Diode laser thermokeratoplasty: application strategy and dosimetry

Assessment of Laser Parameters to Improve Lid Tension-A Proof of Concept towards Lasercanthoplasty

BACKGROUND

Preliminary clinical work indicates that increasing eyelid tension improves the function of the meibomian glands. The aim of this study was to optimize laser parameters for a minimally invasive laser treatment to increase eyelid tension by coagulation of the lateral tarsal plate and canthus.

METHODS

Experiments were performed on a total of 24 porcine lower lids post mortem, with six lids in each group. Three groups were irradiated with an infrared B radiation laser. Laser-induced lower eyelid shortening was measured and the increase in eyelid tension was assessed with a force sensor. A histology was performed to evaluate coagulation size and laser-induced tissue damage.

RESULTS

In all three groups, a significant shortening of the eyelids after irradiation was noticed (p < 0.0001). The strongest effect was seen with 1940 nm/1 W/5 s, showing -15.1 ± 3.7% and -2.5 ± 0.6 mm lid shortening. The largest significant increase in eyelid tension was seen after placing the third coagulation.

CONCLUSION

Laser coagulation leads to lower eyelid shortening and an increase in lower eyelid tension. The strongest effect with the least tissue damage was shown for laser parameters of 1470 nm/2.5 W/2 s. In vivo studies of this effect have to confirm the efficacy of this concept prior to clinical application.

Surgical Correction of Hyperopia

Surgical attempts to correct hyperopia have yielded varying results over the last 130 years. These techniques include the reshaping of the cornea through incisions, burns, or lamellar cuts with removal of peripheral tissue; the addition of central inlays; laser ablations; and the replacement of the crystalline lens. By examining the success of each surgical technique, the refractive surgeon may be able to make an informed decision on its indications and limitations, based on the specific patient's characteristics. Reporting the outcomes and complications of hyperopic surgery will help refine our approach to the management of an increasingly hyperopic and presbyopic population.

Non-contact Holmium:YAG Laser Thermal Keratoplasty for Hyperopia: Two-year Follow-up

PURPOSE

We evaluated the safety and efficacy of the non-contact holmium:YAG laser thermal keratoplasty (LTK) for the treatment of mild to moderate hyperopia without astigmatism.

METHODS

A prospective, non-comparative case series included 50 eyes of 28 patients (aged > or =40 years) who had stable refraction and an astigmatic component < +0.50 diopters (D). We applied the non-contact pulsed holmium:YAG laser to treat the hyperopic spherical component using the Hyperion LTK System. All patients had minimum 12-month follow-up and 64% (18 patients) had 24-month follow-up.

RESULTS

The mean age of patients was 48.4 +/- 8.23 years (range: 40 to 62 years). The preoperative hyperopic mean spherical equivalent refraction was +2.32 +/- 0.975 D (range: +1.00 to +4.75 D). Postoperatively, the subjective manifest refraction decreased from the preoperative mean value of +2.32 D to a mean -0.09 D at 1 month after surgery and regressed to +0.315 D at the last follow-up examination, resulting in a mean correction of +2.005 +/- 0.81 D at 24 months after surgery. Preoperatively, mean uncorrected visual acuity in LogMAR units was 0.798 +/- 0.353 and at 12 months after surgery, it was a mean 0.108 +/- 0.136. Keratometric power increased from 42.595 +/- 1.949 D before surgery to 44.605 +/- 1.626 D at 24 months after surgery.

CONCLUSIONS

Holmium:YAG LTK was an acceptable alternative for the correction of mild to moderate hyperopia in this middle-aged population.

Semianalytical thermal model for subablative laser heating of homogeneous nonperfused biological tissue: application to laser thermokeratoplasty

We present a semianalytical technique to calculate the temperature in homogeneous nonperfused tissue during subablative laser heating. Analytical expressions of the temperature distribution in time and space are provided for collimated beams with Gaussian and top-hat intensity distributions perpendicularly incident on a finite tissue slab. The temperature distribution produced with a collimated Gaussian beam is the triple sum of the product of four functions of separate variables. The semianalytical technique can be used to rapidly calculate the temperature in laser-irradiated tissue at any point in time and space. The model was used to estimate the corneal temperature during pulsed holmium:YAG laser thermokeratoplasty with various boundary conditions at the anterior and posterior corneal surface. The model demonstrates that the corneal temperature during laser thermokeratoplasty (LTK) with a pulsed Ho:YAG laser may be sufficient to induce superficial vaporization of epithelial cells and local thermal damage to the endothelium. The calculations show that convection at the anterior corneal surface does not have a significant effect on the corneal temperature distribution, but that a better knowledge of the cooling effect of the aqueous is required to better estimate the corneal temperature distribution during LTK.

Diode-pumped fiber lasers: a new clinical tool?

BACKGROUND AND OBJECTIVE

Diode-pumped fiber lasers are a compact and an efficient source of high power laser radiation. These laser systems have found wide recognition in the area of lasers as a result of these very practical characteristics and are now becoming important tools for a large number of applications. In this review, we outline the basic physics of fiber lasers and illustrate how a number of clinical procedures would benefit from their employment.

STUDY DESIGN/MATERIALS AND METHODS

The pump mechanisms, the relevant pump and laser transitions between the energy levels, and the main properties of the output from fiber lasers will be briefly reviewed. The main types of high power fiber lasers that have been demonstrated will be examined along with some recent medical applications that have used these lasers. We will also provide a general review of some important medical specialties, highlighting why these fields would gain from the introduction of the fiber laser.

RESULTS/CONCLUSION

It is established that while the fiber laser is still a new form of laser device and hence not commercially available in a wide sense, a number of important medical procedures will benefit from its general introduction into medicine. With the number of medical and surgical applications requiring high power laser radiation steadily increasing, the demand for more efficient and compact laser systems providing this capacity will grow commensurately. The high power fiber laser is one system that looks like a promising modality to meet this need.

Photorefractive Keratectomy for Hyperopia in 800 Eyes With the Meditec MEL 60 Laser

PURPOSE

To evaluate the refractive results of 800 hyperopic eyes undergoing PRK treatment.

METHODS

Eight hundred hyperopic eyes were treated with PRK. An Aesculap-Meditec MEL 60 scanning ArF excimer laser used. Treatment Group 1 consisted of eyes with a preoperative refractive error of +3.50 D or less (n = 482) and Group 2, of +3.75 D or more (n = 318).

RESULTS

Preoperatively, Group 1 required an average correction of +2.88+/-1.34 D and Group 2 required +5.64+/-2.96 D. One year after PRK, average residual correction was +1.26+/-1.24 D in Group 1, and in Group 2, +2.46+/-1.84 D. In Group 1, uncorrected visual acuity (UCVA) was 20/40 or better in 88.4% (426/482); 20/20 or better in 75.7% (365); 2.1% (10/482) of eyes lost 2 lines, 2.1% (10/482) gained 2 lines; 3.1% (15/482) gained 2 or more lines of BSCVA; 74.4% (359/482) of eyes were within +/-0.50 D of target correction and 84.8% (408/482) were within +/-1.00 D. In Group 2, 47.5% (151/318) had UCVA of 20/40 or better; 34.2% (109/318) saw 20/20 or better uncorrected; 19.1% (61/318) lost 2 lines; 11.6% (37/318) lost 3 lines; none of the eyes gained 2 or more lines of BSCVA; 22.3% (71/318) were within +/-0.50 D and 46.8% (149/318) were within +/-1.00 D of target correction. Refractive stability was achieved after 6 months; a slight regression after 6 months was still observed. In Group 1, 10.5% (42/482) and in Group 2, 21.6% (69/318) complained of problems with daytime vision (glare and ghost image); during night-driving in Group 1, 17.6% (85/482) and in Group 2, 40.5% (129/318) had problems.

CONCLUSION

PRK with the Aesculap-Meditec MEL 60 scanning ArF excimer laser offered the best long-term results with +3.50 D or less preoperative refractive error. With higher corrections, regression, decrease in BSCVA, and daytime visual problems were encountered.

Corneal Endothelial Cell Damage After Experimental Diode Laser Thermal Keratoplasty

PURPOSE

To evaluate the safety of diode laser thermal keratoplasty (LTK) with respect to corneal endothelial cell damage.

METHODS

In an in vitro animal model system, porcine eyes were irradiated with a continuously emitting laser diode at wavelengths (lambda) of 1.85 or 1.87 microm, corresponding to an absorption coefficient (micro(a)) of 1.1 or 2.0 mm(-1). Different irradiation and application parameters were tested serially. To determine the temperature threshold for endothelial damage, corneal buttons were analyzed separately in a waterbath experiment. The endothelial damage was assessed after trypan blue and alizarin red supravital staining under light microscopy.

RESULTS

The thresholds for the 50% probability of thermal damage (ED50) were determined at corneal temperatures of 65 degrees C for a 10-second water-bath immersion, and 59 degrees C for 60 seconds. Coagulations that reached the deeper stromal layers revealed severe endothelial cellular alterations and areas of exposed Descemet's membrane. The thermally induced changes were dependent on laser power and the absorption coefficient (wavelength). Mean diameter of total endothelial cell damage was 245 +/- 154 microm (range, 0 to 594 microm) for an absorption coefficient of 1.1 mm(-1). The maximal lateral extent of endothelial cell damage induced by the laser exposure was 594 microm in diameter. Increasing the absorption coefficient decreased the penetration depth of the laser irradiation, creating a greater temperature rise within the corneal stroma and significantly less endothelial damage (P < .01), when the same laser power was applied. The calculated total area of damage for the paracentral human corneal endothelium ranged from 1.8% to 13.6%.

CONCLUSION

Data obtained in this in vitro study were transferred to an endothelial cell damage nomogram, demonstrating that appropriate parameter improvements can minimize the adverse effects to the corneal endothelium. However, model adjustment to the human cornea indicates the potential for endothelial cell damage after diode laser thermal keratoplasty, and should be considered when performing this elective procedure.

Thermal and biomechanical parameters of porcine cornea

PURPOSE

New methods in refractive surgery require a considerable understanding of the material "cornea" and are often studied by theoretical modeling in order to gain insight into the procedure and an optimized approach to the technique. The quality of these models is highly dependent on the preciseness of its input parameters. Porcine cornea often is used as a model in preclinical studies because of its similarity to man and its availability.

METHODS

The important physical parameters for biomechanical deformation, heat conduction, and collagen denaturation kinetics have been determined for porcine cornea. Experimental methods include densitometry, calorimetry, turbidimetry, tensile tests, stress relaxation, and hydrothermal isometric tension measurements.

RESULTS

The density of porcine cornea was measured as p = 1062+/-5 kg/m3, the heat capacity gave c = 3.74+/-0.05 J/gK. The stress-strain relation for corneal strips is represented by a third order approximation where the secant modulus yields about Esec approximately equal to 0.4 MPa for small strains less than 2%. The normalized stress relaxation is described by an exponential fit over time. The denaturation process of cornea is characterized by specific temperatures which can be related to the change of the mechanical properties. Denaturation kinetics are described according to the model of Arrhenius yielding the activation energy deltaEa = 106 kJ/mol and the phase transition entropy deltaS = 39 J/(mol x K).

CONCLUSIONS

The established set of parameters characterizes the porcine cornea in a reliable way that creates a basis for corneal models. It furthermore gives direct hints of how to treat cornea in certain refractive techniques.

Influence of temperature and time on thermally induced forces in corneal collagen and the effect on laser thermokeratoplasty

PURPOSE

To investigate thermomechanical aspects of corneal collagen denaturation as a function of temperature and time and the effect of the induced forces on refractive changes with laser thermokeratoplasty (LTK).

SETTING

Medical Laser Center Lübeck, Lübeck, Germany.

METHODS

In a material-test setup, porcine corneal strips were denatured in paraffin oil at various constant temperatures for 10 and 500 seconds, and the temporal course of the contractive forces was studied under isometric conditions. Typical LTK lesions were performed in porcine eyes in vitro with a continuous-wave infrared laser diode at a wavelength of 1.87 microm for 10 and 60 seconds. The laser power was chosen to achieve comparable denatured volumes at both irradiation times. The refractive changes were measured and analyzed by histologic evaluations and temperature calculations.

RESULTS

The time course of the induced forces was characterized by a maximal force, which increased almost linearly with temperature, and a residual lower force. After 500 seconds of heating, the highest force was achieved with a temperature of 75 degrees C. With a limited heating period of only 10 seconds, the forces steadily increased with temperature over the entire observation period. Laser thermokeratoplasty produced less refractive change after 10 seconds of irradiation than after 60 seconds, although the laser power was 25% higher in the short heating period. Polarization light microscopy of LTK lesions revealed different stages of thermal damage.

CONCLUSION

The course of the contractive forces during and after heating is a complicated function of the spatial time/temperature profile. Laser thermokeratoplasty lesions produced with 2 irradiation times showed different stages of denaturation and induced refractive change.

Continuous-wave diode laser thermokeratoplasty: first clinical experience in blind human eyes

PURPOSE

To evaluate the safety and stability of laser thermokeratoplasty (LTK) with a continuous-wave diode laser in blind human eyes and to optimize parameters for a study in sighted eyes.

SETTING

Department of Ophthalmology, Medical University Lübeck, Germany.

METHODS

A continuous-wave diode laser was set to emit radiation with a wavelength of 1.854 microns (Group 1, n = 4) or 1.870 microns (Group 2, n = 4) and 100 to 150 mW power for 10 seconds. A focusing handpiece was coupled with an application mask and fixed by partial vacuum to the conjunctiva or cornea. The radiation was focused into the corneal stroma between 400 and 600 microns in Group 1 and set to 1000 microns in Group 2. Eight (Group 1, single ring) or 16 (Group 2, double ring) coagulations were applied.

RESULTS

The refractive change increased with higher laser power and smaller ring diameters. Two rings of coagulations provided higher and more stable refractive changes of up to 5.66 diopters (D) than a single ring. The refractive effect stabilized between 3 and 6 months postoperatively. At 1 year, mean refractive change was +0.99 D +/- 0.39 (SD) in Group 1 and +2.32 +/- 2.24 D in Group 2. Extensive endothelial damage occurred in Group 1 but was minimal in Group 2.

CONCLUSIONS

Diode LTK was used to treat hyperopia safely and effectively. Regression occurred mainly in the first 3 postoperative months. With a wavelength of 1.870 microns, corneal endothelial damage was limited.