Pivotal bone anchor assembly with circumferential multi-directional increased angulation
A pivotal bone anchor assembly includes a shank with a head, an anchor portion, and a neck extending between the head and the anchor portion, and a receiver with upright arms defining a channel for receiving a rod and a central bore extending downward to a lower portion of the receiver. The assembly also includes a retaining structure rotatably coupled with the receiver with an internal seating structure pivotally supporting the head while providing for independent rotation of the shank prior to locking the assembly with the rod and a closure. The retaining structure includes an external surface and a slot extending between the seating structure and the external surface with a concave cut-out configured to closely receive the neck so as to provide for increased articulation between the shank and the receiver along the slot, and with the shank and the retaining structure being rotatable together around a centerline axis to align the increased articulation in any radial direction around the circumference of the receiver.
1. A pivotal bone anchor assembly for securing an elongate rod to a bone of a patient via a closure, the pivotal bone anchor assembly comprising:
a shank comprising a longitudinal axis, a head portion with a partial spherical outer surface, an anchor portion opposite the head portion configured for fixation to the bone, and a neck portion extending between the head portion and the anchor portion;
a receiver having a vertical centerline axis, a pair of upright arms defining a U-shaped channel configured to receive the elongate rod, and a central bore extending downward through the U-shaped channel to a lower portion of the receiver;
a retaining structure operably rotatably coupled with the receiver and having a central opening defined by an internal seating structure configured to receive and pivotally support the partial spherical outer surface of the head portion of the shank while providing for independent rotation of the shank about its longitudinal axis relative to the retaining structure prior to locking the assembly with the elongate rod and the closure, the retaining structure having a discontinuous external cylindrical surface defining an outer periphery of the retaining structure, a single slot extending along at least a bottom surface of the retaining structure from the internal seating structure through to the discontinuous external cylindrical surface, and a concave cut-out portion of the single slot configured to closely receive and constrain the neck portion of the shank so as to provide for increased angular articulation between the shank and the vertical centerline axis of the receiver in a direction of the single slot relative to the U-shaped channel of the receiver; and
an insert at least partially positionable within the U-shaped channel and having an upward-facing surface configured to engage the elongate rod and a downward-facing concave surface configured to engage the head portion of the shank, the insert having a through-bore, an upper portion with a first cylindrical outer surface extending to an uppermost top surface of the insert and defining a first outer diameter, and a lower portion with a second cylindrical outer surface extending to a bottom of the insert and defining a second outer diameter that is larger than the first outer diameter,
wherein the downward-facing concave surface of the insert further comprises a circumferentially-continuous concave surface configured to engage the partial spherical outer surface of the head portion of the shank along a circumferentially-continuous circular surface, and
wherein when the neck portion of the shank is received within the concave cut-out portion of the single slot of the retaining structure, the shank and the retaining structure are configured for circumferential rotation together around the vertical centerline axis of the receiver to align the direction of the increased angular articulation of the shank with respect to the vertical centerline axis of the receiver with any of a plurality of directions around a circumference of the receiver.
2. The pivotal bone anchor assembly of claim 1 , wherein the shank and the retaining structure are configured to be continuously rotated together 360 degrees around the axis of the receiver to align the increased angular articulation of the shank with respect to the vertical centerline axis of the receiver in any desired direction around the circumference of the receiver.
3. The pivotal bone anchor assembly of claim 1 , wherein the upper portion of the retaining structure includes a top surface rotatably engageable with a downward facing surface formed on the receiver.
4. The pivotal bone anchor assembly of claim 1 , wherein the insert is configured to be top loaded into the receiver and positioned at least partially under downward facing surfaces formed in the central bore of the receiver.
5. The pivotal bone anchor assembly of claim 1 , wherein the neck portion of the shank has a cylindrical outer surface.
6. The pivotal bone anchor assembly of claim 5 , wherein the concave cut-out portion of the single slot of the retaining structure further comprises a downwardly opening partially cylindrical surface configured to closely receive the cylindrical outer surface of the neck portion of the shank.
7. The pivotal bone anchor assembly of claim 1 , wherein the retaining structure is operably rotatably coupled to the receiver within a cavity formed in a lower portion of the central bore of the receiver.
8. The pivotal bone anchor assembly of claim 1 , wherein the retaining structure includes at least one recess formed in the retaining structure.
9. The pivotal bone anchor assembly of claim 1 , wherein the retaining structure includes at least one lateral protrusion formed in the retaining structure.
10. The pivotal bone anchor assembly of claim 1 , wherein the uppermost top surface of the insert further comprises a planar surface extending inwardly from the first outer cylindrical surface perpendicular to the vertical centerline axis of the receiver.
11. The pivotal bone anchor assembly of claim 1 , wherein the head portion of the shank includes a non-spherical upper surface surrounding an internal drive feature and configured to provide additional clearance between the head portion and the downward-facing surface of the insert during pivoting of the shank with respect to the receiver.
12. The pivotal bone anchor assembly of claim 11 , wherein the non-spherical upper surface is frusto-conical.
13. The pivotal bone anchor assembly of claim 1 and further comprising the elongate rod and the closure, wherein the closure is configured for positioning entirely within the U-shaped channel of the receiver above the elongate rod and in engagement with a discontinuous mating structure formed into interior surfaces of the upright arms, so as to apply a downward pressure toward a top of the elongate rod to secure the elongate rod to the bone of the patient.
14. The pivotal bone anchor assembly of claim 1 , wherein the retaining structure is configured to be operably rotatably coupled with the receiver with a friction fit prior to locking the pivotal bone anchor assembly with the elongate rod and the closure.
15. The pivotal bone anchor assembly of claim 1 , wherein the head portion of the shank is configured to be pivotally supported within the retaining structure with a non-floppy friction fit prior to locking the pivotal bone anchor assembly with the elongate rod and the closure.