Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1997-11-17
2001-02-13
Milano, Michael J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S017160
Reexamination Certificate
active
06187048
ABSTRACT:
BACKGROUND OF THE DISCLOSURE
The present invention relates to prosthetic devices and in particular to such devices which deform when in use (eg. when under compressive load). Here particularly the present invention relates to disc nucleus prostheses.
The intervertebral disc comprises three regions, the annulus fibrosus, the nucleus pulposus within the annulus fibrosus, and the cartilaginous end plates. Herniation of the disc may cause the nucleus pulposus to leak through a rupture point in the annulus fibrosus and bear against the ganglia of the spinal nerve, causing severe pain in the back as well as the legs. Surgical procedures to alleviate this condition include excision of the disc, chemo nucleolysis and spinal fusion. Another approach has been to replace the nucleus pulposus with a synthetic material. Thus in U.S. Pat. Nos. 5,047,055 and 5,192,326 it is proposed to replace the nucleus pulposus with a hydrogel in a hydrated form. The axial compression stresses acting upon an intervertebral disc which can vary between 0.15 MPa and 1.5 MPa depending upon the posture and activity involved. With highly hydrated material there is a propensity for the free water with hydrogel to be expelled.
In U.S. Pat. Nos. 4,772,287 and 4,904,206 it is, inter alia, proposed to at least partially replace the annulus pulposus with a prosthesis comprising two capsules arranged side-by-side, each filled with a therapeutic fluid. Similarly in European Patent No.480954 there is disclosed a balloon which is insertable into the cavity formed after removal of nucleus pulposus and which is fillable with fluid thereby to expand within the cavity. Such devices require a valve to prevent excess of the fluid where the disc is under an axial compression load, e.g. by bending.
In U.S. Pat. No. 3,875,595 there is disclosed a collapsible plastic bladder-like prosthesis of generally the same external form as the nucleus pulposus which is provided with a stem such that it can be filled with an unspecified liquid or plastic. The bladder also carries a stud for engagement within a recess formed in the cartilage end-plate or the over/underlying bone of the spinal column to retain the bladder in place. This disadvantage associated with such devices are the surgical procedure needed to form the recess sufficient to receive the stud and the need to prevent excess fluid when the disc is compressed.
The present invention seeks to avoid the disadvantages of the prior art proposals by the provision of a prosthesis which is simpler and cheaper to install and is not subject to mechanical failure.
SUMMARY
Accordingly, there is provided an implant for forming an intervertebral disc nucleus pulposus prosthesis comprising a conformable material adapted to fill cavities within the intervertebral disc and to be cured in-situ to form a shaped resiliently deformable prosthesis.
The invention further provides a method for replacing at least part of an intervertebral disc nucleus pulposus by implanting, within a cavity in the disc, an implant material in accordance with the invention and curing said material in-situ.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The term curing as used herein means any chemical reaction to form covalent or other bonds, for example hydrogen bonds.
The prosthesis may be used to replace or supplement body components which deform in the natural state. The prosthesis may thus be used for example, in breast implantation in cosmetic surgery, (such as in replacement of soft tissue for cosmetic purposes) in tissue expansion, in plastic surgery, etc.
Preferably, however the prosthesis is an intervertebral disc nucleus prosthesis.
The curable material desirably forms a solid or semi-solid elastomeric structure when cured and may be injected directly into the nucleus cavity. Alternatively a flexible cover or envelope may be used and can be provided so that it is impermeable to such toxic materials and they can therefore be maintained enclosed within the cover.
Desirably the curable material and the cured material are biocompatible, especially if the material is to be cured in-situ without a cover.
The curable material is flowable and is preferably in liquid form or in the form of a low viscosity gel (although it may be in the form of a particulate solid). This enables the implant material to flow easily so as to be able to conform to the desired shape before curing occurs.
The curable material may be, for example, a polyfunctional isocyanate based prepolymer so that water can be used to effect curing by causing the formation of urea linkages. The number of isocyanate groups available to form such linkages can be kept low if it is desired to minimise CO
2
evolution and exotherm.
Blocked isocyanate prepolymers which, on crosslinking with active prepolymer, can cure about or below body temperature may also be used. An example of this type of system is Desmocup II (polyurethane resin containing blocked isocyanate groups based on toluene diisocyanate and p-isononyl phenol) reacted with a polyfunctional amine terminated polymer such as polyalkylene oxide amine terminated polymer (eg. Jeffamine D2000 sold by Texaco Chemicals). The hydrophilicity of these systems may be varied by reaction of the blocked isocyanate resin with polyfunctional amine terminated polymers which contain a high proportion of ethylene oxide (eg. Jeffamine ED-600). Alternatively the blocked isocyanate polyurethane prepolymers may be prepared using polyols with high ethylene oxide content.
The curable implant materials may also include catalysts to promote or accelerate the curing reaction. Suitable catalysts for the isocyanate systems include tin catalysts such as Metatin 812ES.
Alternatively the material may comprise mixtures of poly(hydroxyalkyl(meth)acrylates) and poly(alkyl(meth)acrylates) crosslinked using polyfunctional (meth)acrylate monomers or oligomers, (eg. triethyleneglycol dimethacrylate. The reagent may be cured at low temperature (eg. 37° C. to ambient) by using a free radical initiator and an amine activator (eg. benzoyl peroxide and dimethyl p-toluidene). Preferably the alkyl groups contain from 1 to 4 carbon atoms.
The curable material may also consist of a mixture of tetra and trifunctional epoxy resin blend reacted with multifunctional amines and amino terminated elastomers such as an epoxy terminated silane and an amino terminated nitrile rubber. The material may comprise a monomer oligomer or polymer which contains ethylenic unsaturation. The ethylenic unsaturation may be acrylic or methacrylic unsaturation. Epoxy resins can also be added to the blocked isocyanate prepolymer (carbamic aryl ester capped urethane polymer—Desmocap II) polyfunctional amine system previously described for strengthening purposes.
Polymer complexes may also be used eg. complexes formed between the following polyanions, poly (sodium acrylate), poly (sodium vinyl sulphate) sodium poly phosphates, sodium polystyrene sulphonate and the following polycations: poly (N,N,N-trialkylammonioalkylacrylate), poly (N-alkylpyridinium) cation. There are several natural polymers which are capable of forming complexes. Anionic polymers include: sodium carboxymethyl cellulose, sodium cellulose sulphate, sodium alginate, sodium hyaluronate. Cationic polymers include: chitosan, quaternised chitosan, amino alkylated and subsequently quarternised cellulose, and poly-L-lysine.
Another alternative is to use siloxanes comprising functional groups which allow curing of the siloxanes with water to occur (eg. alkoxy, acyloxy, amido, oximo or amino groups). Acyloxy, acetoxy and alkoxy functionalities are most frequently employed. The number of siloxane groups can be determined such that the cured polymer is a resiliently deformable material.
Scavengers such as magnesium oxide may be advantageously employed if it is desired to reduce or eliminate any adverse effects of by-products of the curing reaction.
Inhibitors may also be included to control the exotherm generation in some systems such that the temperature of the implant material upon curing, does not increase much above that
Arrowsmith Paul
Millan Edith Jane
Milner Richard
Milano Michael J.
Surgical Dynamics Inc.
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