Pivotal bone anchor assembly with twist-in-place insert having radially offset receiver engaging structures

1. A pivotal bone anchor assembly for securing an elongate rod to a bone via a closure top, the pivotal bone anchor assembly comprising:

a receiver having a longitudinal axis, a base, and an integral upper portion defining a channel extending transverse to the longitudinal axis configured to receive the elongate rod, the base including a cavity in communication with a bottom surface of the base through a bottom opening and having a spherical seating surface proximate the bottom opening, the receiver including an axial bore centered about the longitudinal axis and extending from the bottom opening to a top of the integral upper portion, the axial bore defined in part by the bottom opening, an inner cylindrical surface in the cavity, and an at least partially cylindrical inner surface in the channel having a discontinuous helically wound thread formed therein, the axial bore including a structure with a downwardly-facing surface positioned beneath the discontinuous helically wound thread;

a shank having a proximal at least partially spherical shaped head and an integral anchor portion extending distally from the head for fixation to the bone, the head being positionable within the receiver cavity with the anchor portion extending downwardly through the bottom opening, the head including an internal drive structure;

a pressure insert having a longitudinal axis and an upwardly-facing seating surface configured to receive at least an underside portion of the elongate rod, a central opening for a tool to pass through to engage the shank head internal drive structure, a lower surface configured to directly engage the shank head to apply downward pressure thereto, and at least one structure with an upwardly-facing planar surface perpendicular to the insert longitudinal axis and positioned radially outward from the seating surface, the pressure insert being installed into a first position within the receiver channel,

wherein upon rotation of the pressure insert about the receiver longitudinal axis into a second position within the receiver channel, the insert at least one structure with the upwardly-facing planar surface is rotated under the receiver downwardly-facing surface so as to inhibit upward movement of the pressure insert within the receiver channel along the receiver longitudinal axis.

2. The pivotal bone anchor assembly of claim 1 , wherein at least one receiver inwardly-protruding integral structure is formed into the receiver channel to engage the pressure insert after rotation into the second position so as to inhibit further rotation of the pressure insert in the receiver channel.

3. The pivotal bone anchor assembly of claim 2 , wherein the at least one receiver inwardly-protruding integral structure further comprises a pair of opposed inwardly-protruding integral structures formed into opposite sides of the receiver channel.

4. The pivotal bone anchor assembly of claim 2 , wherein the at least one receiver inwardly-protruding integral structure further comprises a resilient spring tab.

5. The pivotal bone anchor assembly of claim 1 , wherein the insert at least one structure with the upwardly-facing planar surface positioned radially outward from the insert seating surface is disposed in the second position so that the at least one upwardly-facing planar surface is entirely below a top side surface of the rod positioned on the insert seating surface.

6. The pivotal bone anchor assembly of claim 1 , wherein the insert seating surface is further defined and flanked by a pair of upright arms.

7. The pivotal bone anchor assembly of claim 6 , wherein each of the insert upright arms has a top surface defining an upwardly-facing planar surface perpendicular to the insert longitudinal axis when the pressure insert is installed within the receiver channel.

8. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert has a top surface that is spaced apart from a bottom surface of the closure top when the closure top is fully engaged with the receiver channel helically wound thread to secure the elongate rod within the channel of the receiver.

9. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert further comprises a pair of oppositely spaced flanges projecting radially outward, each flange having a top surface defining an upwardly-facing planar surface and extending, upon rotation of the pressure insert into the second position, under the receiver downwardly-facing surface configured to at least partially overlap and be engageable with the flange upwardly-facing planar surface.

10. The pivotal bone anchor assembly of claim 1 , wherein the insert central opening is configured to allow the tool to pass therethrough so that the tool engages the internal drive structure formed on the shank head when the shank is co-axially aligned with the receiver longitudinal axis.

11. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert is top loaded into the receiver channel when installed in the first position.

12. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert further comprises oppositely spaced apart notches that are configured, upon rotation of the pressure insert into the second position, to engage opposed integral structures protruding inwardly toward the receiver longitudinal axis to prevent the further rotation of the pressure insert within the receiver channel.

13. The pivotal bone anchor assembly of claim 12 , wherein the notches are located on the insert adjacent the insert at least one radially outward upwardly-facing planar surface.

14. The pivotal bone anchor assembly of claim 1 , wherein the insert lower surface further comprises a downwardly-facing spherical surface.

15. The pivotal bone anchor assembly of claim 1 , wherein the shank head is installed into the receiver cavity prior to the pressure insert being installed into the receiver channel.

16. The pivotal bone anchor assembly of claim 1 , wherein the shank head includes a spherical outer surface slidably engageable with the receiver cavity spherical seating surface.

17. The pivotal bone anchor assembly of claim 1 , wherein the shank is cannulated along an entire length thereof.

18. The pivotal bone anchor assembly of claim 1 , wherein the receiver channel helically wound thread comprises a discontinuous flange form.

19. The pivotal bone anchor assembly of claim 1 , wherein the receiver helically wound thread comprises one of a discontinuous square thread, buttress thread, or reverse angle thread.

20. The pivotal bone anchor assembly of claim 1 , wherein a separate retainer is positioned in the receiver cavity so as to be in pivotal engagement with the cavity spherical seating surface, the retainer configured to hold the shank in the receiver.

21. A pivotal bone anchor assembly for securing an elongate rod to a bone via a closure top, the bone anchor assembly comprising:

a receiver having a longitudinal axis, a base, and a pair of upstanding arms extending upwardly from the base with opposed inner surfaces defining an upwardly-opening rod-receiving channel extending through outwardly-facing front side and back side surfaces of the receiver and configured to receive the elongate rod, the arm inner surfaces having a closure top mating feature formed thereon, the base including a cavity in communication with the rod-receiving channel and with a bottom surface of the base through a bottom opening, the receiver including an axial bore centered about the longitudinal axis and extending from the bottom opening to a top of the upstanding arms, the axial bore defined in part by the bottom opening, an inner cylindrical surface in the cavity, and an at least partially cylindrical inner surface in the channel having a discontinuous helically wound thread formed therein, the axial bore including at least one downwardly-facing surface formed in an interior surface thereof and a pair of opposed integral structures facing inwardly toward the longitudinal axis between the closure top mating feature and the cavity, the cavity having a spherical seating surface proximate the bottom opening;

a shank having a longitudinal axis and a proximal capture portion with a spherical outer surface on a spherically shaped head and an anchor portion extending distally from the capture portion for fixation to the bone, the shank capture portion being positionable within the receiver cavity with the shank anchor portion extending downwardly through the bottom opening, the spherically shaped head having a central internal driving structure formed around the shank longitudinal axis and centered thereon; and

a pressure insert having an upper seating surface configured to receive at least an underside portion of the elongate rod, at least one upwardly-facing surface positioned radially outward from the insert seating surface, a central opening for a tool to pass through so as to be engageable with the internal driving structure formed in the shank head, a lower surface configured to engage the shank capture portion spherical outer surface, and an outer side surface having diametrically opposite notches formed therein, the pressure insert being top loaded within the receiver axial bore into a first position within the receiver axial bore with the insert seating surface in a non-alignment orientation with respect to the receiver rod-receiving channel,

wherein upon rotation of the pressure insert about the receiver longitudinal axis into a second position within the receiver axial bore, with the insert seating surface in a co-linear alignment with the receiver rod-receiving channel, the at least one insert upwardly-facing surface is configured to be positioned under the receiver axial bore downwardly-facing surface so as to inhibit the pressure insert from moving back up within the receiver axial bore, and the insert outer side surface diametrically opposed notches are configured to accept the receiver inwardly facing integral structures so as to prevent further rotation of the pressure insert within the receiver axial bore.

22. The pivotal bone anchor assembly of claim 21 , wherein the receiver inwardly facing integral structures are formed into the receiver axial bore prior to top loading the pressure insert into the receiver axial bore.

23. The pivotal bone anchor assembly of claim 21 , wherein the receiver inwardly facing integral structures further comprise a pair of opposed non-cylindrically shaped inwardly-protruding integral structures formed into opposite sides of the receiver axial bore.

24. The pivotal bone anchor assembly of claim 21 , wherein the receiver inwardly facing integral structures further comprise resilient spring tabs.

25. The pivotal bone anchor assembly of claim 21 , wherein the receiver closure top mating feature further comprises a discontinuous helically-wound guide and advancement structure.

26. The bone pivotal anchor assembly of claim 25 , wherein the discontinuous helically-wound guide and advancement structure is one of a flange form, square thread, buttress thread, or reverse angle thread.