and  244   b  each have a corresponding eccentric cam section  246   a  and  246   b.    
         [0123]      FIG. 13 (A) shows a perspective cross-section of this embodiment&#39;s configuration. Referring to  FIG. 13 (B), a degree of rotational movement or adjustment may be allowed as indicated by arrows  210  and  210 ′. In addition, a degree of rotational movement or adjustment may be allowed, out of the plane defined by arrows  210  and  210 ′, this degree of rotational movement indicated by  FIG. 13 (C) as arrows  220  and  220 ′. As noted above, once the proper adjustment is made, generally for reasons of patient geometry accommodation, the system is tightened, preventing further movement. Of course, various degrees of freedom may be non-tightened if desired to allow movement with respect to that degree of freedom.  
         [0124]     The cam position may be indicated by markers  256  and  258 , located on the bar and on the screws (see  FIG. 13 (B)). Top and bottom views are also shown in FIGS.  14 (A) and (B). A detail of the head  244   a  is shown in  FIG. 14 (C). This figure shows cam section  246   a , marker  258 , as well as intended direction of rotation  260 , for left-handed threads. The marker on the screw and the marker on the bar may be employed to align starting positions, ending positions, etc. For example, aligned markers may indicate a starting position, where the cam is not engaged with the rod, and a 90° rotation may then be employed to capture the rod.  
         [0125]     A captured rodis placed in juxtaposition with the cross-connector in a functional manner, such as for example, in juxtaposition with the rod attachment element of the cross-connector. A captured rod may be permitted free movement, limited movement, or no movement. In some embodiments, and depending on the level to which tightening of, e.g., screws, is performed, the rod may be permitted no movement, sliding movement, limited rotational movement, significant rotational movement, and so on.  
         [0126]     In use, after the rods are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by placing the rods against rod-contacting surfaces  216   a  and  216   b , and tightening screws  238   a  and  238   b  into their respective holes. With this tightening, the cam section  246   a  locks against the rod, both frictionally arresting and mechanically preventing movement of the rod out of the rod attachment element. That is, the cam section  246   a  may force the rod against the hook section  216 , and thus frictionally secure the same against movement. The cam section  246   a  can also, with appropriate design, take a position under the rod and force the same upward against the rod attachment element, thus mechanically preventing removal, at least removal via a downward motion. The degree of frictional arrest and mechanical movement prevention may be adjusted by choice of geometry of the cam, the rod, and rod attachment element hook section, and to a lesser degree by the type of materials chosen for construction. In all installation techniques, the physician may be aware of the positioning of the cam section via the markers.  
         [0127]     The bars  222   a  and  222   b  are then inserted in an overlapping fashion into the void  232  of cross connector  236 , such that the nub  226  engages the recess  228 . A degree of orientation may be performed by the physician, to accomplish a particular treatment goal, following which the screw  250  is threadingly inserted into the hole  234 . As noted above, in typical installations, the screw  250  is inserted first, but not fully tightened. The physicians orients the system properly, and then fully tightens screw  250 . Once tightened, the system is secured and the procedure concluded. In an alternative embodiment, the bars are engaged to the cross connector first, and then the rod attachment elements are secured to the rods.  
         [0128]     FIGS.  15 (A)-(C) show another embodiment of the invention. In this embodiment, rods  270   a  and  270   b  are shown coupled to two-part rod attachment elements  272   a  and  272   b , this coupling occurring as will be described in a different manner than the above-described embodiments. Rod-locking screws  274   a  and  274   b  assist in creating this coupling. A cross connector  280  is shown with a housing  276  and further employing screws  278   a  and  278   b , these screws clamping directly on respective swivels  282   a  and  282   b  and/or on respective bars  281   a  and  281   b.    
         [0129]     Additional details of this embodiment are shown in  FIGS. 16-18 .  
         [0130]     First, details of the rod-locking screw system are shown in  FIG. 16 (A)-(B). Referring to  FIG. 16 (A), the rod attachment element  272   a  includes a rod-locking screw  274   a  which is inserted into a clamp  286   a . The clamp  286   a  includes an upwardly-projecting screw receiver  288   a  with internal threads  292   a . The clamp  286   a  is inserted into a housing  273   a , the housing having a rod-receiving channel  290   a  and a bar  281   a , on which is mounted the swivel  282   a  as will be described. The clamp  286   a  has a curved rod-receiving lower surface  291   a  which acts to surround the rod  270   a.    
         [0131]     Referring to  FIG. 16 (C), the swivel  282   a  is mounted on the bar  281   a . In particular, radially-inward projections  296   a  and  298   a  may together be inserted into a groove  285   a  in the bar  281   a . This engagement may serve as a retaining feature, maintaining the swivel on the bar but still allowing sliding of the swivel on the bar for, e.g., width adjustment.  
         [0132]     Referring to FIGS.  16 (A) and  17 (A)-(D), the orientation of the clamp  286   a  may be adjusted to ease rod insertion.  FIG. 17 (A) shows the orientation prior to insertion of the rod. The surface  291   a  and the rod-receiving channel  290   a  are rotationally-oriented such that the rod may be easily inserted, the configuration just after insertion shown in  FIG. 17 (B). The clamp  286   a  may then be rotated as shown in  FIG. 17 (C), at which point a portion of the clamp is forced against the rod and the same transmits a force against the rod-receiving channel  290   a , frictionally securing the components together. That is, in one orientation, first and second portions of the rod attachment element are arranged such that a rod may enter the rod-receiving channel. In another orientation, the first and second portions of the rod attachment element are arranged such that the rod is locked in the rod-receiving channel.  
         [0133]     To maintain the frictional engagement, the screw  274   a  may be rotated in a direction shown by arrow  302 , causing a downward movement of the screw indicated by arrow  300 . The tightening of the rod-locking screw causes the clamp to compress around the rod, in the directions indicated by arrows  304  and  306 , and the tightening may be maintained until the rod is rigidly attached, both axially and rotationally. The rotation of the screw and clamp causes the rod to be clamped between portions  307  and  309  (see  FIG. 17 (D)). This is termed a “scissor” design. When the screw  274  is tightened, the clamping force is enhanced.  
         [0134]     The bar clamping screws  278   a  and  278   b  may act directly on the swivels  282   a  and  282   b , and on the bars  281   a  and  281   b , and may serve to frictionally secure the combination against movement following installation. To install the swivels onto the bar, the same may be either slid on or snap-fit over. To install the swivels and bars into the cross connector  280 , the swivel, bar, and rod attachment element combination may be rotated to the position shown in  FIG. 18 (A). The swivel may then be inserted through a swivel insertion slot  293 , which as shown in  FIG. 18 (B) has a horizontal dimension X and a vertical dimension Y. The vertical dimension Y is less than the outer radius of the swivel. The swivel has a substantially spherical surface to accommodate polyaxial orientations prior to tightening of all screws. The housing  276  is provided with a slot on its general underside to accommodate the bar in this position.  
         [0135]      FIG. 18 (B) shows the swivel partially inserted in the cross connector  280 , and  FIG. 18 (C) shows the swivel fully inserted in the cross connector  280 . Following this full insertion, the swivel, bar, and rod attachment element combination may be rotated to the position shown in  FIG. 18 (D), which is approximately the appropriate position for use in a patient.  
         [0136]     In use, after the rods  270   a  and  270   b  are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by placing the rods in the rod-receiving channels  290   a  and  290   b , following the procedures of FIGS.  17 (A)-(D), and tightening screws  274   a  and  274   b . The bars  281   a  and  281   b  along with their corresponding swivels  282   a  and  282   b  may then be disposed in the voids of the cross connector  280  in the manner described by FIGS.  18 (A)-(D). The screws  278   a  and  278   b  may then be tightened, securing the swivels and bars in the cross connector. In an alternative embodiment, the cross connector is connected to the bars first, and then the rod attachment elements are secured to the rods.  
         [0137]     In some embodiments, the bars may be omitted, and the rod attachment elements may attach directly to a cross connector. For example, referring to FIGS.  19 (A)-(B) and  20 (A)-(C), a cross-connector system  300  is shown with rod attachment elements  302   a  and  302   b , a cross connector  306  spanning them. The rod attachment element  302   a  includes a housing  303   a  with a throughhole  305   a  defined therein, a biasing section or hook section  318   a , and a post hole  320   a  defined therein. The rod attachment element  302   b  has similar components, although the structure of rod attachment element  302   b  may be entirely different if dictated by the requirements of the user.  
         [0138]     A base  312   a  is provided corresponding to each rod attachment element  302   a , the base  312   a  including a threaded hole  314   a  and a post  316   a . When constructed, a c-clip  310   a  is disposed between the rod attachment element housing  303   a  and the base  312   a . The c-clip  310   a  may snap onto a thread or groove on the screw, so that, in combination with the head on the screw, the cross connector and the rod attachment element are frictionally engaged. The rod attachment element  302   b  may employ similar components.  
         [0139]     The cross connector  306  has one or more holes defined therein, which are shown in  FIG. 19 (B) as holes  308   a  and  308   b . The holes may be elongated, as shown, to allow a set of screws  304   a  and  304   b  to occupy a variety of locations along the elongated hole, as may be required (see  FIG. 20 (A)). The holes  308   a  and  308   b  may be provided with a depression, so that when a screw is inserted therethrough, the screw head is flush with or below the level of the cross connector  306  (see  FIG. 20 (C)). The screws  304   a  and  304   b  serve to attach the cross connector to the rod attachment elements and also to attach the rods to the rod attachment elements. The hook section  318   a  includes an interior surface  324   a , preferably with a roughened surface. In  FIG. 19 (A), the interior surface  324   a  is shown with a number of teeth disposed thereon. Other roughened forms may also be using, including serrations, grit-blasted surfaces, textured coatings and the like. The post  316   a  engages the post hole  320   a  so that the base  312   a  maintains a fixed, e.g., unrotating, position with respect to the housing  303   a  as the screw  304   a  is threadingly inserted into the hole  314   a  in the base  312   a.    
         [0140]     While the post and post hole maintain the relative positions of the rod attachment element housing and base, the entire rod attachment element may be rotated if desired about the screw  304   a . In particular, the longitudinal axis of rod attachment element  302   a  need not be collinear with the longitudinal axis of rod attachment element  302   b . As seen in  FIG. 20 (A), the rod attachment element  302   a  may be rotated relative to the cross connector  306 , in the angular directions indicated by the arrows  323  and  323 ′.  
         [0141]     The rod attachment elements&#39; pivot, in the directions indicated by the arrows  323  and  323 ′, may be in part arrested by rounded edges  326  on the underside of the cross connector  306 . The rounded edges  326  may be disposed on one or both sides of the cross connector, and at one or two places on each side (to accommodate both directions  323  and  323 ′). The rounded edges may by configured to gradually increase the stopping force present as the rod attachment elements are pivoted to extreme angles.  
         [0142]     As seen in  FIG. 20 (C), the depression of hole  308   a  may have a spherical shape  327  on which sits the screw head of screw  304   a , and the screw head itself may have a spherical shape. Thus, the screw head and screw may pivot adjacent and with respect to the depression of hole  308   a . To accommodate the screw shank movement during pivoting, an underside  325  of the hole  308   a  may be tapered as shown. Due to these cooperating engaged surfaces, the rod attachment elements may be able to pivot along the directions shown by arrows  329  and  329 ′, as shown in  FIG. 20 (B).  
         [0143]     Referring to  FIG. 21 (A)-(B), threading insertion of the screw  304   a  causes the base  312   a  to move in a direction indicated by arrow  328   a . Such displacement brings an angled surface  331   a  of the base  312   a  into engagement with the rod, securing the same against removal. The rod may be further secured by employment of an undercut  330   a  which may form a distal end of the hook  318   a  (see  FIG. 21 (B)). Contact or locking points for the system are shown in  FIG. 21 (B) by black dots.  
         [0144]     In use, after the rods are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by first placing the rods in the rod-receiving channels within hook sections  318   a  and  318   b . Then, posts  316   a  and  316   b  are placed in post holes  320   a  and  320   b . The screws  304   a  and  304   b  are then at least partially tightened, and c-clips  310   a  and  310   b  may be disposed on the threads or grooves of screws  304   a  and  304   b . As noted above, the use of posts ensures that angled surfaces  331   a  and  331   b  remain directed against the rods, securing the same from movement or removal. As above and in other embodiments, the system geometry, such as fine adjustments of widths and angles between components, may be adjusted prior to final screw tightening.  
         [0145]     A related embodiment is shown in  FIG. 22 , in which a cross connector  306 ′ is shown with a single slot  308 ′. In this embodiment, the single slot  308 ′ still allows a degree of width adjustment to achieve a desired distance between rods. The single slot  308 ′ can of course be provided for attachment to either rod attachment element. The method of use of the embodiment of  FIG. 22  is analogous to the method of use of the preceding embodiment, except that the width adjustment is accomplished via only one screw, in particular, screw  304   b  sliding in slot  308 ′.  
         [0146]     FIGS.  23 (A)-(B) show another related embodiment. In this embodied system  340 , a cross connector  344  spans two rod attachment elements  342   a  and  342   b . The rod attachment element  342   a  includes a body  345   a  with a throughhole  347   a  defined therein and a biasing section or hook section  343   a . The rod attachment element  342   b  has similar components, although the structure of rod attachment element  342   b  may be entirely different if dictated by the requirements of the user.  
         [0147]     A base  348   a  is provided corresponding to each rod attachment element  342   a , the base  348   a  including a threaded hole  349   a  and an angled rod-locking surface  360   a . When constructed, a c-clip  352   a  is disposed between the rod attachment element housing  345   a  and the top of the base  348   a , for the same purpose as is described above. The rod attachment element  342   b  may employ similar components.  
         [0148]     The cross connector  344  has one or more holes defined therein, which are shown in  FIG. 23 (B) as holes  362   a  and  362   b . The holes may be elongated, as shown, to allow a set of screws  354   a  and  354   b  to occupy a variety of locations within the elongated hole, as may be required to accommodate different patient spinal dimensions. The holes  362   a  and  362   b  may be provided with depressions  363   a  and  363   b , as seen in  FIG. 23 (B), so that when a screw is inserted therethrough, the screw head may be made flush with or below the level of the cross connector  344 . The screws  354   a  and  354   b  serve to attach the cross connector  344  to the rod attachment elements  342   a  and  342   b  and also to attach the rods to the rod attachment elements, as will be shown.  
         [0149]     The hook section  343   a  includes an interior surface  356   a  with a roughened surface. In  FIG. 23 (A), the interior surface  356   a  is shown with a number of teeth disposed thereon. Other roughened forms may also be using, including serrations and the like. As may be seen in  FIG. 23 (A), the rod-contacting surface  356   a  has a substantially cylindrical cross-section; at an extremal point on the circumference of this cylindrical cross-section, at the end nearest the hole  347   a , a flange  358   a  may downwardly depend, the flange  358   a  having an angled surface  351   a  for sliding frictional engagement with an angled surface  360   a  of the base  348   a . Unlike the previous embodiment, no post or post hole is employed in this embodiment; instead, the interaction and engagement between angled surfaces  351   a  and  360   a  maintain the fixed, e.g., relatively unrotated, position with respect to the housing and the base as the screw is threadingly inserted into the hole in the base. The screw, in this embodiment as well as others, may incorporate a distal thread section that is deformed or otherwise configured so as to prevent disassembly. The distal thread section or the threaded hole may alternatively or in addition incorporate a locking feature such as a polymer insert or the like.  
         [0150]     As in the previously-described embodiment of  FIGS. 19-21 , the screw head may be made spherical, and the depression appropriately configured as described in connection with those figures, to allow a degree of pivot to accompany this embodiment.  
         [0151]     On the side of the flange  358   a  opposite that of the surface  351   a , an undercut  359   a  may be formed (see  FIG. 24 ), which assists in the securing of the rod to the rod attachment element. The flange may form a flexible hinge, allowing the hook section to be snap-fit around the rod.  
         [0152]     As in the previous embodiment, the entire rod attachment element may be rotated if desired about the screw  354   a . In particular, the longitudinal axis of rod attachment element  342   a  need not be collinear with the longitudinal axis of rod attachment element  342   b  or with the cross connector  344 .  
         [0153]     In a similar way as noted above in connection with  FIGS. 19-20 , the rod attachment elements&#39; pivot angle, in the plane parallel to the cross connector  344 , may be in part arrested by rounded edges  346  on the underside of the cross connector  344 . The rounded edges  346  may be disposed on both sides of the cross connector, and at two places on each side (to accommodate both clockwise and counter-clockwise). The rounded edges may be configured to gradually increase the stopping force present as the rod attachment elements are pivoted to their extreme angles.  
         [0154]     Contact or locking points for the system are shown in  FIG. 24  by black dots.  
         [0155]     In use, after the rods are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by first placing the rods in the rod-receiving channels within hook sections  343   a  and  343   b . Then base  348   a  and  348   b  are threaded onto screws  354   a  and  354   b  such that surfaces  351   a  and  360   a , as well as  351   b  and  360   b , are adjacent. The screws  354   a  and  354   b  are then tightened, and c-clips  352   a  and  352   b  may be disposed on the threads or grooves of screws  354   a  and  354   b . In an alternative embodiment, the cross-connector system may be assembled, or partially assembled, prior to attachment of the rod attachment elements to the rods.  
         [0156]     Referring to FIGS.  25 (A)-(B), an embodiment is shown with certain similarities to prior-described embodiments, although the rod attachment mechanism is different.  
         [0157]     In this embodied system, a cross connector  370  spans two rod attachment elements  368   a  and  368   b . The rod attachment element  368   a  includes a body  369   a  with a throughhole  384   a  defined therein and a biasing section or hook section  371   a . The rod attachment element  368   b  has similar components, although the structure of rod attachment element  368   b  may be entirely different if dictated by the requirements of the user.  
         [0158]     The cross connector  370  has one or more holes defined therein, which are shown in  FIG. 25 (B) as holes  382   a  and  382   b . The holes may be elongated, as shown, to allow a set of screws  380   a  and  380   b  to occupy a variety of locations within the elongated hole, as may be required. The holes  380   a  and  380   b  may be provided with a depression, as in the prior embodiments, so that when a screw is inserted therethrough, the screw head may be made flush with or below the level of the cross connector  370 . The screws  380   a  and  380   b  serve to attach the cross connector  370  to the rod attachment elements  368   a  and  368   b  and indirectly also assist in the attachment of the rods to the rod attachment elements, as will be shown.  
         [0159]     The hook section  371   a  includes an interior surface  374   a  with a roughened surface. In  FIG. 25 (A), the interior surface  374   a  is shown with a number of teeth disposed thereon. Other roughened forms may also be using, including serrations and the like. As may be seen in  FIG. 25 (A), the rod-contacting surface  374   a  has a substantially cylindrical cross-section; at an extremal point on the circumference of this cylindrical cross-section, at the end nearest the hole  384   a , a flange  376   a  may downwardly depend. On the side of the flange  376   a  opposite the rod-contacting surface, the flange  376   a  may incorporate a projection  377   a . On the side of the flange  376   a  adjacent the rod-contacting surface, the flange  376   a  may incorporate an undercut  379   a . The undercut  379   a  assists in securing the rod to the rod attachment element; in particular, the flange may form a flexible hinge, allowing the hook section to be partially snap-fit around the rod.  FIG. 26 (A) shows a perspective cross-sectional view of this embodiment. The snap-fit is enhanced, or in some cases may be supplanted, by the action of the screw on the flange. In particular, as the screw  380   a  is inserted through hole  382   a  and further threadingly inserted into hole  384   a , a distal end  378   a  of the screw  380   a  contacts the projection  377   a  and forces the same towards the rod. As the projection  377   a  is forced in that direction, so is the undercut  379   a , and the undercut  379   a  further contacts and surrounds the rod, and secures the same against removal.  
         [0160]     No base need be employed in this embodiment. As in the previously-described embodiments, the screw head may be made spherical, and the depression appropriately configured as described in connection with those figures, to allow a degree of pivot (prior to final screw tightening) to accompany this embodiment.  
         [0161]     In a similar way as noted above in connection with  FIGS. 19-20  and  23 , the rod attachment elements&#39; pivot angle, in the plane parallel to the cross connector  370 , may be in part arrested by rounded edges  372  on the underside of the cross connector  370 . The rounded edges  372  may be disposed on both sides of the cross connector, and at two places on each side (to accommodate both clockwise and counter-clockwise). The rounded edges may be configured to gradually increase the stopping force present as the rod attachment elements are pivoted to their extremal angles.  
         [0162]     Contact or locking points for the system are shown in  FIG. 26 (B) by black dots.  
         [0163]     In use, after the rods are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by first placing the rods in the rod-receiving channels within hook sections  371   a  and  371   b . In many procedures, for this embodiment and for the others, a rod attachment element is attached to one rod, and the system is partially assembled. The width between the cross-connector is then accommodated by modification of the cross-connector and/or one or more rod attachment elements. A rod attachment element is secured to the second rod, and a final screw tightening may then occur. Then screws  380   a  and  380   b  are installed such that their distal ends deflect projection  379  and undercut  377  such that the undercut is forced against the rod, securing the same against movement.  
         [0164]     Another embodiment of the invention is shown in  FIGS. 27-30 . Referring in particular to FIGS.  27 (A)-(B), a system  390  is shown with two rod attachment elements  392   a  and  392   b . Within each is defined a substantially cylindrical opening  398   a  and  398   b  for receipt and securing of a rod. The cylindrical openings may further include a serrated portion  400   a  and  400   b  with the same purpose as above, to assist in the securing of a rod. The rod attachment elements  392   a  and  392   b  further incorporate threaded holes  418   a  and  418   b  for threading insertion of rod-locking screws  396   a  and  396   b . The rod-locking screws  396   a  and  396   b  have threads  426   a  and  426   b  disposed thereon. The rod attachment elements  392   a  and  392   b  further incorporate first and second bars  394   a  and  394   b . The bar  394   a  may extend directly out from the rod attachment element  392   a , while the bar  394   b  may be vertically displaced a distance d from rod attachment element  392   b  via diagonal section  412   b . Of course, in an alternative embodiment, rod attachment element  392   a  may incorporate the vertically-displaced bar, or both may have vertically-displaced bars, where the amount of vertical displacement differs and/or is in opposite directions.  
         [0165]     Each bar may have a through-hole defined therein. In  FIG. 27 (B), the bar  394   a  has through-hole  422 , and the bar  394   b  has through-hole  420 . One or both through-holes may be elongated to accommodate a range of widths between the rods. In  FIG. 27 (B), the through-hole  422  is shown elongated.  
         [0166]     A bar clamping screw  402  having threads  406  holds the bars in a secure fashion via a nut  404 . The nut  404  includes a base section  423  through which is defined a threaded hole  424 . Depending upwardly from the base section  423  are two projections  414   a  and  414   b . The projections  414   a  and  414   b , which may vary in number, are received within the elongated hole  422  of the bar  394   a  and serve to prevent the nut  404  from turning when the screw  402  is threadingly inserted.  
         [0167]     The construction as described above allows a number of degrees of freedom to be obtained by the system  390  (these degrees of freedom may all be removed by final screw tightening—or one or more may remain “free”—such as to allow a degree of motion after implantation).  FIG. 28 (A) shows arrows  416   a - 416   d , which indicate a degree of rotational freedom about an axis defined by the longitudinal axis of the screw  402 . An arrow  417  indicates a degree of translational freedom due to the elongated hole  422 .  FIG. 28 (B) shows another rotational degree of freedom enjoyed by the system  390 , this degree of freedom transverse to the plane defined by the bars. The degree of freedom indicated by arrows  430  and  432  is afforded by the construction of the system  390  as indicated in  FIG. 28 (C) and FIGS.  29 (A)-(B).  
         [0168]     As seen in  FIG. 28 (C) and  FIG. 27 (B), a bottom surface  437  of a head  435  of screw  402  may be constructed to be substantially spherical, and the same may rotationally engage a spherical taper  421  at the top of the hole  420 . In the same way, the bottom of the hole  420  may be provided with a spherical taper  427 , and the same may rotationally engage a spherical taper  425  at the top of the hole  422 . The hole  420  may itself incorporate a wall  439  having a taper as indicated by lines  436  and  438  of  FIG. 28 (C).  
         [0169]     In other words, to further assist in providing the degree of freedom indicated by arrows  430  and  432 , the bottom of the bar  394   b , and the top of the bar  394   a , may be provided with mating spherical tapers so that one may be slidingly rotated on top of the other, as indicated in FIGS.  28 (C) and  29 (A). In many procedures, the degrees of freedom may be used to orient a device in a proper position and then the screw  402  may be threadingly tightened, locking the system in that position, as shown in  FIG. 29 (B).  
         [0170]     Contact or locking points for the system are shown in  FIG. 30  by black dots.  
         [0171]     In use, after the rods are attached to the installed pedicle screws, the rod attachment elements may be attached to the rods by first placing the rods in the rod-receiving channels defined by surfaces  398   a  and  398   b . Then screws  396   a  and  396   b  are installed such that their distal ends deflect projection  410  and undercut  408  such that the undercut is forced against the rod, securing the same against movement. The bars are then positioned such that the screw  402  may extend through the hole in each. The nut  404  is then positioned such that the screw  402  may be threadingly inserted into the same, with the projections  414   a  and  414   b  inserted into the hole  422  to arrest rotational movement of the nut. Of course, in an alternative embodiment, the bars may be secured together first, and the same later attached to the rods.  
         [0172]     In all of the above-described embodiments, where descriptions are provided for a group of elements suffixed by the letter ‘a’, a similar description may apply for the group of elements suffixed by the letter ‘b’; however, in all cases, a different type of group of elements may also be employed. There is no requirement that the same elements be employed. For example, a cross-connector system may employ two different rod attachment elements of entirely different type, if dictated by the requirements of the user. Whether the rod attachment elements are of the same or of differing types, the way in which the same couple to the rods may differ. One may couple at a different angle than the other. One may couple in a dynamic way, while the other couples in a static way. One may couple in a reversible fashion, while the other couple irreversibly. They may attach to different size rods, including rods of different lengths or diameters or both. The materials of construction of the rod attachment elements may differ. The bars may attach to the rod attachment elements at virtually any angle, and as noted may be pre-attached or integral therewith.  
         [0173]     Further, in all of the above-described embodiments, various types of locking screws may be employed to protect against disassembly. Such locking screws may include polymer inserts, deformed or other high-resistance threads, or other types of locking mechanisms. The heads of the screws may incorporate insertable or removable fills or inserts so as to prevent contamination from entering a portion of the head where engagement with a tool may occur. In this way, follow-up adjustments and removal procedures may be made more convenient.  
         [0174]     The rods described above may be of the type disclosed in U.S. patent application Ser. No. 11/362,366, filed on Feb. 23, 2006, entitled “Systems and methods for stabilization of bone structures” and incorporated by reference in its entirety herein.  
         [0175]     The materials used in construction of all of the components are typically biocompatible and may be metal, such as titanium, although rigid plastics may also be employed.  
         [0176]     Components disclosed above may be employed in various combinations.  
         [0177]     Each rod attachment element may further include a hydraulic or pneumatic component, e.g., a hydraulic assembly that compresses the clamp portion to grip a corresponding rod. Other devices conveying a mechanical advantage to improve the gripping force may also be employed, such as cams, gear assemblies, and the like.  
         [0178]     While the invention has been described in the context of spinal fusion, the same may be employed in dynamic systems, and indeed may include dynamic elements either in the cross-connector or as parts of the stabilization rods to which the rod attachment elements connect. Embodiments of the invention may also be employed in various other systems, such as facet replacement or facet augmentation systems.