WO2005074823A1

WO2005074823A1 - Bone fixation apparatus

Info

Publication number: WO2005074823A1
Application number: PCT/KR2005/000325
Authority: WO
Inventors: Kyung-Tae Kim; Jung-Sung Kim; Byung-Soo Kim; Tae-Jin Shin; Joon-Kyu Park; Weon-Wook Park; Ki-Chan An; Sang-Hoon Lee
Original assignee: Medixalign Co., Ltd.
Priority date: 2004-02-09
Filing date: 2005-02-03
Publication date: 2005-08-18
Also published as: KR20050080493A; KR100589728B1

Abstract

Disclosed herein is a bone fixation apparatus, comprising a bone screw having a head; a cap for supporting the top of the head of the bone screw; a compression bush for pushing the cap; a receiver member having a bore in which the cap and the compression bush are accommodated and the head of the screw is hitched, and a U-shaped channel through which a support bar is accommodated; a bar fixing nut threadedly coupled to the receiver member to fix the support bar, wherein the cap has a shape of a belleville spring in which a diameter of a lower end thereof is smaller than that of an upper end, and an upper surface thereof is concavely inclined toward an inner radial direction and a lower side thereof is formed with a curvature contacting an outer circumferential surface of the head.

Description

BONE FIXATION APPARATUS

FIELD OF THE INVENTION

The present invention relates to a bone fixation apparatus adapted for use in fixing and

stabilizing bones such as spines and the like after the bones are corrected into their

normal state.

BACKGROUND OF THE INVENTION

Various bone fixation apparatuses have been provided in the known arts such as those

disclosed in Korean Patent Laid-open Publication No. 2000-48562.

Referring to FIG. 1, the bone fixation apparatus disclosed in Korean Patent Laid-open

No. 2000-48562 includes a bone screw (10) having a spherical head (12), a cap for

supporting the head (12) of the bone screw (10), a receiving member (18) and a set

screw (20). The receiving member (18) has a center bore for limiting a tapered recess

(16) for accommodating the cap (14). The receiving member (18) further has a U-

shaped channel communicating with the recess (16) and accommodating a support bar

(R) therein. The set screw (20) is threadedly coupled to the receiver member (18) to fix the support bar (R).

The head (12) of the bone screw (10) is formed with a tool-engaging recess (22) in

which a tool can be engaged. The tool engagement recess (22) is defined on a flat

upper end surface (24) of the head (12), the flat upper end surface being defined by

truncating the head (12). A portion of the receiver member (18) that is formed with the

U-shaped channel is defined with internal threads (26) for the set screw (20) to be

threadedly coupled to the internal threads (26). The cap (14) is formed at a rear surface

thereof with a contoured recess (28) in which the head (12) is partially accommodated.

If the set screw (20) is tightened, the support bar (R) compresses the cap (14), and the

cap (14) is in turn squeezed within the tapered recess (16) of the receiver member (18),

whereby the bone screw (10) is fixedly held at a vertical or inclined position of a

predetermined angle.

However, there is a problem in the bone fixation apparatus as described in the Korean

Patent Laid-open Publication No.2000-48562 in that the head (12) accommodated in

the tapered recess (16) cannot fully adhere to the recess (16) such that the bone

fixation apparatus can be easily moved by an external force or torsional force applied

to an affected part because a fixing force is weak under assembled state, thereby deteriorating the initial support state.

SUMMARY OF THE INVENTION

The present invention is disclosed to solve the afore-mentioned problem and it is an

object of the present invention to provide a bone fixation apparatus configured to

increase a supporting force of a bone screw for preventing movement of the bone, to

improve the assemblability of the bone fixation apparatus and to enable various sizes

of support bars to be used.

In accordance with an embodiment of the present invention, there is provided a bone

fixation apparatus comprising: a bone screw having a head; a cap for supporting the

top of the head of the bone screw; a compression bush for pushing the cap; a receiver

member having a bore in which the cap and the compression bush are accommodated

and the head of the screw is hitched, and a U-shaped channel through which a support

bar is accommodated; a bar fixing nut threadedly coupled to the receiver member to fix

the support bar, wherein the cap has a shape of a belleville spring in which a diameter

of a lower end thereof is smaller than that of an upper end, and an upper surface

thereof is concavely inclined toward an inner radial direction and a lower side thereof

is formed with a curvature contacting an outer circumferential surface of the head. In accordance with another embodiment of the present invention, the compression

bush is formed thereon with a channel having inclined surfaces at both inner sides

thereof so as to be compressed by various sizes of a support bar.

In accordance with another embodiment of the present invention, the compression

bush is formed at both external sides thereof with a recess of a predetermined depth,

and a punch groove of a predetermined depth is formed at an external surface of the

receiver member in a manner such that an inner portion of the receiver member can be

easily protruded toward the recess by a punch

BRIEF DESCRD7TION OF THE DRAWINGS

For a better understanding of the nature and objects of the present invention, reference

should be made to the following detailed description with the accompanying drawings,

in which:

FIG. 1 is a an assembled exemplary cross-sectional view illustrating a conventional

bone fixation apparatus;

FIG. 2 is an assembled cross-sectional view illustrating a bone fixation apparatus

according to an embodiment of the present invention; FIG. 3 is an elevation illustrating a partially-cut upper end of a bone screw of FIG. 2;

FIG. 4 is a cross-sectional view illustrating a receiver member of FIG. 2;

FIG. 5 is a cross-sectional view illustrating a cap of FIG. 2;

FIG. 6 is a plan illustrating a compression bush of FIG. 2;

FIG. 7 is a cross-sectional elevation illustrating the compression bush of FIG. 2; and

FIG. 8 is a right side view illustrating the compression bush of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention will now be described in detail

with reference to the annexed drawings, where the present embodiment is not limiting

the scope of the present invention but is given only as an illustrative purpose.

Referring to FIG. 2, the bone fixation apparatus includes a bone screw (50) having a

head (52), a cap (60) supporting the top of the head (52) of the bone screw (50), and a

compression bush (70) for compressing the cap (60). A receiver member (80)

accommodates the cap (60), compression bush (70) and support bar (R), and is fixed

by a bar fixation nut (90). The bone screw (50) has the head (52) and a threaded

portion (54) which is embedded into a bone, as shown in FIG. 3. The head (52) of the bone screw (50) has a truncated sphere-shaped configuration.

A hexagonal groove (56) of a predetermined depth is defined on the flat upper end

surface of the head (52) so that a tool such as a wrench or the like can be inserted

thereinto to screw the bone screw (50) into the bone. It is to be noted that the head may

have a groove of cross-sectional shapes other than the hexagonal shape.

Furthermore, a plurality of inclined grooves (58) are circumferentially defined on the

outer surface of the head (52), each groove spaced at a predetermined distance apart, in

order to increase frictional force between the head (52) and the cap (60). The outer

surface of the head (52) may be formed with knurls in order to increase the frictional

force with the cap (60).

A lower end of the threaded portion (54) is pointed for easy penetration into the bone.

Although each root diameter of the external threads of the threaded portion is

gradually decreased toward the lower end, the same outer diameter of the external

threads is maintained to remain unchanged.

If the external threads of a bone screw have the same outer diameter and the same

thread height throughout the entire length of the bone screw, a problem occurs in that

the bone screw is apt to loose because the fixing force is gradually decreased due to repetitive application of external force and the like.

While a structure is disclosed in the known art in which an outer diameter is gradually

decreased toward the lower end of the bone screw, this structure encounters a

drawback in that, due to the reduction in strength, the bone screw is likely to break. In

an effort to cope with this problem, in the present invention, the root diameter of the

external threads is gradually toward the lower end of the bone screw but the external

threads have the same crest diameter. As a result, the external threads are not only

securely fixed with respect to the bone, but it is also possible to prevent the strength of

the bone screw from being reduced.

Referring now to FIG. 5, the cap (60) is centrally formed with a hole (62)

communicating with the hexagonal groove (56) and is also formed thereunder with a

curvature (64) being brought into contact with a circumferential surface of the head

(52). The cap (60) is further formed thereon with an inclined upper surface (66)

recessed toward an inner radial direction. Furthermore, the cap (60) is inclined in such

a manner that an external surface (68) thereof has a radius decreased toward the lower

end.

In other words, the cap (60) substantially has a belleville spring-shape configuration in which the diameter of the lower end is smaller than that of the upper end. As is well

known in the operation of the belleville spring, if a circumference of the upper surface

(66) is pushed down by the compression bush (70) to apply a vertical force, a drooping

occurs where the upper surface (66) is lowered at a predetermined level to allow the

curvature (64) to be tightened along the circumferential surface of the head (52).

For reference, when an upper end margin of the belleville spring is pressed down by a

linear contact, a drooping of the upper end and a tightening of the lower end occur.

In the present invention, the cap (60) is formed thereon with an inclined upper surface

(66) recessed toward an inner radial direction, and is further inclined in such a manner

that an external surface (68) thereof has a radius decreased toward the lower end,

whereby an external end of the upper surface (66) is brought into linear contact with

the lower surface of the compression bush (70).

For example, the curvature (64) of the cap (60) may be formed with knurls in order to

increase the frictional force with the head (52). For further example, the cap (60) may

be manufactured with a stainless steel or an alloy steel comprising titanium, cobalt,

chrome and the like. Referring to FIGS. 6 to 8, the compression bush (70) is brought into contact with the

external end of the cap (60) to push down the cap (60). The compression bush (70) is

formed thereon with a channel (72) having an incline (72a) so as to be compressed by

various sizes of support bars. The compression bush (70) is centrally formed with a

hole communicating with the hole (62) of the cap (60) and the hexagonal groove (56)

of bone screw (50). The compression bush (70) is further defined at both external sides

thereof with recesses (76) each of a predetermined depth. The channel (72) is

accommodated with the support bar (R) and fixed by the bar fixation nut (90).

Referring now to FIG. 4, the receiver member (80) has a bore (82) which extends in an

axial direction and in which the cap (60) and the compression bush (70) are

accommodated and fixedly held, and at which the head (52) of the bone screw (50) can

be hitched. The receiver member (80) further has a U-shaped channel (84) through

which the support bar (R) extends. The receiver member (80) still further has at an

external surface thereof a punch groove (86) of a predetermined depth in such a

manner that an inner portion of the receiver member (80) can be easily protruded by a

punch toward the recess (76) of the compression bush (70).

The bore (82) includes a head hitching unit (82a) formed at an inner lower surface

thereof and having a curvature at which the head (52) is hitched, a receptible unit (82b) formed on the head hitching unit (82a) in which the cap (60) and the compression bush

(70) are accommodated, and a screw unit (82c) on which the bar fixation nut (90) is

threaded. Furthermore, the head hitching unit (82a) is formed thereunder with a

chamfer.

Each internal thread of the screw unit (82c) has a trapezoidal configuration in order to

be more resistant to deformation of the receiver member and to provide optimal results

in terms of force transfer, locking efficiency and the likelihood of unscrewing. For

reference, in the case of a triangular thread, as the bar fixation nut (90) is coupled,

while excellent locking force can be obtained, the axial force transfer rate becomes low.

Furthermore, because the receiver member (80) widens in a radial direction after

locking of the bar fixation nut (90), a disadvantage is caused in that a separate cap

should be coupled to the receiver member (80). Still furthermore, a square thread

suffers from defects in that, while an excellent axial force transfer rate is obtained, the

likelihood of unscrewing is increased. In the present invention, due to the fact that

trapezoidal threads are adopted, optimal results are provided in terms of force transfer

and locking efficiency, and it is not necessary to couple a separate cap to an external

side of the receiver member.

Preferably, the trapezoidal thread has an upper surface which is formed to have a slope (FI) greater than a slope (F2) of a lower surface, whereby the bar fixation nut (90) can

be easily assembled while being prevented from being unintentionally unscrewed. In

other words, since the upper slope (FI) is greater than the lower slope (F2), the bar

fixation nut (90) can be easily screwed. Also, unscrewing is avoided even when force

is upwardly applied, namely, when the force is applied in an opposite direction of the

coupling.

The bar fixation nut (90) has a cylinder-shaped configuration with threads formed on

the circumferential outer surface thereof. The threads are threadedly coupled to the

internal threads of the screw unit (82c) at the receiver member (80). A hexagonal

groove is defined at the center of the bar fixation nut (90).

Hereinafter, the assembly of the bone fixation apparatus according to the present

invention, constructed as mentioned above, will be described.

First, in a state where the bone screw (50) is positioned above the receiver member

(80), the bone screw (50) is inserted into the bore (82) of the receiver member (80) in a

manner such that the head (52) is seated on the head hitching unit (82a). Then the cap

(60) and the compression bush (70) are sequentially positioned in the receptible unit

(82b). Next, a wrench is fitted into the hexagonal groove (56) of the head (52) through the

holes of the compression bush (70) and the cap (60), and the wrench is manipulated to

allow the bone screw (50) to be fixed to the bone. The support bar (R) is made to pass

through the U-shaped channel (84) of the receiver member (80) and the channel (72) of

the compression bush (70) and is allowed to be accommodated on the upper side of the

compression bush (70). The bar fixation nut (90) is tightened to fix the bone screw (50).

The locking force of the bar fixation nut (90) is transferred to the head (52) through the

support bar (R), compression bush (70) and the cap (60). At this time, due to the fact

that the compression bush (70) is brought into linear contact with the external surface

end of the cap (60) to move the cap (60) downward along the circumferential outer

surface of the head (52), the upper surface (66) of the cap (60) is drooped as much as a

predetermined height to allow the curvature (64) thereunder to be inwardly tightened at

a radial direction.

Due to the fact that the curvature (64) underneath the cap (60) is tightened to be

securely fixed against the circumferential outer surface of the head (52), a strong

locking force is generated in such a manner that the supporting force of the bone screw

(50) is increased through which the bone screw is prevented from moving, and is securely fixed against movement.

Meanwhile, a portion of the inner surface of the receiver member (80) is protruded

toward the recess (76) by punching work through the punch groove (86), thus

preventing movement of the compression bush (70).

Furthermore, due to the fact that the upper surface of the compression bush (70) is

formed with the channels (72) having slopes at both inner upper sides thereof, it is

possible to accommodate various sizes of support bars. The head (52) is defined with

the groove (58) in such a manner that frictional force can be increased to improve the

supporting force of the bone screw (50).

The foregoing description of the preferred embodiment of the present invention has

been presented for the purpose of illustration and description. It is not intended to be

exhaustive or to limit the invention to the precise form disclosed, and modifications

and variations are possible in light of the above teachings or may be acquired from

practice of the invention. It is intended that the scope of the invention be defined by

the claims appended hereto, and their equivalents.

As apparent from the foregoing, the bone fixation apparatus according to the embodiment of the present invention provides advantages in that a cap has a shape of a

belleville spring in which a diameter of a lower end is smaller than that of an upper

end in such a manner that a lower curvature is radially and inwardly tightened when an

upper end of the cap is compressed by a compression bush to allow an upper curvature

to fully adhere to a circumferential outer surface of a head. In other words, the cap in

the present invention is fully brought into contact with the head to generate a strong

locking force in such a manner that a supporting force of a bone screw is increased to

prevent the bone screw from being moved by an external force or twist applied to an

affected part when the apparatus is assembled.

Another advantage is that a compression bush compresses a cap to be fixedly brought

into contact with a head by tightening a fixing nut, thus improving the assembly of the

bone fixation apparatus.

Still another advantage is that channels having slopes at both inner sides thereof are

formed at an upper surface of a compression bush, thus allowing use of various sizes

of supporting bars.

Claims

WHAT IS CLAIMED IS:

1. A bone fixation apparatus comprising: a bone screw having a head; a cap for supporting the top of the head of the bone screw; a compression bush for pushing the cap; a receiver member having a bore in which the cap and the compression bush

are accommodated and the head of the screw is hitched, and a U-shaped channel

through which a support bar is accommodated; a bar fixing nut threadedly coupled to the receiver member to fix the support

bar, wherein the cap has a shape of a belleville spring in which a diameter of a lower

end thereof is smaller than that of an upper end, and an upper surface thereof is

concavely inclined toward an inner radial direction and a lower side thereof is formed

with a curvature contacting an outer circumferential surface of the head.

2. The apparatus as defined in claim 1, wherein the compression bush is formed

thereon with channel having both slopes at an inner side thereof.

3. The apparatus as defined in claim 1, wherein the compression bush is formed at both

external sides thereof with a recess of a predetermined depth, and a punch groove of a

predetermined depth is formed at an external surface of the receiver member in a manner such that an inner portion of the receiver member can be easily protruded

toward the recess by a punch

4. The apparatus as defined in claim 1, wherein the head is externally formed with a

plurality of grooves in order to increase a frictional force with the cap.

5. The apparatus as defined in claim 1, wherein the head is externally formed with

knurls in order to increase a frictional force with the cap.

6. The apparatus as defined in claim 1, wherein the curvature of the cap is formed with

knurls in order to increase a frictional force with the head.