Abstract:
A pair of bone plates adapted for attachment to cranial bone segments, the cranial plates being mounted to a distraction mechanism such that the cranial plates can be separated gradually in very small increments. Each of the cranial plates is mounted to the distraction mechanism with a ball and socket connector assembly, such that each of the plates can move in multiple directions and orientations relative to the distraction mechanism to better accommodate a convex, planar or concave surface topography presented during the distraction procedure.

Description:
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/516,557, filed Apr. 5, 2011, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates broadly to the field of medical devices known generally as bone distractors, which are devices designed to allow the distance between two bone segments to be gradually increased while bone regenerates in the area between the bone segments. In particular, the invention relates to cranial bone distractors, which are devices applied to two bone segments of the cranium. The distractors comprise in general a pair of bone plates mounted to a distraction assembly. 
     Bone regeneration or osteogenesis is a medical technique for repairing defective or damaged bones, such as for example when a bone fails to fully develop to its proper shape or size. An osteotomy or cut is made in the bone to produce two sides of exposed bone tissue and a distractor mechanism is attached to both bone segments. As new bone tissue is produced by the bone segments to bridge the gap, the distractor mechanism is used to gradually increase the distance between the bone segments. New bone tissue continues to be produced and eventually the proper shape or size for the bone is achieved. The distractor mechanism is then removed and the bone is allowed to heal and rigidify. 
     Bone distractors are well known in the art, but are primarily designed for linear distraction. Linear distraction is suitable for many bones, such as the mandible or the long bones of the legs or arms, but is not optimal for bones possessing curved topographical configurations, such as for example the cranium. A problem with known distractors when used with cranial distraction is that the linear distractors fail to accommodate the curvature of the cranium, such that the distraction does not result in formation of new bone having or approximating the appropriate curvature, instead producing a more planar area of new bone. It is an object of this invention to provide a cranial distractor that addresses this problem, wherein the distraction mechanism has a linear operation but the bone plates affixed to the opposing bone segments are mounted in a manner allowing free multi-directional movement to allow and account for the curvature and changes in the topography of the cranium during the osteogenesis process. 
     SUMMARY OF THE INVENTION 
     The invention comprises in general a pair of cranial bone plates adapted for attachment to cranial bone segments, the cranial plates being mounted to a linear distraction mechanism such that the cranial plates can be separated gradually in very small increments. Each of the cranial plates is mounted to the distraction mechanism with a ball and socket connector assembly, such that each of the plates can move in multiple directions and orientations relative to the distraction mechanism to account for the topographical changes of the cranium during osteogenesis such that the configuration of the regenerated bone better matches the desired final configuration of the cranium while simultaneously reducing stress on the cranial distractor. One cranial plate is mounted to the distraction mechanism in stationary position while the other cranial plate is mounted in a manner that allows for movement along the longitudinal axis of the distraction mechanism in the direction away from the other cranial plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a representative embodiment of the cranial distractor, with arrows provided to illustrate the range of motion of various elements and structures. 
         FIG. 2  is a side view of the embodiment of  FIG. 1 , with arrows provided to illustrate the range of motion of various elements and structures. 
         FIG. 3  is an end view of the embodiment of  FIG. 1 , with arrows provided to illustrate the range of motion of various elements and structures. 
         FIG. 4  is a side view of an alternative embodiment of the cranial distractor, wherein the distal cranial plate moves axially. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to the drawings, which are meant to be illustrative and not limiting, the invention will now be described in detail with regard for the best mode and the preferred embodiment. In general, the invention is a cranial distractor comprising two cranial bone plates structured for affixation to opposing cranial segments using mechanical fasteners, adhesive bonding or other suitable means, the bone plates being connected to a linear distraction mechanism defining a longitudinal axis whereby operation of the distraction mechanism increases the distance between the bone plates. Each of the bone plates is connected to the distraction mechanism by a ball and socket connector assembly such that each bone plate may tilt up and down, rotate about the axis defined by the ball and socket connector assembly, and swivel side to side. With this structure the cranial plates of the distractor will properly orient relative to whatever convex, planar or concave surface topology is presented by a particular area of the cranium during the distraction procedure. 
     The cranial distractor comprises a distal cranial bone plate member  21  and a proximal cranial bone plate member  22  connected or mounted to a linear distraction mechanism  10 , the distraction mechanism  10  operating in known manner such that the distance between the distal cranial plate  21  and the proximal cranial plate  22  can be gradually increased. The distal and proximal cranial plates  21  and  22  are relatively thin in cross-section, and may be composed of a malleable material to allow the cranial plates  21  and  22  to be shaped to conform to the surface topography of the cranial segments to which they will be affixed. The cranial bone plates  21  and  22  are provided with bone screw receiving apertures  24  that allow the plates  21  and  22  to be affixed to opposing cranial bone segments in known manner using mechanical screw fasteners, either permanent or bioresorbable. The cranial bone plates  21  and  22  are sufficiently dimensioned in length and width so as to be readily attachable to the exterior of the cranial bone segments such that localized stresses are reduced. The cranial bone plates  21  and  22  are connected to the distraction mechanism  10  in a manner that allows for independent movement of each cranial plate  21  and  22  in multiple directions relative to the distraction mechanism  10 . Depending abutment or tab members  24  may be provided which extend from the proximal side of the distal cranial plate  21  and from the distal side of the proximal cranial plate  22 . These abutment members  24  abut against the sides or edges of the opposing cranial bone segments during the distraction process to further minimize local stresses. 
     In the embodiment shown in  FIGS. 1 through 3 , the linear distraction mechanism  10  comprises a distal end  14 , a proximal end  15  and an elongated sleeve member  11  defining a longitudinal axis  99  and provided with a longitudinal slot  16 . A threaded drive rod  12  is received coaxially within the sleeve member  11  in a manner that allows for rotation of the drive rod  12  relative to the stationary sleeve member  11 . A head member  13  attached to the proximal end of the drive rod  12  extends beyond the proximal end  15  of the sleeve member  11  and is used to rotate the drive rod  12 , whether by finger manipulation or with the use of a drive tool. 
     The distal cranial plate  21  is joined to the distal end of the sleeve member  11  with a stationary ball and socket connector assembly  31 , such that the stationary ball and socket assembly  31  does not move axially along the distraction mechanism  10 . The ball and socket connector assembly  31  allows the distal cranial plate  21  to move in multiple directions relative to the axis  99  of the distraction mechanism  10 , as shown by the arrows in the drawings. The distal cranial plate  21  may tilt up and down relative to the axis  99  of the distraction mechanism  10 , swivel left and right relative to the axis  99  of the distraction mechanism  10 , and rotate about the distraction mechanism axis  99 , the axis defined by the ball and socket assembly  31  being coaxial to the distraction mechanism axis  99 , such that the cranial plate  21  can be properly oriented for affixation to the cranial bone segment without creating detrimental stresses on the distraction mechanism  10  or the bone screws or other mechanical fasteners. 
     The proximal cranial plate  22  is joined to the drive rod  12  with a moving ball and socket connector assembly  32 , and the moving ball and socket connector assembly  32  is connected to an internally threaded riding collar member  33  which is coaxially mounted onto the externally threaded drive rod and which extends through the slot  16  of the elongated sleeve member  11 . As the drive rod  12  is rotated, the riding collar member  33  and the moving ball and socket connector assembly  32  moves longitudinally along the drive rod  12  in the axial direction. The moving ball and socket connector assembly defines a lateral or radial axis  98  extending outward to the distraction mechanism axis  99 , and the moving ball and socket connector assembly  32  allows the proximal cranial plate  22  to move in multiple directions relative to the axis  98 , as shown by the arrows in the drawings. The proximal cranial plate  22  may tilt up and down relative to the axis  98 , swivel left and right relative to the axis  98 , and rotate about the axis  98 , such that the cranial plate  22  can be properly oriented for affixation to the other cranial bone segment without creating detrimental stresses on the distraction mechanism  10  or the bone screws or other mechanical fasteners. Furthermore, the cranial plates  21  and  22  are independently adjustable relative to each other. 
     With this structure, the cranial plates  21  and  22  are readily mountable to the cranial segments in proper conformation and are free to change orientation as they are gradually separated during the distraction procedure. This freedom accommodates the curvature of the cranium such that undesirable stresses are not imparted to the cranial bone segments and the configuration of the bone regeneration is able to approximate a natural curvature. 
     As shown in the alternative embodiment of  FIG. 4 , the affixation of the cranial plates  21  and  22  may be reversed such that the proximal cranial plate  22  is mounted by a fixed ball and socket connector assembly  31  to the proximal end  15  of the distraction mechanism and the distal cranial plate  21  is mounted by a moving ball and socket connector assembly  32  onto the internally threaded collar  33  positioned on the externally threaded drive rod  12 , such that the distal cranial plate  21  moves longitudinally along the axis  99  away from the proximal cranial plate  22 . 
     It is contemplated and understood that the cranial distractor may comprise distraction mechanisms having different operational mechanisms as long as the cranial plates are connected to the distractor assembly using ball and socket connector assemblies. Furthermore, while axial and lateral mountings for the ball and socket assemblies are preferred in order to minimize the overall height of the device such that the distractor mechanism remains close to the cranium, it is understood that the ball and socket assemblies could also be mounted beneath the distractor mechanism in an alternative embodiment, and the longitudinal slot could be disposed on the underside of the distractor mechanism. 
     It is to be understood that equivalents and substitutions to certain elements set forth above may be obvious to those ordinary skill in the art, and therefore the true scope and definition of the invention is to be as set forth in the following claims.