Abstract:
This disclosure describes example an surgical fixation anchor and methods for implanting the surgical fixation anchor on the spine as part of a spinal fixation construct. The surgical fixation anchor can be coupled to a spinal rod. The coupling between the spinal rod and the anchor can be offset from an axis of the anchor. The position and orientation of the coupling to the spinal rod can also be transnationally and rotationally adjusted relative to the anchor to facilitate the coupling and/or to optimize space usage adjacent to the fixation construct.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a non-provisional application claiming the benefit of priority under 35 U.S.C. §119(e) from U.S. Provisional Application Ser. No. 61/888,480 filed on Oct. 8, 2013, the entire contents of which is hereby incorporated by reference into this disclosure as if set forth fully herein. 
    
    
     FIELD 
     The present application describes a bone anchor for use as part of a spinal fixation construct. 
     BACKGROUND 
     The spine is formed of a column of vertebra that extends between the cranium and pelvis. The three major sections of the spine are known as the cervical, thoracic and lumbar regions. There are 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae, with each of the 24 vertebrae being separated from each other by an intervertebral disc. A series of about 9 fused vertebrae extend from the lumbar region of the spine and make up the sacral and coccygeal regions of the vertebral column. 
     The main functions of the spine are to provide skeletal support and protect the spinal cord. Even slight disruptions to either the intervertebral discs or vertebrae can result in serious discomfort due to compression of nerve fibers either within the spinal cord or extending from the spinal cord. If a disruption to the spine becomes severe enough, damage to a nerve or part of the spinal cord may occur and can result in partial to total loss of bodily functions (e.g., walking, talking, breathing, etc.). Therefore, it is of great interest and concern to be able to treat and correct ailments of the spine. 
     When conservative efforts fail, treating spinal ailments very often includes a combination of spinal fusion and fixation. Generally, spinal fusion procedures involve removing some or all of an intervertebral disc, and inserting one or more intervertebral implants into the resulting disc space. Introducing the intervertebral implant serves to restore the height between adjacent vertebrae (“disc height”) and maintain the height and/or correct vertebral alignment issues until bone growth across the disc space connects the adjacent vertebral bodies. Fusions may be performed across a single level or multiple levels. 
     Often during spinal fusion procedures a posterior fixation construct is implanted to immobilize the vertebrae to be fused until the fusion is complete. The posterior fixation construct generally includes at least two bone anchors (e.g. pedicle screws, laminar hooks) connected together with a rod. Like the fusion, the fixation construct can be implanted across a single level or across multiple levels, and typically, the fixation system is positioned to at least span each level to be fused. These pedicle screw systems are very effective. However, there can also be challenges associated with them. For example, connecting the rod to each anchor can be difficult since the anchors are not necessarily coplanar. When this happens the rod must be bent to match the position of the anchor, or the entire vertebra must be moved to move the anchor to the rod. There are also instances when the position of the rod and/or the connecting portion of the anchor can obstruct access to spine in close proximity to the anchor. 
     The devices and methods described in the present application are directed at address these challenges. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a top view of a bilateral spinal fixation construct utilizing a quartet of offset bone anchor assemblies, according to an example embodiment; 
         FIG. 2  is a perspective view of an offset bone anchor assembly, according to one example embodiment; 
         FIG. 3  is an exploded view of the offset bone anchor assembly of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the offset bone anchor assembly of  FIG. 2 ; 
         FIG. 5  is a cross-sectional view of the offset bone anchor assembly of  FIG. 2 , wherein the cross-section extends through a tulip of the bone anchor assembly; 
         FIG. 6  is another perspective view of the offset bone anchor assembly of  FIG. 2 ; 
         FIG. 7  is a perspective view of a connector of the bone anchor assembly of  FIG. 2 ; 
         FIG. 8  is a perspective view of a base of the connector of  FIG. 7 ; 
         FIG. 9  is another perspective view of a base of the connector of  FIG. 7 ; 
         FIG. 10  is a perspective view of a tulip of the connector of  FIG. 7 ; 
         FIG. 11  is another perspective view of the tulip of the connector of  FIG. 7 ; 
         FIG. 12  is another perspective view of the tulip of the connector of  FIG. 7  shown without a compressible member that is situated within the tulip; 
         FIG. 13  is another perspective view of the tulip of the connector of  FIG. 7  shown without the compressible member; 
         FIG. 14  is a perspective view illustrating two of the offset bone anchor assemblies of  FIG. 2  coupled together by a laterally offset spinal rod; 
         FIG. 15  is a perspective view illustrating one of the offset bone anchor assemblies of  FIG. 2  coupled together with a polyaxial screw by a spinal rod medially offset from the offset bone anchor assembly; 
         FIGS. 16A-16E  are top view illustrations depicting certain steps of an example method for constructing the bilateral fixation construct of  FIG. 1 ; 
         FIGS. 17-18  are lateral and axial views of a vertebra depicting one example of a medialized trajectory for implanting the bone anchors according the example method of  FIGS. 16A-1E ; and 
         FIG. 19  is a perspective view illustrating one of the offset anchor assemblies of  FIG. 2  fixed to the ilium at the caudal end of a fixation construct, the cephalad anchor location being offset medially relative to the ilium anchor locations. 
         FIGS. 20-21  illustrate perspective views of another example embodiment of an offset bone anchor assembly; and 
         FIG. 22  is a front view of the bone anchor assembly of  FIGS. 20-21 . 
     
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of devices and techniques for spinal fixation are described herein. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The spinal fixation anchor and methods described herein boast a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     With reference to  FIG. 1 , a single level, bilateral spinal fixation construct utilizing a quartet of offset bone anchor assemblies  10  is depicted. More specifically, a first anchor assembly  10 A is anchored through the pedicle P 1  of lumbar vertebrae V 1  and a second anchor assembly  10 B is anchored through the contralateral pedicle P 2  of V 1 . A third anchor assembly  10 C is anchored through the pedicle P 3  of lumbar vertebra V 2  adjacent to V 1 , and a fourth anchor assembly  10 D is anchored through pedicle P 4  of V 2 . The anchor assemblies  10 A and  10 C are connected by a spinal rod  24 A and the anchor assemblies  10 B and  10 D are connected by the spinal rod  24 B in order to fix the vertebrae V 1  and V 2  in position relative to each other. The anchor assemblies  10 A- 10 D include a rod connector that is offset and adjustable with respect to position relative to the shank, in effect, de-coupling the rod position from the shank position. Accordingly, the shank can be placed in the desired location (e.g. pedicle) while the position of the rod connector and rod can be adjusted to accommodate other surgical considerations (e.g. anatomical obstructions, clearance for other implants, grafts, etc. . . . ). By way of example, in the configuration shown, the anchor assemblies  10 A- 10 D are implanted using a medialized trajectory, that is, with a starting point that is more medial to that of a standard pedicle screw starting point (e.g. the starting point may be just medial and inferior to the articulating surface of the superior facet) and angled slightly laterally such that the distal end of the shank lies lateral to the proximal end of the shank at the starting point, and preferably anchored into the cortical bone along the periphery of the vertebral body. The rod position however is offset laterally from the shank which affords a greater opportunity between the shanks for the medial placement of graft material, spinous process plates, or other fusion and fixation elements. In alternative configurations, the offset anchor  10  may be used to achieve a medial offset (e.g. for connecting a construct to the ilium ( FIG. 19 )) and/or various other anchor types (e.g. standard polyaxial screws, fixed screws, hooks, etc. . . . ) may be used at other anchor locations with one or more offset anchors  10  to achieve the desired fixation construct. 
       FIGS. 2-13  illustrate an example embodiment of the offset bone anchor assembly  10  that may be used in at least one anchor position along a fixation construct. The anchor assembly  10  includes a shank  12  configured to anchor in bone and a rod connector  14  coupleable to the shank  12  and coupleable to a fixation rod  24 . The shank  12  includes a spherical head  26 , a neck  28 , and a threaded shaft  30 . The head  26  includes an engagement recess  32  formed therein that is configured to engage with a suitable driver instrument (not shown). The neck  28  is a generally smooth (e.g. non-threaded) surface extending circumferentially around the bone anchor shank  12  and having a diameter smaller than the diameter of the head  26 . The threaded shaft  30  extends distally from the neck  28  and may be any length suitable to achieve the desired bone purchase. As pictured, the threaded shaft  30  employs a triple zoned thread pattern that is designed to maximize purchase with a medialized screw trajectory. The shank  12  may be cannulated to allow for insertion of the shank  12  over a k-wire or other guide instrument, or the shank may be non-cannulated. 
     The rod connector  14 , shown in  FIG. 7 , includes a base  18  that couples to the shank  12  and a tulip  16  that receives the rod  24 . The base  18  further includes a receptacle  34  with an aperture  36  extending axially therethrough and an arm  46  extending from the receptacle  34  perpendicularly to a longitudinal axis of the aperture  36 . The aperture  36  has an upper threaded pocket  38  and a conically tapered lower pocket  40 . A cylindrical collet  42  having a plurality of flexible fingers  44  is situated within the lower pocket  40  and a spherical pocket  45  within the collet  42  is configured to receive the spherical head  26  of the shank  12 . The anchor locking cap  22  is advancable through the threaded upper pocket  38  into engagement with the collet  42  and forces the collet  42  deeper into the lower pocket  40  such that the flexible fingers  44  are directed inwards as they advance along the conical taper  39 . Thus, when the shank head  28  is captured within the spherical pocket  45  of the collet  42  and the anchor locking cap  22  is engaged, the flexible fingers  44  collapse inwards and close about the spherical head  26 , locking the connector  14  to the shank  12  and fixing the position of the base  18  relative to the shank. Prior to locking, the spherical head  26  can rotate within the spherical pocket  45  of the collet  42  such that the base  18  is polyaxially coupled to the shank  12 . According to one example, in use the shank  12  may be anchored into the vertebra first, free of the connector  14 . Thereafter, the receptacle  34  can be advanced onto the shank  12  until the shank head  26  rests in the spherical pocket  45  of the collet  42 . 
     With reference to  FIGS. 8-9 , the arm  46  of base  18  has an upper surface  50  and a lower surface  52  which are separated by sidewalls  54  and endwall  56  such that the arm has a length L 1  between the endwall  56  and the receptacle  34 , a height H 1  between the upper surface  50  and lower surface  52 , and a width W 1  between the sidewalls  54 . The tulip  16  slidably and rotatably couples to the arm  46  and a pin  58 , which may be press fit, welded, or otherwise securely engaged in a hole  60  adjacent the endwall  56  maintains the tulip  16  on the arm  46  after assembly. 
       FIGS. 10-13  illustrate the tulip  16 . The tulip  16  includes a pedestal  62  and a pair of upstanding arms  64  extending from the pedestal and separated by a rod channel  66 . The pedestal  62  defines a passage  68  through which the arm  46  is situated. The passage  68  runs perpendicular to the rod channel  66  such that a rod captured in the tulip will lie transverse to the arm length L 1  and offset from the shank  12 . A spherical compression element  70  situated within the passage  68  includes an upper compression element  72  and a lower compression element  74 . The lower compression element has a spherical lower surface that rests in a spherical cavity  76  in the passage floor and a planar upper surface that contacts the lower surface  52  of the arm  46 . The upper compression element  72  has a spherical upper surface that rests in a spherical cavity  78  on the underside of the load ring  80  upon which the rod  24  sits and a planar lower surface that contacts the upper surface  50  of the arm  46 . The lower surface of the upper compression element  72  can slide along the upper surface  50  of the arm and the upper surface of the lower compression element  74  can slide along the lower surface  52  of the arm  46  such the compression element  70  can translate along the arm  46  to adjust the position of the tulip along the length of the arm  46  and relative to the shank  12 . The tulip  16 , via spherical cavity  76  and spherical cavity  78 , can also rotate around the spherical compression element  70  such that the tulip  16  can rotate about the arm  46 . The passage  68  through pedestal  62  includes a height H 2  that is greater than the arm height H 1  and a width W 2  that is greater than the arm width W 1  such that the rotation around the compression element  70  is permitted in all direction to a limited extent. Similarly, the passage has a length L 2  that is less than the arm length L 1  so that the tulip  16  can translation along the arm  46 . 
     The upstanding arms  64  are equipped with a guide and advancement feature  82 , such as by way of example, a helically wound flange feature disposed on the interior face of each arm  64 . The guide and advancement feature  82  mates with a complementary guide and advancement feature  84  on the rod locking cap  20  ( FIG. 3 ). The upstanding arms also include an engagement feature  85  that provides an attachment point for coupling the tulip  16  to various instruments that may be used during the procedure (e.g. a rod reduction instrument, compressor, distractor, etc. . . . ). The load ring  80  has a concave, semi-cylindrical upper surface  88  that forms a cradle to receive the spinal rod  24 . As the rod locking cap  20  engages the upstanding arms  64  via the complementary guide and advancement features  82 ,  84 , the locking cap forces the rod  24  down into the load ring  80  which in turn presses down against the upper compression element  72 . The upper compression element presses down against the arm  46  which loads the lower compression element against the pedestal. Fully engaging the rod locking cap  20  seizes the mechanism and locks the position and orientation of the tulip relative to the arm  46 , and thus too, to the shank  12 . 
     With reference to  FIG. 14-15 , the offset anchor assembly  10  described above can be utilized in any number of ways and configurations when constructing a spinal fixation construct. By way of example,  FIG. 14  depicts a pair of offset anchor assemblies  10  used at each end of a single level construct with the tulips  16  offset laterally. As previously noted this configuration may be particularly useful when applied in a medialized posterior fusion surgery wherein anchors are medialized, as will be described below, or by way of another example, for fusion of the posterior elements and spinous process fixation.  FIG. 15  depicts a construct in which a single offset anchor assembly  10  is used at the caudal end of a single level construct and the tulip offset medially. A regular polyaxial pedicle screw  90  is used at the caudal end of the construct. This configuration may be useful for example, to fix the cephalad end of the construct to the ilium where the desired anchor location lies more lateral than the spinal levels above it. This is depicted, for example in  FIG. 19 . Additionally, the offset anchor  10  may be used at any one or more anchor locations of a multilevel construct and offset medially or lateral depending on the goal to be achieved. 
     Now with reference to  FIGS. 16A-16E , a preferred method for employing anchor assembly  10  with a spinal fixation construct is described. First, after the affected spinal level is identified, a midline incision is made and the necessary anatomy, including pedicles P 1  and P 2  of vertebrae V 1  and pedicles P 3  and P 4  of vertebra V 2 , is exposed. A tissue retractor may be employed to maintain access to the exposed spinal anatomy and hold tissue out of the operative corridor. Next, and with reference to  FIG. 16A , a shank is anchored into each pedicle. For example, a first shank  12 A is anchored through pedicle P 1 , a second shank  12 B is anchored through pedicle P 2 , a third shank  12 C is anchored through pedicle P 3 , and a fourth shank  12 D is anchored through the pedicle P 4 . The shanks  12 A- 12 D are implanted along a medialized trajectory, which is depicted by way of example as trajectory T M  in  FIGS. 17-18 . The medialized trajectories between the shanks anchored in the same vertebra (e.g.  12 A- 12 B in V 1  and  12 C- 12 D in V 2 ) diverge and may also be directed slightly superiorly. Inserting screws along this trajectory generally allows for placement of a shorter screw shank than those placed along traditional trajectories because the medialized trajectory takes advantage of the anatomical location of cortical bone within the vertebral body. Once the shanks  12 A- 12 D are inserted any therapy to be performed on the spine (e.g. decompression, discectomy, interbody implant insertion, etc. . . . ) may be completed. Then, as illustrated in  FIG. 16B , the base  18 A of rod connector  14 A is coupled to the head of shank  12 A with the tulip  16 A offset laterally to the shank. The connector may be rotated about the shank head until the desired position is achieved and then the anchor locking cap  22  is engaged to lock the connector.  14 A relative to the shank  12 A. These steps are repeated for each additional rod connector  14 B- 14 D, as shown in  FIG. 16C . Coupling the rod connectors after performing the therapy on the spine allows for more space within the surgical exposure to operate on the spine and perform the desired therapy as there is less hardware present during the procedure. Turning to  FIG. 16D , once the connectors  14 A-D are coupled to their respective shanks  12 A-d, a first rod  24 A is inserted into position to connect tulips  16 A and  16 C and a second rod  24 B is inserted into position to connect tulips  16 B and  16 C. To facilitate rod capture, the tulips  16  can translate along the base arm  46  and/or rotate relative to the arm  46  to adjust the position and orientation of the tulip. With the rods  24 A- 24 B appropriately situated, the rod locking caps  20 A- 20 D are fully engaged in the tulips to complete the construct, as shown in  FIG. 16E , arresting any further movement of the tulips and fixing the position of vertebra V 1  and Vertebra V 2  relative to each other. 
       FIGS. 20-22  illustrate a second example embodiment of an offset anchor assembly  10 ′ to be utilized at least one anchor location of a spinal fixation construct. The offset connector  10 ′ includes a shank  12 ′ and connector  14 ′ that further includes a tulip  16 ′ and base  18 ′. The shank  12 ′ and base  18 ′ are identical to the shank  12  and base  18  previously described. The tulip  16 ′ is also identical to the tulip  16 , except in that the passage  68 ′  68  runs parallel to the rod channel  66 ′ such that a rod captured in the tulip  16 ′ will lie generally in line with the arm length. Under typical circumstances in this configuration the offset of the tulip becomes one of a cranial or caudal offset as opposed to the medial or lateral offset provided by anchor assembly  10 . 
     It is further contemplated that the compressible member  70  of anchor assembly  10  or compressible member  70 ′ of anchor assembly  10 ′ may be configured such that when the rod locking cap is fully engaged, the compressible member provides resistance against movement, but does arrest movement entirely. Or alternatively, the compressible member will resist all movement up to a certain force, above which the compressible member will give and some movement will be achieved. In effect, the provides for a semi-rigid connection that provides controlled motion in that tulip (and thus the rod captured within the tulip) has some freedom to translate and/or rotate relative to the arm which can accommodate natural shifting and/or realignment pressure that may occur after the construct is completed. It is believed that this ability to accommodate some degree of controlled movement, particularly with the anchor assembly  10  or  10 ′ positioned at the end of a construct, an especially a long construct, may have beneficial effects in preventing the development of adjacent segment pathology like Proximal Junctional Kyphosis (PJK) which is thought to possibly be caused by excess strain and stress on the proximal instrumented spinal segment which is then at least partially transferred to the bone structures, disc, ligaments and other soft tissues, causing a loss of normal structural integrity and mechanical properties. 
     While specific embodiments have been shown by way of example in the drawings and described herein in detail, it will be appreciated that the invention is susceptible to various modifications and alternative forms (beyond combining features disclosed herein). The description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.