Pivotal bone anchor with tool engagement grooves and break-off extensions
A polyaxial bone screw assembly includes a threaded shank body having an integral upper portion receivable in a receiver, the receiver having an upper channel for receiving a longitudinal connecting member and a lower cavity cooperating with a lower opening. The upper portion expands a retaining member in the receiver cavity to capture the shank upper portion in the receiver. In some embodiment either the retaining member or an insert provide for a friction fit of the shank upper portion in the receiver resulting in non-floppy placement of the shank with respect to the receiver. Some retainers and inserts have a lock-and-release feature. Final locking of the polyaxial mechanism is provided by frictional engagement between the shank upper portion and the retaining member. A pre-assembled receiver, retaining member and optional insert may be popped-on or snapped-on to the shank upper portion prior to or after implantation of the shank into a vertebra.
1. A pivotal bone anchor assembly for anchoring to patient bone and coupling with an elongated implant via a closure, the pivotal bone anchor assembly comprising:
a shank including a head and a bone engagement portion below the head;
a receiver including a central bore formed around a longitudinal axis, a front surface and a back surface opposite the front surface, the receiver coupled with, or configured to couple with, the shank head and including:
a base defining a lower cavity portion of the central bore in communication with a bottom surface of the receiver through a bottom opening, the lower cavity portion including a cylindrical sidewall surface having a first inner diameter and an inwardly curved surface between the cylindrical sidewall surface and the bottom opening having a second inner diameter less than the first inner diameter, the shank head being positionable into the lower cavity portion with the bone engagement portion extending downward through the bottom opening, the first inner diameter of the cylindrical sidewall surface being greater than an outer diameter of the shank head to allow the shank head to articulate within the lower cavity portion in an unlocked configuration;
first and second upright arms extending upward from the base, each upright arm including a cylindrical outward facing surface and an inner threaded surface;
a channel between the arms and extending transverse to the longitudinal axis between the front surface and back surface, the channel configured to receive the elongated implant, the channel configured for threaded engagement with the closure in coupling the elongated implant with the pivotal bone anchor assembly;
a break-off extension extending from each upright arm; and
a non-threaded radiused tool engagement groove extending horizontally and circumferentially entirely around the cylindrical outward facing surface of each upright arm to the front surface and the back surface of the receiver, each tool engagement groove being isolated from the central bore and located below a top surface of the respective upright arm that exists at least subsequent to the break-off extension having been removed from the respective upright arm; and
a compression insert that is inside the receiver and is compressed between the elongated implant and the shank head when the elongated implant is coupled with the pivotal bone anchor assembly via the closure.
2. The pivotal bone anchor assembly of claim 1 , wherein the threaded engagement with the closure employs a square-shaped thread.
3. The pivotal bone anchor assembly of claim 1 , wherein the threaded engagement with the closure employs a reverse angle thread.
4. The pivotal bone anchor assembly of claim 1 , wherein the tool engagement grooves are undercut.
5. The pivotal bone anchor assembly of claim 1 , wherein the shank is cannulated.
6. The pivotal bone anchor assembly of claim 1 , wherein the compression insert engages the head of the shank when the compression insert is compressed between the elongated implant and shank.
7. The pivotal bone anchor assembly of claim 1 , further comprising a retainer located in the lower cavity portion of the receiver central bore and in frictional engagement with the shank head prior to the compression insert being compressed between the elongated implant and the shank, the frictional engagement being sufficient to result in non-floppy placement of the shank with respect to the receiver while allowing for angular adjustment of the shank with respect to the receiver prior to the compression insert being compressed between the elongated implant and the shank head.
8. The pivotal bone anchor assembly of claim 7 , wherein the retainer and compression insert are spaced apart from each other in a non-contacting arrangement within the receiver central bore prior to the compression insert being compressed between the elongated implant and the shank head.
9. The pivotal bone anchor assembly of claim 7 , wherein the retainer is disposed within an internal recess of the lower cavity portion of the receiver central bore before the shank is received in the receiver.
10. The pivotal bone anchor assembly of claim 7 , wherein the retainer is resilient.
11. The pivotal bone anchor assembly of claim 10 , wherein the retainer includes a slit or slot.
12. The pivotal bone anchor assembly of claim 10 , wherein the retainer frictionally engages the head of the shank above a spherical hemisphere of the head.
13. The pivotal bone anchor assembly of claim 1 , further comprising the closure and wherein, when the elongated implant is coupled with the pivotal bone anchor assembly via the closure, the closure engages the elongated implant but does not contact the compression insert.
14. The pivotal bone anchor assembly of claim 1 , further comprising the closure and wherein, when the elongated implant is coupled with the pivotal bone anchor assembly via the closure, the closure engages both the elongated implant and the compression insert.
15. The pivotal bone anchor assembly of claim 1 , wherein the receiver has a cylindrical shape.
16. The pivotal bone anchor assembly of claim 1 , wherein the compression insert is top-loaded into the receiver.
17. The pivotal bone anchor assembly of claim 1 , wherein, when the break-off extensions have been removed from the upright arms and the compression insert is fully compressed between the elongated implant and the shank head by application of the closure against the elongated implant, each top surface and each tool engagement groove is uncovered.
18. The pivotal bone anchor assembly of claim 1 , wherein each tool engagement groove further comprises a continuous and uninterrupted downward facing surface extending to the front surface and the back surface of the receiver.
19. The pivotal bone anchor assembly of claim 1 , wherein each upright arm has an upper portion and a lower portion below the upper portion, the upper portion having a first length between the front surface and back surface, and the lower portion having a second length between the front surface and back surface, the first length being less than the second length.
20. The pivotal bone anchor assembly of claim 19 , wherein the first length extends across a top surface of each upright arm.
21. The pivotal bone anchor assembly of claim 1 , wherein the grooves have surfaces that are isolated from the receiver channel.
22. The pivotal bone anchor assembly of claim 1 , further comprising an outwardly-facing planar surface formed in the outward facing surface of each upright arm below the tool engagement groove.