Bone screw
The invention concerns a bone screw comprising a head ( 2 ), a stem ( 3 ) and a thread ( 4 ) provided in the form of turns on the stem ( 3 ). A plurality of first turns ( 6 ) comprising smooth cutting edges ( 7 ) extend towards the head ( 2 ) from the free end ( 5 ) of the stem ( 3 ). Second turns ( 8 ) adjacent to the first turns ( 6 ), arranged in steps towards the head ( 2 ), comprise recesses ( 9 ) provided in the cutting edges ( 7 ). The size of the recesses ( 9 ) of the second turns ( 8 ) gradually increases from the free end ( 5 ) while the are of the edges in the peripheral direction gradually decreases.
1. A bone screw comprising:
an elongated cylindrical shank having a free end and at an opposite end a screw head;
a screwthread extending generally helically along the shank from the free end toward the screw head and itself having
a first screwthread portion extending from the free end toward the screw head, comprising three turns, and forming a generally continuous and smooth cutting edge,
a second screwthread portion extending from the first portion toward the screw head and formed of a plurality of turns each formed with at least one respective outwardly open recess, the recesses being of increasing angular dimension away from the free end, and
a third screwthread portion extending from the second portion to the screw head and formed of a plurality of turns each formed with three outwardly open recesses equispaced every 360°, all of the recesses of the third portion being of generally the same angular dimension.
2. The bone screw according to claim 1 wherein in the second screwthread portion and in the third screwthread portion every 360° several recesses are formed.
3. The bone screw according to claim 2 wherein every 360° three recesses are formed.
4. The bone screw according to claim 1 wherein each recess extends as a chord toward the screw shank from the cutting edges of the second screwthread portion and of the third screwthread portion and joins in an arc again the respective cutting edge.
5. The bone screw according to claim 1 wherein the second screwthread portion comprises three turns.
6. The bone screw according to claim 1 wherein lengths of the cutting edges of the turns of the second screwthread portion are constant.
7. The bone screw defined in claim 1 wherein the angular dimension of the recesses of the third portion corresponds substantially to the angular dimension of that turn of the second portion closest to the third portion.
8. The bone screw defined in claim 1 wherein the recesses of the third screwthread portion have a radial depth that is less than a radial height of the turns of the screwthread portion.
9. The bone screw defined in claim 1 wherein the turns of the second and third screwthread portions are each formed with at least three of the respective recesses.