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Biomedical subjects

F H Silver

Publications and source records attributed to F H Silver.

At least 37 records · Page 2Linked to original sources

A review of the etiology and treatment of skin ulcers with wound dressings: comparison of the effects of occlusive and nonocclusive dressings.

This article reviews the etiology and treatment of skin ulceration caused by external pressure, vascular insufficiency, and diabetes. In the case of pressure sores, compression of skin against bone may cause ischemic injury to underlying fat and muscle that precedes necrosis of dermis and epidermis. Venous and arterial insufficiency lead to leg ulcers as a result of incompetency of the valves in the veins connecting the superficial to the deep venous systems and atherosclerosis, respectively. Diabetics are susceptible to foot ulcers because of atherosclerosis and the resulting occlusive arterial disease and peripheral neuropathy. Once the underlying medical condition is solved, occlusive and nonocclusive wound dressings can be used in an attempt to promote healing. A review of the literature of animal and clinical studies suggests that both occlusive and nonocclusive wound dressings promote healing compared with air-exposed wounds. Dressings that absorb wound fluid offer some advantages over those that do not absorb large quantities of fluid in heavily exudative wounds and may require less frequent dressing changes. However, the chemistry of the material that comprises the wound dressing seems unimportant unless the material is biologically active. It is likely that the next generation of wound dressings will be composed of a moisture-retaining material coupled with material that has biological activity.

Animals↗

Use of viscoelastic solutions in ophthalmology: a review of physical properties and long-term effects.

Solutions of flexible macromolecules are used in ophthalmic surgery for insertion of intraocular lens, during corneal transplantation, in cataract, corneal, glaucoma, trauma, and vitreo-retinal surgery. Polymeric materials used in formulating these solutions include hyaluronic acid, chondroitin sulfate, polyacrylamide, collagen, and mixtures of some of these materials. Although the physical properties of these materials have been studied extensively, the long-term effects resulting from transitory increases in intraocular pressure and loss of even a small number of corneal endothelial cells are still unknown. The purpose of this paper is to review the literature that is available on these topics.

Acrylic Resins↗

Tissue compatibility of tyrosine-derived polycarbonates and polyiminocarbonates: an initial evaluation.

Compression-molded disks of two tyrosine-derived polymers [poly(desaminotyrosyl-tyrosine-hexyl ester carbonate) and poly(desaminotyrosyl-tyrosine hexyl ester iminocarbonate)], two polymers derived from Bisphenol A [poly(Bisphenol A iminocarbonate) and poly(Bisphenol A N-phenyliminocarbonate)], and two clinically used standard materials [poly(D,L-lactic acid) and high-density polyethylene] were implanted subcutaneously in the back of Sprague-Dawley rats. The tissue response elicited by these materials was evaluated histologically at 7, 30, and 120 days postimplantation, based on the total cell density (including fibroblasts, monocytes, giant cells, and macrophages) at the implantation site. The tissue response observed for the two tyrosine-derived polymers was mild, comparable to the two standard materials, medical-grade poly(L-lactic acid) and high density polyethylene. The two Bisphenol A-containing polymers elicited significantly more severe tissue responses. These results indicate that the use of derivatives of the natural amino acid L-tyrosine in the synthesis of degradable implant materials improved the tissue compatibility of these materials relative to chemically related polymers that contain Bisphenol A, an industrial diphenol. The tyrosine-derived polyiminocarbonate and polycarbonate are therefore promising candidates for a detailed evaluation of their biocompatibility, including long-term implantation studies in higher mammals.

Animals↗

Cell growth on collagen: a review of tissue engineering using scaffolds containing extracellular matrix.

Collagen has been used as a substrate for growth of cells derived from skin and cardiovascular tissue. Experimental results reported in the literature suggest that cell differentiation and orientation is enhanced in the presence of collagen. Extensive progress in cultivation of autogenous and allogeneic cells in vitro on collagen has led to advances in the transplantation of cultured materials. The purpose of this paper is to review the literature regarding cell culture on collagen substrates and to analyze the state of knowledge concerning the long-term effects. In the area of skin, long-term replacement of tissue has been studied extensively. Progress in this area has led to the development of technologies that will be used in the future for producing replacements for a wide variety of tissues and organs.

Animals↗

Analysis of mammalian connective tissue: relationship between hierarchical structures and mechanical properties.

It is widely accepted that the mechanical properties of implants must match those of the surrounding connective tissue to prevent stress concentration and premature failure. The purpose of this paper is to review the structure-mechanical property relationships that exist for connective tissue. The mechanical properties of connective tissue depend on the content of collagen, elastic tissue, and proteoglycans, as well as the geometric arrangement of the fibrous components, age, and location of the specimen. To a first approximation the geometry and loading pattern of the collagen networks in these tissues dominate the mechanical response at high strains. The behavior of the elastic fiber networks dominate the low-strain mechanical response in tissues where energy and shape recovery are critical. Proteoglycans are involved in resisting tissue compressive forces. Since the stiffness of connective tissue increases with age, it is necessary to attempt to match this property by designing implants that have similar behaviors to insure that stress concentration does not occur at the interface between the implant and host.

Biomechanical Phenomena↗

Use of collagen film as a dural substitute: preliminary animal studies.

Cadaver grafts, laminated metallic materials, and synthetic fabrics have been evaluated as dural substitutes. Use of cadaver tissues is limited by fear of transmission of infectious disease while use of synthetic materials is associated with implant encapsulation and foreign body reactions. The purpose of this study is to evaluate the use of collagen film as a dural substitute. Collagen films prepared from bovine skin were used to replace the dura of rabbits and histological observations were made at 16, 28, 42, and 56 days postimplantation. Controls consisted of dura that was removed and then reattached. Control dura showed no signs of inflammation or adhesion to underlying tissue at 16 and 28 days postimplantation. By 56 days postimplantation, extensive connective tissue deposition was observed in close proximity to adjacent bone as well as pia arachnoid adhesions. Implanted collagen film behaved in a similar manner to control dura showing minimal inflammatory response at all time periods. At 56 days postimplantation collagen film appeared strongly infiltrated by connective tissue cells that deposited new collagen. The results of this study suggest that a reconstituted type I collagen film crosslinked with cyanamide acts as a temporary barrier preventing loss of fluid and adhesion formation. It is replaced after approximately 2 months with host collagen with limited inflammatory and fibrotic complications. Further studies are needed to completely characterize the new connective tissue formed as well as long-term biocompatibility and functioning of a reconstituted collagen dural substitute.

Animals↗

Effects of fibroblasts and basic fibroblast growth factor on facilitation of dermal wound healing by type I collagen matrices.

Healing of large open dermal wounds is associated with decreased values of the tensile strength even up to 6 months post-wounding. Results of previous studies have shown that healing is facilitated in the presence of a type I collagen sponge by promoting deposition of newly synthesized large-diameter collagen fibers parallel to the fibers of the sponge. In this study healing is evaluated in dermal wounds treated with a collagen sponge seeded with fibroblasts or coated with basic fibroblast growth factor (bFGF). Experimental results indicate that the presence of a collagen sponge results in increased wound tensile strength and increased collagen fiber diameters in the upper dermis 15 days post-wounding in an excisional guinea pig dermal wound model. In comparison, dermal wounds treated with collagen sponges seeded with fibroblasts or coated with bFGF showed increased tensile strengths 15 days postimplantation and increased degree of reepithelialization. These results indicate that fibroblast seeding and bFGF coating in conjunction with a type I collagen sponge matrix facilitate early dermal and epidermal wound healing.

Animals↗

Anterior cruciate ligament replacement: a review.

The anterior cruciate ligament (ACL) is the major intra-articular mechanical element that limits motion of the tibia with respect to the femur. It is a multi-fasciculated structure composed of crimped aligned collagen fibers. The purpose of this paper is to review the literature on ACL structure and mechanical properties in an effort to stimulate the development of a new generation of more effective replacement devices. Replacement of the ACL is achieved using biologic and synthetic grafts. Biologic grafts include illiotibial band, semitendinosus and gracilis tendons, patellar tendon, and meniscus. Bone-patellar-bone complexes used to replace the ACL are revascularized and ultimately replaced by neo-ligament. Synthetic implants including the Integraft, Leads-Keio ligament, Gore-Tex¿ ligament and Kennedy Ligament Augmentation Device (LAD) have either not been approved or approved by the FDA for limited use as a replacement for the ACL. The Kennedy LAD has been found to increase the strength of autogenous tissue during revascularization. Based on the success of autografts and the Kennedy LAD, we conclude that the next generation of ACL replacement devices will consist of a scaffold and a biodegradable augmentation device. The scaffold will have a structure that mimics the normal ACL as well as stimulates revascularization and healing. A biodegradable augmentation device will be employed to mechanically reinforce the scaffold without stress shielding the neo-ligament. By combining the advantages of autografts and a biodegradable augmentation device, a new generation of ACL replacements will be achieved.

Anterior Cruciate Ligament↗

The use of Collagen Matrix to enhance closure of facial defects.

Facial defects created by removal of various types of skin cancer are usually closed primarily. There are some areas of the face where primary closure produces less than optimal results. We have utilized a non-reactive collagen sponge (Collagen Matrix) to enhance closure by secondary intention following removal of skin cancers. This presentation describes the properties of Collagen Matrix as well as the technique we utilized for closure of facial defects following tumor removal.

Aged↗

Regeneration of Achilles tendon with a collagen tendon prosthesis. Results of a one-year implantation study.

We previously reported on the short-term biocompatibility of a reconstituted type-I collagen prosthesis that had been tested in the Achilles tendons of rabbits. Preliminary results indicated that, by ten weeks after implantation, carbodiimide-cross-linked implants had been replaced by neotendon in a manner that was similar to that of autogenous tendon grafts that had been used as controls. Also by ten weeks after implantation, glutaraldehyde-cross-linked collagen implants were encapsulated and appeared to have caused an acute inflammatory response. In the present study, carbodiimide and glutaraldehyde-cross-linked collagen implants and autogenous grafts that served as controls were implanted for fifty-two weeks as a replacement for a three-centimeter section of the Achilles tendon of rabbits. The absence of a crimp in a cross-linked implant and the presence of a crimp in normal tendon and in tendon that formed after an implant had been resorbed made it possible to distinguish between a cross-linked implant and new host tendon that had replaced the implant after it was resorbed. New collagen that had replaced the implant and autogenous (control) tendon graft were compared with normal Achilles tendon with respect to the angle and length of the crimp. The autogenous grafts and the carbodiimide-cross-linked collagen implants had been completely resorbed and replaced by neotendon. The neotendon that was present fifty-two weeks after implantation was similar, but not identical, to normal tendon. In contrast, the glutaraldehyde-cross-linked implant was essentially inert, had not been resorbed, and was surrounded by a capsule of collagenous connective tissue. The neotendon in the capsule was also similar, but not identical, to normal tendon. There were more cells in the capsule than in the autogenous grafts or in the carbodiimide-cross-linked implants. The results of the present study indicate that rapid repair is achieved with a carbodiimide-cross-linked collagenous implant that has a structure and mechanical properties that are similar to those of an autogenous tendon graft and that biodegrades at a similar rate. Prolonged biodegradation of a glutaraldehyde-cross-linked collagenous implant results in formation of a capsule and only limited formation of neotendon.

Achilles Tendon↗

Noninvasive assessment of mechanical properties of peripheral arteries.

An ultrasound examination was used to noninvasively determine the changes in mechanical properties associated with age for the common carotid, brachial, popliteal, femoral, and tibial arteries. Forty-two normal male subjects, ranging in age from 8 to 60 years of age, were examined. The subjects were placed in one of three age groups: less than 29 years of age, 29 to 38, and greater than 38. Mechanical properties including percentage variation in diameter, pressure-strain, and circumferential elastic modulus were determined from changes in wall thickness and pulse pressure. Percentage variation in diameter (PVD) was seen to decrease with age for all arteries except the brachial, which remained relatively constant. Pressure-strain (Ep) and circumferential elastic moduli (Eo) were seen to increase with age in all arteries except the brachial, which remained relatively constant. Values of Ep and Eo were normalized into a stiffness index by dividing by the value found for the brachial artery. Stiffness indexes for the common carotid and femoral arteries were observed to increase more rapidly with age than the indexes obtained for the popliteal and tibial arteries. It is proposed that the stiffness index and changes in this parameter that occur with age may be useful in noninvasively assessing the progression of atherosclerosis.

Adolescent↗

Formation of continuous collagen fibres: evaluation of biocompatibility and mechanical properties.

Reconstituted collagen fibres have potential applications in repair of soft and hard tissues. Preliminary studies conducted in our laboratory suggest that discontinuous reconstituted type I fibres have strengths similar to those of fibres teased from tendons. The purpose of this paper is to report a method for continuous collagen fibre production and the properties of fibres produced. Ultimate tensile mechanical properties and biocompatibility of continuous type I collagen fibres were studied and compared with the properties of fibres produced manually (discontinuous fibres). In general, continuously made cyanamide cross-linked fibres show slightly inferior mechanical properties and faster biodegradation rates compared with manually made fibres because of minor differences in the fibreformation protocol introduced by design constraints. However, continuous and discontinuous fibres crosslinked with glutaraldehyde had comparable properties. These results demonstrate that production of 50 microns diameter continuous collagen fibre is possible.

Animals↗

Immunogenicity of collagenous implants.

Collagenous biomaterials have been used in our laboratory for treatment of decubitus ulcers, tendon/ligament repair and nerve regeneration. Results of previous studies suggest that implants containing bovine type I collagen enhance repair and regeneration of connective tissue found in different organs. The purpose of this paper is to evaluate the immunological response to type I collagen that is cross-linked using either glutaraldehyde or cyanamide treatment. Humoral and cell mediated responses to type I collagen are evaluated in a rabbit model. Results obtained in this study suggest that antibody levels and cell-mediated response to type I collagen are highest in animals exposed to uncross-linked implant materials and these responses are increased by booster injections of the antigen. Antibody titres to cross-linked collagen are significantly lower than those observed for uncross-linked material. Extensive implant cross-linking does not totally eliminate the humoral response and may lead to a cell-mediated reaction.

Animals↗

Method of growing vaginal mucosal cells on a collagen sponge matrix. Results of preliminary studies.

A method of obtaining and growing vaginal mucosal epithelial cells in culture on plastic and on a collagen sponge was developed. The results indicate that simultaneous culture of skin and vaginal epithelial cells from the same donor showed morphologically similar growth potential in culture on plastic and on a collagen sponge. Vaginal cells grown on a collagen sponge formed elongated clumps of cells similar to those observed for skin epithelial cells. The results suggest that vaginal cells grown in culture on a collagen matrix may be a suitable biomaterial for use in reconstructive surgery on women with the Rokitansky syndrome or in those who have had a malignancy.

Cells, Cultured↗

An evaluation of purified reconstituted type 1 collagen fibers.

Collagen fibers composed of type I collagen molecules were studied for biocompatibility and mechanical properties. These fibers were crosslinked using two different processes: 1) glutaraldehyde, 2) dehydration followed by exposure to cyanamide (DHT/C); the latter method produces only urea as a by-product of the crosslinking process and is postulated to be more biocompatible. An in vitro model using rat tendon fibroblasts growing on individual fibers was used to evaluate outgrowth rates, cell/fiber interactions, and cell morphology. These studies showed an advantage with DHT/C crosslinking, relative to glutaraldehyde crosslinking, in promoting fibroblast growth. In vivo intramuscular implantation in rats showed excellent biocompatibility for both kinds of collagen implants. In addition, aligned ingrowth into the implant from the medial collateral ligament when applied in that location was demonstrated. Mechanical testing demonstrated the higher strength of dry fibers; however, upon hydration, there was a marked decrease in stress to failure. This reduction in strength was due principally to an increase in cross section due to swelling. These collagen fibers appear to be very biocompatible even in the presence of low concentrations of glutaraldehyde. They promote fibrous aligned ingrowth in a setting of ligament healing. Thus, they represent a strong candidate as a scaffold ligament or tendon prosthesis if crosslink density can be increased.

Animals↗

Mechanical properties of collagen fibres: a comparison of reconstituted and rat tail tendon fibres.

This study involves comparison of the mechanical properties of reconstituted collagen fibres with those of collagen fibres obtained from rat tail tendons. Reconstituted collagen fibres were cross-linked in the presence of glutaraldehyde vapour for 2 and 4 d or using a combination of severe dehydration and carbodiimide treatment. Ultimate tensile strengths for reconstituted fibres cross-linked with glutaraldehyde ranged from 50 to 66 MPa while those cross-linked by severe dehydration and carbodiimide treatment had ultimate tensile strengths between 24 and 31 MPa. Rat tail tendon fibres had tensile strengths that ranged from 33 to 39 MPa. These results indicate that high-strength collagen fibres can be reconstituted in vitro and that these fibres may be useful in repair of dermal, dental, cardiovascular and orthopaedic defects.

Animals↗

Effect of proteoglycans on type I collagen fibre formation.

Collagen fibrillogenesis is a multistep process involving assembly of molecules into fibrils and bundles of fibrils. The exact role of proteoglycans in collagen fibrillogenesis is unclear. The purpose of these studies is to study the effect of proteoglycans on collagen fibrillogenesis in vitro. Results of these studies suggest that the proteoglycans dermatan sulphate and chondroitin sulphate do not change the final turbidity and hence the diameter of fibrils formed during the early stages of fibrillogenesis. This suggests that proteoglycans may not influence the early phases of collagen assembly, such as nucleation. However, proteoglycans added during the final stages of collagen fibre formation in vitro cause changes in ultimate tensile strength. In the presence of the high-molecular-weight proteoglycan, the ultimate tensile strength is increased by a factor of 1.5 above that of the control, whilst in the presence of low-molecular weight chondroitin sulphate proteoglycan the tensile strength is significantly decreased. It is concluded that proteoglycans influence the later stages of fibre formation. The presence of high-molecular-weight chondroitin sulphate proteoglycan leads to efficient stress transfer between collagen fibrils, altering the ultimate tensile strength. The results of these studies will be useful in optimizing the design of collagen tendon-ligament prostheses.

Animals↗

Development of a reconstituted collagen tendon prosthesis. A preliminary implantation study.

A reconstituted collagen tendon prosthesis was developed and implanted in rabbit Achilles tendons. The prosthesis was prepared by extruding type-I collagen into fibers and crosslinking it either with glutaraldehyde or with dehydrothermal treatment followed by exposure to carbodiimide. A tendon prosthesis was assembled by coating a longitudinal array of the fibers with uncrosslinked collagen. In one leg of the rabbit, the Achilles tendon was replaced with the synthetic tendon; in the contralateral leg of the animal, the tendon was excised, devascularized, and anastomosed as an autogenous graft. The autogenous tendon grafts were seen to be infiltrated centrally by fibroblasts and capillaries ten weeks postoperatively and to have been partially replaced by repair tissue twenty weeks postoperatively. Three weeks after implantation, all collagen implants were noted to have been infiltrated with fibrous tissue. At ten weeks, reorganization of collagenous tissue was observed in and around the prostheses, and the carbodiimide-crosslinked implants had been resorbed and replaced by normal-appearing neotendon. The implants that had been treated with glutaraldehyde were resorbed more slowly and were surrounded by more inflammatory cells, compared with the prostheses that had been treated with carbodiimide. Neotendon in the glutaraldehyde-treated prostheses matured more slowly. When the implants were examined at intervals after the operation, their mechanical properties approached those of fresh tendon. The initial strength of the carbodiimide-treated implants was lower than that of the fresh autogenous grafts. Twenty weeks after implantation, the strength and modulus of the carbodiimide-treated implants approached those of fresh tendon.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗