Spondylolisthesis screw and instrument for implantation

1. A screw for use in spinal surgery, comprising:

a screw shaft;

a screw head; and

at least one extension tab extending proximally from said screw head,

said extension tab being defined relative to said screw head by a stress concentration feature,

said extension tab having a proximal end and an opposing distal portion, said extension tab tapers from said stress concentration feature at said distal portion to said proximal end wherein said extension tab at said proximal end is smaller than said extension tab at said distal portion thereof.

2. The screw of claim 1 , wherein said stress concentration feature is a groove.

3. The screw of claim 1 , wherein said screw shaft is integral with said screw head.

4. The screw of claim 1 , wherein said screw shaft has a shaft head, and said screw head cooperates with said shaft head so as to permit a defined amount of angular movement of said screw head relative to said screw shaft.

5. A screw for use in spinal surgery, comprising:

a screw shaft;

a screw head having a longitudinal axis along a length thereof; and

at least one extension tab extending from said screw head,

said extension tab being defined relative to said screw head by an external stress concentration feature,

wherein said external stress concentration feature includes an undercut axially recessed along said longitudinal axis relative to a top surface of said screw head adjacent said extension tab wherein said at least one extension tab extends from said undercut.

6. The screw of claim 5 , wherein said screw shaft is integral with said screw head.

7. The screw of claim 5 , wherein said screw shaft has a shaft head, and said screw head cooperates with said shaft head so as to permit a defined amount of angular movement of said screw head relative to said screw shaft.

8. A screw for use in spinal surgery, comprising:

a screw shaft;

a screw head having a longitudinal axis and a radial axis perpendicular to said longitudinal axis; and

at least one extension tab extending from said screw head,

said extension tab being defined relative to said screw head by an external stress concentration feature,

wherein, in a planar cross-section taken in a plane passing through a longitudinal axis of said screw head, said external stress concentration feature has a profile,

wherein said profile has a proximal edge and a distal edge, and said distal edge has a most proximal portion and second portion, said second portion is at a location more radially inward than a radial location of said most proximal portion, and said second portion of said distal edge is more distal than said most proximal portion.

9. The screw of claim 8 , further comprising an internal stress concentration feature positioned more distally than said external stress concentration feature along said longitudinal axis, wherein said internal stress concentration feature is axisymmetric.

10. A screw for use in spinal surgery, comprising:

a screw shaft;

a screw head having a longitudinal axis; and

at least one extension tab extending proximally along said longitudinal axis from said screw head,

said extension tab being defined relative to said screw head by a stress concentration feature,

said extension tab having a proximal end and a distal end and a tool connection feature between said proximal end and said distal end, wherein said distal end of said extension tab is adjacent said stress concentration feature, said extension tab tapers from a position more distal than said tool connection feature towards said proximal end, wherein said extension tab at said proximal end is smaller than said extension tab at said position thereof.

11. The screw of claim 10 , wherein said extension tab tapers from said stress concentration feature to said proximal end.

12. The screw of claim 10 , wherein said tool connection feature is one or more external indentations.

13. A screw for use in spinal surgery, comprising:

a screw shaft;

a screw head; and

at least one extension tab extending proximally from said screw head,

said extension tab being internally threaded,

said extension tab being defined relative to said screw head by a stress concentration feature, and said extension tab formed by a cutting plane intersecting said extension tab creating a taper wherein a proximal end of said extension tab is smaller than a distal end of said extension tab; and

said cutting plane intersects said internal thread of said extension tab.

14. The screw of claim 13 , wherein said cutting plane intersects said stress concentration feature.

15. A screw for use in spinal surgery, comprising:

a screw shaft;

a screw head defining a longitudinal screw head axis and having a distal end adjacent said screw shaft and a proximal end opposite thereto, wherein said proximal end of said screw head includes a top surface with an undercut, said undercut further comprising a groove extending circumferentially around said top surface and extending axially along said longitudinal screw head axis in a first direction from said top surface towards said distal end, wherein said groove includes a bottom; and

at least one break-away extension tab projecting from said groove bottom in a second direction opposite said first direction.

16. The screw of claim 15 , wherein said at least one break-away extension tab decreases in size in said second direction away from said groove bottom.

17. The screw of claim 15 , wherein a first distance from said screw head distal end to said screw head top surface is larger than a second distance from said screw head distal end to said groove bottom.

18. The screw of claim 17 , wherein said screw head further comprises an internal stress concentration feature, said internal stress concentration feature is axisymmetric and positioned at a third distance from said screw head distal end, wherein said third distance is smaller than said second distance.