Patent Publication Number: US-11653954-B2

Title: Bone anchor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/365,383 filed Nov. 30, 2016, which claims the benefit of U.S. Provisional Application Ser. No. 62/262,530, filed 3 Dec. 2015, each of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates generally to medical devices, more specifically to the field of spinal surgery and spinal fixation devices. Such devices as well as systems and methods for use therewith are described. 
     BACKGROUND 
     The spine is critical in human physiology for mobility, support, and balance. The spine protects the nerves of the spinal cord, which convey commands from the brain to the rest of the body, and convey sensory information from the nerves below the neck to the brain. Even minor spinal injuries can be debilitating to the patient, and major spinal injuries can be catastrophic. The loss of the ability to bear weight or permit flexibility can immobilize the patient. Even in less severe cases, small irregularities in the spine can put pressure on the nerves connected to the spinal cord, causing devastating pain and loss of coordination. 
     Surgical procedures on the spine often include the immobilization of two or more vertebra. Immobilizing the vertebrae may be accomplished in many ways (e.g. fixation plates and pedicle screw systems). One of the most common methods for achieving the desired immobilization is through the application of bone anchors (most often introduced into the pedicles associated with the respective vertebra to be fixed) that are then connected by rigid rods locked to each pedicle screw. These pedicle screw systems are very effective. Pedicle screws generally include an anchor component and a rod-housing component (or “tulip”) that is often coupled to the anchor component in a manner that permits angular adjustability of the tulip relative to the anchor component in one or more planes. Once the pedicle screws are implanted in the desired positions a spinal rod is seated in each tulip and locked in position. The angular adjustability of the tulips is also locked, either through the locking of the rod, or independently thereof, to thus fix the connected vertebrae relative to each other. Pedicle screw configurations which allow increased angulation of the housing component in one direction are useful in certain situations where an increased pivot angle is needed (e.g. where there is an acute angle between the anchor component and rod trajectories, such as occurs, for example, with S2-Alar screws). However, configurations that permit the increased angulation also tend to reduce the strength of the connection between the anchor component and rod-housing component. Therefore a need exists for new and improved anchors with increased angulation housings but without the reduction in connection strength suffered in current solutions. 
     SUMMARY 
     The needs described above, as well as others, are addressed by embodiments of a bone anchor described in this disclosure (although it is to be understood that not all needs described above will necessarily be addressed by any one embodiment). 
     A bone anchor with increased range of angulation is provided having a rod-housing connected to a bone fastener. The bone fastener includes a head and a bone engagement feature (such as screw threads) that serve as a means to fasten the anchor to a bone structure (e.g., a pedicle). The rod housing connects the bone anchor to a spinal fixation rod that may in turn be connected to one or more additional bone anchors on other bone structures. The rod housing has a longitudinal axis that is adjustable through a range of angles (including 0. degree.) to the longitudinal axis of the bone fastener. The degree of angulation achievable may vary depending on the direction of angulation. This may be facilitated in part by the use of a capture ring with an oblique orientation relative to the longitudinal axis that contains a head section of the bone fastener on this distal side. 
     A general embodiment of the bone anchor comprises a rod housing including a base, a pair of upright arms extending from the base to an upper proximal end, and a longitudinal axis extending though a distal opening in the base and a proximal opening in the upper proximal end, the pair of upright arms separated by a rod channel, and the base including an internal groove oriented at an angle oblique to the longitudinal axis; a capture ring situated within the groove and oriented at an angle oblique to the longitudinal axis, the capture ring having a capture surface; and a bone fastener extending through the distal opening in the rod housing and including a fastener head and a bone engagement feature, the head situated within the base and having a surface that mates with the capture surface of the capture ring to maintain a connection between the bone fastener and the rod housing. 
     The above presents a simplified summary in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview. It is not intended to identify key or critical elements or to delineate the scope of the claimed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1   . A perspective view of an embodiment of the bone anchor. 
         FIG.  2   . A side view of the rod housing shown in  FIG.  1   . The maximum deflection angle (.alpha..sub.M) of the bone fastener relative to the longitudinal axis of the rod housing (L) is shown. 
         FIG.  3   . A side view of the rod housing shown in  FIG.  1   . The nominal deflection angle (.alpha..sub.N) of the bone fastener relative to the longitudinal axis of the rod housing (L) is shown. 
         FIG.  4   . A cross-sectional view of the embodiment of the bone anchor shown in  FIG.  1   . 
         FIG.  5   . A front view of the rod housing shown in  FIG.  1   . 
         FIG.  6   . A top view of the rod housing shown in  FIG.  1   . 
         FIG.  7   . A perspective view of the rod housing shown in  FIG.  1   , without the bone fastener shown, revealing detail of the recess on the inner wall of the rod housing. 
         FIG.  8   . A perspective view of an embodiment of the capture ring. 
         FIG.  9   . A perspective view of an embodiment of the load ring. 
         FIG.  10   . A side cross-sectional view of the embodiment of the rod housing shown in  FIG.  1   , with the bone fastener and rod omitted. 
         FIG.  11   . A front cross-sectional view of the embodiment of the rod housing shown in  FIG.  1   , with the bone fastener and rod omitted. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments of a spinal fixation anchor are described below. 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 a 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 anchor assembly disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
     A bone anchor  2  is provided having a rod housing  16  connected to a bone fastener  58 . The bone fastener  58  includes a shank  4  with a bone engagement feature  64  (such as screw threads) that serve as a means to fasten the anchor  2  to a bone structure (e.g., a pedicle). The rod housing  16  connects the bone anchor  2  to a spinal fixation rod  58  that may in turn be connected to one or more additional bone anchors  2  on other bone structures. The rod housing  16  has a longitudinal axis that is adjustable through a range of angles (including 0. degree.) to the longitudinal axis L of the bone fastener  58 . The degree of angulation achievable may vary depending on the direction of angulation. This may be facilitated in part by the use of a capture ring  42  with an oblique orientation relative to the longitudinal axis L that constrains the translocation of a head section  10  of the bone fastener  58  on the distal direction. 
     The rod housing  16  comprises a base  18 , a pair of upright arms  20  extending from the base  18  to an upper proximal end, and a longitudinal axis L extending though a distal opening  36  in the base  18  and a proximal opening  104  in the upper proximal end. The rod channel  22  runs between the pair of upright arms  20 . When in use, the rod channel  22  contains the spinal fixation rod  58 ; therefore the rod channel  22  may be dimensioned to accommodate a spinal fixation rod  58  (the rod may be of any suitable dimensions known in the art). The rod channel  22  may take a variety of shapes, and in some embodiments has a U-shaped profile that is complementary to a cylindrical rod  58  on the proximal side (i.e., the bottom portion of the “U”). Some embodiments of the rod channel  22  are open at the proximal end in order to allow the rod  58  to be emplaced from the proximal direction, although other configurations are possible. As shown in  FIGS.  1 - 3    a locking element  68  (such as a locking cap  28  as illustrated) may be placed proximal to the rod channel  22 , to constrain the rod  58  from proximal displacement. 
     The upright arms  20  extend on either side of the rod channel  22 . The arms  20  may include attachment features  24  for coupling to various tools useful during implantation of the bone anchor  2  and associated fixation construct (e.g., inserters, reducers, and other such tools as are known in the art). In the specific embodiment shown in  FIGS.  1 - 5  and  10 - 11   , the attachment features  24  comprise a circumferential slot  72  on both arms  22  just below the proximal end of each arm  22 , and an indentation  74  that meets the distal side of the circumferential slot  72 . Together the circumferential slot  72  and the indentation  74  allow a tool to connect to the bone anchor  2  in such a way that the tool will neither translate longitudinally nor rotate circumferentially while attached. Other configurations of course may be used. 
     The upright arms  20  may comprise a locking element engagement feature  76  that cooperates with a locking element  68  to capture and lock a rod  58  in the rod channel  22 . In a certain embodiment of the bone anchor  2 , the upright arms  20  comprise helical guide features  78  that cooperate with complementary helical guide features  80  of a locking cap  28 . Alternatively, the upright arms  20  could have internal helical flanges  82  that cooperate with the threads of a locking cap  28 . The locking element engagement feature  76  is configured such that the locking element  68  functions to exert force with a distal vector on the rod  58 , providing a means to reduce the rod  58  and seat it in the channel. In the illustrated embodiment the locking cap  28  has a driver engagement feature  14  to allow a driving tool to engage and to drive the cap. In the particular illustrated embodiment the driver engagement feature  14  is a hexalobular internal feature. 
     The base  18  functions to mate with one end of the bone fastener  58  and comprises an internal groove  38  oriented at an angle oblique to the longitudinal axis L. As will become apparent in the discussion below, the obliqueness of the internal groove  38  allows the bone fastener  58  to deflect over a wider angular range than would otherwise be possible. In some embodiments of the base  18  the distal opening  36  is defined by an internal wall portion  92  of the base  18 . The geometry of the base can be varied to allow more or less deflection of the bone fastener  58  relative to the base&#39;s  18  longitudinal axis. One example of such useful geometry is shown in  FIG.  7   . It takes the form of a recess  40  that is formed in a bottom surface of the base. The recess  40  is formed on one side of the base, as in  FIG.  7   , to align with a high side of the angled capture ring. A recess  40  on one side has the advantage of allowing an increase in deflection in a specific direction while only minimally detracting from the strength of the rod housing  16 . In some embodiments the recess  40  may be scalloped to complement a shank  4  or neck  8  of the bone fastener  58  that may be narrower in diameter than the distal opening  36  itself. The recess  40  will generally form an angle with the longitudinal axis L that is at least as great as the angle of the internal groove  38 . In some embodiments the recess  40  forms an angle with the longitudinal axis L that is greater than the angle of the internal groove  38 . The greater the angle formed between the longitudinal axis L and the recess  40 , the more freedom to deflect the bone fastener  58  will have. Absent such a recess  40 , the maximum angle formed between the longitudinal axis L and the bone fastener  58  (.alpha..sub.M) will be limited. 
     The housing  16  includes an internal ring groove  38  oriented at an angle (.alpha..sub.R) oblique to the longitudinal axis L. The ring groove  38  is oriented at an angle .alpha..sub.R relative to the longitudinal axis L to shift the nominal (.alpha..sub.N) and maximum angle (.alpha..sub.M) of the shank  4  relative to one side of the housing  16 . In the example shown, the angle .alpha..sub.R is approximately 12′, the angle.alpha..sub.M is approximately 50. degree., and the angle.alpha..sub.N is approximately 10′. The surface of the internal ring groove  38  will generally complement the shape of the capture ring  42 . For example, in embodiments in which the capture ring  42  has a frusto-spherical external contour  46 , the surface of the internal groove  38  will have at least a portion that is complementary to the frusto-spherical external contour  46 . As used herein, the prefix “frusto” denotes a frustum of a specified shape and “frustum” means part of a solid (such as a cone or sphere) intersected between two planes that are either parallel or roughly parallel. For example, a frustoconical solid is a frustum of a cone, and a frusto-spherical solid is a frustum of a sphere. 
     The capture ring  42  itself sits in the internal groove  38  and serves to prevent displacement of the fastener head  58  from the rod housing  16  in the distal direction. Some embodiments of the capture ring  42  contain the fastener head  58  by virtue of having a minimum diameter that is less than a maximum diameter of the fastener head  58 . In the illustrated example in  FIG.  4   , the fastener head  58  has a uniform diameter around the equator of the frustum of the sphere that is greater than the internal diameter of the capture ring  42 . The capture ring  42  may also have an external contour  46  that allows it to articulate relative to the rod housing  16 . For example, a capture ring  42  with a frustoconical external contour  46  can rotate around its own center within the groove  38 . As another example, a ring with a frusto-spherical external contour  46  can rotate around its own center and deflect relative to the rod housing  16 . A specific embodiment of the capture ring  42  has a frusto-spherical outer contour  46  and a frusto-spherical inner contour  44 . The capture ring  42  may have additional features to facilitate easy installation. One such feature is a slit  48  through the ring that allows it to be compressed for easy insertion into the groove  38 . The slit  48  will generally penetrate entirely through one section of the ring  42 . 
     The bone anchor  2  may further comprise a load ring  50  positioned within the base distal to the rod channel  22  and proximal to the capture ring  42 . The load ring  50  is positioned to exert compressing force on the fastener head  10  with a distal vector when the load ring  50  receives compressing force with a distal vector. In an exemplary embodiment of the anchor comprising the load ring  50 , the locking element  68  exerts a compressive force with a distal vector on the spinal rod  60  when the locking element  68  is tightened using the helical guides on the upright arms  20 , which in turn exerts compressive force with a distal vector on the load-ring  50 . The upper (proximal) surface  60  of the load ring  50  may be shaped to prevent it from interfering with the rod  58 . In the example shown in  FIG.  9   , the proximal surface  52  of the load ring  50  has two dips opposite one another to provide space for the rod  58 . The load ring  50  in turn exerts compressing force against the head  10  of the bone fastener  58 , which locks the bone fastener  58  in a desired orientation. Prior to the exertion of such compressing force by the load ring  50  on the bone fastener head  10 , the bone fastener&#39;s  58  orientation may be changed by rotation, deflection, or both. 
     The bone fastener  58  has a fastener head  10  and a bone engagement feature  64 . The fastener head  10  has a surface that contacts a surface on the capture ring  42 . In the embodiment illustrated in the figures the distal surface  12  of the head  10  is complementary to the internal contours  44  of the capture ring  42 , both of which are frusto-spherical. The fastener head  10  may include a driver engagement feature  14 , similar to the driver engagement feature  76  that may be found on the locking element  68 . The driver engagement feature  14  functions to receive a driving tool, such as a screwdriver, to rotate or otherwise drive the fastener into the bone. In the embodiment illustrated in  FIG.  4    the fastener engagement feature  64  in the fastener head  10  is a hexalobular internal feature, but any known fastener engagement feature  64  could be used (e.g., flathead, hex-head, and Phillips engagement features). 
     The bone engagement feature  64  secures the fastener  58  to the bone. An example of such a feature is a shank  4  comprising one or more screw-threads. The shank  4  may have a threaded point to facilitate attachment. The shank&#39;s  4  diameter is independent of the head&#39;s  10  diameter, which is made possible by the use of the narrow capture ring  42  to retain the head  10 . Some embodiments of the shank  4  have a diameter that exceeds that of the head  10 . In other embodiments the diameter of the shank  4  is equal to or less than that of the head  10 . 
     The bone anchor  2  may be constructed of any suitable materials, including biocompatible materials. Some embodiments of the bone anchor  2  are constructed of non-absorbable biocompatible materials. Specific examples of such suitable materials include titanium, alloys of titanium, steel, and stainless steel. Parts of the bone anchor  2  could conceivably be made from non-metallic biocompatible materials, which include aluminum oxide, calcium oxide, calcium phosphate, hydroxyapatite, zirconium oxide, and polymers such as polypropylene. 
     It is to be understood that any given elements of the disclosed embodiments of the invention may be embodied in a single structure, a single step, a single substance, or the like. Similarly, a given element of the disclosed embodiment may be embodied in multiple structures, steps, substances, or the like. 
     The foregoing description illustrates and describes the processes, machines, manufactures, compositions of matter, and other teachings of the present disclosure. Additionally, the disclosure shows and describes only certain embodiments of the processes, machines, manufactures, compositions of matter, and other teachings disclosed, but, as mentioned above, it is to be understood that the teachings of the present disclosure are capable of use in various other combinations, modifications, and environments and are capable of changes or modifications within the scope of the teachings as expressed herein, commensurate with the skill and/or knowledge of a person having ordinary skill in the relevant art. The embodiments described hereinabove are further intended to explain certain best modes known of practicing the processes, machines, manufactures, compositions of matter, and other teachings of the present disclosure and to enable others skilled in the art to utilize the teachings of the present disclosure in such, or other, embodiments and with the various modifications required by the particular applications or uses. Accordingly, the processes, machines, manufactures, compositions of matter, and other teachings of the present disclosure are not intended to limit the exact embodiments and examples disclosed herein. Any section headings herein are provided only for consistency with the suggestions of 37 C.F.R. .sctn. 1.77 or otherwise to provide organizational queues. These headings shall not limit or characterize the invention(s) set forth herein.