Abstract:
Methods, systems, devices and tools for placing bone stabilization components in a patient are provided. The systems and devices have a reduced number of discrete components that allow placement through small incisions and tubes. More particularly, the present invention is directed to screws for use in systems and methods of treating the spine, which eliminate pain and enable spinal motion, which effectively mimics that of a normally functioning spine. Methods are also provided for installation of the screw and other subject systems.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 11/427,738 entitled “Systems and methods for posterior dynamic stabilization of the spine” filed on Jun. 29, 2006, which is a continuation-in-part of U.S. patent application Ser. No. 11/436,407 filed on May 17, 2006, which is a continuation-in-part of U.S. patent application Ser. No. 11/033,452 filed on Jan. 10, 2005, which is a continuation-in-part of U.S. patent application Ser. No. 11/006,495 filed on Dec. 6, 2004, which is a continuation-in-part of U.S. patent application Ser. No. 10/970,366 filed on Oct. 20, 2004; U.S. patent application Ser. No. 11/427,738 is also a continuation-in-part of U.S. patent application Ser. No. 11/362,366 filed on Feb. 23, 2006, which is a continuation-in-part of U.S. Patent Application Ser. No. 60/701,660 filed on Jul. 22, 2005, all of which are incorporated herein by reference in their entirety. This application is also a continuation-in-part of U.S. patent application Ser. No. 11/586,849 entitled “Systems and methods for stabilization of bone structures” filed on Oct. 25, 2006, which is a continuation-in-part of U.S. patent application Ser. No. 11/362,366 filed on Feb. 23, 2006, which is a continuation-in-part of U.S. Patent Application Ser. No. 60/701,660 filed on Jul. 22, 2005, all of which are incorporated herein by reference in their entirety. 
     
    
     FIELD  
       [0002]     The present invention generally relates to surgical instruments and methods for using these instruments. More particularly, but not exclusively, minimally invasive methods of stabilizing one or more bone structures are disclosed.  
       BACKGROUND  
       [0003]     Systems, methods and devices for stabilizing one or more bone structures of a patient have been available for many years. Prior art procedures typically require large incisions and also significant tissue manipulation to adequately expose the areas intended for the attachment. The procedures are associated with long recovery times and increased potential for adverse events, such as infection, usually associated with muscle and other tissue trauma and scarring.  
         [0004]     Currently available minimally invasive techniques and products are limited. These procedures are difficult to perform, especially in spinal applications in which the attachment points are deeper in tissue, and damage to neighboring tissue must be avoided. Many of the currently available less invasive products remain somewhat invasive due to component configurations, and required manipulations to be performed during the attachment.  
         [0005]     In reference specifically to treatment of the spine,  FIGS. 1A-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 joints  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 .  
         [0006]     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 axis, both in rotation and in translation. The various spinal motions are illustrated in  FIGS. 1C-1D . In particular,  FIG. 1C  illustrates flexion and extension motions, anterior translation, and axial loading,  FIG. 1D  illustrates lateral bending motion and lateral translation 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.  
         [0007]     Various 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. Often, a disorder in one location or spinal component can lead to eventual deterioration or disorder, and ultimately, pain in the other.  
         [0008]     Spine fusion (arthrodesis) is a procedure in which two or more adjacent vertebral bodies are fused together. While spine fusion generally helps to eliminate certain types of pain, it has been shown to decrease function by limiting the range of motion for patients in flexion, extension, rotation and lateral bending. Furthermore, the fusion creates increased stresses on adjacent non-fused motion segments and accelerated degeneration of the motion segments.  
         [0009]     Various technologies and approaches have been developed to treat spinal pain without fusion in order to maintain or recreate the natural biomechanics of the spine. To this end, significant efforts are being made in the use of implantable artificial intervertebral discs. Unfortunately, the currently available artificial discs do not adequately address all of the mechanics of motion for the spinal column.  
         [0010]     It has been found that the facet joints can also be a significant source of spinal disorders and debilitating pain. Current interventions for the treatment of facet joint disorders have not been found to provide completely successful results.  
         [0011]     Most recently, surgical-based technologies, referred to as “dynamic posterior stabilization,” have been developed to address spinal pain resulting from more than one disorder, when more than one structure of the spine have been compromised. An objective of such technologies is to provide the support of fusion-based implants while maximizing the natural biomechanics of the spine. Dynamic posterior stabilization systems typically fall into one of two general categories: (1) interspinous spacers and (2) posterior pedicle screw-based systems.  
         [0012]     Examples of interspinous spacers are disclosed in U.S. Pat. Nos. Re. 36,211, 5,645,599, 6,695,842, 6,716,245 and 6,761,720.  
         [0013]     Examples of pedicle screw-based systems are disclosed in U.S. Pat. Nos. 5,015,247, 5,484,437, 5,489,308, 5,609,636 and 5,658,337, 5,741,253, 6,080,155, 6,096,038, 6,264,656 and 6,270,498. These types of systems involve the use of screws which are positioned in the vertebral body through the pedicle. Certain types of these pedicle screw-based systems may be used to augment compromised facet joints, while others require removal of the spinous process and/or the facet joints for implantation. One such system, employs a cord which is extended between the pedicle screws and a fairly rigid spacer which is passed over the cord and positioned between the screws. While this system is able to provide load sharing and restoration of disc height, because it is so rigid, it is not effective in preserving the natural motion of the spinal segment into which it is implanted. Other pedicle screw-based systems employ joints between the pedicle screws that provide some discreet amounts of movement in different directions to somewhat simulate the complex movement of the spine.  
         [0014]     There remains a need for minimally invasive methods and devices for bone stabilization procedures, including but not limited to spinal segment stabilization procedures such as dynamic spinal segment stabilization procedures  
         [0015]     Furthermore, there is an ongoing need for systems that provide easier insertion for the clinician. Systems that allow simplified multiple degree of freedom of adjustment during implantation that then can be securely fixed are needed as are systems that can reduce fatigue failures, avoid large stresses between components under all load conditions and generally have a long implant life.  
       SUMMARY  
       [0016]     According to one aspect of the invention, a bone screw system is provided. The bone screw includes a threaded section and a screw head section integrally connected to the threaded section. The system includes a coupler having a screw head receiving portion configured to receive at least a portion of the screw head section of the screw. The coupler also includes a rod receiving portion integral with the screw head receiving portion. The rod receiving portion is configured to receive a rod. The system includes a seat having a first end and a second end. The seat further includes at least one sidewall extending between the first end and the second end, a cap receiving portion configured to receive a cap at the first end and a coupler receiving portion configured to receive the coupler. The seat includes a bottom opening at the second end and a top opening at the first end. At least one rod channel defined by the sidewall and the at least one rod channel is interconnected with the top opening. The system includes a cap configured to close the top opening of the seat. The cap has a top surface and a bottom surface interconnected by an outer surface and an inner surface. The inner surface defines a threaded set screw receiving portion and the outer surface defines at least one seat-engagement feature for engagement with the cap receiving portion of the seat. The system includes a set screw having a top surface and a bottom surface interconnected by a threaded outer surface. The set screw is configured to be threadingly engaged with the set screw receiving portion of the cap. The system further includes a retainer configured to retain the coupler inside the seat. A rod is provided. The rod has a first end and a second end. The first end of the rod is configured to connect to the rod receiving portion of the coupler. At least a portion of the screw head section is disposed inside the screw head receiving portion of the coupler. The screw is inserted in the bottom opening of the seat and the coupler is retained inside the seat via the retainer pressed between the coupler and the seat. The first end of the rod is removably connected to the rod receiving portion of the coupler. The cap is removably inserted into the cap receiving portion of the seat and retained therein via the at least one seat-engagement feature on the outer surface of the cap. The set screw is disposed in the set screw receiving portion of the cap. The rod is disposed in the rod channel with the cap disposed in the cap receiving portion of the seat. Upon advancement of the screw, the bottom surface of the set screw contacts at least a portion of the rod within the seat and the bone screw and rod is locked into position with advancement of the set screw into the seat.  
         [0017]     According to another aspect of the invention, a bone screw system having a locked configuration and an unlocked configuration is provided. The bone screw system includes a rod and a seat. The seat has a first end and a second end and is configured to receive at least a portion of the rod inside the seat such that the rod has a range of motion relative to the seat while in the unlocked configuration. A bone screw having a first end and a second end is also provided. At least a portion of the first end of the bone screw is disposed inside the seat. The screw has a range of motion relative to the seat when in the unlocked configuration. A lock down mechanism is removably disposed inside the seat at the first end of the seat. At least a portion of the rod is located between the lock down mechanism and the first end of the screw. The lock down mechanism is operable between a locked configuration and an unlocked configuration such that both the rod or the screw is locked into position as the lock down mechanism operates from the unlocked configuration to the locked configuration.  
         [0018]     According to yet another aspect of the invention, a bone screw system is provided. The bone screw system includes a rod and a bone screw having a first end and a second end. The system includes a seat having a bottom opening. Also included is a coupler having a rod receiving portion and a bone screw receiving portion. The rod receiving portion of the coupler is configured to connect to the rod and the bone screw receiving portion is configured to house at least a portion of the first end of the bone screw. At least a portion of the bone screw is retained in the bone screw receiving portion of the coupler which is retained inside the seat such that the bone screw is connected to the seat via the coupler and such that the bone screw extends through the bottom opening of the seat. The rod is connected to the rod receiving portion of the coupler.  
         [0019]     According to another aspect of the invention a system for housing at least a portion of a bone screw is provided. The system includes a seat having a first end and a second end, and an inner surface and an outer surface. At least a portion of the bone screw is retained inside the seat. The seat includes a top opening at the first end and a bottom opening at the second end. At least one sidewall extends between the first end and the second end. The seat includes a closure mechanism receiving portion and a screw receiving portion.  
         [0020]     According to another aspect of the invention, a method for inserting a screw system is provided. The method includes the step of providing a system that includes a bone screw and a seat. The seat has a first end, a second end, an inner surface and an outer surface. At least a portion of the bone screw is retained inside the seat. The seat includes a top opening at the first end and at least one sidewall extending between the first end and the second end. A flange is formed on the outer surface of the seat such that the flange extends outwardly from the seat; the flange has an upper surface, a lower surface and an outer surface. The method includes the step of providing an instrument having a first portion and a second portion at a distal end of the instrument. The first and second portions are controllable at the instrument&#39;s proximal end by a user. The method includes the steps of contacting at least a portion of the lower surface of the flange with the first portion of the instrument and contacting at least a portion of the upper surface of the flange with the second portion of the instrument. The second portion of the instrument is advanced to apply a force on the flange. The force is biased by the first portion of the instrument to secure the instrument to the seat. The system includes the step of delivering the system into the patient with the instrument.  
         [0021]     According to another aspect of the invention, a method is provided. The method includes the step of providing a system comprising a seat having a first end and a second end. The seat includes at least one sidewall extending between the first end and the second end and at least one rod channel formed in the sidewall. The seat also has a top opening at the first end. The system includes a coupler retained inside the seat. The coupler has a rod receiving portion and a screw receiving portion. The system further includes a bone screw that is retained inside the screw receiving portion of the coupler. The method includes the steps of delivering the system into a patient and inserting the bone screw into a bone of the patient. The method includes the step of providing a rod having a first end and a second end and delivering the rod into the patient. The first end of the rod is pivotally connected to the rod receiving portion of the coupler. The method includes the step of pivoting the rod into position. A closure mechanism is provided. The closure mechanism is delivered into the patient to close the top opening of the seat.  
         [0022]     According to another aspect of the invention, a bone screw system is provided. The system includes a bone screw and a seat having a first end and a second end. The seat retains the bone screw inside the seat. The seat includes at least one sidewall extending between the first end and the second end. The seat has a top opening at the first end and a cap receiving portion in the at least one sidewall. The cap receiving portion is configured to receive a cap within the seat at the first end. The cap receiving portion has at least one wing groove that has an upper surface. The system includes a cap that is configured to close the top opening of the seat. The cap has a top surface and a bottom surface interconnected by an outer surface and an inner surface. The inner surface defines a set screw receiving portion. The outer surface of the cap defines at least one wing lug extending outwardly from the outer surface of the cap. The wing lug is configured to mate with the at least one wing groove of the seat by rotation of the wing lug into the wing groove. The system includes a set screw having a top surface and a bottom surface interconnected by a threaded outer surface. The set screw is configured to be threadingly engaged with the set screw receiving portion of the cap. The set screw is inserted into the set screw receiving portion of the cap. The cap is removably inserted into the cap receiving portion of the seat closing the top opening. With the cap in the seat, the cap is rotated to position the at least one wing lug inside the at least one wing groove. The set screw is advanced into the seat. Set screw advancement is biased by the seat raising the cap such that the at least one wing lug contacts the upper surface of the wing groove, secures the cap to the seat and prevents splaying of the seat sidewall.  
         [0023]     Advantages of the invention may include one or more of the following. Insertion of certain of the described screws and pivoting rods may be performed with reduced insertion forces, and may feature simplified usage. Rotational locking may be employed to secure the pivoting rod against movement. Embodiments of the invention allow reduced stress on the pivoting rod. Embodiments of the invention are compatible with other pedicle screw systems and/or spinal implants. Embodiments of the invention may be applicable to patients with degenerative disc disease, spinal stenosis, spondylolisthesis, spinal deformities, fractures, pseudarthrosis, tumors, failed prior fusions, or other vertebral segment trauma and disease.  
         [0024]     It is noted that perfect alignment of the screws with one another is quite difficult and requires great skill on the part of the surgeon to accomplish. Alignment of the screws is even more difficult in minimally invasive/percutaneous procedures. Alignment may further be complicated by the patient&#39;s condition such as damaged or diseased bone or other anatomical condition. Screws can be out-of-alignment not only in one plane but in two and in some cases three planes. However, the polyaxial seat of the screw of the present invention advantageously allows the seat to swivel on top of the screw such that they may be lined up regardless of the orientation of the screws&#39; axes and can even thus be made to accommodate a certain amount of misalignment from difference in height once they are inserted into bone. The polyaxial motion of the seat allows the rod channels to be lined-up so that the rod can be placed or attached between the screws without having their axes perfectly aligned to do so. Then the seat can be modified to eliminate motion and stabilize one or more vertebral segments.  
         [0025]     Other advantages will be apparent from the description that follows, including the drawings and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     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:  
         [0027]      FIGS. 1A and 1B  illustrate perspective views of a portion of the human spine having two vertebral segments, where the spinous process and the lamina of the superior vertebra have been resected in  FIG. 1B .  
         [0028]      FIGS. 1C, 1D  and  1 E illustrate left, dorsal and top views, respectively, of the spinal segments of  FIG. 1A-1B  under going various motions.  
         [0029]      FIG. 2A  illustrates a perspective exploded and perspective view of a screw system which may be employed in an embodiment of the present invention.  
         [0030]      FIG. 2B  illustrates a perspective view of a pivoting rod which may be employed in an embodiment of the present invention.  
         [0031]      FIG. 2C  illustrates a perspective exploded and perspective view of a cap and set screw system which may be employed in an embodiment of the present invention.  
         [0032]      FIG. 2D  illustrates a perspective view of a pivoting rod which may be employed in an embodiment of the present invention.  
         [0033]      FIG. 2E  illustrates a perspective view of a pivoting rod which may be employed in an embodiment of the present invention.  
         [0034]      FIGS. 3 and 3 A illustrate top and enlarged top views of a rod attachment mechanism which may be employed in an embodiment of the present invention.  
         [0035]      FIGS. 4 and 4 A illustrate top and enlarged top views of another rod attachment mechanism which may be employed in an embodiment of the present invention.  
         [0036]      FIGS. 4B and 4C  illustrate side cross-sectional view of details of a set screw system for use in the system of  FIGS. 4 and 4 A.  
         [0037]      FIG. 5A  is a perspective detailed view of a seat and retaining ring according to an embodiment of the present invention.  
         [0038]      FIG. 5B  is a perspective view of a seat, coupler, retaining ring and screw according to an embodiment of the present invention.  
         [0039]      FIGS. 6A and 6B  illustrate bottom perspective and top perspective views of a cap and set screw system according to an embodiment of the invention.  
         [0040]      FIG. 7  illustrates a perspective view of a coupler system according to an embodiment of the invention.  
         [0041]      FIG. 8A  illustrates a perspective exploded view of a screw system according to an embodiment of the present invention.  
         [0042]      FIG. 8B  illustrates a perspective exploded view of a screw system according to an embodiment of the present invention.  
         [0043]      FIG. 8C  illustrates a side elevational view of a screw system according to an embodiment of the present invention.  
         [0044]      FIG. 9A  illustrates a side exploded view of a screw/seat/cap and set screw combination.  
         [0045]      FIGS. 9B-9D  illustrate the combination system of  FIG. 9A  in unlocked, partially locked, and fully locked configurations.  
         [0046]      FIGS. 10A-10C  illustrate a screw/seat/pivoting rod combination system in exploded, connected but not deployed, and post-rotation configurations, respectively.  
         [0047]      FIGS. 11A and 11B  show the installed device in top and perspective views, respectively.  
         [0048]      FIG. 12  illustrates a side cross-sectional view of the seat with a seat-gripping instrument engaging the seat according to one aspect of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0049]     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.  
         [0050]     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.  
         [0051]     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.  
         [0052]     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.  
         [0053]     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.  
         [0054]     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, inferior facets  10   a  and  10   b , lamina  5   a  and  5   b , posterior arch  16  and spinous process  18  of superior vertebra  2  of  FIG. 1A  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.  
         [0055]     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 multiple bone stabilization components such as 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 may include 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. Where multiple bone segments such as 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.  
         [0056]     The superior and inferior components (and any medial components therebetween), when operatively implanted, may be engaged or interface with each other in a manner that enables the treated spinal motion segment to mimic the function and movement of a healthy segment, or may simply fuse the segments such as to eliminate pain and/or promote or enhance healing. The interconnecting or interface means include one or more structures or members that enables, limits and/or otherwise selectively controls spinal or other body 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.  
         [0057]     In certain embodiments, the multiple components, such as superior and inferior spinal components, are mechanically coupled to each other by one or more interconnecting or interfacing means. In other embodiments, components interface, in a manner that constrains their relative movement and enables the treated segment to mimic the function or partial function and/or movement or partial movement of a healthy segment. Typically, spinal interconnecting means is a dorsally positioned component, i.e., positioned posteriorly of the superior and inferior components, or may be a laterally positioned component, i.e., positioned to the outer side of the posterior and inferior components. The structures may include one or more struts and/or joints that provide for stabilized spinal motion. The various system embodiments may further include a band, interchangeably referred to as a ligament, which provides a tensioned relationship between the superior and inferior components and helps to maintain the proper relationship between the components.  
         [0058]     Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In addition, each of the inventive embodiments described herein may be employed in a percutaneous procedure, a mini-open procedure or an open procedure. Utilization of minimally invasive techniques can shorten the procedure&#39;s time and speed recovery by the patient. The application of these inventions in a minimally invasive manner is not a requirement.  
         [0059]      FIG. 2A  illustrates a perspective exploded and perspective view of a screw system which may be employed in an embodiment of the present invention. In this figure, a screw system  20  is shown having a screw  30  with threads  28 . The threads  28  are appropriate for entering the bone of a patient. At a proximal end of screw  30  is a ball end  32 . While a ball end  32  is shown, various other shapes may also be employed. A hex socket  34  that is interconnected with a guidewire lumen (not shown) extends thru the general axial center of screw  30 , and also can extend through the retaining ring  22 , coupler  24  and seat  26 . The system is suitable for being installed in a patient for treating at least one or more of the following: degenerative disc disease, spinal stenosis, spondylolisthesis, spinal deformities, fractures, pseudarthrosis, tumors, failed previous fusions, other vertebral segment trauma or diseases.  
         [0060]     The ball end  32  of screw  30  is fitted into the bottom of the coupler  24 , which has a spherical interior shape, as will be described in greater detail below. If end  32  has a different shape, the shape of the interior of the bottom of the coupler  24  may be similarly complimentary. No matter the shape, when the ball end is fitted into the bottom end of end of the coupler  24 , the coupler  24  may be fitted into the “bucket-shaped” seat  26 . Retaining ring  22  ensures that coupler  24  does not escape from the interior of seat  26 , and is described in greater detail below.  
         [0061]      FIG. 2B  illustrates a perspective view of a pivoting rod  40  which is employed in an embodiment of the present invention. The pivoting rod  40  in  FIG. 2B  is shown to be straight, however, the invention is not so limited and a curved pivoting rod that conforms to the natural curve of the spine may be employed. The pivoting rod  40  has a shaft  42 , a ball end  44 , and two pins  46  (one of which is not shown in  FIG. 2B ) for insertion into the coupler  24 . The shaft  42  may vary in length, or may be adjustable by the physician, either by a telescoping mechanism or by being cut to measure. The end of the shaft  42  opposite that of ball end  44  may be straight, as shown, or may itself incorporate a ball end (not shown) or other type of end to enable trapping and capture in a seat of a pedicle screw system mounted to another spinal segment. The ball end  44  need not actually be a ball, and may be a square or rectangular solid, or other such shape, so long as the shape allows rotation of the pivoting rod. In general, the rod  40  and coupler  24  include mating features adapted to connect together.  
         [0062]     As shown in  FIG. 2B , sides  47  of the ball end, perpendicular to the pins  46 , are flattened. The flat sides  47  that are substantially perpendicular to the pins  46  also assist in confining the range of motion of the rod substantially within a single plane. Furthermore, the flattened sides  47  surrounding the pins  46  provide for a greater surface area that is in contact with the coupler  24  and thereby create an advantageous snug-fit engagement with the coupler. Because of the flattened sides  47 , when the rod  40  is inserted and the pins  46  are connected into the coupler  24 , the greater surface area of contact of the flattened sides  47  with the coupler provides for a snug-fit engagement with the coupler that would otherwise be the case with a rounded rod end. As a result, the rod advantageously does not pivot on its own in a direction away from the insertion angle or other angle at which the rod is positioned subsequent to insertion into the coupler. This feature facilitates insertion for the surgeon.  
         [0063]     In another variation, additional flat portions  45  are formed on the ball end  44  of the rod  40  as shown in  FIG. 2B . The flat portions  45  are substantially perpendicular to the flattened sides  47  on the ball end  44 . The pins  46  are substantially parallel to the flat portions  45 . When the rod is inserted into the seat and pivoted into a generally horizontal position, the flat portions  45  face upwardly and downwardly and as a result provide a lower profile for the rod within the seat. Furthermore, the flat portions  45  provide a flat contact surface for the set screw on the upper surface of the rod and a flat contact surface for the coupler on the bottom surface of the rod. The rod is not limited to having two flat portions  45  and/or two flattened sides  47  and any number and combination of flat portions  45  and sides  47  are within the scope of the present invention.  
         [0064]      FIG. 2C  illustrates a perspective exploded and perspective view of a closure mechanism  50 . The closure mechanism is shown as a cap and set screw system  50  which may be employed in an embodiment of the present invention. It should be noted that the closure mechanism is not limited to a cap and set screw combination as shown and described herein and various other closure mechanisms evident to a person having ordinary skill in the art are within the scope of the invention. Some examples of closure mechanisms include but are not limited to a threaded closure mechanism having external threads that engage with a threaded internal portion of the seat, a sliding closure mechanism, a compression fit closure mechanism, and a snap fit closure mechanism. The closure mechanism  50 , when fully installed in the seat  26 , secures the rod  40  against movement. In one variation, the system  50  includes a cap  48  and a set screw  52 . The cap  48  includes a set screw receiving portion. The cap and set screw are configured to close the top opening of the seat  26 . The external shape of the cap  48  is substantially cylindrical. The cap  48  includes a top surface and a bottom surface interconnected by an outer surface and an inner surface. The cap  48  includes external flanges or features  54 , discussed in more detail below, and internal screw threads  56 .  
         [0065]     The set screw  52  includes external screw threads  58  and a socket  62  for a driving tool, the socket  62  being substantially coaxial with the screw threads  58 . The set screw  52  also includes a flange  51  at the bottom end of the set screw  52 . The flange  51  is configured to extend outwardly from the circumference of the set screw to serve as a stop and to prevent the set screw  52  from being backed-out as the set screw is retracted upwardly with respect to the cap  48 .  
         [0066]     Various aspects and alternative embodiments of this basic system are described below. In this regard, it is noted that the above system of  FIGS. 2A-2C  would typically be employed in the following fashion: a first pedicle screw assembly would be installed in a patient, this first pedicle screw assembly having a hinge assembly that attaches to a proximal end of the pivoting rod. A second pedicle screw assembly would also be installed in a patient, this second pedicle screw assembly having a receiving cradle that accepts the distal end of the pivoting rod. Except for the hinge assembly and the receiving cradle, other parts of the screw assemblies may be the same and/or interchangeable. Each pedicle screw assembly also includes a set screw and cap assembly as described above and below. Further, the pivoting rod may include a dynamic element at some point along its length if desired, as described, e.g., in U.S. patent application Ser. No. 11/427,738, filed Jun. 29, 2006, U.S. patent application Ser. No. 10/970,366 filed Oct. 20, 2004, U.S. patent application Ser. No. 11/006,495 filed Dec. 6, 2004, U.S. patent application Ser. No. 11/033,452 filed Jan. 10, 2005, and U.S. patent application Ser. No. 11/436,407 filed on May 17, 2006, all of which are incorporated by reference herein in their entirety for all purposes.  
         [0067]     First, referring back to  FIG. 2B , the pivoting rod  40  is shown with integral pins  46  that are configured to snap-fit with the coupler  24  to secure the rod  40  in place and permit rotation of the rod. In one variation, the pins  46  are chamfered to ease the insertion of the pivoting rod by the clinician into the coupler and also ease removal of the pivoting rod from the coupler, if desired. Pins are not the only means of attachment of the rod  42  to the coupler  24  and other variations and means are within the scope of the invention. For example, as shown in  FIG. 2D , the ball end  44  of the rod can include cutout portions  43  that result in the ball end not having a completely spherical shape but permits attachment to and rotation relative to the coupler  24 . Another example is shown in  FIG. 2E . In  FIG. 2E , a rod  40  is provided with a ball portion  41  having a bore  39  passing through it. The rod  40  of  FIG. 2E  is positioned inside the bore  39  of the ball portion  41  such that the ball portion  41  is allowed to rotate and slide relative to the rod as indicated by the arrows in  FIG. 2E . This embodiment advantageously provides yet another degree of freedom of motion and facilitates installation by the surgeon. Furthermore, the embodiment of  FIG. 2E  advantageously permits the bone screw to be locked into position independently of rod and in another variation it permits the rod to be locked into position independently of the bone screw. More details of the independent lock down capability of this embodiment will be described in greater detail hereinbelow.  
         [0068]     An alternative way in which the pivoting rod  40  may be attached to the coupler that employs pins is shown in  FIG. 3  and  FIG. 3A . In particular, a set of two pins  64  may be employed which mate with a corresponding set of holes  66  in the coupler. As shown in  FIG. 3A , the pin  64  may be spring-loaded with springs  68 . This spring-biased hinge pin allows pivoting of the rod and also allows the pin to move radially inward during insertion, and then “pop” out when in place. The pin may then be retracted for removal. In an alternative embodiment, the pin may be permanently locked in place by injecting cement or glue or another such material into the travel volume of the pin.  
         [0069]     Another variation for the rod-end system is shown in  FIG. 4  and the enlarged views of  FIG. 4A-4C  in which the pivoting rod  40  is attached to the coupler via pins  72 . In this case, pins  72  may again be spring-biased but may be movable via action of a set screw  76 . In particular, and as shown in  FIGS. 4A and 4B , pins  72  are biased by springs  74  in a retracted state; i.e., they are not extended so as to engage or mate with holes in the coupler. Instead, a portion of the pins  72  extends into a threaded hole  70  within a proximal end of pivoting rod  40 , i.e., ball end  44 . When a set screw  76 , with conical distal surface  82 , is advanced into the threaded hole  70 , the distal surface  82  impinges on pins  72  and drives the same outward, such that they may engagedly mate with the holes in the coupler. The set screw  76  may have a slot  78  and/or other tool engagement means (not shown) in its proximal (top) surface to allow for such driving. In this way, the extendable/retractable hinge pins extend after insertion to pivotally lock the pins in place.  
         [0070]     In an alternative embodiment to  FIGS. 4 and 4 A- 4 C, the extension and retraction may be accomplished with hydraulics or pneumatics, rather than springs and set screws. To this end, a fluid injection port may be provided which is integral, or not, to the pivoting rod. In a further alternative embodiment, the pins may be permanently locked in place, if desired by the physician, by injecting cement or glue into the pin travel volume. Although the rod has been described as being configured to connect to the coupler the invention is not so limited and in alternative variations, the rod is configured to connect to the seat or retainer.  
         [0071]     Various aspects of the seat, coupler, and retaining ring are now discussed. Referring to  FIG. 5A , a seat  26  and retaining ring  22  are shown in an exploded view. Retaining ring  22  is shown with two projections  86 , also known as keys, which engage features on the seat  26 , to hold the coupler in place (the coupler is not shown in the figure for clarity). Ring rod channel bevels  84  are shown on opposite ends of a diameter of the ring, adjacent the projections  86 , although in alternative embodiments they need not be adjacent. Ring rod channel bevels  84  are depressed areas along an upper surface of the circumference of the ring  22 , and assist in receiving the pivoting rod (also not shown for clarity). Another variation of the retaining ring  22  is shown in  FIG. 8B . The retaining ring  22  of  FIG. 8B  includes a split such that the retaining ring  22  is approximately C-shaped. The split retaining ring  22  snaps into place inside the seat  26  to secure the assembly.  
         [0072]     Still referencing  FIG. 5A  and with particular reference to  FIG. 5B , the seat  26  includes an inner surface and an outer surface and a first end  81  and a second end  83 . At least one sidewall  79  extends between the first end  81  and the second end  83  forming a top opening at the first end  81  and at least one “U”-shaped void or rod channel  90  into which the pivoting rod may pivot when installed. Two rod channels  90  or voids and shown in  FIG. 5A  in which voids  90  are defined in part by seat rod channel bevels  94 . A void or keyway  98  is provided near the base of the seat to engage each projection  86  to orient the ring in a press-fit fashion with respect to the coupler and seat. In  FIG. 5A , the keyways  98  are adjacent the rod channel bevels  84  and  94  because the keys  86  are adjacent the same; however, the keys and keyways need not be along the same diameter as the bevels. The ring and seat rod channel bevels may generally match each other in shape, pitch, angle, slope, etc., and assist in orienting the rod pivot arc as well as orienting the rod channel to receive the rod on the cradle or receiving assembly.  
         [0073]     The seat  26  includes a closure mechanism receiving portion or a cap receiving portion  75  configured to receive a cap at the first end  81  and a coupler receiving portion  73  configured to receive a coupler. The coupler receiving portion  73  includes a tapered ramp that corresponds to a tapered ramp on the coupler. The cap receiving portion  75  includes a locking lug groove  88  that is provided near the top of the seat  26  to slidingly receive a corresponding locking lug or projection of the cap, described below. Cap rotation of, for example 90 degrees, secures the cap in place. The locking lug groove  88  may further include an anti-rotation mechanism, such as a mechanical stop. In this way, the locking lugs may be fixed in the amount of rotation needed to secure them in place. A wing groove  92  is also provided on the seat  26 , to slidingly receive and engage a corresponding wing lug or projection on the cap, as described below. The wing groove  92  may also be provided with a mechanical stop that prevents further rotation of the wing within the wing groove similar to the locking lug groove. On the outside surface of the seat  26 , a flange  21  and two recesses  23  in opposed locations are formed as shown in  FIGS. 5A and 5B . The flange has an upper surface, lower surface and an outer surface.  
         [0074]      FIGS. 6A-6B  show bottom and top perspective views, respectively, of the cap system  50  having cap  48  and set screw  52 . The cap  48  incorporates at least one groove  112  and recess  113  for engagement with an inserter or driving tool to accomplish the partial rotation needed to lock the cap  48  into the seat  26 . The inserter or driving tool may grip the cap for rotation: the recess provides room for “tangs” of the inserter tool, and the groove allows the “tangs” to clear the inner surfaces of the seat. A flange  116  may be provided which is an annular projection at the top surface. The flange  116  acts as a mechanical stop, to limit the amount of insertion of the cap into the seat. The outer surface of the cap includes at least one seat-engagement feature for engagement with the cap-receiving portion of the seat.  
         [0075]     One seat-engagement feature on the cap is at least one locking lug  110  that is provided in at least one location around the circumference of the cap  48  and extending from the outer surface of the cap. As shown in  FIGS. 6A and 6B , two locking lugs  110  are provided on or are integral with the flange  116  opposite from one another. The locking lugs  110  are sized for insertion into the rod channel  90 . Also, the locking lugs  110  are configured to be rotatably inserted into the locking lug groove  88  in the seat  26 . Typically, the locking lugs  110  are first inserted into the rod channel  90  and then rotated into position inside the locking lug groove  88 .  
         [0076]     Another seat-engagement feature on the cap is at least one wing  54  that is provided in at least one location around the circumference of the cap  48  extending outwardly from the outer surface of the cap. As shown in  FIGS. 6A and 6B , two wings  54  are provided in opposed locations around the circumference of the cap  48 . The two wings  54  are aligned with the two locking lugs  110  wherein the wings  54  are located below locking lugs  110 . The wings are sized for insertion into the rod channel  90 . Also, the wings are configured to be rotatably inserted into the wing groove  92 . Generally, the cap  48  is placed into the seat  26  with the two wings  54  and the two locking lugs  110  in alignment with the rod channel  90  such that the cap  48  drops into the seat until the flange  116  abuts a surface of the locking lug groove  88 . After the cap is seated, it is capable of being turned. Turning of the cap  48  rotates the wings  54  and the locking lugs  110  into the wing grooves  92  and locking lug grooves  88 , respectively. To effect the rotation, a tool is used to engage the groove  112  and/or recess  113  of the cap to turn the cap  48  while it is inside the seat.  
         [0077]     In one variation, as shown in the perspective close-up view of the seat  26  in  FIG. 5B , the cap (not shown) is turned until rotation is stopped by a wall  85  located in the locking lug groove  88  against which the locking lugs  110  abut. A second locking wall (not shown) may also formed in the opposite locking lug groove generally diagonally from wall  85 . The degree of rotation is preferably approximately 90 degrees but the invention is not so limited and any degree of rotation is within the scope of the invention. The wall serves as an anti-rotation mechanism that prevents the cap from turning past a locked position. Other anti-rotation mechanisms may also be employed.  
         [0078]     In one variation, after the cap  48  is seated and rotated such that the wings  54  and locking lugs  110  are in the wing grooves  92  and locking lug grooves  88 , respectively, a set screw  52  located inside the cap  48  is tightened. As the set screw  52  is tightened, the cap  48  rises relative to the seat  26 , that is, the cap will move upwardly relative to the seat. This rise is arrested by the wings  54 , also known as wing lugs, contacting the upper surface of the wing groove  92 . In an alternative variation, the locking lugs alone or in conjunction with the wings are employed to arrest the rise of the cap as the set screw is advanced and a force, resulting from the set screw being biased against the seat, is applied to the rod below the set screw.  
         [0079]     Furthermore, in one variation, counter-rotation of the cap  48  is prevented as the set screw is advanced and the locking lugs  110  rise relative to the seat  26  into a recess  89  or window formed inside the locking lug groove  88  as shown in  FIG. 5B . The recess or window  89  includes a stop  91  against which the locking lugs  110  abut to prevent counter-rotation. After the cap  48  has moved upwardly upon set screw advancement such that the locking lugs  110  have substantially entered the recess or window  89 , the locking lugs  110  are substantially moved out of the locking lug groove  88  and they cannot be moved back into the groove  88 , and thus the cap cannot be removed, until the set screw is “backed off” and the cap drops or is “lowered” such that the locking lugs  110  reside again in the groove  88 . It should be noted that a corresponding recess  89  and a corresponding wall  91  is formed in the other side of the cap receiving portion of the seat.  
         [0080]     The wing  54  has a reverse angle surface  114  to inhibit spreading of the seat. The wing or wing lug groove  92  defined by the interior of seat  26  slidingly receives the wing  54  or wing lug of the cap  48 , and the cap is locked into the seat when the cap is rotated, for example, by 90 degrees. The reverse angle surface  114  keeps the seat  26  from splaying as the set screw  52  is rotated. In particular, as the set screw  52  rotation forces the cap upwards, the reverse angle surface  114  keeps the walls of the seat  26  from spreading outward. Otherwise, the forces of the cap upward movement would tend to spread the seat.  
         [0081]     In an alternative embodiment, the wings may snap into recesses of the wing lug groove  92  when an appropriate or predetermined degree of rotation has been achieved. Appropriate spring-loading may be employed to achieve this snapping feature.  
         [0082]     As shown in  FIGS. 6A and 6B , in one variation, the bottom surface of the set screw  52  includes a dome  118  that protrudes from the bottom surface of the set screw  52 . As the set screw  52  is advanced, the feature  118  contacts the rod  40  and creates a single point, line or smaller surface area of contact than would otherwise be the case between the cap system  50  and the rod  40 . This restrains less of the rod, allows some flexion and thus reduces the stiffness of the total device between the screws, leading to a better stress distribution through the construct, a lower stress concentration and enhanced fatigue performance. Examples of other features in the bottom surface of the set screw include but are not limited to any one or more of the following used alone or in combination: a dome, nipple, aperture, raised surface, and a dome with an aperture.  
         [0083]      FIG. 7  shows additional details of the coupler  24 . The coupler  24  generally has a bone screw receiving portion  128  and a rod receiving portion  122 . The rod receiving portion is shown in one variation as two upstanding forks  122  each of which has a receiving hole  66  for receiving the rod pin. The upstanding forks  122  may have a tapered end, a closed end and/or an open end.  FIG. 7  shows a beveled region  124  radially exterior of each fork that lessens the amount of material in each fork, allowing greater amounts of flex. The coupler access bore hole  108  provides access to the engagement means of the screw such as a hex socket. The inner surfaces of the rod receiving portion of the coupler and the screw head receiving portion are provided with grit-blasting to increase the surface roughness and resultant friction coefficient between the coupler and/or the rod.  
         [0084]     Still referencing  FIG. 7 , a lip  102  is provided to mate with the retaining ring  22 . An approximately spherical bore  128  or screw head receiving portion is provided in the interior of the bottom of the coupler  24  that “snap-fits” over the head  32  of the screw  30  to allow a limited amount of rotation, for example 60 degrees of polyaxial rotation. The exterior surface of the coupler, exterior of the spherical bore  128 , may be a generally tapered ramp  126 . Slits  109  may further be provided to allow circumferential compression around the screw head.  
         [0085]     With the cap in the cap-receiving portion of the seat and as the set screw is advanced within the cap, the screw contacts the rod and the cap rises relative to the seat until the wing lugs contact the upper surface of the wing lug groove and the cap is thereby biased into a locked configuration by the seat. Further advancement of the set screw exerts additional force onto the rod and it is transferred to the coupler and drives the coupler downward. As the set screw drives the coupler downward, e.g., through a force transmitted through the rod, the coupler is pushed downward, further into the seat. The tapered ramp of the coupler engages the corresponding tapered ramp in the seat. The coupler is radially compressed (which is possible because of the slits  109 ), thus gripping the screw head securely and simultaneously locking the bone screw and the rod into the desired position. In one variation, the lockdown of the bone screw does not occur simultaneously with the lockdown of the rod. For example, if the rod embodiment of  FIG. 2E  is employed, advancement of the set screw contacts the ball portion  41  that slides and rotates with respect to the rod  40 . The contact with the set screw transmits force directly to the coupler to effect the lockdown of the bone screw relative to the seat without locking down the rod relative to the seat, thereby, allowing the rod to slide and rotate with respect to the ball portion  41 . Further advancement of the set screw compresses the ball portion  41  locking the rod into position relative to the ball portion  41  after the bone screw has been locked. In an alternative variation, the advancement of the set screw contacts the ball portion  41  and compresses the ball portion  41  to effect lockdown of the rod with respect to the seat without locking down the bone screw relative to the seat. Further advancement of the set screw transmits force to the coupler to effect lockdown of the bone screw relative to the seat after the rod has been locked first. This independent lockdown mechanism permits the selective lockdown of the rod relative to the seat and bone screw relative to the seat. Prior to the set screw being tightened, the bone screw and rod each were allowed movement relative to the seat. After the set screw is tightened, movement of both the bone screw and rod is generally eliminated. Hence, the cap set screw system in combination with the seat and coupler provide a lockdown mechanism just described that operates between a locked configuration in which the rod and the bone screw are locked into position and an unlocked configuration in which the rod and the bone screw each have a range of motion relative the seat. It is noted that with the cap positioned in the seat, the rod is not in vertical alignment with the seat, but instead, at least a portion of the rod extends through the rod channel  90 . However, the rod still retains a range of motion while disposed in the rod channel and in the unlocked configuration. It should be noted that in one variation of the present invention, the system permits some degree of motion of the rod and bone screw even when the system is in the locked configuration. Also, there may even be additional structural elements employed to permit some degree of motion while in the locked configuration. Some examples of such elements include, a reduced point of contact with the set screw as described herein and shock absorption elements deployed between the coupler and the seat for example. Hence, the term “locked” is used to describe the restriction of motion of the rod and/or screw relative to the unlocked configuration. Also, the term “locked” is used with respect to the cap to describe the cap being seated inside the seat whether or not the set screw is advanced to the locked configuration to set the position of the rod and/or screw.  
         [0086]     A recess or keyway  106  is provided in which a driving tool may be disposed to receive the keys or projections  86  on the retaining ring  22 . Especially for use on the hinged assembly version, a “lead-in” ramp  104  may be employed as a chamfered edge, providing a mechanical advantage to spread the coupler forks. To this end, the forks may be configured to flex and also be resiliently biased. All of these features allow the hinge pins to more conveniently slide in and snap securely into the receiving holes  66 .  
         [0087]     Especially for use on the receiving cradle assembly embodiment, the coupler may be generally the same and may further include a smooth surface  133  which incorporates a radiused edge which increases the contacting surface area and reduces high stress concentrations. In this way, the rod may be even more tightly received between the forks reducing the stress concentration on the rod and coupler.  
         [0088]      FIG. 8A  is a more expanded view of  FIG. 2A . The basic four set of components, ring, coupler, seat, and screw, may be the same or similar for both the hinged assembly and the receiving cradle. The seat snaps onto the screw, the coupler is placed into the seat, and the ring is press-fitted into the seat to retain the coupler. To this end, the seat  26  may have an internal tapered bore to hold the coupler and screw in a snug configuration.  
         [0089]      FIG. 8B  is a perspective view of another variation of the present invention showing the screw  30 , seat  26 , coupler  24  and retaining ring  22  in an exploded view and  FIG. 8C  is a side-elevational view of the system wherein like elements are referenced with like numerals. In the variation shown in  FIGS. 8B and 8C , the screw includes a flange  111  located below the screw head  32 . The flange  111  extends outwardly from and around the screw; however, the invention is not so limited and the flange  111  may be noncontinuous forming two or more flange pieces around the circumference of the screw for example. The flange  111  is configured to serve as a stop and prevent the screw from being inserted into the bone beyond the flange. Hence, the flange  111  is sufficiently broad that it does not dig into the bone as the screw is attempted to be advanced beyond the flange-to-bone contact. Preferably, the flange surrounds the circumference of the screw at a distance below the screw head that permits maximum angular and polyaxial adjustment and rotation of the seat. The flange  111  may be formed or located closer to the screw head to constrain the degree of freedom of the polyaxial adjustment and rotation of the seat if it is so desired. Furthermore, the flange advantageously provides a tactile signal to the clinician when the flange  111  abuts the bone when inserting the screw into the bone. Without the tactile signal provided by the screw flange, the clinician must verify advancement of the screw under fluoroscopy to avoid the screw head being completely driven to the surface of the bone which would impede the ability of the polyaxial seat to rotate and angulate freely. Because the flanged screw facilitates screw insertion, it is particularly advantageous in minimally invasive procedures.  
         [0090]     As mentioned above, the retaining ring  22  of  FIG. 8B  includes a split such that the retaining ring  22  is approximately C-shaped. To assemble the screw system shown in  FIG. 8B , the bone screw head  32  is inserted into the coupler  24  and the split retaining ring  22  is inserted into the seat  26 . The screw and coupler assembly is passed through the top of the seat  26  and attached together to complete assembly of the screw system.  
         [0091]     Various methods of use are now described with respect to  FIGS. 9A-9D . In use, the set screw  52  is partially advanced into the cap  48  and the same is situated above the seat  26  as shown in  FIG. 9A . In  FIG. 9A , for clarity, the locking lugs and the wings of cap  48  are half in-the-page and half out-of-the page. In the same way, one rod channel is below the page and the other is above the page.  
         [0092]     As shown in both views in  FIG. 9B , the cap is rotated 90 degrees with respect to the orientation of  FIG. 9A , and the same is placed into the seat. The locking lugs and wings are shown in the general area of the rod channel  90 . Using the cap inserter groove, the cap is then rotated to lock the same into position.  FIG. 9B  shows an approximately 45 degrees turn counter-clockwise. In this configuration, the cap is partially locked onto the seat. In  FIG. 9D , the cap has been rotated 90 degrees with respect to the orientation of  FIG. 9B , and the cap is now locked onto the seat. A mechanical stop  91  may be employed to prevent further rotation.  
         [0093]      FIG. 10A-10C  illustrates a method of rod installation which is typically accomplished just prior to the cap and set screw fixation shown in  FIG. 9A-9D . Pivoting rod system  40  is disposed in the screw system  20 . The chamfered nature of pins  46  eases the installation into the receiving holes of the coupler, as does the ramp  104  on the coupler. Once installed, as shown in  FIG. 10B , the same may be rotated to approximately the position shown in  FIG. 10C , though the actual amount of rotation will vary based on the shape of the patient&#39;s spine.  
         [0094]     It should be noted that although  FIGS. 10A and 10B  illustrate the rod  40  in a vertical orientation that is substantially parallel to the screw orientation, the invention is not so limited. According to one variation of the invention, the rod  40  may be oriented 90 degrees in  FIGS. 10A and 10B  such that it is substantially perpendicular to the screw orientation and be capable of insertion into the seat. In another method of practicing the invention, the rod  40  can be oriented at any angle relative to the screw that permits insertion into the seat. The screw system  20  allows for the bone screw to be first set into the bone and then, following the insertion of the bone screw, the rod is attached in any of the various orientations of the rod relative to the screw just discussed. This two step process is advantageous in the event that patient anatomy makes it difficult to insert the rod-plus-screw combination at the same time. The system is versatile such that the rod may be attached first to the seat in the various orientations of the rod relative to the screw just described and then the entire system  20  (the rod-plus-screw combination) set into the bone simultaneously using instrumentation that grips the seat  26  at the flange  21  and/or recesses  23 , for example, or the rod  40  itself. After the rod is attached, the cap is seated and locked. Prior to the locked configuration, that is a complete tightening of the set screw, and with or without the cap in position, the system retains two levels of freedom of motion. Firstly, the rod is free to be adjusted with respect to the seat and secondly, the seat is free to be adjusted relative to the screw. Hence, both the rod and the screw retain a degree of motion relative to seat, with or without the cap in place, which allows the clinician to custom orientate the seat with respect to the bone screw. The freedom of motion also permits the clinician to custom orientate the rod with respect to the seat with the system deployed inside the patient and in the unlocked configuration. This freedom of motion advantageously provides the surgeon with a much-needed, increased ease of implantation.  
         [0095]      FIGS. 11A and 11B  show a typical installation environment of the devices in two spinal segments of a patient.  
         [0096]      FIG. 12  illustrates a side cross-sectional view of the seat  26  with a sectional view of a seat-gripping instrument  130  engaging the seat. The flange  21  of the seat  26  includes an upper surface  136 , an outer surface  138  and a lower surface  140  and extends outwardly from the outer surface of the seat. Although  FIG. 12  shows a single circumferentially continuous flange  21 , the invention is not so limited and more than one flange may be formed.  
         [0097]     Still referencing  FIG. 12 , the instrument  130  includes a first portion  132  and a second portion  134  interconnected with a handle portion (not shown). The first portion  132  of the instrument is configured to contact at least a portion of the flange outer surface  138  and/or at least a portion of the flange lower surface  140 . The second portion  134  of the instrument is configured to slide with respect to the first portion  132  and contact the upper surface  136  of the flange as shown in  FIG. 12 . The second portion  134  is advanced to tighten against the upper surface and securely retain the seat within the instrument  130  for deployment into the patient.  
         [0098]     The preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of present invention is embodied by the appended claims.