Abstract:
A pedicle screw provides for elimination of splaying forces caused by an internal closure member. A locking member dovetails with longitudinal channels in wall sections of the receiver, locking the wall sections together and preventing the splaying. The closure member when disposed within a bore of the receiver tightens the locking member onto an elongated member, preventing removal of the locking member and the elongated member from the receiver. The locking member and the closure member may be permanently rotatably inter-engaged for ease of insertion and removal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to medical devices and in particular to a pedicle screw.  
           [0003]    2. Description of the Related Art  
           [0004]    A pedicle screw is a form of specialized screw used in surgery for the stabilization and immobilization of spinal segments to relieve instability for trauma purposes, and for the relief of pain due to degenerative disc disease. The screw is composed of multiple components generally including a screw shank, a receiver or screw head, a closure cap, and a rod which is used to connect two or more screws together in a series. The screw shank typically extends through the bottom of the screw head and is held in place in a polyaxial joint that allows for flexibility to adapt to the desired position of the vertebra. The closure cap is then typically inserted down on top of the rod and threaded into the receiver or head, locking the rod down tightly to the receiver as well as locking down the polyaxial joint between the receiver and screw shank.  
           [0005]    There are several different types of pedicle screws with various closure mechanisms currently available. Internal closure caps offer advantages that have made internal closure caps widely used in pedicle screws. One such advantage is the ease of use. Internal closure caps provide a clean, visible, protected spot to place the closure cap. However, internal closure caps inherently act as a wedge, splaying or spreading the “horseshoe” shape of the receiver as the cap tightens down on the rod, or afterwards, tending to splay in response to stress caused by patient movement. This splaying can lead to a compromise of the connection between the closure cap and the receiver. This is a known problem. There have been documented instances where closure caps have popped out of or dislocated within receivers causing immediate and catastrophic failure of the pedicle screw system. A number of patents are directed to solving this problem, but the search for a better solution continues.  
           [0006]    Because of the splaying problem inherent to an internal closure cap, some pedicle screws have utilized an outer nut to avoid the splaying problem. However, an outer nut leads to its own set of problems. The outer nut tends to bind in soft tissue that surrounds the vertebra which can cause complications and increase the difficulty in attempting to attach the nut. Outer nuts can also lead to increased difficulty and application of other instruments that commonly used to perform correction maneuvers. These instruments are often used in other parts of the surgery and are often used in conjunction with tightening and loosening of the closure caps.  
           [0007]    Pedicle hooks, similar to pedicle screws but using a hook mechanism instead of a screw shank, are also used in certain surgical situations. The hook is typically formed as a single unit with the receiver.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    Various embodiments implementing aspects of the invention are disclosed, generally directed to an internally closed receiver with a locking member to prevent splaying of the receiver. The locking member locks into a longitudinal channel in the walls of the receiver. In one embodiment, the locking member and a closure member are rotatably attached to each other for insertion into the receiver. In various embodiments, the receiver is attached to the screw shank for universal movement, the shank can be affixed to the receiver, or the receiver is attached to a hook for attachment to a bone. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0009]    A better understanding can be obtained when the following detailed description of several disclosed embodiments is considered in conjunction with the following drawings in which  
         [0010]    [0010]FIG. 1 is a cutaway view of a pedicle screw according to one embodiment;  
         [0011]    [0011]FIG. 2 is a cutaway view of a portion of the embodiment of FIG. 1 from an angle perpendicular to the view of FIG. 1;  
         [0012]    [0012]FIG. 3 is a top view of a locking member engaged with the screw head of the pedicle screw according to one embodiment;  
         [0013]    [0013]FIG. 4 is a top view of the embodiment of FIG. 3 showing a closure member holding the locking member in place;  
         [0014]    [0014]FIG. 5 is an exploded view of the screw head locking member and closure member according to one embodiment;  
         [0015]    [0015]FIG. 6 is a view of three pedicle screws connected with a rod into a single unit;  
         [0016]    [0016]FIG. 7 a  is a cutaway view of a pedicle hook according to one embodiment; and  
         [0017]    [0017]FIG. 7 b  is a view of the pedicle hook of FIG. 7 a  from a different angle. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    A pedicle screw of an internal plug design with an additional locking member actively resists the aforementioned splaying or spreading forces. Disclosed embodiments provide for easy insertion of the closure cap and prevent any compromise of strength due to splaying of the screw head. FIG. 1 is a cutaway view of a pedicle screw S according to one embodiment assembled together. The screw shank  105  is a threaded portion of a screw that is driven or otherwise inserted into the pedicle of a vertebra to provide an anchor point. Screw shanks  105  come in a variety of outer thread diameters, typically between 4.5 mm and 8.5 mm. The length of the screw shank  105  is determined by measuring from right below the head of the screw shank  105  to the tip of the screw shank  105 . The head of the screw shank  105  is not typically included in the length measurement. Lengths typically range between 25 mm and 60 mm. A thread pitch of the screw shank  105  can vary. In one embodiment, the head  110  of the screw shank  105  is spherical in nature. A drive slot (not shown) may be milled into end of the head  110  in line with the midline of the screw shank  105  in order to drive the screw shank  105  into and out of the bone of the vertebra. Other techniques for driving the screw shank  105  into the bone can be used.  
         [0019]    A receiver or head  115  as in a disclosed embodiment may perform multiple functions. The polyaxial head  115  includes a floating saddle  112  where the polyaxial screw head  110  articulates, giving the polyaxial screw shank  105  polyaxial capability. The receiver  115  also acts as a receiver for a rod (not shown in FIG. 1) for connecting multiple screws S to each other. The receiver  115  forms a female portion or recess to receive the closure cap  145  and locks the entire construct down. The receiver  115  typically is generally shaped as a cylinder between 13 mm and 16 mm in diameter and between 15 mm and  17  mm in length. The receiver  115  may contain features to allow for mechanical assistance in seating the rod into the receiver  115 . These features are well known in the art and will not be further discussed. A bore is formed into the upper portion of the receiver  115 . A transverse channel  155  is also formed perpendicular to the cylindrical axis of the receiver  115  for accepting the rod that connects multiple screws S together. Rods may have different outer diameters, with a 5.5 mm rod being typical. As best shown in FIG. 5, the receiver  115  thus contains a plurality of walls  120 A and  120 B surrounding the transverse channel  155 . Typically the upper end of the inside surface of the plurality of walls  120 A and  120 B will have threads  500  for screwing an internal closure member into the polyaxial head  115 . As best shown in FIG. 5, each of the plurality of walls  120 A and  120 B contains a longitudinal channel  310 A and  310 B. This channel is typically perpendicular to the transverse channel for the rod and typically extends into the threaded portion  500  of the walls  120 . However, other channel orientations may be used, such as spiral orientation.  
         [0020]    A locking member  135  can be inserted into the receiver  115 . The lower surface of the locking member  135  may contain a channel  130  for receiving the rod and holding the rod in place. The channel  130  typically mirrors the channel  155  of the receiver  115 . In one embodiment, a pin or other protrusion may extend perpendicular to an upper surface of the locking member  135  for engagement with the closure member  145 . As shown in FIG. 1, the pin  140  can be splayed outwardly after engagement with the closure member  145  to allow the closure member  145  and the locking member  135  to be held together for ease of use in surgery, while allowing rotation of the two pieces with respect to each other. The closure member  145  may contain threads  150  to mate with the threaded portion  500  of the plurality of walls  120  for securely tightening the locking member  135  onto the rod  200 . Other techniques for mating the closure member  145  with the walls  120  can be used.  
         [0021]    As shown in FIG. 2, when the locking member  135  is placed into the receiver  115 , it can saddle the rod  200  in the channel  130 , thus allowing fixing the rod  200  in place.  
         [0022]    As shown in the top view of FIG. 3, tenons  300   a  and  300   b  of the locking member  135  are mortised into longitudinal channels  300   a  and  300   b  upon insertion of the locking member  135 . The locking member  135  is thus dovetailed into the plurality of walls  120   a  and  120   b , preventing splaying of the walls  120   a  and  120   b , when the closure member  145  is threaded into or otherwise inserted into the receiver  115 , as shown in the top view of FIG. 4. As shown in the top view of FIG. 4, the closure member  145  may have a hexagonal shaped opening for engaging with an insertion tool. Although as shown in FIG. 4, the opening is hexagonal in shape, other shapes may be used, corresponding to the insertion tool (not shown). Although as shown, the channels  300   a  and  300   b  are roughly triangular in cross-section, other cross-sectioned shapes in which the width of the channel  300  distal from the bore in the receiver  115  is larger than the width of the channel  300  proximal to the bore may be used. For example, a T-shaped channel may be used.  
         [0023]    As best shown in FIG. 5, the locking member  135  may have a vertical pin  140  for insertion into an opening  147  of the closure member  145 . In another embodiment, the locking member  135  is not attached to the closure member  145  but can be separately inserted into the receiver  115 . Other forms of attachment allowing rotation between the locking member  135  and the closure member  145  may be used.  
         [0024]    [0024]FIG. 6 illustrates the use of pedicle screws S with a common rod  200  locking multiple vertebra in a desired alignment.  
         [0025]    In some situations, a pedicle hook  700  as shown in FIGS. 7 a - 7   b  may be preferable to use of a pedicle screw S as discussed above. FIG. 7 a  is a cutaway view of the pedicle hook  700  from an angle parallel to the rod  200 , shown in phantom. FIG. 7 b  is a view of the pedicle hook  700  from an angle perpendicular to the  200 . The pedicle hook  700  is typically a unitary receiver  720  as in the pedicle screw S, substituting a hook  710  for the screw shank  105  and floating saddle  112 . However, other techniques for attaching the hook  710  to the receiver  720  can be used. The shape, size, and orientation of the hook  710  shown in FIGS. 7 a - 7   b  is exemplary and illustrative only, and other shapes, sizes, and orientations can be used. Both open and closed hooks are known in the art, with closed hooks typically open for insertion and closed with a surgical tool upon proper placement. The receiver  720  of the pedicle hook in a disclosed embodiment is otherwise identical to the receiver  115  of the pedicle screw S of FIGS. 1-6.  
         [0026]    The foregoing disclosure and description of the various embodiments is illustrative and explanatory thereof, and various changes in the elements and details of the illustrated elements, construction and method of operation may be made without departing from the spirit of the invention.