Pivotal bone anchor assembly with bottom loaded shank and insert engaging retainer
View Patent ↗A pivotal bone anchor assembly includes a receiver defining a channel for receiving an elongate rod and a central bore with an internal engagement surface adjacent a bottom opening, as well as a shank having an anchor portion and a capture structure opposite the anchor portion positionable within the central bore through the bottom opening. The pivotal bone anchor assembly further includes a retainer configured to capture and hold the capture structure within the central bore and having an outer surface with a lower portion that is engageable the internal engagement surface of the receiver, together with a compression insert having an upper surface engageable with the elongate rod and a lower surface in overlapping engagement with an upper portion of the outer surface of the retainer, with the compression insert being spaced apart from the capture structure of the shank upon assembly of the separate components within 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 receiver having a vertical axis, a central bore centered around the vertical axis, and a pair of integral upright arms having top surfaces and opposed interior surfaces defining an upwardly-open channel configured to receive the elongate rod, the central bore communicating with a bottom surface of the receiver through a bottom opening and extending upward through the channel to the top surfaces of the upright arms, the central bore including a discontinuous mating structure integrally formed therein adjacent the top surfaces of the upright arms and configured for mating with the closure, and an internal engagement surface adjacent the bottom opening;
a shank having a longitudinal axis, an anchor portion configured for attachment to the bone, and a capture structure opposite the anchor portion configured for positioning within the central bore of the receiver through the bottom opening, the capture structure having an upper end with an internal socket formed therein configured for engagement by a driving tool;
a retainer positionable within the central bore prior to the capture structure of the shank and configured to capture and hold the capture structure within the central bore with the shank extending downward through the bottom opening, the retainer having a lower portion of an outer surface configured to engage the internal engagement surface of the central bore, the shank having pivotal movement with respect to the receiver prior to a locking of the pivotal bone anchor assembly via the closure; and
a compression insert positionable within the central bore and having an upper surface configured for engagement with the elongate rod, a lower surface configured for overlapping engagement with an upper portion of the outer surface of the retainer, and a central aperture configured to provide access to the internal socket by the driving tool,
wherein after assembly of the retainer, the compression insert, and the capture structure of the shank into the receiver, the compression insert is configured to remain spaced apart from the capture structure of the shank during pivotal movement of the shank with respect to the receiver.
2. The pivotal bone anchor assembly of claim 1 , wherein the compression insert is configured to be spaced apart from the closure when the elongate rod is positioned within the channel of the receiver and against the upper surface of the compression insert and the pivotal bone anchor assembly is locked with the closure.
3. The pivotal bone anchor assembly of claim 1 , wherein the upper surface of compression insert further comprises a U-shaped channel defined by a pair of upstanding arms, the upstanding arms having uppermost top surfaces configured to be positioned below a midpoint of the elongate rod when the elongate rod is positioned within the channel of the receiver and against the U-shaped channel of the compression insert and the pivotal bone anchor assembly is locked with the closure.
4. The pivotal bone anchor assembly of claim 1 , wherein the compression insert is configured to have a non-snap-fit engagement with the elongate rod.
5. The pivotal bone anchor assembly of claim 1 , wherein the upper end of the capture structure of the shank does not extend above a top surface of the retainer upon the capture of the capture structure by the retainer.
6. The pivotal bone anchor assembly of claim 1 , wherein the shank remains spaced apart from the internal engagement surface of the central bore of the receiver when the longitudinal axis of the shank and the vertical axis of the receiver are co-axial.
7. The pivotal bone anchor assembly of claim 1 , wherein the shank is cannulated along an entire length thereof.
8. The pivotal bone anchor assembly of claim 1 , wherein the retainer includes a slit or slot.
9. The pivotal bone anchor assembly of claim 1 , wherein the retainer further comprises a plurality of pieces configured to cooperate with the geometry of the capture structure of the shank.
10. The pivotal bone anchor assembly of claim 1 , wherein the retainer further comprises a plurality of pieces configured to be positioned between the capture structure and the internal engagement surface of the central bore.
11. The pivotal bone anchor assembly of claim 1 , wherein the retainer is configured to be top loaded into the central bore of the receiver.
12. The pivotal bone anchor assembly of claim 1 , wherein the retainer is configured to be bottom loaded into the central bore of the receiver.
13. The pivotal bone anchor assembly of claim 1 , wherein the retainer is configured to be threadably engaged with the shank upper capture structure to capture and hold the shank upper capture structure within the central bore of the receiver.
14. The pivotal bone anchor assembly of claim 1 , wherein the discontinuous mating structure of the central bore further comprises a discontinuous partial helically wound guide and advancement structure configured to resist splaying of the upright arms upon mating with the closure.
15. The pivotal bone anchor assembly of claim 14 , wherein the partial helically wound guide and advancement structure is a buttress thread.
16. The pivotal bone anchor assembly of claim 14 , wherein the partial helically wound guide and advancement structure is a square thread.
17. The pivotal bone anchor assembly of claim 14 , wherein the partial helically wound guide and advancement structure is a reverse angle thread.
18. The pivotal bone anchor assembly of claim 1 , further comprising the closure, wherein the closure comprises a cylindrical body having an outer side surface with a continuous mating structure formed therein and complementary with the mating structure.
19. The pivotal bone anchor assembly of claim 18 , wherein the cylindrical body includes a solid, closed off bottom surface configured to engage a top surface of the elongate rod.
20. The pivotal bone anchor assembly of claim 18 , wherein the top surfaces of the upright arms of the receiver are configured to remain uncovered when the pivotal bone anchor assembly is locked with the closure.