NZ548772A

NZ548772A - Tapered multiple diameter bone screw

Info

Publication number: NZ548772A
Application number: NZ548772A
Authority: NZ
Inventors: Andre Schlienger; Peter Senn
Original assignee: Synthes Gmbh
Priority date: 2004-02-23
Filing date: 2004-02-23
Publication date: 2009-09-25

Abstract

A device for bone fixation comprising, a) at least one bone screw 1 with a shaft carrying a thread, a front end and a rear end, the external diameter "d' of the thread or the core diameter "k" of the shaft tapering in a middle region L4 of the shaft from a larger dimension "dz" or "kz" respectively which it exhibits on a length L3 towards the front end to a smaller dimension "d1" or "k1" respectively, "d1" or "k1" not being equal to zero, and b) a medullary pin 20, whereby c) the shaft is provided with a concentrically disposed, radially expandable sleeve 10, d) the sleeve 10 being positioned on that part of the shaft, which has the smaller external diameter "d1", e) the sleeve 10 is expandable to a diameter dS-L5 by screwing into the middle region L4 and to a diameter dS-L3 by screwing over the length L3, f) the medullary pin 20 has the diameter ON and a transverse borehole extending transversely to its longitudinal axis, the transverse borehole having a diameter Os, which is larger than dz as well as larger than dS-L5 but smaller than dB-L3.

Description

548772 1 BONE SCREW The invention relates to a bone screw, as well as to a device for fixing bones.

Such a bone screw is known from DE-A 35 38 238, which furthermore also shows a dowel, which interacts with the bone screw. However, in the case of this known device, the dowel is anchored first of all separately in the bone, after which the bone screw is screwed into the dowel. The purpose of this known device accordingly is to connect two bone fragments in accordance with the dowel/screw principle, which is already known from general fastening techniques.

In the case of the invention, a fixation of two bone fragments is not the primary objective and is realized only secondarily by means of a medullary pin. Instead, the primary objective is to anchor a bone screw stably in the transverse borehole of a medullary pin, or to at least provide a useful choice to the public. The known transverse locking screws namely all have some clearance with respect to the transverse borehole of the medullary pin.

Thus, it is an object of the invention to provide a bone screw, which can be brought into the transverse borehole of a medullary pin without any clearance, or to at least provide a useful choice to the public.

Pursuant to one aspect of the invention, there is provided a device for bone fixation comprising at least one bone screw with a shaft carrying a thread, a front end and a rear end, the external diameter "d1 of the thread or the core diameter "k" of the shaft tapering in a middle region (L4) of the shaft from a larger dimension "d2" or "k2" respectively which it exhibits on a length (L3) towards the front end to a smaller dimension "di" or intellectual property office of n.2. 2 0 AUG 2009 RECEIVED 548772 la 'V respectively, "di" or "k-T not being equal to zero, and a medullary pin, whereby the shaft is provided with a concentrically disposed, radially expandable sleeve, the sleeve being positioned on that part of the shaft, which has the smaller external diameter "dr, the sleeve is expandable to a diameter dB-i_5 by screwing into the middle region (L4) and to a diameter de-L3 by screwing over the length (L3), the medullary pin has the diameter DN and a transverse borehole extending transversely to its longitudinal axis, the transverse borehole having a diameter DB, which is larger than d2 as well as larger than dB-i_5 but smaller than db-l3- 7 inihllhc'i'ual property office of n.z. 2 0 AU6 2009 RECEIVED The advantages, achieved by the invention, consist essentially of its simplicity (pre-instalied sleeve on the bone screw) and of the absence of clearance with respect to the medullary pin, achievable therewith.

In the case of a special embodiment, the thread of the bone screw has a constant depth, the diameter of the core of the shaft, in the middle region (U), decreasing towards the front end from a larger value "kf to a smaller value "kV without assuming the value of zero. The radial expansion accordingly takes place by way of the expansion of the of the core diameter. This guarantees a certain clamping force in the transverse borehole of the medullary pin. In comparison to an expansion of the external diameter of the thread, the expansion of the core diameter results in a larger mass, which is available for locking the screw.

In the case of a special embodiment, the sleeve has an internal thread. The axial movement, which produces the radial expansion of the sleeve due to the particular geometry of the conical section of the core in the middle part (Ld), is generated by the pitch of the internal thread. The rotational or screwing movement is required anyhow for introducing the screw into the transverse borehole of a medullary pin.

The sleeve may have one or more longitudinal slots, resulting in the advantage that the radial expansion of the sleeve can be achieved with a lesser expenditure of force or with a smaller torque. The pre-installation of the sleeve on the screw is simpler, the shear forces are equalized and the sleeve is subjected to a lower internal stress. If a single longitudinal slot is provided, it may extend over the whole height of the sleeve. However, the longitudinal slot may also extend only over a portion of the height of the sleeve. In the case of this embodiment, the slot may be open at one side or ciosed at both sides. In the case of such partial slots, several longitudinal slots may also be provided, which preferably are open alternately at the top and at the bottom.

In the case of a further embodiment, the sleeve, with the diameter DH, has at one of its ends, a peripheral flange of larger diameter DF > Dh. The flange functions as a stop or as a positioning means for the sleeve at the medullary pin, so that the bone screw, with its comically expanding middle part, must be screwed through the sleeve, expanding the latter, radially, while the sleeve itself is in contact with the middle part.

The sleeve may, for example, consist of a non-absorbable plastic, preferably of PEEK. The advantages of such a sleeve are the biocompatibility, the ready produceability, the simple installation and the fact that, if the pin is pulled out, the sleeve can be compressed in the event that it remains hanging in the hole of the pin.

The sleeve may, however, also consist of a metallic material. The advantages here are the higher strength, the biocompatibility and the fact that the sleeve may consist of the same material as the medullary pin and the screw. However, the metallic sleeve is also more ductile, so that it can flow somewhat, if it is inserted at an angle. Finally, the metallic sleeve has the advantage that fragments are not formed.

The sleeve may also consist of an absorbable plastic. A first advantage lies therein that the angularly stable locking decreases with time. A second advantage lies therein that, in the event that such a sleeve is lost, it is not a serious disadvantage that the sleeve is absorbed overtime. 548772 4 In the case of a further embodiment, a shaft part is disposed in the region "L2" with an enlarged external diameter d3 > D2 between the rear end of the bone screw and that part of the screw with an enlarged external diameter "d2".. This arrangement has the advantage that it closes off the hole drilled in the front corticalis of the bone, so that the flange of the sleeve can be brought through the hole in the first corticalis without much expenditure of force. Moreover, the shaft part may be provided in the region "L2" with an external diameter d3 > d2 with an external thread. The externa! thread with the external diameter d3 results once again in a screw-like anchorage in the bone.

The medullary pin, which is to be used with the bone screws described above, has a diameter DN and a transverse borehole, which extends transversely to its longitudinal axis and advisably has a diameter DB, which is larger than d2 as well as larger than dB_L5. Advantageously, the diameter DH of the sleeve satisfies the condition that DH < 1.2 DB. Advantageously, the length HL of the sleeve should be less than 1.5 DN and more than 0.2 DN.

In the case of a further embodiment, the medullary pin has a continuous cannulation and a wall thickness WN. The length HL of the sleeve advantageously corresponds to the wall, thickness WN of the medullary pin.

The invention and further developments of the invention are described in even greater detail in the following by means of partially diagrammatic representations of several examples, of which Fig. 1 shows a longitudinal section through a bone screw with the sleeve mounted, 548772 Fig. 2 shows a longitudinal section through the bone screw of Fig. 1 inserted in a medullary pin with a transverse borehole, Fig, 3 shows a longitudinal section through the bone screw of Fig. 1 and Fig. 4 shows a perspective view of the sleeve of Fig. 1.

An embodiment of the bone screw is shown in Figs. 1 and 3. It comprises a shaft 3, a front end 4 and a rear end 5 with a screw head 6. The bone screw 1 has five segments, which are disposed axially behind one 3 another, these segments being formed as follows: • the screw head 6 extends on a length L1? measured from the rear end 5; • adjoining thereon, a shaft part 8 with a constant external diameter d3 and an externa! thread 9 having a constant core diameter k3 extends on a length L2; • adjoining thereon, a segment of the shaft 3 with a constant external diameter d2 < k3 and a thread 2 having a constant core diameter k2 extends on a length L3; • adjoining thereon, a segment of the shaft 3, tapering at an angle a ^ towards the front end 4, extends on a length U so that the external J - ■ diameter d2 of the thread decreases in this segment to an external diameter di. Likewise, the core diameter k2 decreases in this segment to a core diameter k-i and • a segment of the shaft 3 with the one constant external diameter d-f and also a thread 2 having a constant core diameter k^ extend between the segment of the length L4 and the front end 4.

The thread 2 is configured so that the thread depths ti and t2 on the segments with lengths L5 and L3 are identical. Furthermore, in Fig. 1, the sleeve 10 is screwed from the front end 4 so far over the shaft 3, that its end, which is provided with the flange 12, extends into the conical segment of length L&. Due to the conicity, the sleeve 10 is expanded somewhat and has a diameter db.l5 > Dh (Fig. 4). If the sleeve 10 is screwed further towards the rear end 5 (shown by a broken line), it is expanded further by the conical segment of length U, until it reaches a diameter dB-L3 on the segment of length L3 on the one external diameter d3.

The embodiment of the bone screw 1, shown in Fig. 1, is shown in Fig. 2 together with a sleeve 10, introduced into a transverse borehole 22 of a medullary pin 20.. The medullary pin 20 is provided with a continuous cannulation 23, which extends coaxially with the longitudinal axis 21 of the medullary pin 20 and has an external diameter DN and a wall thickness WN. The transverse borehole 22 has a diameter DB, which is approximately 90% of the diameter dB-L3 of the sleeve 10. The sleeve 10 is inserted completely into the transverse borehole 22 and, with a flange 12, is in contact with the wall of a facet 24 of the transverse borehole 22.

An embodiment of the radially expandable sleeve 10 with a continuous diameter DH is shown in Fig. 4. The outer casing of the sleeve 10 is formed with a continuous longitudinal slot 11, from which the sleeve 10 gains its elasticity. Furthermore, the cavity of the sleeve 10 is provided with an internal thread 13. At one end of the sleeve 10, a flange 12 with an external diameter DF > DH is disposed for axial contact with a suitable supporting surface.

Claims

548772 7 What we claim is:

1. A device for bone fixation comprising a) at least one bone screw with a shaft carrying a thread, a front end and a rear end, the external diameter "d' of the thread or the core diameter "k" of the shaft tapering in a middle region (L4) of the shaft from a larger dimension "d/ or "k2" respectively which it exhibits on a length (L3) towards the front end to a smaller dimension "d-i" or "ki" respectively, "di" or 'V not being equal to zero, and b) a medullary pin, whereby c) the shaft is provided with a concentrically disposed, radially expandable sleeve, d) the sleeve being positioned on that part of the shaft, which has the smaller external diameter "di", e) the sleeve is expandable to a diameter dB-i_5 by screwing into the middle region (L4) and to a diameter dB.L3 by screwing over the length (L3), f) the medullary pin has the diameter Dn and a transverse borehole extending transversely to its longitudinal axis, the transverse borehole having a diameter DB, which is larger than d2 as well as larger than dB-i_5 but smaller than dB-L3.

2. The device of claim 1, wherein the thread has a constant depth and the diameter of the core of the shaft decreases in the middle region (L4) in the direction of the front end from a larger value "k2" to a smaller value "ki" without assuming the value of zero,

3. The device of claims 1 or 2, wherein the sleeve has an internal thread.

4. The device of any one of claims 1 to 3, wherein the sleeve has at least one longitudinal slot. ln7euictual property office of n.z 2 0 AUG 2089 RECEIVED 548772 8

5. The device of claim 4, wherein the longitudinal slot extends over the whole height of the sleeve.

6. The device of claim 4, wherein the longitudinal slot extends only over a portion of the height of the sleeve.

7. The device of claim 6, wherein several longitudinal slots are present.

8. The device of claim 6, wherein the longitudinal slots are open alternately at the top and at the bottom.

9. The device of any one of claims 1 to 8, wherein the sleeve with the diameter DH has a peripheral, encircling flange with an enlarged diameter DF > Dh at one of its ends.

10. The device of any one of claims 1 to 9, wherein the sleeve consists of a non-absorbable plastic.

11. The device of claim 10, wherein the plastic is PEEK.

12. The device of any one of claims 1 to 9, wherein the sleeve consists of a metallic material.

13. The device of any one of claims 1 to 9, wherein the sleeve consists of a bio-absorbable plastic.

14. The device of any one of claims 1 to 13, wherein adjoining the end of the bone screw there is a shaft part in a region "L2" with a diameter d3 and adjoining to this region "L2" is a region "L3" with the diameter d2 and wherein Intellectual property office of m.z 2 0 AUG 2009 RECEIVED 548772 9 d3 is larger than d2.

15. The device of claim 14, wherein the shaft part is provided with an external diameter d3 > d2 with an external thread in the region "L2".

16. The device of any one of claims 1 to 15, wherein A) the bone screw comprises a screw head, which extends over a length (Li), measured from the rear end; B) a segment of the shaft, with a thread having a constant external diameter "d2" and a constant core diameter "k2", extends over a length (U); C) adjoining thereon, a segment of the shaft, tapering with an angle a towards the front end, extends over a length (L4), so that the external diameter "d2" of the thread decreases in this segment to an external diameter "di" and D) on a length (L5), between the segment with the length (U) and the front end, a segment of the shaft extends with the thread having a constant external diameter "di" and also a constant core diameter "ki"; E) the lengths and diameters of these segments being larger than zero.

17. The device of claim 16, wherein the core diameter "k2" decreases to a core diameter 'V in the segment of the shaft having the length (L4).

18. The device of claim 16 or 17, wherein a shaft portion with a constant external diameter "d3 > d2" and an external thread having a constant core diameter "k3" extends on a length (L2), which adjoins the length (Li).

19. The device of any one of claims 16 to 18, wherein the depths of the threads "tr and "t2" are identical on the segments with the length (L5) and (U). intellectual property office of n.z. 2 0 AUG 2009 RECEIVED 548772 10

20. The device of any one of claims 1 to 17, wherein the diameter DH of the sleeve fulfills the condition that DH^1,2DB.

21. The device of any one of claims 1 to 20, wherein the length HL of the sleeve is more than 1.5DN.

22. The device of any one of claims 1 to 21, wherein the length HL of the sleeve is greater than 0.2DN.

23. The device of any one of claims 1 to 22, wherein the medullary pin has a continuous cannulation and a wall thickness WN.

24. The device of claim 23, wherein the length HL of the sleeve corresponds to the wall thickness WNof the medullary pin.

25. A device for bone fixation substantially as herein described with reference to any embodiment shown in the accompanying drawings. intellectual property office of n.2. 2 0 AUG 2009 RECEIVED