An element with a thread is created, in which both partial flanks enclose a negative angle with a plane extending perpendicular to the screw axis. Further, a screw or a hook for connecting to a rod, as used in particular in spinal column surgery, is created, wherein the inner thread of a holder, to which the rod is connected, and the associated thread of a nut comprise the thread according to the invention.
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The invention relates to an element to be used in spinal column or accident surgery for connecting to a rod.
From U.S. Pat. No. 5,129,388 a pedicle screw is known which comprises a head with a U-shaped recess defining two free legs to accommodate a rod, wherein the legs are provided with an inner thread to receive a fixing screw for fixing the rod. To prevent spreading of the legs a cap is provided embracing the legs.
From U.S. Pat. No. 5,005,562 it is known with a pedicle screw according to the preamble of Patent claim 1 to construct the inner thread and the thread of the screw cooperating with it as a saw-tooth thread, so as in this way to avoid spreading out of the two flanks by completely eliminating the radial components of the force during screwing in.
A saw or else saw-tooth thread of this kind is known from Meyer's Enzyklopädisches Lexikon, Mannheim, 1974, page 302. From both the specifications U.S. Pat. Nos. 5,605,458 and 5,607,304, concerning implants, it is known with implants of this kind to be introduced into the body, in particular hip implants, also to construct the saw thread in such a way that the flank bearing the load is constructed not only at 90° to the thread axis but even at a negative angle. This is supposed to achieve that the connection obtained with the thread resists a radial relative movement of the connected parts and thus guarantees better fixing of the implant. From EP-A-1 128 773 a pedicle screw is known in which the inner thread of a head accommodating a rod has a negative thread and the screw cooperating with it is correspondingly constructed.
The object of the invention is to create an element of the kind initially described, which is suitable for avoiding the above described disadvantage. In particular bending up of the open ends of a mount of this kind and therefore loosening of the connection between screw and rod should be avoided.
This object is achieved by the element characterised in Patent claim 1.
This construction has the advantage that the action of force between the exposed legs and the inner thread happens in such a way that the open ends of the holding element are not pressed outwards, but instead are drawn inwards, so an intensive connection takes place between the cooperating elements.
Further features and advantages of the invention emerge from the description of the embodiment examples using the figures.
The pedicle screw according to
In operation a rod 10, to be connected to the screw, is placed in the U-shaped recess 4 in such a way that the rod comes to rest on the base of the U-shaped recess. The screw 9 is then screwed in far enough for it to exert sufficient stopping pressure on the rod 10 located in the U-shaped recess.
As can be seen more clearly in
In the thread in each case two adjacent partial flanks 12, 13 form a thread pitch. The two partial flanks 12, 13 enclose the flank angle β. The flank side 13 facing the open end 7 encloses a positive angle β1 with a plane 40, extending perpendicular to the longitudinal axis 3 and intersecting the flank side 13 on the base 23 of the thread (see
The embodiment shown in
By contrast with the first embodiment here there is a so-called polyaxial screw, in which the thread shank 1 is connected to a receiving part 15 via a spherical head 14. The receiving part has on one end an axially symmetrically aligned first bore 16, the diameter of which is larger than that of the thread section 1 and smaller than that of the head 14. The receiving part further has a coaxial second bore 17, which is open on the end located opposite the first bore and the diameter of which is large enough for the screw element to be guided through the open end with its thread section through the first bore and with the head 14 to the base of the second bore. Between the first and the second bore a small coaxial section us provided, which directly borders on the first bore and is constructed as spherical towards the open end, wherein the radius is substantially identical to the radius of the ball-segment-shaped section of the head 14. The inner thread 8 is again provided bordering on the open ends 7 of the legs 5, 6. The receiving part 15 further has a U-shaped recess 18, arranged symmetrical to the centre of the part, the base of which is directed towards the first bore, the two side legs 5, 6 of which extend towards the open end 7. Further, in the second bore 17 a pressure element is placed, which is situated on the spherical head 14. In operation the rod 10 is inserted into the U-shaped recess. Then the screw 9 is turned and screwed, in the same way as shown in
Instead of the above described embodiments, in which the head or the receiving part are connected in each case to a bone screw, the head or the receiving part can also be connected to a hook, as used in spinal column surgery for hooking in behind bone projections of the spinal column.
When a saw thread is spoken of above this is only supposed to characterise the direction of the partial flanks. Threads with rounded tips and rounded base, pointed threads and pointed thread base and flat thread edges and flat thread base should be also be included, as is also the case with a triangular thread, round thread and trapezoidal thread.
Whereas with known screws with a metric thread, in which the screw presses on a rod in the way shown in
As can be seen in particular in
In a preferred embodiment the difference between the measurement of radii r1 and r2 is about 1 to 5% of the outer diameter of the thread or 0.1 to 0.5 mm with a screw diameter of 5 to 12 mm.
3. A device comprising:
- a shank;
- a holding component for engaging and holding a rod, the holding component comprising a recess having two legs, a U-shaped cross-section for accommodating the rod, an open end for receiving the rod into the U-shaped cross-section between the legs, and an inner thread on the legs; and a screw, which cooperates with the inner thread and directly or indirectly acts on the rod when positioned in the recess, the screw having a central axis;
- wherein the inner thread comprises an inner thread base, first and second partial flanks adjacent to one another, and an internal thread edge between the first and second partial flanks;
- wherein the screw has an outer thread for engagement with the inner thread of the legs, the outer thread comprising an outer thread base, third and fourth partial flanks adjacent to one another, and an external thread edge between the third and fourth partial flanks;
- the first and third partial flanks, which face away from the open end, enclose a negative angle β2 with a plane which extends through the respective thread base and extends perpendicular to the central axis;
- the second and fourth partial flanks, which face toward the open end, enclose a positive angle β1 with a plane which extends through the respective thread base and extends perpendicular to the central axis.
4. The device according to claim 3, wherein the internal thread base is rounded between adjacent first and second partial flanks.
International Classification: A61F 2/30 (20060101);