Benedikt Jean

Mid-IR Laser Applications in Medicine

This chapter reviews medical applications of a variety of mid-infrared lasers. These applications are based on strong absorption of laser light in human tissue due to the presence of naturally occurring chromophores, specific and unspecific absorbers. Medically relevant laser-tissue interactions are described. Experimental data, obtained with free electron lasers describe photoablation quantitatively in the mid-IR as well as collateral adverse effects. Feedback technologies for online therapy control are presented; they enhance the selectivity of the laser—tissue interaction. Typical medical and surgical applications in gynecology, otorhinolaryngology, neurosurgery, dermatology, urology, dental surgery, ophthalmology and cardiovascular surgery are briefly summarized.

Mesopic vision in myopia corrected by photorefractive keratectomy, soft contact lenses, and spectacles

Purpose: To evaluate contrast vision and glare sensitivity under mesopic conditions in eyes having uncomplicated excimer laser photorefractive keratectomy (PRK) for myopia and in eyes corrected by disposable soft contact lenses, soft contact lenses, and spectacles. Setting: Division of Experimental Ophthalmic Surgery, University of Tübingen, Germany. Methods: The Mesoptometer II test was used to evaluate mesopic vision (glare sensitivity and contrast vision) in 28 eyes of 14 patients wearing disposable soft contact lenses, 20 eyes of 10 patients wearing soft contact lenses, 39 eyes of 20 patients wearing spectacles, 30 eyes of 15 emmetropic patients, and 33 eyes of 22 patients after PRK with a 5.0 mm optical zone. Follow‐up was between 15 and 60 months after PRK (mean 34.5 months). Results: The guidelines of the German Ophthalmologic Society state that patients must recognize Mesoptometer II contrast levels of 1:5 or better with and without glare to meet the minimum legal night‐driving standards for private cars. All eyes with disposable soft contact lenses and soft contact lenses, all emmetropic eyes, and 38 eyes corrected by spectacles recognized contrast levels of 1:5 or better without glare. In contrast, 18 eyes in the PRK group were unable to recognize contrast level 1:5 without glare. With glare, 1 eye in the disposable soft contact lens group, 1 in the soft contact lens group, and 7 with spectacles were unable to recognize the 1:5 contrast level. All emmetropic eyes recognized contrast levels of 1:5 or better; 22 PRK eyes were unable to recognize contrast level 1:5 with glare. Conclusion: Myopic PRK may lead to long‐term impairment of mesopic vision, while soft contact lens use does not seem to markedly influence mesopic vision in eyes with low to moderate myopia.

Laser Thermal Keratoplasty Using a Continuous Wave Diode Laser

BACKGROUND Laser thermal keratoplasty is currently performed with a pulsed Ho:YAG laser at 2.07 microns wavelength. Long-term stability depends critically on the coagulation depth of each cone and thus on emission wavelength (absorption in corneal tissue) and focusing, all contributing to controlled stable collagen shrinkage. To achieve this, a temperature range of 65 degrees to 90 degrees C is needed. A continuous wave laser source meets the coagulation requirements more effectively by avoiding tissue cooling by thermal diffusion as well as the peak temperatures of pulsed lasers, which counteracts the intended central corneal steepening. METHODS A continuous wave diode laser was developed, emitting at 1.885 microns with a maximal energy output of 450 mW. In a contact focusing application, the absorption depth in water as a function of wavelength was measured. Using laser parameters, comparable to those used for a pulsed Ho:YAG laser in contact mode, coagulation spots in human cornea were applied for the continuous wave diode laser. RESULTS The macroscopic and microscopic effects of the diode laser coagulation on corneas in vitro and in situ were comparable to those of the Ho:YAG laser, if a comparable amount of total energy per spot was applied. CONCLUSION Due to better optimized laser-collagen interaction, higher corrections and more stable clinical refractive effects appear achievable using the continuous wave diode laser.

Customized aspheric intraocular lenses calculated with real ray tracing.

PURPOSE: To calculate the exact geometry of custom intraocular lenses (IOLs) for pseudophakic eyes and theoretically predict the residual wavefront error by real ray tracing based on Snells law. SETTING: Centre for Ophthalmology, University Hospital, Tübingen, Germany. METHODS: Individual computer models were constructed based on measurements, including corneal topography and axial length. The geometry of custom spherical, aspheric, toric, and toric aspheric IOLs was calculated in an optimization process with real ray tracing to provide the minimum root mean square wavefront error. The geometric optical properties in terms of residual wavefront error was simulated and approximated by Zernike polynomials. RESULTS: Data from 45 pseudophakic eyes were used to construct the models. Defocus was almost completely corrected by the spherical IOL and astigmatism, by the toric IOL. The aspheric IOL strongly reduced spherical aberration but only slightly reduced total higher‐order aberrations (HOAs); both theoretical predictions corresponded to clinical investigations of wavefront measurements in pseudophakic eyes with a spherical or aspheric IOL. CONCLUSIONS: Real ray tracing calculated the exact geometry of custom IOLs to provide the minimum wavefront error, going beyond simple diopter information. Results show spherical aberration can be significantly reduced with aspheric IOLs. However, the limited possible reduction of total HOAs, even perfectly positioned custom aspheric IOLs, may be a reason for the unclear results in studies assessing the potential benefit to visual performance of currently used aspheric IOLs.

Calculating intraocular lens geometry by real ray tracing.

PURPOSE An implementation of real ray tracing based on Snells law is tested by predicting the refraction of pseudophakic eyes and calculating the geometry of intraocular lenses (IOLs). METHODS The refraction of 30 pseudophakic eyes was predicted with the measured corneal topography, axial length, and the known IOL geometry and compared to the manifest refraction. Intraocular lens calculation was performed for 30 normal eyes and 12 eyes that had previous refractive surgery for myopia correction and compared to state-of-the-art IOL calculation formulae. RESULTS Mean difference between predicted and manifest refraction for a 2.5-mm pupil were sphere 0.11 +/- 0.43 diopters (D), cylinder -0.18 +/- 0.52 D, and axis 5.13 degrees +/- 30.19 degrees. Pearsons correlation coefficient was sphere r = 0.92, P < .01; cylinder r = 0.79, P < .01; and axis r = 0.91, P < .01. Intraocular lens calculation for the normal group showed that the mean absolute error regarding refractive outcome is largest for SRK II (0.49 D); all other formulae including ray tracing result in similar values ranging from 0.36 to 0.40 D. Intraocular lens calculation for the refractive group showed that depending on pupil size (3.5 to 2.5 mm), ray tracing delivers values 0.95 to 1.90 D higher compared to the average of Holladay 1, SRK/T, Haigis, and Hoffer Q formulae. CONCLUSIONS It has been shown that ray tracing can compete with state-of-the-art IOL calculation formulae for normal eyes. For eyes with previous refractive surgery, IOL powers obtained by ray tracing are significantly higher than those from the other formulae. Thus, a hyperopic shift may be avoided using ray tracing even without clinical history.

Pupillary distortion after contact transscleral diode laser cyclophotocoagulation

Editor,—In recent years, transscleral contact diode laser cyclophotocoagulation (TCDLC) has been shown to be efficient in successfully lowering the intraocular pressure in different types of glaucoma.1-5 Reported success rates by various criteria ranged from 38% to 85%. Mostly, a fixed distance from the corneoscleral limbus with a specially designed contact probe without visualisation of the ciliary body is used. Complications reported so far include phthisis, chronic hypotony, corneal graft decompensation, macular pucker, cystoid macular oedema, hyphaema, vitreous haemorrhage, loss of visual acuity, retinal detachment, conjunctival burns, uveitis, and ocular pain.1-5 However, with the increasing use of TCDLC, more complications may be observed. This report describes pupillary distortion, a previously unreported complication. ### CASE REPORT A 32 year old man with bilateral juvenile glaucoma since 1987 was referred …

Ablation in teeth with the free-electron laser around the absorption peak of hydroxyapatite (9.5 μm) and between 6.0 and 7.5 μm

Pulsed IR laser ablation on dental hard substances was studied in the wavelength range between 9.5 and 11.5 micrometers with the Free-Electron Laser (FEL) in Nieuwegein/NL and between 6.0 and 7.5 micrometers with the FEL at Vanderbilt University in Nashville/TN. Depth, diameter and volume of the ablation crater were determined with a special silicon replica method and subsequent confocal laser topometry. The irradiated surfaces and the ejected debris were examined with an SEM 9.5 - 11.5 micrometers : depth, diameter and volume of the ablation crater are greater and the ablation threshold is lower for ablation with a wavelength corresponding to the absorption max. of hydroxyapatite (9.5 micrometers ), compared to ablation at wavelengths with lower absorption (10.5 - 11.5 micrometers ). For all wavelengths, no thermal cracking can be observed after ablation in dentine, however a small amount of thermal cracking can be observed after ablation in enamel. After ablation at 9.5 micrometers , a few droplets of solidified melt were seen on the irradiated areas, whereas the debris consisted only of solidified melt. In contrast, the surface and the debris obtained from ablation using the other wavelengths showed the natural structure of dentine 6.0 - 7.5 micrometers : the depth of the ablation crater increases and the ablation threshold decreases for an increasing absorption coefficient of the target material. Different tissue components absorbed the laser radiation of different wavelengths (around 6.0 micrometers water and collagen, 6.5 micrometers collagen and water, 7.0 micrometers carbonated hydroxyapatite). Nevertheless the results have shown no major influence on the primary tissue absorber.

THREE YEARS OF BIOMEDICAL FEL USE IN MEDICINE AND SURGERY : HOW FAR HAVE WE COME?

Abstract Since the FEL has been made available for biophysical research in the IR, it has revolutionized the optimization strategies of laser-tissue interaction and the minimizing of adverse effects, in particular for photoablative use in surgery. Its tunability together with the free combination of wavelength and energy made it an efficient research tool, allowing the reduction of risks and costs of preclinical biomedical research. New computer-assisted surgical techniques evolved and the broader data basis of IR photothermal ablation allows more accurate predictive modelling of the efficiency and the adverse effects of photoablation. New applications for diagnostic imaging as well as the first clinical applications in neurosurgery lay ahead.

Depth-adjusted thermal keratoplasty using a cw diode laser and a new focusing handpiece

Thermal keratoplasty is currently performed with a Ho:YAG laser at 2.1 micrometer wavelength. Long term stability critically depends upon the coagulation depth of each cone and thus upon emission wavelength, absorption and focusation, all contributing to controlled collagen shrinkage in order to induce central corneal steepening. Cw properties meet coagulation requirements more effectively by avoiding tissue cooling by thermal diffusion as well as peak temperatures of pulsed lasers. Both of these factors interfere negatively with collagen shrinkage. Therefore, a cw diode laser has been developed, emitting at 1.9 micrometer (absorption in water, comparable to Ho:YAG) with a maximal energy output of 400 mJ. For corneal application, the laser was coupled into a depth adjustable handpiece which allows the user to react to corneal thickness (maximal in the 6 h position) and its interindividual variation, covering a depth range between 350 micrometer and 800 micrometers. The histology of 10 porcine and 6 human cadaver corneas coagulated in vitro, revealed a similar spot geometry compared to conventional Ho:YAG radiation if the same total energy is applied. Higher corrections and better long term stability appear to be clinically achievable.

Silicon Cast Method for Quantification of Photoablation

BACKGROUND Topometry and measurement of photoablation patterns are key questions for keratorefractive photoablation. Ablation rates have been determined previously by either tissue perforation or by micrometry performed on histologic sections. METHODS A three-dimensional cast of cornea after irradiation was made by using a two-component silicon gel that polymerizes within minutes, thus preserving the corneal topography immediately after photoablation. Polymerization is athermal and nontoxic. The resulting silicon blocks were cut perpendicularly to the anterior surface and measured by calibrated light microscopy. RESULTS The silicon surface is extremely smooth and the accuracy of the cast is better than 0.25 micron. Reproducibility and long-term stability were demonstrated for casts of photoablated polymethylmethacrylate. Thus, ablation rates and profile, volumetry, and topometry can be determined following laser ablation. The method has been applied for 193-nanometer excimer laser in vitro irradiation of the human cornea. Ablation rates in Bowmans layer and stroma for various radiant energies and distinct pulse numbers were found to be in agreement with published data, and an incubation effect for the first laser pulses could be demonstrated. CONCLUSIONS The method is nondestructive, accurate, inexpensive, practical, and reduces requirements for laboratory animals.