Polyaxial bone screw assembly with partially spherical screw head and twist in place pressure insert
View Patent ↗A pivotal bone anchor assembly for a securing an elongate rod to a bone includes a shank having a capture portion and an anchor portion extending distally from the capture portion, and a receiver having an upper portion defining a receiver channel for receiving a rod and a base defining a cavity for receiving the shank capture portion. The assembly also includes a retainer configured to capture and hold the shank capture portion within the receiver cavity, as well as a pressure insert having a central drive tool opening and upright arms forming an insert channel. The pressure insert is loaded into a first position within the receiver channel with the insert channel in a non-alignment orientation with respect to the receiver channel, and then rotated about a longitudinal axis into a second position with the insert channel in alignment with the receiver channel.
1. A pivotal bone anchor assembly for a securing a cylindrically-shaped elongate rod to a bone of a patient via a closure, the pivotal bone anchor assembly comprising:
a receiver comprising a longitudinal axis of rotation, an upper portion defining an open first channel configured to receive the elongate rod, and a base defining a cavity communicating with the first channel and with a bottom surface of the receiver through a bottom opening, the first channel having a discontinuous closure mating structure formed therein, the receiver including a substantially cylindrical interior surface extending downward below the closure mating structure with an internal abutment surface formed therein, and the cavity further defined by an interior seating surface adjacent the bottom opening;
a shank comprising a capture portion with a partial spherical surface and a central drive tool-engaging structure formed therein, and an anchor portion opposite the capture portion configured for fixation to the bone, the capture portion being positionable within the cavity of the receiver with the shank extending downward through the bottom opening;
a retainer configured for positioning within the cavity of the receiver so as to capture and hold the capture portion within the cavity, with an outer surface of the retainer engaged against the interior seating surface of the cavity while allowing for pivotal movement between the receiver and the shank; and
a pressure insert configured for top loading into the receiver separate and apart from the shank, the pressure insert having a central opening centered on the longitudinal axis of rotation of the receiver configured for a drive tool to pass through to engage the drive tool-engaging structure of the shank, outer side surfaces having alignment notches formed therein and protruding surfaces adjacent the alignment notches, and a partially-cylindrical upward-facing curved surface entirely surrounding the central opening to define a second channel extending between the outer side surfaces and sized and shaped to receive the cylindrically-shaped elongate rod, the pressure insert being top loaded into a first position within the substantially cylindrical interior surface portion of the receiver with the second channel of the pressure insert in a non-alignment orientation with respect to the first channel,
wherein after being top loaded into the first position within the substantially cylindrical interior surface portion of the receiver, the pressure insert is rotatable about the longitudinal axis of rotation of the receiver, with the outer side surfaces of the pressure insert rotating below the discontinuous closure mating structure of the channel, until the protruding surfaces of the pressure insert abut the abutment surface in the substantially cylindrical interior surface to inhibit further rotation of the pressure insert within the receiver and to define a second position in which the second channel is in alignment with the first channel.
2. The pivotal bone anchor assembly of claim 1 , wherein the receiver includes outer sidewall crimp holes with thin inner walls configured to deform and engage the alignment notches of the pressure insert to hold the second channel in alignment with respect to the first channel.
3. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert is top loadable into the receiver prior to the shank.
4. The pivotal bone anchor assembly of claim 1 , wherein the capture portion of the shank is bottom loadable into the cavity of the receiver through the bottom opening.
5. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert further comprises upright arms defining an upper portion of the second channel and having top surfaces that remain spaced from the closure when the closure is used to lock the elongate rod within the pivotal bone anchor assembly.
6. The pivotal bone anchor assembly of claim 5 , wherein the pressure insert upright arm top surfaces are planar.
7. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert further comprises upright arms defining an upper portion of the second channel and having top surfaces that engage the closure when the closure is used to lock the elongate rod within the bone anchor assembly.
8. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert further includes an at least partial spherical lower surface configured to engage the partial spherical surface of the capture portion of the shank.
9. The pivotal bone anchor assembly of claim 1 , wherein the pressure insert further comprises upright arms defining an upper portion of the second channel, and wherein the alignment notches and protruding surfaces adjacent the alignment notches are formed into outer side surfaces of the upright arms.
10. The pivotal bone anchor assembly of claim 1 , wherein the discontinuous closure mating structure is a discontinuous helically wound thread.
11. The pivotal bone anchor assembly of claim 1 , wherein the retainer is configured to engage the spherical surface of the capture portion of the shank within the cavity of the receiver.
12. The pivotal bone anchor assembly of claim 1 , wherein the interior seating surface of the cavity further comprises a recess configured for receiving and retaining the retainer within the cavity in a non-pivotal relation.
13. The pivotal bone anchor assembly of claim 12 , wherein the recess of the cavity is a circumferential groove.
14. The pivotal bone anchor assembly of claim 1 , wherein the retainer includes a partially spherical inner surface having substantially a same radius as that of the partial spherical surface of the capture portion of the shank.
15. The pivotal bone anchor assembly of claim 14 , wherein the retainer includes a chamfer or beveled non-radiused surface extending from the partially spherical inner surface down to a bottom of the retainer.
16. The pivotal bone anchor assembly of claim 1 , wherein the retainer is uploadable into the cavity of the receiver through the bottom opening.
17. The pivotal bone anchor assembly of claim 1 , wherein the retainer further comprises first and second spaced ends, the retainer being compressible and expandable with the first and second ends being movable toward and away from one another.
18. The pivotal bone anchor assembly of claim 17 , wherein the retainer is a split ring.
19. The pivotal bone anchor assembly of claim 17 , wherein the first and second spaced apart opposed ends have surfaces that are obliquely oriented with respect to each other.
20. The pivotal bone anchor assembly of claim 1 , wherein the spherical surface of the capture portion of the shank extends above the retainer to engage a lower surface on the pressure insert.
21. The pivotal bone anchor assembly of claim 1 , wherein the central drive-tool engaging structure of the capture portion of the shank is adapted for non-slip engagement by the drive tool for driving the shank into the bone.
22. The pivotal bone anchor assembly of claim 1 , wherein the shank is cannulated with a central opening extending through an entire length thereof.
23. The pivotal bone anchor assembly of claim 1 , wherein the capture portion of the shank is positionable within the cavity of the receiver prior to the pressure insert.
24. A pivotal bone anchor assembly for a securing an elongate rod having a cylindrical shape to a bone of a patient, the pivotal bone anchor assembly comprising:
a receiver comprising a longitudinal axis of rotation, an upper portion defining an open first channel configured to receive the elongate rod, and a base defining a cavity communicating with the first channel and with a bottom surface of the receiver through a bottom opening, the first channel having a discontinuous closure mating structure formed therein, the receiver including a substantially cylindrical interior surface extending downward below the closure mating structure with opposed abutment surfaces formed therein, and the cavity further defined by an interior seating surface adjacent the bottom opening;
a shank comprising a capture portion with a partial spherical surface and a central drive tool-engaging structure formed therein, and an anchor portion opposite the capture portion configured for fixation to the bone, the capture portion of the shank being positionable within the cavity of the receiver with the shank extending downward through the bottom opening;
a retainer configured to be positioned in the cavity of the receiver so as to capture and hold the capture portion within the cavity, with an outer surface of the retainer engaged against the interior seating surface of the cavity while allowing for pivotal movement between the receiver and the shank; and
a pressure insert configured to be top loaded into the receiver separate and apart from the shank, the pressure insert having a central opening centered on the longitudinal axis of rotation of the receiver configured for a drive tool to pass through to engage the drive tool-engaging structure of the shank, outer side surfaces having a pair of opposite alignment notches formed therein and a pair of opposite protruding surfaces adjacent the alignment notches, and a partially-cylindrical upward-facing curved surface entirely surrounding the central opening of the pressure insert to define a second channel configured to receive the elongate rod,
wherein the pressure insert is configured to be top loaded into a first position within the substantially cylindrical interior surface portion of the receiver with the second channel being in a non-alignment orientation with respect to the first channel, after which the pressure insert is rotatable about the longitudinal axis of rotation of the receiver, with the outer side surfaces of the pressure insert rotating below the discontinuous closure mating structure of the channel and with the second channel entering into alignment with the first channel, until the pair of opposite protruding surfaces of the pressure insert abut the opposed abutment surfaces of the substantially cylindrical interior surface to inhibit further rotation of the pressure insert within the receiver and to define a second position with the second channel being aligned with the first channel.
25. The pivotal bone anchor assembly of claim 24 , wherein the retainer is configured to be resiliently compressed during uploading into the cavity of the receiver through the bottom opening.
26. The pivotal bone anchor assembly of claim 24 , wherein the capture portion of the shank is configured for bottom loading into the cavity of the receiver through the bottom opening.
27. The pivotal bone anchor assembly of claim 26 , wherein the shank is configured for first implantation of the anchor portion into the bone followed by assembly of the capture portion with the receiver.