JP2002052031A - Bone screw - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bone screw capable of being screwed into a bone within a limited range of thickness such as a skull to fix a member to the bone. SOLUTION: This bone screw 10 has a screw part 15 having a thread ridge formed by a front face 12 of the screwing-in direction, vertical to an axis 11 of the bone screw 10, and an inclined rear face 13, and having a self-tappable tapered structure, and a head part 16 positioned at a rear end of the screw part 15 and having a diameter larger than the screw part 15 for fastening the member, an elastic linear material is approximately circular bent to form a stop ring 25 having an outwardly projecting end part on a rear end with respect to the screwing direction, and the stop ring 25 is mounted on a boundary between the screw part 15 and the head part 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bone screw for screwing into a bone having a limited thickness such as a skull and fixing a member.

[0002]

2. Description of the Related Art Screws must be implanted in bone to fix members used for purposes such as correcting and supporting the arrangement of bone. Conventionally used screws for that purpose include:
For example, PCT / US94 / 00950 (WO 94/1
There is a symmetric type having a relatively shallow thread described in the invention of 8917). This type of screw is suitable when sufficient depth is obtained, as in the bone of the spine,
It is unsuitable for bones that have only a very limited thickness, such as skulls. For other types of screws, see JP-A-7
No. 2,557,739 is also known which has a relatively deep trapezoidal thread found on the shaft of a bone screw. Screws of this form exhibit greater fixation forces than screws of the above form, but sufficient fixation forces cannot be expected with bones of limited thickness, such as skulls. This is disclosed in
The same applies to the screw described in No. 3201.

[0003] As a result of studying these screws, the inventor has come to the conclusion that the conventional screw has a problem in the shape of the thread. The reason is as follows. FIG. 4 (a)
Assuming that the angle of the thread is α and the load in the tightening axial direction is P with respect to the triangular screw shown in, the force acting at right angles to the slope of the thread is P / cos (α / 2), and the friction coefficient is μ. The friction force is represented by μP / cos (α / 2).
That is, in the shape of the screw thread, the slope of the screw thread on the front side in the screwing direction affects the force required at the time of screwing, and the slope of the screw thread on the rear face in the screwing direction has an effect on the loosening of the tightened screw. From this viewpoint, it can be seen that the asymmetrical thread shape as disclosed in Japanese Patent Application Laid-Open No. 7-255739 has an adverse effect on the prevention of loosening.

On the other hand, some conventional bone screws which have been used for screwing into the skull may not fit into the bone having a limited thickness and the tip thereof may fall into the skull. Even if it penetrates the skull, the tip is blocked by the cerebral meninges and does not enter the brain, so it is a prescription from the viewpoint that there is no problem. But even so, if a patient undergoing such an operation bangs or falls over the head, the impact can damage the cerebral meninges at the tips of the bone screws and have serious consequences. The coming is easily predictable.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object thereof is to fix a required member by screwing into a bone having a limited thickness such as a skull. It is to provide a bone screw that can do.

It is another object of the present invention to provide a bone screw which can prevent a serious accident due to accidental removal of a bone screw after being attached to a bone. Most desirably, the bone screw fits within a bone of limited thickness and does not penetrate. However, it can be considered that there is no need to be concerned with whether the brain is actually penetrated, even if the brain is not damaged by the bone screw even after the impact.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention has a thread in which a front surface in a screwing direction is perpendicular to an axis and a rear surface to a coaxial line is inclined,
And has a screw portion having a tapered structure capable of self-tapping, and a head at the rear end of the screw portion and formed with a larger diameter than the screw portion to tighten the member, furthermore, the wire rod is substantially circular Forming a retaining ring having a tangentially outwardly protruding end at the rear end in the screwing direction, and arranging the retaining ring at a boundary between the screw portion and the head. It is.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION A bone screw according to the present invention is screwed into a bone having a limited thickness such as a skull to fix required members.

In order to obtain the force for fixing the required members, it is necessary to firmly connect to the bone tissue. One of the requirements for the present invention is that the front surface in the screwing direction is perpendicular to the axis. The rear face to the coaxial line is provided with inclined threads. The front face in the screwing direction is the face toward the tip of the screw, and the rear face is the opposite face, that is, the face toward the head of the screw. The outer diameters of the screws shown in FIGS. 1B, 2 and 4B correspond to this. Front vertical means substantially vertical, and substantial means not causing loosening.

Although the asymmetrical thread itself is known to be called, for example, a saw tooth screw, a conventionally known thread has a front surface inclined in the screwing direction as disclosed in JP-A-7-255739. , The rear surface is perpendicular to the axis, so that the structure of the present invention is reversed.
The fact that a thread having such a configuration has a rather adverse effect on preventing loosening of a threaded screw and the basis thereof have already been described.

The thread having the above-mentioned thread has a tapered structure capable of self-tapping. The self-tapping thread into the bone allows a plurality of pitches, preferably several pitches, to penetrate into the bone tissue and obtain the necessary force for fixation with several threads. Another requirement is the ratio of the diameter to the length of the threaded portion, and if it is too small, the required fastening force cannot be exerted. The ratio of diameter to length is usually on the order of 1: 2 or 2: 3 or 1: 1. Therefore, the bone screw according to the present invention has a thin and short appearance.

The rear end of the threaded portion has a head formed with a larger diameter than the threaded portion for screwing in the threaded portion and fastening required members. The head thus has means for screwing. The means is, for example, a hexagonal head or a wrench hole. The head also has a portion (means) for tightening and fixing the member itself by inserting a screw portion into a screw hole drilled in the member and screwing it into bone. This is a portion corresponding to a so-called seat surface. It is desirable that the contact surface between the screw head and the member has a close contact with the head and the counterbore, rather than a planar contact. By such close contact, the screwing direction of the bone screw is defined in a direction substantially perpendicular to the bone surface, and a firm fixation is ensured.

[0013] Further, a bending ring is formed on the elastic wire material to form a retaining ring having a radially outwardly projecting end at an end rearward with respect to the screwing direction. And the head. The retaining ring fits inside the screw hole drilled in the member, while a concave arrangement for placing the retaining ring can be specially formed at the boundary between the screw and the head, When the stop ring is arranged here, it is desirable that the outer diameter of the stop ring is fitted with the inner diameter of the screw hole formed in the required member with a tolerance of zero. A tolerance of zero means such a practical critical dimension that a retaining ring can enter without tolerance. Since the stop ring fitted with the tolerance 0 has a protruding end facing outward, the bone screw is allowed to rotate in the screwing direction,
If you try to rotate in the opposite direction, the elastic reaction force cuts into the fitting wall of the screw hole drilled in the member, significantly increasing the frictional force, preventing it from rotating, and removing the bone screw by mistake. To prevent

[0014]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a bone screw according to the present invention; FIG. 1 shows a plan, front and lower surface of a bone screw 10 of the present invention.

The bone screw 10 has a front surface 12 which is substantially perpendicular to the axis 11 in the screwing direction, and a rear surface 13 which has an inclined thread.
4 has a tapered screw portion 15 having a 4 and a head portion 16 formed at a rear end of the screw portion 15 to have a larger diameter than the screw portion 15 for tightening the member 20. The head 16 is tapered, and a screw hole 21 formed in the member 20 at the tapered surface 17 thereof.
Of the bone B
Are defined substantially at right angles to the outer surface. Reference numeral 18 denotes a means for screwing a bone screw, which is shown as a hexagon wrench hole. The dimensions and the like of the screw portion 15 of the bone screw 10 shown as an example are shown in a non-limiting manner. The maximum diameter D of the screw portion 15 and the length L to the tip of the screw portion 15 including the arrangement portion 19 of the retaining ring 25 are shown. Is set to 2: 3, and the thread angle α is set to 45 °.

Further, a concave arrangement portion 19 for disposing the retaining ring 25 is formed at the boundary between the screw portion 15 and the head 16. The outer diameter of the arranging portion 19 is set so that the outer diameter of the retaining ring 25 arranged in the arranging portion 19 is fitted to the inner diameter of the screw hole 21 formed in the member 20 with a tolerance of zero. The retaining ring 25 is formed by bending a wire having appropriate elasticity into a bent circle, and at the rear end in the rotational direction at the time of screwing,
For example, the protrusion 26 is formed tangentially outward. It is desirable that the protruding end portion 26 be sharp enough that the end portion can bite into the surface constituting the outer diameter of the arrangement portion 19.

The illustrated arrangement portion 19 is formed in a groove shape, so that a brim portion 23 is formed at the boundary with the screw portion 15. The collar portion 23 may have a difference in dimension such that it can easily pass through the screw hole 21, and may or may not extend beyond the screw hole 21 at the time of mounting.

The bone screw 10 is screwed into the skull B, and the member 2
FIG. 2 shows a state where 0 is fixed. Bone B comprises outer and inner hard layers c and c 'and a soft layer d sandwiched between them. In FIG. 2, the bone screw 10 penetrates the hard layer c on the outer surface, is screwed into the soft layer d and stops.
Is fixed to the outer surface of the bone under pressure. In this state, the force acting at right angles to the rear surface of one thread is P / cos 45 °, where P is the load acting in the axial direction. On the other hand, the force acting on the face of a thread having a thread of 60 degrees is P / cos (60 ° / 2).
Therefore, the acting force according to the present invention is much larger,
Moreover, since it is not necessary to consider the influence of the pressure on the front surface as in the case of the target thread, it can be seen that a strong fixing force can be obtained.

The soft layer d into which the screw portion 15 is screwed.
In particular, the strength is insufficient for osteoporosis patients, and it is difficult to obtain the required strength with conventional bone screws. However, according to the present invention, the front surface in the screwing direction is vertical, and the slanted threads on the rear surface can be screwed into the fragile bone of an osteoporosis patient without destroying the bone tissue to obtain the required strength. Can be. The member 20 fixed in this way is
For example, as shown in FIG. 5, the skull E is used to fix the skull E to the spine G.

Further, the bone screw 10 of the present invention and the member 20 fixed thereby cannot be easily removed by the action of the retaining ring 23. FIG. 3A shows a state in which the stop ring 23 fitted in the arrangement portion 19 is located between the bone screw 10 and the screw hole 21 of the member 20. In this state, the friction due to the tight fitting reduces the stop ring 23. And screw hole 2
And 1. If a force acts in the direction in which the bone screw 10 is removed in this state, the friction between the protruding end portion 26 of the retaining ring 25 and the screw hole 21 increases, and the role of a detent is fulfilled, so that the bone screw 10 is erroneously removed. Can be prevented.

[0021]

Since the present invention is constructed and operated as described above, it has a limited thickness like a skull by having a thread which is perpendicular to the axis at the front in the screwing direction and inclined at the rear. The member can be firmly fixed even to bones that only have it, and it fits within the thickness of the bone, so there is no danger that the tip will protrude into the skull and cause an accident, and it will be fixed once Serious accidents can be prevented because the bone screws and members that are in use are less likely to be accidentally removed.

[Brief description of the drawings]

FIG. 1 (a) is a plan view showing an embodiment of a bone screw according to the present invention. (B) Front view of the same. (C) Similarly, the lower surface explanatory drawing of a screw part.

FIG. 2 is a sectional view showing a use state.

FIG. 3A is a plan view showing a state where a retaining ring is attached. (B) A plan view of a retaining ring.

FIG. 4 (a) is an explanatory view of an operation of a conventional screw thread. (B) Operation explanatory drawing of the screw thread of this invention.

FIG. 5A is a rear view showing an example of using a bone screw. (B) Side sectional drawing which shows the example of use of a bone screw.

Claims (3)

[Claims] 1. A bone screw for screwing into a bone having a limited thickness such as a skull and fixing a member, wherein a front surface in a screwing direction is perpendicular to an axis and a rear surface to a coaxial line is inclined. A threaded portion having a thread which has a tapered structure and a self-tapping capable of tapering, and a head formed at the rear end of the threaded portion and having a larger diameter than the threaded portion to tighten the member. Further, the elastic wire is bent into a substantially circular shape to form a stop ring having a protruding end portion at the rear end in the screwing direction, and this stop ring is formed between the screw portion and the head. A bone screw having a configuration disposed at a boundary. 2. A concave arrangement portion for arranging a retaining ring is formed at the boundary between the screw portion and the head, and the diameter of the arranging portion is equal to the outer diameter of the retaining ring when the retaining ring is disposed there. 2. The bone screw according to claim 1, wherein the screw is fitted with an inner diameter of a screw hole formed in the member with a tolerance of zero. 3. The head according to claim 1, wherein the head is tapered so as to be in close contact with the countersink of the screw hole formed in the member, whereby the screwing direction is defined substantially perpendicular to the bone surface. Bone screw as described.