Abstract:
An implant system for spinal fixation includes a fastener having an upper portion, a lower portion configured to engage a vertebra, and a shoulder between the upper and lower portions. A connector is provided for engaging the fastener to an elongated rod that is positionable along the spinal column laterally from a line containing the axis of the fastener. The connector includes a first portion for engaging the upper portion of the fastener adjacent the shoulder, and an integral second portion having a surface for engaging the elongated spinal rod. The connector further includes an elongated slot between the first portion and the second portion to permit relative lateral adjustment between the rod and the upper portion of the fastener. A threaded fastener is provided for clamping the rod against the surface of the connector. In one embodiment, the fastener includes an eyebolt defining an aperture for receiving the spinal rod. In another embodiment of the invention, the connector includes an elongated plate that defines a slot through which the second portion of the fastener is received. This slot includes a plurality of grooves on a surface of the plate facing the rod, each of the grooves configured to receive a portion of the rod therein.

Description:
RELATED APPLICATION 
     This application is a continuation of application Ser. No. 08/106,910, filed Aug. 13, 1993, now abandoned, which is a divisional of application Ser. No. 07/826,839, filed Jan. 28, 1992, now U.S. Pat. No. 5,300,073, which is a continuation-in-part of application Ser. No. 07/593,196, filed Oct. 5, 1990, now U.S. Pat. No. 5,127,912. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to sacral implants, and more particularly to an improved implant system for fixing a stabilizing appliance to the sacrum and to the lumbar vertebrae. 
     Spinal fusion, especially in the lumbar and sacral region is regularly employed to correct and stabilize spinal curves, to prevent recurrence of spinal curves and to stabilize weakness in trunks that result from degenerative discs and joint disease, deficient posterior elements, spinal fracture, and other debilitating problems. Spinal implant systems have been used regularly to stabilize the lumbar and sacral spine temporarily while solid spinal fusions develop. 
     Several temporary stabilization systems are currently in use. All perform adequately, however leave room for improvement. For example, an implant system for attaching the superior most lumbar vertebra (L 1 ) to the implant without interfering with normal motion of the next superior vertebra needs to be developed. Additionally, implant systems that achieve stronger sacral fixation, easier use for multiple segment fixation, and easier use with spinal deformity are needed. Further, better implant systems for rigidly tying the base of the system to the sacrum must be developed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sacral implant system that rigidly affixes the base of the implant system to the sacrum while allowing ease of installation and flexibility of design. Moreover, the present system provides apparatus for securing the upper portion of the implant system to, for example, the L- 1  vertebra, without interfering with the next superior most vertebra (T- 12 ) and any or all vertebrae in between. The sacral implant system of the present invention comprises first and second sacral plates for mounting on opposite sides of the sacrum adjacent the lumbosacral junction. Each of the sacral plates has at least a pedicle and oblique mounting means for rigidly affixing each of the sacral plates to the sacrum. The system also includes first and second rods extending in a superior direction and generally parallel relationship from respective ones of the sacral plates. The rods are situated on opposite sides of the sagittal plane. Means are also provided for rigidly affixing the rods to respective sacral plates. At least one connecting member is employed to rigidly interconnect the rods at a location superior to the sacral plates. Finally, a superior fixation plate having a lateral portion and a medial portion is employed to affix the superior most vertebra to be fused to the implant system. A pedicle screw is fixed to and through the pedicle of the vertebra. The lateral portion of the fixation plate is rigidly affixed to the pedicle screw. The medial portion of the fixation plate is offset in an inferior direction sufficiently far so that it avoids the inferior articulate process of the next superior vertebra. In this manner the next superior vertebra can move in a normal fashion relative to the vertebra to be fused during the temporary stabilization. Preferably, a lateral fixation plate is also used for pedicle fixation of intermediate vertebrae. 
     In another aspect of the invention, a specialized pedicle screw is provided for attachment of the offset and lateral fixation plates to the vertebra. The screw includes a first threaded portion for threading into the vertebra, a subhead portion and a second threaded portion projecting above the subhead. The second threaded portion is adapted to receive a nut. The subhead has a diameter greater than the second threaded portion and an upwardly facing shoulder lying in a plane substantially orthogonal to the axis of the screw. In use, the shoulder engages the anterior surface of the fixation plate while the nut is threaded on the second threaded portion and bears down against the posterior surface of the plate to secure the plate and screw together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an isometric view of the spinal implant system of the present invention as applied to the lumbar spine; 
     FIG. 2 is an isometric view of a right lateral sacral plate constructed in accordance with the present invention; 
     FIG. 3 is a plan view (in the sacral plane) of a sacral plate shown in FIG. 2; 
     FIG. 4 is a cross-sectional view taken along section line  4 — 4  of FIG. 3; 
     FIG. 5 is a cross-sectional line taken along broken cross-sectional line  5 — 5  of FIG. 3; 
     FIG.  6  and FIG. 7 are elevation views of fixation screws for use with the sacral plate; 
     FIG. 8 is a plan view of the offset fixation plate that is constructed in accordance with the present invention; 
     FIG. 9 is a elevation view of a pedicle screw for use with the fixation plate of FIG. 8; 
     FIG. 10 is an exploded isometric view of the fixation plate and screw of FIG. 9 shown in conjunction with a fixation rod and fastening system used in accordance with the present invention; 
     FIG. 11 is a plan view of a straight fixation plate; 
     FIG. 12 is an enlarged dorsal view of a superior portion of the sacrum showing the sacral plates implanted in accordance with the present invention; 
     FIG. 13 is an enlarged cross-sectional view taken along section line  13 — 13  of FIG.  1  through the sacrum looking in an inferior direction at the sacral implant system of the present invention; 
     FIG. 14 is an enlarged cross-sectional view taken along section line  14 — 14  of FIG. 1 of the pedicle screw and offset fixation plate implanted in accordance with the present invention looking in an inferior direction; 
     FIG. 15 is a lateral view looking from right to left of the offset fixation plate shown in FIG. 14; 
     FIG. 16 is a plan view (in the sacral plane) of a second embodiment of the sacral plate shown in FIGS. 2 and 3; and 
     FIG. 17 is a cross-sectional view taken along broken cross-sectional line  17 - 17  of FIG.  16 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, the spinal implant system  20  constructed in accordance with the present invention is affixed to the lumbar spine, generally designated  22 . The implant system includes a pair of sacral plates  24  and  26  affixed to the sacrum  28  adjacent the lumbosacral joint. A pair of fixation rods  30  and  32  extend in a superior direction on opposite sides of the sagittal plane from the sacral plates posterior to the lumbar vertebrae L 5 , L 4 , L 3 , L 2  and L 1 . Rods  30  and  32  terminate adjacent the superior portion of vertebra L 1 . Conventional fasteners  34  and  36  securely affix the rods  30  and  32 , respectively, to the sacral plates  24  and  26 . 
     At the superior end of the rods, a pair of offset fixation plates  40  and  42  affix the upper ends of the rods to the L 1  vertebra. Inferior to that location, a pair of conventional inferior hooks  44  and  46  grasp the inferior portion of the L 1  vertebra to secure it relative to the rods  30  and  32 . At intermediate locations a straight fixation plate  48  is employed to affix vertebra L 3  to the rods  30  and  32 . Immediately superior to the sacral plates, a connecting member  50  rigidifies the rods  30  and  32  relative to each other. One of ordinary skill in this technique will readily recognize that one or more connecting members  50 , straight fixation plates  48 , and hooks  44  can be employed as needed. 
     The implant system  20  constructed and employed in accordance with the present invention provides a rigid stabilization system for the lumbar spine. The system rigidly ties the sacrum to one or more of the lumbar vertebrae. Moreover, the offset fixation plates  40  and  42  allow the upper portion of the implant system to be rigidly affixed to the superior lumbar vertebra L 1  while avoiding contact with the inferior processes of the next superior vertebra T 12 . In this manner the T 12  vertebra can move in a normal manner while stabilization of the lumbar spine occurs. 
     Referring now to FIGS. 2,  3 ,  4  and  5 , the right sacral plate is illustrated. The right sacral plate is a mirror image of the left sacral plate; therefore, only the right plate will be described in detail. The right sacral plate  26  has a base  60  having a posterior surface and an anterior surface. The anterior surface of the plate is designed to intimately contact the posterior surface of the sacrum adjacent the lumbosacral joint. In position, the base  60  lies generally in a plane generally tangential to the portion of the sacrum adjacent the lumbosacral joint. For purposes of this description, that plane will be referred to as the sacral or dorsal plane. 
     A U-shaped flange  62  extends posteriorly from the medial portion of the base  60 . The medial surface of the flange  62  carries a groove  64  oriented in a superior/inferior direction for receiving a fixation rod  32 . A conventional rod clamp  68  is employed to securely and rigidly affix the rod  32  in the groove  64  on the flange  62 . The lateral portion of the sacral plate  26  carries three bores that extend from the posterior surface of the base  60  in a generally anterior direction. These bores are the pedicle bore  70 , the lateral bore  72  and the oblique bore  74 . The bores  70 ,  72  and  74 , while extending in an anterior direction, are not orthogonal to the sacral plane. 
     Instead, the pedicle bore  70  has a cylindrical section  80  having an axis  82  extending in an anterior and medial direction that is offset in the medial direction preferably at an angle of 15 degrees to a line orthogonal to the sacral plane. A countersink bore  81  is located posterior to the cylindrical section  80  and emerges onto the posterior surface of the sacral plate. This angle can be varied from 0 degrees to 20 degrees, depending upon the particular sacral anatomy being fixed. However, it is understood that the screw that extends through this opening extends through the pedicle of the sacrum and must always lie within the pedicle. It has been found that 15 degrees is the angle most universally acceptable for this orientation. In the present embodiment, the axis  82  is not inclined in a superior or inferior direction relative to a plane perpendicular to the sacral plate. It however can be inclined superiorly so that the vertebral end plate, rather than the anterior cortex, can be engaged by the end of the screw. 
     The lateral bore  72  has a cylindrical section  84  having an axis  86  extending in an anterior and lateral direction that is preferably offset in the lateral direction at an angle of 30 degrees from a line orthogonal to the sacral plane. If desired, one of ordinary skill may also vary the lateral angle from 30 degrees up to 45 degrees. Preferably the axis  86  is not canted in either an inferior or superior direction relative to the sacral plate. However, depending upon the sacral anatomy, the axis can be canted from 0 degrees to 15 degrees in the superior direction when viewed in the sacral plane. A countersink bore  85  is located posterior to the cylindrical section  85  and emerges onto the posterior surface of the sacral plate. 
     The oblique bore  74  also has a cylindrical section  88  having an axis  90  having two offsets in the lateral and inferior directions. The axis  90  when viewed in the sacral plane is first preferably offset 45 degrees from a lateral line, but may be varied from 30 degrees to 60 degrees. Secondly, the axis  90  is offset in the lateral direction preferably 30 degrees from a line orthogonal to the sacral plane but again may be varied from 30 degrees to 45 degrees. A countersink  89  is located posterior to the cylindrical section  88  and also emerges onto the posterior surface of the sacral plate. 
     Referring now to FIG. 6, the pedicle screw  94  employed with the sacral plate has a unique construction. It has a lower threaded portion  96 , an upper flared head  98  and a cylindrical section  100  immediately below the head  98 . The head also carries an allen socket  102  so that the screw can be rotated into a hole drilled in the pedicle. The bone engaging threads on the lower threaded portion  96  are of conventional design. The cylindrical section  100  has a diameter slightly less than the diameter of the cylindrical section  80  of pedicle bore  70 . The diameters are chosen such that when the cylindrical section  100  is in the cylindrical section  80 , the screw  94  can rotate and reciprocate. However, the tolerances are such that the screw cannot angulate or toggle relative to the axis  82 . The upper flared portion  98  is configured to mate with countersink  81  when the screw is completely threaded into the sacrum. 
     Referring to FIG. 7, the same screw  106  is employed in both the lateral bore  72  and the oblique bore  74 . Screw  106  also has a lower threaded portion  108 , a flared head  110  and a cylindrical section  112 . Cylindrical section  112  is sized relative to the cylindrical sections  84  and  88  to allow rotation and reciprocation but not angulation. The flared head  110  is configured to mate with the countersinks  85  and  89  when the screws are completely threaded into the sacrum. 
     Referring now to FIGS. 8 and 10, the offset fixation plate  40  includes a medial portion  122  and a lateral portion  124 . The fixation plate of FIG. 8 is employed on the left side of the fixation system. A similar fixation plate, having the mirror image of plate  40 , is employed on the right side; however, it is not shown in the drawings. The lateral portion  124  carries a bore  126  that extends in a posterior/anterior direction when installed. The medial portion  122  is offset in an inferior direction from the lateral portion  124 . The medial portion  122  carries a lateral slot  127 . The anterior surface of the medial portion  122  carries a plurality of grooves  128  that extend in an inferior/superior direction and intersect the slot  127 . These grooves have a diameter equivalent to the fixation rod  30 . A conventional rod to clamp fastener  58  is employed to secure the fixation plate  40  to the fixation rod  30 . 
     A special pedicle screw  129  is employed with the offset fixation plate. Referring to FIG. 9, the pedicle screw includes a lower threaded portion  130 , a subhead portion  132  and an upper threaded portion  134 . The upper threaded portion  134  has an allen socket  136  extending axially into its upper end. The subhead has a diameter larger than the upper threaded portion  134  and terminates in its upward end in a shoulder  138  that is positioned in a plane orthogonal to the axis of the screw. Referring now to FIG. 10, the pedicle screw  129  is received by the bore  126 , which is sized just slightly larger than the upper threaded portion  134  so that the pedicle screw can reciprocate relative to the offset fixation plate  40 , but cannot angulate relative to the screw axis when engaging the bore  126 . A conventional nut  140  is threaded onto the upper portion  134  of the pedicle screw  129  securing the shoulder  138  against the anterior surface of the fixation plate while the nut  140  snugs against the posterior surface, thus rigidly interlocking the pedicle screw  129  and the fixation plate. 
     A straight fixation plate  48  is illustrated in FIG.  11 . The straight fixation plate  48  is similar in construction to the offset fixation plate  40  except that it does not contain the offset. It carries a similar bore  144  for receiving a pedicle screw similar to screw  129 , a lateral slot  146  and rod engaging grooves  148  for securing the plate  48  to a fixation rod. 
     Referring to FIGS. 12 and 13, in use, the sacral plates  24  and  26  are affixed to the sacrum  28  adjacent the lumbosacral junction. As desired and as necessary, the anterior surface of the sacrum can be smoothed so as to receive the anterior surface of the sacral plates  24  and  26  in snug relationship. The pedicle screws  94  and  94 ′, for use in the pedicle bores of the sacral plates, are threaded into appropriate bores made by the surgeon through the pedicle of the sacrum. The pedicle screws are snugged down so that the flared heads are seated firmly in the countersinks in the respective plates. A torque ranging from 6 to 10 in./lb. can be used to snug the screws. The physician also makes appropriate bores into the sacrum that are aligned with the lateral bores  72  and  72 ′ and with the oblique bores  74  and  74 ′. Screws  106  are inserted through the lateral and oblique bores  72  and  74  in the right plate, and bores  72 ′ and  74 ′ in the left plate. All the screws  106  are snugged down so that the flared heads seat snugly in the countersinks in the anterior surface of the sacral plates. Again a torque of 6 to 10 in./lb. is appropriate for snugging the screws into the plate. 
     In this manner, the three screws in each sacral plate all diverge from each other. As a result, the screws cannot be easily pulled from the bores in the bone. A force in the direction of the axis of one of the screws will be partially distributed over the bone on which the remaining two screws bear. In this manner, full force cannot be exerted in the direction of the axis of a single screw and thus a single screw cannot be sheared from its bore in an easy manner. This construction provides significant advantages over the prior art while allowing independent placement of a sacral plate on each side of the sacrum. For example, screw placement is designed to achieve fixation in the proximal part of the sacrum, which has the strongest bone. The oblique screw is designed to be proximal to and parallel the S 1  foramin, thereby avoiding damage to the S 1  nerve. The medial screw is inclined medially to allow bicortical fixation while avoiding neurovascular structures directly anterior to the S 1  pedicle. The lateral screw is also designed to allow bicortical fixation lateral to the significant neurovascular structures. 
     Referring now to FIGS. 14 and 15, offset fixation plates  40  and  42  are shown affixed by conventional fasteners  58  to fixation rods  30  and  32 . The pedicle screws  129  are threaded into suitable bores in the left and right pedicle  150  and  152  of the L 1  vertebra. Nuts  140  are threaded onto the upper portions of the pedicle screws  129  and tightened against the anterior surfaces of the fixation plates  40 . The fasteners  58  thereafter are tightened to secure the other end of the plate to the fixation rods  30  and  32 . In this manner, the upper end of the lumbar spine implant system can be secured to the L 1  vertebra without interfering with the next superior vertebra. 
     Referring now to FIGS. 16 and 17, a second embodiment of a right sacral plate  150  according to the present invention is shown. A left sacral plate is configured as a mirror image of the right sacral plate; therefore, only the right sacral plate will be described in detail. The sacral plate  150  has a base  152  having a posterior surface and an anterior surface. The anterior surface of the plate is designed to intimately contact the posterior surface of the sacrum adjacent the lumbosacral joint. In position, the base  152  lies in a plane generally tangential to the portion of the sacrum adjacent the lumbosacral joint. 
     A U-shaped flange  154  is configured in the same way as flange  64  shown in FIG.  2 . The medial surface of the flange  154  carries a groove  156  oriented in a superior/inferior direction for receiving a fixation rod  32  that is held in place with a conventional rod clamp  68  as shown in FIG.  2 . The lateral portion of the sacral plate  150  carries two bores that extend from the posterior surface of the base  152  in a generally anterior direction. These bores are the pedicle bore  158  and the oblique bore  160 . The bores  158  and  160 , while extending in the anterior direction, are not orthogonal to the sacral plane. 
     The pedicle bore has a cylindrical section  162  having an axis  164  extending in an anterior and medial direction that is offset in the medial direction preferably at an angle of 15 degrees to a line orthogonal to the sacral plane. A countersink bore  166  is located posterior to the cylindrical section  162  and emerges onto the posterior surface of the sacral plate. The angle of the axis  164  can be varied from 0 degrees to 20 degrees, depending upon the particular sacral anatomy being fixed. However, it is understood that a screw that extends through this opening into the pedicle of the sacrum must always lie within the pedicle. In the present embodiment of the sacral plate  150 , the axis  164  is not inclined in the superior or inferior direction relative to a plane perpendicular to the sacral plate. However, the axis can be inclined superiorly so that the vertical end plate rather than the anterior cortex can be engaged by the end of the screw. 
     The oblique bore  160  also has a cylindrical section  166  having a axis  168  having two offsets in the lateral and inferior directions. The axis  168  when viewed in the sacral plane is first preferably offset at 45 degrees from a lateral line, but may be varied from 30 degrees to 60 degrees. Secondly, the axis  168  is offset in the lateral direction preferably 30 degrees from a line orthogonal to the sacral plane but again may be varied from 30 degrees to 45 degrees. A countersink  170  is located posterior to the cylindrical section  166  and also emerges onto the posterior surface of the sacral plate. 
     The sacral plate  152  is designed for patients having a lumbosacral joint that is too small to accept the sacral plate  26  shown in FIGS. 2 and 3. By providing a sacral plate  150  having only the pedicle bore and oblique bore, the sacral implant system according to the present invention can be adapted to fit patients having smaller skeletal structures. 
     The present invention has been described in connection with the preferred embodiment. However, one of ordinary skill will be able to effect various alterations, substitutions of equivalents and other changes without departing from the broad concepts imparted herein. It is, therefore, intended that the letters patent issued hereon be limited only by the definition contained in the appended claims and equivalents thereof.