Pivotal bone anchor screw with nested two-piece closure and independent locking twist-in-place insert
View Patent ↗A pivotal bone anchor assembly includes a receiver having a base defining a cavity for receiving the capture portion of a shank and a first pair of upright arms defining a first channel for receiving an elongate rod, and a pressure insert with a second pair of upright arms defining a second channel. The pressure insert is configured for initial positioning into the receiver, with the first channel and the second channel in a malaligned orientation, and afterwards being rotated so that radially offset upward-facing surfaces of the insert rotate under downward-facing abutment surfaces of the receiver and outer rotation blocking surfaces of the insert engage inward-facing rotation blocking surfaces of the receiver, so as to resist the rotation between the pressure insert and the receiver, until the first channel and the second channel are in alignment and further rotation of the pressure insert is inhibited by abutment between the outer rotation blocking surfaces against the inward-facing rotation blocking surfaces.
1. A pivotal bone anchor assembly for securing an elongate rod to a bone of a patient, the pivotal bone anchor assembly comprising:
a shank comprising a proximal capture portion having a spherical enlargement and an anchor portion opposite the capture portion configured for attachment to the bone of a patient;
a receiver comprising a base defining a cavity configured to receive the capture portion of the shank with the shank extending downward through a lower opening, and a first pair of upright arms extending upward from the base to define an open first channel configured to receive the elongate rod, the first channel opening upwardly onto top surfaces on the first pair of upright arms and opening transversely onto front and back outer surfaces of the upright arms, the cavity communicating with the first channel to define a central bore centered about a vertical centerline axis and extending upwardly from the lower opening to the top surfaces of the first pair of upright arms, the central bore further including:
a discontinuous helically wound thread formed into an upper portion of the central bore;
downward-facing abutment surfaces formed below the discontinuous helically wound thread; and
opposed inward-facing rotation blocking surfaces pre-formed into interior surfaces of the central bore;
a pressure insert comprising a body with a centralized through-bore configured to receive a driving tool and a second pair of upright arms defining an open second channel configured to receive the elongate rod, the second pair of upright arms configured to extend above a top surface of the elongate rod and having opposite radially offset upward-facing surfaces and outer rotation blocking surfaces, the pressure insert configured for initial positioning within the central bore of the receiver with the first channel and the second channel in a malaligned orientation relative to each other and afterwards being rotatable around the vertical centerline axis of the receiver, with the radially offset upward-facing surfaces rotating at least partially under the downward-facing abutment surfaces of the receiver and the outer rotation blocking surfaces engaging the inward-facing rotation blocking surfaces to resist the rotation between the pressure insert and the receiver, until the first channel and the second channel are in alignment relative to each other and further rotation of the pressure insert is inhibited by abutment between the outer rotation blocking surfaces against the inward-facing rotation blocking surfaces; and
a closure comprising a closed outer ring fastener having a continuous helically wound thread engageable with the discontinuous helically wound thread in the central bore of the receiver and a threaded central through-bore, and a threaded inner set screw engageable with the threaded central through-bore and having an internal tool abutment surface near to and opposite a bottom surface of the set screw, the closure configured for positioning within the central bore of the receiver with a bottom surface of the closed outer ring fastener engageable with the second pair of upright arms of the pressure insert and the bottom surface of the set screw extending below the bottom surface of the closed outer ring fastener to engage a top surface of the elongate rod to bias the elongate rod toward and against the pressure insert,
wherein an angular position of the shank relative to the receiver is locked by the closed outer ring fastener independent of the elongate rod being secured within the first and second channels by the inner set screw.
2. The pivotal bone anchor assembly of claim 1 , wherein the cavity defines a static retaining structure, and the pivotal bone anchor assembly further comprises a pivoting retaining structure positioned in the static retaining structure and disposed around the capture portion of the shank, the pivoting retaining structure cooperating with the capture portion to hold the shank in the receiver and thereby providing selective angular positioning of the shank with respect to the receiver when the assembly is in an unlocked configuration.
3. The pivotal bone anchor assembly of claim 2 , wherein the pivoting retaining structure is pivotal with the shank and cooperates with the capture portion to hold the shank in the receiver and thereby provides selective angular positioning of the shank with respect to the receiver.
4. The pivotal bone anchor assembly of claim 2 , wherein the pressure insert is configured to engage the capture portion of the shank positioned in the static retaining structure of the receiver.
5. The pivotal bone anchor assembly of claim 2 , wherein the pressure insert is spaced apart from the shank along the vertical centerline axis.
6. The pivotal bone anchor assembly of claim 1 , wherein the capture portion of the shank is configured for uploading into the static retaining structure of the receiver through the lower opening.
7. The pivotal bone anchor assembly of claim 1 , wherein the bottom surface of the inner set screw is flat.
8. The pivotal bone anchor assembly of claim 1 , wherein a top surface of the inner set screw is flat.
9. The pivotal bone anchor assembly of claim 7 , wherein the top surface of the inner set screw does not extend above a top surface of the closed outer ring fastener when the assembly is in a locked orientation.
10. The pivotal bone anchor assembly of claim 1 , wherein the opposite radially offset upward-facing surfaces of the pressure insert further comprise top surfaces of a pair of flanges projecting radially outward from the upright arms of the pressure insert and configured for rotation under the downward-facing abutment surfaces of the central bore of the receiver to prohibit upward movement of the pressure insert out of the receiver.
11. The pivotal bone anchor assembly of claim 1 , wherein the receiver further comprises radiused horizontal upper tool engaging grooves near the top surfaces of the upright arms and extending from the front outer surface to the back outer surface thereof.
12. The pivotal bone anchor assembly of claim 1 , wherein the shank is cannulated with a central opening extending an entire length of the shank.
13. The pivotal bone anchor assembly of claim 1 , wherein the opposed inward-facing rotation blocking surfaces are formed on inwardly protruding structures.
14. The pivotal bone anchor assembly of claim 1 , wherein the outer rotation blocking surfaces on the upright arms of the pressure insert further comprise a plurality of surfaces substantially extending perpendicular to the vertical centerline axis.
15. The pivotal bone anchor assembly of claim 1 , wherein the outer rotation blocking surfaces on the upright arms of the pressure insert are formed in recesses.
16. A method of assembling a pivotable bone anchor assembly configured for securing an elongate rod to a bone of a patient via a closure that includes a closed outer ring fastener having a continuous helically wound outer thread and a threaded central through-bore, and a threaded inner set screw engageable with the threaded central through-bore and having an internal tool abutment surface near to and opposite a bottom surface of the set screw, the method comprising:
positioning a proximal capture portion of a shank having a spherical enlargement into a cavity of a receiver through a lower opening, the receiver comprising a base defining a cavity in communication with a bottom of the receiver through a lower opening, and a first pair of upright arms extending upward from the base to define an open first channel configured to receive the elongate rod, the first channel opening upwardly onto top surfaces on the first pair of upright arms and opening transversely onto front and back outer surfaces of the upright arms, the cavity communicating with the first channel to define a central bore centered about a vertical centerline axis and extending upwardly from the lower opening to the top surfaces of the first pair of upright arms, the central bore further including:
a discontinuous helically wound thread formed into an upper portion of the central bore;
downward-facing abutment surfaces formed below the discontinuous helically wound thread; and
opposed inward-facing rotation blocking surfaces pre-formed into interior surfaces of the central bore; and
positioning a pressure insert into an initial position within the central bore of the receiver, the pressure insert comprising a body with a centralized through-bore configured to receive a driving tool and a second pair of upright arms defining an open second channel configured to receive the elongate rod, the second pair of upright arms configured to extend above a top surface of the elongate rod and having opposite radially offset upward-facing surfaces and outer rotation blocking surfaces, the first channel and the second channel being in a malaligned orientation relative to each other when the pressure insert is in the initial position, the pressure insert afterwards being rotatable around the vertical centerline axis of the receiver, with the radially offset upward-facing surfaces rotating at least partially under the downward-facing abutment surfaces of the receiver and the outer rotation blocking surfaces engaging the inward-facing rotation blocking surfaces to resist the rotation between the pressure insert and the receiver, until the first channel and the second channel are in alignment relative to each other and further rotation of the pressure insert is inhibited by abutment between the outer rotation blocking surfaces against the inward-facing rotation blocking surfaces;
wherein the discontinuous helically wound thread of the central bore of the receiver is configured for threadable engagement by the continuous helically wound outer thread of the closed outer ring fastener of the closure upon the closure being downwardly rotatably positioned into the central bore, until a bottom surface of the closed outer ring fastener engages the second pair of upright arms of the pressure insert to lock a position of the shank relative to the receiver independent of the elongate rod being secured within the first and second channels by the inner set screw.
17. The method of claim 16 , wherein the bottom surface of the set screw is configured to extend below the bottom surface of the closed outer ring fastener to engage a top surface of the elongate rod positioned within the first and second channels upon the inner set screw being downwardly rotatably positioned within the threaded central through-bore of the closed outer ring fastener, so as to bias the elongate rod toward and against the pressure insert.
18. The method of claim 16 , further comprising positioning a pivoting retaining structure within a static retaining structure formed into the cavity of the receiver, the pivoting retaining structure configured to engage the capture portion of the shank to hold the shank in the receiver and thereby providing selective angular positioning of the shank with respect to the receiver when the assembly is in an unlocked configuration.
19. The method of claim 18 , wherein the static retaining structure further comprises a seating surface adjacent to the lower opening.