Patent Publication Number: US-2012041490-A1

Title: Variable offset spine fixation system and method

Description:
BACKGROUND OF THE INVENTION 
     Minimization of surgical steps during posterior screw and rod placement during some spinal surgeries is desirable. Current top loading polyaxial pedicle screw systems, in which the bone anchor is loaded manually into the down through the top of the anchor seat just prior to the surgery or, more often, in which the combination comes preassembled, typically requires several surgical steps after an incision and access path are provided. These steps may include implantation of multiple polyaxial screw and anchor seat assemblies, insertion of a spinal rod into the multiple screw and anchor seat assemblies insertion of a locking cap into each screw and anchor seat assembly and tightening of the locking cap onto each screw and anchor seat assembly for locking the construct. Seating of the rod and insertion of the multiple locking caps can be challenging due to the anatomical placement of the bone anchors, the nature of the anatomical correction required, less than optimal rod bending, and other factors that can complicate the reduction of the rod into the anchor seats. Complicated rod reduction may result incross-threading of the locking screw, anchor seat damage and potential replacement of the pedicle screw and anchor seat assembly mid-surgery. Preassembled screw and anchor seat assemblies have a profile that reduces the surgeon&#39;s visibility during its implantation and, further, its footprint, once implanted, decreases the amount of working space available in the anatomy. Surgeons often struggle to achieve full rod reduction or anatomical correction even using bottom-loading, pop-on head type pedicle screw systems. While powerful rod reduction instruments are available for use with the rod and screw construct systems, the surgeon is often not able to completely reduce the rod into the anchor seat without pre-bending and re-bending the rod to the point at which the rod may become weakened. Further, the application of too much force during the reduction of the spinal rod may also cause one or more of the implanted bone anchors to be dislodged from the vertebral bodies in which they are implanted. It is thus desirable to develop a system and associated method that provides for a reduction in the difficulty and number of surgical steps necessary during posterior screw and rod placement during spine surgery when patient anatomy results in a complicated construct. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates generally to orthopedics. More specifically, the present invention relates to a posterior spine fixation system and method including an assembly of variable height anchor seats. 
     In a preferred embodiment of the present invention, a kit includes a plurality of bone anchors, each bone anchor having a polyaxial head and a shaft, and a plurality of anchor seat assemblies. Each anchor seat assembly includes a proximal end and a distal end, and each of the plurality of anchor seat assemblies further includes a rod receiving channel that has a channel proximal end adjacent the proximal end of the anchor seat assembly and a channel distal end terminating between the anchor seat proximal end and the anchor seat distal end. At least two of the plurality of anchor seat assemblies have different heights between the distal end of the anchor seat assembly and the channel distal end of the rod receiving channel. The kit further includes a plurality of locking caps couplable to the plurality of anchor seat assemblies adjacent the anchor seat proximal end adapted to secure a spinal rod with respect to the plurality of bone anchors within one of the rod receiving channels of the plurality of anchor seat assemblies. 
     A method includes anchoring a plurality of bone anchors into the plurality of vertebral bodies and selecting a plurality of anchor seat assemblies, where at least two of the plurality of anchor seat assemblies have different heights. The method further includes coupling the plurality of anchor seat assemblies to the plurality of bone anchors, coupling a spinal rod to the plurality of anchor seat assemblies, and locking the anchor seat assemblies to the spinal rod with a plurality of locking caps. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the preferred embodiments of the application, will be better understood when read in conjunction with the appended drawings. For the purposes of illustrating the kit of the present application, there is shown in the drawings preferred embodiments. It should be understood, however, that the application is not limited to the precise arrangements and instrumentalities shown. In the drawings: 
         FIG. 1  illustrates front elevational views of several pedicle screw assemblies of a spine fixation system, in accordance with one or more embodiments of the present application; 
         FIG. 2  illustrates a side elevational view of the spine fixation system including pedicle screw assemblies in accordance with  FIG. 1 ; 
         FIG. 3  illustrates an exploded perspective view of a spine fixation system in accordance with one or more embodiments of the present application; 
         FIG. 4  illustrates a cross-sectional view of a portion of a spine fixation system in accordance with one or more embodiments of the present application; 
         FIG. 5  illustrates a cross-section view of an anchor seat in accordance with one or more embodiments of the present application; 
         FIG. 6  illustrates a cross-section view of an anchor seat in accordance with one or more embodiments of the present application; 
         FIG. 7  illustrates a cross-section view of an anchor seat in accordance with one or more embodiments of the present application; and 
         FIG. 8  illustrates a perspective view of a spine fixation system in accordance with one or more embodiments of the present application. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain terminology is used in the following description for convenience only and is not limiting. The words “right”, “left”, “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” or “distally” and “outwardly” or “proximally” refer to directions toward and away from, respectively, the geometric center of the spine fixation system and related parts thereof. The words, “anterior”, “posterior”, “superior,” “inferior” and related words and/or phrases designate preferred positions and orientations in the human body to which reference is made and are not meant to be limiting. The terminology includes the above-listed words, derivatives thereof and words of similar import. 
     Referring to  FIGS. 1 and 2 , a spine fixation system  100  is provided that includes a plurality of bone anchors  110 A,  110 B,  110 C, a plurality of anchor seats  130 A,  130 B,  130 C, a plurality of collets  152 , and a plurality of locking caps  140 A,  140 B,  140 C. The system  100  also preferably includes at least one spinal rod  120 . The bone anchor  110 A,  110 B,  110 C, anchor seat  130 A,  130 B,  130 C, locking cap  140 A,  140 B,  140 C and collet  152  are generally considered to make-up a pedicle screw construct. 
     Each of the plurality of bone anchors  110 A,  110 B,  110 C includes a threaded shaft  126  and a partially spherical or otherwise enlarged heads  124 , as shown in  FIG. 3 . The frustaspherical head  124  assists in facilitating rotation with respect to the collet  152 . The head portion  124  includes a driver portion that receives a portion from a driver for rotating the bone anchor  110  into engagement with a patient&#39;s vertebra V ( FIGS. 2 and 8 ). The bone anchor  110  may include a reduced diameter neck portion  128  between the head  124  and the shaft  126 . One or more of the bone anchors  110 A,  110 B,  110 C may include an interior cannulation to accommodate implantation of the bone anchor  110 A,  110 B,  110 C over a guide wire. 
     Each of the plurality of anchor seats  130 A,  130 B,  130 C includes an axial bore  206  and a rod receiving channel  208  oriented transversely with respect to the axial bore  206 . The anchor seat  130 A,  130 B,  130 C preferably includes interior threading  207  for receiving the externally threaded locking cap  140 A,  140 B,  140 C, but may alternatively include external threading or a locking feature (not shown) to engage the locking cap  140 A,  140 B,  140 C. 
     The distal portion of the axial bore  206  is configured to house the collet  152 . The collet  152  optionally includes a partially spherical interior volume at a distal end for capturing and retaining the head portion  124  of the bone anchor  110 A,  110 B,  110 C and a plurality of deflectable fingers that are positioned at the distal end. The collet  152  is preferably configured to pop over the head of the bone anchor  110 A,  110 B,  110 C via the deflection of the fingers and to retain the head of the bone anchor  110 A,  110 B,  110 C therein, while permitting articulation and rotation of the anchor seat  130 A,  130 B,  130 C, collet  152  and locking cap  140 A,  140 B,  140 C with respect to the bone anchor  110 A,  110 B,  110 C. 
     The fingers  172  of the collet  152  are configured to collapse around the head  124  of the bone anchor  110 A,  110 B,  110 C and lock the articulation and rotation of the anchor seat  130 A,  130 B,  130 C with respect to the bone anchor  110 A,  110 B,  110 C as a result of a downward force applied to the top of the collet  152  that causes the exterior surface of the fingers to interact with the interior surface of the distal portion of the anchor seat portion  130 A,  130 B,  130 C and thereby crush lock the collet  152  around the head of the bone anchor  110 A,  110 B,  110 C. An outer surface of the fingers  172  includes a spherical convex surface  151  of the collet  152  that contacts an inner surface of the anchor seats  130 A, B, C. 
     The preferred collet  152  includes one or more grooves  175  on an outer surface that engage a projection or dimple  216  of the anchor seat  130 A, B, C. The collet  152  optionally floats within the axial bore  206  of the anchor seat between a loading position and a locked position. Interaction between the one or more grooves  175  and the projection or dimple  216  assists in preventing the collet  152  from moving out of the upper end of the anchor seat when in the loading position. The collet  152  optionally further includes one or more rod-locking features such that the spinal rod  120  is coupled to the collet  152 , and relative to the anchor seat. 
     The locking cap  140 A,  140 B,  140 C is preferably externally threaded, includes an instrument engagement feature and may further include an axial bore. The locking cap  140 A,  140 B,  140 C may further include a rotatably coupled saddle element  310  configured to contact the top surface of the rod  120  and freely rotate with respect to the locking cap  140 A,  140 B,  140 C. The saddle  310  further includes a recess that receives at least a portion of the rod  120 . The locking cap  140 A,  140 B,  140 C is configured to be engaged and rotated with respect to the anchor seat  130 A,  130 B,  130 C using a driver instrument. As the locking cap  140 A,  140 B,  140 C is driven down into the anchor seat  130 A,  130 B,  130 C, the bottom surface of the locking cap  140 A,  140 B,  140 C bears against the top of the rod  120  and applies a downward force to the top of the collet  152  to lock the angulation of the anchor seat  130 A,  130 B,  130 C with respect to the bone anchor  110 A,  110 B,  110 C. An intermediary wedge member may be included between the collet  152  and the rod  120  or between the collet  152  and the locking cap  140 A,  140 B,  140 C to assist in directing the downward force from the advancement of the locking cap  140 A,  140 B,  140 C to the top of the collet  152 . Alternatively, the collet  152  may include arm members or other features that extend upwardly and interact directly with the bottom of the locking cap  160 . 
     As shown in  FIGS. 1 and 2 , the spine fixation system  100  includes variable height pedicle screw assemblies wherein multiple anchor seats  130 A,  130 B,  130 C are provided that have different heights between the bottom  209  of their rod receiving channels and the distal end  204  of each anchor seat  130 A,  130 B,  130 C when the rod  120  is in a seated position. Similarly, the corresponding collets  152  positioned in the anchor seats  130 A,  130 B,  130 C between the rod-contacting surfaces of the collets  152  and the collapsible bone anchor head-grasping portion of each collet  152  have different heights. Alternately, in the case in which an intermediary wedge member is disposed within each anchor seat  130 A,  130 B,  130 C between the collet  152  and the rod  120 , the lengths of the collets  152  and/or the intermediary wedge members can vary between each anchor seat  130 A,  130 B,  130 C.  FIG. 1  illustrates three anchor seats  130 A,  130 B,  130 C having heights from near a center of the bone anchor  110 A,  110 B,  110 C to a longitudinal axis  250  of the spinal rod  120 . In an assembled configuration, the heights may be comprised of five, nine and thirteen millimeters (5 mm, 9 mm and 13 mm), as can be seen in  FIG. 1 . The pedicle screw assemblies are not limited to these heights and the kits of the spinal fixation system  100  of the present application are not limited to having three heights and the kits may include pedicle screw assemblies having nearly any number of heights and dimensions that are adaptable to the anatomy encountered in patients who receive spinal surgery such that the rod receiving channels of the constructs may be linearly aligned in a final construct, as will be described in greater detail below. The constructs can be used for a curved rod and/or the kits can be used for sagittal correction or coronal correction. 
     The pedicle screw assemblies are preferably provided to a surgeon in a kit or tray including a plurality of bone anchors  110 A,  110 B,  110 C having different lengths and/or diameters, a plurality of anchor seats  130 A,  130 B,  130 C having various heights with collets  152  therein, a plurality of locking caps  140 A,  140 B,  140 C, which are, in an option, mountable to any of the anchor seats  130 A,  130 B,  130 C and a plurality of rods  120  having various lengths and/or diameters. Following insertion of the bone anchors  110 A,  110 B,  110 C into the vertebrae V the surgeon selects the appropriate anchor seats  130 A,  130 B,  130 C to mount to the bone anchors  110 A,  110 B,  110 C, selecting the appropriate height anchor seat  130 A,  130 B,  130 C that permits alignment of the rod receiving channels of the anchor seats  130 A,  130 B,  130 C, which minimizes bending of the spinal rod  120 . That is, the heights of the anchor seats  130 A,  130 B,  130 C are selected such that the rod receiving channels of the anchors seats  130 A,  130 B,  130 C are generally linearly aligned such that bending of the spinal rod  120  is minimized. Similarly, in cases where the spinal rod  120  is further from the bone anchors  110 A,  110 B,  110 C than the height of the standard anchor seat, selection of an anchor seat  130 A,  130 B,  130 C of an appropriate height can serve to align the spinal rod  120  in the receiving channel and thus minimize, or eliminate, the need to back out the bone anchor  110 A,  110 B,  110 C (and thus avoid potential loss of bone purchase) or over-persuade the rod into the receiving channel (and thus avoid the potential pullout of the bone anchor  110 A,  110 B,  110 C). Proper alignment of the spinal rod  120  within the rod receiving channel through selection of an appropriate height anchor seat  130 A,  130 B,  130 C further reduces “false” locking, which can result in screw toggling or rod slippage. 
     Due to patient anatomy, typical pedicle screw assemblies having uniform height anchor seats do not permit linear alignment of the rod receiving channels, often in multiple level constructs, and significant spinal rod  120  bending is necessary. The ability to select various height anchor seats  130 A,  130 B,  130 C facilitates increasing or decreasing of lordosis, kyphosis, and coronal plane curves with a straight spinal rod  120 , as the curvature of the spinal rod  120  does not need to match the anatomical curve achieved. This can be especially beneficial in Minimally Invasive Spine Surgery and for correcting adult deformities. In another option, the spinal rod  120  is comprised of a curved spinal rod  120 , as shown in  FIG. 8 . 
     In operation, and in continuing reference to  FIGS. 1 and 2 , a plurality of bone anchors  110 A,  110 B,  110 C are implanted into the pedicles of a plurality of vertebral bodies. A plurality of anchor seats  110 A,  110 B,  110 C are chosen from a kit that includes at least two which have different heights between the channel distal end  209  of the rod receiving channel and the distal end  204  of the anchor seat  130 A,  130 B,  130 C. For example, the heights can be 5 mm, 9 mm, 13, mm, as shown in  FIGS. 5 ,  6 , and  7 , respectively. The plurality of anchor seats  130 A,  130 B,  130 C are popped over the heads of the plurality of bone anchors  110 A,  110 B,  110 C. In an option the heads of the bone anchors  110 A,  110 B,  110 C, can be preassembled, and/or modular and provided in a kit. The anchor seats  130 A,  130 B,  130 C are polyaxially angulated with respect to the heads of the bone anchor  110 A,  110 B,  110 C prior to or during the insertion of the spinal rod  120  through the rod receiving channels  208  of the anchor seats  130 A,  130 B,  130 C. 
     The plurality of locking caps  140 A, B, C are coupled to and rotatably advanced within the plurality of anchor seats  130 A,  130 B,  130 C to crush lock the collets  152  around the heads of the bone anchors  110 A,  110 B,  110 C and, thereby, lock the angulation of the anchor seats  130 A,  130 B,  130 C with respect to the bone anchors  110 A,  110 B,  110 C and lock the position of the spinal rod  120  with respect to the plurality of bone anchors  110 A,  110 B,  110 C. The distance between the spinal rod  120  and the heads of the bone anchors  110 A,  110 B,  110 C and, thus, the vertebral bodies to which they are implanted, differs between one or more of the spinal levels due to the surgeon&#39;s intraoperative choice of different anchor seats  130 A,  130 B,  130 C having different heights to accommodate optimal offset for a particular spinal level and, thereby, ease the difficulty of complex spinal corrective surgery and achieve full rod reduction and anatomical correction. The amount of the offset can be controlled by the selection of the height of the anchor seats  130 A,  130 B,  130 C. For example, while a standard anchor seat has an approximately 4 mm offset at 25 degrees when used with a 6 mm spinal rod  120  and a 1 mm bone anchor head, an anchor seat with a 4 mm height would have an approximately 5.8 mm offset at 25 degrees (approximately equivalent to a 36 degree offset of the standard anchor seat), and an anchor seat with a 8 mm height would have an approximately 7.5 mm offset at 25 degrees (approximately equivalent to 50 degree offset of the standard anchor seat). In another option, the anchor seat has at least a 1 mm offset. In another option, the offset is in the range of about 4-15 mm. 
     Further, due to the modularity of the bottom-loading, pop-on anchor seats  130 A,  130 B,  130 C, if the surgeon is unable to completely reduce the spinal rod  120  into a particular anchor seat  130 A,  130 B,  130 C, the surgeon can remove the particular anchor seat  130 A,  130 B,  130 C, such as a first anchor seat assembly, from its bone anchor  110 A,  110 B,  110 C, e.g., using a push-button instrument or a forceps-type grasping instrument, and replace it with an anchor seat  130 A,  130 B,  130 C, such as a second anchor seat, having an appropriate height without necessitating the removal or replacement of its corresponding bone anchor  110 A,  110 B,  110 C. The flexibility to remove and replace any particular anchor seat  130 A,  130 B,  130 C with respect to its corresponding bone anchor  110 A,  110 B,  110 C, without necessitating the removal and replacement of the corresponding bone anchor  110 A,  110 B,  110 C eliminates one of the most time consuming and risky aspects of conventional pedicle screw and rod construct implantation procedures. 
     The spinal rod  120  can be introduced through the plurality of anchor seats  130 A,  130 B,  130 C through a single small incision due to the flexibility afforded by the different heights and the bottom-loading nature of the polyaxial pedicle screw assemblies. 
     The various vertical offsets provided to each anchor seat  130 A,  130 B,  130 C of the spine fixation system  100  may increase allowable medial/lateral offset of the spinal rod  120  with respect to a bone anchor  110 A,  110 B,  110 C head. Specifically, the variable height anchor seats  130 A,  130 B,  130 C may permit constructs with less bending of the spinal rod  120  in order to increase or reduce lordosis, kyphosis, and/or coronal plane curves during spinal deformity corrections. 
     While the present invention has been described with respect to bottom-loading pop-on pedicle screw assemblies, the teachings and the advantages provided by the present invention are applicable to various other pedicle screw systems, including top-loading, preassembled, twist-on, and push-and-turn pedicle screw assemblies. Further, the pedicle screw assemblies of the present invention may include spinal hooks as opposed to bone anchors. Further, the rod receiving channels in the anchor seats, as well as the collets  152   130 A,  130 B,  130 C may be offset with respect to the longitudinal axis of the anchor seat  130 A,  130 B,  130 C. 
     The accompanying drawings that form a part hereof, show by way of illustration, and not of limitation, specific embodiments in which the subject matter may be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments may be utilized and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. This Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled. 
     Such embodiments of the inventive subject matter may be referred to herein, individually and/or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. Thus, although specific embodiments have been illustrated and described herein, it should be appreciated that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the above description. 
     The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.