POLYAXIAL BONE ANCHOR WITH POP-ON MULTI-THREAD SHANK, SOME WITH DIAMETRIC INTERFERENCE FIT INSERTS
Polyaxial and uni-planar bone screw assemblies include a multi-threaded shank body having an integral upper portion receivable in a one piece receiver, the receiver having an upper channel for receiving a longitudinal connecting member and a lower cavity cooperating with a lower opening. A compression insert and open retaining ring located in the receiver may be made of cobalt-chrome and the compression insert may provide a diametric interference fit with the receiver. Receivers or inserts may include resilient arm portions. The open ring cooperates with the shank to provide for pop- or snap-on assembly of the shank with the receiver either prior to or after implantation of the shank into a vertebra.
1 . In a medical implant, the improvement comprising:
a) a bone anchor shank with a body for fixation to a bone and an integral upper portion having a curved first surface with a first radius and a hemisphere;
b) a bone anchor receiver having a base and a pair of upright arms forming an open channel, the channel partially defined by arm inner surfaces for receiving a closure, the base defining a chamber and having a lower opening, the channel communicating with the chamber, the receiver base having a central axis and a loading surface located near the lower opening;
c) an open retainer captured within the chamber and expandable about the shank upper portion, the retainer having a bottom surface sized and shaped for engagement with the receiver loading surface, the retainer being positioned entirely below the shank upper portion hemisphere when in a final locked position with respect to the shank and the receiver with the retainer bottom surface frictionally engaging the receiver loading surface, the retainer frictionally engaging the shank curved surface when in the final locked position, the retainer being in expansion-only locking engagement with both the shank upper portion and the receiver;
d) an insert located within the receiver, the insert having an inner lower surface in a friction fit with the shank upper portion during temporary manipulation of the shank with respect to the receiver, the shank upper portion being movable with respect to the insert with some resistance when a force is applied to the shank to pivot the shank with respect to the receiver, the insert being in locked frictional engagement with the shank upper portion when a force is applied on the insert that in turn presses the retainer into the expansion only locked position; and wherein
e) the retainer is made from a first material and the receiver is made from a second material, the first material being harder than the second material.
2 . The improvement of claim 1 wherein the first material is a cobalt-chrome alloy and the retainer has one and up to a plurality of grooves formed in one of an inner and outer surface thereof.
3 . The improvement of claim 1 wherein the second material is a titanium alloy.
4 . The improvement of claim 1 wherein the retainer and the insert are made from a first material and the receiver and the shank are made from a second material, the first material being harder than the second material.
5 . The improvement of claim 4 wherein the closure is made from the first material.
6 . The improvement of claim 1 wherein the shank body has two thread forms at a bottom thereof and three thread forms near the upper portion.
7 . The improvement of claim 1 wherein the insert has diametrically opposed outer surfaces in frictional engagement with inner front and rear surfaces of the receiver spaced from the receiver arms.
8 . The improvement of claim 1 wherein the insert has a pair of opposed arms, each arm having a slot running downwardly from a top surface of the respective arm, the slot forming a resilient outer arm portion that is spaced from and pressable toward a remainder of the arm.
9 . The improvement of claim 8 wherein the insert has a pair of opposed projections located on outside surfaces of the resilient outer arms portions and each receiver arm has a ledge for receiving one of the outer projections.
10 . The improvement of claim 1 wherein the insert has a pair of opposed arms, each arm having an outer surface with a shallow aperture formed therein and the receiver has opposed resilient arm tabs frictionally engaging the insert at the shallow apertures.
11 . The improvement of claim 10 wherein each insert arm further has a groove located below each shallow aperture, each receiver arm tab sliding along one of the grooves during assembly of the insert with the receiver.
12 . The improvement of claim 11 wherein the receiver arm tabs are located in the grooves during assembly of the receiver with the shank.
13 . The improvement of claim 1 wherein the shank upper portion has a first planar surface adjacent the curved surface and wherein the insert has a second planar surface, the first and second planar surfaces being adjacent and limiting pivoting of the shank to a single plane.
14 . The improvement of claim 13 wherein the retainer has a third planar surface adjacent the shank upper portion first planar surface.
15 . The improvement of claim 1 wherein the receiver has opposed resilient arm tabs frictionally engaging the insert.
16 . A bone anchor comprising:
a) a shank with a body for fixation to a bone and an integral upper portion having a curved first surface with a first radius and a hemisphere;
b) a receiver having a base and a pair of upright arms forming an open channel, the channel partially defined by arm inner surfaces for receiving a closure, the base having inner front and rear surfaces located between the receiver arms, the base defining a chamber and having a lower opening, the channel communicating with the chamber, the receiver base having a central axis and an inner seating surface located near the lower opening;
c) an open retainer captured within the chamber and expandable about the shank upper portion, the retainer having a bottom surface sized and shaped for engagement with the receiver seating surface, the retainer being positioned entirely below the shank upper portion hemisphere when in a final locked position with respect to the shank and the receiver with the retainer bottom surface frictionally engaging the receiver seating surface, the retainer frictionally engaging the shank curved surface when in the final locked position, the retainer being in expansion-only locking engagement with both the shank upper portion and the receiver; and
d) an insert located within the receiver, the insert having an inner lower surface in frictional engagement with the shank upper portion, the insert having diametrically opposed outer surface portions in frictional engagement with the receiver inner front and rear surfaces.
17 . The bone anchor of claim 16 wherein the retainer and the insert are made from a first material and the shank and the receiver are made from a second material, the first material being harder than the second material.
18 . The bone anchor of claim 16 wherein the retainer and the shank are made from a first material and the insert and the receiver are made from a second material, the first material being harder than the second material.
19 . The bone anchor of claim 16 wherein the shank body has one and up to a plurality of thread forms near a bottom thereof and greater than two thread forms near the upper portion.
20 . The bone anchor of claim 16 wherein the shank body has two thread forms near a bottom thereof and three thread forms near the upper portion.
21 . The bone anchor of claim 16 wherein the shank upper portion has a first planar surface adjacent the curved surface and wherein the insert has a second planar surface, the first and second planar surfaces being adjacent and limiting pivoting of the shank to a single plane.
22 . The bone anchor of claim 21 wherein the retainer has a third planar surface adjacent the shank upper portion first planar surface.
23 . In a pedicle screw having a lower threaded shank for insertion into bone and a head, the improvement comprising a shank body having a first threaded portion having one and up to a plurality of thread forms, a second threaded portion with more than two thread forms, and a non-interleaved transition portion located between the first and second portions, the transition portion including both the first portion thread forms and the second portion thread forms and wherein a minor diameter of the shank remains substantially constant at the transition portion and a first major diameter measured at the transition portion is one of equal to and less than a second major diameter of the first portion and a third major diameter of the second portion.
24 . The improvement of claim 23 wherein the second portion is an upper portion of the shank located between the transition portion and the head.
25 . The improvement of claim 23 wherein the first portion has two thread forms and the second portion has three thread forms.