Abstract:
A screw fixation system includes a plate having opposed first and second surfaces, and a hole defined in the plate which extends between the first and second surface. A screw is included having a head and a shaft extending from the head and defining a longitudinal axis for the screw, the shaft of the screw being configured and adapted to be inserted into the hole. A retainer is circumferentially positioned around the shaft of the screw and proximate to the screw head. The retainer is configured to engage with the plate proximate the hole when the shaft of the screw has been inserted into the hole so as to resist removal of the screw from the hole and to allow removal of the screw from the hole when a preset torque is applied to the screw.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Application No. 61/229,525, filed Jul. 29, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fixation plates. More particularly, the invention relates to controlling the backing out of screws from such fixation plates. 
     2. Description of Related Art 
     A wide variety of bone fixation plates exist. An exemplary plate carries a number of screws, each extending through an associated aperture in the plate and attached to bone. In the art of orthopedic fixation, it is now common for surgeons to utilize fixation plates for the treatment of spinal disorders which can include spinal anomalies, spinal injuries, disc conditions, and bone conditions. 
     While the procedure of implanting a bone fixation plate carries its own potential for complications, an added risk of complication is in the tendency of fixation screws to back out or loosen with respect to the remaining implant components. The loosening of the screws can lead to failure of the device. In the case of cervical spine fixation, for example, a loose screw could penetrate the esophagus causing infection. 
     The tendencies of screws to back out of plates has led to proposals for a wide variety of mechanisms for retaining screws against back-out. However, most of the systems employ some form of secondary locking screw. Such a secondary locking screw inherently increases the complexity and cost of the device while further increasing the implantation time. Also, these secondary locking screws themselves can loosen and back out of the plate over time. 
     Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for an implant system that provides a fixation plate screw locking system that allows for improved prevention of fixation screws from backing out while having fixation screws that can still be removed from the implant device if necessary. There also remains a need in the art for such an implant system that is easy to make and use. The present invention provides a solution for these problems. 
     SUMMARY OF THE INVENTION 
     The subject invention is directed to a new and useful screw fixation system. The screw fixation system includes a plate having opposed first and second surfaces, and a hole defined in the plate which extends between the first and second surface. A screw is included having a head and a shaft extending from the head and defining a longitudinal axis for the screw, the shaft of the screw being configured and adapted to be inserted into the hole. A retainer is circumferentially positioned around the shaft of the screw and proximate to the screw head. The retainer is configured to engage with the plate proximate the hole when the shaft of the screw has been inserted into the hole so as to resist removal of the screw from the hole and to allow removal of the screw from the hole when a preset torque is applied to the screw. 
     In accordance with certain embodiments, the plate includes a plurality of holes and each hole receives a screw and a retainer associated therewith. The retainer can be a split ring to permit the retainer to radially compress when a preset torque is applied to the screw to allow insertion of the screw. The retainer can be a split ring with a vertical or diagonal slit to permit the retainer to radially compress when a preset torque is applied to the screw to allow removal of the screw. The retainer can have a substantially quadrilateral radial cross-section. 
     In certain embodiments, the shaft of the screw includes a circumferential channel which is configured to receive the retainer. The channel can include a bottom surface which is formed at an oblique angle with respect to the longitudinal axis of the screw, wherein the channel is configured to receive the retainer. The retainer can have an inner peripheral surface which is adapted to mate with the bottom surface of the channel when the retainer is radially compressed into the channel. At least one corner of the retainer can be rounded to facilitate engagement of the retainer in the channel. The bottom surface of the channel can be at an angle of about eight degrees to about fifteen degrees with respect to the longitudinal axis of the screw, wherein the channel is configured to receive the retainer. It is also contemplated that the bottom surface of the channel can be at an angle of about twelve degrees, or any other suitable angle, with respect to the longitudinal axis of the screw. 
     In accordance with certain embodiments, the plate includes a circumscribing undercut surface inset from the second surface of the plate formed about the hole to accommodate angled engagement of the screw. The screw shank can have a keyway and the retainer can have a corresponding key configured to engage the keyway and restrict relative rotation of the screw and retainer. The retainer can have a band portion and a plurality of lateral projections extending from the band portion wherein the plurality of projections are configured to engage a portion of the second surface of the plate adjacent the hole. The plate can include a circumscribing plurality of radially extending channels inset from the second surface of the plate adjacent to the hole and configured to engage lateral projections of the respective retainer. 
     The invention also provides a screw fixation system with a plurality of flexible radially inward extending petals in the plate. The plate has opposed first and second surfaces, and a hole defined in the plate which extends between the first surface and second surface. A plurality of flexible radially inward extending petals are circumferentially spaced apart from one another around the periphery of the hole. A screw having a head and a shaft extending from the head and defining a longitudinal axis is included. The shaft of the screw is configured and adapted to be inserted into the hole and to engage with the flexible petals, the flexible petals being configured to engage the screw to resist backing out of the screw from the plate. 
     These and other features of the systems and methods of the subject invention will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject invention appertains will readily understand how to make and use the devices and methods of the subject invention without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG. 1  is a perspective view of an exemplary embodiment of a fixation plate system constructed in accordance with the present invention, showing a plate with a plurality of screws seated in the plate; 
         FIG. 2  is a cross-sectional side elevation view of the fixation plate system of  FIG. 1 , taken along line  2 - 2 , showing a screw with a retainer seated in the plate; 
         FIG. 3  is an enlarged view of a portion of the plate assembly of  FIG. 2 , showing the engagement of the retainer and screw seated in the plate; 
         FIG. 4  is a side elevation view of a portion of the plate assembly of  FIG. 2 , showing the screw with a retainer split having a diagonal slit; 
         FIG. 5  is a cross-sectional side elevation view of the fixation plate of  FIG. 1 , showing the screw and retainer ring assembly during insertion into the plate; 
         FIG. 6  is a perspective view of another exemplary embodiment of a fixation plate system constructed in accordance with the present invention, showing a screw with a split ring retainer having a vertical slit; 
         FIG. 7  is a perspective view of another exemplary embodiment of a fixation plate system constructed in accordance with the present invention, showing a plate with inward extending petals engaging a screw; 
         FIG. 8  is a cross-sectional elevation view of the screw and plate of  FIG. 7 , taken along line  8 - 8 . showing a plate with petals engaging the screw; 
         FIG. 9  is a plan view of the plate of  FIG. 7 , showing the petals of the plate with the screw removed; 
         FIG. 10  is a perspective view of another exemplary embodiment of a fixation plate system constructed in accordance with the present invention, showing a plate and screw with a retainer and lateral projections from the retainer engaging the plate; 
         FIG. 11  is a cross-sectional elevation view of the fixation plate system of  FIG. 10 , taken along line  11 - 11 , showing the plate and screw with the retainer removed; 
         FIG. 12A  is a plan view from above of the fixation plate system of  FIG. 10 , showing the heat of the screw; 
         FIG. 12B  is a plan view from above of a portion of the fixation plate system of  FIG. 10 , showing the retainer with lateral projections; 
         FIG. 12C  is a perspective view from above of a portion of the fixation plate system of  FIG. 10 , showing the plate; 
         FIG. 13A  is a plan view from below of a portion of the fixation plate system of  FIG. 10 , showing the screw; 
         FIG. 13B  is a perspective view from below of a portion of the fixation plate system of  FIG. 10 , showing the plate with a plurality of grooves circumscribing the hole; 
         FIG. 13C  is a plan view from below of a portion of the fixation plate system of  FIG. 10 , showing the retainer with lateral projections; 
         FIG. 14A  is a perspective view of a portion of the fixation plate system of  FIG. 10 , showing the keyway of the screw; 
         FIG. 14B  is a perspective view of the fixation plate system of  FIG. 10 , showing the key of the retainer for engaging the keyway of the screw; and 
         FIG. 14C  is a perspective view of the fixation plate system of  FIG. 10 , showing the plate with a plurality of grooves circumscribing the hole for engaging the lateral projections of the retainer. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject invention. For purposes of explanation and illustration, and not limitation, a perspective view of an exemplary embodiment of a fixation plate system in accordance with the invention is shown in  FIG. 1  and is designated generally by reference character  20 . Other embodiments of fixation plate systems in accordance with the invention, or aspects thereof, are provided in  FIGS. 2-14C , as will be described. The systems of the invention can be used to provide a fixation plate screw locking system that allows for improved prevention of fixation screws from backing out while having fixation screws that can still be removed from the plate if necessary, for example. 
       FIG. 1  shows a bone fixation plate assembly  20  including a plate  22  and a plurality of screws  24 . The exemplary plate is formed as a unitary single metal piece having a second face  26  for contacting the bone(s) to be secured thereto and an opposite first face  28 . A lateral perimeter  30  circumscribes the plate. 
     Referring now to  FIG. 2 , each screw is accommodated by an associated hole (aperture)  32  in the plate. As is discussed further below, each hole  32  has a central axis  500 . As is discussed further below, the axis  500  may coincide with the axis  502  of the screw with allowance for relative excursions. An exemplary angle between the axes  500  and  502  is shown as θ 1 . Hole  32  is bounded by a surface including a convex bowl/dish  40  divergently opening toward the first face  28 . The bowl curvature is complementary to the curvature of a convex peripheral surface portion  42  of an underside of the screw head  44 . A shank or shaft  46  depends from screw head  44 . Shaft  46  extends from a proximal end near head  44  to a distal end or tip  47  and bears an external thread  48  for engaging the bone. 
     With screw  24  in an installed condition, the convex peripheral surface portion  42  may be in sliding engagement with the bowl  40  to accommodate orientational tolerance of screw  24  relative to plate  20 . Tool-engaging features, e.g. facets  50 , are formed in the upper surface  52  of head  44  for engaging a screwdriver, allan wrench, or other tool as may be known in the art or yet developed. In a transition region  60  between head  44  and shaft  46 , screw  24  includes a radially inwardly directed channel  62  having a base  64 , an upper side surface  66 , and a lower side surface  68 . As is discussed further below, channel  62  carries a split ring retainer  70 . The exemplary retainer has an angular/diagonal (off-longitudinal) split  72 , shown in  FIG. 4 . 
     Referring now to  FIG. 3 , the exemplary retainer  70  has a radial cross-section characterized as a modified rectangle with inboard and outboard faces  74  and  76  along the longer legs of the rectangle and upper and lower rim faces  78  and  80  along the shorter legs of the rectangle. An optional rounded transition  82  is provided between the inboard face  74  and the lower rim face  80  to facilitate engagement of retainer  70  in channel  62 . The shape of retainer  70  is essentially complementary to that of channel  62 . The exemplary retainer  70  and channel  62  are oriented at an angle that is off-radial (i.e., the surface normals, channel base  64 , and retainer inboard face  74  are off-radial by an angle θ 2  (e.g., about 12°, more generally 8-15°. The surface normals of the channel sides and retainer rims are similarly off-longitudinal. Higher angles θ 2  can be achieved by increasing the wall thickness of the retainer and making the groove in the screw deeper. Those skilled in the art will readily appreciate that any suitable angles can be used for channel  62  and retainer  70  without departing from the spirit and scope of the invention. 
     The exemplary hole  32  is further bounded by a circumscribing undercut  86  in second face  26 , leaving an annular lip  88  between the undercut  86  and an adjacent portion of bowl  40 . Undercut  86  may accommodate the retainer during angular excursions of screw  24  relative to plate  20 . 
     In the installed condition, an initial backing out of screw  24  causes retainer upper rim face  78  to contact an adjacent portion, e.g., a bevel  90 , of lip  88 . With the lower rim face  80  of retainer  70  engaging lower side  68  of channel  62 , further backing out is resisted. However, the off-radial angle of upper rim surface  66  may be selected to permit screw extraction provided that the backing out force reaches a sufficient level (e.g., associated with intentional unscrewing rather than incidental vibration). When the force reaches a sufficient amount, a camming action between retainer  70  and the surface bounding/defining holes  32  will tend to radially contract retainer  70 , causing retainer  70  to snap outward through hole  32 . The relatively shallow (near longitudinal) angle of the surface  76  provides relative ease of compression of retainer  70  during insertion. The more abrupt (radial) angle of surface  78  provides relatively higher resistance to spring compression during retraction and, thereby, relatively higher required extraction force. The variability of toggle between screw and plate installation/retraction forces required can also be varied by varying the tightness with which retainer  70  locks onto plate  22 . 
     The components may first be manufactured, for example, via machining from appropriate metal stock, or by any other suitable process. Exemplary materials for the components of system  20  include appropriate implantable materials such as titanium, cobalt, Hastelloy (available from Haynes International of Kokomo, Ind.), nitinol, PEEK, and the like. Exemplary plate, screw, and retainer materials are medical grade titanium, titanium alloys, and the like. Those skilled in the are will readily appreciate that any suitable materials can be used without departing from the spirit and scope of the invention. 
     The retainers may be assembled to their respective screws by inserting the screw tips through the retainers. A camming engagement between screw threads  48  and angled inboard face  74  of retainer  70  will expand retainer  70  allowing retainer  70  to be brought into alignment with channel  62  whereupon retainer  70  may relax into channel  62 . Thereafter, each assembled screw/retainer may be inserted into the associated hole  32 . Typically, plate  20  will have been prepositioned on the bone and pilot holes may have been drilled for screws  24 . As each screw  24  is screwed into the bone, its retainer  70  will come into contact with bowl  40  (e.g., a junction  100  of outboard face  76  and lower rim face  80  contacting bowl  40 ). The shape of the bowl  40  may produce a caroming interaction contracting retainer  70 , as shown in  FIG. 5 , and allowing retainer  70  to be snapped passed lip  88 . 
     Further embodiments are shown and discussed below with only partial plates modeled (i.e., showing only one hole). However, the hole and screw/retainer configurations may be applied to single or multi-hole plates as described above or below.  FIG. 6  shows a system similar to that of  FIGS. 2-5  but wherein the ring is longitudinally split at slot  172  rather than diagonally split as described above. 
       FIGS. 7-9  show a retainer ring-less system  200  wherein the lip is segmented into individual, inwardly extending petals  220 . Petals  220  can engage thread  48  to resist backing out. It is also possible to use such petals with a retainer ring such as those described herein. During insertion, petals  220  may flex to allow passing of the screw. An inward/downward (toward the bone surface) angle of the petals may cause relative ease of flexing upon insertion compared with backing out for extraction. Petals  220  are angled toward the tip of the screw and flex outward at insertion. At screw removal, petals  220  flex in the opposite direction but greater force is needed to flex them. 
       FIGS. 10-14C  show a system having a crown-like retention ring  370 . The crown may be formed of sheet metal (e.g., as a single piece) and having a wrapped band portion  304 , as shown in  FIG. 14B , having a lower rim  306  and an upper rim  308 . A series of projections  310  extend upward and outward from the upper rim  308  to tips  312 . A pair of ends of the band fold inward and are bent to form an inward extending key  314  which can fit into a longitudinal slot  316  along a boss  318  of screw  324  just below its head to resist or prevent relative rotation of screw  324  and retainer  370 . Boss  318  includes a lower flange  320  which may engage surface  306  to longitudinally retain retainer  370  to screw  324 . Bent end portions  322 , shown in  FIG. 13C , of key  314  can contact the underside  324  of flange  321  to resist relative upward longitudinal movement of retainer  370 . The underside of the lip of plate  320  has a series of blind radial channels  330  separated by lands  332 , as shown in  FIGS. 10 ,  1311 , and  14 C.  FIGS. 12A-12C ,  13 A- 13 C, and  14 A- 14 C show screw  324 , retainer  370 , and plate  320  separately from above, below, and in perspective, respectively. In  FIG. 11 , the engagement of screw  324  with plate  320  is shown as in  FIG. 10 , but with retainer  370  removed for clarity. 
     Upon installation of the screw/retainer combination, projections  310  flex through the lip of plate  320  and become captured on the underside of the lip, as shown in  FIG. 10 . Cooperation of tips  312  with grooves  330  resists relative rotation of the screw/retainer assembly against plate  320 . Similarly, cooperation of projections  310  with the underside of the lip resists longitudinal extraction. However, projections  310  may have sufficient flexibility, which can be elastic or inelastic, that a sufficient extraction force can bend projections  310  over onto the outer surface of band portion  304  and, thereby, allow extraction of the screw/retainer with sufficient extraction force (e.g., from intentional unscrewing). 
     The methods and systems of the present invention, as described above and shown in the drawings, provide for implant systems with superior properties including improved prevention of backing out of fixation screws. The methods and systems described herein allow for variability in the angle of the screw relative to the plate during its application. This is a distinct advantage over fixed locking plates which have a strict angle for attachment of the screws to the plate. 
     While the apparatus and methods of the subject invention have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject invention.