Multi-axial double locking bone screw assembly

[0001] This application claims the benefit of U.S. Provisional Application No. 60/789,459, filed Apr. 5, 2006, the disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to devices and implants used in osteosynthesis and other orthopedic surgical procedures such as devices for use in spinal surgery, and, in particular, to an posterior pedicle screw, connector/rod assembly which is implantable within a patient for stabilization of the spine. Specifically, the present invention relates to a top loading bone anchor assembly capable of achieving multiple angular axial orientations with respect to an elongated member extending along bone tissue.

[0003] Several techniques and systems have been developed for correcting and stabilizing damage or malformation of bones, especially the long bones and the spine. In one type of system, an elongated member such as a bendable rod is disposed longitudinally along a length of the bone(s). In spinal applications, the rod is preferably bent to correspond to the normal curvature of the spine in the particular region being instrumented. For example, the rod can be bent to form a normal kyphotic curvature for the thoracic region of the spine, or a lordotic curvature for the lumbar region. In accordance with such a system, the rod is engaged to various vertebrae along a length of the spinal column by way of a number of fixation elements. A variety of fixation elements can be provided which are configured to engage specific portions of the vertebra and other bones. For instance, one such fixation element is a hook which is configured to engage the laminae of the vertebra. Another very prevalent fixation element is a screw that can be threaded into various parts of the vertebrae or other bones.

[0004] In one typical spinal procedure utilizing a bendable rod, the rod is situated on opposite sides of the spine or spinous processes. A plurality of bone screws are threaded into a portion of several vertebral bodies, very frequently into the pedicles of these vertebrae. The rods are affixed to this plurality of bone screws to apply corrective and stabilizing forces to the spine.

[0005] One example of a rod-type spinal fixation system includes elongated rods and a variety of hooks, screws, and bolts, all configured to create a segmental construct throughout the spine. In one aspect of the system, the spinal rod is connected to the various vertebral fixation elements using eyebolts. In this configuration, the fixation elements are engaged to the spinal rod laterally adjacent to the rod. In another aspect of the system, a variable angle screw may be engaged to the spinal rod with an eyebolt. The variable angle screw allows pivoting of the bone screw in a single plane parallel to the plane of the spinal rod. Details of this type of system can be found in U.S. Pat. No. 5,261,909 to Sutterlin et al., the disclosure of which is incorporated by reference herein. This type of system allows a surgeon to apply vertebral fixation elements, such as a spinal hook or a bone screw, to the spine in appropriate anatomic positions and also allows the surgeon to easily engage a bent spinal rod to each of the fixation elements for final tightening.

[0006] Another rod-type fixation system provides a variety of fixation elements for engagement between an elongated rod and the spine. In one aspect of the system, the fixation elements themselves include a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to clamp the rod within the body of the fixation element. The system uses hooks and bone screws having this “open-back” configuration. Details of this type of system can be found in U.S. Pat. No. 5,005,562, the disclosure of which is incorporated by reference herein.

[0007] The fixation elements of these types of systems are capable only of pivoting about the spinal rod to achieve variable angular positions relative to the rod. While this limited range of relative angular positioning may be acceptable for many spinal pathologies, many other cases require more creative orientation of a fixation element (such as a bone screw relative) to a spinal rod. While certain aspects of this problem may be addressed by the variable angle screw of the '909 patent type of system, there remains a need for a bone screw capable of angular orientation in multiple planes relative to the spinal rod, as well as multiple spherical head orientations. Preferably, the bone screw axis is capable of various three dimensional orientations with respect to the spinal rod. Screws of this type of angular orientation in multiple planes relative to the spinal rod have been referred to as poly-axial or multi-axial bone screws. The use of both angular orientations in multiple planes relative to the spinal rod allows for virtually unlimited axial angulations of the bone engaging screw member within the design parameters as well as an ultra-low profile of the said device utilizing a minimum of components without sacrificing the security of the interfaces of the invention components.

[0008] Others have approached the solution to this problem with various poly-axial screw designs. For example, U.S. Pat. No. 5,466,237 to Byrd et al., the disclosure of which is incorporated by reference herein, describes a bone screw which includes a spherical projection on the top of the bone screw. An externally threaded receiver member supports the bone screw and a spinal rod on top of the spherical projection. An outer nut is tightened onto the receiver member to press the spinal rod against the spherical projection to accommodate various angular orientations of the bone screw relative to the rod. While this particular approach utilizes a minimum of components, the security of the fixation of the bone screw to the rod is lacking. In other words, the engagement or fixation between the small spherical projection on the bone screw and the spinal rod is readily disrupted when the instrumentation is subjected to the high loads of the spine, particularly in the lumbar region.

[0009] Another approach is shown in U.S. Pat. No. 4,946,458 to Harms et al., the disclosure of which is incorporated by reference herein. A spherical headed bone screw is supported within separate halves of a receiver member. The bottoms of the halves are held together by a retaining ring. The top of the receiver halves are compressed about the bone screw by nuts threaded onto a threaded spinal rod. In another approach taken by Harms et al., in U.S. Pat. No. 5,207,678, the disclosure of which is incorporated by reference herein, a receiver member is flexibly connected about a partially spherical head of a bone screw. Conical nuts on opposite sides of the receiver member are threaded onto a threaded rod passing through the receiver. As the conical nuts are threaded toward each other, the receiver member flexibly compresses around the head of the bone screw to clamp the bone screw in its variable angular position. One detriment of the systems in the two Harms et al. patents is that the spinal rod must be threaded in order to accept the compression nuts. It is known that threading rods can tend to weaken the rods in the face of severe spinal loads. Moreover, the design of the bone screws in these patents requires a multiplicity of parts, which makes it fairly complicated to achieve complete fixation of the bone screw.

[0010] A further approach is illustrated in U.S. Pat. No. 5,797,911 to Sherman et al., the disclosure of which is incorporated by reference herein. A U-shaped holder is provided, through the top of which a bone fastener topped with a crown member is loaded. The holder accommodates a rod in a channel above the crown member and a compression member above the rod. The compression member presses on the rod and crown member to lock the fastener against the holder in any of a number of angles in three dimensions with respect to the rod. This approach has proven to be quite effective in addressing the above-identified problems. However, it does not permit bottom-loading of the fastener. Additionally, the holder is somewhat bulky in order to accommodate the other structural components.

[0011] Yet a further approach is shown in U.S. Pat. No. 5,733,285 to Errico et al., the disclosure of which is incorporated by reference herein. In this system, a holder is provided with a tapered and colletted portion at the bottom into which a bone fastener head is inserted. A sleeve is provided that slides down around the colletted portion to crush lock the colletted portion around the head of the bone fastener. This apparatus is believed to be relatively bulky and difficult to manipulate given the external sliding locking mechanism. It is further dependent on the fit of the external sleeve and the relative strength of the collet and its bending and crushing portions for secure locking of the bone fastener head.

[0012] There is therefore a need remaining in the industry for an ultra-low profile, multi-axial/double-locking bone anchor that can be readily and securely engaged to an elongated member of any configuration—i.e., smooth, roughened, knurled or even threaded—which achieves greatly improved angulations of the bone anchor, improved strength, and reduced size, including profile and bulk, of the components used to engage the bone anchor to the elongated member in any of a variety of angular orientations.

[0013] In one illustrative embodiment, the present invention includes a top-loading multi-axial, double locking bone anchor system or assembly. A receiver member receives an internal saddle member, a bi-polar member, and a bone anchor through an open bottom, which are retained by an internal threaded ring member threaded onto the receiver member. A double-locking compression retaining member includes an external nut-like fastener and an internal setscrew-like fastener. The nut-like fastener attaches to an upper portion of the receiver member and interfaces with the internal saddle member to retain a desired angular position on the bone anchor and the setscrew-like fastener interfaces with an elongated member placed in a channel of the receiver member to retain it to the assembly. The apposing forces created by these fasteners create a locking force within the double-locking compression retaining member thus reducing the likelihood of involuntary loosening of the double-locking compression retainer member. The bone anchor is capable of multi-axial as well as multi-polar positioning with respect to the receiver member.

[0014] Additional embodiments, examples, advantages, and objects of the present invention will be apparent to those of ordinary skill in the art from the following specification.

[0039] For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein, being contemplated as would normally occur to one skilled in the art to which the invention relates.

[0040] Referring generally to FIGS. 1 and 2, there is shown one illustrative embodiment of a multi-axial/double-locking bone anchor assembly 20 in accordance with the principles of the present invention. In the illustrated embodiment, assembly 20 includes a receiver member 30, a bone anchor 50, an internal saddle member 70, a bi-polar member 101, an internal threaded ring member 90, and a retainer assembly 120. The assembly 20 of the present invention is designed for use with an elongated member R, such as a spinal rod, bar or other orthopedic construct.

[0041] Referring now generally to FIGS. 3a-3d, one embodiment of the receiver member 30 depicted in FIGS. 1 and 2 is shown in more detail. Receiver member 30 is a generally circular member having at least one sidewall 37 surrounding a central aperture 32. Sidewall 33 defines an upper portion 31a and a lower portion 31b of the receiver 30. Central aperture 32 extends through receiver member 30 from an upper aperture 33 in top end 34 of upper portion 31a to a lower aperture 35 in bottom end 36 of lower portion 31b. Central aperture 32 may include, in one specific embodiment, a chamber/void 38 defined by a chamber wall 39 in lower portion of receiver 30. The diameter of the central aperture 32 at chamber/void 38 is larger than the diameter of the central aperture 32 in upper portion 31a. It will be appreciated that in other embodiments, the upper and lower portions 31a, 31b can have a variety of configurations, such as each having one or more sections of differing diameter.

[0042] Receiver member 30 may have a chamfered or rounded edge 40a at top end 34, and may have a surrounding chamfered or rounded edge 40b at the bottom end 36. Proximate to bottom end 36, the exterior surface of the receive member 30 may include threads 41 and an associated ledge 41a around sidewall 37. In the illustrated embodiment, threads 41 extend around the entire perimeter of lower surface 32, although it will be appreciated that threads 41 could extend only partially around the perimeter of lower surface 32. Thread 41 has a thread depth A (FIG. 9) and a thread diameter B (FIG. 3a).

[0043] Receiver member 30 may include one or more pairs of upright branches 42, 43 in upper portion 31a, through which central aperture 32 extends. Branches 42, 43 may further define a generally U-shaped channel 45 transverse to opening 32 that communicates with upper portion 31a and lower portion 31b of opening 32, and that accommodates an elongated member R (FIG. 9). In one specific embodiment, external threads 44 are formed in branches 42, 43, and may be a modified acme buttress thread. The width 47 of upper portion 31a of receiver member 30 (which includes branches 42, 43) may be narrower than the width 48 of bottom portion 31b of receiver member 30, thereby reducing the bulk and profile of receiver member 30.

[0044] Referring now generally to FIGS. 4a-4c, one illustrative embodiment of a bone anchor 50 which may be used with systems in accordance with the present invention is illustrated. The illustrated bone anchor 50 is a bone screw. Bone anchor 50 includes an anchorage portion 52 and a head portion 54. As illustrated, anchorage portion 52 includes at least one thread 56, which may be a cancellous self-tapping thread, around an elongated shaft. Head portion 54 is disposed at a proximal end of the shaft and has a curvate cross-sectional shape. As illustrated, this curvate shape may form a portion of a sphere, though alternative curvate and other configurations may be employed. Head 54 may include structures for improving purchase with the surrounding members of system 20. For example, head 54 may feature a series of ridges 58 for improving purchase with the lower inside of internal saddle member 70 (described below), or may have alternative friction-increasing surface configurations, such as roughening or knurling. Further, head 54 may include a tool-engaging print 60 (not shown) with which a tool (not shown) may be engaged to drive anchorage portion 52 into a bone. Tool-engaging print 60 may be an interior print or an exterior print, and it may have any of a number of configurations, such as hexagonal, hexalobate, X-shaped, or other known torque-transferring configurations.

[0045] Other embodiments of bone anchor 50 with a suitable head 54 may be used in systems in accordance with the principles of the present invention. For example, bone anchor 50 could be a bone-engaging hook rather than a screw. In such an embodiment, anchorage portion 52 would be configured with a hook rather than an elongated section with thread 56.

[0046] Head 54 of bone anchor 50 may be shaped and sized to fit within at least interior portion 78 of internal saddle member 70 (FIG. 1 and FIGS. 5a-5d) and chamber 38 of receiver member 30. Specifically, head 54 may have a width that is smaller than the width of lower aperture 35 and chamber 38 of receiver member 30. As more fully described below, bone anchor 50 may be inserted into receiver member 30, with anchorage portion 50 entering through opening 80 and interfacing with surface 78 of internal saddle member 70 or 70A.

[0047] Referring now to FIGS. 5a-5d, there is depicted one illustrative embodiment of an internal saddle member 70 in accordance with the principles of the present invention. Internal saddle member 70 may be generally shaped as a hollow cylinder, having an exterior surface 72 with a beveled top edge 74 surrounding a central channel 76 which runs from top edge 74 to a bottom opening. The central channel 76 includes an enlarged portion 77 sized to retain at least a portion of head 54 of a bone anchor 50 near the bottom opening. The enlarged portion 77 may taper from a narrower top to a wider bottom near the bottom opening, generally mirroring the portion of the head 54 of bone anchor 50. For example, the illustrated embodiment of enlarged portion 77 has the shape of part of a sphere. The interior surface 78 of enlarged portion 77 may be provided with a friction or purchase enhancing surface configuration for cooperation with head 54 of bone anchor 50. As depicted, the interior surface 78 has ridges or steps 80 in the enlarged portion 77. In other embodiments, the surface 78 may be roughened or knurled.

[0048] In the illustrated embodiment of internal saddle member 70, a screw driving tool can access the bone anchor 50 through central channel 76, in order to drive the bone anchor 50. Internal saddle member 70 is sized and shaped to fit receiver member 30 by loading through lower aperture 35 and chamber 38. The outer dimension of the lower portion of internal saddle member 70 may be slightly smaller than the inner dimension of chamber 38 so the saddle member 70 is slidably movable within chamber 38 and opening 32. Further, in the illustrated embodiment the outer dimension of the lower potion of the internal saddle member 70 is larger than the inner dimension of central aperture 32 in the upper portion 31a, so that internal saddle member 70 cannot move into upper portion 31a. As depicted, this may be accomplished by an external shelf 79 on the outer surface of the lower portion of saddle member 70.

[0049] In the upper portion of saddle member 70, sidewall 71 defines two or more pairs of branches 72 and 73, through which central channel 76 extends. Branches 72, 73 may further define one or more generally U-shaped transverse channels 75, which run generally perpendicularly to central channel 76 to accommodate an elongated member R (FIG. 9). Upon assembly, transverse channel 75 may align with U-shaped channel 45 to allow for the top loading of the elongated member R. The interior surface of the transverse channel 75 may be provided with a friction or purchase enhancing surface configuration for cooperation with an elongated member R. As depicted, the interior surface of transverse channel 75 may have ridges or steps, or it may be roughened or knurled.

[0050] Referring now to FIGS. 8A-8C, there is shown one illustrative embodiment of bi-polar member 101 in accordance with the principles of the present invention. In the depicted embodiment, bi-polar member 101 is formed as a circular disc, having an exterior surface 102 with a top edge 104 and an interior surface 108. Interior surface 108 is configured to accommodate head 54 of bone anchor 50. Accordingly, the illustrated embodiment of interior surface 108 in FIGS. 8A and 8B has the shape of part of a sphere. It will be appreciated that in other embodiments, the shape may differ, in order to accommodate other head 54 shapes. For example, see the conical interior surface 108′ of FIG. 8C. Interior surface 108 can be provided with a friction or purchase-enhancing surface configuration (e.g. roughening or knurling) for cooperation with head 54 of bone anchor 50.

[0051] Bi-polar member 101 also includes a hole 110 faced by interior surface 108. Hole 110 is provided so that bone anchor 50 may be partially passed therethrough, allowing the bone engaging threads 56 of bone anchor 50 to be available through bi-polar member 101, while head 54 is retained therein. The dimension of hole 110 of the bi-polar member 101 is preferably slightly larger than the outer dimension of bone anchor head 54 so that the bone anchor head 54 is slidably and rotatably movable within hole 110 and bipolar member 101.

[0052] Bi-polar member 101 is sized and shaped to fit within at least lower portion 31b of central aperture 32 and chamber 38 of receiver member 30. The outer dimension of bi-polar member 101 is preferably slightly smaller than the inner dimension of chamber 38 and lower portion 31b of central aperture 32 so that bi-polar member 101 is slidably and rotatably movable within chamber 38 and central aperture 32. Further, in the illustrated embodiment, the outer dimension of bi-polar member 101 is larger than the inner dimension of upper opening portion 31a, so that bi-polar member 101 cannot move into upper portion 31a of receiver member 30.

[0053] Referring now to FIGS. 6a-6b, there is shown one embodiment of an internal threaded ring member 90 in accordance with the teachings of the present invention. In the illustrated embodiment, internal threaded ring member 90 is a generally ring-shaped member with a central aperture 94. Internal threaded ring member 90 includes a top surface 92 and a bottom surface 93. An internal surface 96 of sidewall 98 substantially surrounds central aperture 94. In a lower portion of central aperture 94, internal surface 96 forms a portion of a sphere of radius substantially identical to the radius of head 54 of bone anchor 50 near bottom opening 102 of aperture 94. At bottom opening 102 the internal surface 96 is conical and angled outward at angle 100 to allow a greater range of angular positioning of bone anchor 50. In alternative embodiments, there may be single or multiple internal angles to the internal surface 96 in the lower portion of aperture 94 near bottom opening 102, such that internal surface 96 may have portions with a cylindrical, conical, spherical or of other appropriate configuration. The diameter of bottom opening 102 is smaller than the diameter of head 54 of bone anchor 50.

[0054] In an upper portion of central aperture 94, internal surface 96 may include threads 97 allowing for attachment to counterpart threads 41 proximate the lower end of receiver member 30.

[0055] Turning to FIG. 7, there is shown one embodiment of a compression member assembly 120 for use in a system in accordance with the teachings of the present invention. In one embodiment, compression member 120, includes a set screw or plug 121 a having external threads and a print 124 for applying torque, and a nut-like cap 121b having internal threads 122 on an interior surface of an outer sidewall 123 and an inner sidewall 125 forming a threaded central socket 126 with an open bottom 127 for receiving plug 121b. Inner sidewall 125 forms a lower surface 129 adjacent open bottom 127, which may be planar. The outer surface of outer sidewall 123 may include a tool engaging print 128, such as slots for receiving a particular driver, or planar facets for interaction with a wrench. In one alternative embodiment, where receiver member 30 is internally threaded, nut-like cap 120 may be externally threaded on outer sidewall 123.

[0056] Generally referring to FIGS. 1, 2 and 9, assembly 20 may be assembled in the following manner: a bone anchor 50 may be inserted through a bi-polar member 101. An internal saddle member 70, the bone anchor 50 and bipolar member 101 may be inserted into a receiver member 30 through lower aperture 35. An internal threaded ring member 90 may then be place over bone anchor 50 and the bipolar member 101 and secured to the receiver member 30. This assembly may be performed as a series of individual steps or substantially in one step.

[0057] At this point, internal saddle member 70 remains slidably positioned in the interior surface of receiver member 30 and bi-polar member 101 and bone anchor 50 remain multi-axially moveable with respect to internal saddle member 70 and receiver member 30. Bone anchor 50, bi-polar member 101 and internal saddle member 70 are retained in receiver member 30 by internal threaded ring member 90. The head 54 of bone anchor 50 is supported between internal saddle member 70 and the bi-polar member 101, which is supported by internal surface 96 of internal threaded ring member 90.

[0058] Assembly 20 may be assembled to this point prior to use in a surgical procedure, although it will be appreciated that it may be assembled during the procedure. In using the embodiment of assembly 20, where bone anchor 50 is a bone screw, the bone anchor 50 may be threaded into an appropriately prepared hole in a bone (not shown). Threaded anchoring portion 52 may be inserted into the hole and an appropriate screwing tool used with tool-engaging print 60 to drive bone anchor 50 into the bone. It will be understood that in alternative embodiments of the invention, for example where bone anchor 50 is a bone hook, drilling a hole in bone and threading the anchor therein may not be necessary.

[0059] When bone anchor 50 is attached to a bone, receiver member 30 may be positioned so that bone anchor 50 forms a desired angle θ with the long axis of receiver member 30, as depicted in FIG. 1. The angle, θ, between bone anchor 50 and the long axis of receiver member 30 may be any value from about 0 to about 57 degrees in any direction up to a total of about 112 degrees total angulation. It will be seen that the maximum angle of bone anchor 50 relative to receiver 30 can be changed, for example by angling bone anchor 50 to its maximum in relation to the long axis of receiver member 30 and by angling bi-polar member 101 with respect to the long axis of receiver member 30. Typically, current pedicle screw system allow for up to approximately 20 to 30 degrees angulation (for a total angle of 40 to 60 degrees). In embodiments of systems in accordance with the present invention, similar angulation may be achieved by bone anchor 50 around a first spherical axis of rotation, with bi-polar member 101 adding from about 27 to about 37 additional degrees of angulation around a second spherical axis of rotation (for a total of approximately about 112 degrees total angulation).

[0060] As described above, receiver member 30 may be angled as the surgeon desires with respect to bone anchor 50. An elongated member, R such as a spinal rod, connector, or other orthopedic surgical implant, may be coupled to assembly 20. Elongated member R may be placed in U-shaped channel 45 of receiver member 30 (and transverse channel 75 of saddle member 70) and in contact with the interior surface of transverse channel 75. The nut-like cap 121b of a compression member 120 assembly may be threaded into/onto threads 44 of receiver member 30 the lower surface 129 of the inner sidewall 125 contacts the top edge of the internal saddle member 70.

[0061] As component 121b of compression member 120 is tightened, internal saddle member 70 is forced downward against bone anchor 50. Head 54 is thereby clamped between the bi-polar member 101 and the internal threaded ring member 90 and internal saddle member 70. In this way, bone anchor 50 may be locked into a desired angular position with respect to the remainder of assembly 20 (and elongated member R).

[0062] The set screw or plug 121a of a compression member 120 assembly, may be threaded into the threaded central sock 126 of the nutlike component 121b, passing through the open bottom thereof to contact the elongated member R. As component 121a of compression member 120 is tightened, elongated member R is forced downward against internal saddle member 70, which locks the elongated member R in place with respect to assembly 20 independently from the bone anchor 50.

[0063] Since each different component 121a and 121b of compression member 120 is separately tightened to compress different components, they generate apposing forces which lock the assembly, reducing the likelihood of involuntary loosening.

[0064] Preferred materials for constructing the various components of the present invention include stainless steel, titanium, and alloys containing the same. It will be recognized that any sturdy biocompatible material may be used to accomplish the osteosynthesis and other orthopedic surgical goals of the present invention.

[0065] While the present invention has been shown and described in terms of preferred embodiments thereof, it will be understood that this invention is not limited to any particular embodiment and that changes and modifications may be made without departing from the true spirit and scope of the invention as defined and desired to be protected.