Biomedical subjects

S Vijayasekaran

Publications and source records attributed to S Vijayasekaran.

33 records · Page 2Linked to original sources

PubMed1996

Production of neocollagen by cells invading hydrogel sponges implanted in the rabbit cornea.

BACKGROUND: Poly(2-hydroxyethyl methacrylate) sponges are artificial tissue-equivalent matrices with potential value as materials for the peripheral zone of artificial corneas. A keratoprosthetic device was developed incorporating a poly(HEMA) spongy skirt which allowed cellular invasion. The present in vivo study investigated the biosynthetic activity of stromal fibroblasts growing within a poly(HEMA) sponge implanted into the rabbit cornea. METHODS: A porous poly(HEMA) hydrogel was synthesized by polymerization in a large excess of water. Specimens with a pore size larger than 10 microns were impregnated with collagen type I and then implanted into the limbal region of cornea in four rabbits. The animals were followed clinically for 28 days, when they were anaesthetized and new sponge specimens were implanted in their second eye. After 2 h, both eyes were enucleated. The 28-day and 2-h explants were subjected to autoradiographic analysis following labelling with tritiated proline and to an immunostaining technique using antibodies to collagen types I-VI. RESULTS: The autoradiographic analysis showed that the fibroblasts within the 28-day explants continued to be synthetically active and deposited proteins. Using the immunostaining technique, the deposition was most clearly demonstrated by the localization of collagen type III in the tissue invading the sponge. Both techniques failed to indicate any cellular activity in the short-time implants. CONCLUSIONS: The presence of collagen type III is consistent with a normal healing response of the stromal fibroblasts and indicates that poly(-HEMA) sponges are able to function as tissue-equivalent matrices.

Animals↗

PubMed1996

Keratoprosthesis: preliminary results of an artificial corneal button as a full-thickness implant in the rabbit model.

PURPOSE: To develop a prototype artificial cornea and evaluate it in the rabbit model. METHODS: Hydrogel core-and-skirt keratoprostheses were made and were inserted as full-thickness implants covered with conjunctival flaps in the right eyes of eight rabbits. RESULTS: Peroperative complications related to inadequate mechanical strength led to failure in the early postoperative period in three animals, one was euthanased for an unrelated reason and the remaining four have been successful for up to 16 weeks' follow-up. CONCLUSIONS: Full-thickness implantation of an artificial cornea, analogous to penetrating keratoplasty, has been achieved in the rabbit model. Histological findings confirm that integration of the prosthesis with host tissue occurs. The main complications encountered in this preliminary series were related to inadequate strength of the sponge skirt of this prototype device. Work in our laboratories is now concentrated upon improving the mechanical qualities of the hydrogel skirt and on the enhancement of biointegration.

Animals↗

PubMed1996

Poly(1-vinyl-2-pyrrolidinone) hydrogels as vitreous substitutes: histopathological evaluation in the animal eye.

A homopolymer of 1-vinyl-2 pyrrolidinone and its copolymer with 2-hydroxyethyl methacrylate, both cross-linked with divinyl glycol, were produced as possible substitutes for the vitreous body of the eye. The hydrated polymers behaved like viscoelastic gels, displaying excellent physical and optical properties. The sterile gels (0.7-1.5 ml) were injected into the vitreous cavity of rabbits, which previously underwent gas-mediated vitrectomy. Clinically, the eyes were quiet, with the exception of transient opacities in the vitreous. After 4 weeks, the operated eyes were enucleated and subjected to histopathological analysis using light and transmission electron microscopy. The common feature in all sections was the invasion of inflammatory cells. Vacuoles containing granular material, assumed to be polymer, were seen in the intercellular spaces of the neural retina, in the retinal pigment epithelium cells, and in macrophages. These findings indicated the fragmentation and phagocytosis of synthetic gels. It appeared that the biodegradation of the internalized polymers did not proceed further, however, the fate of polymers and their usefulness as vitreous substitutes should be investigated through long-term experiments.

Animals↗

PubMed1996

Initiation of impaired outer segment degradation in vivo using an antisense oligonucleotide.

This paper describes the first successful in vivo application of antisense DNA technology to induce the accumulation of photoreceptor outer segment derived debris in the retina. An antisense oligonucleotide (CatSC), which was previously demonstrated to be an effective tool to induce debris accumulation in vitro, was injected into the vitreous of pigmented and non-pigmented rats. The animals were euthanased 7 days after the injections. The number of inclusions significantly increased in the RPE layer of Long Evans and RCS-rdy + rats injected with 66 ug of CatSC to 96.2 +/- 13.6 (SD) (p < 0.0003) and 204.2 +/- 39.3 (SD) (p < 0.0001), respectively. The difference between the number of phagosome-like inclusions present in control saline, 6.6 ug of CatSC or 66 ug of sense oligonucleotide (S1) injected animals was not statistically significant. There were no abnormalities observed in the inner layers of the retina but the accumulation of phagosome-like inclusions was accompanied by disorganisation in the apices of outer segments. The large number of inclusions found in CatSC treated animals showed the characteristics of phagosomes containing stacks of undigested photoreceptor outer segment membranes which suggest that the lysosomal digestion process was halted or at least slowed down by the antisense oligonucleotide.

Animals↗

PubMed1995

Melanin-containing hydrogel intraocular lenses: a histopathological study in animal eyes.

Poly(2-hydroxyethyl methacrylate) hydrogel intraocular lenses, containing adrenochrome-melanin, were manufactured and implanted in animal eyes in order to assess the effect of melanin upon (a) biocompatibility of implants with the eye tissues, and (b) fibrous proliferation of lens epithelium responsible for the opacification of the posterior capsular membrane. An equal number of control lenses were also implanted. The animals were followed up for durations up to two years, and a detailed histopathological examination of the eyes was performed subsequent to their enucleation. The postoperative complications were minor and probably caused by surgical trauma. The study failed to give any indication of the postulated antiproliferative activity of adrenochrome-melanin since minimal capsular opacification occurred in the operated eyes, regardless of the presence of melanin.

Animals↗

PubMed1995

Optimal conditions required for the creation of an iatrogenic chorioretinal venous anastomosis in the dog using argon green laser photocoagulation.

We have previously reported iatrogenic retinal to choroidal vein anastomosis, developed as a potential method of by-passing the site of obstruction to venous outflow in retinal venous occlusion in dogs (1), and in rats (2). In order to minimise tissue damage to the retina and choroid and increase the rate of success in these experiments, we investigated in the dog model the factors that would promote an anastomosis and compare the effects of three different power levels. A small spot size (50 microns) argon green laser beam of 514 nm at power levels of 0.5w, 1.5w and 2.5w were used. Spaced serial sections from each lesion were examined by light and by transmission electron microscopy. Morphometric measurements of the corresponding retinal pigment epithelium (RPE)/glial scar was computed using IBMPC digitising pad and sigma scan software and the extent of tissue damage at the different power levels assessed. At the lowest power level of 0.5w the damage to the retina was mild and there was an absence of anastomosis formation. At the 1.5w power level an anastomosis formed in 40% of the lesions. At the highest power level of 2.5w a 71% rate of success was obtained however, the damage to the retina tended to be severe. The results of this study also indicate that disruption of Bruch's membrane and vein rupture at the time of irradiation are essential for anastomosis formation, which may be further enhanced by necrotic tissue, retinal pigment epithelial and glial scar formation and inflammation. These findings are useful in establishing optimal conditions for the creation of a chorioretinal venous anastomosis, for consideration in human trials.

Anastomosis, Surgical↗

PubMed1995

The use of Fourier transform infrared spectrometry for monitoring the retention of polymers in the vitreous humour.

A frequent cause of the failure of various materials tested as artificial vitreous substitutes is their short retention in the vitreous humour of the eye. Previous techniques to monitor the residence time of the vitreous substitutes have been traditionally complicated and usually involved the chemical modification of the test fluids. Fourier transform infrared (FTIR) spectrometry is used in this study to determine the rate of disappearance of a crosslinked poly(1-vinyl-2-pyrrolidinone) (PVP) hydrogel after injection into the vitreous humour of rabbits. The fluid removed from the eye at various postoperative intervals of time (1 day, 1 week, and 1 month) was placed on a horizontal attenuated reflectance (ATR) unit, and the infrared spectrum from 700 to 3000 cm-1 was obtained directly. The absorption band at 1295 cm-1 due to C-N stretching vibration does not overlap with bands of any other species (natural vitreous, blood) present in the test fluid. By estimating the intensity of this band, it was found that about 80% of the injected polymer was removed from the vitreous cavity within 1 month. This study shows that (1) FTIR spectrometry is a convenient method to assess the residence time of foreign materials in the vitreous humour, and (2) the PVP gel is unsuitable in this current form as a permanent vitreous substitute, regardless of the pathways of its removal, such as phagocytosis of fragments produced by the injection process, or enzymatic biodegradation.

Animals↗

PubMed1994

Interpenetrating polymer network (IPN) as a permanent joint between the elements of a new type of artificial cornea.

The combination at the interface between two chemically identical polymers was investigated by light and electron (scanning, transmission) microscopy. The polymers constitute elements of a new type of artificial cornea in which the peripheral skirt is made from spongy poly(2-hydroxyethyl methacrylate) (PHEMA) and the central optical zone from homogeneous, transparent PHEMA. Their two-phase combination along the boundary fulfill formally the requirements for an interpenetrating polymer network (IPN). The procedure for the manufacture of prosthesis was described in detail. Thin and ultrathin sections excised from the interface region were investigated using microscopic techniques. Light microscopy allowed the measurement of the diffusion path length of transparent PHEMA into sponge, which was approximately 0.5 mm. Transmission electron microscopy revealed a cellular-like morphology as well as larger segregated zones, which indicated network interpenetration on a molecular level and also a relatively poor miscibility of the two polymers despite their identical chemical structure. The latter was interpreted as a result of the submicroscopic restraints imposed by polymer I (sponge) upon polymer II. This study provides evidence that the interface combination of the prosthetic elements should be regarded as a gradient homo-IPN. This system offers a union between elements much stronger than those previously reported in artificial corneas.

Biocompatible Materials↗

PubMed1994

Synthetic polymers as materials for artificial vitreous body: review and recent advances.

In spite of a century of recorded attempts to replace the vitreous body of the eye with a foreign material, there is no permanent substitute currently available, and the success of some materials as temporary substitutes is still very limited. Among the large number of materials proposed or tested as vitreous substitutes, synthetic polymers have been episodically considered during the past four decades. This article will review these attempts, their outcomes, and recent progress in this field. There were only seven synthetic polymers and a few semisynthetic polymers (modified biopolymers) so far proposed or tested as vitreous substitutes. It appears that probably a synthetic hydrogel with very high water content would eventually be the material of choice for a permanent vitreous substitute, but the ideal material has not yet been found.

Animals↗

PubMed1994

Significance of Bruch's membrane in the creation of iatrogenic chorioretinal venous anastomosis.

Iatrogenic retinal to choroidal vein anastomoses, as a method of bypassing retinal venous occlusions has been reported in dogs in which Bruch's membrane is poorly formed. In order to determine whether chorioretinal venous anastomoses can be induced in an animal with a Bruch's membrane that is well developed as in humans, pigmented rats were chosen. A high intensity, small spot size argon green laser beam of 514 nm was used to induce the anastomosis. Three out of 5 rat eyes developed a retinal vein to choroidal vein anastomosis. The success rate of iatrogenic retinal to choroidal vein anastomoses in the rat was comparable to that obtained in the dog. This study suggests that Bruch's membrane was not significant in the creation of an iatrogenic chorioretinal venous anastomosis.

Anastomosis, Surgical↗

PubMed1993

Poly(2-hydroxyethyl methacrylate) sponges as implant materials: in vivo and in vitro evaluation of cellular invasion.

The pore size and the in vivo behaviour of four poly(2-hydroxyethyl methacrylate) sponges were investigated. The sponges were synthesized by polymerization of monomer in 70, 80 and 90 wt% water, respectively. In one of the formulations, a high amount of initiator was added. The average pore diameter was calculated with Ferry's equation and the results compared to those obtained by examination of samples using environmental scanning electron microscopy. The use of the equation greatly underestimated the size of pores. We also showed that the pores in polymers obtained in 70 wt% water were not interconnected, whilst the pores in polymers obtained in 80 and 90 wt% water, respectively, were larger and interconnected throughout the polymer. When implanted subcutaneously in rabbits, only the latter polymers allowed invasion and proliferation of cells. Penetration and proliferation of cells in these sponges were also assessed by an in vitro method using cultured human fibroblasts. The procedure included the overlaying of a glass plate covered by confluent cultured cells on to the surface of polymer impregnated with collagen. The depth of migration and number of sections needed to be cut to count a fixed number of invading cells were measured after incubation for 2 wk and used as indicators in comparing the ability of various sponges to allow cellular invasion. The assay showed that more cells invaded a hydrogel sponge produced in 80 wt% water than one produced in 90 wt% water. It also showed that the cut polymer surfaces allowed a greater cellular invasion than the moulded ones.

Animals↗

PubMed1993

Tissue interaction with hydrogel sponges implanted in the rabbit cornea.

We proposed poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel sponges as potential materials for the peripheral zone of a composite keratoprosthesis. It was previously shown that such sponges allowed cellular invasion when implanted s.c. in rabbits. To evaluate the reaction elicited by these materials in the corneal tissue, a PHEMA sponge, with pore size diameters of 10-30 microns, was produced, impregnated with collagen, and implanted in the rabbit cornea in both central and limbal regions of separate eyes. The eyes were examined by slit-lamp biomicroscopy up to 6 months postoperatively. Enucleation was performed at 3 weeks, 6 weeks, and 6 months, then excised implants were examined by light and transmission electron microscopy. Both clinical and histopathological examination indicated that sponges were well tolerated by the stromal and limbal tissues. No capsule was formed around any of the implants. Corneal fibroblasts and capillaries readily invaded the internal voids in the sponge to all depths. The invading cells remained viable and active up to the end of the follow-up period. Our findings suggest that hydrophilic PHEMA sponges may be successful as peripheral keratoprosthetic materials able to provide a permanent and tight fusion of the keratoprosthesis with the host tissue.

Animals↗

PubMed1992

Induced chorioretinal venous anastomosis in experimental retinal branch vein occlusion.

Iatrogenic retinal vein to choroidal vein anastomoses were created using laser photocoagulation in six of seven dog eyes in which a partial branch retinal vein occlusion had previously been created photochemically. A similar attempt to create an anastomosis was made in six control eyes in which no branch vein occlusion was present. In the eyes in which a branch retinal vein had been created, a venous chorioretinal anastomosis appeared to be present by 3 to 6 weeks. In three control eyes similar venous anastomosis was created; however this took 6 to 8 weeks to develop and was of much smaller calibre than the one that developed in the presence of a partial branch retinal vein occlusion. No adverse complications were noted in the period of the study (3 months). This study demonstrates that chorioretinal venous anastomoses can be created and may be of use in the treatment of partial retinal vein occlusions that show signs of progression.

Anastomosis, Surgical↗

PubMed1991

"Black prosthesis" revisited: a study of epinephrine-induced pigment deposits on poly(methyl methacrylate).

To investigate in detail the "black prosthesis" syndrome, experimental production of melanin from epinephrine was performed both in bulk and onto the surface of a common prosthetic material, poly(methyl methacrylate) (PMMA). The study by ultraviolet/visible spectrometry showed that the radiation-absorptive properties of PMMA were significantly enhanced; a sample treated for 20 days in epinephrine absorbed all ultraviolet radiation up to a 344-nm wavelength and transmitted only 4.9% from the ultraviolet spectrum at 400 nm and 16.2% from the visible spectrum at 500 nm. Transmission electron microscopy studies suggest that melanogenesis occurs on the surface of PMMA, and the pigment does not penetrate the polymer. Using infrared spectrometry, it was confirmed that the pigmentation is caused by a melanin formed through the oxidative polymerization of epinephrine.

Epinephrine↗

PubMedDate not supplied

Preliminary evaluation of a hydrogel core-and-skirt keratoprosthesis in the rabbit cornea.

BACKGROUND: We developed a core-and-skirt keratoprosthesis, with both components made from poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels. The identical chemical nature of both spongy skirt and transparent core assures a permanent union between them. We have previously shown that PHEMA sponges, within a certain range of pore size, can support cellular invasion and neovascularization when implanted into the rabbit cornea. The present study is the first to evaluate the behavior of the whole prosthesis after implantation into the rabbit cornea. METHODS: Hydrogel keratoprostheses were inserted intrastromally into the corneas of seven rabbits and histologically examined by light microscopy in five eyes enucleated at 8, 12, and 14 weeks. RESULTS: None of the implants extruded over this period. Both clinical and histopathologic examination showed that the keratoprostheses were well tolerated by the host tissue. The porous skirt was fully integrated into the stroma by fibrovascular invasion, and no capsule formed around the implants. Stromal melting anterior to the implant occurred in two cases, but this did not affect the fixation of the keratoprostheses. CONCLUSIONS: This study indicates that our keratoprosthesis can prevent extrusion in the short term when inserted into an intrastromal pocket of the rabbit eye.

Animals↗