DE102012112150A1 - Self-tapping bone screw for connecting bone cover with remaining cranial bone after craniotomy for neurosurgery in head of patient, has shaft middle region including core diameter lower than diameter in shaft region that is adjacent to end - Google Patents

Abstract

The screw (10) has a screw shaft (12) including an end (14) designed as a screw head (16), and another end (18) designed as a screw tip (20). An outer thread i.e. self-tapping thread, extends at an outer side of the shaft. The screw shaft includes a middle region (24), which has a core diameter (d1) lower than a core diameter (d2) in a shaft region that is adjacent to the latter end. The middle region is extended over approximately quarter to three-quarters, preferably third to two-thirds of a shaft length. The outer thread includes a constant outer diameter (Ds) over the entire shaft length.

Description

The invention relates to a bone screw, in particular for neurosurgery. The bone screws according to the invention are also particularly suitable for oral and maxillofacial surgery. The bone screws have a screw shaft having a first and a second end, the first end of the screw shaft is formed as a screw head and the opposite second end of the screw shaft as a screw tip, wherein the screw shaft has a screw thread extending on its outer side.

When creating a neurosurgical access to the head of a patient, a craniotomy is usually performed after a trepanation. Here, a part of the skull bone is removed as bone cover for the duration of the operation, more precisely, sawed out, and normally then again position-oriented inserted into the skull.

After insertion, the bone cover is connected to the skull bone so that it is stabilized in its position. For this purpose, different connection systems can be used. According to a very common technique, the bone cover is connected to the remaining cranial bone with bone plates and bone screws.

For this purpose, one or more bone plates are screwed to the bone lid, which does not usually happen to the patient. The position-oriented positioning on the skull bone is done afterwards, and the bone lid is then screwed with the help of bone plates on the skull bone.

For this purpose, bone screws are used, which are self-tapping, or screws that must be pre-drilled for their use.

The screwing is a critical step in both cases, since it can come here from the screw head of the bone screw when applying the screw-in torque for slipping the screwdriver. The screw-in moments are greater, the deeper the bone screw has penetrated into the bone material. The surgeon faces similar challenges in the field of oral and maxillofacial surgery.

From the US patent publication US 2001/004694 A1 a bone screw is known which has a trained as a drill screw tip.

A self-drilling and self-tapping screw screw with a tapered screw tip is disclosed in the US patent US 5,925,048 A disclosed.

Another self-drilling and self-tapping bone screw, the screw tip is formed with a flute is in the DE 295 20 312 U1 described.

From the DE 10 361 044 B4 a bone screw is known in which the screw tip is formed off-center.

For all these bone screws results the problem described above.

The object of the invention is to propose an improved bone screw of the type described above, with which the risk of slipping when applying the bone screw is reduced.

This object is achieved by a bone screw with the features of claim 1.

According to the invention, the shaft has a smaller core diameter in its central region than at its end regions adjacent to the first and second ends. The fact that the diameter of the shaft is smaller in this central region, the friction of the screw when screwing into the bone material over a certain time remains substantially constant, since the central portion of the shaft no or only a negligible direct contact with the has surrounding bone material. However, the holding force of the screw is not reduced thereby, since the central region of the shaft of the bone screw is located in the cancellous part of the cranial bone.

The diameter of the bone screw, on the outer circumference of the screw thread (outer contour) determined to the diameter of the shaft at the first and second end can vary within wide ranges, as is the case with the above-mentioned bone screws in the prior art.

The bone screws according to the invention are typically made of metal, in particular titanium or titanium alloys, ceramics and plastic composite materials.

Preferably, the central region of the shaft with a reduced shaft diameter, hereinafter also called the core diameter, extends over approximately one quarter to approximately three quarters, in particular over approximately one third to approximately two thirds of the shaft length.

Preferably, the core diameter of the shaft in the central region has about 50% to about 80%, in particular about 60% to about 70%, of the core diameter of the shaft in the region adjacent to the second end of the shaft, which does not belong to the screw tip in which a taper of the outer contour of the screw is given.

Even a slight reduction of the core diameter to about 90% brings a noticeably smaller increase in the screwing when screwing the bone screw with it.

Preferably, the screw thread over the entire length of the shaft has a substantially constant diameter (outer contour). This ensures that the bone screw receives a good guidance in the bone material.

More preferably, it can be provided that the screw shaft has in its region adjacent to the first end a core diameter which is greater than the diameter in the region adjacent to the second end. This ensures a particularly secure fit of the bone screw in the cortical region of the bone material.

In this preferred embodiment of the bone screw according to the invention, it is particularly advantageous if the core diameter of the screw shaft is continuously progressively formed at its region adjacent to the first end in the direction of the screw head.

It can certainly be provided that the core diameter of the shaft at its first end adjacent area substantially reaches the diameter of the screw thread (outer contour).

The bone screw according to the invention can also be formed with a self-tapping screw thread.

Preferably, in particular in the formation of the screw thread as a self-tapping screw thread, the screw tip is formed with a flute.

The present invention will be explained in more detail with reference to the drawing. Show it:

1 a first embodiment of a bone screw according to the invention;

2 a second embodiment of a bone screw according to the invention; and

3A and 3B : two conventional bone screws.

1 shows a designed according to the invention for neurosurgery bone screw 10 with a shaft 12 , one at the first end 14 of the shaft 12 trained screw head 16 , one at the second end 18 of the shaft 12 trained screw tip 20 as well as essentially from the screw head 16 to the screw tip 20 ersteckenden external thread 22 ,

The shaft 12 points in a middle area 24 a core diameter d ₁ , which is about 66% of the core diameter d ₂ in the region of the shaft adjacent to the second end 18 is. The diameter D _{S of} the screw thread 22 (Outer contour) is substantially the same over its entire length, except for the screw tip 20 counting part, leaving the bone screw 10 has a substantially cylindrical geometry. Based on the core diameter d _{2 of} the shaft, the core diameter d _{1 is} about 66%. The core diameter d ₂ is about 83% of the diameter of the screw thread. In absolute terms, in one example, the diameter D _{S is} about 1.5 mm, the core diameter d _{1 is} about 1 mm, and the core diameter d _{2 is} about 1.25 mm.

The central region of reduced core diameter d ₁ extends over approximately half the length of the stem 12 ,

In the screw head 16 adjacent area, the shaft 12 a varying core diameter d ₃ , which increases substantially continuously from the diameter d _{1 of} the central region to the diameter D _{S of} the screw thread.

2 shows a likewise designed for neurosurgery bone screw according to the invention 40 that essentially features the bone screw 10 having, with a shaft 42 , one at the first end 44 of the shaft 42 trained screw head 46 , one at the second end 48 of the shaft 42 trained screw tip 50 as well as essentially from the screw head 46 to the screw tip 50 ersteckenden external thread 52 ,

The shaft 42 points in a middle area 54 a core diameter d ₁ , which is about 66% of the core diameter d ₂ in the region of the shaft adjacent to the second end 48 is. The diameter D _{S of} the screw thread 52 is substantially the same over its entire length, except for the screw tip 50 counting part. The core diameter d ₂ is again about 83 % of the outer diameter D _{S of} the screw thread.

In terms of numbers, for example, the diameter D _{S can be} about 1.5 mm, the core diameter d ₁ about 1 mm and the core diameter d ₂ about 1.25 mm.

The screw tip 50 is here with a flute 56 formed, which is the self-tapping screwing the bone screw 40 supported in a bone material.

The central region of reduced core diameter d ₁ again extends over approximately half the length of the stem 52 ,

In the screw head 46 adjacent area, the shaft 42 a varying core diameter d ₃ , which is substantially continuous from the diameter d _{1 of} the central region 44 on the diameter D _{S of} the screw thread 50 increased.

In the 3A and 3B are two conventional bone screws 60 and 70 shown.

At the bone screw 60 is a substantially uniform core diameter of the shaft 62 given, except for the screw head 64 subsequent area. Also, the outer diameter of the bone screw 60 is substantially constant over its entire length, except for the screw tip 66 attributable part.

The bone screw 70 is in contrast designed as a conical screw whose shaft 72 has a diameter that is continuous with the screw tip 74 rejuvenated.

If one compares the screwing moments which occur in practice when applying a bone screw as a function of the respectively achieved penetration depth into a bone material in the case of the bone screws 10 . 60 and 70 of comparable length, the following results:
The increase of the screw-in torque with increasing penetration depth is with the conical screw 70 into the bone material most steeply and substantially linearly, except for the portion where the shaft end adjacent to the screw head penetrates into the bone material, which is associated with a steeper rise in this force / displacement curve.

With the screws 40 and 60 one compares with the curve of the screw 70 when penetrating the screw tip in the bone material for a short time a slightly steeper increase in the force / displacement curve fixed, but at still relatively low screw-in moments. Then the curve is at the screw 60 a little less steep than the screw 70 , so that the screwing in when reaching the fully screwed position is significantly lower than in the conical screw 70 ,

Again, a significant reduction in the steepness of the force / displacement curve is in the screw according to the invention 40 compared to the screw 60 observed, shortly before the complete screwing, the screw-in torque increases a little more steeply. Overall, however, the Einschraubmomente remain over the entire course of the curve, except for the beginning of the penetration of the screw tip into the bone material, well below the curves of the screws 60 and 70 ,

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2001/004694 A1 [0007]
• US 5925048 A [0008]
• DE 29520312 U1 [0009]
• DE 10361044 B4 [0010]

Claims (9)

Bone screw, in particular for neurosurgery, with a screw shaft having a first and a second end, wherein the first end of the screw shaft is formed as a screw head and the opposite second end of the screw shaft as a screw tip, wherein the screw shaft has a screw thread extending on its outside, characterized in that the shaft has a central portion which has a smaller diameter than the end portions of the shaft adjacent the first and second ends. Bone screw according to claim 1, characterized in that the central region of the shaft extends over approximately one quarter to approximately three quarters, in particular over approximately one third to approximately two thirds, of the shaft length. Bone screw according to claim 1 or 2, characterized in that the diameter of the shaft in the central region about 50% to about 80%, in particular about 60% to about 70%, of the diameter of the shaft portion adjacent to the screw tip. Bone screw according to one of claims 1 to 3, characterized in that the screw thread over the entire length of the shaft has a substantially constant outer diameter. Bone screw according to one of claims 1 to 4, characterized in that the screw shaft in its region adjacent to the first end has a diameter which is greater than the diameter of the region adjacent to the second end. Bone screw according to claim 5, characterized in that the diameter of the screw shaft at its first end adjacent region in the direction of the screw head is continuously increasing. Bone screw according to claim 5 or 6, characterized in that the diameter of the shaft reaches at its first end adjacent portion substantially the diameter of the screw thread. Bone screw according to one of claims 1 to 7, characterized in that the screw thread is designed as a self-tapping thread. Bone screw according to one of claims 1 to 8, characterized in that the screw tip is formed with a flute.

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