Patent Abstract:
An anatomic posterior lumbar system comprising of a thin plate and fasteners for securing the plate to vertebra or other osseous material. The plate is preferably a thin rectangular structure having an opening running lengthwise in its central portion. At least one pedicle screw having a hollow threaded interior is fastened to a vertebra, and the plate located on the screw using the opening. A washer spans the width of the plate, and accommodates an inner screw, which is threaded into the pedicle screw, and captures the head of the inner screw. Miscellaneous shapes of plates are also disclosed which are fitted to specific portions of the spine.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit from U.S. Provisional Patent Application Serial No. 60/279,157, filed Mar. 27, 2001, which application is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to devices for use in spinal surgery, and, in particular, to an anatomic posterior lumbar plate which is implantable within a patient for stabilization of the spine.  
           [0004]    2. Description of the Related Art  
           [0005]    Eighty-five percent of the population will experience low back pain at some point. Fortunately, the majority of people recover from their back pain with a combination of benign neglect, rest, exercise, medication, physical therapy, or chiropractic care. A small percent of the population will suffer chronic low back pain. The cost of treatment of patients with spinal disorders plus the patient&#39;s lost productivity is estimated at 25 to 100 billion dollars annually.  
           [0006]    Seven cervical (neck), 12 thoracic, and 5 lumbar (low back) vertebrae form the normal human spine. Intervertebral discs reside between adjacent vertebra with two exceptions. First, the articulation between the first two cervical vertebrae does not contain a disc. Second, a disc lies between the last lumbar vertebra and the sacrum (a portion of the pelvis).  
           [0007]    Motion between vertebrae occurs through the disc and two facet joints. The disc lies in the front or anterior portion of the spine. The facet joints lie laterally on either side of the posterior portion of the spine. The osseous-disc combination of the spine coupled with ligaments, tendons, and muscles are essential for spine function. The spine allows movement (flexation, lateral bending, and rotation), supports the body, and protects the spinal cord and nerves.  
           [0008]    The disc changes with aging. As a person ages the water content of the disc falls from approximately 85 percent at birth to 70 percent in the elderly. The ratio of chondroitin sulfate to keratin sulfate decreases with age. The ratio of chondroitin 6 sulfate to chondroitin 4 sulfate increases with age. The distinction between the annulus and the nucleus decreases with age. These changes are known as disc degeneration. Generally disc degeneration is painless.  
           [0009]    Premature or accelerated disc degeneration is known as degenerative disc disease. A large portion of patients suffering from chronic low back pain is thought to have this condition. As the disc degenerates, the nucleus and annulus functions are compromised. The nucleus becomes thinner and less able to handle compression loads. The annulus fibers become redundant as the nucleus shrinks. The redundant annular fibers are less effective in controlling vertebral motion. The disc pathology can result in: 1) bulging of the annulus into the spinal cord or nerves; 2) narrowing of the space between the vertebra where the nerves exit; 3) tears of the annulus as abnormal loads are transmitted to the annulus and the annulus is subjected to excessive motion between vertebra; and 4) disc herniation or extrusion of the nucleus through complete annular tears. Disc herniation can also cause arthritis of the facet joints, which, in turn may cause back pain.  
           [0010]    The problems created by disc degeneration, facet arthritis, and other conditions such as spondylolysis, spondylolisthesis, scoliosis, fracture, tumor, or infection are frequently treated by spinal fusion. Such problems may include pain in the back or legs, nerve injury, risk of future nerve injury, or spinal deformity. The goal of spinal fusion is to successfully “grow” two or more vertebrae together. To achieve this, bone from the patient&#39;s body (spine or iliac crest) or from cadavers, is grafted between vertebrae. Alternatively, bone graft substitutes, such as hydroxyapatite and bone morphogenetic protein, may be used. The bone graft is placed between the vertebrae in the disc space and/or over the posterior elements of the vertebrae (lamina and transverse processes). The surgeon scrapes the vertebrae to create bleeding. Blood flows into the bone graft. The scraped bone, blood clot (hematoma), and the bone graft simulates a fracture. As the patient heals, the “fracture” causes the vertebrae to be fused and heal together.  
           [0011]    Spinal instrumentation may be placed onto or into the spine to immobilize the vertebrae that are going to be fused. Immobilization leads to a higher fusion rate and speeds a patient&#39;s recovery by eliminating movement. The use of spinal fixation plates or rods for correction of spinal deformities and for fusion of vertebrae is well known. Typically, a rigid plate is positioned to span bones or bone segments that need to be immobilized with respect to one another. Bone screws may be used to fasten the plate to the bones. Spinal plating systems are commonly used to correct problems in the lumbar and cervical portions of the spine, and are often installed posterior or anterior to the spine.  
           [0012]    One technique of treating these disorders is known as surgical arthrodesis of the spine. This can be accomplished by removing the intervertebral disk and replacing it with bone and immobilizing the spine to allow space to connect the adjoining vertebral bodies together. The stabilization of the vertebrae to allow fusion is often assisted by a surgically implanted device to hold the vertebral bodies in proper alignment and allow the bone to heal, much like placing a cast on a fractured bone. Such techniques have been effectively used to treat the above described conditions and in most cases are effective at reducing the patient&#39;s pain and preventing neurologic loss of function.  
           [0013]    Several types of anterior spinal fixation devices are currently in use. One technique involves placement of screws completely through the vertebral body, called bicortical purchase. The screws are placed through a titanium plate but are not attached to the plate. This device is difficult to place, and over-penetration of the screws can result in damage to the spinal cord. The screws can back out of the plate into the surrounding tissues as they do not fix to the plate. Several newer generation devices have used a unicortical purchase of the bone, and in some fashion locking the screw to the plate to provide stability and secure the screw from backout. Problems have resulted from over rigid fixation and stress shielding, resulting in nonunion of the bony fusion, chronic micromotion during healing, resulting in stress fracture of the fixation device at either the screw to the plate resulting in screw backout, or inadequate fixation strength and resultant collapse of the graft and angulations of the spine.  
           [0014]    Another technique involves formation of a medical construct using surgical rods and connectors. Such systems include a pair of rods which are placed on opposite sides of the portion of the spine which is intended to be fused. Pedicle, lateral, and oblique mounting means are used to secure the rods relative to the desired portion of the spine which will be fused by the fixation system. However, this construct extends outwardly further than a plate/screw system, potentially affecting the surrounding muscle, and causing pain to the patient.  
           [0015]    A typical device which is used for spinal fixation is taught in U.S. Pat. No. 4,611,581. This device consists of a simple plate having a series of openings for receiving threaded portions of force transmitting members which securely lock in a part of the bone of the vertebra in which they are mounted and a threaded portion which projects outwardly from the vertebrae. The vertebra is pulled into the desired relationship with adjacent vertebrae by tightening a nut on the outwardly projecting end portion of the force transmitting member.  
           [0016]    Another typical device used is shown in U.S. Pat. No. 6,306,136. This patent discloses an implant which is used particularly as an anterior cervical plate, having a solid plate consisting of two sliding parts, each of which has holes for anchoring screws in two adjacent vertebrae. The sliding parts are provided with a screw and slot for limiting the sliding travel between the parts.  
           [0017]    Another vertebrae connecting plate is taught in U.S. Pat. No. 5,147,361. This plate has a small thickness, and uses set screws which are screw engaged in threaded holes within the connecting plate to prevent any loosening of the screws within the plate.  
           [0018]    One problem that sometimes arises in the use of known plate systems is when the points on the vertebrae defined by the screws are not collinear, i.e. they do not line up in a straight line. This creates a problem when the openings in the plate do not align with the position where the screws are to be inserted. The plate must then be contoured intraoperatively, which is often a difficult and time consuming task. A device is taught in U.S. Pat. No. 6,336,927 that attempts to overcome this problem. This device includes a plurality of link members which can be screwed to adjacent vertebrae in chain-like fashion using pedicle 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 using a single link member.  
           [0019]    Still another problem which arises when using spinal plates is the differences in spinal curvature depending on the location of the vertebrae to be stabilized. U.S. Pat. No. 5,647,872 addresses this problem by providing a preformed plate having a spine opposing face incorporating two patterns or components of curvature.  
         SUMMARY OF THE INVENTION  
         [0020]    It is therefore an object of the present invention to provide a plate system for spinal surgery which comfortably fits a patient&#39;s anatomy.  
           [0021]    It is a further object of the present invention to provide a plate system which is easily implanted within a patient.  
           [0022]    It is a further object of the present invention to provide a plate system which is easily positioned and establishes a secure connection between vertebrae.  
           [0023]    It is a still further object of the present invention to provide a plate which is easily adaptable to the lumbar region of the spine.  
           [0024]    These and other objects and advantages of the present invention will be readily apparent in the description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is an exploded view of a spinal plate system of the prior art;  
         [0026]    [0026]FIG. 2 is an assembled view of the system of FIG. 1;  
         [0027]    [0027]FIG. 3 is a plan view of a spinal plate system according to the present invention;  
         [0028]    [0028]FIG. 4 is a side view, partly in cross section, of the system of FIG. 3;  
         [0029]    [0029]FIG. 5 is a plan view of several spinal plates manufactured according to the present invention;  
         [0030]    [0030]FIG. 6 is a plan view of one of the plates shown in FIG. 5 in the installed position;  
         [0031]    [0031]FIG. 7 is a plan view of another set of spinal plates manufactured according to the present invention;  
         [0032]    [0032]FIG. 8 is a plan view of one of the plates shown in FIG. 7 is the installed position;  
         [0033]    [0033]FIG. 9 is a cross-sectional view of a vertebra showing the relative position of a pedicle screw installed according to the present invention;  
         [0034]    [0034]FIGS. 10A and B show a spinal plate according to the present invention, along with a cross-sectional view;  
         [0035]    FIGS.  11 A-C, taken together, represent a sequence of tightening a spinal plate into position;  
         [0036]    [0036]FIG. 12 shows a plan view, partly in phantom, of a pedicle screw for use in the present invention;  
         [0037]    [0037]FIG. 13 shows a plan view of an inner screw for use in the present invention;  
         [0038]    [0038]FIG. 14 shows a plan view of a spinal system of the present invention using an extended nut;  
         [0039]    [0039]FIG. 15 shows a plan view of a screw locking system according to the present invention;  
         [0040]    [0040]FIG. 16 shows a plan view of the system of FIG. 15 in the installed position;  
         [0041]    [0041]FIG. 17 shows an exploded view of an alternative embodiment of a screw system for use in the present invention;  
         [0042]    [0042]FIG. 18 is a side view of the screw system of FIG. 17 in the assembled position;  
         [0043]    [0043]FIG. 19 is a top view of the system shown in FIG. 18;  
         [0044]    [0044]FIG. 20 is a cross-sectional view of the system shown in FIG. 18;  
         [0045]    [0045]FIG. 21 is a fragmentary perspective view of the pedicle screw of the present invention;  
         [0046]    [0046]FIG. 22 is a top view of the pedicle screw in position with the spinal plate according to the present invention;  
         [0047]    [0047]FIG. 23 is a fragmentary side view, partly in cross section, of a plate system according to the present invention, using a washer;  
         [0048]    [0048]FIG. 24 is a die view of an inner screw according to the present invention; and  
         [0049]    [0049]FIG. 25 is a fragmenting view, partly in cross section, of a plate system using the inner screw of FIG. 24. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]    [0050]FIGS. 1 and 2 represent a typical prior art plate system currently used for spinal stabilization. Referring now to FIG. 1, there is shown a spinal plate  10 . Plate  10 , which is generally rectangular or elliptical in shape, contains an opening  12  in its central portion. A pedicle screw  14  is shown having a threaded shaft  16  and an upper region  18 . Screw  14  contains an extended section  20  shaped to easily receive a tool for threading screw  14  into the bone. Upper region  18  is shown having a threaded section  24  and a head  26 . Head  26  is of a lesser diameter than threaded section  24  such that a nut  28  can be threaded onto threaded portion  24  of inner screw  22 .  
         [0051]    Referring now to FIG. 2, the plate system of FIG. 1 is shown in its assembled position. Plate  10  is positioned on pedicle screw  14  through opening  12  after it has been installed into the bone such that section  20  contacts the bottom side of plate  10 . Nut  28  is then threaded onto threaded shaft  24  and against the upper surface of plate  10  to secure the system. Generally, several of pedicle screws  14  are used to secure plate  10  to vertebrae of the spine for stabilization.  
         [0052]    One problem that often occurs when using these prior art plates is that the preferred locations of the pedicle screws for attaching the plate to the spine do not lie collinearly. For example, the pedicle screw in L5 often lies lateral to the screws in L4 and S1, forcing the L5 pedicle screw to fit into a rectangular plate and risking fracture of the L5 pedicle screw due to stress.  
         [0053]    A solution to this problem is to enlarge the opening in the spinal plate and use a washer to position the pedicle screw within the opening. Referring now to FIGS. 3 and 4, there is shown a plate  40  having an opening  42  which is wider than the opening in prior art plates. Pedicle screws  44  have been placed into the proper vertebrae, and inner screws  46 , each having a head  48 , have been threaded into the interior section of pedicle screws  44  to hold plate  40  securely in place. Between head  48  of each pedicle screw  46  and plate  40 , there is positioned a washer  50 . Washer  50  contains a flat section  50   a  and side sections  50   b , which depend in a perpendicular direction downwardly from section  50   a , along with an opening  51  that screw  46  passes through. Opening  51  may be slotted such that inner screw  46  may be accurately positioned for insertion. Sections  50   b  overhang the edges of plate  50  such that inner screw  46  can be optimally positioned within opening  42  for holding plate  40  in its proper location without putting undue stress on pedicle screws  44 . To assist in the stable positioning of washer  50  along plate  40 , a frictional surface  56  may be added to the top surface of plate  40  and the bottom surface of washer  50 . Plate  40  is particularly suited for use as a posterior lumbar plate. Installation of plate  40  is easily accomplished, as it is top loading and top tightening.  
         [0054]    Another solution to the problem of non-collinearity of the screw placement in the lower lumbar region of the spine involved the use of specially shaped spinal plates. FIGS. 5 and 6 show these plates. Referring now to FIG. 5, a pair of nonlinear plates  60   a ,  60   b  are shown having a first linear section  62   a ,  62   b  and a second linear section  64   a ,  64   b  connected by an angled section  65   a ,  65   b . Plates  60   a ,  60   b  contain an opening  66   a ,  66   b  which runs along the length of the plates. Plate  60   a  is designed to be implanted on the left side of the spinal column, while plate  60   b  is designed to be implanted on the right side. One of plates  60   a ,  60   b  is preferably used in spine fusion procedures spanning two or more levels which include L5 or L5 and S1. FIG. 6 shows a two level fusion plate, which spans this region of the spine. Plate  60   a  is shown in position implanted on the spine with a pedicle screw  80  shown secured to S1, a pedicle screw  82  secured to L5, and a pedicle screw  84  secured to L4.  
         [0055]    [0055]FIG. 7 shows a set of plates which is designed for use in a three level fusion. A pair of non-linear plates  90   a ,  90   b  are shown having a first linear region  92   a ,  92   b  and second linear region  94   a ,  94   b  joined by a curved region  96   a ,  96   b . Plates  90   a ,  90   b  contain an opening  98   a ,  98   b  which runs along the length of the plate. Note that the cephalad portion  99   a ,  99   b  of regions  94   a ,  94   b  which lie above the L4 vertebra which, when implanted on the spine, is essentially linear. If a plate is desired for use for a 4 or 5 level fusion, a plate having portion  99   a ,  99   b  extended linearly may be constructed.  
         [0056]    Referring again to FIG. 6, the non-collinearity of the pedicle screws can be clearly seen. A line  100  is shown extending through screw  80  at S1 and screw  84  at L4, while a parallel line  102  is shown passing through screw  82  at L5. The average distance between parallel lines  100  and  102  varies between 5 mm and 7 mm. The average distance between S1 and L5 is approximately 3 cm, while the average distance between L5 and L4 is approximately 3.5 cm.  
         [0057]    Referring again to FIG. 7, the non-collinearity of a three-level plate is shown. Line  100  extends through screw  80  at S1 and screw  84  at L4. Parallel line  102  extends through screw  82  at L5, while another parallel line  104  extends through a pedicle screw  106  which is fastened at L3. The distance between line  100  and line  102  is an average of 5 mm to 7 mm, while the distance between line  100  and line  104  is approximately 3 mm to 5 mm. Thus, it will be necessary to offer different size plates while would be used in surgery depending on the particular sizes applicable to different patients. For example, plates  60   a ,  60   b  and plates  92   a ,  92   b  could be available with either 3 mm, 5 mm, or 7 mm between the L5 offset lines, and 3 mm and 5 mm between the L4/L3 offset line, or can be available for any combination thereof.  
         [0058]    Another problem which arises in the implantation of spinal plates involves the angle of pedicle screws. Pedicle angulation increases in the caudal portion of the spine, as is shown in FIG. 9. Referring to FIG. 9, there is shown a vertebra  120  having a pedicle screw  122  inserted. A line  124  passes through the center of vertebra  120 , while a second line  126  extends angularly from center line  124  from vertebra  120 . An angle  128 , called the transverse pedicle angle, defines the angular relationship between lines  124  and  126 . Angle  128  changes for each vertebra in the caudal area. For example, the average angle  128  for L2 is 12°; for L3, it is 14°; for L4, it is 18°; for L5, it is 30°; and for S1, it is greater than 30°. Unfortunately, the angle of screw insertion does not always match the transverse pedicle angle. Therefore, when the angle of screw insertion is significantly different from the plate angle, the inner screws can be difficult to insert through the plate, and as the screws are tightened, the pedicle screw alignment in the bone can be changed and could conceivably be forced out of the lateral portion of pedicle as the inner screw is tightened.  
         [0059]    Several solutions to address this problem are proposed in the present invention. One solution is to increase the width of the opening within the spinal plate through which the inner screw is inserted into the pedicle screw. This can be accomplished by use of washer  50  which is shown and described with respect to FIGS. 3 and 4. By using washer  50 , the central opening with the spinal plate can be enlarged as washer  50  spans across the opening and the inner screw can be inserted into the pedicle screw with greater leeway.  
         [0060]    Another solution to this problem involves a twisting of the spinal plate in the area of L5. FIGS. 10A and B show a spinal plate having this twist. Referring now to FIGS. 10A and 10B, a spinal plate  170  is shown having an upper linear section  170   a  and a lower linear section  170   b  which are connected by a curved section  170   c . In use, section  170   c  is positioned at L5 along the spine. At this section, plate  170  is twisted from the horizontal at a 10° angle, as can been seen at  172  in the cross-sectional view FIG. 10B, while plate  170  is flat at  170   a , as seen at  174  in FIG. 10B. In this manner, the upper surface of the pedicle screw can align better with the bottom surface of plate  170  such that the inner screw can be inserted without additional stress to the pedicle screw/inner screw coupling.  
         [0061]    If the angle of the screw insertion is significantly different from the plate angle, the inner screw can be difficult to insert through the plate, and as the screw is tightened the pedicle screw alignment in the bone can be changed, even to the point where pedicle screw extends from the bone. This scenario can be seen in FIGS.  11 A-C. Referring now to FIG. 11A, a pedicle screw  175  has been inserted into a vertebra  176  and a plate  177  is tightened into position by an inner screw  178 . It can be seen that screw  178  contacts the sides of opening  179  through plate  177 , as screw  178  is at an angle to plate  177 . In FIG. 1B, a nut  180  is placed onto screw  178  and is torqued to fasten plate  177  into place. As nut  180  is screwed onto screw  178 , the stress placed on pedicle screw  175  is great enough to cause screw  175  to be forced through the lateral portion of vertebra  176 , as can be seen in FIG. 11C.  
         [0062]    A solution for reducing this problem involves the use of a pedicle screw having a rounded head. Referring now to FIG. 12, there is shown a pedicle screw  181  for use in the present invention. Pedicle screw  181  contains a threaded portion  182  which is screwed into bone, and a head portion  184 . Head portion  184  consists of a spherical top portion  186  and an outwardly extending lower portion  188 . Portion  188  is preferably hexagon shaped, or a similar structure, such that it accommodates a wrench or similar tool to insert threaded portion  182  into the bone. Screw  181  also includes a hollow threaded interior portion  190 .  
         [0063]    When pedicle screw  181  is installed in bone and a spinal plate inserted over it, and an inner screw  192  is threaded into interior portion  190 , spherical top portion  186  of screw  181  allows the plate to be tightened into position without creating undue stress on the system. The thread interaction between inner screw  192  and interior portion  182  of pedicle screw  181  should help lock the two together when torqued to the proper amount. If it is not possible to create enough torque using current inner screws, a special head could be adapted onto the inner screw, as can be seen in FIG. 13. Referring now to FIG. 13, inner screw  192  contains a lower threaded section  202  and a head section  204 . Head section  204  further has a spherical lower section  206  and a larger section  208  configured to accommodate a wrench or similar tool which can apply considerable torque to screw  192 .  
         [0064]    Another device which would ease insertion of a spinal plate system is shown in FIG. 14. Referring now to FIG. 14, an extended nut  220  is used to securely fasten a spinal plate  222  in position on a pedicle screw  224  and an inner screw  225 . Nut  220  contains an upper portion  226  configured to accommodate a wrench or similar tool, a central spherical section  228 , and a lower cylindrical section  230 . Section  230  is configured to be positioned within an opening within plate  222 , while section  228  acts to accurately position nut  220  relative to pedicle screw  224  and inner screw  225 . Ideally, the length of cylindrical section  230  should be less than the width of plate  222 , or plate  222  and a washer, if a washer is to be used.  
         [0065]    Still another device which would ease insertion of a spinal plate system is shown in FIG. 15. Referring now to FIG. 15, there is shown a multiaxial screw system  250  including a pedicle screw  251  having a lower threaded section  252  which is shaped to be threaded into bone, and an upper section  254 . Upper section  254  contains a lower portion  256  configured to accommodate a wrench for threading screw  251  into the appropriate vertebra. Pedicle screw  251  also has an internally threaded portion  258  for receiving an inner screw  260 . A multiaxial seat  262  is threaded onto the threaded portion  252  of pedicle screw  251 . Seat  262  has a hollow spherical section  264  which accommodates inner screw  260 . In operation, which can be clearly seen in FIG. 16, when a plate  270  is positioned over seat  262  and pedicle screw  251 , and inner screw  260  is threaded in position, section  264  of seat  262  allows for a range of motion while inserting inner screw  260  to secure screw  260  safely. A washer  272 , as described earlier, can be installed over plate  270  to stabilize system  250 , and a nut  274  is tightened to tightly secure system  250  in the proper position. As nut  274  is tightened over plate  270 , multiaxial seat  262  locks into position, and system  250  is pulled securely into the proper orientation.  
         [0066]    An alternative embodiment of the spinal plate system of the present invention is shown in FIGS.  17 - 22 . Referring now to FIG. 17, there is shown a novel screw system, generally indicated at  300 . Screw system  300  includes a pedicle screw  302  having a lower threaded section  304  capable of locking system  300  into bone, and an upper section  306  having an unthreaded portion  308  which is shaped to accommodate a wrench for threading screw  302  into a vertebra. Section  306  also includes an extension  310  which extends from a flat upper surface  312  of portion  308 . Extension  310  is preferably elliptical or oval shaped to fit nicely in a slotted opening  313  within a spinal plate  314 . Pedicle screw  302  also contains a threaded hollow portion  316  capable of receiving the threaded portion of an inner screw  320 . Screw  320  includes a lower threaded portion  322  which is sized to mesh with portion  316  of pedicle screw  302 , and an upper head  324 . Head  324  is preferably spherical shaped, and contains a driving socket  326  within head  324 . FIG. 18 shows screw system  300  in its assembled orientation, while FIG. 19 shows a top view of system  300 , and FIG. 20 shows a cross-sectional view of FIG. 18.  
         [0067]    Spherical shaped head  324  of inner screw  320  provides a smooth surface for the part of system  300  which would contact tissue with the patient&#39;s body, making it less likely for system  300  to cause irritation to the surrounding tissue. Head  324  also has a lower profile than any of the screw systems in use today, which also decreases the risk of tissue irritation for screw system  300 , and FIG. 20 shows a cross-sectional view of FIG. 18.  
         [0068]    Oval shaped extension  310  of pedicle screw  302  is shaped such that it may be accommodated nicely within slot  313  of plate  314  such that it prevents rotation of screw  302  when inner screw  320  is inserted, as can be seen in FIGS. 21 and 22. In the present embodiment, slot  313  measures approximately 6 mm, while the width of extension  310  is approximately 5 mm. In addition, the height of extension  310  is approximately 4 mm, while the thickness of plate  314  is approximately 5 mm, which allows inner screw  320  to firmly tighten plate  314  into position.  
         [0069]    One problem that exists within all spinal plate systems is the risk of pedicle screw fracture when installing the inner screw. However, this risk decreases as the length of the inner screw increases. Pedicle screws rarely break in the anteriormost section. One cannot count on the inner screw to bottom out in the internal threads of the pedicle screw, as plates do not always sit flush against the spine. Often, washers are added to improve the plate/pedicle screw fit.  
         [0070]    [0070]FIG. 23 shows a washer in use with the previously described plate system  300 . Referring now to FIG. 23, a washer  330  is used to relieve stress caused by plate  314  not fitting flat against pedicle screw  302 . Washer  330  consists of a flat lower surface  332 , a raised edge  334  and an angled top surface  336 . Washer  330  also has a rectangular opening  338  to accommodate extension  310  of pedicle screw  302 . In operation, pedicle screw  302  is fixed into a vertebra by virtue of threaded section  304 . Washer  330  is then installed by placing it onto pedicle screw  302  by inserting extension  310  through opening  338 . Plate  314  is then placed over washer  330  and extension  310  such that extension  310  projects through opening  313  in plate  314 . Inner screw  320  is finally inserted into the hollow threaded section  316  within pedicle screw  302  and tightened. Upper surface  336  of washer  330  sits flush against the bottom surface of plate  314 . while lower surface  332  sits flush against surface  312  of screw  302 .  
         [0071]    Another device which may be helpful in reducing the stress which may affect the efficiency of the screw/plate system consists of a special inner screw, as can be seen in FIGS. 24 and 25. Referring now to FIG. 24, an inner screw  340  is shown having a lower threaded section  342  and head  324  as previously taught. At the end of section  342  there is a non-threaded aligning tip  344 . As inner screw  340  is threaded into pedicle hollow section  316  and aids in the alignment of inner screw  340  as it enters section  316 . Tip  344  extends into threaded section  316  of pedicle screw  302  almost to the bottom to assist in stress reduction within plate system  300 , as can be seen in FIG. 25.  
         [0072]    While the present invention has been shown and described in terms of preferred embodiments thereof, it will be understood that this invention is not limited to any particular embodiment, and that changes and modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.

Technology Classification (CPC): 8