Abstract:
A polyaxial screw is used to secure orthopedic appliances to bone, particularly the spine. The polyaxial screw has a shaft including a helical screw thread for securing the screw to bone. One end of the shaft has a reverse tapered head tapering toward and away from the shaft from an enlarged median. A spherical shaped swivel, having an opening smaller than the median, is snapped over the head to rotatably engage the shaft. A connector with a dome shaped cavity on one end and bifurcated tabs on the other is connected with the swivel and tightened in place with a set screw and guide ring. An appliance is held between the bifurcated tabs. The tabs are frangible to reduce the profile above the appliance.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to the field of orthopedic surgery and, particularly to modular screws having a polyaxial head.  
         [0003]     2. Description of the Prior Art  
         [0004]     It is conventional to install surgical appliances, such as rods or plates, to correct skeletal deformities and injuries in which the appliance spans several vertebrae. The surgical appliance is secured to the vertebrae and maintains a stable spatial relationship between them. Pedicle screws are inserted into the vertebrae to anchor the appliance in place. Because of the non-uniform anatomy of the vertebrae, the screws are usually misaligned relative to each other. To compensate for these variances, the rods may be bent to match the location of the screws or the screws may have connectors capable of angular displacement to engage the rods or both.  
         [0005]     For example, U.S. Pat. No. 6,669,697 to Pisharodi teaches the use of eccentric shims between the bone screws and the surgical appliance to correct the angle between the rod and the screw. Shluzas, U.S. Pat. No. 6,648,888, teaches a bone screw with a spherical head rotating in a tapered connector. The connector has opposing tabs to capture the spinal rod and a set screw to hold the rod in the yoke. Reissued patent, U.S. Pat. No. RE37,665, teaches another spherical headed screw with a swiveling connector having an upstanding yoke capturing a spinal rod. Varieur, U.S. Pat. No. 6,641,586, teaches another spherical headed bone screw and an upstanding yoke retainer for the surgical appliance. The retainer has an internal nut and an external ring threaded onto the yoke to secure the appliance. These devices all have a multitude of small parts that must be assembled at the surgical site.  
       SUMMARY OF THE PRESENT INVENTION  
       [0006]     The polyaxial screw of the instant invention has a helical screw shaft for securing the screw to bone. One end of the shaft has a reverse tapered head tapering toward and away from the shaft from an enlarged median. A spherical shaped swivel, having an opening smaller than the median, is placed over the head to rotatably engage the shaft. The swivel frictionally engages the head. A connector with a dome shaped cavity on one end and bifurcated tabs on the other is connected with the swivel and tightened in place with a set screw and guide ring. An appliance is held between the bifurcated tabs. The tabs are frangible to reduce the profile above the appliance.  
         [0007]     It is therefore an objective of this invention to provide a modular polyaxial screw assembly having a few large parts which can be easily handled and quickly connected during surgery.  
         [0008]     It is another objective of this invention to provide a polyaxial screw assembly having the ability of being angularly fixed in place and connected to a surgical appliance with one tool.  
         [0009]     It is another objective of this invention to provide a low profile surgical appliance.  
         [0010]     It is yet another objective of this invention to provide a device to prevent cross threading of components.  
         [0011]     It is a further objective of this invention to provide an assembly permitting a small screw head.  
         [0012]     It is still another objective of this invention to provide a bone screw that creates a buttress seat in the vertebrae through rotation of an integral flange.  
         [0013]     Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an exploded perspective of the polyaxial screw of this invention;  
         [0015]      FIG. 2  is an exploded side view of the polyaxial screw with an off-set head;  
         [0016]      FIG. 3  is a longitudinal cross section of the assembled polyaxial screw of  FIG. 2 ;  
         [0017]      FIG. 4  is a perspective, partially in section, of the swivel of the polyaxial screw;  
         [0018]      FIG. 5  is a cross section of the rod connector of the polyaxial screw; and  
         [0019]      FIG. 6  is a top plan view of the lock nut and guide ring. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The polyaxial screw assembly  10 , shown in  FIGS. 1-2 , is composed of a bone or pedicle screw  11 , a collet or swivel  12 , a ring or collar  13 ,  13 ′, a connector  14 , a guide or locking ring  15 , and a set screw or nut  16 . In  FIG. 3 , a surgical appliance, such as a spinal rod  17  is illustrated positioned between the connector  14  and the nut  16 . The polyaxial screw assembly may be used in conjunction with spinal rods, hooks or other surgical appliances that require securement to the skeletal structure.  
         [0021]     The bone screw  11 , shown in  FIGS. 1-3 , has a shaft  18  having a helical thread  19  for drilling into the skeleton of a patient. A reverse tapered head  20  has a bore  21  for engagement with a tool (not shown) for turning the screw into the bone. The leading end  22  of the screw may be self tapping or used with pre-drilled holes. The head  20  has its greatest diameter at the shoulder  23 . The head has a taper  24  from the shoulder toward the leading end and another taper  25  from the shoulder toward the head end. A cylindrical groove  26  circumscribes the screw  11  and forms a retention ring for the swivel  12 . Between the groove  26  and the threads  19 , a buttress flange  36  extends circumferentially around the head with radial serrations  37 . As the pedicle screw is turned into the vertebrae, the buttress flange  36  of the screw will contact the cortical bone and rotationally groom the bone to accept the seating of the screw against the vertebra body and provide space between the bone and the head. This serves to stabilize the head of the screw against the hard outer shell of the vertebrae and prevents the bone from interfering with the completion of the assembly. The serrations  37  shear away any bony protuberances resulting in a flat bed for the screw. The flange  36  also provides a depth gauge and stop for the screw.  
         [0022]     The swivel or collet  12 , shown in  FIGS. 1, 2  and  4 , is generally spherical in shape, with an internal cavity, and has longitudinal slots  27  providing adjustability in size through compression. The swivel has a mouth  28  which is interrupted by the slots  27 . The top of the collet, opposite the mouth, has an opening for insertion of a tool. The diameter of the mouth  28  is less than the diameter of the shoulder  23  on the head  20 . The mouth has a lip  29  extending inwardly of the collet. The lip  29  is shaped to snap into the groove  26 . The swivel and the screw will be held together by the elasticity of the collet.  
         [0023]     The connector  14  and the swivel  12  can be pre-assembled as a subassembly to reduce the work load at the surgical site. Of course, this modular construction provides the flexibility of changing the size screw to fit the anatomical requirements of the patient. Once the screw and collet are assembled, the collet can rotate around the longitudinal axis of the screw. This construction permits a smaller screw head than the conventional solid spherical screw heads. The modularity allows different sized screws and different sized collets to be mixed and matched, given that the shoulder  23  of the screw and the mouth  28  of the collet are relatively sized to snap fit.  
         [0024]     A universal connector  14 , as shown in  FIG. 1 , has a dome shaped cavity  30  on one end. The ring  13  is inserted into the cavity  30 . The cavity  30  has an opening  31  in one end to accept the collet. Before or after the assembled polyaxial screw is fixed in the bone, a collar  13 , shown in  FIG. 1 , is inserted between the tabs  32 ,  33  into cavity  30  and in contact with the edge of the opening in the dome shaped cavity  30  of the connector  14 . The collet  12  is inserted into cavity  30  and retains collar  13 . The collet has a diameter greater than the diameter of the opening. The collar  13  has a central opening  41  with a circular extension  42  depending therefrom. The extension  42  is sized to contact the collet  12 . The collar may also be pre-assembled with the connector assembly  40 . The diameter of the circular extension  42  is somewhat less than the outer dimensions of the spherical collet  12 . When the collet, the intermediate ring  13 , and the connector are assembled, the collet is compressed to firmly secure the connector and the collet and the screw together. The top of the dome shaped cavity  30  is open and formed with directly opposed, parallel, semicircular tabs  32  and  33  extending normal to the opening  31 . The outer surfaces  34  and  35  of the tabs are substantially smooth. Near the dome shaped cavity  30 , both tabs  32  and  33  have sections  57  of a frangible ring. The ring may be a groove or separated dimples with the function of locating the level at which the tabs may be broken or otherwise separated to reduce the profile of the apparatus. The internal surfaces have threads  38  and  39  to translate a threaded locking screw along their length.  
         [0025]     The swivel  12 , ring  13 , and the connector  14  may be pre-assembled with the screw  11  to provide a conveniently sized component for the surgeon&#39;s use. The continuous pathway through the tabs, the top of the dome, the ring, and the collet to the bore in the screw permits the assembly to be driven into the bone with one tool in one operation. The connector can then be rotated about the screw by means of the collet. The ball and socket fit between the connector and the collet allow universal movement in an arc of approximately 30 to 40 degrees.  
         [0026]     In this manner, a surgical appliance, such as a spinal rod  17 , may be attached between the tabs of several misaligned screws. A guide ring  15 , shown in  FIG. 6 , is telescoped over the tabs  32  and  33 , after the spinal rod  17  is in place. The guide ring has an inner diameter approximately the same as the outer diameter of the tabs. The guide ring may orient the lock nut  16  in position relative to the tabs to prevent cross threading of the components. As an example, shown in  FIG. 6 , arms  50  and  51  are diametrically opposed and the arms are aligned with the gap between the tabs. In pre-assembly, the threads  55  of the lock nut  16 , shown in  FIG. 5 , may be threaded into buttress screw threads  38  in these arms. The lock nut  16  is translated along the threads  38  to engage the rod  17  by a tool (not shown) engaged with the aperture  56 . Arms  50  and  51  retain the set screw  16  by means of a groove  77  eliminating disassembly.  
         [0027]     The guide ring  15  is placed about the tabs  32  and  33  and secured in place to reinforce the connector  14  diametrically. The lock nut  16  is tightened to positively fix the connector and the swivel  12  in a particular angular orientation with the surgical appliance held in place by the lock nut. However, it is within the purview of this apparatus to use more than one lock nut or collars or circular shims to vertically adjust the level of the surgical appliance within the tabs. The collar  13  serves as a compression ring with an area contact on the collet.  
         [0028]     Once the proper sized screws and connectors are selected for a surgical procedure, the pre-assembly of modules permits the surgeon to manipulate and install a relatively large component having the bone screw, the collet, the connector and the collar as a unit. A tool similar to a screw driver may be inserted through the tabs of the connector the opening in the collar, and the opening in the collet to drive the screw into the bone. The connector may then be maneuvered to put the surgical appliance between the tabs. The guide ring  15  is then placed over the tabs and the locking nut is threaded onto the tabs and tightened in place against the surgical appliance forcing the surgical appliance the collar and the connector into a fixed position.  
         [0029]     In the preferred embodiment, the pedicle screw is placed in the vertebrae and the assembly  40  composed of the collet  12 , the collar  13  and the connector  14  is snapped on the tapered head into groove  26 . Collet ridge  29  will expand beyond shoulder  23  and settle into groove  26 . This action is uni-directional and will not reverse back off screw body. A tool (not shown)can accomplish the assembly and disassembly, if necessary. The collet  12  has screw threads  43  in the top to permit a tool to be inserted to disassemble the device if removal becomes necessary. The connector assembly  40  can now swivel in a cone of approximately 50 degrees around the screw head. To achieve the desired position and lock it in place, rod  17  is placed between tabs  32 ,  33  and makes contact with collar  13 . Following this action, the lock nut  16  and guide ring  15  are threaded down over tabs  32 ,  33  until seated firmly against rod  17 . Pressure created and translated through rod  17  to collar  13  and collet  12  causes collet  12  to compress against cavity  30 . As a result of screw head  20  being inserted into collet  12  during attachment, it will not allow collet  12  to compress to a diameter that will allow it to slip through cavity opening  31  because the diameter of collet  12  is greater than the diameter of the opening  31  due to the shoulder making the collet  12  a solid structure rather than a collapsible structure.  
         [0030]     The tabs  32  and  33  are frangible at a point  57  along their length above the guide ring  15  to reduce the profile of the assembly after placement in the body. The tabs may be weakened by reduced thickness or other modification of the tabs. Further, there may be more than one level of frangibility to compensate for different surgical appliances and vertical adjustability of the placement.  
         [0031]     Due to anatomical considerations, it is rare that a series of pedicle screws will be closely aligned once they are screwed into the vertebrae. To compensate for such misalignment, an off-set connector  14 ′ is shown in  FIGS. 2 and 3 . In the preferred embodiment, the elements of sub-assembly  40 ′ are pre-assembled. The connector  14 ′ can be oriented through a 360 degree arc about the collet  12  and locked in place with a collar  13 ′. The collar  13 ′ has an upstanding shoulder  45  with external threads  46  that engage internal threads  47  in connector  14 ′. The upper end of the collar  13 ′ has a slot  48  to accept a tool (not shown) used to tighten the collar  13 ′ and dome shaped cavity  30 ′ of connector  14 ′ onto the collet  12  for final assembly.  
         [0032]     The connector  14 ′ has an integral ring  60  with a bore  61  parallel to the opening  31 ′ of the dome shaped cavity. The tabs  32 ′,  33 ′ are formed with a closed bottom  62  of a diameter to pass through the bore  61 . The closed bottom  62  has a groove  63  about its circumference. After assembly, the bottom protrudes beyond ring  60  and a retaining clip  64  is resiliently seated in the groove  63  to complete the assembly. The off-set connector and screw may be preassembled before surgery to avoid handling small components in the surgical field. As mentioned above, the preferred embodiment contemplates the pedicle screw being placed in the vertebrae, alone, with subsequent assembly of the device.  
         [0033]     In the off-set screw head  14 ′, lock down between the screw  11  and the head  20  is accomplished by driving set screw  46  down to engage attached collar  13 ′. Pressure created and translated through set screw  46  onto collar  13 ′ and collet  12 ′ causes collet  12 ′ to compress against cavity  30 ′. As a result of screw head  20 ′ being inserted into collet  12 ′ during attachment, it will not allow collet  12 ′ to compress to a diameter that will allow it to slip through cavity opening  31 ′.  
         [0034]     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment but only by the scope of the appended claims.