The effect of radiation therapy on the kinetics of in vitro lymphocyte response to mitogens

Iridotomies were made in rabbit and human eyes using a continuous wave argon laser. Several of these have been studied histologically to determine the short- and long-term effects on the iris, cornea, lens and retina. Thirty-three eyes of patients with angle-closure glaucoma were treated by argon laser iridotomy and followed for periods of up to twelve months. In two eyes, planned cataract surgery was performed shortly after the laser iridotomy and histologic specimens of the iris were examined. From the experimental and clinical studies, an evaluation was made of the complications and clinical usefulness of the continuous wave argon laser as a means to produce iridotomies for the treatment of angle-closure glaucoma. Further long-term controlled studies are recommended to document the role of laser iridotomy.     

Photodegradation of furosemide solutions

PURPOSE: To evaluate the therapeutic effect of combined argon and Nd: YAG laser peripheral iridectomy. METHODS: 151 cases (200 eyes) of primary angle-closure glaucoma and combined glaucoma were treated by combined procedure. Argon laser was the first used to create partial iridectomy in 2/3-3/4 thickness without penetrating pigmented epithelium, and then Nd: YAG laser was required to complete a patent iridectomy. RESULTS: A successful iridectomy was achieved in all patients independent of any type of irides (100%). Iris penetration rate in one session presented in 95.0%. Complications appeared to be less common in our patients: iris bleeding was only seen in 5.0% cases; the postoperative inflammation was mild; corneal burns, localized lenticular opacities and closure of iridectomies were less frequent. The follow-up ranged from 3m to 6.5 years. The success rate in controlling IOP with/without medications were 99.0% except 2 eye failure. No attack occurred in 129 eyes with prophylactic laser therapy during the periods of follow-up. CONCLUSION: Combined procedure had much more advantage over either the argon or Nd: YAG laser iridectomy. It tended to be more preferable for Asian patients with heavily pigmented and thick irides.     

Types of medical lasers

The knowledge about different types of lasers and their potential use in medicine is presented. A very rapid development of laser technology in the world imposes a need for up-to-date information about the characteristics of different laser instruments. Without this kind of information it would be difficult to keep in touch with the latest developments in the world's technology. Different types of lasers have different indication range in the medical practice. An inquiry into the fundamental principles of lasers physics is an important prerequisite for successful application of this technology in medicine. Laser as a surgical knife has shown certain advantages over scalpel, electrocautery and cryosurgery, as the laser surgery is a noncontact method, bloodless, precise, with better visualization, minimal postoperative edema, painless healing, without complications. Although laser cannot entirely replace conventional surgical instruments, it is still the instrument of choice for treatment of numerous pathological conditions. The carbon dioxide laser is a highly precise, bloodless light scalpel used for incising and excising tissues and sealing small blood vessels. The infrared beam at 10,600 nm wavelength is absorbed by water and tissue destruction is due to the instantaneous vaporization at relatively low temperature of 100 degrees C. The beam seals blood vessels of up to 0.5 mm in diameter and if the beam is defocused, larger vessels may be controlled. The beam also seals lymphatics, possibly reducing the spread of tumour cells by this route, and seals nerve endings: there is no incidence of neuroma formation. Carbon dioxide laser has shown a great efficiency in otorhinolaryngology, in maxillo-facial surgery and plastic surgery, in urology and gynecology. Provides true "no touch" surgery, and is used increasingly in neurosurgery for the precise atraumatic removal of tissue and for creation of precise lesions for the control of pain. The carbon dioxide laser beam cannot, at present, be transmitted via a flexible fibre, although a number of fibres are being investigated. Delivery of laser energy to microscope, colposcope or handpiece is via an articulated arm which is a hollow tube with mirrors at the articulations. The argon laser produces blue-green coherent light at a number of wavelengts but 80% of the energy is at wavelengths of 488 and 514 nm. This laser was first used in ophtalmology to treat diabetic retinopathy through, and without damage to, the clear anterior parts of the eye. The argon laser is used for blood vessel coagulation but can be used to perform slow, thermal tissue destruction at higher power levels. Argon laser is most commonly used in ophthalmology for otological micro-surgery, particularly in the treatment of otosclerosis and tympanosclerosis. Very good results have been achieved in the argon laser treatment of gastrointestinal bleeding ulcers, vascular lesions and polyps. Dermatology is another field where argon laser has shown great efficiency: hemangyomas, telangiectasias, tattoos, small benign and malignant tumours are amenable to argon laser treatment. In neurosurgery it is used to control both normal and abnormal blood vessels but at present much work on treatment of arteriovenous malformations and aneurysms is experimental. Both the argon laser energy can be transmitted via flexible fibre optic delivery system which can then be attached to an operating microscope, slit lamp, endoscope delivery fibre or handpiece. The Neodymium-YAG laser is used both for tissue destruction with good haemostasis and for the control of normal and abnormal blood vessels. This laser produces infrared coherent light at 1060 nm wavelength, which is deeply absorbed in the tissues without colour or tissue specificity. Neodymium-YAG laser is mostly used in tracheobronchial, gastrointestinal and urologic pathology in the treatment of stenoses, granulomas, benign tumours, and for reduction of malignant tumours. (ABSTRACT TRUN     

Expression of vascular endothelial growth factor mRNA in non-small-cell lung carcinomas

PURPOSE: To determine the distribution of matrix metalloproteinase-1 (MMP-1) in the uveoscleral outflow pathway and other anterior segment tissues of normal human eyes. METHODS: Normal human eyes were fixed in methacarn and sectioned and immunostained using a specific polyclonal antibody to MMP-1. Immunoreactivity was visualized using diaminobenzidine. To compare the staining intensity in various tissues, the mean optical density within the ciliary body, mid-iris stroma, iris root, uveal trabecular meshwork, cornea, and sclera was determined using imaging densitometry. To determine the cellular distribution of MMP-1 in ciliary muscle, additional sections were double-immunostained using antibodies to MMP-1 and calponin. These sections were examined by confocal laser scanning microscopy. Specificity of the antibody to MMP-1 in ocular tissues was confirmed by western blot analysis with uveal tract homogenates. RESULTS: Moderate-to-strong MMP-1 immunoreactivity was observed in ciliary muscle, iris, sclera, corneal endothelium, and ciliary nonpigmented epithelium. Lighter immunoreactivity was observed in corneal epithelium, blood vessels, trabecular meshwork, Schlemm's canal, and associated collector channels. Confocal microscopy showed that ciliary muscle MMP-1 was primarily inside ciliary muscle cells. Densitometry showed that net optical density was approximately fivefold greater in ciliary muscle, iris root, and sclera than in trabecular meshwork. CONCLUSIONS: MMP-1 was prominently identified in regions of the anterior segment of normal human eyes associated with the uveoscleral outflow pathway and in the iris, corneal endothelium, and ciliary nonpigmented epithelium. These data support the hypothesis that MMP-1 activity is involved in regulating uveoscleral outflow facility.     

Identification and characterization of cells infiltrating the graft and aqueous humour in rat corneal allograft rejection

In a rat model of corneal transplantation, Fischer 344 (RT1(lv1)) rats received orthotopic corneal isografts or Wistar-Furth (RT1(u)) donor allografts. Rejection was observed in 25 of 26 allograft recipients, at a median time of 18 days, with all isografts surviving > 100 days. Flow cytometric analysis of aqueous humour identified cellular infiltration of the aqueous at the time of allograft rejection, in contrast to the acellular aqueous found in isografts at corresponding times following transplantation. A higher proportion of CD8+ than CD4+ cells was found at days 1-3 following rejection, whereas there was a higher proportion of CD4+ cells at days 5-8. No changes in peripheral blood T cell subsets were found at the time of rejection. Immunohistochemical analysis of cells infiltrating recipient iris and grafted cornea undertaken at days 1-2, 4 and 7-10 following onset of rejection, demonstrated inflammatory cells in the graft epithelium, stroma and aggregated on the endothelium. Large numbers of macrophages, T cells (CD4+ > CD8+ at all time points), natural killer (NK) cells and neutrophils were detected in graft tissue at days 1-2 and 4, diminishing after that time. Most infiltrating cells expressed MHC class II antigen, and a smaller number expressed IL-2R. Expression of the co-stimulatory marker B7 was identified in a few cells at day 4 in the region of the graft-host wound. The immune response in graft rejection was characterized at day 4 also by expression of intercellular adhesion molecule-1 (ICAM-1) on endothelial cells of iris and corneal vessels, demonstration of interferon-gamma on mononuclear cells in the peripheral (recipient) cornea, and tumour necrosis factor-alpha on aggregated mononuclear cells on the graft, but not recipient, endothelium. Only sparse cellular infiltrates were found in isograft controls, with inflammation located at the graft-host wound. These findings suggest that inflammatory cells reach a corneal allograft by two routes--from vessels in the peripheral recipient cornea, and from vessels in the recipient iris via the aqueous humour. Different aqueous and intragraft T cell subset proportions were seen early in rejection, although a preponderance of CD4+ cells was found in both aqueous and graft at later times.     

Host-graft circulation and vascular morphology in pancreatic tissue transplants in rats

One of the most determining factors for the survival of tissue grafts is an intact vasculature. This vasculature must, however, be linked to the circulation of the host animals for survival and growth to occur. The mechanism(s) of revascularization of pancreatic tissue grafts is still unclear and more so the process by which the host blood vessels anastomose with those of the graft. The microvasculature and revascularization of neonatal pancreatic tissue fragments transplanted into the anterior eye chamber of rats were investigated using conventional light and electron microscopy as well as the india ink perfusion method. Light microscopy demonstrated that the blood vessels of the host iris revascularized the transplants within 24 h of transplantation. Sinusoidal blood capillaries were observed to invade the peripheral parts of the grafts. The capillary encroachment from the iris into the graft continued through the second day of transplantation. The host-graft anastomosis developed completely and became prominent and conspicuous after the third day of transplantation. Many capillaries were observed to encroach into the graft from the iris. The vasculature of the graft was increased in comparison with that of normal pancreatic tissue. This increase was prominent especially around pancreatic islet and ductal cells which survived after transplantation. Blood cells observed in these vessels of the graft indicated a functional state. The ultrastructure of the intrinsic blood vessels of the graft was intact and showed the peculiar fenestrations normally seen in the blood vessels of endocrine tissues. These observations indicated that pancreatic tissue fragments transplanted into the anterior eye chamber of rats were revascularized within 24 h. The revascularization was completed at the end of the third day of transplantation when definitive blood vessels linked the circulation of the graft with that of the iris.     

Fuchs' heterochromic cyclitis. Clinico-histopathologic findings of nodular iritis

BACKGROUND: Despite the fact that in 1906 Fuchs described the first case of Fuchs' heterochromic cyclitis histopathologic reports of this disease are still rare. PATIENT AND METHODS: A clinicopathologic correlation of findings in Fuchs' heterochromic cyclitis is presented. In a patient with a history of Fuchs' heterochromic cyclitis for 15 years a secondary open-angle glaucoma developed. Several operations were performed including intracapsular cataract extraction, goniotrephanation (Elliot) and repeated cyclocryotherapy. The eye finally had to be enucleated because of a painful absolute glaucoma. RESULTS: Clinical hallmarks of Fuchs' heterochromic cyclitis in this patient include cataract formation and secondary open-angle glaucoma. Histology revealed accumulations of mononuclear cells on the surface of the iris and the corneal endothelium, as well as sparse inflammatory cells within the anterior chamber. The trabecular meshwork showed an infiltration of mononuclear inflammatory cells, chiefly lymphocytes and plasma cells. DISCUSSION: Accumulations of mononuclear cells on the surface of the iris, which histopathologically have not been described before, could represent the clinicopathologic correlate of Koeppe and Busacca nodules. Histologically, the cell deposits on the iris were similar to those in the anterior chamber and to larger corneal precipitates. It is supposed that the cells, which originally emigrated from the iris vessels, may form primary cell deposits on the iris surface or endothelium. Such iris precipitates may also be secondarily attracted by the endothelium in whole. An autoimmune-reaction against the corneal endothelium may be the underlying reason.     

Microvascular injection studies in rubeosis iridis and neovascular glaucoma

Microvascular silicone injection, tissue clearing, and histologic examination were used to demonstrate the origin, distribution, and interconnections of newly formed iris and chamber angle blood vessels in four eyes with rubeosis iridis and neovascular glaucoma associated with diabetic retinopathy and central retinal vessel occlusion. The newly formed iris vessels that formed either a tight, evenly distributed (diabetic) or loose, irregularly distributed (central vessel occlusion) network in the iris originated from the normal iris arteries that were branches of either the major arterial circle or of the perforating branches of the anterior ciliary arteries, and drained into the normal iris and ciliary body veins and occasionally into the paralimbal episcleral veins. These newly formed iris vessels appeared to shunt intravascular fluid from arteries to veins. The newly formed anterior chamber angle vessels that formed tufts and arcades at the trabecular meshwork also originated from the roots of the iris arteries and the ciliary body arteries and connected with the peripheral neovascular iris network. In addition, the circumferentially running angle vessels that coursed within the trabecular meshwork branched into and coursed within a fibrosed Schlemm's canal and into two of its intrascleral collector channels. No open communication between these newly formed vessels and the Schlemm's canal-aqueous outflow system was seen.     

Early changes in iris blood vessels in exfoliation syndrome

PURPOSE: To look for possible changes in iris blood vessels that might precede the formation of true exfoliation material, and to correlate these changes, if present, to aging of the iris. METHODS: Iris blood vessels of 10 clinically normal irises from 10 eyes (ages: 1 day-80 years), 4 irises with exfoliation (ages: 64-80 years) and 3 irises from primary open-angle glaucoma (POAG) (ages: 27-65 years) were examined with an electron microscope. RESULTS: Pronounced multilayering of the basal lamina around the vessels, abnormal excessive formation of microfibrils, presence of atrophic invaginations in the outer cell membranes of endothelial cells containing interlacing basal lamina, and an apparent decrease in the perivascular collagen fibers were evident in exfoliative iris vessels devoid of true exfoliation material. Similar, but much milder, changes were evident in normal aged and POAG irises. CONCLUSIONS: Abnormal elastic tissue and/or basal lamina production plays a role early in the formation of exfoliation material around iris blood vessels.     

Ablation dynamics in laser sclerostomy Ab externo by means of pulsed lasers in the mid-infrared spectral range

BACKGROUND AND OBJECTIVE: Sclerostomy ab externo with pulsed laser systems is currently in phase II clinical trials. The authors investigated the ablation dynamics of tissue treated with pulsed laser systems in the mid-infrared range to estimate the extent of thermo-mechanical damage to the sclera and the anterior chamber. MATERIALS AND METHODS: Freshly harvested porcine eyes were used. A bare 400-micron fiber in direct contact with tissue was used for fistulization. Polarization light microscopy, fast-flash photography, as well as optical and acoustic transients were performed for analysis. RESULTS: Substantial mechanical tissue deformation and dissections were found during pulsed laser ablation. The mechanical damage range within tissue far exceeds the pure thermal damage zone. Aspheric cavitation bubbles of up to 3 mm in length penetrate the anterior chamber after perforation. The cavitation demonstrates a significantly larger time constant in tissue than in water. CONCLUSIONS: Early fistula occlusions due to iris adherences may be attributed to iris trauma caused by cavitation. In response to the findings of this study, the authors propose an automatic feedback system to control the ablation process and minimize secondary ocular tissue effects. With respect to the overall damage zones, a new continuous-wave, mid-infrared diode laser system seems to be superior to pulsed laser systems.     

A prospective multicenter trial of excimer laser phototherapeutic keratectomy for corneal vision loss. The Summit Phototherapeutic Keratectomy Study Group

PURPOSE: The 193-nm argon fluoride excimer laser can remove corneal scars and smooth corneal irregularities, obviating corneal transplantation. We conducted a prospective multicenter trial of excimer laser phototherapeutic keratectomy for corneal vision loss as a basis for Food and Drug Administration premarket approval. METHODS: We treated 232 eyes of 211 patients with corneal vision loss. All had corneal pathology in the anterior 100 microns of the stroma. Mean postoperative follow-up was 10 +/- 8 months. The primary outcome variable was change in best spectacle-corrected visual acuity. RESULTS: At postoperative month 12, best spectacle-corrected visual acuity improved in 46 (45%) of 103 eyes and worsened in nine (9%) of 103 eyes by 2 or more Snellen lines. Best spectacle-corrected visual acuity improved by a mean of 1.6 +/- 2.8 Snellen lines (95% confidence interval, 1.1 to 2.1 lines). Every postoperative visit confirmed statistically significant improvement of mean best spectacle-corrected acuity. At month 12, treated eyes had a mean hyperopic shift in refraction of 0.87 diopter and a mean reduction in astigmatism of 0.36 diopter. Treatment appeared most effective in eyes with hereditary corneal dystrophies, Salzmann's nodular degeneration, and corneal scars, and least effective in eyes with calcific band keratopathy. Complications included recurrence of underlying pathology, corneal graft rejection, and bacterial keratitis. CONCLUSIONS: Argon fluoride excimer laser phototherapeutic keratectomy is effective, with relatively few complications, for treating vision loss from corneal opacification or irregularity. Efficacy, however, varies widely depending upon individual eyes and underlying diagnoses.     

Corneal sensitivity in people with the same and with different iris color

Corneal touch thresholds were determined for both eyes of 15 people with different colored irides (a condition known as heterochromia) and of 25 people with similar colored irides. It was found that corneal sensitivity is not significantly different in both eyes of either group. Therefore it is suggested that the fact previously demonstrated that corneal sensitivity is related to iris color is not of corneal origin.     

Postoperative complications of subconjunctival THC-YAG (Holmium) laser sclerostomy

BACKGROUND: The THC-YAG (Holmium) laser is currently being used to create thermal sclerostomies, an alternate method of intraocular pressure control with poorly regulated glaucoma. A significant advantage of this procedure is the ab externo approach, which utilizes a small fiberoptic laser probe that is advanced subconjunctivally to the limbus. Surgical intraocular trauma is minimized, decreasing the amount of scarring and ultimate bleb failure. Complications may be seen with the THC-YAG sclerostomy and include: conjunctival burn, filtration bleb leak, posterior corneal striae, peaked pupil, iris incarceration, infection, hyphema, cataracts and ocular hypotony. METHODS: This paper presents four glaucoma patients, co-managed at the Illinois Eye Institute, who developed complications following THC-YAG laser sclerostomy. A discussion of other possible postoperative complications and their management is included. RESULTS: While the holmium laser has shown positive results in lowering intraocular pressure in patients with uncontrolled refractory glaucoma, complications are inherent in this procedure. The most common are posterior corneal striae, iris incarceration with secondary peaked pupil, and ocular hypotony. Conjunctival burn with secondary filtration bleb leak, limbal pigment deposition and hyphema may also be seen. CONCLUSION: Early diagnosis and treatment of postoperative complications may help in maintaining the long term filtration ability of the sclerostomy.     

Immediate argon laser peripheral iridoplasty as treatment for acute attack of primary angle-closure glaucoma: a preliminary study

OBJECTIVE: This study aimed to examine the intraocular pressure (IOP)-lowering effects and safety of immediate argon laser peripheral iridoplasty (ALPI) as a first-line treatment for acute primary angle-closure glaucoma (PACG). DESIGN: A prospective cohort study. PARTICIPANTS: Ten consecutive patients with their first attack of PACG, with IOP of 40 mmHg or greater, were recruited into the study. INTERVENTION: On presentation, each patient received topical pilocarpine (4%) and timolol (0.5%) and immediate ALPI as primary treatment. The IOPs at 15, 30, and 60 minutes after ALPI were documented by applanation tonometry. When the corneal edema had settled, laser peripheral iridotomy was performed as a definitive treatment. MAIN OUTCOME MEASURES: The IOP, corneal edema, and complications from ALPI were measured. RESULTS: The mean IOP of this group of patients was reduced from 59.5+/-10.4 mmHg to 28.7+/-14.9 mmHg at 15 minutes, 21.7+/-13.1 mmHg at 30 minutes, and 16.0+/-9.4 mmHg at 60 minutes after ALPI. No complications from the laser procedure were encountered during the study period. In nine of the ten patients, the corneal edema cleared up 1 hour after ALPI. In the remaining patient, the cornea cleared up 2 hours after ALPI. CONCLUSION: From this preliminary study, immediate ALPI, without adjunctive systemic antiglaucoma treatment, appeared to be very effective in controlling the IOP and returning corneal clarity in acute PACG. Its safety also appeared reassuring and did not have the risks associated with conventional systemic therapies.     

Laser welding of fascial grafts and its potential application in tympanoplasty: an animal model

This study evaluates the welding effect of argon laser on fascial grafts and its potential application in tympanoplasty. An animal model was designed so that surgically induced perforations of the lumbar fascia of the animals were grafted in an underlay fashion. Each graft was "spot welded" around the circumference of the perforation using a hand-held argon laser. Various lasing parameters were used on different grafts. Control perforations were grafted but not welded. A total of 96 perforations was studied. Weld sites were evaluated both histologically and by tensile strength measurement at the end of surgery, and at 7, 14, and 21 days postoperatively. Comparison with controls showed a higher tensile strength for welded grafts, which was most significant in the early postoperative period (p < 0.001). Histologically, it seems that the basic mechanism of fascial welding involves liquification and denaturation of extravasated proteins at the weld sites. Correlation between the extent of thermal damage on the tissues and the various lasing parameters is described. Based on the well-documented safety of argon laser in otologic surgery, and the successful welding of fascial grafts in this study, argon laser may prove beneficial in welding temporalis fascia grafts in tympanoplasty.     

Breast cancer in older women

Successful laser treatment of haemangiomas requires selective photocoagulation of subsurface targeted blood vessels without thermal damage to the overlying epidermis. We present an in vivo experimental procedure, using a chicken comb animal model, and an infrared feedback system to deliver repetitive cryogen spurts (of the order of milliseconds) during continuous Nd:YAG laser irradiation. Gross and histologic observations show deep-tissue photocoagulation is achieved, while superficial structures are protected from thermal injury due to cryogen spray cooling. Experimental observation of epidermis protection in chicken comb animal models suggests selective photocoagulation of subsurface targeted blood vessels for successful treatment of haemangiomas can be achieved by repetitive applications of a cryogen spurt during continuous Nd:YAG laser irradiation.     

Binding of the N-terminal 63 kDa portion of connectin/titin to alpha-actinin as revealed by the yeast two-hybrid system

BACKGROUND: Corneal blood staining may occur as a serious complication of persisting hyphema. In our two cases the corneal blood staining is a result of direct bleeding in the corneal stroma. HISTORY AND SIGNS: A female patient presented with a corneal blood staining through rupture of reopened vessels in interstitial keratitis of congenital syphilis after physical effort with high blood pressure. The second patient presented with a blood staining caused by a vessel errosion after corneal ulceration. THERAPY AND OUTCOME: The clearing of the blood staining is thought to be a result of the phagocytic action of the keratocytes and from a diffusion of hemoglobin into the conjunctival circulation and the anterior chamber (2). The therapeutic efforts are directed toward prevention of corneal blood staining. At the first sign of microscopic blood staining of the cornea a surgical evacuation of hyphema is necessary. In our two cases there wasn't a hyphema, so it was only possible to treated the corneal ulcer, entropion and systemic hypertension. CONCLUSION: The clinician has to wait for a spontaneous clearing, although it may take 2 or 3 years or more. In this case a penetrating keratoplasty is indicated.     

Wound healing following intrastromal photorefractive keratectomy with the Nd:YLF picosecond laser in the cat

BACKGROUND: We studied the histopathology of the stromal wound healing response in the cat cornea following intrastromal photorefractive keratectomy (IPRK) with the Nd:YLF picosecond laser. METHODS: Intrastromal PRK was performed in the anterior stroma of cat corneas with the Nd:YLF picosecond laser. The cats were sacrificed at predetermined intervals ranging from immediately to 6 months postoperatively. Effects of the laser treatment on the epithelium, Bowman's layer, stroma, and the endothelium were evaluated using light and scanning electron microscopy. No anti-inflammatory agents were used. RESULTS: Intrastromal PRK resulted in no perceptible damage to the corneal epithelium or Bowman's layer either acutely or at 6 months. The corneal stroma showed multiple cavitations immediately after intrastromal PRK, which collapsed over several hours, followed by thinning of the cornea over 2 weeks. At 1 month, the stromal collagen was abnormal with surrounding hypercellularity. The endothelium showed no injury, acutely or at 6 months. No thermal effects on stromal collagen were observed at 6 months, and disruption of the lamellar pattern was not apparent after the cavitation bubbles were reabsorbed. CONCLUSION: Intrastromal PRK can effectively remove stromal tissue without acute damage to the adjacent lamellae, epithelium, or endothelium. There is a transient cellular wound healing response associated with a transient stromal collagen abnormality at 2 weeks to 1 month, which was not apparent 2 months after the procedure.     

Diode laser thermokeratoplasty: application strategy and dosimetry

PURPOSE: To investigate suitable application parameters for efficient hyperopic correction by laser thermokeratoplasty (LTK) using mid-infrared laser diodes. SETTING: Medical Laser Center Lübeck, Lübeck, Germany. METHOD: A tunable continuous-wave laser diode in the spectral range between 1.845 and 1.871 microns was used. Transmitted by waveguides, the laser energy was used to induce coagulations on freshly enucleated porcine eyes to increase corneal curvature. The coagulations were equidistantly applied by a fiber-cornea contact and a noncontact focusing device that were adjusted on a ring concentric to the corneal apex. Different laser parameters and application geometries were evaluated. Refractive changes were measured by computer-assisted corneal topography before and after treatment. Polarization light microscopy and temperature calculations were used to analyze the coagulations. RESULTS: Because of the tunability of the laser diode, the influence of the corneal absorption coefficient (between 0.9 and 1.6 mm-1) on the refractive change could be measured. A laser power between 125 and 200 mW was adequate to achieve refractive changes up to 10.0 diopters. In the preferable focusing device, the refractive change increased almost logarithmically with the irradiation time up to 15 seconds. The number of coagulations on a fixed application ring showed no significant influence on refractive change; however, it showed an almost linear decrease with increasing ring diameter from 5.0 to 10.0 mm. Histological analysis revealed 3 stages of thermal damage. CONCLUSION: Diode LTK provided defined and uniform coagulations when using a well-adapted focusing device, resulting in sufficient refractive change. The results indicate that diode LTK is superior to pulsed holmium LTK.