Multi-axial screw assembly

1. A multi-axial screw assembly to connect a longitudinal member to a vertebral member, the multi-axial screw assembly comprising:

a screw comprising a threaded shank and an enlarged head; and

a unitary body comprising an upper channel sized to receive the longitudinal member, a lower chamber to receive the head and being in communication with the upper channel, an upper constriction, and a lower constriction;

both the upper and lower constrictions being narrower than the head and disposed on generally opposite sides of the lower chamber, the upper channel including a threaded section;

the head being wider than the threaded section when the head is positioned in the lower chamber;

the screw extending into the lower chamber through the lower constriction;

the lower constriction 360° bounded by continuous material of said body.

2. The multi-axial screw assembly of claim 1 , wherein the multi-axial screw assembly comprises a first state wherein the screw is selectively angularly positionable relative to the body and a second state wherein the screw is angularly locked relative to the body.

3. The multi-axial screw assembly of claim 1 , wherein the chamber is sized such that a portion of the head extends outward from the chamber and beyond the body.

4. The multi-axial screw assembly of claim 1 , wherein the lower constriction is spaced inward in the body and away from a lower end of the body.

5. The multi-axial screw assembly of claim 1 , wherein the lower constriction is positioned at a lower end of the body.

6. The multi-axial screw assembly of claim 1 , further comprising a crown positioned within the body, the crown including a first end with an inner surface that contacts the head and a second end that contacts the longitudinal member when the multi-axial screw assembly is in a locked, second state.

7. The multi-axial screw assembly of claim 6 , wherein the crown further comprises a flange that contacts against a ledge in the lower chamber, the ledge including a smaller width than the lower chamber.

8. The multi-axial screw assembly of claim 1 , wherein the lower constriction includes a circular shape.

9. The multi-axial screw assembly of claim 1 , further comprising a contact surface proximate to a lower end of the body, the contact surface being positioned at an angle of about 12° relative to the lower end.

10. The multi-axial screw assembly of claim 1 , wherein at a vertical level of the lower chamber between the upper and lower constrictions, the body defining an outer extent of the multi-axial screw assembly and an inner wall of the lower chamber.

11. A multi-axial screw assembly to connect a longitudinal member to a vertebral member, the multi-axial screw assembly comprising:

a screw comprising a threaded shank and an enlarged head;

a unitary body comprising an upper channel sized to receive the longitudinal member, and a lower chamber to receive the head of the screw, the upper channel and the lower chamber being in communication; and

the lower chamber comprising a first constriction narrower than the head and positioned on an upper vertical side of the chamber, a second constriction narrower than the head and positioned on a lower vertical side of the chamber, and a central section located between the first and second constrictions and including an enlarged width greater than either the first and second constrictions;

the head disposed in the central section;

the upper channel including a threaded section that is narrower than the head of the screw when the head is positioned in the central section;

at a vertical level of the central section, the body defining an outer extent of the multiaxial screw assembly and an inner wall of the lower chamber.

12. The multi-axial screw assembly of claim 11 , wherein the body is circumferentially un-reinforced at the vertical level of the central section.

13. The multi-axial screw assembly of claim 11 , further comprising a crown that includes a neck that extends through the second constriction and a contact surface with a spherical surface that contacts the head.

14. The multi-axial screw assembly of claim 11 , wherein the lower constriction is continuous and forms a 360° boundary around the screw.