POLYAXIAL BONE ANCHOR WITH RECEIVER WITH SPHERIC EDGE FOR FRICTION FIT
Polyaxial bone screws include a receiver and a shank, the receiver having in inner concave surface defining a spherical void and the shank having an upper spherical portion, the shank upper portion and receiver concave surface being in moveable friction fit engagement prior to locking of the shank with respect to the receiver. The receiver includes a circular edge formed by a spheric section, the edge capturing the shank upper portion within the receiver. Bone screws may include pressure inserts, shank upper portions having separate retainer rings, receiver arms having break-off extensions, and one or two-piece closures.
1 . In a polyaxial bone anchor, the improvement comprising:
a) a receiver having an inner surface portion partially defining a chamber communicating with a first channel defined by opposed upstanding arms, the first channel sized and shaped for receiving a portion of a longitudinal connecting member, the chamber communicating with a lower opening, the inner surface portion being concave and defining a spherical void having a first hemisphere with a first diameter passing through a center of the first hemisphere, the surface portion also having an upper edge defining a spheric section, the edge having a spheric section diameter that is smaller than the first diameter; and
b) a shank having a threaded body and an upper portion, the upper portion having a convex radiused surface with a second hemisphere and a second diameter passing through a center of the second hemisphere, the radiused surface second diameter being substantially equal to the receiver first diameter, the shank upper portion captured within the receiver chamber beneath the receiver edge and the shank upper portion radiused surface being movable with respect to the receiver inner surface portion in a friction fit relation requiring some force to pivot the shank with respect to the receiver prior to locking of the shank in a desired angular orientation with respect to the receiver.
2 . The improvement of claim 1 wherein the shank upper portion is integral with the shank body.
3 . The improvement of claim 1 wherein the shank upper portion comprises first and second parts, the first part being integral with the shank body and having an outer thread and the second part having an inner thread mated with the outer thread, the second part having the radiused surface.
4 . The improvement of claim 1 wherein the shank upper portion comprises first and second parts, the first part being integral with the shank body and the second part being a resilient open retainer ring expandable about at least a portion of the first part in the receiver chamber during assembly and then is captured by the first part within the receiver chamber, the second part having the radiused surface.
5 . The improvement of claim 1 wherein the shank upper portion radiused surface has a surface treatment.
6 . The improvement of claim 5 wherein the surface treatment is a plurality of ridges.
7 . The improvement of claim 1 wherein the receiver opposed upstanding arms each have an integral break-off extension with a notch formed in an outer surface located at a juncture of the arm and the break-off extension, a closure guide and advancement structure extends helically about and along inner surfaces of both the arm and the extension.
8 . The improvement of claim 7 wherein a recess is formed in the guide and advancement structure at a location opposite the notch, the recess cutting into the structure in a direction transverse to a slope of the structure.
9 . The improvement of claim 1 further comprising an insert in engagement with the shank upper portion and the longitudinal connecting member.
10 . The improvement of claim 1 wherein the receiver opposed upstanding arms each have a closure guide and advancement flange extending helically about and along an inner surface thereof and further comprising an insert in engagement with the shank upper portion and the flange.
11 . The improvement of claim 1 wherein the receiver opposed upstanding arms each have a closure guide and advancement flange extending helically about an along an inner surface thereof and are configured for mating engagement with a closure having a two start flange form thereon.
12 . The improvement of claim 1 wherein the shank is top loaded into the receiver.
13 . The improvement of claim 1 wherein the shank is bottom loaded into the receiver.
14 . The improvement of claim 1 further comprising an insert located within the receiver, the insert in frictional engagement with the shank upper portion.
15 . The improvement of claim 14 further comprising a closure structure having an outer piece and an inner set screw, the outer piece pressing exclusively on the insert, the inner set screw pressing exclusively on a longitudinal connecting member captured within the receiver.
16 . The improvement of claim 15 wherein the outer piece has a break-off head.
17 . The improvement of claim 15 wherein the outer piece has a drive structure that includes radially extending slots formed in a top surface of the outer piece.
18 . The improvement of claim 17 wherein the inner set screw has a drive structure that is multi-lobular.
19 . The improvement of claim 18 wherein the inner set screw forms a through bore.
20 . In a polyaxial bone anchor, the improvement comprising:
a) a receiver having an inner partially spherical concave surface portion having a hemisphere and partially defining a chamber communicating with a rod-receiving channel defined by opposed upstanding arms, the inner spherical concave surface portion terminating at a spheric edge, the spheric edge located below the rod-receiving channel, the spheric edge having a width smaller than a diameter at the hemisphere of the concave surface portion, the chamber communicating with a lower opening; and
b) a shank having a threaded body and an upper portion, the upper portion having a convex radiused surface sized and shaped for being pressed downwardly through the spheric edge and into the chamber and thereafter being movable with respect to the receiver inner concave surface in a friction fit relation requiring some force to pivot the shank with respect to the receiver prior to locking of the shank in a desired angular orientation with respect to the receiver.