Abstract:
An improved distraction bone screw and a method for its use are described. The distraction screw is comprised of an implantable distal segment and a detachably secured proximal segment. The distal segment includes a head portion and a threaded shank portion. The proximal segment is represented as an elongated body having an internal bore that extends through its length. A deployable member is disposed within the bore, which is extendible outside the internal bore to securely couple to the distal segment. As an assembly, the distraction screw is used to affix and realign bone during surgical reconstruction. Upon completion of the surgical work, the proximal segment is removed and the distal segment is left attached to the reconstructed bone. Securely affixed, the distal segment provides an additional point of fixation for the skeletal plates that are used to preserve the bony alignment while bone healing occurs. The affixed distal segment will also provide a ready mechanism for distraction screw replacement at the time of surgical revision without obligatory plate removal. Different embodiments of the proximal segment, distal segment and the rotational locking mechanisms which inhibit the rotation of one segment relative to the other during deployment were also described. In addition, in cases where the distraction screw must be placed into the bone at an inclined angle, poly-axial heads were provided so that proper skeletal plate placement can still be accomplished.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of the U.S. Provisional Application No. 60/417,776 filed Oct. 11, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to skeletal plating systems and components thereof, which can be used to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during healing and fusion after surgical reconstruction of a mammalian bone structure. Such systems can comprise skeletal plates, bone screws and/or distraction screws and plate-to-screw locking mechanisms.  
         BACKGROUND OF THE INVENTION  
         [0003]    The surgical removal of a herniated disc, whether from degenerative disease or traumatic disruption, is a common procedure in current medical practice. In the cervical spine, the procedure involves placement of a large temporary bone screw, which is also known as the distraction screw, into each of the vertebral bones above and below the diseased disc space. These screws are used to realign the vertebral bones into the desired anatomical relationship and to temporarily distract them so as to permit work within the intervening disc space. The disc is removed and a bone graft or suitable graft substitute is placed into the evacuated space. The temporary distraction screws are then removed from the vertebrae and a metallic skeletal plate is used to maintain the position of the vertebral bones while bone healing occurs. The bones are fixed to the skeletal plate using implantable bone screws (usually two screws per vertebrae), which are separate and distinct from the distraction screws.  
           [0004]    Removal of the distraction screws from the vertebral bodies usually produces robust bone bleeding and requires that the bone holes be filled with a hemostatic agent. The empty bone holes also act as stress concentration points within the vertebral bodies, as would any empty opening or crack within a rigid structural member, and predispose the vertebral bodies to bone fracture, screw/plate migration and construct failure. Further, the empty holes often interfere with proper placement of the implantable screws and the associated skeletal plate, making proper alignment of the plate along the anatomically desired plane more difficult. This is especially problematic since the plate is placed at the end of the operative procedure and the preceding surgical steps have distorted the anatomical landmarks required to ensure proper plate alignment.  
           [0005]    Lastly, once placed, the plate will effectively cover the vertebral bodies of the reconstructed segment. Extension of the operation to an adjacent level at a future date will require placement of a distraction screw within a covered vertebrae and, thus, necessitate plate removal. The latter requires re-dissection through the scarred operative field of the initial procedure and significantly increases the operative risk of the second procedure for the patient.  
           [0006]    In view of the above, it would be desirable to design an improved distraction screw. The new device should minimize blood loss, reduce the potential for stress concentration, maximize the likelihood of proper plate alignment, provide an additional point of fixation for the skeletal plate and provide a ready mechanism for distraction screw replacement at the time of surgical revision without obligatory plate removal.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is one of an improved distraction screw and a method for its use. The design substantially enhances the functional capability of distraction screws used in the surgical reconstruction of mammalian bones. In this invention, the multi-segmental distraction screw comprises an implantable distal segment and a detachably secured proximal segment. The distal segment includes a head portion and a threaded shank portion. The proximal segment is represented as an elongated body having an internal bore that extends through its length. A deployable member is disposed within the proximal segment, which is extendable beyond the distal end of the internal bore to engage and secure the distal segment, thus forming a unitary distraction screw. Once assembled, the screw is used to realign and distract the bones during surgical reconstruction of a degenerated skeletal segment. Upon completion of that work, the proximal and distal segments are disengaged leaving the latter attached to bone. Securely affixed, the distal segment provides an additional point of anchoring and/or fixation for the skeletal plate and facilitates its proper placement. It also provides a ready mechanism for distraction screw replacement at the time of surgical revision without obligatory plate removal.  
           [0008]    In other embodiments of the present invention, different proximal and distal segment designs are provided as well as an optional rotational locking means to inhibit the rotational movement of the proximal and distal segments relative to each other. Further, where the distal segment is affixed to the underlying bone at an inclined angle, a poly-axial head adapter is provided to ensure proper aligment during placement of the skeletal plate.  
           [0009]    The distraction screw design of the present invention provides significant advantages over the current and prior art. These and other features of the present invention will become more apparent from the following description of the embodiments and certain modifications thereof when taken with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a partial side view of a distraction screw of the present invention, together with a tool driver to effect a rotational movement therefor;  
         [0011]    [0011]FIG. 2 is a partial sectional side view of an assembled distraction screw of the present invention affixed onto a mammalian bone substrate;  
         [0012]    [0012]FIG. 3 is a partial sectional side view of a distal segment of the distraction screw implanted onto a mammalian bone substrate;  
         [0013]    [0013]FIG. 4 is a sectional side view of the distal segment affixing a skeletal plate onto the mammalian bone substrate;  
         [0014]    [0014]FIG. 5 is a partial sectional side view of another embodiment of the present invention, which incorporates a rotational locking means as represented by a key-receptacle arrangement;  
         [0015]    [0015]FIG. 6 a  is a partial sectional side view of a further embodiment of the present invention, which incorporates another variation of a rotational locking means as represented by a hex insert-socket arrangement;  
         [0016]    [0016]FIG. 6 b  is a partial sectional side view of the assembled distraction screw shown in FIG. 6 a  affixed onto a mammalian bone substrate;  
         [0017]    [0017]FIG. 7 a  is a sectional side view of another embodiment of the proximal segment;  
         [0018]    [0018]FIG. 7 b  is a sectional side view of the assembled proximal segment shown in FIG. 7 a;    
         [0019]    [0019]FIG. 7 c  is a sectional side view of the assembled proximal segment shown in FIG. 7 a,  together with the distal segments and a tool driver to effect the rotational movement thereof;  
         [0020]    [0020]FIG. 8 a  is a partial sectional side view of another embodiment of the proximal segment, together with a tool driver used to effect its rotation;  
         [0021]    [0021]FIG. 8 b  is a sectional side view of one embodiment of the distal segment used with the proximal segment shown in FIG. 8 a;    
         [0022]    [0022]FIG. 8 c  is a sectional side view of another embodiment of the distal segment used with the proximal segment shown in FIG. 8 a;    
         [0023]    [0023]FIG. 9 a  is a partial sectional side view of another embodiment of the proximal and distal segments, together with a tool driver used to effect their rotation;  
         [0024]    [0024]FIG. 9 b  is a top view of the distal segment illustrated in FIG. 9 a;    
         [0025]    [0025]FIG. 10 a  is a partial sectional side view of another embodiment of the proximal and distal segments, together with a tool driver used to effect their rotation;  
         [0026]    [0026]FIG. 10 b  is a top view of the distal segment illustrated in FIG. 10 a;    
         [0027]    [0027]FIG. 11 a  is a partial sectional side view of another embodiment of the proximal and distal segments, together with a tool driver used to effect their rotation;  
         [0028]    [0028]FIG. 11 b  is a partial top view of the distal segment illustrated in FIG. 11 a;    
         [0029]    [0029]FIG. 12 a  is a partial sectional side view of the distal segment of the embodiment illustrated in FIG. 11 a;    
         [0030]    [0030]FIG. 12 b  is a partial top view of the distal segment of the embodiment illustrated in FIG. 11 a;    
         [0031]    [0031]FIG. 13 is a partial sectional side view of another embodiment of the present invention, which incorporates a poly-axial feature;  
         [0032]    [0032]FIG. 14 is a partial sectional side view of the embodiment of FIG. 13 shown as an assembly;  
         [0033]    [0033]FIG. 15 is a partial sectional side view of a distal segment with a poly-axial feature implanted onto a mammalian bone substrate on which a skeletal plate is affixed;  
         [0034]    [0034]FIG. 15 a  is a partial top view of a mounting plate used to secure the skeletal plate onto the distal segment;  
         [0035]    [0035]FIG. 15 b  is a side view of the mounting plate of FIG. 15 a;    
         [0036]    [0036]FIG. 16 a  is a partial sectional side view of another embodiment of the present invention incorporating another variation of the poly-axial feature;  
         [0037]    [0037]FIG. 16 b  is a top view of the screw cap shown in FIG. 16 a;    
         [0038]    [0038]FIG. 17 is a partial sectional side view of the assembled distraction screw of the embodiment shown in FIG. 16 a;  and  
         [0039]    [0039]FIG. 18 is a partial sectional side view of a distal segment with poly-axial feature implanted onto a mammalian bone substrate on which a skeletal plate is affixed. 
     
    
     DETAILED DESCRIPTION  
       [0040]    The present invention provides an improved distraction screw and a method for its use. FIG. 1 shows an embodiment of the present invention, as represented by a distraction screw  10 , which comprises a distal segment  120  and a removable proximal  130  segment. The distal segment  120  is implantable on a vertebral bone as part of the surgical procedure. The distal segment  120  has a head portion  122 , and a threaded shank portion  124  which can be securely fastened unto the bone structure and which may be self-tapping and/or self-drilling.  
         [0041]    As shown in FIGS. 1 and 2, the proximal segment  130  has an elongated body  132  with an internal bore  134  extending through its length from its proximal end portion  135  to its distal end portion  137 . The elongated body  132  houses a deployable member  136 , which is disposed within the internal bore  134 . The deployable member  136  is adapted to be retractably deployed beyond the opening  138  of the internal bore  134  at the distal end portion  137  of the elongated body  132 .  
         [0042]    Along the wall  140  of the interior bore  134  of the elongated body  132  are cooperating threads  142 , which complement threads  144  of the deployable member  136  such that rotation of the deployable member  136  relative to the elongated body  132  in one direction extends it beyond the opening  138  of the internal bore  134  in a deployed position, as shown in FIGS. 1 and 2. Conversely, rotation of the deployable member  136  in the opposite direction effects its retraction from the deployed position. Thus the deployable member  136  can be rotated independently of the elongated member  130 .  
         [0043]    For the embodiment shown in FIG. 1, the threads are made as right-hand thread, that is, viewing from the proximal end portion  135  of the elongated body  132 , a clockwise rotation of deployable member  136  causes it to extend beyond the opening  138  of the internal bore  134 . Conversely, a counter-clock wise rotation of the deployable member  136  effects its retraction into the internal bore  134 .  
         [0044]    The proximal segment  130  is adapted to be attached to the distal segment  120 . As shown in FIGS. 1 and 2, the deployable member  136  has a threaded end portion  138 , with threads  150 , which are adaptably securable to interfit and interlock with complemental threads  162  of the threaded well  158  of the distal segment  120 . Threads  150  and  162  are oriented in the same turn direction and have the same pitch (number of threads per unit length) as those of threads  142  and  144 . This enables the threaded portion  138  to advance into the threaded well  158  when turned clockwise.  
         [0045]    Construction of the threads  142  and  150 , and their respective counterpart complemental threads  144  and  162  can be accomplished by various means. For example, threads  142  and  144  can be constructed as a screw drive arrangement to facilitate the relative movement between the elongated body  132  and the deployable member  136  in deployment or retraction. Likewise, threads  150  and  162  can be constructed for effective mutual engagement. As a matter of design preference, threads  142  and  144  may be of any length and may be placed at any point throughout the internal bore of the elongated member. In addition, though not necessary, threads  150  of the deployable member  136  can be an extension of its threads  144 .  
         [0046]    At its proximal end portion  152 , the deployable member  136  is adapted to be manipulated to effect its extension beyond the opening  138  of the internal bore  134  in a deployed position or retraction. For the embodiment as shown in FIGS. 1 and 2, the rotational movement of the deployable member  136  can be effected by tools such as a wrench, socket wrench, screwdriver, or the like. In one embodiment of the present invention as shown in FIG. 1, the proximal end portion  152  of the deployable member  136  has a hex-shaped configuration, which is engageable by a socket or a wrench to effect a rotational action. In alternative embodiments, proximal end portion  152  has an intersecting depression (not shown) adapted to accommodate the driving tip of a “Phillips” screwdriver to effect a rotational action. Any alternative means and arrangements for engaging and rotating the deployable member  136  can be employed including, but not limited to, a driver or “Allen” wrench configuration.  
         [0047]    As referenced above, rotation of the deployable member  136  relative to the elongated body  130  extends the deployable member for its threads  150  to engage the threads  162  of the head portion  160  of the distal segment  120 . Once threads  150  are engaged with threads  162 , both the proximal and the distal segments are coupled as a unit.  
         [0048]    The deployable member  136  can be removed from the elongated body  132 , allowing for different sizes, threads and/or shapes for the head portion and/or tool attachment portions. Thus, the attachment and/or arrangement of the elongated body  132  and the deployable member  136  can be a screw-fit, or snap-fit arrangement, which does not interfere with the rotation of the deployable member  136 .  
         [0049]    The proximal segment  130  is provided with a tool attachment end portion  180  that is adaptable to receive a rotational torque to effect a rotational action of the elongated body  132 . As shown in FIG. 1, a “hex-head” end configuration is provided, on which a socket  187  can be fitted to effect the rotational action of the elongated body  132 . Optionally, the proximal end can incorporate a flange  154  to limit the extension of the deployable member  136  beyond the distal end of the proximal segment.  
         [0050]    The coupled proximal and distal segments employing the above-described means of engagement provide a detachably coupled distraction screw, which functions as a unitary device. In a surgical application, a socket (coupled to a wrench, not shown)  187  is attached to the tool attachment portion  180 , and the distraction screw is positioned at a site of a bone structure  20 . By applying a rotational torque to the elongated body  132  in a clockwise direction, both the proximal and distal segments rotate in unison so that thread  110  of the distal segment  120  may engage opening  22  of the underlying bone. Shank  124  is advanced and secured onto the bone structure as shown in FIG. 2.  
         [0051]    As shown in FIGS. 1 and 2, the distal segment  120  comprises a threaded shank portion  124  and a head portion  160 . As referenced above, threads  110  of the shank portion  124  would preferably, but not necessarily, be self-tapping and/or self-drilling. The threads  110  would also follow the same turn direction as those of threads  150  and  144 . Depending on the particular application, the shank portion  124  can be of variable lengths and threads  110  may be of any known configurations. One of ordinary skill in the art would understand that the threads can be of any design that is understood and well known to be applicable for screwing and inserting into mammalian bone. In the embodiment shown in FIGS. 1, 2 and  3 , the internal diameter of the threaded shank portion is progressively tapered from the head portion to the distal tip.  
         [0052]    The shape of the head portion  160  may be of any geometric design, including but not limited to, rectangular, trapezoidal, cylindrical, circular, spherical, hybrid configurations and the like. Further, the head may be absent altogether, placing the engagement adapter directly into the body of the screw shank (FIG. 7 c ). In the embodiment as shown in FIGS. 1, 2 and  3 , the head portion  160  is mono-axial, remaining in a fixed plane relative to the threaded shank. As used herein, “mono-axial” refers to rotation of the head portion and shank along a common arbitrary axis. This is defined by the placement of the head portion in a fixed geometric relationship to the threaded shank such that when the shank is rotated, the head portion also rotates along the same axis. Thus, in the embodiment as shown in FIGS. 1, 2 and  3 , the head portion is arranged with its diameter perpendicular to the length of the shank, which defines a common mono-axial relationship.  
         [0053]    The distal segment  120  can be made of any biologically adaptable or compatible materials. Materials considered acceptable for biological implantation are well known and include, but are not limited to, stainless steel, titanium, combination metallic alloys and the like, various plastics, ceramics, biologically absorbable materials and the like. It would be understood by one of ordinary skill in the art that the distal segment  120  can be made of any materials acceptable for biological implantation and capable of withstanding the torque required for insertion and the load encountered during use. Any components may be further coated/made with osteo-conductive (such as deminerized bone matrix, hydroxyapatite, and the like) and/or osteo-inductive (such as Transforming Growth Factor “TGF-B,” Platelet-Derived Growth Factor “PDGF,” Bone-Morphogenic Protein “BMP,” and the like) bio-active materials that promote bone formation. The proximal segment  130  may be made from any non-toxic material capable of withstanding the torque required for insertion and the load encountered during use. Materials used in the proximal segment  130  need not be limited to those acceptable for implantation, since it functions to deliver the implatable distal segment  120  but is not, itself, implanted.  
         [0054]    As shown in FIG. 2, the distraction screw  10  is placed at a predefined location of the vertebral bone. As rotational torque is applied to the distal segment  120  by the tool attachment, both the segments rotate in unison, which inserts the threaded portion  110  of the distal segment into the bone opening  22 . The coupling of the proximal and distal segments provides the longitudinal stability and the structural integrity of the coupled segments as a distraction device. In another embodiment of the present invention as shown in FIGS. 5, 6 a  and  6   b,  in addition to coupling the deployable member  136  with the distal segment  120 , the elongated body  132  of the proximal segment  130  is also engageable to the distal segment  120  to prevent their relative rotational movement by a rotation locking means.  
         [0055]    As shown in FIG. 5, at the distal end portion  137  of the proximal segment  130 , a key  170  is provided. The key  170  is fitted to be inserted into a receptacle  172  as defined by a depression located at the head portion  160  of the distal segment  120 . When the key  170  is inserted into receptacle  172 , the elongated body  132  of the proximal segment  130  is engaged with the distal segment  120 , which prevents the relative rotational movement between the two segments. FIGS. 6 a  and  6   b  show another embodiment of the present invention in which a variation in the design of the rotation locking means is presented. The distal end portion  137  of the proximal segment  130  incorporates a hex extension  190 , which can be fitted into the well  158  of the head portion  160  with a complemental hex socket receptacle  194 . When so fitted, rotation of the distal segment  120  proximal segment  130  relative to each other is inhibited. As shown in FIGS. 6 a  and  6   b,  hex extension  190  has an internal bore  192  through which the deployable member  150  passes for engagement with the distal segment by way of their cooperating threads  150  and  162 .  
         [0056]    While the rotation locking means is illustrated in a key-receptacle arrangement and hex extension-socket configuration, it is not limited to these examples. It is understood that any engageable arrangement can be used as a rotational locking means. These include, but are not limited to, one or more extended protuberances of the elongated body  132  to seat within complemental bored depressions on the head portion of the distal segment  120 . Similarly, square-jaw or spinal jaw clutch arrangements, and serrated or saw tooth edges can be incorporated to mate or interlock with similar features on the head portion (not shown).  
         [0057]    In embodiments that incorporate such rotation locking means, assembly of the proximal and distal segments can be easily accomplished. The deployable member  136  is fitted within the internal bore  134  of the elongated body  132  in a retracted configuration by effecting a relative rotational movement between elongated body and the deployable member along their cooperating threads. The proximal segment  130  is then held adjacent to the head portion  160  of the distal segment  120  to insert key  170  into the receptacle  172 . For the embodiment shown in FIGS. 6 a  and  6   b,  the hex extension  190  is seated within the socket  194  of the head portion  160 . A suitable tool such as a screw driver, wrench, pliers, or the like is used to engage the proximal end portion  152  of the deployable member  136  in a rotating action to extend the threaded end portion  152  beyond the end opening  138  of the bore  134  (or bore  192  of the hex extension) to engage the internal threads  162  of the head  160  of the distal segment  120 . Their actions secure the proximal and distal segments in a coupled relationship and inhibits any relative longitudinal and rotational movements between the segments.  
         [0058]    As discussed above, the proximal segment  130  is securably coupled to the distal segment  120  as a distraction device while being anchored onto the bone structure. After the need for the distraction has been met, the proximal segment  130  is detached from the distal segment  120 . From the coupled configuration, the elongated body  132  is held stationary and, using segment  152 , the deployable member  136  is rotated in a direction opposite to that which was used to effect its coupling to the internal threads  162  of the head  160 . This rotation disengages threads  150  from threads  162  of the distal segment  120 . The rotation also releases the friction between the distal portion of the elongated body and the head portion of the distal segment. Detachment of the proximal and the distal segment is thus effected, leaving the latter securely implanted onto the vertebral structure, as shown in FIG. 3.  
         [0059]    The deployable member can be retracted and stowed into the internal bore  134  of the proximal segment. For the embodiment as shown in FIG. 5, once the complemental threads  150  and  162  are disengaged, the proximal segment  130  can be dislodged with the key  170  disengaged from the receptacle  172  to separate from the distal segment  120 . In a similar manner, for the embodiment shown in FIGS. 6 a  and  6   b,  once threads  150  and  162  are de-coupled, the hex extension  190  can be withdrawn from hex socket  194  of the distal segment. In this way, the use of the rotation locking means further ensures that the distal segment  120  would not be inadvertently rotated and de-coupled from the skeletal bone while rotating the deployable member  136  during detachment of the proximal and distal segments.  
         [0060]    [0060]FIGS. 7 a - 7   c,    8   a - 8   c,    9   a,    9   b,    10   a,    10   b,    11 ,  12   a,    12   b  illustrate other embodiments of the modular distraction screw. Since a thorough description of the device has been presented above, only the relevant design differences of the other embodiments will be described in detail.  
         [0061]    [0061]FIG. 7 a  demonstrates another embodiment of the proximal segment. This embodiment employs an elongated proximal segment  130  with a smooth internal bore  134  and no internal threads. A deployable member  133  has a threaded tip  150  on its distal end and proximal segment  152  which is adapted so as to be engaged by a screw driver, wrench or the like in order effect its rotation. A flange  154  is placed immediately distal to the engageable proximal end. FIG. 7 b  demonstrates the assembled proximal segment wherein the outer elongated body and the deployable member are each independently rotatable from the other. FIG. 7 c  shows the proximal segment  130 , distal segment  160  and the wrench  187 . As threads  150  of the proximal segment are engaged with threads  162  of the distal segment, flange  154  limits the extension of the deployable member and applies a compressive force across the elongated element  130 , thus forming a rigid distraction screw. As before, the screw is inserted into bone by application of a rotational force onto element  180  using wrench  187 . Rotation may be achieved by any engageable means and is in no way limited to the hex-wrench arrangement illustrated. After completion of the bone work, the distal segment is disengaged from the proximal segment by rotation of element  152  in the direction opposite to that used for engagement while segment  130  is held stationary using element  180 . Optionally, a rotation locking means can be incorporated as part of the distal tip of the proximal segment in order to ensure that the distal segment  120  does not inadvertently rotate and de-couple from the skeletal bone during distraction screw disassembly.  
         [0062]    [0062]FIGS. 8 a - 8   c  shows another embodiment of the present invention in which a proximal/distal interface is defined by a threaded extension  157  disposed on the head portion of the distal segment. The threaded extension  157  is fitted within the complmental threaded female receptacle  156  of the proximal segment. It is undersood that the head of the distal segment beneath extension member  157  may be of any geometric configuration. Further, in these or any of the other embodiments presented herein, the proximal/distal interface is not limited to the screw and screw receptacle arrangement depicted. Thus, for example, FIGS. 9 a  and  9   b  demonstrate a sprocket arrangement  159  (male member) and a complementary receptacle  143  (female member), and FIGS. 10 a  and  10   b  show a smooth male member  161  with a key which is used to engage the complementary receptacle  145 . These two design arrangements demonstrate the adaptation that any engageable means can be used.  
         [0063]    [0063]FIGS. 11 a,    11   b,    12   a  and  12   b  demonstrate a sprocket arrangement which permits a locking engagement with the complementary receptacle. As illustrated in FIGS. 12 a  and  12   b,  the cylindrical head  163 , which is a smaller-diameter continuation of the screw shank  124 , is fitted with engageable teeth  167  in the parallel plane (along the long axis of shank  124 ) and engageable teeth  165  in the perpendicular plane. The distal end portion of the elongated body  132  is provided with a receptacle  147  which is complimentary to the cylindrical head  163  of the distal segment  120 . Receptacle  147  has a central bore and engageable teeth in both the parallel and perpendicular planes relative to the long axis of the proximal segment to accommodate and engage teeth  163  and  165  of the distal element  120 .  
         [0064]    The elongated body  132  of the proximal segment  130  has an engageable proximal end portion  181 , which is adapted to be rotated, as for example, by means of a wrench  191 . Similarly, the proximal segment  130  is rotatable by means of a wrench  189 . With rotation, the proximal segment  130  advances along threads  119  to the receptacle  147  of the proximal segment around the cylindrical head  163  of the distal segment to produce a rigid distraction screw.  
         [0065]    Wrench  191  is used to engage the end portion  181  of the proximal segment  132  to effect its rotation. The teeth within receptacle  147  of the proximal segment engage the complimentary teeth  165  and  167  of the distal segment, which rotates the distal segment and drive threads  111  into the underlying bone. Once the bone work has been completed, wrench  189  is used to rotate the proximal segment  130  in the direction opposite to that used during engagement causing it to retreat along threads  119 . In this way, the head portion  163  can be disengaged from the receptacle  147  thus leaving the distal segment  120  attached to the bone. One of ordinary skill in the art will understand that the engageable arrangements described herein are illustrative and not restrictive, and that any engageable means may be alternatively used at any of these points of contact.  
         [0066]    The distal segment  120  of the distraction screw  10 , which remains securely affixed onto the vertebral bone, provides enhanced structural integrity of the bone by reducing the stress concentration generally expected of an empty opening in a structural member. Leaving the distal segment  120  in place further eliminates the robust bone bleeding encountered after removal of current, commercially-available distraction screws and obviates the need to fill the holes with a hemostatic agent.  
         [0067]    The distal segment  120  can also provide a point of anchoring for a skeletal plate  30  or other prosthetic devices to adjust, align and maintain the spatial relationship(s) of adjacent bones or bony fragments during healing and fusion after surgical reconstruction, as shown in FIG. 4. Since placement of the distraction screws is performed as the first step in the surgical procedure, the anatomical landmarks required to ensure proper alignment of the plate or other prosthetic device in the desired anatomical plane are still intact.  
         [0068]    Plate fixation using the affixed distal segment is largely similar for the many mono-axial embodiments illustrated. For simplicity, it will be described in detail for the first embodiment alone. As shown in FIG. 4, a skeletal plate  30  is mounted onto the distal segment, where head portion  160  is adapted with peripheral surface contour to fit an opening  32  of the skeletal plate. A mounting plate  212  having a tapered opening  214  centers the screw  210  in alignment and engagement with the threads  162  of the head portion. The mounting plate  212  also serves as a washer to assert the necessary force onto the skeletal plate  30  to be secured onto the bone substrate  20 . In this way, the distal segment guides the placement of the plate and maximize the likelihood of correct anatomical alignment. It will also provide an additional point of attachment for the plate or device and enhances the structural integrity of bone/plate interface.  
         [0069]    It is accepted that fusion of a specific spinal level will increase the load on the disc space immediately above and below the fused segment. Over time, the increased load will promote degeneration of the adjacent discs and may ultimately require that they be removed and the fusion extended to the adjacent bony level. In that event, the mounting plate  212  can be removed, permitting access to the distal segment  120 . The proximal segment  130  and the elongated member  136  can be reattached to the distal segment  120  and, thus, reconstitute the distraction screw without removal.  
         [0070]    A second distraction screw is placed into the bone of the new operative level and the surgical reconstruction is performed. After the necessary work, the proximal segments  130  are removed from each distraction screw, leaving distal segments  120  securely affixed to the vertebral bodies. A bone plate or device is affixed to maintain the spatial relationships of the new operative level while bone healing and fusion progress. Again, each distal segment  120  so affixed provides an additional point of attachment for the plate or device.  
         [0071]    In other embodiments of the present invention, the distal segment incorporates a poly-axial design feature, which further facilitates the mounting of the skeletal plate  30  onto the vertebral bone. As used herein, “poly-axial” refers to the ability for the head portion of the distal segment to rotate about an axis that is other than that of the longitudinal axis of the threaded shank. This design provides a ready mechanism through which a skeletal plate maybe affixed onto an implantable distal segment that has been placed into the skeletal bone at an angle other than the perpendicular. This situation arises when the degenerated bony elements have suffered significant mal-alignment, requiring that the distraction screws be placed at an angle to the bone surface in order to achieve the trajectory needed to realign the bones.  
         [0072]    Examples of the poly-axial head design are illustrated in FIGS. 13, 14,  15   a - c,    16   a,    16   b,    17  and  18 . With such a feature, a poly-axial distal segment  220  incorporates a head portion  222 , which generally assumes the geometric shape of a spherical segment, or cup shape, and a neck portion  224  with a narrower cross-sectional profile that tapers to the shank portion  226 . A poly-axial head adapter  230  is swivelably fitted over the head portion  222 . The poly-axial head adapter  230  has a ring body  232 , which has an internal ring opening with a smaller internal diameter at its lower portion  234  than its upper portion thus forming a socket arrangement. The lower portion  234  also has a smooth concave external contour  236 .  
         [0073]    Poly-axial head adapter  230  is installed over the head portion  222  by way of the opening at its lower ring portion. A rotational space between the poly-axial head adapter  230  and the head portion  222  is provided to allow the poly-axial head adapter to move. This type of connection can be considered a ball joint, or socket connection, though other means for providing a connecting relationship between the poly-axial head adapter and the head portion while permitting varying degrees of rotational flexibility (swivelability) can also be adapted.  
         [0074]    A flange  228  is located between the neck portion  224  and the shank portion  226 , on which the poly-axial head adapter  230  can be rested. Flange  228  also provides as a stop when the shank  226  is inserted onto the bone structure, as well as a measure of the depth of the shank implant. A concave curvature in the lower portion of the flange  228  allows the maximum thread/bone contact and support when the distal segment  220  is affixed in an inclined angle relative to the surface of the bone  20 .  
         [0075]    Coupling of the proximal segment  130  and the distal segment  220  in this embodiment can employ any of the coupling designs described in detail for the mono-axial distal segment. These methods include, but are not limited to, the design illustrated in FIGS. 13 and 14. Once coupled, the segments will function as a unitary device. By applying a rotational force to the proximal segment  130 , the threaded shank of the distal segment  226  can be advanced and secured into the underlying bone, as described for the mono-axial design.  
         [0076]    Following the distraction work and detachment of the proximal segment from the implantable distal segment, the skeletal plate  30  can be mounted onto the implantable distal segment. As shown in FIG. 15, the adapter ring  230  is peripherally contoured for it to be fitted within an opening  32  of the skeletal plate  30 .  
         [0077]    Poly-axial head adapter  230  has an open top with internal circumferential thread  238  for receiving a mounting plate  242  with complemental threads  244 . As shown in FIGS. 15, 15 a  and  15   b,  mounting plate  242  has a circular-shaped top flange  246 , which is seated on the rim  34  of the opening  32  of the skeletal plate  30 . After the skeletal plate  30  is mounted onto the adapter ring  230 , the mounting plate  242  is threaded onto its internal thread  238 , thus forming a unitary piece. Before the treads  242  and  238  are completely engaged, the skeletal plate can be tilted or rotated for it to be aligned in proper placement. Since the adapter ring  30  is swivelable in relation to the head portion  222 , skeletal plate  30  can be easily manipulated to assume the desired position in relation to the bone structure despite the other than normal or vertical entry of the distal segment onto the bone structure.  
         [0078]    After placement of the bone plate  30 , the mounting plate  242  is tightened against the thread  238 . The force asserted by the thread engagement draws the head adapter close to the mounting plate, which in turn closes the space between the lower portion  234  of the adapter ring and the head portion  222  and to firmly secure the head adapter onto the distal segment as well as the skeletal plate. As shown in FIG. 15 a,  the mounting plate has a central opening  250  into which a turning devise can be inserted to facilitate its turning. Although illustrated as a hexagonal opening into which an “Allen” wrench driver may be deployed, any engagement method consisting of a driver and complimentary receptacle can be employed.  
         [0079]    [0079]FIGS. 16 a,    16   b,    17  and  18  show another variation in the poly-axial design feature. The poly-axial head adapter  310  is provided with a cap  312 , which is coupled to the head adapter by means of threads  318 . In assembly, screw  274  is fitted into the poly-axial head adapter  310  and cap  312  is used to engage threads  318 . The screw  274  has a head portion  276  and a flat top  278 . The cap  312  has a central opening  316  with internal threads  320 , which is adapted to receive the threaded, rounded distal end  402  of the deployable member  136 . Cap  312  may be further adapted to receive an optional rotation locking means. While the key design (opening  314 ) is illustrated for simplicity, it is understood that the rotation locking means may be of any engageable configuration.  
         [0080]    After key  170  is fitted into the key opening  314 , the deployable member is extended to pass through the threaded opening  316  and to push against the top surface  278  of the screw  274 . As the threaded distal portion is rotated further in relation to threads  320 , the force exerted by the rounded end  402  on surface  278  causes the under surface of the screw head  276  to firmly engage portion  322  of head adapter  310 , forming a unitary distraction screw. With distraction screw assembly, its important that the long axis of the proximal segment  130  be the same as the long axis of screw  274 , permitting uniform rotation of both segments along a common axis. In use, a rotational torque is applied to the proximal segment  130 , which is translated by the key  170  to the head adapter  310  and, in turn, to screw  274 . The shank rotates and engages the underling bone.  
         [0081]    Following distraction and bony realignment, the proximal portion is detached from the poly-axial head adapter, leaving the implantable distal segment affixed to bone. The skeletal plate  30  is mounted with its opening  32  to fit over the peripherally contour of the distal segment and is manipulated to assume the desired position. The swivel action of the poly-axial head adapter permits proper placement of the skeletal plate even with angled placement of bone screw  274 . A mounting plate  324  is seated on the stepped rim  34  of the opening  32  of the skeletal plate  30 . It has a central opening  326  though which a mounting screw  328  can be passed to engage threads  320  of screw cap  312 . As the threads are tightened, force is exerted onto surface  278  by the rounded end of screw  328  causing the under surface of the screw head  276  to firmly engage portion  322  of head adapter  310 , and locking the poly-axial head portion to screw  274 . The same action also effects a force on the mounting plate, bearing against the step rim  34  of the skeletal plate  30  for it to be securely anchored. For this embodiment, it is understood that a space is provided between the screw cap and the mounting plate to provide for the engagement of the poly-axial head adapter and the head portion.  
         [0082]    From the above, it is apparent that the poly-axial design will produce a highly versatile distraction screw and can be used even with significantly mal-aligned bony structures. The ability of adapter ring  310  to rotate and swivel permit it to accommodate and orient the skeletal plate  30 , thus ensuring proper alignment and correct plate fixation.  
         [0083]    As described above, the present invention is that of a distraction screw and its use. It provides a significant design advantage over existing art by decreasing the bone stress encountered at the empty bone holes and reducing the extent of operative bleeding. The present design also provides an additional point of fixation for the implantable plate/prosthesis, maximizes the likelihood of proper plate/prosthesis alignment, and provides a ready mechanism for modular extension of the surgical reconstruction to adjacent levels at a future date. While the different embodiments of the present invention have been illustrated as consisting of a proximal and distal segment, it is understood that a modular distraction screw may be constructed from more than two components. The preceding descriptions and accompanying drawings are to be considered as illustrative and not restrictive in character. Further understanding of the present invention, and other embodiments as described herein can be obtained through a review of the claims: