Patent Publication Number: US-2012029577-A1

Title: System and method for bone fixation using biodegradable screw having radial cutouts

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     This application claims priority to U.S. Provisional Application Ser. No. 61/368,277, filed Jul. 28, 2010, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE  
     The present disclosure generally relates to biodegradable polymer screws and systems and methods for utilizing the screws for bone fixation procedures. In particular, the present disclosure relates to a biodegradable screw having radial cutouts in the screw head adapted to couple with a driver element having corresponding prongs that securably attach the screw in a displacement fit for insertion into bone. 
     BACKGROUND  
     Biodegradable screws are becoming more prevalent in medical procedures because they can eliminate the need for a second removal operation after a first implantation operation, reduce stress shielding at the fixation site, reduce the opportunity for hardware migration and also reduce or eliminate post-operative artifact imaging. 
     Considerable forces are exerted on a screw during rotation of the screw as it is applied in bone fixation procedures. Where the screw head is shaped to include a centrally located recess to receive the drive element, these forces can deform the central recess of the biodegradable screw resulting in a loss of purchase between the screw head and the drive element (i.e., stripping). Additionally, even where the drive element remains engaged in the central recess, the rotational forces exerted on the polymer material that comprises the biodegradable screw may be so great as to shear the screw apart during insertion of the screw, leaving a fragmented screw shaft embedded in a bone fixation site. This can be especially true for extremely small or thin screws of the type commonly used in craniomaxillofacial procedures. 
     For example, when mandibular osteotomies are performed using conventional biodegradable screws, MMF (Maxillomandibular Fixation) devices are also typically used due to the lack of strength of the biodegradable screw. The MMF devices typically cause the patient&#39;s mandible be wired to the patient&#39;s maxilla for a period of time immediately following surgery. This MMF is not currently required when performing the procedure with traditional metallic screw fixation. Accordingly, the surgeon is disincentivized from using the conventional biodegradable screw over metallic fixation because of the additional procedure of wiring the jaw closed when using conventional polymeric screws. 
     What is therefore desired is an improved biodegradable screw. 
     SUMMARY  
     Accordingly, the present disclosure relates to a system and method for bone fixation utilizing a biodegradable screw and a driver adapted to couple with the screw and insert the screw into an underlying bone. Any bone fixation procedure can be accomplished with the screw and driver disclosed herein, but particularly, bone fixation for craniofacial osteotomies, and more particularly for osteotomies related to orthognathic procedures involving the maxilla and mandible such as sagittal split osteotomies, vertical ramus osteotomies, inferior border osteotomies, sub apical osteotomies and genioplasties. 
     A biodegradable screw according to the present disclosure has a central axis and includes a head, shaft and distal end. The screw head has regularly spaced radial notches on its periphery for receiving a driver and distributing the forces of rotation away from a concentrated central point of the screw. 
     The present disclosure also relates to a driver for inserting the biodegradable screw into bone. The driver includes a driver body that defines a proximal end and a distal end, the driver body extending along a central axis from the proximal end to the distal end, and the driver body defining an outer surface. The proximal end is adapted to mate with a drive element, such as a handle, and a distal end that is adapted to couple with the biodegradable screw. The distal end of the driver has regularly spaced prongs spaced along the periphery of its distal end that can correspond to the notches of the screw head. In order to better relieve the stress placed on the material of the screw during rotation it is advantageous to place the notches on the periphery of the screw head rather than having a centrally located recess. By employing multiple prongs on the driver, the force exerted on the polymer material during application of the screw is more evenly distributed across the screw head. Even force distribution can be particularly desirable in small, thin screws typical in cranio-maxiofacial applications. The notches can couple with corresponding prongs from the driver in a unique secure displacement fit that prevents excess stress on the polymeric material of the screw head in the direction of rotation. In other words, the secure fit is accomplished by a displacement of the polymeric material of the screw head by the prongs in a direction normal to the direction of rotation. This displacement fit allows the screw to remain coupled to the driver permitting the surgeon to more easily apply the screw. A further advantage to the coupling is included where the outer surface of the distal end of the driver defines a first maximum cross-sectional dimension of the distal end of the driver and an outer perimeter of the screw head defines a second maximum cross-sectional dimension of the screw so that, according to one embodiment, the second maximum cross-sectional dimension is not less than the first maximum cross-sectional dimension when the notches and prongs are coupled in the secure displacement fit. This design allows the driver to fully apply the screw to a bone fixation site while preventing the outer surface of the driver from engaging bone and damaging the fixation site or over-widening the bone fixation site and possibly compromising the proper seating of the screw into the bone. It additionally prevents the driver from possible disruption of the bone fixation site or dislodging of the screw during withdrawal of the driver after the screw has been seated. 
     Additionally, the biodegradable screw can be provided with a central raised plateau on the screw head, located in an inner region of a proximal surface of the screw head from the notches. A corresponding recess located on the distal end of the driver can be sized to receive the raised plateau during coupling of the screw and the driver. When the distal end of the driver is placed proximally to and in contact with the screw head, the raised plateau acts as a self-centering mechanism by remaining within the prongs during axial rotation of the driver in instances where the prongs do not initially align with the corresponding notches in the screw head. By permitting the driver to remain centered on the screw head while a user rotates the driver to align the prongs with the notches, the raised plateau relieves the user from “forcing” the driver to remain in contact with the screw head with unnecessary application of axial force that could disrupt the polymeric material comprising the screw. Once the prongs and notches are in alignment, an axially directed force moves the driver distally with respect to the screw and engages the prongs and notches in the above-mentioned secure displacement while the central raised plateau is received within the recess. 
     Moreover, the screws disclosed herein can be treated to optimize the strength and rigidity of the polymer through a process called polymer orientation. There is occasionally a desire to utilize properties of polymers in applications where their strength and stiffness are not sufficient from conventional manufacturing methods such as injection molding, or machining of conventionally formed polymer stock. For example, a particular polymer may be desired as a bone screw due to its degradation profile and preferred bioaffinity but lacks the structural integrity necessary to withstand the forces encountered in such an application. In these cases, it may be advantageous to modify the polymer morphology from a spherulitic state, as is the case for a polymer that has cooled from the molten state, to a fibrillar (orientated) state. Increasing the yield strength and the elastic modulus becomes important as the use of polymer materials move from the role of a simple positioning device into areas of use where larger and larger forces are seen. In these load sharing and/or load bearing applications the conventionally formed polymers are simply not strong enough and therefore any product made from these methods can not be used. By realizing these increases, polymers and their advantages can be considered in load sharing and load bearing applications. 
     According to one embodiment of the disclosure the shaft of the biodegradable screw has an outer surface including a continuous helical threading. The shaft has a minor diameter and a major diameter. The threading has a proximal surface and a distal surface, and optionally a ridge. Alternatively, the shaft can have a non-continuous threading, or a series of protrusions oriented on the outer surface of the shaft in a generally helical pattern. In a preferred embodiment, the screw thread is adapted to be neither self-drilling, nor self-tapping, as those terms are understood in the art. In a more preferred embodiment, the threaded shaft is configured as a coarse buttress thread configuration. 
     According to another embodiment of the disclosure, the proximal end of the driver is adapted to couple in a standard hex coupling and in another embodiment the proximal end is adapted to couple in a snap-fit coupling to a ninety degree driving tool. 
     Further, a method of coupling the bone screw and the bone screw driver of the present disclosure includes: 
     a) centering the distal end of the driver over the proximal surface of a screw head such that the distal end of the driver is in physical contact with the screw head; and 
     b) applying an axially directed force to the driver such that the prongs engage with and couple to the notches in a secure displacement fit. 
     Where the screw includes the central raised plateau, the method can include the steps of: 
     a) centering the distal end of the driver over the proximal surface of a screw head such that the distal end of the driver is in physical contact with the screw head and the central raised plateau is maintained within the prongs of the driver; 
     b) axially rotating the driver while the central raised plateau is maintained within the prongs of the driver until the prongs are aligned with the notches; and 
     c) applying an axially directed force to the driver such that the prongs engage with and couple to the notches and the central raised plateau is received within the recess. 
     Additionally, a method for bone fixation is provided that includes the above disclosed steps of coupling the screw and driver and can optionally include the further steps of: 
     d) placing the distal tip of the screw at a bone fixation site; 
     e) axially rotating the driver to apply the screw into bone; and 
     f) disengaging the driver from the applied screw. 
     The method for bone fixation as described above can also optionally include placing a bone plate having a plurality of apertures at the fixation site and further include rotating the driver to apply the screw through one of the bone plate apertures into bone. In another embodiment of the above method, the screw is configured such that the method includes drilling at least one hole at the bone fixation site and threading (or tapping) the hole prior to applying the screw into the bone fixation site. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The foregoing summary, as well as the following detailed description of an example embodiment of the application, will be better understood when read in conjunction with the appended drawings, in which there is shown in the drawings an example embodiment for the purposes of illustration. It should be understood, however, that the application is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG. 1  is a side elevation view of a biodegradable screw constructed in accordance with one embodiment; 
         FIG. 2  is a top plan view of the screw illustrated in  FIG. 1 ; 
         FIG. 3  is perspective view of the screw illustrated in  FIG. 1 ; 
         FIG. 4  is another perspective view of the screw illustrated in  FIG. 1 ; 
         FIG. 5  is another top view of the screw illustrated in  FIG. 1 ; 
         FIG. 6  is a sectional side elevation view of the screw taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a side elevation view of a driver constructed in accordance with one embodiment; 
         FIG. 8  is a perspective view of a distal end of the driver illustrated in  FIG. 7 ; 
         FIG. 9  is a perspective view of the driver illustrated in  FIG. 7 ; 
         FIG. 10  is a bottom plan view of the driver illustrated in  FIG. 7 ; 
         FIG. 11  is a sectional side elevation view of the distal end of the driver taken along line  11 - 11  of  FIG. 10 ; 
         FIG. 12  is a broken enlarged bottom plan view of a portion of the distal end of the driver at the dashed circle region illustrated in  FIG. 10 ; 
         FIG. 13  is a perspective view of the distal end of the driver illustrated in  FIG. 7 ; 
         FIG. 14  is another perspective view of the distal end of the driver illustrated in  FIG. 7 ; 
         FIG. 15  is another bottom plan view of the driver illustrated in  FIG. 7 ; 
         FIG. 16  is a side elevation view of a bone fixation system including the screw illustrated in  FIG. 1  and the driver of  FIG. 7 , wherein the screw is illustrated in pre-engagement alignment with the driver; 
         FIG. 17  is a side elevation view of the bone fixation system illustrated in  FIG. 16 , wherein the is in a displacement fit with the driver; 
         FIG. 18  is a bottom plan view of the bone fixation system illustrated in  FIG. 17 ; 
         FIG. 19  is a perspective view of the bone fixation system illustrated in  FIG. 1 , showing the screw being driven into a bone fixation site; and 
         FIG. 20  is a perspective view of the bone fixation system illustrated in  FIG. 16 , showing the screw being driven into a bone plate at the bone fixation site. 
         FIG. 21  is a perspective view of a driver constructed in accordance with an alternative embodiment; 
         FIG. 22  is a side elevation view of the driver illustrated in  FIG. 16 ; and 
         FIG. 23  is a side view with partial cross-section of the distal end of the driver illustrated in  FIG. 22 . 
     
    
    
     DETAILED DESCRIPTION  
     Referring to  FIGS. 1-6 , a biodegradable screw  25  includes a proximal head  29 , a distal tip  37  axially opposed from the proximal head  29  along a central axis  26 , and a shaft  33  that extends along the axis  26  between the head  29  and the distal tip  37 . The screw  25  can be made from any suitable polymer, or polymeric blend; however, biodegradable polymers and/or blends thereof are the preferred starting material(s). 
     Biodegradable polymers contemplated as suitable for use as the starting material can include both homopolymers, and copolymers as wells as blends and combinations of both, such as polycaprolactone, polylactide, polyglycolide, poly(L-lactide), poly(D-lactide), poly(D,L-lactide), poly(L-lactide-co-D,L-lactide), poly(L-lactide-co-glycolide), poly(L-lactide-co-ε-caprolactone), poly(D,L-lactide-co-glycolide), poly(D,L-lactide-co-ε-caprolactone), polydioxanone and polycarbonates. In the case where the biodegradable polymer is a copolymer the monomer base unit ratio can be present in any range from 50:50 up to 96:4. Example biodegradable polymers are poly(L-lactide-co-glycolide) and poly(L-lactide-co-D,L-lactide). A preferred base unit range for poly(L-lactide-co-D,L-lactide) is 70:30 to 96:4. A preferred base unit range for poly(L-lactide-co-glycolide) is 80:20 to 90:10 and particularly preferred is 85:15. 
     Additionally, the screw  25  can be treated to optimize the strength and rigidity of the polymer through a known process called polymer orientation. Common methods of performing this change are a drawing operation, hydrostatic extrusion, and ram extrusion. All of these operations are mechanical operations that begin with a cross sectional area of polymer which is larger than the cross sectional area of the outlet of the process, commonly referred to as a die. In any of these processes, the draw ratio (ratio of beginning cross section to ending cross section) may be varied to impart different degrees of orientation into the polymer, and also ease in the processing. Another variable that may be used at some or all of the points in any of these operations is the application of heat. The vessels which contain the polymer may be heated. The die, the polymer itself, the ram, or any other part of this machinery may be heated to varying levels to impart different degrees of orientation to the polymer. Yet another factor in these processes is the force that is applied to the ending cross section after it has been drawn down; this force resists the natural tendency of the polymer to rebound during cooling to its original cross sectional shape and size. One skilled in the art can select any one of the above mentioned processes depending upon the characteristics of the preferred biodegradable polymer material. 
     With continuing reference to  FIGS. 1-6 , the head  29  of the screw  25  defines a proximal surface  41 , a distal surface  45 , and a side surface  49  that extends between the proximal surface  41  and the distal surface  45 . The side surface  49  defines an outer perimeter of the head  29  and extends axially between proximal surface  41  and distal surface  45 . The outer perimeter of head  29  can, according to one embodiment, define a maximum cross-sectional dimension of the screw. The proximal surface  41  extends substantially perpendicular to central axis  26  and can slope as desired either toward or away from the side surface  49  along a radial direction outward from the central axis  26 . The distal surface  45  extends distally from the side surface  49  to the shaft  33 . In accordance with the illustrated embodiment where the head  29  is represented as having a substantially circumferential outer perimeter, the diameter of the head  29  is greater than the major diameter  93  of the shaft  33 . Accordingly, the distal surface  45  tapers radially inward from the side surface  49  towards the shaft  33  in a distal direction along the central axis  26 , resulting in a head  29  configuration known in the art as a counter-sink. Other configurations are contemplated and depend upon the radial difference between the diameter of the head  29  and shaft  33  as well as the desired depth that screw  25  is intended to be driven into underlying bone. 
     According to one embodiment, the head  29  can also define an inner region having a central raised plateau  53 . The central raised plateau  53  has a side wall  57  and a proximal face  61 . The side wall  57  defines an outer perimeter of central raised plateau  53  and extends proximally from the proximal surface  41  to the proximal face  61 . Side wall  57  can extend proximally substantially normal to the proximal surface  41  and alternatively can extend proximally from the proximal surface in a direction having an inward sloping radial component. The proximal face  61  extends radially in a direction substantially perpendicular to central axis  26 . 
     The screw  25  also includes a plurality of (i.e., at least two) notches  65  that extend radially inward from the side surface  49  and are open at the side surface. The notches  65  are peripherally defined by an inner face  69  of the head  29  that extends into the side surface  49 . The inner face  69  can be curved or rounded as illustrated, or can define any geometry as desired. The notches  65  have a height  67  extending distally from the proximal surface  53 , through the side surfaces  49  towards the distal surface  45 . The notches  65  also have a radial depth  68  extending radially inward from side surface  49 , or otherwise stated toward the central axis  26 . The notches  65  can terminate at the wall  57  of the central raised plateau  53  in accordance with the illustrated embodiment, however it should be appreciated that the notches can define any depth as desired. For instance, the notches  65  can terminate radially outward or inward of the wall  57 . The notches  65  further have a cross-sectional width  66  that can decrease in a radial direction along the depth  68 . Alternatively, the width  66  can increase or remain substantially constant in the radial direction along the depth  68 . The width  66  can be constant along the height  67 , or can increase or decrease along the proximal or distal direction. 
     The notches  65  can be spaced regularly along the periphery of head  29  as illustrated. For example, regular spacing of the notches can include spacing that is equidistant along the perimeter of head  29 , as well as spacing that is equiangular such that at least two pairs of notches form equivalent central angles with respect to one another. Alternatively, one or more of the notches  65  can be spaced irregularly about the head  29 . In accordance with the illustrated embodiment, the head  29  defines four notches  65  spaced ninety degrees apart from one another about the periphery of head  29 . The head  29  has at least two notches  65  and preferably four, but can have any number based upon the physical properties of the biodegradable polymeric material used and the distribution of rotational forces that screw  25  will be subject to during application in bone. 
     With continuing reference to  FIGS. 1-6 , the shaft  33  of the screw  25  has an outer surface  34  that defines a minor diameter  89  measured radially through central axis  26 . The shaft  33  extends distally along the central axis  26  from the distal surface  45  of head  29  to the distal tip  37 . The shaft  33  is illustrated having a substantially cylindrical geometry (i.e., a constant minor diameter  89 ). It should be appreciated however that the shaft  33  can alternatively have a tapered configuration with a larger minor diameter  89  near the distal surface  45  of head  29  and a gradually decreasing minor diameter  89  as it extends towards the distal tip  37 . 
     The outer surface  34  of the shaft  33  can include external threads  73 . Threads  73  can be in a substantially continuous helical pattern or alternatively can be non-continuous or fragmented thread pattern. As another alternative, outer surface  34  may not include a thread, but rather a series of protrusions, for example teeth, that can either extend distally along the outer surface  34  in a generally helical pattern, or else in a linear or random distribution depending on the particular application or procedure that screw  25  is intended to be used. When the outer surface  34  of the shaft  33  contains threads  73  or some other type of protrusion, the shaft  33  will have a major diameter  93  measured as the radial distance of shaft  33  including threads  73 . The threads  73  are illustrated in a continuous helical pattern, and include a proximal side  77  that faces the head  29 , a distal side  81  that faces the distal tip  37 , and can further include a ridge  85  that extends between the proximal side  77  and the distal side  81 . The threads  73  further include a thread depth  97  that can be one-half the difference between the major diameter  93  and the minor diameter  89 . The threads  73  further include a pitch  101  (or what is sometimes referred to as a lead) that is measured as the axial distance covered by threads  73  during one complete axial rotation of screw  25  and are typically categorized in the art as coarse threads for those with larger pitch lengths, and fine threads for those with smaller pitch lengths. 
     The threads  73  can be designed as desired, but most typical designs for use as metallic bone screws include self-drilling, and self-threading. In the embodiment illustrated in  FIGS. 1-6 , the threads  73  are configured as a non self-drilling, non-self-tapping configuration. The particular configuration illustrated is a coarse buttress thread design. A buttress thread configuration is known in the art and designed to withstand high axial load and high axial thrust in one direction making it well-suited for bone fixation and osteotomy procedures. In such a configuration, the proximal side  77  is the load bearing surface, oriented substantially perpendicular to the central axis  26 , and extends from outer surface  34  to ridge  85  generally in an angular range of zero to twenty degrees with respect to the radial direction that extends perpendicular to the central axis  26 . The ridge  85  extends substantially parallel to central axis  26 , and the distal side  81  extends from ridge  85  back towards outer surface  34  generally in an angular range of thirty to sixty degrees with respect to the radial direction. Accordingly, the cross-sectional thread shape for a buttress design is illustrated as trapezoidal, which distinguishes a buttress design from those of self-drilling or self-tapping thread designs that are generally triangular in cross-section. 
     The distal tip  37  of the screw  25  has an outer surface  38 . In accordance with the illustrated embodiment, the distal tip  37  is tapered being generally concave, wider where it meets the shaft  33  and gradually tapering inwards as it extends distally from the shaft  33 . The distal tip  37  can also be designed as a blunt tip having a generally cylindrical or frusto-conical configuration, or a more pointed tip having a conical configuration. 
     It should be appreciated that the system of bone fixation described herein can include a plurality of screws  25  having various dimensional configurations according to the particular clinical indication and anatomical region to which they are intended to be used. For example, the screws  25  can have a range of lengths anywhere from about 6 mm up to about 100 mm, and for indications typical for craniomaxialfacial and orthognathic procedures, the screw lengths can be in the range of about 10 mm to about 18 mm. Additionally, the major diameter  93  of the screw  25  can have a range of diameters anywhere from about 1 mm to about 5 mm, and for indications typical for craniomaxialfacial and orthognathic procedures, the major diameter  93  of the screw  25  can have a range of about 2 mm to about 3 mm. It should be appreciated that these dimensions are provided as examples only, and the present disclosure is not intended to be limited to the dimensions provided. 
     Referring now to  FIGS. 7-15 , a driver instrument  120 , according to the system of bone fixation described herein, includes a driver body  121  that extends along a central axis  132 , and defines a proximal end  124  and an axially opposed distal end  128 . The proximal end  124  of the driver body  121  is adapted to engage a drive element or actuator, such as a handle, that imparts a rotational force to the driver instrument  120  so as to rotatably drive the driver instrument  120 . The drive element can be manually or automatically actuated as desired. As illustrated in  FIGS. 7 and 9 , the proximal end  124  has a coupling  180  designed to be a male couple for a standard hex coupling engagement known in the art, though it should be appreciated that the coupling can be male or female and configured to mate with the drive element in any manner as desired. 
     The driver body  121  defines an outer surface  136  (which as illustrated is substantially circumferential) at the distal end  128 , an inner surface  140  that is opposite to the outer surface  136  that defines a recess  144 , and a distal surface  148  that can be axially directed between the inner and outer surfaces  136 ,  140 . The driver  120  further includes prongs  152  that extend distally from the distal surface  148 . The outer surface  136  extends axially along distal end  128  and defines an outer periphery of the driver body  121  at the distal end  128 . The outer surface  136  further defines a first maximum cross-sectional dimension of the distal end of the driver and can define a maximum cross-sectional dimension of the prongs  152 . The inner surface  140  extends axially along distal end  128  within and substantially parallel to outer surface  136 . The inner surface  140  defines an outer periphery of the recess  144 . The distal surface  148  extends radially between inner surface  140  and outer surface  136 , and can extend perpendicular with respect to the axis  132 , or can be sloped with respect to the axis  132  as desired. 
     A plurality of (i.e., at least two) prongs  152  extend distally from the distal end  128  and are spaced regularly apart from one another along distal surface  148 , such that each prong can be aligned with a complementary notch  65  of the screw  25 . According to one embodiment, there are an identical number of prongs and notches such that each prong  152  can couple with a complementary notch  65  of the head  29 . In an alternative embodiment, there can be a greater number of notches  65  than prongs  152  such that there can be multiple complementary orientations of the prongs  152  with notches  65 . In this type of embodiment, axial rotation of the driver  120  will permit multiple alignments where coupling of prongs  152  and notches  65  can occur. 
     Each prong  152  defines an inner face  168 , and a radially opposed outer face  172 . Each prong  152  further extends axially along a direction substantially parallel to the central axis  132  so as to define a height  156  extending axially from the distal surface  148 , and depth  160  extending radially inward from the outer face  172  along a direction substantially perpendicular to the central axis  132 . The outer face  172  of each prong  152  can be circumferential or alternatively shaped, and substantially continuous with the outer surface  136  of distal end  128 . Otherwise stated, the outer face  172  can be aligned with the outer surface  136  such that the outer surface  136  and the outer face(s)  172  define an identical maximum cross-sectional dimension. Alternatively, the outer face  172  can be radially inwardly or outwardly offset with respect to the outer surface  136 . Each of the prongs  152  defines a width  164  that can vary radially inward along the depth of the prong  152 . For instance, in accordance with one embodiment, the width can be defined by a linear distance that extends between opposed radially outer ends of the inner face  168 . In accordance with the illustrated embodiment, the width  164  decreases along its depth  160 , for instance along the radially inward direction, though it should be appreciated that the width can remain constant or increase. 
     The inner face  168  can be shaped so as to correspond with the inner face  69  of the corresponding notch  65  of the screw  25  as described above. Thus, the inner face  168  can be shaped such that the prongs  152  have a radial cross-section that is substantially semicircular or can define the shape resembling a sector of a circle having defined by any angle as desired. Alternatively still, the inner face  168  can be shaped such that the radial cross-section can be substantially triangular or any geometry as desired so as to engage the screw head  29  in the complementary notches  65 . As illustrated in  FIGS. 10 ,  12  and  15 , the inner face  168  has a shape such that prongs  152  have a blended semicircular/triangular radial cross-section wherein inner face  168  is shaped substantially semicircular near outer face  172  and as the depth  160  of prong  152  increases as it crosses through distal surface  148  the inner face  172  is shaped substantially planar such that the radial cross-section of prongs  152  assumes a more triangular configuration near recess  144 . This particular configuration is best seen in  FIG. 12 . As shown in  FIGS. 7-15 , four prongs  152  are spaced regularly at ninety degree intervals along distal surface  148  and extend axially away from distal surface by height  156 . While four prongs is a preferred embodiment, any number of two or more equiangular spaced prongs can be utilized depending upon the particular screw configuration driver  120  will be engaging. 
     The prongs  152  can further include a distal edge  176  formed at the distal most boundary of outer face  172  and inner face  168 . In this embodiment, as best shown in  FIGS. 11 and 22 , outer face  172  slopes radially inward as it extends distally while inner face  168  slopes radially outward as it extends distally, thus forming edge  176 . Thus, the distal edge  176  can further be referred to as a distal tip. The angle of slope for both the outer face  172  and inner face  168  can be variable and not necessarily the same for the faces. It should thus be appreciated that the slope of both the outer face  172  and the inner face  168  can be configured so as to accommodate the complementary geometry of the screw  25  to which driver  120  will couple. In accordance with one embodiment, the outer face  172  is sloped so as to properly align with a side surface  49  and tapered distal surface  45  of the screw head  29 . 
     Referring now to  FIGS. 16-20 , a bone fixation system  123  includes the screw  25  and the driver  120  constructed as described herein. In particular, the distal end  128  of the driver  120  is configured (or adapted) to couple with the head  29  of the screw  25  such that the screw  25  is securely coupled to the driver  120  in a displacement fit that allows the driver  120  to implant the screw  25  into an underlying bone  190  at a bone fixation site  194 . 
     In order to facilitate coupling between the driver  120  and the screw  25  according to one embodiment, the screw includes a plurality of notches  65  that are regularly spaced around the periphery of the head  29  while the driver  120  includes a plurality of regularly spaced prongs  152  at the distal end  128  along the distal surface  148  such that the regular spacing of notches  65  and prongs  152  permits an alignment of notches and prongs with each other. The outer surface  136  defines a first maximum cross-sectional dimension of distal end  128 , including the prongs  152 , while side surface  49  of the head  29  defines an outer perimeter of the head  29  which further defines a second maximum cross-sectional dimension of head  29  such that the second maximum cross-sectional dimension is not less than the first maximum cross-sectional dimension when the notches and prongs are coupled in the secure displacement fit. 
     This can be seen in  FIGS. 17-18  where the outer perimeter of head  29  defined by the side surface  49 , the outer surface  136  of distal end  128 , and the outer face  172  of prong  152  are in alignment, which as illustrated is a substantially circumferential alignment. It is also shown that the inner face  168  of the prong  152  is adjoined with the inner face  69  of the notch  65 . A secure displacement fit between the screw  25  and the driver  120  occurs because the prong  152  has a first radial depth  160  that is greater than a second radial depth  68  of the notch  65 . When coupled, the inner face  168  of the prong  152  applies a radially inwardly directed force (which is normal to the tangential force applied during rotation) to the inner face  60  of the notch  65 , thereby causing displacement of the polymeric material in head  29 . This displacement will secure head  29  of screw  25  to prongs  152  of driver  120 . Additionally, as best shown in  FIG. 17 , the outer face  172  can be sloped along a portion of its length extending to the distal edge  176  to correspond to an equivalent slope of the distal surface  45  of the head  29 . 
     Additionally, the distal end  128  of the driver  120  can also include a recess  144  that is defined by an inner surface  140 . The recess  144  can be sized to accommodate the corresponding central raised plateau  53  of the screw  25 , or stated another way, the plateau  53  can be sized to be received within the recess  144 . This interface between the plateau  53  and the recess  144  provides a self-centering mechanism for the screw/driver coupling prior to the displacement fit of prongs  152  and notches  65 . When the distal end  128  of the driver  120  is placed proximally to and in contact with the head  29  of the screw  25 , there exists the possibility that the notches  65  will not be in alignment with the corresponding prongs  152 . The central raised plateau  53  allows the prongs  152  remain in contact with the proximal surface  41  with the plateau  53  remaining within the prongs  152 . This configuration allows a user to refrain from unnecessarily applying an axial force (and possibly damaging the polymeric material) in order to prevent the prongs  152  from slipping off of the head  29 , which can allow the user to rotate driver  120  relative to the screw  25  to align the prongs  152  with the corresponding notches  65 . When the prongs  152  are aligned with the notches  65 , the user can then apply the necessary axial force to move the driver  120  distally and engage the prongs  152  into a secure displacement with the corresponding notches  65 . The recess  144  is thus spaced to receive the central raised plateau  53  when the driver  120  moves distally relative to screw  25 . 
     The bone fixation system  123  can also include at least one, including a plurality of bone plate(s)  198  having at least one apertures  202  therethrough an example of which is shown in  FIG. 20 . Such bone plates can be of any configuration suitable for the particular bone fixation procedure being performed. According to such an embodiment, the bone plate  198  is placed on a surface of the bone  190  at the bone fixation site  194  such that at least one of the apertures  202  of plate  198  is in alignment with fixation site  194  such that driver  120  can drive the screw  25  through the aperture  202  into fixation site  194  so as to fix the bone plate to underlying bone. 
     It should be appreciated that the bone fixation system  123  provides a method for coupling the bone screw  25  and driver  120  as well as the utilization of the system  123  for implanting the bone screw  25  into the underlying bone  190  at a target fixation site  194 . While listed in a particular sequence, the following steps need not necessarily be performed in the exact manner as listed below. For example, a particular step in the method may be performed before, after, or simultaneously with another listed step of the method. 
     According to one embodiment, a method of coupling the screw and driver of the bone fixation system  123  can include the steps of: 
     a) centering the distal end of the driver over the proximal surface of a screw head such that the distal end of the driver is in physical contact with the screw head; and 
     b) applying an axially directed force to the driver such that the prongs engage with and couple to the notches in a secure displacement fit. 
     According to another embodiment, a method of coupling the screw and driver of the bone fixation system  123  can include the steps of: 
     a) centering the distal end of the driver over the proximal surface of a screw head such that the distal end of the driver is in physical contact with the screw head and the central raised plateau is maintained within the prongs of the driver; 
     b) axially rotating the driver while the central raised plateau is maintained within the prongs of the driver until the prongs are aligned with the notches; and 
     c) applying an axially directed force to the driver such that the prongs engage with and couple to the notches and the central raised plateau is received within the recess. 
     The bone fixation methods utilizing the system  123  disclosed herein can be performed on any malunion or non-union of bones or bone fragments both in vivo and ex vivo, on a human or on a non-human animal. One example method is for bone fixation following an osteotomy. As shown in  FIGS. 19-20 , a particular bone fixation method is for the repair of the mandible following a sagittal split osteotomy. 
     One example method of bone fixation includes carrying out the step previously identified to couple the bone screw and driver and further including the following steps: 
     d) placing the distal tip of the screw at a bone fixation site; 
     e) axially rotating the driver to apply the screw into bone; and 
     f) disengaging the driver from the applied screw; 
     Where the screw shaft and the distal tip are configured such that the screw  25  is not self-drilling, for example as a coarse buttress thread, the method can include the following step of drilling at least one bore hole into bone at a bone fixation site. Further, where the screw shaft and distal tip are configured such that the screw is not self-tapping (or self-threading), for example a coarse buttress thread, the method can include the step of tapping (or threading) the bore hole such that the bore hole can receive the particular thread pattern of the screw. Additionally, where the system includes a bone plate having at least one, or alternatively a plurality of apertures therethrough, the method can further include the steps of placing a bone plate at the surface of a bone fixation site, aligning the plate with bone such that at least one of the apertures is aligned with at least one bore hole in the bone, and axially rotating the driver to apply the screw through the aperture and into bone. 
     It should be appreciated that the bone fixation system  123  has been described in accordance with the illustrated screw  25  and driver  120 , though it should be appreciated that the bone fixation system  123  and its components can be constructed in accordance with alternative embodiments without departing from the scope of the present disclosure, for instance as defined by the appended claims. For instance, referring now to  FIGS. 21-23 , the driver  120  is illustrated as described above, however the coupling  184  is configured as a male couple for a snap-fit engagement with a ninety-degree driver element known in the art. 
     Having described various embodiments of a system and method of bone fixation utilizing a biodegradable screw and corresponding driver, it is believed that other modifications, variations and changes will be appreciated by one skilled in the art in view of the teachings set forth in this disclosure. It is therefore understood that all such modifications, variations and changes would fall within the scope of the disclosure as defined in the appended claims.