Abstract:
The present invention provides a stabile construct to facilitate connecting a spinal fixation rod to a plurality of vertebral bodies. The construct or implant comprises a housing and an insert. The housing and insert are shaped to cooperatively engage the bone screw and rod to provide a stable construct that allows for polyaxial orientation between the bone screw and the remainder of the construct.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to spinal fixation devices and more particularly to a pedicle screw and rod fixation assembly useful in stabilizing a spine of a patient.  
       BACKGROUND OF THE INVENTION  
       [0002]     Over the years, several techniques and systems have been developed for correcting spinal injuries and/or degenerative spinal processes. Spinal correction frequently requires stabilizing a portion of the spine to facilitate fusing portions of the spine or other correction methodologies. Medical correction of this type is frequently employed for many spinal conditions, such as, for example, degenerative disc disease, scoliosis, spinal stenosis, or the like. Frequently, these corrections also require the use of implants, such as, bone grafts. Stabilizing the spine allows bone growth between vertebral bodies such that a portion of the spine is fused into a solitary unit.  
         [0003]     Several techniques and systems have been developed for correcting and stabilizing the spine and facilitating fusion at various levels of the spine. In one type of system, a rod is disposed longitudinally along the length of the spine in the region of concern. The rod is arranged according to the anatomy and the correction desired. In this system, the rod is aligned along the spine and engages various vertebrae along its length. The rod engages, or more typically the parallel rods engage, the spine using fixation elements, such as, anchors attached to vertebral bodies by a bone screw.  
         [0004]     Anatomy and correction frequently require aligning the rod and screw at various angles along the length of the portion of correction. In order to provide this alignment, polyaxial screws/anchors have been developed. Many variations of polyaxial screw and rod fixation systems exist on the market today.  
         [0005]     Despite the fact that numerous polyaxial rod fixation systems are on the market, improvement of the devices is desirable. Accordingly, there is a need for a screw and rod fixation system that provides a strong, effective, and secure lock of the screw and rod in the desired position and angle. Additionally, there is a need for a screw head securing mechanism or device that is minimal in size and has a reduced amount of components to provide for a simpler, more effective, and less cumbersome device for fixing screws.  
       SUMMARY OF THE INVENTION  
       [0006]     To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a screw and rod fixation system is provided. The screw and rod fixation system comprises a bone screw having a threaded portion, a transition, and a head comprising a housing engaging surface and an insert engaging surface. The bone screw is provided in a housing with a bone facing surface and a top edge and a sidewall extending therebetween. A through hole extends from the top edge to the bone facing surface and a pair of opposing slots reside proximate the top edge. The housing has a screw head seating surface that is shaped to cooperatively engage the housing engaging surface to allow polyaxial orientation of the bone screw relative to a vertical axis of the housing. A housing couples to the bone screw and housing. The insert comprises a screw head engaging surface to cooperatively engage the insert engaging surface to allow polyaxial orientation of the bone screw relative to a vertical axis of the housing and an elongate rod rest shaped to seat a portion of the elongated rod, the elongated rod rest traversing the housing between the pair of opposing slots. An elongated rod traverses the housing between the pair of opposed slots and seated in the elongated rod rest.  
         [0007]     The present invention further provides an insert useful in a device that couples spinal rods to vertebral bodies. The insert comprises a bone screw engaging surface and an elongated rest coupled to the bone screw engaging surface having an outer edge and a bottom surface opposite the elongated rest. A transition portion connects the bone screw engaging surface and the elongated rest. The insert cooperatively engage a bone screw to allow polyaxial orientation of the bone screw relative to the insert.  
         [0008]     The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]     The accompanying drawings illustrate various embodiments of the present invention and are a part of the specification. The illustrated embodiments are merely examples and illustrations of the present invention and do not limit the scope of the invention.  
         [0010]      FIG. 1  shows a perspective view of a screw and rod fixation system in accordance with an embodiment of the present invention;  
         [0011]      FIG. 2  shows a cross-sectional view of the system of  FIG. 1 ;  
         [0012]      FIG. 3  shows a perspective view of a housing associated with an embodiment of the present invention shown in  FIG. 1 ;  
         [0013]      FIG. 4  shows a cross-sectional view of the housing of  FIG. 3 ;  
         [0014]      FIG. 5  shows a perspective view of a bone screw associated with an embodiment of the present invention shown in  FIG. 1 ;  
         [0015]      FIG. 6  shows another perspective view of the bone screw of  FIG. 5 ;  
         [0016]      FIG. 7  shows a perspective view of an insert associated with an embodiment of the present invention shown in  FIG. 1 ;  
         [0017]      FIG. 8  shows a cross-sectional view of the insert of  FIG. 7 ;  
         [0018]      FIG. 9  shows a top elevation view of the insert of  FIG. 7 ; and  
         [0019]      FIG. 10  shows a detailed cross-sectional view of the construct shown in  FIG. 2 .  
     
    
     DETAILED DESCRIPTION  
       [0020]     Referring now to  FIG. 1  and  FIG. 2 , and in accordance with certain embodiments of the present invention, a screw and rod fixation system  100  is shown.  FIG. 1  shows a perspective view of system  100  while  FIG. 2  shows a cross-sectional view of system  100 . System  100  includes a bone screw  102 , a housing  104 , an insert  106 , a rod  108 , and a compressive member, such as, a setscrew  110 . Housing  104  may contain one or more first mating surfaces  112 . First mating surfaces  112  are designed to mate with a tool (not specifically shown). Also, setscrew  110  typically has one or more second mating surface  114  to mate with a tool (also not shown). As shown in  FIG. 1 , first mating surfaces  112  are actually slots on an outer surface  116  of housing  104 . While shown as slots, first mating surfaces  112  may be any number of designs including one or more dimples, hex detents, or other equivalent mechanisms as are known in the art. Second mating surface  114  is shown with a hex shape to accept a hex driver useful in threading the setscrew. Of course, one of ordinary skill in the art would recognize other and equivalent first and second mating surfaces  112 ,  114  are possible.  
         [0021]     Referring now to  FIGS. 3 and 4 , housing  104  is described in more detail. Housing  104  may be referred to as a coupling device, seat, or anchor. Housing  104  has a bone facing surface  302 , at least one sidewall  304  having an outer surface  116  and an inner surface  306  (best seen in  FIG. 4 ), first mating surfaces  112 , a pair of opposed slots  308  in sidewall  304 , a top edge  310 , and a through hole  312  extending from top edge  310 , to bone facing surface  302 . Housing  104  can be considered to have a rod receiving section  402  and a screw receiving section  404 .  
         [0022]     Notice, housing  104  is shown with one cylindrically shaped sidewall  304 . It is believed providing housing  104  as a cylindrical shape reduces the profile of the device, but other shapes are possible, such as cubic or the like. If housing  104  had multiple sidewalls  304 , the edges between the multiple sides should be beveled or rounded to inhibit tissue trauma.  
         [0023]     Referring first to rod receiving section  402 , inner surface  306  has first threads  406  extending partially along inner surface  306 . As will be explained below, first threads  406  allow setscrew  110  to be threaded on housing  104 . Slots  308  generally have a width W 1  sufficiently wide to accept rod  108 . Slots  308  have an insert seating surface  408 , which will be explained further below. An insert groove  410  is provided about insert seating surface  408 . Insert groove  410  allows provides a mechanism to lock and stabilize insert  106 , as will be explained below.  
         [0024]     As shown in  FIG. 4 , rod receiving section  402  provides a channel for a single rod  108  to traverse housing  104  from slot  308  to slot  308 . In some instances, it may be difficult to provide a single rod  108  that can traverse the entire length of the spine necessitating multiple rods. Housing  104  could be easily converted to provide multiple rod guides. For example, housing  104  could have a second set of opposed slots substantially parallel to slots  308 . Alternatively (but not shown), a rod cap/seat could be installed below threads  406  and above insert seating surface  408 . The rod cap/seat would provide a cap side operatively sized and shaped to fit about rod  108  opposite the insert seating surface  408 . The rod cap/seat provide a second seating surface operatively sized and shaped to fit about a second rod aligned substantially above rod  108 . Setscrew  110  would provide compressive force to the substantially aligned rod, which would correspondingly provide compressive force to rod  108  and insert  106 , etc.  
         [0025]     Referring to screw receiving section  404 , inner surface  306  has a screw head seating surface  412 . Surface  412  has a shape corresponding to the screw head, described below. Surface  412  has an upper lip  414  and a lower lip  416 . Upper lip  414  and lower lip  416  could comprise a beveled edge and/or protrusions. Upper lip  414 , lower lip  416 , and surface  412  cooperative engage the screw head to allow a polyaxial engagement of the screw  102 , which will be described below. As shown, upper lip  414  has a first diameter d 1  and lower lip  416  has a second diameter d 2 . First diameter d 1  is larger than second diameter d 2 . Inner surface  306  terminates at bone facing surface  302 . As shown, inner surface  306  flares out from second diameter d 2  such that the bone side of through hole  312  has a third diameter d 3  larger than second diameter d 2 , but generally smaller than first diameter d 1 . Outer surface  116  is generally perpendicular to top edge  310  and bone facing surface  302  to conserve space. As shown, bone facing surface  302  may have a rounded transition  418  between bone facing surface  302  and outer surface  116  to inhibit trauma to tissue. Also, top edge  310  has beveled or rounded edges. Lastly, through hole  312  at top edge  310  has a fourth diameter d 4 . Diameter d 4  is sufficiently large to accept the head of screw  102  and setscrew  110 .  
         [0026]     Bone screw  102  will now be described with reference to  FIGS. 5 and 6 . Bone screw  102  has a threaded portion  502 , a transition portion  504 , and a head portion  506 . Threaded portion  502  can use any conventional thread, but as shown, threaded portion  502  has a shaft  508  and threads  510  machined such that shaft  508  has an increasing diameter from the tip  512  to transition portion  504 . Further, threads  510  become relatively thicker towards transition portion  504 . Designing threaded portion  502  in this fashion increases the frictional engagement of bone screw  102  in bone and generally increases the screw strength.  
         [0027]     Transition portion  504  comprises the portion of bone screw  102  between threaded portion  502  and head portion  506 . Transition portion  504  could be integrated into threaded portion  502 . Transition portion  504  may be straight, curved, bowed, flared, or the like to transition threaded portion  502  to head portion  506 .  
         [0028]      FIG. 6  shows head  506  of screw  102  in more detail. Head  506  has a housing engaging surface  602 , an insert engaging surface  604 , a top edge  606 , and at least one third mating surface  608 , which in this case comprises a plurality of grooves. Insert engaging surface  604  has a shape corresponding to the insert, as is explained further below. Third mating surface  608  is designed to allow a surgeon to thread screw  102  into bone using a driver.  
         [0029]     Housing engaging surface  602  has a shape corresponding to screw head seating surface  412 . As shown, screw head seating surface  412  is convex in shape to mate with the concave shaped housing engaging surface  602 . Housing engaging surface  602  can rotate relative to screw head seating surface  412 . If designed as protrusions, upper lip  414  and lower lip  416  may provide limits on the rotation or angle of screw  102  relative to housing  104 .  
         [0030]     Insert  106  will now be described with reference to  FIGS. 7, 8 , and  9 . Insert  106  comprises a screw head engaging surface  702 , a transition section  704 , and an elongated rod rest  706 . Screw head engaging surface  702  has a shape that corresponds to insert engaging surface  604 . In this case, insert engaging surface  604  is concave shaped to mate with convexly shaped screw head engaging surface  702 . The corresponding surfaces  604  and  702  allow polyaxial orientation of screw  102  relative to insert  106 . Moreover, insert  106  and housing  104  remain aligned or substantially aligned. Transition section  704  is a provides a transition between screw head engaging surface  702  and elongated rod rest  706 . Transition section  704  is somewhat narrower than the largest diameter of surface  702  to increase the rotational engagement between the screw and insert.  
         [0031]     Elongated rod rest  706  traverses housing  104  between opposing slots  308 . Elongated rod rest  706  has a bottom surface  708  shaped to fit in insert seating surface  408 . Elongated rod rest  706  is shaped to receive rod  108 . Optionally, rod rest  706  may be solid as shown or substantially void as a matter of design choice; however, it is believed providing a substantially solid rest provides a more stable construct.  
         [0032]     Referring now to  FIG. 9 , a top elevation view of insert  106  is provided. As shown, elongated rod rest  706  has a plurality of tabs  902  formed by slots  904 . Each tab  902  has a protrusion  906 . Insert  106  is placed in housing  104  by compressing tabs  902  as allowed by slots  904  toward rest  706 . When compressed, protrusions  906  are a distance d 5  apart, where d 5  is less than W 1 . When compressed, insert  106  can be fitted into through hole  312  of housing  106  until screw head engaging surface  702  rests on insert engaging surface  604 . Once surface  702  and surface  604  are in place, the compression on tabs  902  is released and tabs  902  return to the uncompressed state such that protrusion  906  are a distance d 6 , where d 6  is greater than d 5  and W 1 . Protrusions  906  engage groove  410  ( FIG. 4 ) to lock and stabilize insert  106  in housing  104 .  
         [0033]     With reference to  FIG. 10 , placement of system  100  will be explained. First, screw  102  is inserted through through hole  312 . Screw  102  is partially driven into bone, not specifically shown in the FIG, until bone facing surface  302  seats on the bone and housing engaging surface  602  engages screw head seating surface  412 . Screw  102  may be angled relative to a vertical axis V of housing  104 . Also, housing is aligned such that opposing slots  308  will align with rod  108 . Screw  102  also may have a polyaxial orientation relative to insert  106 . Rod  108  is placed on elongated rod rest  706  and setscrew  110  is threaded into through hole  312  until setscrew  110  contacts rod  108  placing the construct under sufficient compression to stabilize and lock the device in place. As shown, threads  406  substantially terminate at a top surface  1002  of rod  102 . This allows sufficient thread to provide the necessary force without providing unnecessary thread. Thread  406  could stop substantially at or above top surface  1002 , but in that case, setscrew  110  should be designed with a boss. The thread design reduces the spreading of the housing  104  when the setscrew is tightened to rod  108 .  
         [0034]     The above construct is very stable in part because of the surface-to-surface contacts provided by various engaging surface. Further stability is provided in some instances by the insert coupling with a groove in the housing an providing a surface-to-surface contact between the rod and insert.  
         [0035]     While the invention has been particularly shown and described with reference to an embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention.