Spinal Intervertebral Disk Replacement Material for Nucleus Pulposus and Production Method Thereof

Abstract

To provide a spinal intervertebral disk replacement material for nucleus pulposus which is relatively superior in load resistance that causes smaller damage of the anulus fibrosus by insertion during operation, remains in the anulus fibrosus stably after operation, and also allows prevention of the damage of the vertebral body cartilage endplate and the anulus fibrosus surrounding the prosthetic portion by sliding, and a production method thereof. The present invention relates to a spinal intervertebral disk replacement material for nucleus pulposus, comprising a dry polyvinylalcohol gel having a percentage of saturated water content of 18 to 50 wt % that changes its shape at a shape deformation rate of 150 vol % or more by water absorption. The present invention also relates to a method of producing the spinal intervertebral disk replacement material for nucleus pulposus above, comprising a step of preparing a polyvinylalcohol gel having a percentage of saturated water content of 20 to 53 wt %, a step of converting the polyvinylalcohol gel into a polyvinylalcohol hydrogel in the hydrated state and forming it into a shape having a hollow region, a step of irradiating the polyvinylalcohol hydrogel with γ ray at an intensity of 20 to 100 kGy, and a step of drying the PVA hydrogel in such a manner that the resulting hydrogel is so deformed that the hollow region disappears.

Description

TECHNICAL FIELD[0001]The present invention relates to a spinal intervertebral disk replacement material for nucleus pulposus that is used in the fields such as of orthopedics and neurosurgery, in particular to a spinal intervertebral disk replacement material replacing the nucleus pulposus, the main component of the intervertebral disk, that is used in treatment of the intervertebral disk damaged, for example, by disease such as hernia of intervertebral disk or by accident, and the production method thereof.BACKGROUND ART[0002]The intervertebral disk is a joint having a function and a structure very complicated functionally and anatomically. It consists of functional structures of anulus fibrosus, vertebral body endplate, and nucleus pulposus. The anulus fibrosus is tough fibrous ring bound to a neighboring vertebral body or a vertebral body endplate. The anulus fibrosus is generally said to have a height of 10 to 15 mm and a thickness of 15 to 20 mm. The nucleus pulposus is placed ...

AI Technical Summary

Benefits of technology

[0019]In the present invention, the spinal intervertebral disk replacement material is obtained as a material in the rod shape by processing a replacement material of polyvinylalcohol into a shape returning (deforming itself) to the shape of the nucleus pulposus to be replaced so that the replacement material can be inserted in the dehydrated state through the incision opening for insertion into the prosthetic space, and it is inserted as it is into the prosthetic space. Then, the spinal intervertebral disk replacement material deforms itself by absorption of body fluid, occupying the entire prosthetic space. It is thus possible to prevent damage of the insertion site (e.g., anulus fibrosus) during operation, without concern about separation of migration. It is also possible to raise the load resistance and eliminate the concern about breakage, excessive deformation, separation, and others under application of load, by controlling the percentage of saturated water content after water absorption in a particular range. It is thus possible to reduce the load to the patient, without need for an additional instrument for fixing the vertebral body. It is also possible to prevent the damage of the vertebral body cartilage endplate and the anulus fibrosus surrounding the prosthetic portion.

[0020]In addition, the replacement material according to the present invention remembers the shape after water absorption and allows control of the shape in the prosthetic space, and thus, there is no unexpected highly or scarcely filled site formed in the prosthetic space. Further, there is no abrasion powder generated by abrasion among the replacement materials.

Problems solved by technology

The intervertebral disk may be degraded or damaged by external wound, disease, or aging.

The nucleus pulposus sticking out pressurizes the spinal cord, causing pain and paralysis of the neighboring structures or pain and paralysis of the lower limbs.

The nucleus pulposus then cannot retain its favorable interbody distance any more, and cause damage of the upper or lower vertebral body.

However, these therapies are aimed at fixation of the vertebral body, and the mechanical properties inherent to the intervertebral disk such as flexibility and load-buffering action are neglected.

As a result, such a therapy exerts an adverse effect on the neighboring upper and lower vertebral bodies, causing secondary damages of the intervertebral disks.

Although these devices allowed interbody mobility, they also had some problems.

These prostheses are large implants demanding partial removal of the neighboring upper and lower vertebral bodies and entire replacement of the intervertebral disk and the neighboring structures, and thus, had a possibility of imposing a very high load on the patient during operation.

In addition, such a device was higher in rigidity and thus, undesirable from the point of material mechanics.

If it is separated, the intervertebral disk that had the separated synthetic nucleus pulposus prosthesis may not have sufficient load resistance and damage the intervertebral disk and the neighboring upper and lower vertebral bodies.

In addition, the implant itself has a possibility of damaging the neighboring structures and nerves.

As a result, the resulting prosthesis is lower in load resistance, possibly causing recurrence of the hernia condition by deformation under application of high load.

Thus, incomplete condition of the anulus fibrosus may result in separation of the prosthesis without functioning as the intervertebral disk.

In addition, a prosthesis with insufficient load resistance demands an instrument for fixing the vertebral body for compensation of low load resistance, which may lead to increase of the load on the patient.

In addition, it may prevent healing of, or even worsen, the anulus fibrosus opening that was cut open for insertion.

Further in some of them, the separation occurred from the sites other than the inserted anulus fibrosus opening site, indicating that the load resistance was insufficient (see Non-patent Literature 1).

However, because the polymeric woven-fabric jacket used was relatively rigid and almost resistant to shrinkage, swelling, denaturation, or deformation, the prosthesis had a problem that it was larger also in the dry state.

However, because the core hydrogel has a high water content, the prosthesis also raises a concern about its low load resistance.

During movement of the prosthesis in anulus fibrosus, the relatively rigid polymeric woven-fabric jacket may damage the upper and lower vertebral body cartilages significantly and thus, such a prosthesis is not favorable as a prosthesis for replacement of nucleus pulposus surrounded by the endplate cartilage and the anulus fibrosus.

Further, the two prostheses inserted into the anulus fibrosus raises concerns about abrasion between them, generation of unexpected abrasion powder, and breakage of the polymer woven fabric jacket.

Currently in the rapidly graying society, diseases caused by the intervertebral disk denatured or damaged by aging, external wound, or disease are significant threat to patients.

However, because the synthetic nucleus pulposus prosthesis described above has a practical water content of approximately 55 to 99 wt %, it raised a concern about low load resistance and recurrence of the hernia condition by deformation under application of high load.

Thus, incomplete condition of the anulus fibrosus may result in separation of the prosthesis from anulus fibrosus without functioning as the intervertebral disk and possible occurrence of damage caused thereby.

In addition, a prosthesis with insufficient load resistance demands an instrument for fixing the vertebral body for compensation of low load resistance, which may lead to increase of the load on the patient.

Further, abrasion with the neighboring vertebral body cartilage endplates may also cause damage of the peripheral structures.

However, it was not possible to control the shape of the hydrophilic xerogel rod in anulus fibrosus, and disadvantageously, the densely filled site raised a concern about recurrence of hernia, while the coarsely filled site raised a concern about insufficient load resistance.

It also caused a concern about generation of unexpected abrasion powder by abrasion among xerogel rods inserted and bent in the anulus fibrosus.

Images