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Biomedical subjects

I J Constable

Publications and source records attributed to I J Constable.

At least 109 records · Page 6Linked to original sources
PubMed1993

Fungal corneoscleritis complicating beta-irradiation-induced scleral necrosis following pterygium excision.

Four cases of fungal corneoscleritis complicating beta-irradiation-induced scleral necrosis after pterygium excision have presented to our institution in recent years. Two cases were due to Petriellidium boydii and one each to Fusarium and Scedosporium inflatum. The condition may remain undiagnosed for weeks to months and becomes chronic with perforation or incipient perforation. The infections may masquerade as a chronic red eye, posterior scleritis or serous retinal detachment. Penetrating or lamellar keratoplasty is required following debridement of necrotic tissue. Prolonged systemic antifungal therapy may still fail to eradicate infection. Visual outcome is usually poor, and one of our patients required enucleation. Removal of calcific plaques from ulcer beds should be accompanied by disinfection, debridement and culture, since these beds and plaques are frequently a nidus of infection. Subsequently these ulcers may be covered with lamellar grafts or conjunctiva. We would caution against the use of radiotherapy to prevent recurrence of pterygia.

Aged↗
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↗
PubMed1993

Pterygium excision with conjunctival autografting: an effective and safe technique.

The optimum mode of treatment for symptomatic pterygia would combine efficacy (a low recurrence rate) with safety (freedom from sight threatening complications), and would not affect visual acuity adversely. The efficacy of pterygium excision with conjunctival autografting in a sun exposed population in which pterygia are prevalent has previously been questioned. A cross sectional review of 93 eyes of 85 patients was carried out by slit-lamp examination a minimum of 6 months (range 6-76 months) after pterygium excision and free conjunctival autografting. Case notes were reviewed to obtain details of complications and visual acuity changes related to surgery. Of six recurrences (6.5%) four of these were asymptomatic with minor recurrences. Two patterns of recurrence were identified: cross graft recurrence (three cases) and outflanking (three cases). Complications (wound dehiscence, three cases; Tenon's granuloma one case; conjunctival cyst, one case) were all corrected by minor surgical revision without sequelae. Unaided acuities were unchanged or improved 3 months after surgery in 86 cases, with a minor diminution (1 Snellen line) in seven cases. This study demonstrates a low recurrence rate for a safe technique in an area in which ongoing ultraviolet light exposure levels are high and pterygia are prevalent.

Adult↗
PubMed1993

Transient preservation of photoreceptors on the flanks of argon laser lesions in the RCS rat.

In the RCS rat there is a progressive degeneration of the photoreceptors which starts at two to three weeks after birth. We have demonstrated that there is prolonged survival of photoreceptors on the flanks of argon laser lesions made at post-natal day 23, just as degeneration begins. The cells in the outer nuclear layer retained a relatively normal appearance on the flanks of the lesions for the first two weeks and there was a very low percentage of pyknotic cells. By one month some cell loss occurred but much less than in non-irradiated regions where very few cells remained. At two months after lesion there were still elevated numbers on the flanks but only 2-3 rows of cells remained. The prolonged survival correlated with disruption to the pigment epithelial layer, increased migration of phagocytic cells into the outer segment/debris layer and a reduction in thickness of the debris layer at late stages. The mechanisms of this effect are unknown however laser lesions provide a well controlled and reproducible situation in which to study these mechanisms.

Animals↗
PubMed1993

Expression of basic fibroblast growth factor and its receptor in the retina of Royal College of Surgeons rats. A comparative study.

PURPOSE: The aim of this study was to identify whether abnormalities in the synthesis of basic fibroblast growth factor (bFGF) or its receptor (bFGF-R) were responsible for the photoreceptor dystrophy in Royal College of Surgeons (RCS) rats. METHODS: The polymerase chain reaction was used to detect the expression of bFGF and bFGF-R messenger RNA in the retinal pigment epithelial (RPE) cells and the neural retina of RCS dystrophic rats and in PVG/C and RCS-rdy+ control animals. RESULTS: In the RPE, it was found that there was no significant difference in the expression of bFGF and bFGF-R between RCS rats and the controls at the ages of 21 days and 3 mo. In the neural retina, the level of bFGF expression was lower in the 21-day-old RCS rats compared with the control group, but bFGF-R expression was as strong as in the PVG/C and RCS-rdy+ animals. However, in 3-mo-old RCS rat neural retina, the bFGF and bFGF-R expression was found to be significantly lower than in the control animals. CONCLUSIONS: Although the mutant gene in RCS rats is expressed in the RPE cells, these results suggest that there is no significant defect in bFGF or bFGF-R expression in the RPE cells of RCS rats, which would be an initiating factor in the development of photoreceptor degeneration in these animals. The lowered bFGF levels in the neural retina at early stages (postnatal day 21) may explain the prolongation of photoreceptor survival when exogenous bFGF is injected.

Animals↗
PubMed1992

Developmental study of chondroitin-6-sulphate in normal and dystrophic rat retina.

The RCS rat is a widely studied model of human retinal dystrophies including retinitis pigmentosa. Chondroitin-6-sulphate (C6S) in the interphotoreceptor matrix was localised immunocytochemically in both the normal congenic and dystrophic strains of the RCS rat up to 65 days postnatally. From postnatal days 5 to 15 the distribution of C6S in both strains was similar, being localised in the interstices of developing inner and outer segments and adjacent to the RPE surface. In the normal rats, the distribution of C6S did not change with age. In the RCS rats, however, at postnatal days 20 to 35 staining was observed as a dense band at the junction of inner and outer segments and no staining was observed adjacent to the surface of the RPE. At postnatal day 45 onwards there was a decrease and a complete absence of C6S staining in these rats. This change in the pattern of staining correlated with the morphological observation of the progressive degeneration of photoreceptor cells suggesting that C6S may be important in photoreceptor degeneration in the RCS rat.

Aging↗
PubMed1992

Flow patterns of blood cells in the retinal capillaries. Retinal capillary flow patterns.

We present a new technique for retinal blood cell angiography which was used to investigate flow patterns of blood cells in the retinal capillary net of normal cats. Capillary paths were defined to be the path taken by the labelled blood cells from the terminal arteriole to the entry point into a collecting venule. We were able to define two types of capillary paths. The 'simple' paths were minimally coiled and had a high blood cell flux. The 'complex' paths were coiled, structurally complex and had a low blood cell flux in comparison to the simple paths of the same eye. A simultaneous fluorescein angiogram was performed to reveal the complete anatomy of the capillary net in which the functional paths exist. The possibility that such flow patterns exist in the human retina should be considered.

Animals↗
PubMed1992

Laser microsclerostomy for primary open angle glaucoma: a review of laser mechanisms and delivery systems.

A number of different lasers and delivery systems are currently used in experimental sclerostomy procedures. These are discussed with reference to their performance in terms of cutting accuracy and trauma to the adjacent tissues. Lasers emitting wavelengths close to the absorption peaks of water in the mid-infrared region and organic polymers in the far ultraviolet region have a short absorption pathlength in the sclera and produce the least adjacent thermal tissue trauma. These ablating lasers cannot be delivered gonioscopically, and contact endoscopic techniques trap hot expanding gases within the forming sclerostomy channel causing secondary thermal and mechanical damage. Optimal results should be obtained using an ablating laser delivered either through an open mask or a modified endoscopic system incorporating an adequate exhaust mechanism.

Endoscopy↗
PubMed1992

A new magnifying indirect stereo ophthalmoscope.

A new magnifying indirect stereo ophthalmoscope had been designed, built, and tested. Magnification is achieved by means of Keplerian telescopic systems mounted in each eyepiece. Schmidt or Pechan prisms are incorporated to shorten the length of the telescopic systems. Using a +20 diopter condensing lens, the total retinal magnification achieved with this system varies from 5.6x at a 40-cm observation distance to the aerial image to 9.4x at a 25-cm observation distance. When the fundus is viewed through a dilated pupil extraordinary stereopsis is obtained. When combined with the magnification, this stereopsis greatly enhances the view of the posterior segment of the eye.

Depth Perception↗
PubMed1992

In vitro cytotoxicity of melanized poly(2-hydroxyethyl methacrylate) hydrogels, a novel class of ocular biomaterials.

Due to their ability to absorb ultraviolet and visible radiation, we have proposed the melanized poly(2-hydroxyethyl methacrylate) hydrogels as biomaterials suitable for the manufacture of soft artificial intraocular lenses. Their biocompatibility has not been evaluated so far. In this study, poly(2-hydroxyethyl methacrylate) containing various amounts of adrenochrome-melanin were synthesized and the cytotoxicity of their aqueous extracts was assessed by using four in vitro testing techniques (trypan blue dye exclusion, inhibition of DNA synthesis, lactate dehydrogenase release, and inhibition of cell growth). Assays were based on incubation with human choroidal fibroblasts. By the first three techniques, no cytotoxicity was found in the extracts. The inhibition of cell growth test showed a slight cellular protein loss, however only in the extracts of polymers with high melanin content. The results suggest that the release of potentially toxic agents from melanized hydrogels into an aqueous medium is not significant. However, when an assay in collagen gel was carried out in the presence of specimens of melanized hydrogels, a toxic reaction was clearly revealed. This can be caused by a delayed release of toxic molecules from melanin, or by some other mechanism. The use of melanin-containing polymers as implant materials becomes questionable and further research is necessary.

Biocompatible Materials↗
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↗
PubMed1991

Cytotoxic effects of residual chemicals from polymeric biomaterials for artificial soft intraocular lenses.

Development of improved hydrogels for soft intraocular lenses, based on 2-hydroxyethyl methacrylate monomer, requires the use of various other monomers and polymerization additives which have potential ocular toxicity. Three monomers, 2-hydroxyethyl methacrylate, methyl methacrylate, and 2-ethoxyethyl methacrylate, as well as two common inhibitors, hydroquinone and 4-methoxyphenol, were subjected to in vitro cytotoxicity assays as aqueous solutions at different concentrations. A new polymerization initiator, 2,2'-azo-bis-(2,4-dimethyl valeronitrile), was thermally decomposed in water at different concentrations and the products were also assayed for cytotoxicity. Assays were based on incubation with human choroidal fibroblasts. Cell death was evaluated by trypan blue dye exclusion, DNA synthesis inhibition, and lactate dehydrogenase tests. While methyl methacrylate and 2-ethoxyethyl methacrylate were found nontoxic, the other chemicals displayed high cytotoxicity. However, when extracts of synthesized poly(2-hydroxyethyl methacrylate) specimens, differentially treated after polymerization, were subjected to the same assays it was found that toxicity from residual 2-hydroxyethyl methacrylate monomer was lost during steam sterilization and storage in water because of the removal of the monomer through aqueous washing. The lack of toxicity in these specimens suggests that residual contents of inhibitor and initiator are too low to cause toxic effects on choroidal fibroblasts. It is concluded that hydrogels have low cytotoxic effects in vitro.

Anisoles↗
PubMed1991

Further studies on ultraviolet-absorbing hydrogels for intraocular lenses: relationship between concentration of a polymerizable benzophenone, absorption, and extractability.

A tendency to reduce the use of benzophenone absorbers is currently evident in the manufacture of the UV-absorbing IOLs, mainly because the cutoff wavelengths are inferior to those provided by benzotriazoles. In principle, by incorporating large amounts of benzophenones it may be possible to achieve high cutoff wavelengths. A covalently bondable benzophenone UV absorber, Cyasorb UV-2098, was incorporated in poly(2-hydroxyethyl methacrylate) (PHEMA) in increasing concentrations, and certain associated phenomena were investigated. At 4% w/w absorber content, the nonhydrated polymers turned partially opaque. In water-swollen hydrogels, opacification occurred at a lower absorber content (2% w/w). By using extraction techniques and gas chromatography, we also found that up to 8% w/w of the absorber remained unpolymerized and could leach out from any material containing less than 5% w/w absorber. In samples with higher initial content of absorber, the amount of unreacted, leachable absorber was significantly higher (25% to 30%). Based on this study model (hydrogel/Cyasorb UV-2098), we concluded that benzophenone absorbers cannot provide cutoff wavelengths higher than those provided by benzotriazoles.

Acrylates↗
PubMed1991

Improved method for calculation of corneal topography for any photokeratoscope geometry.

Accurate and efficient calculation of the anterior corneal surface topography by photokeratoscopy has become important for the management or surgical treatment of refractive errors. Methods have been published that describe such calculations. The algorithms described here eliminate an assumption made in some of the earlier computational methods without an excessive increase in the computation time or increased sensitivity to measurement errors. This results in an enhanced accuracy for reconstructing corneal topographies. Computer simulation results indicate an improved accuracy in measuring the topography for nonspherical corneas. The equations given here apply to any general photokeratoscope geometry and can easily be used in computer programs. In a notable previous publication concerning the more common flat faceplate keratoscope geometry, several of the equations were presented with errors. These errors are identified as an aid to anyone who had already attempted to use those equations in a computer program.

Algorithms↗