Polyaxial screw

1. A spine stabilization system comprising:

an elongate rod;

a bone fastener having a rounded head;

a coupling element comprising a coupling body and a cap;

wherein the coupling body comprises sidewalls that define two upwardly extending arms which define two slots capable of receiving the elongate rod;

wherein the coupling body and cap interact to capture the elongate rod in the coupling body, wherein a first and second protrusions configured on opposing sides of the cap and extending in a radial direction and a first and second recess configured in the two upwardly extending arms extending in a radial direction and wherein the first and second protrusions are configured and dimensioned to engage the first and second recess to limit movement of the cap with respect to the coupling body, when the cap is rotated from a first position to a second position,

wherein the cap having a threaded opening for receiving a threaded locking element, the locking element capable of securely holding the elongated rod in a fixed position relative to the coupling element,

wherein the cap further comprises at least one rim which extends radially outward from the top end of the perimeter of the cap,

wherein a first of the upwardly extending arms includes a third recess positioned between the first recess and the upper surface of the first of the upwardly extending arms for receiving the at least one rim of the cap and a second of the upwardly extending arms includes a fourth recess for receiving the at least one rim of the cap, the fourth recess positioned between the second recess and the upper surface of the second of the upwardly extending arms and wherein the third recess extends along the length of an inner surface of the first of the upwardly extending arms and the fourth recess extends along the length of an inner surface of the second of the upwardly extending arms.

2. The spine stabilization system of claim 1 , wherein the at least one rim engages the upwardly extending arms to align the first and second protrusion s of the cap and the first and second recesses of the coupling body when the cap is inserted into the coupling body.

3. The spine stabilization system of claim 1 , wherein the height of the first and second recesses are dimensioned to correspond to the thickness of the first and second protrusion.

4. The spine stabilization system of claim 3 , wherein the first and second recesses are configured with a first and second relief area.

5. The spine stabilization system of claim 4 , wherein the first and second relief areas interact with the first and second protrusion when the cap is rotated into a final position.

6. The spine stabilization system of claim 1 , wherein the cap comprises a lower surface configured to exert downward force upon the elongate rod as the cap is rotated from a first position to a second position.

7. A spine stabilization system comprising:

an elongate rod;

a bone fastener having a rounded head;

a coupling element comprising a coupling body and a cap;

wherein the coupling body comprises sidewalls that define first and second upwardly extending arms, said arms defining two slots capable of receiving the elongate rod;

wherein the cap comprises an upper portion and a lower portion, the upper portion of the cap having radially extending first and second protrusion;

wherein the first upwardly extending arm comprises a first recess and a first channel, the first recess configured to engage with the first protrusion of the cap, the second upwardly extending arm comprises a second recess and a second channel, the second recess configured to engage with the second protrusion of the cap,

wherein the first and second protrusions engage with the first and second recesses of the first and second upwardly extending arms as the cap is rotated from a first position to a second position,

wherein the cap is configured with a threaded opening for receiving a locking element, the locking element capable of securely holding the elongated rod in a fixed position relative to the coupling element,

wherein the cap further comprises at least one rim which extends radially outward from the upper portion the cap, wherein the first channel engages with the at least one rim of the cap and the second channel engages with the at least one rim of the cap, wherein the first channel extends along a majority of the length of an inner surface of the first upwardly extending arms and the second channel extends along a majority of the length of an inner surface of the second upwardly extending arms.

8. The spine stabilization system of claim 7 , wherein the coupling body is configured with a first relief area in the first recess.

9. The spine stabilization system of claim 8 , wherein the first relief area comprises a first contact surface.

10. The spine stabilization system of claim 9 , wherein the first protrusion of the cap comprises a second contact surface.

11. The spine stabilization system of claim 10 , wherein the first contact surface and second contact surface abut in a second position to limit rotation of the cap.

12. The spine stabilization system of claim 11 , wherein the first and second contact surfaces are configured such that there is no net outward radial force applied to the coupling body when the cap is in the second position.

13. The spine stabilization system of claim 7 , wherein the cap is inserted into the coupling body in a first position and may be rotated by about 90° to a second position.

14. The spine stabilization system of claim 7 , wherein the at least one rim of the cap is configured to frictionally cooperate with the first and second channel of the first and second upwardly extending arms.

15. The spine stabilization system of claim 7 , wherein the cap comprises a lower surface configured to exert downward force upon the elongate rod as the cap is rotated from a first position to a second position.