Polyaxial pedicle screw
A polyaxial bone anchor including a housing, a bone screw, and a retainer for pivotably coupling the head of the bone screw to the housing. The retainer is positioned into the bore of the housing and includes a plurality of alternating tabs and slots circumferentially arranged to define a cavity for receiving the head portion of the bone screw therein. The retainer is axially moveable in the housing from a first position in which the head portion is not passable through the lower opening of the retainer to a second position in which the head portion is passable through the lower opening of the retainer.
1. A polyaxial bone anchor comprising:
a housing having an upper end, a lower end and a bore extending through the housing from the upper end to the lower end, the bore opening out at the lower end at a lower opening, the housing including a channel configured for receiving an elongate stabilization member therethrough which extends from a first side surface of the housing to a second side surface of the housing opposite the first side surface transverse to the bore;
a retainer positioned into the bore of the housing and having an outermost diameter greater than a diameter of the lower opening, the retainer including:
a lower portion having a plurality of alternating tabs and slots circumferentially arranged to define a cavity therein;
an upper portion extending into the channel of the housing configured to support the elongate stabilization member off a bottom of the channel; and
an intermediate portion residing between the lower portion and the upper portion, wherein the intermediate portion includes a resilient spring that enables the intermediate portion to be compressed between the upper portion and the lower portion; and
a bone screw including a head portion and a shank extending from the head portion, the head portion of the bone screw positionable in the cavity of the retainer with the shank extending from the lower end of the housing by deflecting the tabs radially outward to enlarge a lower opening of the retainer into the cavity from a diameter less than a diameter of the head portion to a diameter greater than or equal to the diameter of the head portion.
2. The polyaxial bone anchor of claim 1 , wherein the retainer is moved toward the upper end of the housing while being positioned in the bore in order to insert the head portion of the bone screw into the cavity of the retainer.
3. The polyaxial bone anchor of claim 2 , wherein the retainer is insertable into the bore of the housing without the head portion of the bone screw positioned in the cavity of the retainer, yet with the head portion of the bone screw positioned in the cavity of the retainer, the retainer is prevented from being removed from the bore of the housing.
4. The polyaxial bone anchor of claim 1 , wherein the housing includes an annular rim defining the lower opening, wherein the diameter of the lower opening at the annular rim is less than a diameter of the bore of the housing toward the upper end of the housing from the annular rim.
5. The polyaxial bone anchor of claim 4 , wherein the retainer includes a lower beveled surface which contacts the annular rim of the housing, thereby preventing the tabs of the retainer from deflecting radially outward in order to secure the head portion of the bone screw in the cavity of the retainer.
6. The polyaxial bone anchor of claim 1 , wherein the retainer is movable in the bore of the housing between a first position and a second position, the retainer being closer to the lower end of the housing in the first position and closer to the upper end of the housing in the second position, wherein the head portion of the bone screw is insertable into the cavity of the retainer from the lower end of the housing when the retainer is in the second position, but is not removable from the cavity of the retainer when the retainer is in the first position.
7. The polyaxial bone anchor of claim 6 , wherein an annular rim defining the lower opening of the housing contacts a lower beveled surface of the retainer to prevent radially outward deflection of a plurality of tabs of the retainer in order to secure the head portion of the bone screw in the cavity of the retainer.
8. The polyaxial bone anchor of claim 1 , wherein the lower portion has a larger outer diameter than the upper portion.
9. The polyaxial bone anchor of claim 1 , wherein the head portion of the bone screw is bottom loaded into the housing.
10. The polyaxial bone anchor of claim 1 , wherein the resilient spring is an integral part of the retainer.
11. The polyaxial bone anchor of claim 10 , wherein the resilient spring comprises a circumferential or helical slot formed in the retainer.
12. The polyaxial bone anchor of claim 10 , wherein the resilient spring comprises a circumferential slot formed in the retainer.
13. The polyaxial bone anchor of claim 12 , wherein the circumferential slot extends less than 360 degrees around a circumference of the retainer.
14. The polyaxial bone anchor of claim 10 , wherein the resilient spring comprises a helical slot formed in the retainer.
15. The polyaxial bone anchor of claim 14 , wherein the helical slot extends greater than 360 degrees around a circumference of the retainer.
16. The polyaxial bone anchor of claim 14 , wherein the helical slot extends less than 360 degrees around a circumference of the retainer.
17. The polyaxial bone anchor of claim 14 , wherein the helical slot extends approximately 360 degrees around a circumference of the retainer.
18. The polyaxial bone anchor of claim 1 , wherein the resilient spring comprises a pseudo-spring.