Pivotal bone anchor assembly with forced downward displacement of a compression insert by a tool
A polyaxial bone screw assembly includes a threaded shank body having an integral upper portion receivable in an integral receiver, the receiver having an upper channel for receiving a longitudinal connecting member and a lower cavity cooperating with a lower opening. A down-loadable compression insert, a down-loadable friction fit split retaining ring having inner and outer tangs and an up-loadable shank upper portion cooperate to provide for pop- or snap-on assembly of the shank with the receiver either prior to or after implantation of the shank into a vertebra. The shank and receiver once assembled cannot be disassembled.
1. A pivotal bone anchor assembly configured for arrangement with a displacement tool prior to securement of an elongate rod to a bone of a patient via a closure, the pivotal bone anchor assembly comprising:
a shank comprising a head and an anchor portion opposite the head configured for fixation to the bone;
a receiver comprising a base defining an axial bore centered about a longitudinal axis and in communication with a bottom of the receiver through a bottom opening, and a pair of upright arms extending upwardly from the base to define an open channel configured for receiving the elongate rod, the axial bore extending upward through the open channel to a top of the receiver and having an interior insert-engaging surface, the receiver further including tool receiving structures configured for engagement by the displacement tool; and
a compression insert configured for positioning into an initial position in the axial bore separate from the head of the shank being received in the axial bore, with the compression insert in the initial position being constrained from vertical movement along the longitudinal axis of the receiver by surface engagements between the axial bore and the compression insert;
wherein after the head of the shank is received within the axial bore and prior to the elongate rod being securely locked in the open channel via the closure, the compression insert is configured for forced downward displacement within the axial bore by direct engagement with the displacement tool that is also engaged with the tool receiving structures of the receiver, the insert being configured for downwardly displacement from the initial position to a deployed position having an interference fit engagement against the interior insert-engaging surface of the axial bore, so as to inhibit the compression insert from moving back up within the axial bore.
2. The pivotal bone anchor assembly of claim 1 , wherein the interior insert-engaging surface of the axial bore and the compression insert in the deployed position have a non-overlapping interference fit engagement with respect to each other.
3. The pivotal bone anchor assembly of claim 1 , wherein the compression insert further comprises an upper saddle surface configured to engage the elongate rod.
4. The pivotal bone anchor assembly of claim 1 , wherein the compression insert is configured to be top-loaded into the receiver.
5. The pivotal bone anchor assembly of claim 1 , wherein the compression insert is configured to be positioned within the receiver axial bore prior to the head of the shank.
6. The pivotal bone anchor assembly of claim 1 , wherein the compression insert is configured to be non-threadably rotated into its initial position within the receiver axial bore.
7. The pivotal bone anchor assembly of claim 1 , wherein the compression insert further comprises a lateral protruding structure configured to engage the interior insert-engaging surface of the receiver in the interference fit engagement.
8. The pivotal bone anchor assembly of claim 1 , further comprising a retainer configured to be positioned within the axial bore of the receiver so as to capture and hold the head of the shank therein.
9. The pivotal bone anchor assembly of claim 8 , wherein the retainer is configured to be non-pivoting with respect to the receiver.
10. The pivotal bone anchor assembly of claim 8 , wherein the retainer includes at least one of a slit or slot extending through a thickness thereof and configured to allow for an expansion of the retainer around the head of the shank upon an uploading of the head into the axial bore through the bottom opening.
11. The pivotal bone anchor assembly of claim 10 , wherein the retainer is configured to be held in an expansion region of the axial bore prior to capturing the head of the shank.
12. The pivotal bone anchor assembly of claim 8 , wherein the retainer is configured to be positioned within the axial bore of the receiver prior to the head of the shank.
13. The pivotal bone anchor assembly of claim 1 , wherein the shank is cannulated along an entire length thereof from a distal tip of the anchor portion to a proximal end surface of the head.
14. The pivotal bone anchor assembly of claim 1 , further comprising the closure and the elongate rod, and wherein the closure is configured to engage the elongate rod but without contacting the compression insert when the elongate rod is securely locked within the pivotal bone anchor assembly via the closure.
15. The pivotal bone anchor assembly of claim 1 , further comprising the closure, and wherein the closure is configured to be mechanically engageable with the receiver to securely lock the elongate rod within the pivotal bone anchor assembly.
16. The pivotal bone anchor assembly of claim 15 , wherein the mechanical engagement between the closure and the receiver further includes rotating the closure into engagement with the receiver.
17. The pivotal bone anchor assembly of claim 15 , wherein the mechanical engagement between the closure and the receiver further comprises a threaded engagement selected from the group consisting of a square-shaped thread, a reverse angle thread, and a buttress thread.