1.  Field of the Invention
The present invention relates generally to apparatus for correction of craniofacial abnormalities, and more particularly to apparatus for affixation to maxillary or mandibular bones to effect distraction of the bones relative to a fracture or osteotomy. 
2.  Background of the Art
Various developmental disorders of the human skull result in craniofacial abnormalities in which certain bones fail to grow in proper proportion to other bones, or in which certain bones fuse prematurely, causing malformation of the midface or mandible.  It is known to correct such abnormalities by separating the bones through osteotomy or fracture, and then slowly distracting the bones relative to each other, thereby inducing bone growth at the separation line.  As the bones are distracted, the newly formed bone at the growth surfaces adjacent is the separation fills in the increasing gap between the bones.  By this method, the midface of the cranium can be advanced forward, or the mandible can be lengthened, for example. 
Apparatus for effecting distraction can involve two affixation members, in which each affixation member is temporarily affixed to the bone on opposite sides of the osteotomy, such as by bone screws, and a screw drive member that threadedly engages at least one of the affixation members.  After a waiting period, or xe2x80x9clatency period,xe2x80x9d of up to seven days following implantation, the screw drive member is rotated incrementally and periodically such that the two affixation members are driven apart at a rate of about one to two millimeters per day until the desired amount of distraction is achieved.  Hence, the bones on either side of the osteotomy are distracted relative to the location of the osteotomy.  Typically, after the desired amount of distraction is achieved, another waiting period, or xe2x80x9cconsolidation period,xe2x80x9d preferably at least twice the length of the period of distraction is required to allow the new bone growth to become sufficiently consolidated.  Subsequently, the distraction apparatus can be removed via a new incision in the gingiva.  While typical distraction rates and associated waiting periods have been discussed, it will be appreciated that faster or slower advancement of the distractor can be performed. 
While the distraction apparatus is in place, the screw drive member must be accessed periodically, typically via the oral cavity, so that a driver can be engaged with the screw drive member to apply torque thereto.  Such applications of torque occur over a period of time.  During that period of time, the surgical wounds made during implantation of the distraction apparatus may heal substantially.  After distraction is completed, it is necessary to explant the distraction apparatus, which requires significant surgery of soft tissue similar to that which occurred during implantation, thereby opening new surgical wounds in the areas of soft tissue that have already healed from the original surgical implantation.  It would be advantageous to provide an improved distractor apparatus that is more easily explanted with less surgical invasion, thereby minimizing additional surgical trauma after distraction is complete and increasing the ease and speed with which the distractor apparatus can be removed.  The present invention, an embodiment of which is described below with reference to the drawings, provides this and other advantages. 
In accordance with one aspect of the present invention, a bone distractor is provided for distracting bone on opposite sides of an osteotomy of the bone.  The bone distractor includes a first affixation member for affixation to the bone on one side of the osteotomy, and a second affixation member for affixation to the bone on another side of the osteotomy.  A distraction element engages the first and second affixation members for distracting the first and second affixation members relative to each other.  At least one of said first and second affixation members comprises a resorbable material. 
In accordance with another aspect of the present invention, a bone distractor is provided for distracting bone on opposite sides of an osteotomy of the bone.  Included are a first affixation member for affixation to the bone on one side of the osteotomy, and a second affixation member for affixation to the bone on another side of the osteotomy.  A distraction element engages the first affixation member and the second affixation member for distracting the first and second affixation members relative to each other.  Bone screws are provided for affixing the first and second affixation members to the bone, wherein the bone screws are constructed of a resorbable material. 
In accordance with yet another aspect of the present invention, a method of distracting bone on opposite sides of an osteotomy of the bone includes the step of providing a bone distractor having a first affixation member for affixation to the bone on one side of the osteotomy, a second affixation member for affixation to the bone on another side of the osteotomy, and a distraction element engaging the first affixation member and the second affixation member for distracting the first and second affixation members relative to each other, wherein at least the first affixation member is constructed of a resorbable material.  Also included is the step of providing means for affixing the first and second affixation members to bone.  The step of implanting the bone distractor and affixing the first and second affixation members to bone with the means for affixing is a further step.  Yet another step involves effecting distraction of the bone.  Explanting the bone distractor without explanting the means for affixing is a further step. 
It is an object of the present invention to provide an improved bone distractor that is readily removable after distraction is completed with minimal surgical invasion of soft tissue. 
Other objects and advantages of the present invention will be apparent from the following description of a preferred embodiment, made with reference to the drawings.