Pivotal bone anchor with snap-on receiver and insert deployment
A polyaxial bone screw assembly includes a threaded shank body having an integral upper portion receivable in a receiver, the receiver having an upper channel for receiving a longitudinal connecting member and a lower cavity cooperating with a lower opening. The upper portion expands a retaining member in the receiver cavity to capture the shank upper portion in the receiver. In some embodiment either the retaining member or an insert provide for a friction fit of the shank upper portion in the receiver resulting in non-floppy placement of the shank with respect to the receiver. Some retainers and inserts have a lock-and-release feature. Final locking of the polyaxial mechanism is provided by frictional engagement between the shank upper portion and the retaining member. A pre-assembled receiver, retaining member and optional insert may be popped-on or snapped-on to the shank upper portion prior to or after implantation of the shank into a vertebra.
1. A pivotal bone anchor assembly for securing an elongate rod to a bone, the bone anchor assembly comprising:
a shank having a shank head at a proximal end and an anchor portion extending distally from the shank head for fixation to the bone;
a receiver having a central bore centered around a longitudinal axis, the central bore extending downward through a base to a distal opening in communication with a bottom surface of the base and extending upward, between a pair of integral arms projecting upwardly from the base, to a proximal opening opposite the distal opening, the pair of integral arms further defining an open channel transverse to the longitudinal axis and configured to receive the elongate rod, the central bore having a sidewall with at least one recess formed therein and defined by upper and lower recess surfaces perpendicular to the longitudinal axis and an end cylindrical surface parallel to the longitudinal axis;
a resilient open ring received within the at least one recess and configured for expansion and contraction therein; and
a shank head holding member receivable within the central bore after the resilient open ring is received within the at least one recess, the shank head holding member having upper and lower outer cylindrical surfaces separated by at least one abutment surface that is perpendicular to the longitudinal axis,
wherein displacement of the shank head holding member, with the shank head secured therein, along the longitudinal axis causes the resilient open ring to snap inward and engage the at least one abutment surface to restrict further displacement of the shank head holding member and the shank head along the receiver longitudinal axis in at least one axial direction.
2. The bone anchor assembly of claim 1 , wherein an inner surface of the resilient open ring is spaced from the shank head.
3. The bone anchor assembly of claim 1 , wherein at least one of the upper and lower recess surfaces engages an upper or lower surface of the resilient open ring.
4. The bone anchor assembly of claim 1 , wherein an inner surface of the resilient open ring is slidingly engaged with one of the upper and lower outer cylindrical surfaces prior to engagement with the at least one abutment surface.
5. The bone anchor assembly of claim 1 , wherein when the resilient open rings snaps inward, the resilient open ring is in a neutral state without application of a compressive force to the shank head holding member.
6. The bone anchor assembly of claim 1 , wherein the resilient open ring is reversible with a substantially rectangular cross-section.
7. The bone anchor assembly of claim 1 , wherein the shank head holding member is uploaded into the central bore through the receiver distal opening.
8. The bone anchor assembly of claim 1 , wherein the shank head holding member is received within the central bore prior to the shank head being uploaded into the central bore through the receiver distal opening.
9. The bone anchor assembly of claim 1 , wherein the at least one abutment surface extends inward from the lower outer cylindrical surface.
10. The bone anchor assembly of claim 1 , wherein the at least one axial direction is upward toward the receiver proximal opening.
11. The bone anchor assembly of claim 1 , wherein an upper surface of the shank head holding member engages the elongate rod when the elongate rod is positioned within the receiver channel.
12. The bone anchor assembly of claim 1 , wherein the shank head holding member engages the shank head both above and below a midline of the shank head.
13. The bone anchor assembly of claim 1 , further comprising a lower retainer received within a lower portion of the central bore proximate the distal opening for preventing the shank head from exiting the central bore through the distal opening.
14. The bone anchor assembly of claim 13 , wherein the lower retainer further comprises an open ring.
15. The bone anchor assembly of claim 13 , wherein the lower retainer is received against a lower annular surface perpendicular to the receiver longitudinal axis to prevent the shank head from exiting the central bore through the distal opening.
16. A bone anchor assembly for securing an elongate rod to a bone, the bone anchor assembly comprising:
a shank having a capture portion with a convex upper surface at a proximal end and an anchor portion extending distally from the capture portion for fixation to the bone;
a receiver comprising a base defining a cavity centered around a longitudinal axis with a distal opening in communication with a bottom surface of the base, and a pair of integral arms extending upwardly from the base to define an open channel transverse to the longitudinal axis and configured to receive the elongate rod, the cavity communicating with the channel and having a circumferential expansion chamber formed therein;
an insert received within the cavity, the insert having a cylindrical outer surface and a concave inner surface for receiving the shank convex upper surface and transferring pressure between the elongate rod and the shank capture portion in a final locked configuration;
a first resilient open ring received within the cavity and having a cylindrical inner surface engaged with the insert cylindrical outer surface to hold the insert in a centralized stationary position within the cavity prior to the uploading of the shank capture portion into the cavity through the distal opening; and
a second resilient open ring received within the expansion chamber of the cavity and configured for radial expansion and contraction therein,
wherein upward movement of the shank capture portion into the cavity during uploading through the distal opening causes the second resilient ring to first expand within the expansion chamber and to then contract around and capture the shank capture portion, and
wherein the second resilient open ring, in the final locked configuration, is spaced below the first resilient open ring and engaged with both the shank capture portion and an annular surface perpendicular to the receiver longitudinal axis to prevent the shank capture portion from exiting the cavity through the distal opening.
17. The bone anchor assembly of claim 16 , wherein the first resilient open ring, in the final locked configuration, is in a neutral state without application of a compressive force to the insert.
18. The bone anchor assembly of claim 16 , wherein the insert is uploaded into the central bore through the receiver distal opening.
19. The bone anchor assembly of claim 16 , wherein the insert concave inner surface engages the shank capture portion both above and below a midline of the capture portion to secure the shank capture portion therein.
20. The bone anchor assembly of claim 19 , wherein the outer surface of the insert further comprises upper and lower outer cylindrical surfaces separated by at least one abutment surface that is perpendicular to the longitudinal axis.
21. The bone anchor assembly of claim 20 , wherein the at least one abutment surface extends inward from the lower outer cylindrical surface.
22. The bone anchor assembly of claim 20 , wherein displacement of the insert along the longitudinal axis, with the shank capture portion secured therein, causes the first resilient open ring to snap inward to engage the at least one abutment surface to restrict further displacement of the insert and the shank head in at least one direction along the receiver longitudinal axis.