Spinal fixation system
A polyaxial pedicle screw includes a housing, a bone screw member, and an anvil. The bone screw member includes a head and a threaded shaft extending from the head. The head is selectively securable within the housing. The anvil is positionable within the housing adjacent to the head of the bone screw member when the anvil and the head of the bone screw member are positioned within the housing. The anvil may define one or more grooves in an outer surface of the anvil. The groove defines a flap that is flexibly attached to the anvil to enable the anvil to flex an amount sufficient to maintain the head of the bone screw in constant contact with the anvil when the bone screw member is moved relative to the anvil. A rod reducer may be secured to the polyaxial pedicle screw to secure a spinal rod within the housing.
1. A method of assembling a polyaxial pedicle screw, comprising:
inserting a head of a bone screw member into a housing through an opening at a distal portion of the housing while a threaded shaft of the bone screw member extends from the head along a shaft axis that is in a first orientation with respect to a longitudinal axis of the housing, in which a first angle is defined between the shaft axis and the longitudinal axis of the housing; and
while the head of the bone screw member is positioned within the housing, rotating the bone screw member with respect to the housing until the shaft axis of the threaded shaft is in a second orientation with respect to the longitudinal axis of the housing, in which a second angle different from the first angle is defined between the shaft axis and the longitudinal axis of the housing, wherein the opening at the distal portion of the housing prevents the head of the bone screw member from being removed from the housing through the opening while the threaded shaft is in the second orientation.
2. The method of claim 1 , further comprising securing a cap to the housing to prevent the bone screw member from rotating back to the first orientation.
3. The method of claim 2 , wherein, after securing the cap to the housing, the bone screw member is polyaxially pivotable such that the shaft axis is positionable among a plurality of angles with respect to the longitudinal axis of housing.
4. The method of claim 3 , wherein the head includes surface texture to increase pivoting friction, such that a force applied by a user stronger than gravitational forces on the polyaxial pedicle screw is required in order to pivot the shaft axis with respect to the housing.
5. The method of claim 3 , wherein the plurality of angles among which the shaft axis is positionable are constrained within a cone angle of up to 80°.
6. The method of claim 1 , wherein a portion of the threaded shaft is received within a recess of the housing when the shaft axis is in the first orientation.
7. The method of claim 6 , wherein the housing includes a collar extending from the distal portion of the housing, the collar defining the recess therein.
8. The method of claim 6 , wherein the portion of the threaded shaft received within the recess is a neck of the threaded shaft disposed between the head and a threaded portion of the threaded shaft.
9. The method of claim 6 , further comprising affixing a cap to the housing to cover the recess.
10. The method of claim 1 , wherein the first angle of the first orientation is substantially 90°, and wherein the second angle of the second orientation is substantially 180°.
11. The method of claim 1 , wherein the head of the bone screw member defines a first outer profile along a plane perpendicular to the longitudinal axis when the shaft axis is in the first orientation, the first outer profile being shaped to be receivable through the opening at the distal portion of the housing when the shaft axis is in the first orientation, and wherein the head of the bone screw member defines a second outer profile along the plane perpendicular to the longitudinal axis when the shaft axis is in the second orientation, the second outer profile being shaped to prevent the head from passing through the opening at the distal portion of the housing when the shaft axis is in the second orientation.
12. The method of claim 11 , wherein the opening at the distal portion of the housing and the first outer profile of the head of the bone screw member have circular shapes, with a diameter of the first outer profile being smaller than a diameter of the opening at the distal portion of the housing.
13. The method of claim 12 , wherein the first outer profile is defined along a cylindrical portion of the head of the bone screw member.
14. The method of claim 11 , wherein the opening at the distal portion of the housing and the second outer profile of the head of the bone screw member have circular shapes, with a diameter of the second outer profile being larger than a diameter of the opening at the distal portion of the housing.
15. The method of claim 14 , wherein the second outer profile is defined along a spherical portion of the head of the bone screw member.
16. The method of claim 11 , wherein the head of the bone screw member has a substantially spherical shape with a cylindrical portion formed therein, such that the first outer profile is defined by the cylindrical portion.
17. The method of claim 1 , further comprising positioning an anvil within the housing adjacent to the head of the bone screw member.
18. The method of claim 17 , wherein the bone screw member is polyaxially pivotable such that the shaft axis is positionable among a plurality of angles with respect to the longitudinal axis of housing, and wherein the anvil is configured to maintain constant contact with the head of the bone screw member.
19. The method of claim 18 , wherein the anvil engages the housing via a flap flexibly attached to the anvil.
20. The method of claim 17 , wherein the anvil is configured to engage the housing so as to maintain an orientation of the anvil with respect to the housing about the longitudinal axis.