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 . 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 . 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 . 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 . 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 . 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 fig1 a 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 . 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 . 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 . 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 . 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 . fig2 a 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 . 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 . fig2 b illustrates a perspective view of a pivoting rod 40 which is employed in an embodiment of the present invention . the pivoting rod 40 in fig2 b 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 fig2 b ) 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 . as shown in fig2 b , 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 . in another variation , additional flat portions 45 are formed on the ball end 44 of the rod 40 as shown in fig2 b . 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 . fig2 c 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 . 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 . various aspects and alternative embodiments of this basic system are described below . in this regard , it is noted that the above system of fig2 a - 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 . first , referring back to fig2 b , 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 fig2 d , 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 fig2 e . in fig2 e , a rod 40 is provided with a ball portion 41 having a bore 39 passing through it . the rod 40 of fig2 e 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 fig2 e . this embodiment advantageously provides yet another degree of freedom of motion and facilitates installation by the surgeon . furthermore , the embodiment of fig2 e 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 . an alternative way in which the pivoting rod 40 may be attached to the coupler that employs pins is shown in fig3 and fig3 a . 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 fig3 a , 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 . another variation for the rod - end system is shown in fig4 and the enlarged views of fig4 a - 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 fig4 a 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 . in an alternative embodiment to fig4 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 . various aspects of the seat , coupler , and retaining ring are now discussed . referring to fig5 a , 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 fig8 b . the retaining ring 22 of fig8 b 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 . still referencing fig5 a and with particular reference to fig5 b , 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 fig5 a 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 fig5 a , 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 . 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 fig5 a and 5b . the flange has an upper surface , lower surface and an outer surface . fig6 a - 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 . 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 fig6 a 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 . 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 fig6 a 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 . in one variation , as shown in the perspective close - up view of the seat 26 in fig5 b , 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 . 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 . 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 fig5 b . 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 . 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 . 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 . as shown in fig6 a 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 . fig7 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 . fig7 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 . still referencing fig7 , 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 . 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 fig2 e 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 . 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 . 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 . fig8 a is a more expanded view of fig2 a . 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 . fig8 b 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 fig8 c is a side - elevational view of the system wherein like elements are referenced with like numerals . in the variation shown in fig8 b 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 . as mentioned above , the retaining ring 22 of fig8 b includes a split such that the retaining ring 22 is approximately c - shaped . to assemble the screw system shown in fig8 b , 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 . various methods of use are now described with respect to fig9 a - 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 fig9 a . in fig9 a , 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 . as shown in both views in fig9 b , the cap is rotated 90 degrees with respect to the orientation of fig9 a , 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 . fig9 b shows an approximately 45 degrees turn counter - clockwise . in this configuration , the cap is partially locked onto the seat . in fig9 d , the cap has been rotated 90 degrees with respect to the orientation of fig9 b , and the cap is now locked onto the seat . a mechanical stop 91 may be employed to prevent further rotation . fig1 a - 10c illustrates a method of rod installation which is typically accomplished just prior to the cap and set screw fixation shown in fig9 a - 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 fig1 b , the same may be rotated to approximately the position shown in fig1 c , though the actual amount of rotation will vary based on the shape of the patient &# 39 ; s spine . it should be noted that although fig1 a 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 fig1 a 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 . fig1 a and 11b show a typical installation environment of the devices in two spinal segments of a patient . fig1 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 fig1 shows a single circumferentially continuous flange 21 , the invention is not so limited and more than one flange may be formed . still referencing fig1 , 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 fig1 . 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 . 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 .
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 .