Abstract:
A growth rod distraction system a domino having first and second bores provided therein. First and second spinal rods are respectively disposed within the first and second bores for movement relative to the domino First and second spur gears are disposed within the domino and respectively cooperate with the first and second growth rods such that rotation of the spur gears causes axial movements of the growth rods relative to the domino The spinal rods may be formed either from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material) or a hybrid combination of such flexible polymer material and a metallic material (such as nitinol or similar medical grade metallic material). The spinal rods can also be used for fusion or non-fusion surgery of the spine to stabilize two or more vertebrae.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/790,117, filed Mar. 15, 2013, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to the field of orthopedic implants, more specifically, a polymer PEEK (polyether ether ketone), carbon fiber PEEK, PEAK or hybrid (polymer and metal) based spinal rods which are used for deformity corrections with growth preservation in early-onset scoliotic patients and for the 8fusion of two or more vertebrae in degenerative spinal disease or deformity. This invention also relates to a mechanism for the minimally invasive distraction of such a rod. 
         [0003]    Surgical techniques for the treatment of early-onset scoliosis are aimed at deformity correction with preservation of growth. The most common approach includes the use of distraction-based growth rods. A typical growth rod fixation has two foundations, namely, proximal and distal, where limited fusion is performed. Pedicle screws, hooks, wires, and other retaining structures may be used at each foundation to anchor the rods with the bony structure. Each foundation has a rod spanning toward the other end, which are connected to each other near the thoracolumbar junction. The rods are connected using a domino, which helps in distraction for serial surgeries until a final fusion is performed. 
         [0004]    In a single growth rod technique, the rods span only one side (the concave side) of the spinal deformity curve. In a dual growth rod technique, the rods span both (both the concave and convex sides) of the spinal deformity curve. For the dual growth rod technique, the region of the rod near the foundation can have a crosslink connecting the rods of both sides. 
         [0005]    In a typical growing rod implant surgery, the rods are attached along one or both sides of the spine above and below the spinal deformity curve using the pedicle screws or other retaining structures. The rod is then extended to correct the spinal deformity curve until the surgeon feels enough compression in the rod to stop the adjustment. The spinal deformity curve can usually be corrected by fifty percent at the time of the initial surgery. During the first operation, the patient usually undergoes invasive surgery. Regular construct lengthening is typically scheduled approximately every six months to a year thereafter. The lengthening procedure usually includes exposing the domino connectors through a small midline incision, loosening either the cranial or the caudal domino-connector setscrews, and distracting across the two rods within the connector. This lengthening process is frequently continued for a period of five to ten years after implantation. 
         [0006]    Although known spinal and growth rod structures and installation procedures have functioned satisfactorily, several potential limitations have been determined. First, current spinal rods are made out of metallic materials, such as stainless steel, cobalt-chromium, or titanium. As a result, these metal spinal and growth rods are very rigid. The high level of rigidity of these metal spinal and rods may restrict the micro-motion of intervertebral disc in the spine, which can cause spontaneous fusion at the intervertebral junction. Spontaneous fusion is an undesirable clinical complication because the aim of the growth rod surgery is to delay or limit spinal fusion so as to allow the spine to grow. Second, current spinal rods are known to experience a relatively high rate of breakage. The material used to form the rod is an important consideration because it is a construct bearing load for a longer duration without spinal fusion, and its durability is vital. Third, the current surgical technique exposes the patient to a chance of infection because of the midline exposure of the tissue during subsequent distraction surgeries. Even though the use of minimally invasive techniques may reduce the chance of infection, none of the current mechanical distraction systems are simple enough for distraction, and some have many sub-units. Therefore, it would be desirable to avoid all of these potential problems. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention relates to a flexible growth rod system that militates against spontaneous fusion at the intervertebral junction, and which also militates against adjacent level degeneration polymer such as PEEK, PEAK, and other medical grade polymer or hybrid (both metal and polymer) rod, which would provide better deformation during physiologic loads and will reduce the incidence of rod breakage due to higher demand for flexibility is innovated. Further the rod has novel minimal invasive mechanisms attachments which would avoid big open reoperations to distract the rod during regular intervals, thereby preventing pain, infection, and morbidity of patients. 
         [0008]    This invention includes a polymer, where a preferred embodiment is one of PEEK, PEAK, carbon fiber PEEK, and other medical grade polymers or hybrid (polymer and metal) spinal rods. 
         [0009]    Polymer (such as PEEK, PEAK, or other medical grade polymers) or hybrid (such as PEEK, PEAK, or carbon fiber with a nitinol core) spinal rods could be made for use in any growth rod technique and with any generic instruments, such as pedicle screws, hooks, dominoes, etc). The hybrid rod can be of two types, namely: (1) a composite rod with hollow outer cylinder made of nitinol and inner rod of the polymer (such as PEEK, PEAK, carbon fiber, or any other medical grade polymer), wherein the outer metal may have serrations to increase the flexibility, or (2) a composite rod with hollow outer cylinder (having a circular, oval, or other desired cross sectional shape) made of polymer (such as PEEK, PEAK, carbon fiber, or any other medical grade polymer) and inner rod of the nitinol. 
         [0010]    Minimally invasive distractible growth rods could be assembled using a compact domino that connects and houses the two ends of the rods at the thoracolumbar junction. The mechanism incorporated within the compact domino may include two spur gears, each for the distraction mechanism of proximal and distal rods. The rod region within the domino may have a special rack-like surface that uses the rotary motion of the spur gear to translate the rod along its central axis. The rod could be made out of metal or polymer or composite materials. The spur gear may be connected to the domino such that it allows the rotation of the gear. Two screw heads can be used as closings to secure and compress the spur gears inside the domino. The compression of the spur gears with the screw head closings retracts the rotation of the gears. Since the spur gears and the associated rods have dependent motion, the rods also stay fixed due to compression on the spur gears. The domino, screw head closings, and the spur gears may be made out of any desired material or materials including, for example, metals such as titanium or stainless steel. 
         [0011]    An in-series distractible growth rod can alternatively be made using a screw mechanism in which the two rods (proximal and distal) extend within a coupler. The coupler can be rotated using a bevel gear on the top and housed inside a domino. The coupler and bevel gear will be in a compact domino. The rotation of the coupler will push the two rods by virtue of a screw mechanism, thereby causing distraction. This mechanism could operated by a micro-electric motor with a microcircuit that is activated by a remote control. 
         [0012]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an elevational view of a first embodiment of a growth rod distraction system in accordance with this invention. 
           [0014]      FIG. 2  is an enlarged elevational view of the first embodiment of the growth rod distraction system illustrated in  FIG. 1  with the screw heads removed. 
           [0015]      FIG. 3  is a perspective view of the first embodiment of the growth rod distraction system illustrated in  FIG. 2  with the spur gears removed. 
           [0016]      FIG. 4  is a sectional elevational view of the first embodiment of the growth rod distraction system illustrated in  FIGS. 1 ,  2 , and  3 . 
           [0017]      FIG. 5  is an enlarged perspective view of one of the screw heads for the first embodiment of the growth rod distraction system illustrated in  FIGS. 1 through 4 . 
           [0018]      FIG. 6  is side elevational view of the screw head illustrated in  FIG. 5 . 
           [0019]      FIG. 7  is an enlarged perspective view of one of the spur gears for the first embodiment of the growth rod distraction system illustrated in  FIGS. 1 through 4 . 
           [0020]      FIG. 8  is an enlarged perspective view of an alternative structure for one of the screw heads for the first embodiment of the growth rod distraction system illustrated in  FIGS. 1 through 4 . 
           [0021]      FIG. 9  is a sectional elevational view of a second embodiment of a growth rod distraction system in accordance with this invention. 
           [0022]      FIG. 10  is a sectional elevational view of a third embodiment of a growth rod distraction system in accordance with this invention. 
           [0023]      FIG. 11  is a perspective view of a first embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0024]      FIG. 12  is a sectional elevational view of the first embodiment of the spinal rod illustrated in  FIG. 11 . 
           [0025]      FIG. 13  is a perspective view of a second embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0026]      FIG. 14  is a sectional elevational view of the second embodiment of the spinal rod illustrated in  FIG. 13 . 
           [0027]      FIG. 15  is a perspective view of a third embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0028]      FIG. 16  is a sectional elevational view of the third embodiment of the spinal rod illustrated in  FIG. 15 . 
           [0029]      FIG. 17  is a perspective view of a fourth embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0030]      FIG. 18  is a sectional elevational view of the fourth embodiment of the spinal rod illustrated in  FIG. 17 . 
           [0031]      FIG. 19  is a sectional elevational view of a fifth embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0032]      FIG. 20  is a sectional elevational view of a sixth embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0033]      FIG. 21  is a sectional elevational view of a seventh embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0034]      FIG. 22  is a sectional elevational view of an eighth embodiment of a spinal rod that can be used in any of the embodiments of the growth rod distraction system or for any other spinal surgery for fusion or non-fusion technique in accordance with this invention. 
           [0035]      FIG. 23  is a perspective view of a portion of a fourth embodiment of a growth rod distraction system in accordance with this invention. 
           [0036]      FIG. 24  is a side elevational view of the portion of the fourth embodiment of the growth rod distraction system illustrated in  FIG. 23 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0037]    Referring now to the drawings, there is illustrated in  FIGS. 1 through 4  a first embodiment of a compact, minimally invasive, and distractible growth rod distraction system, indicated generally at  10 , in accordance with this invention. The growth rod distraction system  10  includes a central domino or housing  11 . In the illustrated embodiment, the domino  11  includes first and second side portions having respective first and second bores  12  and  13  provided therein. Each of the illustrated first and second bores  12  and  13  is generally hollow and cylindrical in shape, having an opened end and a closed end. The illustrated bores  12  and  13  extend generally parallel to one another. However, it will be appreciated that the first and second bores  12  and  13  may have any desired shape or combination of shapes and may be oriented in any desired relationship. In the illustrated embodiment, the domino  11  also includes a central portion that extends between the first and second side portions and that has first and second openings  14  and  15  provided therein. The illustrated first and second openings  14  and  15  are each generally hollow and cylindrical in shape and, as shown in  FIGS. 2 through 4 , extend respectively through the domino  11  into the first and second bores  12  and  13 . Respective posts  14   a  and  15   a  are provided within the first and second openings  14  and  15  for a purpose that will be explained below. The outer ends of the first and second openings  14  and  15  are threaded, as shown at  14   b  and  15   b  in  FIG. 4 , again for a purpose that will be explained below. 
         [0038]    First and second growth rods  16  are respectively disposed within the first and second bores  12  and  13  of the domino  11 , as shown in  FIGS. 1 through 4 . The structures of the growth rods  16  are illustrated in detail in  FIG. 5 . As shown therein, the illustrated growth rods  16  are each generally cylindrical in shape, preferably corresponding with the shapes of the first and second bores  12  and  13  within which they are respective supported. For example, the growth rods can each define outer diameters of 4.5 mm, 5.5 mm, or 6.35 mm depending upon the particular application. However, it will be appreciated that the growth rods  16  may have any desired shape or combination of shapes. Inner ends of the growth rods  16  are supported within the first and second bores  12  and  13  of the domino  11  for axial movement relative thereto, as will be explained in detail below. The outer surfaces of the inner ends of the growth rods  16  are provided with respective pluralities of recesses  16   a,  the purpose of which will also be explained below. The material or group of materials used to form the growth rods  16  will discussed further below. 
         [0039]    First and second spur gears  17  are respectively disposed within the first and second openings  14  and  15  of the domino  11 , as shown in  FIGS. 2 and 3 . The structure of one of the spur gears  17  is illustrated in detail in  FIG. 5 . As shown therein, the illustrated spur gear  17  has a central opening  17   a  and a plurality of outer teeth  17   b.  The central opening  17   a  is adapted to receive one of the posts  14   a  and  15   a  therein so as to allow each of the spur gears  17  to be disposed within the first and second openings  14  and  15 . When so disposed, the spur gears  17  are supported for rotation relative to the domino  11 . Also, some of the outer teeth  17   b  of the spur gears  17  extend into engagement with some of the recesses  16   a  provided in the growth rods  16 . As a result, rotation of the spur gears  17  causes axial movement of the associated growth rods  16  relative to the domino  11 . The purpose for this cooperation between the spur gears  17  and the growth rods  16  will be explained below. 
         [0040]    The spur gears  17  can be retained within the respective openings  14  and  15  of the domino  11  and locked in position relative thereto by respective heads  18 , as shown in  FIG. 1 . The structure of one of the heads  18  is illustrated in detail in  FIGS. 6 and 7 . As shown therein, the illustrated head  18  is shaped generally in the form of a flat disc, having an outer circumferential surface  18   a  that is threaded. The threaded outer circumferential surfaces  18   a  of the heads  18  are sized and shaped to cooperate with the threaded outer ends  14   b  and  15   b  of the first and second openings  14  and  15 , as shown in  FIG. 1 , so as to retain the spur gears  17  within the first and second openings  14  and  15  of the domino  11 . Further rotation of the heads  18  cause them to frictionally engage the associated spur gears  17 , thereby preventing the spur gears  17  from to rotating within their respective openings  14  and  15 . This prevents axial movement of the growth rods  16  relative to the domino  11 . 
         [0041]    In use, the heads  18  of the growth rod distraction system  10  are initially loosened or removed so as to not engage the respective spur gears  17 . As a result, the spur gears  17  are free to rotate within their respective openings, allowing free axial movement of the growth rods  16  relative to the domino  11 . The outer ends (not shown) of the growth rods  16  are then anchored to respective bony structures by means of pedicle screws, hooks, wires, or other conventional retaining structures (not shown) as described above. These anchors form the proximal and distal foundations for the growth rod distraction system  10  by attaching the outer ends of the growth rods  16  to the bony structure of spine. After the desired distraction of the bony structure of spine has been achieved by the surgeon, the heads  18  are installed and/or tightened on the domino  11 . As discussed above, the heads  18  are caused to frictionally engage the associated spur gears  17 , thereby preventing the spur gears  17  from to rotating within their respective openings  14  and  15 , and further preventing axial movement of the growth rods  16  relative to the domino  11 . 
         [0042]      FIG. 8  illustrates an alternative structure for one of the screw heads  19  for the first embodiment of the growth rod distraction system illustrated in  FIGS. 1 and 6 . The alternative head  19  is shaped generally in the form of a flat disc, having an outer circumferential surface  19   a  that is threaded. The threaded outer circumferential surfaces  19   a  of the heads  19  are sized and shaped to cooperate with the threaded outer ends  14   b  and  15   b  of the first and second openings  14  and  15 , as shown in  FIG. 1 , so as to retain the spur gears  17  within the first and second openings  14  and  15  of the domino  11 . Additionally, however, the alternative head  19  has an aperture  19   b  provided therein that is sized and shaped to receive one of the posts  15   a  therein when the head  19  is secured to the domino  11 . 
         [0043]      FIG. 9  illustrates a portion of a second embodiment of a growth rod distraction system, indicated generally at  20 , in accordance with this invention. The growth rod distraction system  20  includes a central domino or housing  21  having a single bore  22  provided therein. In the illustrated embodiment, the domino  21  also includes a single opening  23  having a post  24  provided therein. The outer end of the openings  23  threaded in the same manner as the openings  14  and  15  described above. 
         [0044]    First and second growth rods  25  have respective inner ends that are supported within the bore  22  of the domino  11  for axial movement relative thereto, as will be explained in detail below. The outer surfaces of the inner ends of the growth rods  25  are provided with respective pluralities of recesses  25   a  for the same purpose as described above. The material or group of materials used to form the growth rods  25  will discussed further below. 
         [0045]    A single spur gear  26  is disposed within the opening  23  of the domino  21  and includes a central opening  26   a  and a plurality of outer teeth  26   b.  The central opening  26   a  is adapted to receive the post  24  therein so as to allow the spur gear  26  to be disposed within the opening  23  for rotation relative to the domino  21 . Also, some of the outer teeth  26   b  of the spur gear  26  extend into engagement with some of the recesses  25   a  provided in both of the growth rods  25 . As a result, rotation of the spur gear  26  causes concurrent axial movement of both of the growth rods  25  relative to the domino  21 . The spur gear  26  can be retained within opening  23  of the domino  21  and locked in position relative thereto by a head (not shown) in the same manner as described above. The operation of the second embodiment of the growth rod distraction system  20  is otherwise similar to the operation of the first embodiment of the growth rod distraction system  10  described above. 
         [0046]      FIG. 10  illustrates a portion of a third embodiment of a growth rod distraction system, indicated generally at  30 , in accordance with this invention. The growth rod distraction system  30  includes a central domino or housing  31 . In the illustrated embodiment, the domino  31  includes first and second side portions having respective first and second bores  32  and  33  provided therein. Each of the illustrated first and second bores  32  and  33  is generally hollow and cylindrical in shape, having an opened end and a closed end. The illustrated bores  32  and  33  extend generally parallel to one another. However, it will be appreciated that the first and second bores  32  and  33  may have any desired shape or combination of shapes and may be oriented in any desired relationship. In the illustrated embodiment, the domino  31  also includes a central portion that extends between the first and second side portions and that has first and second openings  34  and  35  provided therein. The illustrated first and second openings  34  and  35  are each generally hollow and cylindrical in shape and extend respectively through the domino  31  into the first and second bores  32  and  33 . Respective posts  34   a  and  35   a  are provided within the first and second openings  34  and  35 . The outer ends of the first and second openings  34  and  35  are threaded as described above. 
         [0047]    First and second growth rods  36  are respectively disposed within the first and second bores  32  and  33  of the domino  31 . The illustrated growth rods  36  are each generally cylindrical in shape, preferably corresponding with the shapes of the first and second bores  32  and  33  within which they are respective supported. However, it will be appreciated that the growth rods  36  may have any desired shape or combination of shapes. Inner ends of the growth rods  36  are supported within the first and second bores  32  and  33  of the domino  31  for axial movement relative thereto. The outer surfaces of the inner ends of the growth rods  36  are provided with respective pluralities of recesses  36   a.  The material or group of materials used to form the growth rods  36  will discussed further below. 
         [0048]    First and second spur gears  37  are respectively disposed within the first and second openings  34  and  35  of the domino  31 . Each of the spur gears  37  has a central opening  37   a  and a plurality of outer teeth  37   b.  The central opening  37   a  is adapted to receive one of the posts  34   a  and  35   a  therein so as to allow each of the spur gears  37  to be disposed within the first and second openings  34  and  35 . When so disposed, the spur gears  37  are supported for rotation relative to the domino  31 . Also, some of the outer teeth  37   b  of the spur gears  37  extend into engagement with some of the recesses  36   a  provided in the growth rods  36 . As a result, rotation of the spur gears  37  causes axial movement of the associated growth rods  36  relative to the domino  31 . 
         [0049]    In this embodiment of the invention, a micro-motor  38  or other actuator mechanism is supported within the domino  31  of the growth rod distraction system  30 . The micro-motor  38  is, of itself, conventional in the art and may be controlled by an external remote control structure (not shown) by means of one or more micro-circuits  39  that are also supported within the domino  31 . The micro-motor  38  includes a rotor that engages each of the spur gears  37 . Thus, when the micro-motor  38  is actuated, the rotor is rotated, thereby causing concurrent rotation of the spur gears  37 . The operation of the third embodiment of the growth rod distraction system  30  is otherwise similar to the operation of the first embodiment of the growth rod distraction system  10  described above. 
         [0050]      FIGS. 11 and 12  illustrate a first embodiment of one of the rods  16 ,  25 , and  36  that can be used in any of the above-described embodiments of the growth rod distraction system  10 ,  20 , and  30 , respectively. In this embodiment of the invention, the rod is formed completely from a solid piece of a flexible polymer material. For example, the rod may be formed from a PEEK (polyether ether ketone), carbon fiber PEEK, PEAK, or similar medical grade polymer material. The rod can be straight or curved as deemed necessary or desirable to correct the particular growth deformity. The rod can be of any desired size or cross sectional shape. 
         [0051]      FIGS. 13 and 14  illustrate a second embodiment of one of the rods  16 ,  25 , and  36  that can be used in any of the above-described embodiments of the growth rod distraction system  10 ,  20 , and  30 , respectively. In this embodiment of the invention, the rod has a circumferential outer portion that is formed from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material) and an inner core that is formed from a metallic material (such as nitinol, titanium, stainless steel, or similar medical grade metallic material). The rod can be straight or curved as deemed necessary or desirable to correct the particular growth deformity. The rod can be of any desired size or cross sectional shape. The inner core may be formed from one piece of material or may be braided from a plurality of pieces of material. 
         [0052]      FIGS. 15 and 16  illustrate a third embodiment of one of the rods  16 ,  25 , and  36  that can be used in any of the above-described embodiments of the growth rod distraction system  10 ,  20 , and  30 , respectively. In this embodiment of the invention, the rod has a circumferential outer portion that is formed from a metallic material (such as nitinol, titanium, stainless steel, or similar medical grade metallic material) and an inner core that is formed from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material). The rod can be straight or curved as deemed necessary or desirable to correct the particular growth deformity. The rod can be of any desired size or cross sectional shape. 
         [0053]      FIGS. 17 and 18  illustrate a fourth embodiment of one of the rods  16 ,  25 , and  36  that can be used in any of the above-described embodiments of the growth rod distraction system  10 ,  20 , and  30 , respectively. In this embodiment of the invention, the rod has a circumferential outer portion that is formed from a metallic material (such as nitinol, titanium, stainless steel, or similar medical grade metallic material), an intermediate portion that is formed from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material), and an inner core that is formed from a metallic material (such as nitinol, titanium, stainless steel, or similar medical grade metallic material). The rod can be straight or curved as deemed necessary or desirable to correct the particular growth deformity. The rod can be of any desired size or cross sectional shape. The inner core may be formed from one piece of material or may be braided from a plurality of pieces of material. 
         [0054]      FIG. 19  illustrates a fifth embodiment of one of the rods  16 ,  25 , and  36  that can be used in any of the above-described embodiments of the growth rod distraction system  10 ,  20 , and  30 , respectively. In this embodiment of the invention, the rod has a circumferential outer surface or portion having one or more serrations provided therein. The serrations can have any desired size, shape, orientation, and/or combinations thereof. The serrations allow the rod to be more flexible towards the serrated side, giving a surgeon more control of the angulation of the rod during installation. 
         [0055]      FIG. 20  is a sectional elevational view of a sixth embodiment of a rod that can be used in any of the embodiments of the growth rod distraction system in accordance with this invention. In this embodiment of the invention, the rod has a special internal rack region for use with the domino in the general manner described above. The rod and the internal rack region are both formed from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material). 
         [0056]      FIG. 21  is a sectional elevational view of a seventh embodiment of a rod that can be used in any of the embodiments of the growth rod distraction system in accordance with this invention. In this embodiment of the invention, the rod has a special internal rack region for use with the domino in the general manner described above. The rod is formed from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material), while the internal rack region is formed from a metallic material (such as nitinol or similar medical grade metallic material). 
         [0057]      FIG. 22  is a sectional elevational view of an eighth embodiment of a rod that can be used in any of the embodiments of the growth rod distraction system in accordance with this invention. In this embodiment of the invention, the rod has an internal core and a special internal rack region for use with the domino in the general manner described above. The rod is formed from a flexible polymer material (such as a PEEK, carbon fiber PEEK, PEAK, or similar medical grade polymer material), while the internal core and the internal rack region are both formed from a metallic material (such as nitinol or similar medical grade metallic material). 
         [0058]      FIGS. 23 and 24  illustrate a portion of a fourth embodiment of a growth rod distraction system, indicated generally at  40 , in accordance with this invention. In this embodiment of the invention, first and second growth rods  41  and  42  and a coupler gear  43  are supported within the domino (not shown). The ends of the first and second growth rods  41  and  42  have respective helical portions  41   a  and  42   a  that cooperate with an internal threaded surface  43   a  of the coupler gear  43 . As a result, rotation of the coupler gear  43  relative to the domino causes axial movement of the first and second growth rods  41  and  42  in a manner that is similar to that described above. The coupler gear  43  is further provided with an external toothed surface  43   b  that cooperates with a bevel gear (not shown) or similar actuator for effecting rotation of the coupler gear  43  (and, therefore, axial movement of the first and second growth rods  41  and  42 ) relative to the domino. 
         [0059]    The major mechanical failure associated with growth rods treatment is rod breakage and screw loosening. It has been found that the parameters of distraction force and distraction frequency can be manipulated to lower the rate of complication by reducing the stresses on rod and load on the screw. The use of electronics would render ease in achieving the required frequency of distraction. It could also be used to set an upper limit on the distraction forces that could results in failure stresses on rod and high loads at screw-bone interface. 
         [0060]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.