Patent Publication Number: US-6336927-B2

Title: Apparatus, method and system for the treatment of spinal conditions and fixation of pelvis and long bones

Description:
This is a division of application Ser. No. 08/898,862 filed Jul. 23, 1997, now U.S. Pat. No. 5,904,682, which in turn is a division of Ser. No. 08/692,821 filed Jul. 29, 1996, now U.S. Pat. No. 5,716,357, which in turn is a continuation of Ser. No. 08/448,566 filed Jun. 7, 1995, now abandoned, and of PCT/US94/11463 filed Oct. 7, 1994. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to apparatus and methods for treating and correcting spinal abnormalities or conditions, stabilizing the position of the spine and vertebrae thereof and fixing or moving the position of bones other than those in the spine. More specifically, the present invention provides an apparatus and system which includes a plurality of links usable together under various circumstances to treat, e.g., different spinal curvature conditions or bone fractures. 
     2. Description of the Relevant Art 
     The prior art includes many different apparatus and methods for treating spinal conditions. Known apparatus utilize elongate plate members having several aligned openings or an elongated slot therein for receiving screws or bolts that are affixed to vertebrae. The plate is secured to the screws or bolts and exerts force on the selected vertebra or vertebrae to move same into a desired position or to maintain same in a desired position. The plate also overlies the majority of the lateral bone surface of the vertebrae. 
     In using such known plate systems, a problem arises when the points on the vertebrae defined by the screws or bolts are not collinear, i.e., they do not lie in a straight line. This creates a problem for the physician because the openings in the plate are collinear and, therefore, the screws do not line up with the plate openings. The physician has several options to compensate for such nonalignment, all of which present additional problems themselves. 
     The plate can be contoured in the frontal plane to attempt to line the screws up with the plate openings. Due to the thickness and high strength of the plate, this is essentially impossible to do intraoperatively. Another option is to bend the screws or bolts so that they fit in the slots or openings in the plate. This creates an immediate high stress region in the screw or bolt which can cause failure of the same upon cyclical loading or, even worse, can lead to a fracture of the pedicle of the vertebra. 
     A third option is to place the screws or bolts in a less than optimum position or trajectory in the pedicle of the vertebrae so that they line up with the plate openings. This too can lead to pedicle fractures or cut-out, as well as nerve root injury. 
     Other known apparatus for treating spinal deformities are disclosed in U.S. Pat. Nos. 5,102,412 and 5,181,917. These apparatus include elongate rod members which have vertebra engaging means secured thereto in an adjustable fashion. The apparatus can be used with bone bolts or screws, or laminar or pedicle hooks. However, the rods are essentially straight which makes utilizing nonlinear points of connection on adjacent vertebrae difficult without the use of specially formed components. In addition, spinal apparatus and systems such as those disclosed in the aforementioned patents include a large number of moving parts and therefore are inherently susceptible to malfunction. 
     Accordingly, there is a need in the art for a method and apparatus for treating spinal conditions which avoids the problems of the prior art, permits attachment to nonlinear points on adjacent vertebrae, and provides increased bone volume for grafts and fusion. 
     The prior art includes various apparatus for the treatment of non-spinal conditions, e.g., in long bones and the pelvis. Such conditions include fractures, joint fusions, osteotomies, etc. The prior art apparatus for treating the aforesaid non-spinal conditions do not permit attachment of the bone fixation device to non-linear attachment points on the bone or bones being treated. Accordingly, there is a need in the art for an improved apparatus and method for the treatment of non-spinal conditions. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for treating spinal conditions by moving a vertebra to a desired position with respect to additional vertebrae or maintaining the vertebra in the desired position. 
     The apparatus of the present invention includes a plurality of link members that can be secured to adjacent vertebrae in chain-like fashion utilizing pedicle bolts or screws that are not collinear with each other. The link members can be used to subdivide multiple nonlinear pedicle fixation points into units of two adjacent points which two points can be interconnected with a single link member. 
     The present invention thus facilitates multiple point fixation using two points at a time to overcome the problem in the prior art of nonalignment between plate openings and pedicle screws. The links form a chain and once they are secured to the pedicle screw or bolt with a locking nut, the result is a rigid construct securely affixed to the vertebrae. 
     The link members are in the form of plates or rods with opposite end portions and a central portion. The opposite end portions each have an aperture therein configured to receive attachment means affixed to the pedicle of adjacent vertebrae. The central portion of each link member is preferably offset from the end portions so that when the link member is attached to adjacent vertebrae portions of the latter are substantially uncovered by the link central portion. However, the present invention also includes links in which the central portion is not offset. 
     This offset provides increased vertebrae bone volume as compared with prior art apparatus which overlie the vertebra surface, which increased bone volume can be used for bone grafts and fusion. In addition, the present invention permits visualization of bony maturation using plain X-rays since the links do not overlie the graft area. 
     The links of the present invention can be used with and secured to pedicle screws, bolts, or pedicle or laminar hooks. A combination of hooks and screws or bolts can be used as well depending on the particular application of the invention. For example, a laminar hook can be used on a lamina that is being fused to avoid damaging its associated facet (joint) such as would be caused by a screw or bolt. 
     The surface of the link member of the present invention is preferably provided adjacent the apertures in the end portions thereof with radial cuts or other interdigitating structure for facilitating and enhancing the locking engagement of the links with a pedicle bolt, screw, hook, or other link(s) at a desired relative position. The bolt or hook has a threaded extension portion that cooperates with a locking nut, and a wedge-shaped washer if needed, to secure an end of the link member to a vertebra. 
     The wedge-shaped washer compensates for a lack of parallelism in the axial plane between adjacent bolts or screws. 
     An additional aspect of the present invention is that link members can be used to secure contralateral chains (formed as described above) to each other at their ends, and/or points intermediate their ends, to form a quadrilateral or ladder-shaped construct having increased torsional stability. 
     A further aspect of the present invention provides a multi-directional attachment assembly which includes a screw portion or member threaded for engagement with a bore formed in a bone, and a bolt portion or member threaded to receive a plate, rod, etc., and a complimentary threaded locking nut. The bolt portion is adjustable with respect to the screw portion and can be positioned in a location that is optimal with regard to receiving and supporting the aforementioned plate, rod, etc. The bolt portion may be angularly positioned relative the screw portion so that the former engages the plate or rod in a desired manner, for example, a perpendicular fashion. The bolt and screw portions preferably include mating hemispherical portions which are joined on a plane that is oblique to the longitudinal axis of both the screw and bolt portions. The multi-directional attachment device may be used with spinal fixation systems to facilitate optimal placement of the screw portion within the vertebra while permitting adjustment of the bolt portion relative thereto for optimal engagement with the plate and/or rod. In addition, the above-described link structures and multi-directional attachment members may be used in the treatment of non-spinal conditions including fractures, joint fusions, osteotomies, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional features of the present invention will be apparent to those skilled in the art from the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
     FIG. 1A is a front elevational view of a link member according to a first embodiment of the present invention; 
     FIG. 1B is a front elevational view of a link member according to another embodiment of the present invention; 
     FIG. 1C is a front elevational view of a link member according to another embodiment of the present invention; 
     FIG. 2 is an exploded view of a pedicle or bone bolt, a washer, and a locking nut according to the present invention; 
     FIG. 3 is an exploded view of a pedicle or laminar hook, the link shown in FIG. 1B, and a locking nut according to another embodiment of the present invention; 
     FIG. 4 is a front elevational view showing a plurality of the link members depicted in FIG. 1A placed together to form a chain-like construct; 
     FIG. 5 is a front elevational view showing a plurality of the link members depicted in FIG. 1B, comparing them to link members in which the central portion is not offset. 
     FIG. 6 is a side elevational view, partly in section, showing two bone bolts and a locking bolt securing together two link members in a delta configuration according to the present invention; 
     FIG. 7 is a side elevational view showing a plurality of link members according to the present invention secured together by pedicle or laminar hook members and looking nuts; 
     FIGS. 8A and 8B are, respectively, front and side elevational views of a curved link member according to another embodiment of the present invention; 
     FIG. 9A is a perspective view of a multi-directional attachment device constructed according to the present invention; 
     FIG. 9B is a perspective exploded view of the multi-directional attachment device depicted in FIG. 9A showing a modification thereto; 
     FIG. 9C is a perspective exploded view of the multi-directional attachment device depicted in FIG. 9A; 
     FIG. 9D is a perspective view of a multi-directional attachment device according to another embodiment of the present invention; 
     FIG. 10A is a front elevational view of the link member and multi-directional attachment device of the present invention used as an external bone fixation apparatus; 
     FIG. 10B is a side elevational view of the link member and multi-directional attachment device of the present invention used as an external bone fixation apparatus in a two plane fixation system; and 
     FIG. 11 is a front elevational view of the link member and multi-directional attachment device of the present invention used as a pelvic bone fixation apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1A, a link member according to a first embodiment of the present invention is indicated generally by the reference numeral  10  and includes first and second end portions  12 ,  14  and a central portion  16 . The end portions  12 ,  14  each have an aperture  18  therein which apertures are configured to receive a threaded bone bolt or screw, or a laminar or pedicle hook, to secure the link  10  to adjacent vertebrae as will be described below. The link  10  is preferably integrally formed so as to comprise a one-piece structure. The link in the embodiment of FIG. 1A is in the form of a plate and is substantially V-shaped with the central portion  16  comprising the apex of the V shape. 
     FIG. 1B shows another embodiment of the link according to the present invention indicated generally at  20 . Link  20  includes first and second end portions  22 ,  24  and a straight central portion  26 . The end portions  22 ,  24  each have an aperture  28  similar to the apertures  18  in link  10 . Link  20  also is in the form of a plate which plate is substantially C-shaped. The C-shape of link  20 , like the V-shape of the link  10  shown in FIG. 1A, provides the link with end portions that are offset from the central portion for reasons discussed below. 
     FIG. 1C shows another embodiment of a link according to the present invention which link is indicated at  30  and includes end portions  32 ,  34  and central portion  36 . The link  30  is in the form of a C-shaped rod member having a cylindrical cross-section. It will be appreciated that the cross-sectional shape of link  30  can be noncylindrical and that link  30  can be a rod member that is V-shaped similar to link  10  in FIG.  1 A. The ends  32 ,  34  have an aperture  38  disposed therein for receiving pedicle bolts, screws or hooks as in the aforementioned embodiments for reasons as will be described below. 
     The present invention also includes link members in which the end portions and the central portion are aligned, i.e., not offset, which links can be connected in chain-like fashion to follow a nonlinear path. For example, see links  240  in FIG.  5 . In an alternative embodiment, the non-offset links can be in the form of a rod with a cylindrical or noncylindrical cross-section or, the links can be in the form of other suitably shaped structure. 
     However, the links preferably have the respective end portions offset from the central portion as in links  10 ,  20  and  30 . This offset is formed such that a line passing through the midpoint of the apertures formed in the end portions of links  10 ,  20  and  30  does not overlie the central portion of the link. See the dashed line in FIGS. 1A or  1 C. While the illustrated links are V-shaped and C-shaped, those of ordinary skill in the art will recognize that other shapes are possible as long as the central portion of the link is at least partially offset from the end portions thereof. 
     FIG. 5 shows two rigid constructs  210 ,  230  formed of a plurality of links  220 ,  240 , respectively. Constructs  210  and  230  are secured to the plurality of vertebrae  200  by means indicated schematically at  222  and  232 , respectively. The securing means can be in the form of bone screws, bolts or hook members as will be explained in detail below. Links  220  of construct  210  have an offset central portion which leaves the lateral margin  202  on one side of the vertebrae  200  (to which the links are secured) substantially uncovered. However, links  240  of construct  230  do not have such an offset and thus the lateral margins on that side of vertebrae  200  are covered by the links  230 . 
     The offset links  220  provide additional bone volume or surface that can be used for grafts, fusion, etc. In addition, because the bone surface is uncovered, a physician can view the bony maturation of the vertebrae being treated with plain X-rays. This is very difficult in prior art systems in which the elongated plates or rods overlie the graft area. 
     FIG. 2 shows a pedicle or bone bolt  50  used with the link members of the present invention to secure the same to vertebrae. While FIG. 2 shows a bone bolt, it will be recognized that a bone screw, e.g. a threaded member having a head which forces the link member against the bone surface, can also be used. The use of bone bolts and screws to secure spinal curvature apparatus to vertebrae by placing the same in bores formed in the vertebrae is generally known and will not be described in detail. As will be discussed below, it is also possible to use laminar or pedicle hook members in place of or in combination with the bone bolts or screws. The engagement of such attachment members with vertebrae is discussed in aforementioned U.S. Pat. Nos. 5,102,412 and 5,181,917 granted to the present applicant, the subject matter of which patents is incorporated herein by reference. 
     The bone bolt  50  shown in FIG. 2 includes a helical portion  52 , i.e., a portion having a helical thread or cutting surface for locking the bolt within a bore formed in a vertebra (not shown). An upper portion  54  of bolt  50  is threaded to receive a locking nut such as that shown at reference numeral  70  in FIG. 2. A shoulder portion  56  is disposed between the aforementioned portions  52  and  54 . 
     After determining the proper position on the vertebrae, a bore is formed for each bolt as is known in the art and the bolt  50  is secured to a first vertebra (not shown) with shoulder  56  and threaded portion  54  extending therefrom. The aperture end of a link member (e.g., link members  10 ,  20  or  30 ) is positioned over the threaded portion  54  with the link member resting on shoulder  56 . The shoulder  56  is provided with means for enhancing the engagement between the link member and the bone screw which means can be in the form of radial cuts  58 , or any other suitable means such as roughened surfaces that aids in locking the link to the screw. With one end of the link member fastened to a first vertebra, the other end of the link member is positioned on a bolt  50  that is similarly secured to a second adjacent vertebra so as to move or lock one vertebra relative to the other vertebra. 
     A washer member  60  is shown which preferably is wedge-shaped and is configured to slide over the threaded portion  54  of bone bolt  50 . The wedge shaped washer  60  can be used to secure a link member (not shown in FIG. 2) to a bolt, screw or hook member that is attached to a vertebra and forms an angle therewith. The washer  60  is placed on the shoulder  56  so that the link member forms substantially a right angle with the longitudinal axis of the bone bolt. Locking nut  70  then is threaded over portion  54  of bolt  5 O. The washer member  60  also preferably has radial cuts or other means for rotationally locking the link to the washer to provide overall torsional stability to the assembly. 
     FIG. 4 shows rigid constructs  100  and  102  formed of a plurality of link members  10  secured to each other in chain-like fashion. As seen therein, the links  10  are positioned with the end of one link overlying the end of an adjacent link. As also seen therein with respect to the construct  100  (on the left hand side of FIG.  4 ), a line passing through the midpoints of connection points A, a, b and B does not follow a linear path. The same is true with regard to the construct  102  and connection points C, c, d and D. 
     It is apparent that the aforementioned four connection points of either construct  100  or  102  are nonlinear and could not be connected using a conventional flat plate with a plurality of, e.g. four, linearly aligned openings. The same is true with regard to a prior art plate having an elongated linear slot or opening in which is positioned a plurality of bone bolts or other attachment structure. It will, of course, be recognized that the four connection points depicted in FIG. 4 are for exemplary purposes only and that it is possible to use more or less vertebrae attachment points. This aspect of the present invention permits a rigid construct to be formed using multiple vertebrae attachment points notwithstanding the fact those points do not lie in a straight line. This was not possible with prior art plates having linearly aligned multiple openings. 
     Also shown in FIG. 4 is a link member  10 A, depicted in phantom, which serves as a cross-tie mechanism that secures the distal most connection points A and C of the constructs  100  and  102 , respectively. While not shown in FIG. 4, a link member preferably is used on the opposite distal end points B and D. This results in a quadrilateral construct that stabilizes the chain-like construct both torsionally and in the frontal plane, as well as increases the strength required to pull the screws out of the vertebrae. Further, such cross-tie mechanisms can be used as well at points intermediate the distal most connection points of the chain-like constructs. 
     A further embodiment of the present invention is shown in FIG,  6  and includes a cross-tie mechanism  300  to provide a quadrilateral construct similar to that discussed above. The cross-tie mechanism  300  includes two links  310  that are produced according to the teachings of the present invention which links are secured to respective bolts  50  substantially as described above. That is, locking nuts  70  cooperate with threaded portions  54  of bolts  50  to attach the tatter to the links  310 . 
     The ends of the links  310  opposite bone bolts  50  are secured to each other by a locking bolt  150  and nut  170 . The ends of the links  310  are overlapped on the bolt  150  and the nut  170  is threaded over and secured to the bolt  150  to lock thee entire assembly. The cross-tie mechanism  300  provides a rigid quadrilateral construct which increases the torsional stability and pull-out strength. As seen in FIG. 6, bone bolts So are disposed in respective vertebrae (not shown) in a converging fashion which, when combined with a cross-tie mechanism, provide a configuration which increases the overall pull-out strength of the assembly. The cross-tie mechanism  300  also can be used at different locations along the length of the two constructs to form a ladder configuration. 
     With reference to FIG. 3, a laminar hook  80  is shown which can be used in addition to or in place of the bone bolts  50  of FIG.  2 . While member  80  is referred to as a laminar hook, those skilled in the art will appreciate that pedicle hooks could be utilized as well. Laminar hook  80  incudes a hook or pad portion  84  which engages portions of the lamina of a respective vertebra. It will be appreciated that the hook portion  84  can be in a form other than the L-shape shown in FIG. 3 and, for example, can take the shapes shown in the aforementioned patents incorporated by reference in this application. 
     Hook member  80  further includes a threaded upper bolt portion  82  which is passed through the aforementioned aperture in one end of a link member, which link in FIG. 3 is in the form of the C-shaped plate member  20  (of FIG.  1 B). Locking bolt  70  is then threaded onto portion  82  of hook member  80  to securely lock the link  20  to the vertebra. 
     FIG. 3 also shows a sleeve member  90  which is disposed over threaded portion  82  of hook  80  to adjust the height of the link member  20  with respect to the hook or pad portion  84 . The proper height sleeve needed for a particular application will typically depend on the laminar thickness of a vertebra and will be determined intraoperatively by the surgeon. The sleeve  90  preferably will be provided in different sizes so that the surgeon can choose the proper size sleeve for the specific application. 
     With attention now directed to FIG. 7, a side elevational view of an exemplary rigid construct assembled from a plurality of link members in accordance with the present invention is indicated generally by reference numeral  400 . The construct includes a plurality of links  410  connected end-to-end in chain-like fashion as described above. The construct  400  includes laminar or pedicle hooks  480  but it will be understood that the above-described bone bolts or screws can be used in addition to or in place of the hooks  480 . 
     As shown in FIG. 7, sleeves  490  of various heights are interposed between the hook or pad portion  484  of each the hooks  480  and link members  410 . In addition, a spacer member  500  is disposed between the two adjacent link members  410  on the right end of the construct to compensate for the difference in height thereof so as to permit the links to be connected substantially in parallel. The aforementioned means for enhancing the locking relationship, e.g., roughening or radial cuts, between the bolts or hooks and the links, the locking nuts  470  and bolts, between adjacent links, etc., can also be used. 
     It will be readily recognized that the construct shown in FIG. 7 is but one example of an application of the present invention. In addition, it will be appreciated that FIG. 7 shows a construct that can be connected to a similar construct by any of the aforementioned cross-tie mechanisms to provide a quadrilateral construct. 
     Another aspect of the present invention is shown in FIGS. 8A and 8B and includes link members which are pre-curved in the sagittal or lateral plane for treating various spinal conditions. The link  610  is plate-shaped and is similar to link  10  except that link  610  is pre-curved in the sagittal plane. As can be seen, concave and convex surfaces  620  and  622  are defined on opposite sides of link  610 . FIGS. 8A and 8B show that the plane in which the link is pre-curved is substantially perpendicular to the plane in which the link is offset, i.e., the plane in which central portion  616  is offset from the ends  612 ,  614 . 
     While link  610  is plate shaped, the present invention also includes rod-shaped links (as described is above and shown in FIG. 1C) which are pre-curved in a similar fashion. The end portions  612 ,  614  of link  610  are preferably flat so that apertures  618  properly engage the vertebrae attachment members. However, the entire link (and not just central portion  616 ) can be curved instead. 
     For example, the link  610  can be placed as shown in FIGS. 8A and 8B to treat lordosis (forward curvature of the lumbar or thoracic spine), while the link could be placed in reverse to treat kyphosis (backward convex curvature of the spine). It will be appreciated that pre-curving the links in a manner other than that shown in FIGS. 8A and 8B is possible to achieve links having shapes and/or curvatures suited for additional applications. 
     The pre-curved links made according to the present invention provide the surgeon with numerous curved spinal links from which can be selected the appropriate link member for a particular application. This is superior to the flat prior art links. The curvature of the present links is set during manufacturing by any suitable process, e.g. a forming or machining process, and thus the link curvature is precise and results in efficient application of force to the selected vertebrae. 
     Referring now to FIGS. 9A-9C, a multi-directional attachment device constructed according to yet another aspect of the present invention is indicated generally by the reference numeral  750 . The multi-directional attachment device  750  may be used with the above-described spinal treatment apparatus, e.g. in place of or in addition to bone bolt  50  shown in FIG. 2, or with various other spinal treatment apparatus or, still further, device  750  may be used with fixation apparatus for treating other bones than those in the spine. For example, the element  750  may be used with external fixation apparatus for treating long bones and the pelvis. 
     Multi-directional attachment device  750  includes two portions, namely, screw portion  752  and bolt portion  754 . Screw portion  752  has an elongated section extending between opposite ends  756 ,  758 , which elongated section has helical threads  754  (or another suitable cutting surface) formed thereon for engaging a bore formed in a vertebra or other bone (not shown). The screw portion  752  preferably has a hemispherically-shaped end  758  with a bore  760  extending therethrough. The end  758  of screw portion  752  has a flat face (FIG. 9C) with serrations or other interdigitating structure  30   780  formed thereon for reasons described below. The bore  760  preferably is centrally located with respect to hemispherically-shaped end  758  as seen in FIGS. 9B and 9C. 
     Bolt portion  762  includes opposite end portions  766  and  768 . A threaded portion  764  extends from end  766  and includes threads (or other means) for engaging a locking nut after an apertured (or slotted) plate, rod, etc. has been positioned thereover as will be described below. Bolt portion  762  also include a bore  770  extending through hemispherically-shaped end  768 , which bore aligns with bore  760  of screw portion  752  to receive means for fastening portions  752  and  762  together. The bore  770 , however, includes a stepped portion  772  so that the enlarged head of a fastening means may be received in countersunk fashion so as to be substantially flush with the rounded portion of hemispherically-shaped end  768  of bolt portion  762 . The end  768  of bolt portion  762  has serrations or other interdigitating structure  780  for locking same to the similarly configured face of end  758  of screw portion  752 . The mating interdigitating surfaces may be in the form of serrations, ramped teeth, roughened surfaces or any other structure for rotationally locking the bolt portion  762  to the screw portion  752 . 
     An example of means for fastening the screw and bolt portions together is shown in FIG.  9 A and includes a lag-type set screw  790  that is threaded so as to mate with the threaded bores  760 ,  770  of the respective screw and bolt portions. The screw and bolt portions may be rotated with respect to each other and locked in position via set screw  790 . 
     Referring to FIG. 9A, the hemispherically-shaped end portions  758 ,  768  of the respective screw and bolt portions  752 ,  762  are joined in face-to-face contact along the plane or equator  800  to form a substantially spherically-shaped central member  774  (with set screw  790  not in place). In a preferred embodiment of the multi-directional screw/bolt according to the present invention, the screw and bolt portions are coaxial, i.e. positioned so that the longitudinal axis of each extends alongline  810 , the pivot axis  820  about which the respective portions are rotatable forms an oblique angle with the aforementioned longitudinal axis  810 . That is, when the screw and bolt portions are positioned so as to be coaxial, the longitudinal axis thereof does not form a right angle with the pivot axis about which the portions may rotate. 
     Another preferred embodiment of the invention is shown in FIG.  9 B and includes a projection  763  on the face of bolt portion  762  and a mating recess  765  on the face of screw portion  752  (or vice-versa). The projection  763  and recess  765  serve to facilitate proper engagement of the components as well as provide the attachment member with added shear strength. 
     This aspect of the invention permits the bolt portion  762  to extend from the center of the sphere  774  formed by the joined screw and bolt hemispherically-shaped ends,  758 ,  768 . In other words, regardless of the angular position of bolt portion  762  relative to the screw portion  752 , the threaded portion  764  of the bolt will always extend along a central axis of the aforementioned sphere. This feature permits the bolt  762  to engage the center of the opening in a plate, sod, etc. For example, it is known in the art to form a plate member with an opening for receiving a threaded attachment member secured to a vertebra. If the attachment member has a cylindrical or semi-cylindrical surface, the plate is formed with a concave depression adjacent the opening which depression seats on the cylindrical surface. Since the bolt portion  762  of the invention extends outwardly from the sphere along a central axis thereof, the plate (or other element) opening can be received on the bolt with the latter centrally located therein, and with the concave depression properly seated on sphere  774 . Locking nut  794  then is threaded over bolt extension  764  to securely fix the plate to the multi-directional attachment device  750 . This provides an extremely stable and secure assembly. 
     However, as seen in FIG. 9D, it also is possible to form a multi-directional attachment device  750 ′ such that the bolt extension  764 ′ does not extend from the center of the spherical portion  774 ′ formed by the mating hemispherically-shaped end portions  768 ′,  758 ′. The elongated screw section  754 ′ also may not extend is from the center of end spherical portion  774 ′. The embodiment shown in FIG. 9B, however, illustrates the preferred construction of the multi-directional attachment device of the present invention. 
     The benefits obtained by the ability to independently position the bolt portion  762  with respect to the screw portion  752  will be apparent to those skilled in the art. It is possible to position the screw portion in an optimal location in the vertebrae (or other bone) without concern as to the angle that the screw forms with the desired position of a plate, rod, etc., because the bolt portion can be adjusted relative to the screw portion so as to engage the plate in a perpendicular manner. 
     In another aspect of the present invention, a bone fixation system is provided for external applications, such as stabilizing bones which have been fractured, e.g., long bones, pelvic bones, etc. FIG. 10A depicts an external bone fixation system secured to a long bone B having a fracture F therein. The system includes a plurality of plates  900 ,  902 ,  904  which may have a structure according to the form of the spinal implant plates discussed above with respect to the aforementioned embodiments. A plurality of multi-directional attachment members having spherical portions  774  are provided, these members preferably having a structure described above in connection with FIGS. 9A-9C. 
     As can be seen in FIG. 10A, plate members  900  and  904  are secured to each portion of bone B on opposite sides of fracture F via respective pairs of multi-directional attachment members. These two plate members are joined to each other by a third plate member  902  the two ends of which respectively overlie an end of each plate  900 ,  904 . The threaded section  754  of the screw portion  752  of each attachment member is positioned in an optimal location in bone B, and the extension  764  of the bolt portion  762  of each respective attachment member is angularly adjusted to engage the plate members in a desired fashion, e.g., perpendicularly. Locking nuts  794  are positioned on bolt portions  762  to lock the plates to the attachment members. This arrangement, which is a one-plane fixation of bone B, stabilizes the bone portions to permit healing of fracture F. 
     FIG. 10B depicts a two-plane fixation of a bone B with a fracture F. Specifically, a first fixation assembly  910  includes a plate member  906  secured to bone B in a first plane via a pair of multi-directional attachment members with spherical portions  774  formed by mating end portions of bolt and screw portions, and locking nuts  794  as described above. A second fixation assembly  920  includes a plate member  908  secured to bone B in a different plane via a second pair of attachment members (the locking washers not being attached to bolt extensions  764  on which plate  908  is mounted in FIG.  10 B). The embodiment in FIG. 10B stabilizes the fractured bone B in two planes and is suitable, e.g., for use in applications requiring considerable stabilization forces. 
     FIG. 11 shows a further application of an external bone fixation system wherein the system is secured to a pelvic bone PB. The system includes a first fixation assembly  930  including a plate member  900  secured to a side of the pelvis via a pair of multi-directional attachment members having spherical portions  774 . As seen in FIG. 11, the screw sections  754  are positioned at a desired location in the bone while the bolt extensions  764  are adjusted relative sections  754  to engage plate  900  in a perpendicular manner. A second fixation assembly  940  is secured to the opposite side of the pelvis via a second pair of attachment members. The respective fixation assemblies  930 ,  940  may be connected to each other by cross-link members (not shown) constructed according to the above-embodiments (e.g., as in FIGS.  4  and  6 ). Such connection of the in plate assemblies may be carried out along the anterior (line A in phantom) or posterior (line P in phantom) surface of the pelvis. 
     Those skilled in the art will appreciate that the embodiments of FIGS. 10A,  10 B and  11  illustrate only exemplary non-spinal applications of the bone-fixation systems according to the present invention. Other applications and uses, of course, will be apparent to persons skilled in the art. 
     It is apparent that the present invention provides a link member that can be secured to other link members in chain-like fashion so as to permit connection of a series of points forming a nonlinear path on several vertebrae. In this manner, force can be selectively exerted on a particular vertebra or vertebrae to move the vertebra to a desired position and to maintain same in that position. 
     The present invention also includes links which overcome the problem with prior art plate systems in which minimal bone volume is available for grafts or fusion by providing an offset link structure. According to one aspect of the present invention, the central portion of the link is laterally displaced relative to the end portions thereof so as to leave the surface of the lateral margin of a vertebra to which the link is attached substantially uncovered. 
     The present invention further provides a multi-directional attachment device which includes a screw portion and a bolt portion. The device permits the screw portion to be secured in bone at a desired angular position, and the bolt portion attached thereto to be angularly adjusted so as to engage a spinal implant plate, rod, or an external bone fixation device in a desired manner. For example, by adjusting the bolt portion relative to the screw portion, the bolt portion can engage the fixation device in a perpendicular manner to enhance the overall stability and effectiveness of the system, as well as substantially prevent trauma to bone tissue. 
     Although the invention has been described with reference to particular embodiments, it is to be understood that the embodiments are merely illustrative of the application of the principles of the invention. Numerous configurations may be made therewith and other arrangements may be devised without departing from the spirit and scope of the invention.