Abstract:
A cranial bone and bone flap fixation device, comprising first and second caps between which portions of the cranial bone and bone flap are to be gripped; a longitudinally extending mounting post located to allow relative cap movement lengthwise of an axis defined by the post at least the first cap having protruding structure to extend into a gap formed between the cranial bone and the bone flap to laterally orient the first cap so that peripheral portions thereof will extend over both the cranial bone and the bone flap during fixation.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to cranial surgery apparatus and method, and more particularly to implant systems and methods for re-fixation of cranial bone flaps after craniotomy. 
   Cranial bone flaps after craniotomy are typically fixed in position with wire, suture material or mini plates and screws. In some cases, fixation with wire or suture material is not secure. Shifting of the bone flap may result in dislocation, causing depression or protrusion of the flap relative to the adjacent cranium. This phenomenon occurs more frequently with the progressive shift to smaller craniotomies for minimally invasive surgical procedures. 
   Fixation of the bone flap using mini plates and screws has improved the attachment of the bone flap. This technique, however, demands a considerable amount of time and added cost. There is need for an improved system for fixation of the bone flap to the cranium, providing for quick and easy application, optimal stability, and self guiding or centering of fixation structure relative to the flap bone to the cranium. 
   SUMMARY OF THE INVENTION 
   It is a major object of the invention to provide improved flap fixation apparatus, system and methods meeting the above needs. Basically, the fixation device of the invention comprises: 
   a) first and second caps between which portions of the cranial bone and bone flap are to be gripped, 
   b) a mounting post located to allow relative cap movement lengthwise of an axis defined by the post, 
   c) at least the first cap having protruding structure to extend into a gap formed between the cranial bone and the bone flap to laterally orient the first cap so that peripheral portions thereof will extend over both the cranial bone and the bone flap during fixation, for optimum retention. 
   An additional object is to provide for cap self centering functionality, by provision of a cap guide configured to extend into a gap formed between the cranial bone and bone flap, to laterally orient the first cap so that portions thereof will overlap the cranial bone and bone flap to approximately equal lateral extents. That guide may have an axially tapering surface or surfaces to project into the gap between edges of said cranial bone and bone flap; and the guide may comprise a cup-shape or tapering tabs, as will be seen. Both caps may incorporate such self-centering functionality. 
   These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 

   
     DRAWING DESCRIPTION 
       FIG. 1  is a perspective view of a cap, post and guide system incorporating the invention; 
       FIG. 2  is an elevational view of the  FIG. 1  system; 
       FIG. 3  is a top plan view of the upper cap as also seen in  FIGS. 1 and 2 ; 
       FIG. 4  is a section taken on lines  4 — 4  of  FIG. 3 ; 
       FIG. 5  is a perspective view of a modified form of the system or device; 
       FIG. 6  is an elevation taken in section through the device of  FIG. 5  during application to cranial bone and flap elements; 
       FIG. 7  is a perspective view of yet another modified form of the system or device; and 
       FIG. 8  is an enlarged section showing clamp-up of the upper and lower caps to flap and cranial bone structure. 
   

   DETAILED DESCRIPTION 
     FIGS. 1–4  show a form of the invention, which is preferred. An upper cap  110  in the form of a disc has an outer portion  110   a  extending about the axis of post  113 . One arcuate segment  110   a ′ of portion  110   a  is intended to engage top surface  119   a  of a cranial bone flap  119 ; and a second arcuate segment  110   a ″ of portion  110   a  is intended to engage top surface  118   a  of cranial bone  118 . See  FIG. 4 . Similarly, a lower cap  130  has an outer portion  130   a  extending about the post, with arcuate segments  130   a ′ and  130   a  that engage bottom surfaces  119   c  and  118   c  of  119  and  118 . See  FIG. 6 . 
   During installation, the upper cap is relatively displaced toward the lower cap, and the caps are tightened against upper and lower surfaces of the cranial bone  118  and bone flap  119 . Typically, the lower cap is pulled upwardly toward and against undersides of the flap and cranial bone, during upper cap lowering. Afterwards, the post is severed, adjacent the top surfaces of the upper cap, as seen at  13   d  in  FIG. 8 . 
   The post  113  may be serrated as shown at  113   e  to enable one-way ratcheting relative movement of cap  110  toward cap  130 , and also to enable positive locking of cap  110  to the post (i.e. serration shoulder angularity to block cap  110  retraction) after desired forcible engagement of both caps with the bone and bone flap  119  and  118 . After such locking, the post is severed at region  13   d  close to the cap  110 . Positive locking of the cap  110  to the post assures positive retention of the caps in engagement with upper and lower sides of the cranial bone  119  and bone flap  118 , and resultant positive positioning of the flap in position relative to cranial bone, during and after replacement of the flap to the skull, promoting healing. 
   A protruding guide  116  is formed on, or carried on, the upper cap  110  and configured to extend or project into a gap or kerf  117  formed between cranial bone  118  and bone flap  119 , at the time of installation. See  FIGS. 4 and 6 . The cap is thereby laterally oriented (by engagement of  116  with edges  118   b  and  119   b ) so that the cap portions  110   a ′ and  110   a ″ of widths indicated at A and B in  FIG. 4  will be caused to automatically lap, to approximately equal lateral extents, the adjacent or subtended cranial bone and flap surfaces  118   a  and  119   a , as a result of clamping installation as seen in  FIGS. 6 and 8 . Note that the guide under surface  116   a  is generally cup-shaped, or frusto-conical, to project into the gap or kerf  117  between edges  118   b  and  119   b  of the bone surfaces at the gap upper mouth. Three post-engaging and ratcheting tabs  120 , are carried by the bottom wall  121   a  of the cup-shaped guide, and a void or cavity exists at  122  above the level of those tabs. Typically, between three and five tabs  120  may be used. 
   The tabs provide one-way ratcheting engagement of tab narrowed inner edges  45   a  seen in  FIG. 3  with the post serrations as the upper cap  110  is displaced along the post relatively toward lower cap  130  during installation. The tabs are individually resiliently flexible in directions generally parallel to axis  14 , and for that purpose they have narrowed width flexure zones  45   b  located between edges  45   a  and the regions  45   c  of tab jointure with the cup-shaped wall  121   a  of the cap. The tab flexure zones  45   b  have widths less than the tab length, and the tabs extend at angles α out of the transverse plane of lower dished region  121   a  of the guide  116  to further such ratcheting engagement with post serrations, and to facilitate locking of the three tab edges  45   a  into the valley or valleys between successive serrations on the post, to hold the cap regions  110   a ′ and  110   a ″ compressively against the upper surfaces of the cranial bone and bone flap. 
     FIGS. 5 and 6  show the provision of a cover  125  extending over cavity  122  and attached to the cap. Such attachment may be effected by finger-like clips  128  projecting downwardly from the periphery  125   a  of the cover plate to clip onto the outer edge  112  or edge extents of cap  110  outer region  101   a , plate  125   a  then seating on the upper surface of the disc-like upper cap outer portion  110   a . Other means of attachment of  125  to  110  may be provided. Such a cover promotes healing of scalp skin over the cap and cover, at generally the same level. The cover is typically applied after the caps are in place, gripping the bone surfaces. 
   The bottom cap  130  may also have an upwardly protruding inverted cup-shaped guide  133 , like guide  116 , and is carried by lower cap  130  to function (with self centering) in the same manner as guide  116 . See  FIG. 6 . The lower end  113   e  of post  113  is centrally attached to the end wall  133   a  of the guide  133 , and it projects upwardly through  116 , whereby tabs  120  can ratchet along post serrations  139 , as the two caps relatively approach one another. The two like guides  116  and  133  co-act to enter the discs  110  and  130  relative to the gap or kerf  117 . 
     FIG. 7  shows another embodiment of the cap orienting guide, in the form of tabs  140  projecting axially upwardly and inwardly toward the post  144  from the disc-like extent  142   a  of a lower cap  142  also having a disc-like outer annular portion  142   b . The tabs provide annularly spaced and tapered surfaces engagable with the bone flap and cranial bone lower edges adjacent kerf surfaces or edges referred to in  FIGS. 4 and 6 , to laterally orient the lower cap, upon assembly. The upper cap  150  may have similar and downwardly projecting tabs to centrally orient it relative to the kerf or gap, upon assembly, for accurately positioning and retaining the flap, relative to the cranial bone. The two caps have outer circular edges  142   c  and  150   c , so that they are disc-like. 
   The caps as used may consist of non corrosive metal or metal oxide, an example being titanium dioxide.