Abstract:
A bone distractor 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 includes a screw having a rotatable member engaging the first and second affixation members for distracting the first and second affixation members relative to each other in response to rotation of the rotatable member. The first and second affixation members comprise resorbable material that is resorbed in the body after distraction is complete, thereby minimizing the size of the surgical wound required to remove the non-resorbable screw. Bone screws for affixing the affixation members to bone comprise a resorbable material, whereby the bone distractor can be explanted without explanting the bone screws.

Description:
BACKGROUND OF THE INVENTION 
     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 “latency period,” 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 “consolidation period,” 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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a bone distractor in accordance with the present invention. 
     FIG. 2 is a plan view of the bone distractor of FIG.  1 . 
     FIG. 3 is a side view of the bone distractor of FIG.  1 . 
     FIG. 4 is a side view of a bone screw useful in connection with the bone distractor of FIG.  1 . 
     FIG. 5 is a perspective view of another embodiment of a bone distractor in accordance with the present invention. 
     FIG. 6 is a plan view of the bone distractor of FIG.  5 . 
     FIG. 7 is a side view of the bone distractor of FIG.  5 . 
     FIG. 8 is a side view of a bone screw useful in connection with the bone distractor of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1,  2  and  3 , a preferred embodiment of a bone distractor apparatus  10 , constructed in accordance with the present invention, is illustrated. Bone distractor apparatus  10  includes a first affixation member  12 , and a second affixation member  14  that is moveable relative to first affixation member  12 . 
     A pair of wing elements  16  and  18  extend transversely from a central bridge portion  20  of first affixation member  12 . Each wing element  16  and  18  is substantially planar and includes a plurality of screw holes  22  for receiving bone screws therethrough to secure wing elements  16  and  18  to bone, with wing elements  16  and  18  lying flat adjacent to the bone surface. Extending longitudinally, generally parallel to the plane in which wing elements  16  and  18  lie, are a pair of parallel rods  24  and  26 . First ends  28  and  30  of rods  24  and  26 , respectively, are friction fitted within corresponding holes in bridge portion  20  of first affixation member  12 . An end member  32  has holes spaced the same as the rod holes in bridge element  20  for receiving second ends  34  and  36  of rods  24  and  26  in similar friction fit. A threaded screw rod  38  has a first non-threaded end  40  received for free rotation within a corresponding hole in bridge member  20  located between the holes  28 ,  30  in which rods  24  and  26  are received. Screw rod  38  has a second non-threaded portion  42  received for free rotation within a corresponding hole in end member  32  located between the holes  34 ,  36  in which rods  24  and  26  are received. Non-threaded portion  42  includes an annular groove disposed within the corresponding hole in end member  32 . A pin  44  received through a crossbore in end member  32  lies perpendicular to screw rod  38  and engages the annular groove in non-threaded portion  42  to restrain screw rod  38  against axial movement relative to end member  32 , and hence (via rods  24  and  26 ) against axial movement relative to bridge member  20  of first affixation member  12 . A driven end  46  of screw rod  38  extends outwardly beyond end member  32 . Driven end  46  has a square cross-section for engaging a driving tool. 
     A pair of wing elements  48  and  50  extend transversely from a central bridge portion  52  of second affixation member  14 . Each wing element  48  and  50  is substantially planar and includes a plurality of screw holes  54  for receiving bone screws therethrough to secure wing elements  48  and  50  to bone, with wing elements  48  and  50  lying flat adjacent to the bone surface. Holes  56  and  58 , spaced to receive rods  24  and  26  therethrough, extend longitudinally through bridge portion  52  and are sized for a free sliding relationship between second affixation member  14  and rods  24  and  26 . A threaded hole  61  extends longitudinally through bridge portion  52  between holes  56  and  58 , and screw rod  38  is threadedly received in threaded hole  61 . In use, distraction apparatus  10  is emplaced through an incision in the gingiva with wing members  16  and  18  of first affixation member  12  on one side of an osteotomy of the mandible, for example, and wing members  48  and  50  of second affixation member  14  on the other side of the osteotomy. Each of wing members  16 ,  18 ,  48  and  50  are bent as necessary to lie flat adjacent the bone, and bone screws  60  are inserted through screw holes  22  and  54  to secure first and second affixation members  12  and  14  to respective bones on each side of the osteotomy. Distraction is effected by engaging square driven end  46  of screw rod  38  with a suitable driver tool. Torque applied to screw rod  38  causes screw rod  38  to turn, rotating freely relative to first affixation member  12  and with respect to end member  32 , but in threaded engagement with second affixation member  14 . Consequently, second affixation member  14  is caused to move longitudinally relative to first affixation member  12 , sliding on rods  24  and  26 . By rotating screw rod  38  in the appropriate direction, the respective bones to which first and second affixation members  12  and  14  are affixed are distracted relative to one another. The amount of distraction per rotation of screw rod  38  is determined by the pitch of the thread. While a threaded screw rod is employed in the disclosed embodiments to effect distraction, it is contemplated that other suitable distraction elements may be employed, including ratchet, rack and pinion, hydraulic, or gear mechanisms. In accordance with the present invention, the preferred embodiment illustrated in FIGS. 1-4 includes features that minimize the surgical trauma to soft tissue that is required for removal, or explantation, of bone distractor  10 . These features include the choice of material from which certain portions of bone distractor  10  are constructed. As preferred, all elements of bone distractor  10 , except screw rod  38 , comprise a biologically resorbable material. The preferred resorbable material is LactoSorb® copolymer, a relatively low molecular weight copolymer that is hydrophilic and amorphous, comprising 82% L-lactic acid and 18% glycolic acid. LactoSorb® copolymer is resorbed within the body after a period of time following implantation, that period typically being about nine to fifteen months. LactoSorb® copolymer retains approximately 70% of its strength for six to eight weeks following implantation. Also, as preferred, bone screws  60  are constructed of the LactoSorb® resorbable material. Screw rod  38 , as preferred, is constructed of a biologically compatible metal, such as titanium, titanium alloy, or stainless steel, capable of receiving a fine screw thread, thereby permitting small longitudinal distractions per revolution of screw rod  38 . The resorbable material of the bone distractor  10  and screws  60  retain sufficient strength and integrity during the period of distraction to perform the function of firmly affixing bone distractor  10  to bone and distracting the bone portions. After distraction is complete, the resorbable portions are resorbed, or at least lose sufficient strength and integrity that the metal screw rod  38  can then be removed surgically with a minimum of trauma by being withdrawn longitudinally through a puncture type surgical wound. 
     In an alternative embodiment, only the bone screws are made of resorbable LactoSorb® copolymer. Resorbable screws do not need to be removed to permit removal of the distractor apparatus, thereby eliminating the multiple percutaneous surgical invasions through the cheek that would be required to align a screw drive tool with the bone screws to effect their removal. Furthermore, it is not necessary to wait for the resorbable LactoSorb® copolymer bone screws to be resorbed or lose their strength before removing the distractor apparatus. Application of heat to the LactoSorb® bone screws via a cauterizing tool will soften the bone screws sufficiently to permit the distractor apparatus to be removed. The bone screws can then be left implanted to be fully resorbed eventually. 
     Alternatively, to provide greater strength and distraction force, bridge portions  20  and  52  of affixation members  12  and  14  can be constructed of biologically compatible metal similar to that from which screw rod  38  is constructed, thereby providing metal-on-metal screw thread engagement. In this alternative embodiment, wing elements  16 ,  18 ,  48  and  50  are constructed of resorbable material. Rods  24  and  26 , screw rod  38 , cross-pin  44 ,as well as end member  32 , can be constructed of a biologically compatible metal, if desired. These alternative embodiments require a somewhat larger surgical wound to extract the remaining metal components after resorption of the resorbable portions, but the required surgical wound is still substantially smaller than would be required if affixation member  12  and  14  were also constructed of metal, as in the prior art. 
     As preferred, bone screws  60  are constructed of resorbable material, thereby eliminating the need for surgical removal of the bone screws following distraction. 
     In an alternative embodiment, the bone distractor  10  and bone screws  60  can be constructed entirely of resorbable material, thereby eliminating the need for explantation surgery. It is preferred, however, that the drive screw be constructed of biocompatible metal for strength. The male threads of the drive screw are more easily stripped than the female threads of the bone affixation members. 
     Referring to FIGS. 5-7, yet another alternative embodiment is illustrated that is similar to the embodiment of FIGS. 1-4, but in which the parallel guide rods  24  and  26 , as well as the end member  32 , of FIGS. 1-4 are omitted. In the embodiment of FIGS. 5-7, bone distractor apparatus  100  includes a first affixation member  120 , and a second affixation member  140  that is moveable relative to first affixation member  12 . 
     A pair of wing elements  160  and  180  extend transversely from a central bridge portion  200  of first affixation member  120 . Each wing element  160  and  180  is substantially planar and includes a screw hole  220  for receiving bone screws therethrough to secure wing elements  160  and  180  to bone, with wing elements  160  and  180  lying flat adjacent to the bone surface. Alternatively, wing elements  160  and  180  can be made larger and provided with a plurality of screw holes  220  through each wing element. 
     A pair of wing elements  480  and  500  extend transversely from a central bridge portion  520  of second affixation member  140 . Each wing element  480  and  500  is substantially planar and includes a screw hole  540  for receiving bone screws therethrough to secure wing elements  480  and  500  to bone, with wing elements  480  and  500  lying flat adjacent to the bone surface. 
     A threaded screw rod  380  has a first non-threaded end  400  received within a corresponding hole in bridge member  200 . A threaded hole extends longitudinally through bridge portion  520 , and screw rod  380  is threadedly received in threaded hole. A driven end  460  of screw rod  380  extends outwardly beyond wing element  480 . Driven end  460  has a square cross-section for engaging a driving tool. 
     Also in accordance with the present invention, the alternative embodiment illustrated in FIGS. 5-7 includes features that minimize the surgical trauma to soft tissue that is required for removal, or explantation, of bone distractor  100 . As with the embodiment of FIGS. 1-4, these features include the choice of material from which certain portions of bone distractor  100  are constructed. As preferred, all elements of bone distractor  100 , except screw rod  380 , comprise a biologically resorbable material. The preferred resorbable material is the LactoSorb® copolymer discussed above. Also, as preferred, bone screws  600  are constructed of the LactoSorb® resorbable material. Screw rod  380 , as preferred, is constructed of a biologically compatible metal, such as titanium, titanium alloy, or stainless steel, capable of receiving a fine screw thread, thereby permitting small longitudinal distractions per revolution of screw rod  380 . The resorbable material of the bone distractor  100  and screws  600  retains sufficient strength and integrity during the period of distraction to perform the function of firmly affixing bone distractor  100  to bone and distracting the bone portions. After distraction is complete, the resorbable portions are resorbed, or at least lose sufficient strength and integrity that the metal screw rod  380  can then be removed surgically with a minimum of trauma by being withdrawn longitudinally through a puncture type surgical wound. Alternatively, to provide greater strength and distraction force, bridge portions  200  and  520  of affixation members  120  and  140  can be constructed of biologically compatible metal similar to that from which screw rod  380  is constructed, thereby providing metal-on-metal screw thread engagement. In this alternative embodiment, wing elements  160 ,  180 ,  480  and  500  are constructed of resorbable material. Screw rod  380  can be constructed of a biologically compatible metal, if desired. These alternative embodiments require a somewhat larger surgical wound to extract the remaining metal components after resorption of the resorbable portions, but the required surgical wound is still substantially smaller than would be required if wing elements  160 ,  180 ,  480  and  500  were also constructed of metal, as in the prior art. 
     As preferred, bone screws  600  are constructed of resorbable material, thereby eliminating the need for surgical removal of the bone screws following distraction. 
     In an alternative embodiment, the bone distractor  100  and bone screws  600  can be constructed entirely of resorbable material, thereby eliminating the need for explantation surgery. It is preferred, however, that the drive screw be constructed of biocompatible metal for strength. The male threads of the drive screw are more easily stripped than the female threads of the bone affixation members. 
     The preferred method of use of the invention involves implantation of a bone distractor in which at least the bone screws are constructed of resorbable material, or as more preferred, both the bone screws and the bone affixation members of the bone distractor are constructed of resorbable materials. The bone distractor is implanted via the oral cavity and through an incision in the gingiva, with the distractor secured to the bone of the mandible with the bone screws on opposing sides of an osteotomy, as in the prior art. The gingiva overlying the bone distractor is sutured and allowed to heal, with only the driver engaging portion of the drive screw protruding through the gingiva for access within the oral cavity. Either immediately following implantation or after a waiting period, or “latency period,” of up to seven days, the drive screw is rotated periodically to effect distraction at a preferred rate of about one to two millimeters per day, until the desired amount of distraction is achieved. After another waiting period, or “consolidation period,” preferably at least about twice as long as the period during which distraction was effected, at least a portion of the implanted bone distractor is explanted through an incision in the gingiva. If only the bone screws are resorbable, then removal of the bone distractor is significantly easier and faster than in the prior art because the bone screws need not be removed and percutaneous access to the screws through the cheek is not required, resulting in less surgical trauma to the patient. If also at least the bone affixation members of the bone distractor are constructed of resorbable material, then the incision in the gingiva through which explantation is effected is substantially smaller than that required to remove wholly non-resorbable prior art devices. If the entire bone distractor is constructed of resorbable material, then the explantation step can be eliminated. 
     Although the above description of a preferred embodiment is given in some detail, limitation of the invention to the described details is not intended. Other configurations and embodiments of the invention will occur to one of skill in the art and yet fall within the scope of the invention as defined in the claims appended hereto.