Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2000-01-07
2002-06-11
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06402784
ABSTRACT:
The present invention relates to a prosthetic intervertebral disc nucleus and to an insertion device for use therewith. Such devices are useful to replace damaged disc nuclei, whether in the lumbar or other region of the spine.
The normal intervertebral disc is a highly specialized joint between the intervertebral bodies from the second cervical vertebra to the first sacral vertebra in the human being. A disc is made up of a strong outer ring called the annulus which is strongly attached to the intervertebral bodies above and below through collagen fibers, and a central nucleus. The nucleus comprises a mesh of collagen fibers to which is attached proteoglycan molecules which are hygroscopic. It can therefore be said to consist of a central portion and an intermediate zone. The central portion comprises 90% proteoglycan and 10% collagen, and the intermediate zone rather less proteoglycan and proportionately more collagen. The annulus comprises 90% collagen with about 10% proteoglycan, which proteoglycan acts to allow a sliding motion between adjacent layers during normal daily use.
Like all other joints in the body, intervertebral discs, particularly lumbar intervertebral discs, are subject to various types of injury, degeneration and disease. Painful disc syndromes can develop due to the destruction of the intervertebral disc structure.
It is often the case that back pain emanating from an intervertebral disc arises from a damaged annulus because the annulus itself is the only part of the disc structure which is innervated. In theory then, the insertion of a prosthesis which has itself a finite structure with inherent strength but at the same time is held in position by the annulus would permit the annulus itself to heal with the commensurate relief of pain.
Various disc prostheses are known in the art for example from U.S. Pat. No. 3,875,595 and WO 95/31948. There are various problems associated with these. In U.S. Pat. No. 3,875,595, there is provided a collapsible plastic bladder prosthesis of the same exterior form as the nucleus pulposis of an intervertebral disc. This is provided with a stem through which liquid/plastic is introduced to inflate the prosthesis to a natural form. The difficulty with this arrangement is that since the exterior of the bladder-like prosthesis is impermeable, the prosthesis is not anchored and hence must be provided with external studs to secure the same in the position relative to adjacent vertebrae. Such an arrangement tends to adversely impact upon the adjacent vertebrae and/or the plastic sheath of the prosthesis, and bearing in mind the trauma associated with insertion and subsequent repair where necessary, such arrangements have not been found to be satisfactory.
In part, these problems have also been addressed in WO 95/31948. This provides an expandable fabric insert for stabilizing spinal motion which allows through growth of living cells, unlike other prior art prostheses. This can be inserted by open operation. This arrangement is designed to be filled with bone graft or bone substitute material with a view to bringing about a solid bone fusion or a fibrous fusion. For anchoring purposes it is of course necessary for the living cells to grow through the woven material of the prosthetic cover and this in fact can occur.
Alternatives have been suggested, for example in U.S. Pat. No. 5,674,295. This describes a nuclear prosthesis which has an outer restraining jacket surrounding a hydrogel, and optionally other materials. In the dehydrated state, the hydrogel has a volume smaller than the constraining jacket but when hydrated is constrained within the jacket so it can conform to a certain extent to external loads placed upon it. The prosthesis is always of a smaller volume than the nuclear space in which it is placed, but on hydration increases the disc height, tensioning the fibers of the annulus.
Another attempt at this has been in WO 92/10982 in that a prosthesis of the type of this invention is revealed with a supported membrane being approximately above 15,000 daltons, or 25A. While this approach may be viable, better results, especially in terms of immunological disturbances may be achieved if significantly lower porosity is utilized without rendering the cover completely impermeable.
What is required therefore is a prosthesis which can be introduced with the minimum of trauma via the lateral percutaneous route, which stays correctly anchored in position, which allows some exchange of fluid and low molecular weight materials, but without immunological problems, and/or acts over the majority of the articulating surfaces of the adjacent vertebrae.
SUMMARY OF THE INVENTION
The present invention is directed towards an intervertebral disc nucleus prosthesis generally comprising a transudative or ion transport material extended over the inner face of a prosthetic woven or porous fabric chosen for its strength and immunological neutrality. With this design, tissue can grow over and incorporate the outer fabric, while water and very low molecular weight materials can pass the prosthetic fabric and the transudative membrane, thus eliminating, or at least substantially alleviating, any immunological problems. This allows the anchoring of the replacement nucleus pulposis within a disc space while preventing the ingrowth of bony trabeculae, thereby providing a more satisfactory long term solution.
An alternative embodiment generally comprises a three layer cover, wherein the outer and inner layer are formed of a strong material in a sandwich construction with a middle layer of a transudative material of a small pore size. Very high pressures are present in use within the normal disc nucleus. To counteract this it is often necessary to introduce a hydrogel or other suitable substance into the cover in a liquid or powdered state. Thus, the hydrogel would have a tendency to escape through the cover were it not so constructed as to combine strength with an ability to retain the hydrogel which might otherwise leak out through the pores in the cover. The alternative jacket construction addresses this concern.
According to a first aspect of the invention therefore there is provided a prosthetic cover shaped to form a replacement nucleus pulposis of an intervertebral disc. The cover comprises a permeable layer of an immunologically neutral material and a transudative material adapted to allow the through flow of selected low molecular weight materials only. The cover terminates in a valve structure configured to allow the normally irreversible introduction of a hydrogel material. In one preferred embodiment, the valve structure includes a one way valve. In another preferred embodiment, the prosthetic cover comprises a three layered cover, preferably forming a sandwich construction having a middle layer of a transudative material.
The term “normally irreversible” it will be understood refers to the situation in use. Clinical situations can arise wherein it is desirable to change the volume of hydrogel in the cover post-operatively. This may be achieved by causing the hydrogel to flow out of the cover by re-opening the valve structure in a fashion analogous to the filling operation, liquidizing the hydrogel and then applying a vacuum.
Regardless of the number of layers comprising the cover, the cover further includes in one preferred embodiment, a strengthening member opposite the valve structure. The strengthening member is preferably integrally formed to the material comprising the cover and is configured to selectively receive an end of an introducer rod during implantation of the prosthesis. In this regard, the strengthening member allows a user to apply a pushing force on the cover without causing any tears.
As is clear from the foregoing, the low molecular weight materials may include water and other low molecular weight materials present in the environment of use. The immunologically neutral material may be woven and is particularly satisfactory if selected from “Dacron”® and “Gortex”®, but other similar materials with the same MW cut off chara
Aberdeen Orthopaedic Developments Limited
Jackson Suzette J.
Laubscher, Jr. Lawrence E.
Willse David H.
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