Laser Thermokeratoplasty Research Articles

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 Lubeck, Lubeck, Germany.Method: A tunable continuous-wave laser diode in the spectral range between 1.845 and 1.871 pm 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.

Imaging of laser thermokeratoplasty lesions by optical low coherence tomography and polarization microscopy after Sirius Red staining

Information on the extent and degree of the thermal effect produced is of great importance for control of the laser dosage in laser thermokeratoplasty (LTK) and for postoperative follow-up. We investigated on acute LTK effects which information images obtained by optical low coherence tomography (OCT) offer compared to those obtained by polarization microscopy. Porcine eyes were irradiated through a 400 microns quartz fiber using light from a laser diode emitting up to 300 mW at a wavelength of 1.86 microns. Thermal lesions of varying strength were scanned using an experimental OCT device with about 25 microns lateral and 20 microns axial resolution. Histologic evaluation of the scanned areas was done by polarization microscopy after Sirlus-Red staining, and similar lesions were also analyzed by TEM. Both methods differentiated three damage zones a transition zone, a zone of moderate coagulation, and a central zone of strong coagulation. In the transition zone, increased birefringence was seen in polarization microscopy, which correlated with increased light scattering seen in the OCT images. In the moderately coagulated zone, a decrease in birefringence was associated with an even stronger increase of the OCT signal. In the central zone, a loss of the fibrillar tissue structure was observed, which led to a complete loss of birefringence and a strong reduction of the OCT signal. Although OCT does not provide the detailed information on thermal changes of tissue seen by the histologic method, it offers information on the extent and degree of tissue changes without preparation artifacts and provides a non-invasive method of immediate and follow-up control of LTK lesions. A quantitative analysis of changes in corneal thickness and curvature is much simpler than by a slit lamp. Time-resolved measurements of corneal light scattering may be used for on-line control of the laser-light dosage during LTK.

Optical coherence tomography of the cornea and the anterior eye segment

The method of optical coherence tomography (OCT) was investigated regarding its suitability and limits for measuring the cornea and the anterior segment of the eye. Furthermore, the stromal expansion of thermally induced lesions in the cornea directly after irradiation was determined within the scope of the laser thermokeratoplasty (LTK). With the experimental scanning OCT system, x-z sections of the anterior eye segment were made with an optical resolution of about 20 microns axially and 25 microns laterally. Freshly enucleated, tonicized porcine eyes were used as model eyes. Thermal lesions were applied with a continuously emitting laser diode (lambda = 1.86 microns) and various radiation parameters. Before and after coagulation, the cornea was viewed from limbus to limbus in a central OCT scan and the individual coagulation source was measured. Global and local changes of the thickness of the cornea as well as the distance between cornea and lens were measured with high precision. Thermal lesions in their expansion can be dearly presented and matching well with the histologically stained sections, but are not as exactly defined at the edges due to the limited optical resolution, as known from histological preparations. With the OCT method quantitative measuring of the anterior eye segment can be performed in vitro and with reduced resolutions also in vivo. Due to the qualitatively good correspondence regarding the dimensions of thermal damage of the cornea with histologically obtained morphometric results, this method can be used for supervision of coagulation directly after LTK as well as for examination of the individual healing process.

Holmium laser thermokeratoplasty for the reversal of hyperopia after myopic photorefractive keratectomy

BACKGROUNDOvercorrection following myopic photorefractive keratectomy, with a target of emmetropia, leaving a spherical equivalent of more than 1.0 D of hyperopia is of the order of 1%. This study analyses...

Histologic analysis of thermal effects of laser thermokeratoplasty and corneal ablation using Sirius-red polarization microscopy

Purpose: To evaluate how well several histologic techniques differentiate degrees of thermally induced changes in corneal tissue after laser thermokeratoplasty (LTK) or corneal ablation.Setting: Medical Laser Center Lübeck, Germany.Methods: Corneas of freshly enucleated porcine eyes were treated with a continuous wave laser diode (1.86 μm) and a pulsed chromium-thulium-holmium:YAG laser (2.1 μm) to produce LTK lesions or ablated with a Q-switched and a free-running chromium-erbium:YSGG laser (2.70 μm), a free-running erbium:YAG laser (2.94 μm), and an argon-fluoride excimer laser (193 nm). The lesions were evaluated by light microscopy (LM) (hematoxylin and eosin, Azan, van Gieson's, and Masson-Goldner's trichrome stains), transmission electron microscopy (TEM), and polarization microscopy after Sirius-red staining. Sirius-red, a strongly elongated, birefringent molecule binding parallel to collagen molecules, was used to enhance corneal birefringence.Results: With routine LM, it was difficult to discriminate the degrees of thermal alterations in LTK lesions. Combined Sirius-red staining and polarization microscopy distinguished between a strongly coagulated central zone and the transition zone to normal tissue. Sirius-red uptake was increased in both zones, reflecting the availability of new binding sites. The central zone appeared darker under polarization than normal collagen because of a loss of birefringence. Intrinsic birefringence was greatly reduced; however, form birefringence partly remained as long as some collagen fibrils were intact. In the center of very strong lesions, where the collagen was hyalinized, birefringence was completely lost because of the complete disintegration of the fibrillar structure, which was visible under TEM. The transition zone toward normal cornea showed increased birefringence because the natural birefringence was largely preserved and enhanced by the increased Sirius-red uptake. Mechanical stretching between neighboring LTK lesions was manifested by increased birefringence.Conclusion: Sirius red offered an improved and simple histologic method for analyzing thermal collagen changes. It may contribute to a better understanding of the working mechanisms of LTK and improve analysis of thermal effects in corneal ablation.

The role of patient's age in regression of holmium:YAG thermokeratoplasty-induced correction of hyperopia.

Hyperopic correction can be induced with holmium:YAG laser thermokeratoplasty, which changes corneal curvature by shrinking stromal collagen. The thermal effectiveness of pulsed Ho:YAG laser depends on the water content of the corneal stroma, which should be related to age. This study investigated the presumed effect of age on the stability of the results of Ho:YAG laser thermokeratoplasty. Ho:YAG laser thermokeratoplasty was done in 63 eyes of 45 patients for low to moderate hyperopic correction (up to 5 diopters). Patients were treated with one or two rings of 8 spots each, with an optical zone of 6 or 7 mm diameters respectively. Regression of the therapeutic effect was evaluated on the basis of the difference between the attempted to achieved correction ratio on the first postoperative day and at the last visit. The regression ratio was 18% in the patients older than 20 years. Patients younger than 20 years showed a ratio of 48%. Achieved correction was +2.40 D in older patients and +2.05 in younger patients. These values are respectively 43% and 35% of the attempted corrections. It appears that age-dependent corneal factors do influence the effectiveness of thermal energy on stromal collagen.

Holmium:YAG laser thermokeratoplasty for astigmatism in rabbits.

Holmium:YAG laser thermokeratoplasty has generated considerable interest as a technique for correcting hyperopia. In this study, the effect of holmium:YAG laser on inducing astigmatism according to application patterns was evaluated. An experimental study based on the results of astigmatic holmium:YAG laser thermokeratoplasty using the Summit OmniMed laser system (Summit Technology Inc, Waltham, Mass) in 36 rabbit eyes is presented. We divided the rabbits into four groups: arcuate, reverse arcuate, linear, and control group according to application patterns. All rabbits were followed for 3 months and cycloplegic refractive measurements were carried out. The average surgically induced astigmatism was 1.86 diopters (D) for the arcuate group, 2.93 D for the reverse arcuate group, and 1.31 D for the linear group. No significant complications related to the operation were noted. The reverse arcuate pattern of holmium:YAG laser thermokeratoplasty is most effective in inducing astigmatism in rabbits.

Holmium:YAG Laser Thermokeratoplasty for Correction of Astigmatism

Holmium:YAG Laser Thermokeratoplasty for Correction of Astigmatism

Holmium: YAG Laser Thermokeratoplasty for Hyperopia

Holmium:YAG laser thermokeratoplasty (LTK), a procedure using a solid-state infrared laser to treat hyperopia, was performed on 10 patients in phase I and 16 patients in phase II--in a total of 29 eyes at the Hunkeler Eye Clinic. Phase II was redesigned after phase I results showed undercorrection and regression. The follow-up period ranged from 1 to 24 months (mean 10.9 months). A total of 79% of phase II patients were within +/- 1.00 D of intended correction at the 6-month visit. Looking at both phases together, no patients had J2 or better near vision preoperatively, but 75% had J2 or better at the 6-month visit. A total of 43% of eyes in phase II lost 1 line and 7% lost two lines of best spectacle corrected visual acuity due to induction of irregular astigmatism. The surgical challenges are to insure appropriate centration of the procedure about the optical axis. Concerns about regression and stability will be defined as these patients are followed through their 2-year visits.

Holmium Laser Thermokeratoplasty

Corneal curvature can be altered by shrinking stromal collagen with a pulsed solid-state holmium: YAG laser in a procedure termed laser thermokeratoplasty. The authors performed laser thermokeratoplasty in 40 human cadaver eyes using a ring pattern of 32 spots, each spot having a diameter of 300 microns. The amount of induced corneal steepening decreased as ring diameter was increased in 1 mm increments, with 22.2 +/- 3.3 and 3.7 +/- 2.0 diopters (D) of central steepening with diameters of 3 and 7 mm, respectively. Results of histologic examination showed a cone-shaped zone of increased stromal hematoxylin uptake extending posteriorly for 90% of stromal thickness. Energy levels greater than those needed to induce topographic changes produced limited endothelial injury in rabbit corneas and, in some cases, intraocular inflammation. A computerized, finite element model of the globe demonstrated central corneal steepening as a result of heat-induced stromal contraction to a depth of 75% corneal thickness. These data support previous studies indicating that central corneal topography can be modified by heating corneal stroma in a controlled fashion with the mid-infrared holmium:YAG laser.

Recent developments in refractive corneal surgery

The 4- and 5-year results of the Prospective Evaluation of Radial Keratotomy Study reveal a hyperopic shift of more than 1.0 D within the follow-up time in 22% and 24%, respectively, of the eyes that underwent radial keratotomy. The number of eyes with this complication increased progressively from 6 months to 4 years. Attempts to correct this progressive hyperopia are discussed. Results of clinical studies of myopic photorefractive keratectomy performed with the excimer laser are reviewed. Regarding safety and short-term efficacy, photorefractive keratectomy is a promising technique. In the past, corneal topography has been interpreted qualitatively. Ray-tracing analysis now allows quantitative estimation of the influence of aspheric corneas on the degradation of the retinal image. Three approaches to correct hyperopia, radial thermokeratoplasty, laser thermokeratoplasty, and hexagonal keratotomy are reviewed and discussed. Additional concave intraocular lenses for correction of high myopia are still undergoing clinical evaluation. Endothelial damage is the most dangerous short-term complication of implantation with these lenses.

Laser Thermokeratoplasty by Means of a Pulsed Holmium:YAG Laser for Hyperopic Correction

A new technique of hyperopic correction similar to radial thermokeratoplasty is presented. A pulsed Holmium:YAG laser was used, emitting a wavelength of 2.06 microns. The laser light was guided by a quartz fiber and focused by means of a handpiece. Eight of 16 coagulations located on rings concentric to the pupil resulted in central corneal steepening. The refractive change increased with the applied pulse energy above a threshold of about 10 mJ per pulse and was constant between 15 and 35 mJ per pulse. The effect decreased linearly with greater distance from the center. Four blind human eyes demonstrated that the parameters evaluated in the human cadaver eyes can be transferred to the living eye. Immediately after surgery, folds in Descemet's membrane parallel to the limbus appeared. With time, they gradually diminished but were still persistent after 4 months. Hyperopic changes of up to 5.00 diopters were obtained, remaining stable for 4 months.