JPH08229051A - Bone connecting screw - Google Patents

Abstract

PURPOSE: To provide a bone connecting screw having large extracting strength from a bone connection section and sufficient fixing force and capable of being decomposed and absorbed in a living body. CONSTITUTION: This screw is made of a copolymer of polylactic acid or lactic acid and glycolic acid, the slant face of a screw ridge 3 and the bottom face of a valley section 4 are formed into a continued recessed curve face, the radius of curvature R1 of the screw tip side slant face 31 of the screw ridge 3 is made larger than the radius of curvature R2 of the bottom face 41 of the valley section 4, and the radius of curvature R3 of the screw rear end side slant face 32 of the screw ridge 3 is made smaller than the radius of curvature R2 of the bottom face of the valley section 4. The screw ridge 3 is formed into a ridge shape tilted to the screw rear end side, the connection between the screw rear end side tilt face 32 having the smaller radius of curvature R3 and a living body bone is strengthened, the extracting strength is increased, and sufficient fixing force is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an osteosynthesis screw made of biodegradable and absorbable polylactic acid or a copolymer of lactic acid and glycolic acid.

[0002]

2. Description of the Related Art Conventionally, in the field of surgery such as orthopedics, plastic surgery, thoracic surgery, and oral surgery, screws and pins made of metal or ceramics have been used as an osteosynthesis device for the purpose of reducing or fixing fractures. Etc. are used. However, since these devices have a high elastic modulus, there are problems such as reducing the strength of surrounding bones. Especially, since a metal device may damage the living body due to elution of metal ions, a fracture or the like may occur. When healed, there was a big problem that the surgery had to be taken out from the body as soon as possible.

Under such circumstances, research using biodegradable and absorbable polymer materials as an osteosynthesis device has become popular, and the applicant of the present invention also molds polylactic acid having a viscosity average molecular weight of 300,000 or more and uses it. Further proposed was a device for osteosynthesis which was further stretched to increase the strength (JP-A-1-198553 and the like). This product has the same initial strength and initial elastic modulus as that of living bone, and maintains strength close to that of living bone until the strength and function of the bone at the fractured part or the damaged part are almost restored, and finally the bone. It was an excellent osteosynthesis device that was completely absorbed in the living body after the formation of the bone was completed.

[0004]

However, the screw among the above-mentioned devices for osteosynthesis has the following points to be improved.

That is, the reference chevron shape of the bone-joining screw is
Depending on the part of the living bone, the pull-out strength of the screw depends on the triangular shape with the apex angle of 60 ° used for normal screws and the ratio of the diameter of the screw thread to the diameter of the valley part. Could become weak and the fixing force might be insufficient.

The present invention has been made in view of the above, and an object of the present invention is to provide an osteosynthesis screw capable of increasing the pull-out strength from the osteosynthesis portion and obtaining a sufficient fixing force. is there.

[0007]

In order to achieve the above object, the screw for osteosynthesis of the present invention is a screw made of polylactic acid or a copolymer of lactic acid and glycolic acid. The bottom surface is formed as a continuous concave curved surface, the curvature radius of the screw tip side slope of the screw thread is made larger than the curvature radius of the bottom surface of the valley part, and the curvature radius of the screw rear end side slope surface of the screw thread is the bottom surface of the valley part. It is characterized in that it is smaller than the radius of curvature of. And, preferably, the apex angle of the screw thread is set to 10 to 50 °, and the inclination angle of the screw rear end side inclined surface near the screw thread apex is set to 70 to 90 °.

The cancellous bone-bonding screw of the present invention is a screw made of polylactic acid or a copolymer of lactic acid and glycolic acid, in which the diameter of the trough is 0.
It is characterized by being 50 to 0.70 times,
The screw for cortical bone or calcaneus of the present invention is a screw made of polylactic acid or a copolymer of lactic acid and glycolic acid, in which the diameter of the valley is 0.65 to the diameter of the screw thread.
It is characterized by being multiplied by 0.85.

[0009]

As in the bone joining screw of the present invention, the slope of the screw thread and the bottom of the valley are formed into a continuous concave curved surface, and the radius of curvature of the slope on the screw tip side of the screw is set to the bottom of the valley. If the radius of curvature is larger than the radius of curvature and the radius of curvature of the slope of the screw thread on the rear end side of the screw is smaller than the radius of curvature of the bottom of the valley, the center of the valley is located closer to the screw tip side, as shown in FIG. As a result, the screw thread has a mountain shape that is inclined toward the rear end side of the screw, and the engagement between the screw rear end side slope having a small radius of curvature of the screw and the living bone becomes strong, so that the screw has a strong fixing force.

Particularly, the apex angle of the screw thread is 10 to 50 °,
A screw having an inclination angle of the screw rear end side slope near the screw crest with respect to the axis of the screw is 70 to 90 ° because the screw rear end side slope near the screw crest engages the living bone at an angle close to a right angle. Since the fixing force is extremely strong and the thread groove into which the screw thread is screwed is narrow and the groove shape can be prevented from being deeply crushed, the slip-out preventing effect becomes remarkable.

The bone-joining screw is used for fixing bone-joint portions and bone fragments of living bones at various sites, and in particular, it is mainly used for cancellous bone, cortical bone and calcaneus. Then, in order to make the fixing force of the screw used for the above-mentioned living bone strong, it is necessary to make the ratio of the diameters of the screw thread and the trough different. As a result of the research, as in the present invention, the cancellous bone-bonding screw has a diameter of the trough of 0.
It is necessary to make it 50 to 0.70 times, and if it is less than 0.50 times, the joint area between the bone fragment and the screw becomes large, but the thread portion becomes thin and the strength becomes weak, and as a result, the bone fragment The fixing force required for joining cannot be obtained, and also
When it is larger than 0.70 times, the bite of the screw thread is insufficient and the fixing force (pulling strength) necessary for joining the fractured part
Can not be obtained.

On the other hand, the screw for cortical bone or calcaneus bone is required to have the diameter of the valley portion 0.65 to 0.85 times the diameter of the screw thread as in the present invention. If it is less than 0.65 times, the strength of the thread portion is weakened and the necessary fixing force cannot be obtained, and if it is more than 0.85 times, the pull-out strength is weakened and sufficient fixing force cannot be obtained.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a front view showing an embodiment of the bone screw of the present invention, FIG. 2 is an enlarged sectional view of the tip of the screw, and FIG. 3 is a plan view of the screw.

This osseointegration screw is made of uniaxially stretched polylactic acid or a copolymer of lactic acid and glycolic acid, and a rotating hexagon wrench is fitted into the head 1 at the rear end (upper end in the figure) of the screw. The hexagonal hole 2 is formed, and the screw thread 3 is formed over the entire length of the screw excluding the head 1.

As shown in FIG. 2, this screw has slopes 31 and 32 of the screw thread 3 and a bottom surface 41 of the valley portion 4 formed into a continuous concave curved surface, and the screw tip side slope 3 of the screw thread 3 is formed.
The radius of curvature R1 of No. 1 is set to be larger than the radius of curvature R2 of the bottom surface 41 of the valley portion, and the curvature radius R3 of the screw rear end side slope 32 of the screw thread 3 is set to be smaller than the curvature radius R2 of the bottom surface of the valley portion. . Then, the apex angle θ1 of the screw thread 3 is set to 1
It is set to 0 to 50 °, and the inclination angle θ2 of the screw rear end side sloping surface 32 near the crest of the screw with respect to the axis L of the screw is set to 70 to 90 °.

When the screw thread 3 is set as described above, the screw thread 3 has a mountain shape inclined toward the screw rear end side, and the center of the valley portion 4 is biased toward the screw front end side. When screwed into the screw hole, the screw rear end side slope 32 having the small radius of curvature R3 of the screw thread 3 and the living bone become stronger in engagement, and especially the screw rear end side slope 32 near the top of the screw thread is perpendicular to the living bone. Since the engagement is performed at an angle close to, the fixing force becomes extremely strong. Since the screw groove into which the screw thread of the screw is screwed can also be formed in a groove shape that is narrow and deep and is hard to be crushed, a strong fixing force can be obtained.

The radius of curvature R1 of the beveled surface 31 on the screw tip side of the thread 3 may be a constant value, but it is set so that the radius of curvature R1 gradually decreases from the top of the thread 3 toward the valley 4. Good. Similarly, the bottom surface 4 of the valley 4
The radius of curvature R2 of 1 may be a constant value smaller than the radius of curvature R1, but may be set so that the radius of curvature R2 gradually decreases as it approaches the screw rear end side slope 32 of the thread 3. The radius of curvature R3 of the bevel 32 on the screw rear end side of the ridge 3 may be a constant value smaller than the radius of curvature R2, but may be set so that the radius of curvature R3 gradually decreases as it approaches the top of the ridge 3. Good.
That is, the radius of curvature of the continuous concave curved surface from the top of the screw thread 3 on the rear end side of the screw to the top of the screw thread 3 on the front end side of the adjacent screw may be gradually reduced. Such a screw is suitable for joining living bones.

The screw in which the screw thread 3 is formed over the entire length as in this embodiment is used for joining cortical bone and cancellous bone. When using a screw for cortical bone bonding, the valley 4
It is necessary to set the diameter A of 0.65 to 0.85 times the diameter B of the screw thread 3. The diameter A of the valley 4 is the diameter B of the thread 3.
If it is smaller than 0.65 times, the screw thread 3 protrudes more than the trough portion 4 and the joint area with the cortical bone increases, but the screw thread 3 becomes thin and the strength against pulling out weakens, and eventually the cortical bone It will not be possible to obtain the fixing force required for joining. On the other hand, if it is larger than 0.85 times, the pull-out strength is weakened due to insufficient biting of the screw threads 3 (insufficient joint surface) and the fixing force is reduced, which is unsuitable for joining cortical bone.

On the other hand, in the case of a cancellous bone joining screw, the diameter A of the valley 4 is 0.50 to 0.7 of the diameter B of the thread 3.
It is necessary to set it to 0 times. Since the cancellous bone is not dense, if the diameter A of the valley 4 is larger than 0.70 times the diameter B of the screw thread 3, the joint area with the cancellous bone is small and the fixing force of the screw becomes weak. When it is smaller than 0.50 times, the joint area becomes large, but the thread 3 becomes too thin and the pull-out strength becomes weak, so that the necessary fixing force cannot be obtained.

To exemplify the concrete dimensions of the screw as described above, in the case of a screw for cortical bone joining, the total length is about 5 to 70 mm, the diameter of the valley portion 4 is about 1.0 to 4.5 mm, and the screw thread is The diameter of 3 is about 1.5 to 6.5 mm, the pitch is about 0.4 to 3.0 mm, and in the case of the cancellous bone screw, the total length is about 10 to 90 mm and the diameter of the valley is 1 .5 to 5.0 mm, the diameter of the screw thread 3 is 3.0 to
The pitch is about 7.0 mm and the pitch is about 0.6 to 3.5 mm.

FIG. 4 is a front view showing another embodiment of the osteosynthesis screw of the present invention. This osteosynthesis screw has a threaded rod portion 5 having a thread 3 from the screw tip to the middle. Between the screw rod 5 and the head 1 at the rear end (upper end in the figure) of the screw, there is a threaded shaft rod 6
It is what

Like the above-mentioned screw, this screw is also formed by forming the slope of the screw thread 3 and the bottom surface of the valley portion 4 into a continuous concave curved surface so that the radius of curvature of the screw tip side slope of the screw thread 3 is equal to the valley portion 4. The hexagonal hole 2 is formed in the head 1 by setting the radius of curvature of the bottom surface of the valley 4 to be smaller than the radius of curvature of the bottom surface of the valley 3 and the radius of curvature of the screw rear end side slope of the screw thread 3.

The screw having such a structure is used for joining calcaneus and cancellous bone. When the screw for calcaneus joint, as in the case of the screw for cortical bone joint,
It is necessary to set the diameter of the valley portion 4 to be 0.65 to 0.85 times the diameter of the screw thread 3, and if it deviates from this range, the pull-out strength becomes weak and the fixing force becomes insufficient for the same reason as above. Needless to say, in the case of using a cancellous bone-bonding screw, the diameter of the valley portion 4 needs to be set to 0.50 to 0.70 times the diameter of the screw thread 3 as described above.

The length L1 of the screw rod 5 and the total length L of the screw
The ratio of 2 is not particularly limited, but the total length is 30 mm, for example.
In the case of a calcaneus screw of a certain degree, it is desirable to set it to approximately 1: 2. Further, in the case of a cancellous bone-bonding screw, it is desirable to set the length L1 of the screw rod portion 5 to about 15 to 35 mm.

To give an example of specific dimensions of such a screw, in the case of a screw for calcaneus joint, the total length L2 is about 20 to 80 mm and the length L1 of the screw rod 5 is 10 to 4.
0 mm, the diameter of the valley portion 4 is about 2.0 to 4.0 mm, the diameter of the screw thread 3 is about 2.5 to 6.0 mm, and the pitch is 0.6.
In the case of a cancellous bone screw, the total length L2 is about 20 to 80 mm, the length L1 of the screw rod portion 5 is about 15 to 35 mm, and the diameter of the valley portion 4 is about 1.5 to
It is about 4.5 mm, the diameter of the screw thread 3 is about 3.0 to 7.0 mm, and the pitch is about 1.0 to 3.0 mm.

The screw shown in FIGS. 1 and 4 is manufactured by melt-molding polylactic acid or a copolymer of lactic acid and glycolic acid into a rod shape, uniaxially stretching in the axial direction, and cutting this. . Polylactic acid is prepared by a conventional method (CE Lov
e, U.S. Pat. No. 2,668,182), lactide, which is a cyclic dimer of lactic acid, is synthesized, and the lactide is subjected to ring-opening polymerization.

It is important that the polylactic acid (PLLA) has an appropriate molecular weight and crystallinity. That is, PL
The higher the molecular weight of LA, the greater the strength. However, if the molecular weight is too high, high temperature and high pressure are required during melt molding, resulting in a significant decrease in the molecular weight, resulting in a low molecular weight after molding. And the strength decreases. On the other hand, the crystalline material has higher bending strength and elastic modulus than the amorphous material, and the permeation of body fluid is slower in the crystalline phase and in the apparent hydrolysis rate. However, when the crystallinity is increased by heat treatment, the strength is improved, but deterioration of the molecular weight decreases due to progress of deterioration due to thermal instability of PLLA, resulting in a slow hydrolysis rate and rapid deterioration of strength. Will happen to you. For these reasons, PLLA having a viscosity average molecular weight of 300,000 to 600,000, preferably about 350,000 to 550,000 and a molded product having a crystallinity of about 10 to 60% is preferably used. .

Further, it is desirable that the PLLA is uniaxially stretched after the melt molding as described above, and the strength is improved by such uniaxial stretching. The preferred draw ratio is 2-5
It is about twice, and when uniaxially stretched at a stretch ratio in this range,
A tough screw having a compression bending strength of about 160 to 250 MPa and a compression bending elastic modulus of about 5500 to 24000 MPa can be obtained.

Further, a copolymer of lactic acid and glycolic acid can be used in place of the above PLLA. This copolymer has a viscosity average molecular weight similar to that of PLLA, and a polymer having a large lactic acid content is suitable. Among them, the weight ratio of lactic acid to glycolic acid is in the range of 99: 1 to 75:25. Some are preferred. When the amount of glycolic acid is more than the above range, the hydrolysis resistance is lowered and the strength is deteriorated at an early stage.

The screw as described above can surely bond and fix the bone even if a large force is applied to the bone bonding portion. After the bone is completely formed and healed, the screw is completely absorbed in the living body, eliminating the need for re-operation for nail extraction and reducing pain and financial burden. it can.

[0032]

EFFECTS OF THE INVENTION As is clear from the above description, the bone-bonding screw of the present invention is easy to screw into the screw hole formed in the bone-bonding portion, and has sufficient fixing force, so that the bones of the living body can be reliably secured. Since it can be bonded and fixed to the body and is not absorbed in the living body after healing and re-operation for nail extraction is not required, it is possible to eliminate the pain and financial burden of re-operation.

[Brief description of drawings]

FIG. 1 is a front view showing an embodiment of a bone screw according to the present invention.

FIG. 2 is an enlarged cross-sectional view of a tip portion of the screw.

FIG. 3 is a plan view of the screw.

FIG. 4 is a front view showing another embodiment of the osteosynthesis screw of the present invention.

[Explanation of symbols]

1 Head 2 Hexagon socket 3 Screw thread 31 Screw tip side slope of screw thread 32 Screw rear end side slope of screw thread 4 Valley 41 Bottom of valley section 5 Screw rod section 6 Shaft rod section R1 Screw tip side slope of screw thread Radius of curvature R2 radius of curvature of the bottom of the valley R3 radius of curvature of the screw rear end side slope of the screw thread θ1 apex angle of the screw thread θ2 inclination angle of the screw rear end side slope near the screw thread top L axis

Claims (4)

[Claims] 1. In a screw made of polylactic acid or a copolymer of lactic acid and glycolic acid, the slope of the screw thread and the bottom of the valley are formed into a continuous concave curved surface, and the curvature of the screw tip side slope of the screw thread is formed. A screw for osteosynthesis, characterized in that the radius is made larger than the radius of curvature of the bottom surface of the valley and the radius of curvature of the screw rear end side slope of the screw thread is made smaller than the radius of curvature of the bottom surface of the valley. 2. The apex angle of the screw thread is set to 10 to 50 °, and the inclination angle of the screw rear end side slope near the screw thread apex is set to 70 to 90 °. Described osteosynthesis screw 3. A spongy bone joint characterized in that in a screw made of polylactic acid or a copolymer of lactic acid and glycolic acid, the diameter of the valley is 0.50 to 0.70 times the diameter of the screw thread. Screw for. 4. A cortical bone joint characterized in that a screw made of polylactic acid or a copolymer of lactic acid and glycolic acid has a diameter of a valley portion 0.65 to 0.85 times a diameter of a screw thread. Screw for calcaneus or calcaneus.