BACKGROUND

1. Technical Field

[0001] The present disclosure relates to orthopedic screw systems for bone fixation, and more particularly, to providing facet joint fixation screw systems with anti-backout features which prevent unintentional withdrawal of the screw.

2. The Relevant Technology

[0002] Loosening is a commonly encountered problem with screw fixation. A screw may work its way loose over time, such that fixation is compromised or the screw head protrudes to an undesirable extent from the surrounding material. Loosening is seen in orthopedic applications, such as facet joint fixation or facet joint fusion, at least partially because normal physiologic movement tends to encourage screw migration, and the bone into which the screw is driven tends to remodel over time. The three-dimensional topography of the bone surface presents an additional challenge in achieving secure fixation. The present disclosure provides a low-profile, self-contained, polyaxial, one-way screw and washer system that automatically and continuously resists any tendency of the screw to unthread from the surrounding material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope.

[0004] Figure 1A illustrates a top perspective view of a system including a screw and a washer assembly; Figure IB is a bottom perspective view of the system of Figure 1A;

[0005] Figure 2A illustrates a top perspective view of a washer of the washer assembly of Figure 1A; Figure 2B illustrates a bottom perspective view of the washer of Figure 2A; Figure 2C illustrates a top view of the washer of Figure 2A; Figure 2D illustrates a side cross- sectional view of the washer of Figure 2 A;

[0006] Figure 3A illustrates a top perspective view of a cap of the washer assembly of Figure 1A; Figure 3B illustrates a bottom perspective view of the cap of Figure 3 A; Figure 3C illustrates a side view of the cap of Figure 3A; Figure 3D illustrates a bottom view of the cap of Figure 3A; Figure 3E illustrates a side cross-sectional view of the cap of Figure 3A;

[0007] Figure 4A illustrates a top perspective exploded view of the system of Figure 1 A; Figure 4B illustrates a bottom perspective exploded view of the system of Figure 1 A;

[0008] Figure 5A illustrates a top view of the system of Figure 1A; Figure 5B illustrates a longitudinal cross-sectional view of the system of Figure 5A; [0009] Figure 6 illustrates a top perspective view of the system of Figure 1 A, showing the washer assembly polyaxially pivoted relative to the screw;

[0010] Figure 7A illustrates a side view of the system of Figure 1A; Figure 7B illustrates a top cross-sectional view of the system of Figure 7A with the system in an unlocked configuration in which the screw can rotate freely in a first direction relative to the washer assembly; and

[0011] Figure 8A illustrates a side view of the system of Figure 1 A; Figure 8B illustrates a top cross-sectional view of the system of Figure 8A with the system in a locked configuration in which the screw and washer assembly are frictionally locked together so that the screw is unable to rotate freely in a second direction relative to the washer assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention relates to systems and methods used in orthopedic surgery, and in particular, to facet joint fixation. Those of skill in the art will recognize that the systems and methods described herein may be readily adapted for other bone or joint fixation procedures. Those of skill in the art will also recognize that the following description is merely illustrative of the principles of the invention, which may be applied in various ways to provide many different alternative embodiments. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts in the appended claims.

[0013] An embodiment of a system includes a screw, wherein the screw includes a spherical head; a washer encircling the head, wherein the washer includes an aperture extending through the washer from a first side of the washer to a second side of the washer opposite the first side, wherein the head is positioned within the aperture, wherein the washer includes a side wall encircling the aperture, wherein the side wall includes a groove extending at least partially around the side wall, wherein the groove extends between a shallow end of the groove and a deep end of the groove; and a ball positioned between the side wall and the head, wherein the ball travels along the groove in response to rotation of the head relative to the washer, wherein the groove has a semicircular cross section complementary to the ball; wherein, when the head rotates in a first direction, the ball travels along the groove to the deep end where the ball is loose between the deep end and the head so that the head rotates freely relative to the washer; wherein, when the head rotates in a second direction opposite the first direction, the ball travels along the groove to the shallow end where the ball jams between the shallow end and the head so that the screw, washer, and ball are frictionally locked together.

[0014] In an embodiment, the aperture includes a socket, wherein the socket receives the head to form a ball-and-socket joint, wherein the side wall includes a ramp, wherein the ramp extends partially around the socket between a first end which is outwardly displaced from the socket and a second end which is tangential to the socket, wherein the groove extends along the ramp.

[0015] In an embodiment, the washer is captive to the head, wherein the aperture includes a socket, wherein the socket receives the head, wherein the side wall includes a ramp, wherein the ramp extends between a first end which is outwardly displaced from the socket and a second end which is tangential to the socket, wherein the groove extends along the ramp.

[0016] In an embodiment, the washer is captive to the head.

[0017] In an embodiment, the aperture includes a spherical socket complementary to the head, wherein the washer is polyaxially pivotable about the head.

[0018] In an embodiment, the ball is continuously biased toward the shallow end, wherein rotation of the head in the first direction temporarily overcomes the bias to urge the ball toward the deep end.

[0019] In an embodiment, when the ball jams between the shallow end and the head, the ball deforms against the head.

[0020] In an embodiment, the system includes a cap, wherein the cap includes a ring secured to the washer, wherein an inner diameter of the ring is smaller than an equatorial diameter of the head, wherein the head is interposed between the ring and the aperture so that the washer is held captive to the head.

[0021] In an embodiment, the system includes a cap, wherein the cap includes a cantilever beam, wherein the beam extends between a fixed end of the beam and a free end of the beam, wherein the fixed end is connected to the ring, wherein the free end continuously biases the ball toward the shallow end, wherein rotation of the head in the first direction deflects the beam to temporarily overcome the bias.

[0022] In an embodiment, the screw includes a shaft extending from the head through the washer, wherein the washer includes a spike pointing along the shaft away from the head.

[0023] Referring to Figures 1A and IB, top and bottom perspective views illustrate a facet fixation screw system 10, comprising a screw 20 and a washer assembly 40. The washer assembly is captive to the head of the screw. The washer assembly includes an anti-backout mechanism which allows the screw to be freely rotated in one direction relative to the washer assembly, but binds the screw to the washer assembly when the screw is rotated in the opposite direction.

[0024] Screw 20 includes a spherical head 22 and a shaft 24. The shaft 24 includes a threaded portion 26 extending along a portion of the shaft. The entire length of the shaft may be threaded, or some portion or portions thereof. The thread pitch may be constant along the threaded portion, or may vary. Preferably, the shaft 24 includes a cannulation 25 to allow placement of the screw over a guidewire, but non-cannulated embodiments may also be provided. A drive feature 28 on the head 22 is shaped to cooperate with a driver instrument to facilitate placement, polyaxial adjustment and/or rotational driving of the screw. In the embodiment shown, the drive feature has a dogbone or bowtie shape; however other drive feature shapes are possible, including but not limited to: hexagon, pentagon, square, triangular, rectangular, cross, star, or other driver shapes known in the art. The drive feature 28 may be a recess as shown; in other embodiments the drive feature 28 may protrude to cooperate with a driver instrument having a complementary recessed driving feature. The head 22 further includes a spherical bearing surface 30. The screw 20 further includes a first end 32 which may be a proximal end, and at which the head 22 is located; and a second end 34 which may be a distal end, and at which a tip 36 of the shaft 24 is located.

[0025] Washer assembly 40 includes a washer 42 and cap 44. The washer assembly may further include a ball 46 (as best seen in Figures 4A and 4B) which is captured between the washer 42, the cap 44, and the head 22 when the washer assembly is operatively assembled with the screw 20 as in Figures 1A and IB. The system 10 may be operatively assembled during manufacture and provided to the end users in the operatively assembled form, in sterile packaging.

[0026] Figures 2A-2D show washer 42 in more detail. Washer 42 is annular, and has a first end 48 and a second end 50. First end 48 terminates with a relatively thin annular lip 49 which may project slightly toward the center of the washer 42. An outer peripheral wall 52 extends exteriorly between the first end 48 and the second end 50. An inner surface 54 extends interiorly between the first end 48 and the second end 50, forming a boundary to an aperture 55. An annular spherical recess 56 occupies a portion of the inner surface 54, and may form a socket to receive screw head 22. A portion of the annular spherical recess 56 is bounded toward the first end 48 by a first shelf 60, and the remainder is bounded toward the first end by a second shelf 62, which is recessed from, and may be described as lower than, the first shelf 60. Between the first shelf 60 and the second shelf 62, a portion of the inner surface 54 forms a ramp 63. The ramp 63 extends partially around the socket, or spherical recess 56, between a first end 57 which is outwardly displaced from the socket and a second end 58 which is tangential to the socket. An alcove 64 having a partial circular cross section is recessed into the inner surface 54; at least a portion of the alcove 64 overlaps the ramp 63, as seen in Figures 2A and 2D. Alcove 64 includes a first shallow portion 65 and a second deep portion 66, the depth of the alcove and the distance of the alcove from the spherical recess 56 increasing between the first and second portions. In this embodiment, alcove 64 has a cross section comprising an arc of a circle with a diameter which is complementary to the outer diameter of ball 46. The diameter of the circle may be slightly less than the outer diameter of ball 46, so that ball 46 is supported on the edges of alcove 64. Alcove 64 may be further described as a semicircular dished face, or a groove.

[0027] At least one bore 67 may extend longitudinally through the washer between the first end 48 and the second end 50, and may, for example, provide access for cleaning during or after manufacturing. The second end 50 of the washer 42 includes a bone engagement surface 68, at least a portion of which may be compressed against bone material when the screw system is implanted. At least one spike 69 protrudes from the second end 50. The spikes 69 may penetrate bone to provide additional fixation and/or rotation prevention. In other embodiments, pegs, nails, pins, keels or other bone-penetrating features may be included in place of or in addition to spikes 69, or no bone-penetrating features may be included. The bone engagement surface 68 may be roughened or porous to promote bone ongrowth or ingrowth; bone growth or other therapeutic agents may also be provided on the bone engagement surface 68.

[0028] Referring to Figures 3A-3E, the cap 44 includes a ring 70, the ring having a first side 72 and a second side 74 with an aperture 75 extending through the ring between the first and second sides 72, 74. A beam 76, which may be a cantilever beam, projects from the second side 74. A fixed end 78 of the beam is fixed to the ring, and a free end 80 is adjacent, but unconnected to, the ring 70. A beam body 82 extends between the fixed end 78 and the free end 80, is curved to follow the curvature of the ring 70, and may be parallel to the ring 70. The beam 76 may have the same radius of curvature as the ring 70, so that it extends along and overlaps a portion of the ring 70. The aperture 75 may include a lip 77 which has a diameter smaller than the equatorial diameter of the screw head 22.

[0029] Figures 4A-4B depict screw system 10 in exploded views, and Figure 5B is a cross-sectional view of the system in an operatively assembled configuration. When system 10 is assembled, screw 20 is received through washer 42, with tip 36 oriented in the same general direction as second end 50, and head 22 toward first end 48. Screw head 22 is retained by washer 42, with spherical bearing surface 30 bearing against annular spherical recess 56. Head 22 and washer 42 may thus form a ball and socket joint, with head 22 polyaxially pivotable relative to washer 42, as seen best in Figure 6. Ball 46 is positioned inside washer 42 on second shelf 62, between head 22 and alcove 46. Cap 44 is positioned on washer 42, with a portion of ring 70 resting on first shelf 60. As best seen in Figures 7B and 8B, beam 76 extends from the second side 74 of the cap and is received in a gap 86 formed between head 22, second shelf 42, and inner surface 54 of washer 42. Free end 80 of beam 76 is adjacent alcove 64 of the washer, and ball 46 is between free end 80 and head 22. The free end 80 continuously spring biases the ball 46 toward the second end of the ramp 63. In other embodiments, ball 46 may be biased toward the second end of the ramp 63 by another kind of known spring, such as a spring clip, retaining ring, compression spring, extension spring, leaf spring, or torsion spring. In additional embodiments, ball 46 may be biased toward the second end of the ramp 63 by other known biasing means, such as magnetic bias, gravitational bias, or shape memory bias. Cap 44 is secured to washer 42 through a press-fit connection; as cap is pressed onto washer 42 in the described position, lip 49 of the washer may be deformed. In other embodiments, cap 44 may be secured to washer 42 by laser welding, a snap fit, a taper fit, a friction fit, threads, or any other known suitable connection means. It is appreciated that head 22 is polyaxially pivotable relative to washer 42 both before and after cap 44 is attached to washer 42.

[0030] Referring to Figures 7B and 8B, when screw system 10 is operatively assembled, screw 20 can rotate freely relative to washer assembly 40 in a first direction, but becomes rigidly locked to washer assembly 40 when rotated in a second direction. As shown in Figure 7B, head 22 may be freely rotated in a first direction 90, which may be clockwise. As head 22 rotates, ball 46 is urged along alcove 64 in ramp 63, from shallow portion 65 toward deep portion 66. As ball 46 encounters free end 80 of beam 76, the beam is deflected toward deep portion 66, and the spring bias of the beam is temporarily overcome, allowing free rotation of the head 22. Although in Figure 7B a small gap is shown between ball 46 and head 22, it is appreciated that ball 46 and head 22 may be in slight contact, yet head 22 is still able to freely rotate relative to washer 42.

[0031] As shown in Figure 8B, rotation of head 22 in a second direction 92, which may be counterclockwise and opposite the first direction, causes a frictional lock to form between head 22 and washer assembly 40. As head 22 is rotated in the second direction 92, ball 46 is urged along alcove 64 in ramp 63 toward shallow end 65. Ball 46 is further urged toward shallow end 65 by the spring bias of beam 76. Ball 46 becomes jammed or wedged between alcove 64 and head 22, and may be deformed against head 22 as a result of the wedging action. In the present embodiment, ball 46 becomes wedged between two points of contact along the edges of alcove 64 and a single point of contact with head 22; the single point of contact may transform into a contact patch or area due to deformation of the ball 46 against the head 22. Further rotation of head 22 in the second direction is prevented; however washer 42 may still be polyaxially pivotable relative to head 22. [0032] In one method of use, screw system 10 may be implanted across a facet joint to provide joint fixation, preventing articulation of the joint. A guide wire may be inserted across the joint, and a cannula inserted over the guidewire to the proximal facet. A cannula insertion instrument providing tissue dilation and cannula insertion, such as that disclosed in U.S. Provisional Patent Application No. 61/374862, which is incorporated by reference in its entirety into this disclosure, may be used to insert the cannula. The cannula may be temporarily docked to the proximal facet. The operatively assembled screw system 10 may be inserted through the cannula to the facet joint, with the screw first end 34 leading. A driver is used to rotate screw 20 in the first direction 90, driving screw 20 through the facets and across the joint. As screw 20 is driven, washer 42 comes in contact with the proximal facet, with at least a portion of bone engagement surface 68 in contact with the bony material of the facet. The polyaxial adjustability of the washer assembly relative to the screw allows the washer to sit at an angled position relative to the screw, which may be dictated by the surface topography of the facet and/or the surrounding environment. As screw 20 is driven further, spikes 69 may penetrate the surface of the facet, providing enhanced fixation and anti- rotation. When desired fixation of the joint and compression of the washer against the facet is achieved, rotation of screw 20 is ceased. Unintentional back-out and/or unintentional loosening of screw system 10 is prevented, as any rotation of screw 20 in second direction 92 results in locking together of screw 20 and washer assembly 40. Since washer assembly 40 is stabilized against the facet by spikes 69, screw 20 is effectively prevented from backing out or loosening.

[0033] Screw system 10 may be used in other applications in which two bone segments are fixed or compressed together to provide fixation or arthrodesis of a joint. The system may also be used in any orthopedic application in which anti-backout capabilities are desired, for example, to attach a prosthesis or implant to a bone. Non-limiting examples include attachment of articulating facet joint prostheses to vertebrae, attachment of intervertebral disk replacement prostheses, attachment of spinal rods, attachment of bone plates, and attachment of other joint prostheses, including knee, hip, shoulder, wrist, and/or ankle prostheses. Screw system 10 may also provide an anchor for anchoring of sutures, or natural or artificial tissues such as muscle, tendon, or ligament. One of skill in the art may appreciate that washer assembly 40, for example, may be modified by replacing washer 42 with an alternate component, which may include some or all of the features described above for washer 42.

[0034] The components of system 10 are preferably formed of titanium or titanium alloy. In other embodiments, system 10 or any of its component parts may comprise cobalt-chrome and its alloys, stainless-steel, titanium and its alloys, titanium carbide, titanium nitride, ion- implantation of titanium, diffusion hardened metals, diamond like coatings, diamond-like carbon, zirconium nitride, niobium, oxinium or oxidized zirconium, ceramics such as alumina and zirconia, polymers, or other biocompatible materials. Any part may comprise a combination of any of the materials listed, and the system 10 may comprise parts made of differing materials.

[0035] Any of the components disclosed herein may include surface treatments or additives in one or more of the component materials to provide beneficial effects such as antimicrobial, analgesic or anti-inflammatory properties. Any of the components disclosed herein may include coatings or treatments to provide surface roughening, including but not limited to knurling or porous coating, among others. Such treatments may be directionally applied to promote movement between component parts in one direction, and/or increase friction between component parts in another direction.

[0036] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. For example, U.S. Provisional Patent Application No. 61/265,614, which is incorporated by reference in its entirety into this disclosure, discloses additional embodiments of facet fixation screw systems. It is appreciated that various features of the above described examples and embodiments may be mixed and matched to form a variety of other combinations and alternatives. It is also appreciated that this system should not be limited simply to facet joint fixation. As such, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.