Abstract:
Disclosed is new type of implantable material for replacement of cartilaginous or fibrous tissue. The material has controlled porosity and is biocompatible. A method for making this material is also disclosed.

Description:
This is a division of application Ser. No. 078 823 filed July 28, 1987. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to implantable materials for replacement of fibrous or cartilaginous tissue. More particularly, the present invention relates to a method of making hydrophilic polymeric implants having controlled pore size which can be shaped and used to replace fibrous or cartilaginous tissue in animals and man. 
     For many years people have been searching for materials useful as replacements for different types of tissue. Materials tried include silicones, acrylates and other plastics, and metals. While each of these materials has certain advantages, some problems have developed in use. For example, silicone becomes hardened and displaced over time so its use as a breast augmentation implant, while tried often in the 1960&#39;s, has declined. Similarly, although metals and some plastic materials have been used for joints and other bone replacements, rejection problems have limited the use to those with no other alternatives. Plastics have also been tried as both soft and hard tissue replacements but there have been similar problems. 
     Because of these problems with the materials tried to date, a great detail of attention has been focused on modified plastics, particularly the acrylates and methacrylates, for implantation. Because of their transparent nature and easy moldability, there has been special emphasis on the dental and optical uses of the acrylate family. The optical uses have included scleral buckles and lens replacements while the dental area has focused on tooth and alveolar ridge replacement. In addition, some work has been done on breast or soft tissue replacement using acrylates and methacrylates. The acrylates and methacrylates, when implanted in a porous form, show tissue ingrowth and/or calcification which may support or harden the implant; in many cases, this ingrowth or calcification has been problem. 
     The Kroder et al. U.S. Pat. No. 3,628,248 is an example of the uses tried for the acrylates and methacrylates. This patent discloses a process for forming artificial implants, preferably for dental uses, using a variety of plastics, most preferably the acrylates and methacrylates. Kroder attempts to encourage tissue growth into the material using a porous surface. They obtain a porous surface by mixing potassium chloride in the acrylate monomer before polymerization. The potassium chloride is then leached out of the outer surface, leaving pores. However, the initiator used by Kroder as a hydrophobic could cause problems with tissue rejection. The use of hydrophobic plastics such as that shown by Kroder could also produce problems with rejection since there cannot be any transfer of electrolytes across the implant. 
     U.S. Pat. No. 4,199,864, issued on application by Ashman, attempted to cure the problems with the Kroder material. Ashman used polymethyl methacrylate as a dental implant material, and tried to form a porous surface by mixing a salt with the hydrophobic material. In fact, Ashman also lined the mold before polymerization with the salt crystals to provide surface porosity. Ashman found that even doing this, a &#34;skin&#34; formed over the plastic so it was necessary to grind off the outer layer of the material to expose the crystals. After exposure, Ashman let the material soak in water in an attempt to leach the salt crystals out. However, this just removed the salt from the outer layer because of the hydrophobicity of the material. Again, the hydrophobic material itself could still cause rejection problems. 
     To U.S. Pat. No. 4,536,158, issued to Bruins and Ashman and assigned to the same assignee as the Ashman patent, the same polymethyl methacrylate material was used for a dental implant. In this patent, the hydrophobic material was coated with a small amount of a hydrophilic methacrylate, hydroxyethyl methacrylate (HEMA) in an attempt to reduce rejection. The Bruins et al. patent describes using the material for replacing bone for dental applications. Very small particles of the coated hydrophobic material is made into a porous filler by packing the particles so that pseudopores are formed between the individual particles. While this approach is fine for a nonweight bearing application, its overall usefulness is limited. This material is sold commercially by Medical Biological Sciences, Inc., under the trade name HTR. 
     Others have used acrylates in various ways in order to replace bone or fibrous tissue. For example, U.S. Pat. Nos. 3,609,867 and 3,789,029 both issued to Hodash, concern an acrylate/ground bone mixture while U.S. Pat. No. 3,713,860, issued to Aushern, discloses a mixture of a porous aluminum oxide and a methyl methacrylate polymer to form a bone replacement substitute. 
     None of these bone or fibrous tissue substitutes have solved all the problems with rejection and controlled ingrowth. Accordingly, HEMA has been one of the newer materials tried for a variety of implantation and surgical uses. For example, in U.S. Pat. No. 4,452,776, issued on an application of Refojo, HEMA is used not only as a replacement for acrylates for contact lenses but also as a scleral buckle. HEMA has also been used as a breast augmentation material and as a dental implant. See Kronman et al., &#34;Poly-HEMA Sponge: A Biocompatible Calcification Implant&#34;, Biomat., Med. Dev., Art. Org., 7(2):299-305 (1979). HEMA has been used as an implant material in both a porous and nonporous state. However, the only method of obtaining porous HEMA has been to polymerize the hydroxyethyl methacrylate monomer about water molecules. For example, the Kronman et al. article discusses both 70/30 and 80/20% HEMA/water mixtures. Polymerizing about water molecules forms micropores within the hydrophilic HEMA but it does not allow any way to control the pore size with accuracy. Further, there is no way of being sure that the pores range throughout the material. 
     Problems with uncontrolled pore size besides the question of whether the pores are evenly distributed throughout the material include the problem that the properties of HEMA after implantation are different depending upon pore size. For example, with a pore size of 60-150μ, calcification takes place, leading to a bone-like implant. If the pore size is between 225 to 275μ, the properties of the material after implantation are similar to those of cartilage while a pore size of 300 to 450μ yields a fibrous-like tissue, similar to fibrous connective tissues. 
     Accordingly, an object of the invention is to provide a method making a material for forming implants of a biocompatible material which does not cause rejection and promotes ingrowth without calcification. 
     Another object of the invention is to provide a method of forming a biocompatible fibrous tissue replacement. 
     A further object of the invention is to provide a biocompatible and shapable cartilaginous implant material with a variety of uses, e.g., nasal augmentation, cartilage reshaping, and ear replacement. 
     A still further object of the invention is to provide a biocompatible synthetic replacement for fibrous, connective tissue which could be used for soft tissue augmentation, e.g., breast augmentation. 
     These and other objects and features of the invention will be apparent from the following description and the drawings. 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein provides a method of making a biocompatible microporous implant having controlled pore size for replacement of fibrous or cartilaginous tissue in an animal, particularly man. The invention also features the implant itself as well as it uses, e.g., breast augmentation, nasal augmentation or reshaping, and ear replacement or augmentation. 
     The microporous implantable material of the invention is formed by dispersing crystals of a water-dissolvable material, preferably a crystalline salt such as sodium chloride, potassium chloride or calcium chloride, in an unpolymerized hydrophilic polymerizable monomer. To make synthetic cartilaginous material, crystals of the water-dissolvable material ranging from about 225 and 275μ in diameter are mixed with the monomer while 300 to 450μ crystals are used for fibrous, connective tissue material. The hydrophilic polymerizable monomer is polymerized about the crystals to form a hydrophilic polymeric material having crystals dispersed therethrough. The resulting material is contacted with a sufficient amount of an aqueous solution to dissolve the crystals, thereby forming a polymerized microporous material having micropores of about 225 to 450μ in diameter at the locations where the crystals had previously been. This microporous material is then formed into the implant of the invention. 
     The monomers may be any hydrophilic acrylates or methacrylates but are preferably hydroxyalkyl acrylates to methacrylates, most preferably hydroxyethyl methacrylate. The hydrophilic polymeric material may consist of a mixture of the hydrophilic materials but a homopolymer of hydroxyethyl methacrylate is preferred. The polymerization reaction is carried out using standard methods, e.g., using an initiator, heat polymerication, or UV radiation. 
     The ratio by weight of the hydrophilic polymeric material after polymerization to the crystal can be controlled to produce the properties desired, e.g., changes in weight ratio can lead to either cartilaginous or fibrous connective tissue. In addition, the ratio of salt volume to polymer volume can be used to determine some properties of the resulting material. 
     The invention also features an implant for replacement of either fibrous or cartilaginous tissue made by the method of the invention. If the implant is made having micropores ranging from about 225 to about 275μ, it forms a cartilaginous-like material. This material is particularly well adapted for a nasal implant and the same type of cartilaginous replacement material may be used for an ear implant. The nasal implants for the invention are useful for augmentation, enhancing, or reshaping of deficient or misformed appendages while the ear implants have similar broad spectrum uses. In addition, the ear implants can form complete ear replacements. 
     If the larger pore sizes are used, e.g., about 300 to 450μ in diameter, a synthetic fibrous connective tissue is formed instead of cartilaginous tissue. This fibrous connective tissue is useful for breast augmentation or reshaping. This method is particularly well adapted for replacement of a breast which has been removed due to a mastectomy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart showing the steps in the manufacturing process for the implant of the present invention; and 
     FIG. 2 shows a cross-section comparison of the material of the present invention with the prior art Ashman material. 
    
    
     DESCRIPTION 
     As noted, the present invention features a method of producing biocompatible implants for replacement of fibrous connective and cartilaginous tissue. The implants of the invention promote tissue ingrowth without undergoing bone-like hardening due to calcification. The invention provides a method of producing micropores of controlled size dispersed throughout a hydrophilic material, yielding an implant which is biocompatible and has pores necessary for tissue ingrowth throughout the implant. Controlling the pore size permits control of the properties of the final material since changes in the amount of fibrous or tissue ingrowth changes the texture of the sponge-like replacement material. In this manner, one can produce synthetic cartilage or fibrous connective tissue under controlled conditions. This method prevents the calcification or other hardening which occurs in many other types of implants and thereby limits their usefulness. 
     FIG. 1 is a schematic diagram of the method of the invention. Crystals of a water-dissolvable material, preferably a salt such as sodium chloride, are milled into a controlled size selected depending on the type of tissue desired to be replaced. If the type of tissue being replaced is cartilaginous, e.g., a nasal implant for replacement of nose cartilage, the pore size of the salt crystals should be about 225 to 275μ in diameter. Salt crystals of that size would be selected so that upon solubilization, they would leave the desired pores. The salt crystals are mixed with an unpolymerized monomer of the implant material, e.g., hydroxyethyl acrylate. 
     One method exemplary of the invention has sodium chloride crystals milled to a size of about 250 μ dispersed throughout a solution of unpolymerized hydroxyethyl methacrylate. Approximately 67.8 g of sodium chloride is dispersed in 150 ml of the hydroxyethyl methacrylate monomer in a 200 ml beaker, yielding a final volume ratio of 75% plastic, 25% salt after polymerization. One method of keeping the salt crystals in a solution is by a magnetic stirrer which disperses the crystals in an even manner throughout the solution while it is undergoing polymerization. Alternatively, the materials are mixed and placed in a beaker or ampule. In either case, the solution is bubbled with nitrogen for thirty minutes, then sealed and polymerized. Polymerization is carried out in a conventional manner, e.g., using an initiator such as benzoyl peroxide but for some uses heat or UV polymerization is preferred since there cannot be any initiator remaining after polymerization which can cause rejection effects in the body. 
     A preferred polymerization technique has the sealed solution polymerized with an initiator in a thermostat at 60° C. for approximately ten hours. Approximately 5-10×10 -3  mole of the initiator, preferably methyl azo-bis isobutyrate is used per 1,000g of monomer. See &#34;Effect of the Structure of Poly(Glycol Monomethacrylate) Gel on the Calcification of Implants&#34;, Sprincl, Kopecek and Lim, Calc, Tiss. Res. 13:63-72 (1973), for exemplary procedures of polymerization. 
     The procedures described will yield a clock of the polymer with the salt crystals dispersed therein. Accordingly, it is necessary to remove the salt crystals in order to form the micropores. Since the polyhydroxyethyl methacrylate is hydrophilic, contacting the material with an aqueous solution leaches the salt crystals. The hydrophilic characteristics of the material allows the aqueous solution to permeate the material and dissolves the salt crystals from the entire body of the implant, as well as allowing the dissolved salt to flow freely from the material. Leaching of the salt crystals can be carried out by placing the material in a large excess of water or another aqueous based solution, preferably at an elevated temperature. It is also possible to use a flow system which constantly replenishes the aqueous solution, keeping the salinity of the surrounding water down and yielding better salt dissolution kinetics. 
     In one exemplary procedure, the block of polymerized material is placed in a 200 ml beaker under running water for about one hour. The block is removed, rinsed and allowed to stand in fresh water for about ten minutes. A pH meter with an ion probe is used to test for ion concentration, indicating whether salt is still leaching. If no salt is detected, the material can be shaped but if ions are detected, further soaking is used to leach the remaining salt. 
     Leaching the salt crystals from the polymerized material leaves a microporous material having pores where the salt crystals were previously. The salt is dissolved throughout the entire material because of the hydrophilicity of the material so the resulting material has pores throughout. In a hydrophobic material, such as that used by Ashman, the water cannot permeate past the outer layers of material and therefore salt crystals are entrapped within the body of the hydrophobic implant material. The entrapped salt cannot be contacted by the liquid and cannot dissolve. Accordingly, a cross-section of the Ashman-type material will have pores only on the outer layer. The portion of FIG. 2 labeled Prior Art illustrates the Ashman material. In fact, since a film or outer coating may form over the material during polymerization, it may be necessary to grind the outer surfaces of the material in order to obtain access to any of the salt crystals if a hydrophobic material is used. This type of grinding is specifically described in Ashman; this is necessitated by his choice of materials. 
     In contrast, the material of the present invention will have pores throughout. The portion of FIG. 2 marked Present Invention shows the extent of the pores using the method described herein. 
     Once one obtains the microporous material, the implant is then shaped in a conventional manner, e.g., cutting or grinding. An advantage of the material of the invention is that once formed, the gel is sponge-like so shaping can be carried out by carving with a scalpel or scissors. The Refojo U.S. Pat. No. 4,452,776, has a description of other ways of forming poly-HEMA into a proper shape. For certain uses, it may be possible to form a mold to the proper shape and polymerize the material directly in that mold. Such preshaping is included within the present invention. 
     The implantation techniques using the material of the invention include those currently known and are not in and of themselves part of the present invention. Common cosmetic surgery techniques for replacement or augmentation of tissue have been described in the literature and need not be described further herein. However, the materials of the present invention is biocompatible and has pores which allows for ingrowth of fibrous or cartilaginous-type tissue which allows implantation without the problems caused by tissue rejection. In part, rejection is minimized because the hydrophilic nature of the material used herein allows free flow of electrolytes and liquids across the implant. In fact, HEMA has been used as a coating on a number of metal or plastic implants in order to minimize rejection. 
     The uses set forth herein, e.g., nasal augmentation and enhancing, ear augmentation and reshaping, and breast augmentation, are exemplary only and others skilled in the art will determine other uses and modifications of the method and implant disclosed herein. Such other modifications and uses are within the scope of the following claims.