Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Meniscus
Reexamination Certificate
2001-03-27
2003-10-07
Snow, Bruce (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Meniscus
Reexamination Certificate
active
06629997
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to surgical implants that are designed to replace meniscal tissue and possibly cartilage in a mammalian joint, such as a knee.
The structure and components of soft tissues are discussed in nearly any textbook on human physiology (e.g., Guyton and Hall,
Textbook of Medical Physiology,
9th edition (1996) at page 186). Very briefly, the cells in most types of “soft tissue” (excluding bones, teeth, fingernails, etc.) are held together by a matrix (i.e., a three-dimensional network) of two types of fibers. One type is composed mainly of collagen, a fibrous protein that provides most of the tensile strength of tissue.
The other type of fiber consists mainly of “proteoglycan filaments”. These filaments contain a small quantity of protein and a much larger amount (roughly 98%) of hyaluronic acid, a natural polymer with alternating saccharide rings of glucosamine and glucuronate. Unlike collagen fibers, which are thick and provide high levels of tensile strength, proteoglycan filaments are extremely thin, and cannot be seen under light microscopes. They cause the watery extra-cellular fluid in soft tissue to form a gel-like material called “tissue gel”. This gel contains water, the proteoglycan filaments, and any other extra-cellular molecules that are suspended in the watery solution.
Roughly ⅙ of the volume of a person's body is made up of tissue gel, and it is essential to proper functioning of any type of soft tissue; among other things, it helps oxygen and nutrients reach cells, it aids in the removal of waste metabolites from tissue, and it helps tissue remain flexible and supple. Because proteoglycan filaments are so thin, molecules dissolved in tissue gel can permeate through the gel material with very little impedance; experiments have indicated that dye molecules can diffuse through tissue gel at rates of about 95 to 99 percent of their diffusion rates in water or saline.
Because nearly any type of soft tissue, in its normal and natural state, can be regarded as a type of hydrogel, many efforts have been made to create and use synthetic polymeric hydrogel materials as tissue implants. Most of these polymers are created by using non-parallel strands of long organic polymeric molecules (usually with chemical structures that are easier to work with and manipulate than glucosamine and glucuronate). Such molecules, to be suitable for use in a hydrogel, must be very hydrophilic (i.e., they must be able to attract and hold large quantities of water). This is most frequently accomplished by polymerizing precursor molecules that will provide large numbers of hydroxy groups (or other hydrophilic groups), on relatively short “side chains” or “side groups” that are bonded in a regular spaced manner to the long “backbone” strands of the final polymer.
An example of a synthetic hydrogel of this nature is PHEMA (an acronym for poly-hydroxy-ethyl-meth-acrylate), which is used to make soft contact lenses, drug-releasing hydrogels, and similar articles. In contact lenses made of PHEMA, the polymer does not actually bend light. Instead, the water that dwells inside the PHEMA polymer when the lens is hydrated does that job. The hydrophilic PHEMA polymer merely holds water molecules together, in the shape of a contact lens. If a polymer such as PHEMA is dehydrated, it typically becomes brittle; as long as it remains filled with water, it stays soft and flexible. However, like most synthetic hydrogels, PHEMA does not have sufficient strength and durability to last for years (or decades) as a permanent surgical implant.
PHEMA is certainly not the only synthetic polymer used to create biocompatible hydrogels; other polymers that can swell and soften when saturated with water include various hydrophilic polyurethane compositions (e.g., Gorman et al 1998 and U.S. Pat. No. 4,424,305, Gould et al 1984), poly(vinyl alcohol) compositions (e.g., Wang et al 1999), and other compounds known to those skilled in this field of art.
The flexible, pliable, gel-like nature of a synthetic hydrogel (when saturated with water) arises mainly from crosslinking attachments between non-parallel fibers in the gel. Depending on the specific polymeric structure that has been chosen, these crosslinking attachments between the long “backbone” chains in a polymer can be formed by covalent bonding, by hydrogen bonding or similar ionic attraction, or by entangling chains that have relatively long and/or “grabby” side-chains.
Regardless of which type of bonding or entangling method is used to bind the backbone chains together to form a hydrogel, the “coupling” points between molecular chains can usually be flexed, rotated, and stretched.
In addition, it should be recognized that the backbone chains in hydrogel polymers are not straight; instead, because of various aspects of interatomic bonds, they are somewhat kinked, and can be stretched, in an elastic and springy manner, without breaking the bonds.
In a typical hydrogel, the fibers usually take up less than about 10% of the volume; indeed, many hydrogels contain less than 2% fiber volume, while interstitial spaces (i.e., the unoccupied spaces nestled among the three-dimensional network of fibers, which become filled with water when the gel is hydrated) usually make up at least 90 to 95% of the total volume. Accordingly, since the “coupling” point between any two polymeric backbone chains can be rotated and flexed, and since any polymeric backbone molecule can be stretched without breaking it, a supple and resilient gel-like mechanical structure results when a synthetic hydrogel polymer is hydrated.
Various methods are known for creating conventional polymeric hydrogels. A number of such methods involve mixing together and reacting precursor materials (monomers, etc.) while they are suspended in water or other solvent. This step (i.e., reacting two or more monomers while they are suspended in a solvent) gives a desired density and three-dimensional structure to the resulting polymerized strands or fibers. The resulting material is then frozen, to preserve the desired three-dimensional structure of the fibers. The ice (or other frozen solvent) is then vaporized and removed, without going through a liquid stage, by a sublimizing process (also called lyophilizing), using high vacuum and low temperature. After the solvent has been removed, any final steps (such as a final crosslinking reaction and/or rinsing or washing steps, to remove any unreacted monomers, crosslinking agents, quenching agents, etc.) are carried out. The polymer is then gradually warmed up to room temperature, and it is subsequently saturated with water, to form a completed hydrogel.
These and other methods for creating synthetic polymeric hydrogels that are biocompatible and intended for surgical implantation are described in numerous patents, including U.S. Pat. Nos. 3,822,238 (Blair et al 1974), 4,107,121 (Stoy 1978), 4,192,827 (Mueller et al 1980), 4,424,305 (Gould et al 1984), 4,427,808 (Stol et al 1984), and 4,563,490 (Stol et al 1986). In addition, various methods of forming hydrogel coatings on the surfaces of other (“substrate”) materials are also described in various patents, such as U.S. Pat. Nos. 4,921,497 (Sulc et al 1990) and 5,688,855 (Stoy et al 1997).
There also have been efforts to reinforce hydrogels with an interpenetrating network (IPN) of fibers to enhance the soft hydrogel's mechanical properties. These fiber reinforcements have been with either chopped or longitudinally aligned fibers within the hydrogel. A number of these efforts to develop “composite hydrogels” apparently have focused on attempts to create synthetic pericardial tissue (i.e., the membrane that surrounds the heart); see, e.g., Blue et al 1991 and Walker et al 1991. Articles which describe these and other efforts to develop “composite” hydrogels are discussed in two review articles, Corkhill et al 1989 and Ambrosio et al 1998. In addition, efforts to develop composite implants with fibers embedded in an “elastomeric matrix”, for use in intervertebral di
Kelly Patrick D.
Miller Cheryl
Snow Bruce
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