Anti-over tightening bone screw fixing arrangement
A fixing arrangement comprises a retaining element and a clamping sleeve which receives a bone screw. The bone screw has a head and a secondary thread and is pivotally mounted in the clamping sleeve. The clamping sleeve has a radially protruding section on the outer casing of the clamping sleeve and at least one track of a clamping thread, which complements the secondary thread, in the interior of the clamping sleeve. In the unclamped state, a free path is formed by a clearance on the clamping sleeve. A protruding section engages into an opening on the receiving seat in a formfitting manner, and the thickened section is shaped to expand towards the rear in the form of a cone. The cone angle is selected such that the free path divided by the tangent of the half-cone angle corresponds to 0.5 to 2.5 times the thread pitch of the secondary thread.
1. A fixing arrangement comprising a holder, a clamping sleeve, and a bone screw received in the clamping sleeve, wherein the bone screw comprises a shank with a front thread and a thickening with a secondary thread at a head end of the bone screw, wherein the holder is provided with a through-opening and a receiving seat in which the clamping sleeve is mounted so as to be pivotably movable, wherein the clamping sleeve comprises on an outside surface of the clamping sleeve a radially protruding projection and in an interior of the clamping sleeve at least one thread of a clamping thread which is complementary to the secondary thread, wherein prior to tightening of the bone screw a free play is formed by a sum of a clearance between the clamping sleeve and the receiving seat and an undersize formed by a gap between the thickening and the interior of the clamping sleeve, wherein the projection engages in a positive locking manner into a recess on the receiving seat, wherein the thickening widens toward the head end of the bone screw in a cone-like manner, and wherein the cone angle is chosen such that the free play divided by the tangent of half the cone angle is between 0.5 and 2.5 times the thread pitch of the secondary thread.
2. The fixing arrangement of claim 1 , wherein the thickening is part of a head of the bone screw.
3. The fixing arrangement of claim 1 , wherein the secondary thread and the front thread on the shank of the bone screw have different pitches.
4. The fixing arrangement of claim 3 , wherein the secondary thread comprises a smaller pitch than the front end.
5. The fixing arrangement of claim 1 , wherein the interior of the clamping sleeve is cylindrical.
6. The fixing arrangement of claim 1 , wherein the interior of the clamping sleeve is conical and the outside surface of the thickening is cylindrical.
7. The fixing arrangement of claim 1 , wherein the pitch of the clamping thread is a maximum of three times the pitch of the front thread on the shank of the screw.
8. The fixing arrangement of claim 1 , wherein the outside surface of the clamping sleeve is formed in a spherical manner and a boundary face between the clamping sleeve and the receiving seat is sphere-like.
9. The fixing arrangement claim 1 , wherein the clamping sleeve is slotted once.
10. The fixing arrangement of claim 1 , wherein the projection is arranged in a region of greatest width of the outside surface of the clamping sleeve.
11. The fixing arrangement of claim 1 , wherein the projection is rotationally symmetrical.
12. The fixing arrangement of claim 11 , wherein the projection comprises a dome-shaped tip.
13. The fixing arrangement of claim 11 , wherein the projection comprises a column-like substructure.
14. The fixing arrangement of claim 1 , wherein the recess in the receiving seat is slot-like and extends in the direction of a center axis of the through-passage.
15. The fixing arrangement of claim 1 , wherein a second projection is arranged diametrically opposite the projection, wherein a corresponding second recess is provided in the receiving seat.
16. The fixing arrangement of claim 4 , wherein the smaller pitch is between 0.4 and 0.7 times smaller than the pitch of the front thread.
17. The fixing arrangement of claim 1 , wherein the pitch of the clamping thread is one and a half times the pitch of the front thread on the shank of the screw.
18. The fixing arrangement of claim 10 , wherein the region of greatest width of the clamping sleeve is on an equator thereof.
19. The fixing arrangement of claim 1 , wherein the projection comprises a semi-spherical tip.
20. The fixing arrangement of claim 1 , wherein the projection comprises a cylindrical substructure.