Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-09-30
2001-07-24
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
Reexamination Certificate
active
06264695
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spinal nucleus implant to replace all or a portion of nucleus pulposus which has been removed form a spinal disc of a living vertebrate, e.g. a human. This spinal nucleus implant is formed of a xerogel which is capable of anisotropic swelling.
2. Information Disclosure Statement
Spinal intervertebral disc is a cartilaginous tissue located between the endplates of adjacent vertebra. The spinal intervertebral disc acts as a flexible joint between the vertebra, allowing bending and twisting of the spine column. Damage to the spinal intervertebral disc can cause spinal dysfunction, crippling pain and short- or long-term disability. Because of the wide occurrence of this problem (5% annual incidence of back pain due to the spinal intervertebral disc is reported), the economic consequences are enormous. Some disc problems require a surgery. Typical current procedure is fusion of the adjacent vertebra using various techniques and devices, such as those described in the U.S. Pat. No., 4,636,217 (Ogilvie, et al.), U.S. Pat. No. 5,489,308 (Kuslich, et al.) and U.S. Pat. No. 5,716,415 (Steffee). All currently available surgical procedures, such as removal of the nucleus or its part (laminectomy), or fusion of adjacent vertebra, compronise spinal function in one way or another.
For this reason, new remedies are being sought, including the development of prosthesis of the disc or its part. This is a difficult undertaking. The spinal column is an extremely intricate body part, and its proper function is dependent on the seamless cooperation of all its components, including the vertebral discs. A vertebral disc has to perform multiple functions. It has to withstand repeated high stresses in very complex modes of deformation including combined bending, torque, shear and compression. In addition, the spinal intervertebral disc acts as an efficient shock absorber and a pump driving a flow of nutrients into and metabolites from the disc. Structurally, the disc is a rather complex composite part involving several types of materials organized in a complex and intricate fashion. Vertebral endplates are covered by a layer of hyaline cartilage composed of a collagen matrix, a glycoprotein component, and water. In addition, about 2-5% of its volume is occupied by living cells producing the components of the cartilage.
The spinal intervertebral disc itself is composed mainly of crystalline collagen fibrils and amorphous hydrophilic proteoglycans. About 3-5% of the volume is occupied by living cells that produce the its constituents. Structurally, the spinal intervertebral disc is composed of a hydrogel-like core called the nucleus pulposus; and an outside ring called the annulus fibrosus. The structure of the spinal intervertebral disc is schematically depicted in FIG.
1
and described below.
The spinal intervertebral disc acts primarily as a weight-bearing and flexible joint. It enables mutual rotation, bending and translation of the adjacent vertebra, while bearing a considerable axial load. In addition, the spinal intervertebral disc attenuates vibrations and mechanical shocks and prevents their propagation through the skeletal system The load bearing capability and flexibility in selected directions is achieved by the combination of the annulus fibrosus and nucleus pulposus. Annulus fibrosus is a layered structure that is rigid in the radial direction but deformable in the axial direction and by torque. The axial load is born by nucleus pulposus that transforms it partly into an axial component that is contained by the annulus fibrosus. The annulus fibrosus is formed mainly by collagen fibrils organized in several layers. Each layer has its collagen fibrils wound at an angle, and subsequent layers have an alternate orientation. The collagen organization closely resembles organization of fiber reinforcement as in composites used for pressure vessels or cords in tires. It guarantees maximum resistance to radial stress (or internal pressure) while allowing a deformation in torque and bending.
The fibril ends are attached to the adjacent vertebra and to the cartilaginous surface of the vertebral endplates. Consequently, the inner space of the annulus fibrosus is virtually sealed. Any liquid penetrating in or out of the core has to pass through the annulus fibrosus tissue or through the vertebral endplates. To achieve sufficient hydraulic permeability, the collagenous structure of the annulus fibrosus is supplemented by proteoglycans embedded between the collagen fibrils. The proteoglycans are hydrated so that the annulus fibrosus forms a sort of a highly organized, anisotropic hydrogel composite. The collagen domains form a microfibrillar mesh. The result of this arrangement is a sufficient deformability in selected directions combined with high mechanical strength, and particularly high tear strength and resistance to fracture propagation that are needed for a load-bearing function.
The nucleus pulposus is connected to the annulus fibrosus, but not to the endplates. It has much a lower concentration of collagen (which concentration increases with age) and a higher concentration of hydrophilic proteoglycans. Consequently, it is a natural composite which is somewhat like a hydrogel and has a very high equilibrium water content (more than 90% by weight in young persons). The water content and volume of nucleus pulposus depends on osmolarity of swelling medium and on the mechanical pressure. The resistance to the decrease of liquid content due to mechanical pressure is called the “swelling pressure”. Swelling pressure is the very key to the function of the nucleus pulposus. As the axial load expels the liquid, the swelling pressure increases until it reaches equilibrium with the external load. Accordingly, the nucleus pulposus is capable of counterbalancing and redistributing the axial stress, converting them to radial components that can be confined by the annulus fibrosus. In addition, the dehydration and rehydration of nucleus pulposus under varying load drives the transport of metabolites and nutrients in and out the spinal intervertebral disc. Therefore, the nucleus pulposus acts as an osmotic pump facilitating transport of nutrient and metabolites to and from the spinal disc and surrounding tissues. This transport function is essential because the cartilaginous components (annulus fibrosus, nucleus pulposus and cartilaginous layer of the vertebral endplates) are neither vascularized nor can be supported with nutrition by mere diffusion.
Since the nucleus pulposus is substantially a macroscopically isotropic tissue, it has to be organized on its molecular and supermolecular levels to perform all these functions.
The nucleus pulposus structure is rather ingenious. The nucleus pulposus is constructed from a two-phase composite consisting of crystalline collagen domains forming a scaffold, and amorphous glycoprotein domains forming hydrophilic filler. The crystalline collagen domains are responsible for a relatively high strength even at high hydration. They form a microfibrillar mesh resembling the fibrous reinforcement in high-performance composites. The result of this arrangement is a sufficient deformability combined with sufficient mechanical strength even at full hydration.
The amorphous domains are responsible for water absorption and for the generation of a swelling pressure. They are formed mainly by high-molecular, water-soluble glycoproteoglycans. Glycoproteoglycans are highly hydrophilic and water-soluble polymers. A small portion of glycoaminoglycans is covalently bound to the coilagenous scaffold, turning it hydrophilic and highly wettable with water (this is necessary for the thermodynamic stability of the two-phase composite). A large portion is unattached to the scaffold and is retained by an entrapment within the scaffold due to the large size of glycoproteoglycans molecules.
To help this physical retention, glycoproteoglycans chains associate to form larger units. Glycoproteoglycans chains are e
Glynn, Esq Kenneth P.
Replication Medical Inc.
Stewart Alvin
Willse David H.
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