Biomedical subjects

C England

Publications and source records attributed to C England.

11 recordsLinked to original sources

PubMed1992

Induction of vascular haemostasis by Nd:YAG laser light in melanin-rich and melanin-free tissue.

Haemostasis was effected in vessels of melanin-rich (MR: choroid) and melanin-free (MF: mesentery) rabbit tissue irradiated with a cw-Nd:YAG laser. The following parameters were employed: - pulse duration: 200 ms (MR) and 100 ms (MF); focal spot diameter: 200 microns (MR) and 80 microns (MF); pulse energies: 100-250 mJ (MR) and 0.5-1 J (MF); irradiances: 1.6-4.0 kW cm-2 (MR) and 1-2 x 10(2) kW cm-2 (MF). In melanin-rich tissue, laser energy is absorbed principally by melanin granules contained within the stromal melanocytes. The heat generated in these structures radiates into the surrounding tissue where it is dissipated. The damage thus incurred by the endothelium of blood vessels encompassed within this field triggers the haemostatic mechanism whereby blood flow is arrested. This effect is realized by the formation of an occluding plug of platelets, which is stabilized by the deposition of fibrin, particularly in capillaries, and to a lesser degree in larger vessels of the vascular lamina. In melanin-free tissue, haemoglobin serves as the primary site of energy absorption, which is thus shifted from the stroma to the vessel lumen. Irradiation of vessels in such tissue leads to thermocoagulation of plasma proteins and consequent stasis of blood flow.

Animals↗

PubMed1992

[Hemostatic effect of the Nd:YAG laser in cw function].

The Nd:YAG laser may be effectively employed to induce blood flow stasis and vessel closure in rabbits. The mechanism by which these effects are achieved depends upon the nature of the primary absorbing structures in the irradiated tissue, as well as upon the energy level of radiation. Vascular wall damage occasioned in consequence of energy absorption in melanin granules triggers blood flow stasis according to the classic clotting cascade, defects in the endothelial cell wall being closed by aggregated platelets. In tissue lacking melanin pigmentation, energy is absorbed primarily by the haemoglobin of erythrocytes, and blood flow is arrested by coagulation of plasma proteins, platelets being absent from the impact centre. The portent of these different haemostatic mechanisms upon repair of the endothelial cell wall and neovascularization have yet to be determined.

Animals↗

PubMed1992

Laser cyclocoagulation.

Since its introduction in 1973 by Beckman and Sugar, cycloablation, using Nd: YAG laser energy has gained increasing attention. Nd: YAG laser energy may both be delivered in the contact and non-contact mode and the energy source may both work in the pulsed or cw mode. The physical and clinical effects of the various design principles differ. A short survey of the present state of the art is presented.

Ciliary Body↗

PubMed1992

A new instrument for controlling pressure exerted on the sclera during contact Nd:YAG laser cyclodestruction.

We tested the device described by Rol et al in their article in this issue, designed to define and precisely control the pressure applied to the sclera during contact cyclodestruction with a cw-Nd:YAG laser. Since scleral transparency, and hence transmission of laser energy, depends on the pressure applied, control of this parameter represents an important advance toward the goal of standardizing the cyclodestructive procedure. In eight of nine cadaver pig eyes, pressure was exerted on the sclera using the pressure-controlling device, with a setting of 0.25 N for two of these eyes, and one of 0.4 N for the other six. In the remaining eye, pressure was controlled intuitively by an experienced surgeon. Applying forces of 0.25 or 0.4 N yielded similar success rates, although these fluctuated greatly even under constant pressure conditions (from 17% to 50% at 0.25 N; from 27% to 42% at 0.4 N). The highest success rate (55%) was achieved by the intuitively controlled pressure irradiations of the experienced surgeon. The variation in coagulation intensity that occurred despite constant pressure suggests that additional factors contribute to the results achieved. Nonetheless, this pressure-controlling probe represents an important achievement. It will permit not only the novice but even experienced operators to adhere unambiguously to a protocol of quantitative, reproducible parameters, and will also help researchers to better understand the relationship between energy delivered and intraocular pressure reduction achieved in contact cyclophotocoagulation.

Animals↗

PubMed1992

Optical principles related to optimizing sclerostomy procedures.

A simple probe, consisting of a 200-micrometer uncladded silica optical fiber, advanced from a protecting 22-gauge hypodermic needle, was used to create sclerostomy fistulas both ab interno and ab externo by means of a combined radiation/mechanical effect. Perforation was achieved by exerting gentle forward pressure on the fiber in synchrony with the delivery of radiation pulses. An irradiation protocol suitable for producing such canals in cadaver porcine eyes was delineated using Ho:YAG, Nd:YAG, and diode laser energy sources. Despite significant differences in the pulse energy required for perforation (0.25, 7, and 8 J for the Ho:YAG, diode, and Nd:YAG lasers, respectively), the extent of collateral damage was comparable for each type of laser. The physical mechanisms underlying these findings are discussed.

Animals↗

PubMed1991

Blood flow stasis induced by cw-Nd:YAG laser irradiation: comparative morphology of mesenteric and choroidal vessels in pigmented rabbits.

cw-Nd:YAG laser radiation was effective in producing blood flow stasis within arteries (35-40 microns in diameter) of pigmented rabbit mesentery (beam spot size: 80 microns; fluence: 2 x 10(4) J cm-2) and choroid (beam spot size: 200 microns; fluence: 6 x 10(2) J cm-2). However, the mechanism by which this was achieved differed in each case, and depended upon the energy absorbing structures present in the irradiated tissue. In non-pigmented tissue, such as the mesentery, haemoglobin represents the primary absorption centre, and the temperature attained within the vessel lumen (as inferred from morphological changes) is sufficient to denature plasma proteins, in particular fibrinogen, which consequently occlude the vessel lumen and arrest bleeding. In pigmented tissue, such as the choroid, melanocyte granules represent the primary absorption centre, which is thus shifted from the vessel lumen to the stroma. The temperature rise achieved within the vessel lumen is consequently lower, as evidenced by the absence of plasma protein denaturation. Blood flow stasis nonetheless occurs, but is triggered according to the normal haemostatic mechanism.

Animals↗

PubMed1990

Transpupillary irradiation of the rabbit retina with the cw-Nd:YAG laser. I. Acute morphologic effects produced using two different pulse forms.

The retinas of six pigmented rabbits (12 eyes) were irradiated with a cw-Nd:YAG laser using two different pulse forms, in conjunction with a constant pulse duration of 200 msec and a retinal focus diameter of 200 microns. The first pulse form (a) was characterized by an initial transient, lasting 15-20 msec, with a peak power twice that of the ensuing plateau phase of 180-185 msec. The second pulse form (b) was almost rectangular in shape. Although the average energies (and powers) for each pulse form were identical, almost 20% of this entity was delivered within the first 20 msec for the first pulse form. Impact lesions were examined at the biomicroscopic, light, and electron microscopic levels, and the characteristic morphologic features described for the cw-mode in general. Comparison of the effects produced using each pulse form revealed a number of differences. At low pulse energies (less than 150 mJ) the rectangular pulse form produced a photocoagulation reaction within the outer sensory retina, but when using the pulse form with the initial transient, this effect was superimposed by a degree of mechanical disruption. Ruptures produced within the elastic component of Bruch's membrane first appeared at lower pulse energies (ie, at 75 mJ, as opposed to 150 mJ for the rectangular pulse form) and were more frequent, and choroidal macrohemorrhaging (observed at pulse energies greater than or equal to 200 mJ) occurred only, when this latter pulse form was employed. Each of these phenomena may be attributed to the higher power and energy densities attained during the initial transient, which has disruptive properties. In order to avoid the risk of severe bleeding, these irradiation parameters should be maintained below approximately 6.4 X 10(3) W/cm2 and approximately 0.13 X 10(3) J/cm2.

Animals↗

PubMed1989

Cyclophotocoagulation in glaucoma therapy.

In a morphological study, the long-term effects of transscleral cyclophotocoagulation on rabbits using the free-running Nd:YAG laser with either a contact or a non-contact delivery system are described. A marked atrophy of the irradiated ciliary processes may be observed. Regeneration of the ciliary epithelial layers is incomplete at least up to 8 months after treatment. The vascular network is also atrophic, and shows a small regenerative capacity only.

Animals↗

PubMed1989

The effect of transscleral laser cyclophotocoagulation on rabbit ciliary body vascularization.

Long-term changes in the vascular network of rabbit ciliary processes induced by Nd:YAG laser cyclophotocoagulation were examined both from a morphological viewpoint and with respect to three-dimensional organization (in vascular casts). Relative changes in intraocular pressure (IOP), monitored within eyes irradiated across one-quarter, one-half, three-quarters, or all of the ciliary body circumference, are discussed in relation to morphological changes. The clinical implications of these findings are considered.

Animals↗

PubMed1988

Diffusion barriers impeding the flow of solutes to the ciliary body epithelium during immersion fixation of rabbit eyes. Assessment of ultrastructural preservation quality by examination of mitochondrial profiles in the electron microscope.

Various manipulations of rabbit anterior eye segments were carried out during the initial stages of glutaraldehyde fixation in order to assess the contribution made by particular tissues in restricting the flow of solutes to the ciliary body epithelium. Ultrastructural preservation quality was monitored by inspection of mitochondrial profiles. These organelles are structurally extremely sensitive to environmental disturbances, and hence changes in their morphological appearance may be used as an early indication of such adverse conditions. The results indicate that the principal barriers to diffusion are represented by the vitreous, posteriorly, and by the cornea at the anterior face. Since the cortical vitreous is strongly attached to the inner basement membrane of the ciliary epithelium, its removal may cause damage to this layer. A simpler and equally effective means of improving flow of fixative to the ciliary epithelium is to remove the cornea.

Animals↗

PubMed1987

Morphology of the trabecular meshwork within monkey (Macaca speciosa) eyes after irradiation with the free-running Nd:YAG laser.

The long-term tissue response of monkey eyes to trabeculoplasty produced at the chamber angle by neodymium:YAG (Nd:YAG) laser (thermal mode) was investigated. Four weeks after irradiation, a monolayer of corneal-like endothelial cells extended over the trabeculum in impact zones, with partial occlusion of intertrabecular spaces; this condition persisted at all subsequent stages of observation (viz, up to 13 weeks). Both primary and secondary degeneration of trabecular endothelial cells occurred predominantly at the posterior border of the juxtacanalicular tissue, and continued throughout the observation period. Lysis and degradation of collagen fibrils were also prominent in this region, and collagen-phagocytosing cells could be seen at all stages. Although activated fibroblasts were marked in this zone, regeneration processes were, in general, less marked than those of degeneration. The significance of these findings in relation to the mechanism by which trabeculoplasty works is discussed.

Animals↗