Patent Publication Number: US-2023142246-A1

Title: Bone Fixation System for Spinal Stabilization

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     In general, the present invention relates to the bone fixation systems that are used to adjust and/or stabilize the vertebrae of the spine. More particularly, the present invention relates to the structure of pedicle screws and alignment rods used in bone fixation systems. 
     2. Prior Art Description 
     It is not uncommon for a person to be born with some form of a skeletal irregularity or to develop some skeletal irregularity as they age. Most irregularities are minor and do not adversely affect a person&#39;s wellbeing. However, some skeletal irregularities, especially spinal irregularities, can cause pain and can limit range of motion. These irregularities can result from trauma, age, disc degeneration, and disease. Often, the treatment for such irregularities is the repositioning and/or immobilizing of a portion of the spine. This treatment commonly involves affixing a plurality of pedicle screws to the vertebrae and interconnecting the pedicle screws with one or more elongated rods. The problem that there are differences in the size and location of vertebrae and the degree to which vertebrae need to be adjusted in each person. 
     Consider the spinal irregularities caused by scoliosis. Scoliosis may cause deviations in all three directions or planes of the spine, i.e. frontal (coronal), lateral (sagittal) and transversal (axial). The list of clinical problems associated with scoliosis is extensive. It includes alteration of normal gait associated with pelvic obliquity, distortion of abdominal and chest organs, and the associated alteration of functional capabilities of the organs. 
     For idiopathic types of scoliosis, there is no congenital anomalies of the vertebrae. Therefore, evaluation of treatment is complicated and often empirical. Historically, the treatment is the surgical correction of the spine. The principles of surgical correction include two basic steps. The first step comprises the acute correction of spinal deformity during the surgery and the insertion of a holding device. The second step comprises the solid fusion of vertebral bodies in the position of gained correction, by insertion of bone graft during the same surgical procedure. Idiopathic scoliosis is not an acute illness, and with time, vertebrae become secondarily deformed. Surgeons who treat scoliosis know about-deformed shape of scoliotic vertebrae. For correction of such scoliotic deformity, acute manual correction of deformity during surgery is used. Positioning is maintained using pedicle screws that attach holding rods or plates to the vertebral body or vertebral prominences. When a surgeon manipulates the pedicle screws, rods and plates, the degree of fine adjustment is limited. Accordingly, there are pedicle screws, rods and plates that have been designed to be finely adjusted after they are surgically implanted. In this manner, a surgeon can make fine adjustments after all the hardware is anchored in place and the effects of the hardware can be observed. 
     Traditionally, pedicle screws are used to mechanically engage the vertebrae. The pedicle screws have anchor heads that are shaped to engage a rod or plate. In the prior art, there are pedicle screws that can be adjusted in length within the body. Such prior art pedicle screws are exemplified by U.S. Pat. No. 9,089,371 to Faulhaber. These prior art pedicle screws have different sections that are internally threaded together. As a result, if one section is rotated relative to a second section, the pedicle screw will either elongate or retract. A problem associated with such prior art pedicle screws is that some of the internal threading used for length adjustment is exposed on the exterior of the pedicle screw as it is elongated. The pitch of the exposed internal threading does not match the pitch of the threading on the exterior of the pedicle screw. As a result, when the pedicle screw is rotated into bone, or removed from bone, the bore in the bone is exposed to threading having two different pitches. This causes the bone bore to become double threaded as the pedicle screw turns. This makes the pedicle screw more difficult to install and/or remove. Furthermore, the double threading causes the lesser of the threads to strip through the bone as the more dominant threading turns. This widens the bone bore and degrades the mechanical connection. Another problem associated with adjustable pedicle screws in the prior art is that the pedicle screws are difficult to adjust so that the screws engage the cortical bone on opposite sides of the vertebral body. Rather, many prior art pedicle screw anchor into the porous cancellous bone within the vertebral body, which creates far weaker bond than would connection with the stringer cortical bone. 
     In addition to adjusting the length of the pedicle screws, the engagement between the rods and the pedicle screws must also be adjustable. In the prior art, there are many rods that are machined or textured in some manner to mechanically interconnect with the head of a pedicle screw in a manner that is adjustable. Such prior art systems are exemplified by U.S. Pat. No. 8,088,149 to White. A problem associated with such adjustment systems is that the head of the pedicle screw only engages the adjustment rod when tightened. When correcting a curvature in the spine, a surgeon must reposition the vertebrae. This requires a surgeon to maintain forces on the vertebrae as the alignment rod is anchored in place. As such, a surgeon must position and maintain tension on the adjustment rod while tightening the adjustment rod to the pedicle screws. This is a complicated task. Accordingly, a need exists in the art for both an adjustable pedicle screw that better engages the cortical bone on a vertebral body. A need also exists for a pedicle screw/adjustment rod system that makes it easier to maintain the adjustment rod in tension prior to being firmly anchored into a fixed position. These needs are met by the present invention as described and claimed below. 
     SUMMARY OF THE INVENTION 
     The present invention is a bone fixation system where an alignment rod is engaged by pedicle screws. The alignment rod has a top surface upon which sections of buttress thread are formed. 
     Each pedicle screw has a threaded shaft that supports a slotted receptacle. The slotted receptacle defines a slot that is sized to receive an alignment rod. The slot has opposing walls that are internally threaded. A set screw that threads into the opposing walls, wherein the set screw can be selectively advanced into the slot to an adjustment position and a deeper locked position. 
     A locking plate is interposed within the slot between the set screw and the alignment rod. The locking plate contains protrusions that engage the sections of buttress thread on the alignment rod when the protrusions are biased against the alignment rod by the set screw. The locking plate prevents the alignment rod from moving in more than one direction through the slot when the set screw is in its adjustment position. Furthermore, the locking plate prevents the alignment rod from moving through the slot in any direction when the set screw is in its locked position. 
     The threaded shaft of the pedicle screw has an upper section and a lower section. A threaded connecting rod interconnects the upper section and the lower section. The threaded connecting rod is exposed to different degrees when the threaded shaft is adjusted in length and the lower section is turned relative to the upper section. The upper section, lower section and threaded connecting rod all have external threads of the same thread pitch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the following description of an exemplary embodiment thereof, considered in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a perspective view of a segment of a bone fixation system that utilizes adjustment rods and pedicle screws; 
         FIG.  2    is a side view of the adjustment rod shown in  FIG.  1   ; 
         FIG.  3    is a cross-sectional view of the alignment rod shown in  FIG.  2    viewed along section line  3 - 3 ; 
         FIG.  4    is an exploded view of the exemplary embodiment of  FIG.  2   ; 
         FIG.  5    is a cross-sectional view of the exemplary embodiment of  FIG.  1   ; 
         FIG.  6    is a cross-sectional view of a vertebral body being engaged by the pedicle screw of the present invention bone fixation system. 
         FIG.  7    is a cross-sectional view of an anchor head assembly of the pedicle screw in a locked position engaging an alignment rod; and 
         FIG.  8    is a cross-sectional view of an anchor head assembly of the pedicle screw in an adjustment position partially engaging an alignment rod. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Although the present invention system can be embodied in many ways, only one exemplary embodiment is illustrated. The exemplary embodiment is being shown for the purposes of explanation and description. The exemplary embodiment is selected in order to set forth one of the best modes contemplated for the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the claims. 
     Referring to  FIG.  1   , a bone fixation system  10  is shown. The bone fixation system  10  consists primarily of a specialized pedicle screw  12  and a specialized alignment rod  14 . The pedicle screw  12  has a shaft assembly  16  that terminates at one end with an anchor head assembly  18 . The anchor head assembly  18  is designed to selectively received and engage the alignment rod  14 . As will be explained, the anchor head assembly  18  works in concert with the alignment rod  14  so that the alignment rod  14  can be pulled in one direction while the anchor head assembly  18  automatically prevents the tensioned alignment rod  14  from moving back in the opposite direction. 
     Referring to  FIG.  2    and  FIG.  3    in conjunction with  FIG.  1   , it can be seen that the alignment rod  14  can be any length as needed by a surgeon. The alignment rod  14  can be either straight or curved. The alignment rod  14  preferably has a rounded profile. However, polygonal shapes can also be used. Regardless of the profile shape, the alignment rod  14  has a top surface  20  and an opposite bottom surface  22 . The bottom surface  22  is smooth and faces the shaft assembly  16  of the pedicle screw  12 . The opposite top surface  20  is machined with segments of buttress thread  24 . Each segment of buttress thread  24  has a vertical surface  26  and a sloped surface  28 , wherein all the sloped surfaces  28  slope in a common direction at the same pitch angle. The segments of buttress thread  24  extend through arc angles  21  on the top surface  20  of the alignment rod  14 . Preferably, the arc angles  21  extends between ten degrees and thirty degrees from the top dead center of the alignment rod  14 . 
     Referring to  FIG.  4   ,  FIG.  5   , in conjunction with  FIG.  1   , it can be seen that the anchor head assembly  18  of the pedicle screw  12  includes a slotted receptacle  30 . The slotted receptacle  30  has an open top  32  and two opposing walls  34 ,  36  that are spaced apart by a slot  38 . The slot  38  is accessible through the open top  32 . The slot  38  is slightly wider than the width of the alignment rod  14 . The opposing walls  34 ,  36  are internally threaded. That is, the opposing walls  34 ,  36  have threads  35  on interior surfaces that face the slot  38 . The slotted receptacle  30  has a bottom surface  40  that is contoured to match the shape of the bottom surface  22  of the alignment rod  14 . In this manner, the alignment rod  14  can be cradled in the slotted receptacle  30 . 
     A locking plate  42  is provided as part of the anchor head assembly  18 . The locking plate  42  is sized to fit into the slot  38  between the opposing walls  34 ,  36  atop the alignment rod  14 . The locking plate  42  has a top surface  41  and an underlying bottom surface  43 . Buttress protrusions  44  are disposed on the bottom surface  43 . The buttress protrusions  44  are sized and spaced to engage the buttress threads  24  on the top surface  20  of the alignment rod  14 . A set screw  46  is also provided. The set screw  46  screws into the threading  35  on the opposing walls  34 ,  36 . As the set screw  46  is tightened, the set screw  46  presses against the top surface  41  of the locking plate  42 . This biases the locking plate  42  against the alignment rod  14 . 
     Referring to  FIG.  7    and  FIG.  8    in conjunction with  FIG.  4    it can be seen that locking plate  42  is biased against the alignment rod  14  by the set screw  46 , the buttress protrusions  44  on the locking plate  42  engage the buttress threads  24  on the alignment rod  14  to some degree. If the set screw  46  is firmly tightened, then the buttress protrusions  44  on the locking plate  42  fully engage the buttress threads  24  on the alignment rod  14 . The alignment rod  14  is therefore firmly locked in place and cannot move independently in relation to the pedicle screw  12 . See  FIG.  5   . However, as can be seen from  FIG.  8   , if the set screw  46  is not firmly tightened, a gap space  48  is created between the locking plate  42  and the alignment rod  14 . If there is a gap space  48 , the locking plate  42  has the ability to move in the gap space  48 . If the gap space  48  is correctly sized, the buttress protrusions  44  on the locking plate  42  will engage the buttress threads  24  on the alignment rod  14  only if the alignment rod  14  is moved in the direction of arrow  50 . If the alignment rod  14  is moved in the opposite direction of arrow  50 , then the locking plate  42  floats atop the alignment rod  14  without engaging the alignment rod  14 . It will therefore be understood that if the gap space  48  is properly sized using the set screw  46 , then the alignment rod  14  can be pulled in one direction using one hand. This can place the alignment rod  14  in tension. Once properly positioned, the alignment rod  14  can be released and it will not move backward and release the tension. A surgeon can then lock the alignment rod  14  in place by tightening the set screw  46  to eliminate the gap space  48 . 
     Referring back to  FIG.  4    and  FIG.  5   , it can be seen that the pedicle screw  12  has a shaft assembly  16 . The shaft assembly  16  has a first upper section  52 , a second lower section  54  and a threaded connection rod  70  that are all vertically aligned. The threaded connection rod  70  is affixed to the lower section  54 . The upper section  52  has a first open end  56  and an opposite second open end  58 . The upper section  52  is hollow between the first open end  56  and the second open end  58 . As such, the upper section  52  is tubular with both an interior surface  60  and an exterior surface  62 . Both the interior surface  60  and the exterior surface  62  are threaded. As such, there are interior threads  64  and exterior threads  66 . The exterior threads  66  have a certain thread pitch. The interior threads  64  have the same thread pitch, albeit with a smaller major diameter. 
     The first open end  56  of the upper section  52  is affixed to the slotted receptacle  30 . The first open end  56  is accessible through the bottom  40  of the slotted receptacle  30 . In this manner, an Allen key or similar tool can be inserted into the interior of the upper section  52  though the bottom of the slotted receptacle  30 . 
     The second lower section  54  of the shaft assembly  16  is solid. The lower section  54  has a first end  74  and an opposite second end  76 . The threaded connection rod  70  extends from the first end  74  of the lower section  54 . The threaded connection rod  70  has a smaller diameter than does the lower section  54 . As such, there is a stepped transition  75  at the point of attachment. The threaded connection rod  70  has external threads  72 . The external threads  72  match the pitch, thread angle and diameter of the interior threads  64  on the interior surface  60  of the upper section  52 . In this manner, the upper section  52  of the shaft assembly  16  threads onto the threaded connection rod  70 . A keyed depression  80  is formed in the threaded connection rod  70 . This keyed depression  80  can be accessed through the slotted receptacle  30 . It will therefore be understood that a surgeon can advance a tool through the slotted receptacle  30  and the upper section  52 . The tool can be used to turn the threaded connection rod  70  and the lower section  54  relative the upper section  52 . As the lower section  54  turns relative to the upper section  52 , the external threads  72  on the threaded connection rod  70  engage the interior threads  64  within the upper section  52 . Accordingly, the turning of the threaded connection rod  70  and the lower section  54  will cause the lower section  54  and the upper section  52  to either move together or spread apart, depending upon the direction of rotation. As the lower section  54  and the upper section  52  move apart, the threaded connection rod  70  becomes more exposed and the overall length of the shaft assembly  16  increases. Conversely, as the lower section  54  and the upper section  52  thread together, the exposed length of the threaded connection rod  70  decreases as does the overall length of the shaft assembly  16 . 
     The lower section  54  of the shaft assembly  16  has exterior threads  82 . The exterior threads  82  match the pitch, thread angle and diameter of the exterior threads  66  on the upper section  52 . Furthermore, when the lower section  54  is fully seated into the upper section  52 , the exterior threads  82  of the lower section  54  seamlessly align with the exterior threads  66  on the upper section  52 , as though the two sections  52 ,  54  were threaded as a single piece. 
     Referring to  FIG.  6    in conjunction with  FIG.  4    and  FIG.  5   , it will be understood that to utilize the bone fixation system  10 , a surgeon sets the pedicle screw  12  into a vertebral body  55 . The length of the pedicle screw  12  can be selectively adjusted by turning the lower section  54  of the shaft assembly  16  relative to the upper section  52 . Fine adjustments can be made after initial implantation. The pedicle screw  12  is adjusted so that both the threaded upper section  52  and the threaded lower section  54  engage the cortical bone  57  on opposite sides of the vertebral body  55 . The pedicle screw  12  passes through the cancellous bone  59  in the center of the vertebral body  55 , however, most of the strength of the mechanical connection is provided by the denser cortical bone  57 . 
     As the length of the pedicel screw  12  is increased, the threaded connection rod  70  becomes increasingly exposed within the vertebral body  55 . However, the threaded connection rod  70  has the same thread pitch as is used on the exteriors of both the lower section  54  and the upper section  52 . Accordingly, all exposed threading follows a single thread path through the vertebral body  55 . Over time, bone grows against the pedicle screw  12 . By utilizing a single pitch thread across all exposed threading, the pedicle screw  12  can be removed from the bone with far less damage to the bone since all threading follows a single thread path. 
     Once a pedicle screw  12  is implanted, the slotted receptacle  30  is oriented and adjusted to the proper elevation, via length adjustments to the pedicle screw  12 . The surgeon selects the proper alignment rod  14  and sets the alignment rod  14  into the slotted receptacle  30  of the pedicle screw  12 . The locking plate  42  and set screw  46  are installed above the alignment rod  14 . The set screw  46  is advanced to an adjustment position where the locking plate  42  only engages the alignment rod  14  in one direction. The surgeon can therefore pull and tension the alignment rod  14  with one hand. Once the alignment rod  14  is properly tensioned, the alignment rod  14  can be released by the surgeon. The surgeon can then fully advance the set screw  46  form the adjustment position to a locked position, wherein the alignment rod  14  is locked in place. 
     It will be understood that the embodiment of the present invention that is illustrated and described is merely exemplary and that a person skilled in the art can make many variations to that embodiment. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.