Journal of Refractive Surgery

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Meeting Program

George O Waring, III, MD, FACS

Abstract

The Third International Congress on Laser Technology In Ophthalmology

May 22-25, 1991

San Francisco, Calif

For the third time, ophthalmologists from around the world gathered to discuss laser surgery of the eye, this time in San Francisco. Under the able organization of the Kugler & Ghedini Congresses, the 99 speakers covered the latest advances in laser surgery of the cornea, retina, iris, ciliary body, and anterior chamber angle. Thirty-five abstracts and 16 posters originating outside the United States were presented. Many speakers, posters, and technical exhibits featured solid state lasers, both diode and otherwise. The 3rd day of the Congress featured eight hands-on workshops, including one on excimer laser corneal surgery sponsored by the VISX corporation. The vigorous discussion periods and collegial societal gatherings provided ample opportunity for exchange of new ideas and maintenance of worldwide friendships.

Excimer Laser Photorefractive Keratectomy

Excimer Laser: Clinical Follow Up. J Marshall. Institute of Ophthalmology, London, UK.

The results of excimer laser photorefractive keratectomy performed on 16 blind eyes and 120 sighted eyes will be presented. Sighted eye patients were divided into six treatment groups of 20 according to their preoperative refractive status. In each group, an identical correction was attempted. These treatment groups ranged from - 2.00 to - 7.00 diopters in whole diopter steps. Follow up was 15 to 18 months for the blind eye study and 2 to 14 months for the sighted eye study. Eighty percent of patients in the - 2-diopter and -3-diopter treatment groups were within ± 1.00 D of intended refraction at 6 months. This figure fell to 33% for the group undergoing a - 6-diopter correction and regression was even more pronounced in those undergoing a -7-diopter correction especially in myopes greater than -10.00 D preoperatively. Most patients had some degree of loss of corneal transparency ("haze") first detected at 4 weeks and maximal at 5 to 6 months with evidence of corneal clearing by 1 year. Best corrected visual acuity was greater than or equal to preoperative levels in 82% of patients. Fifteen percent experienced only a slight reduction and 3% a loss of two lines of Snellen acuity. Eighty percent of patients noticed halos around fights in low levels of illumination although only 12.5% regarded this as a significant problem. The halo effect arises because of the 4-millimeter ablation zone diameter used and with a large pupil both central and mid-peripheral cornea can contribute simultaneously to the retinal image. This is an exaggerated form of positive spherical aberration. The authors will describe this phenomenon and put forward a simple clinical method for monitoring its magnitude. They consider that although there is individual variation in response to this surgery and follow up at the present time is relatively short, photorefractive keratectomy is a safe procedure, which holds considerable promise for refractive surgery in the future.

Excimer Laser Keratectomy: The Toronto Experience. SJ Weinstock, DJ Weinstock. University of Toronto, Canada.

Thirty-six excimer laser keratectomies for the correction of myopia were performed in Toronto between October 12, 1990 and January 26, 1991. This paper will present the results with respect to refraction, visual acuity, corneal clarity, and complications. The follow-up period reported will be between 3 and 6 months.

Excimer Laser Photorefractive Keratectomy in Low, Medium, and High Myopia. D Dausch, RJ Klein, E Schroder. Amberg, FRG.

A 136-case study was performed in 1990. The follow up ranged between 6 and 12 months. The authors have been treating patients with all grades of myopia. They were divided into four groups: 3.00 diopters to 6.00 D, 6.00 D to 10.00 D, 10.00 D to 20.00 D, and over…

The Third International Congress on Laser Technology In Ophthalmology

May 22-25, 1991

San Francisco, Calif

For the third time, ophthalmologists from around the world gathered to discuss laser surgery of the eye, this time in San Francisco. Under the able organization of the Kugler & Ghedini Congresses, the 99 speakers covered the latest advances in laser surgery of the cornea, retina, iris, ciliary body, and anterior chamber angle. Thirty-five abstracts and 16 posters originating outside the United States were presented. Many speakers, posters, and technical exhibits featured solid state lasers, both diode and otherwise. The 3rd day of the Congress featured eight hands-on workshops, including one on excimer laser corneal surgery sponsored by the VISX corporation. The vigorous discussion periods and collegial societal gatherings provided ample opportunity for exchange of new ideas and maintenance of worldwide friendships.

Excimer Laser Photorefractive Keratectomy

Excimer Laser: Clinical Follow Up. J Marshall. Institute of Ophthalmology, London, UK.

The results of excimer laser photorefractive keratectomy performed on 16 blind eyes and 120 sighted eyes will be presented. Sighted eye patients were divided into six treatment groups of 20 according to their preoperative refractive status. In each group, an identical correction was attempted. These treatment groups ranged from - 2.00 to - 7.00 diopters in whole diopter steps. Follow up was 15 to 18 months for the blind eye study and 2 to 14 months for the sighted eye study. Eighty percent of patients in the - 2-diopter and -3-diopter treatment groups were within ± 1.00 D of intended refraction at 6 months. This figure fell to 33% for the group undergoing a - 6-diopter correction and regression was even more pronounced in those undergoing a -7-diopter correction especially in myopes greater than -10.00 D preoperatively. Most patients had some degree of loss of corneal transparency ("haze") first detected at 4 weeks and maximal at 5 to 6 months with evidence of corneal clearing by 1 year. Best corrected visual acuity was greater than or equal to preoperative levels in 82% of patients. Fifteen percent experienced only a slight reduction and 3% a loss of two lines of Snellen acuity. Eighty percent of patients noticed halos around fights in low levels of illumination although only 12.5% regarded this as a significant problem. The halo effect arises because of the 4-millimeter ablation zone diameter used and with a large pupil both central and mid-peripheral cornea can contribute simultaneously to the retinal image. This is an exaggerated form of positive spherical aberration. The authors will describe this phenomenon and put forward a simple clinical method for monitoring its magnitude. They consider that although there is individual variation in response to this surgery and follow up at the present time is relatively short, photorefractive keratectomy is a safe procedure, which holds considerable promise for refractive surgery in the future.

Excimer Laser Keratectomy: The Toronto Experience. SJ Weinstock, DJ Weinstock. University of Toronto, Canada.

Thirty-six excimer laser keratectomies for the correction of myopia were performed in Toronto between October 12, 1990 and January 26, 1991. This paper will present the results with respect to refraction, visual acuity, corneal clarity, and complications. The follow-up period reported will be between 3 and 6 months.

Excimer Laser Photorefractive Keratectomy in Low, Medium, and High Myopia. D Dausch, RJ Klein, E Schroder. Amberg, FRG.

A 136-case study was performed in 1990. The follow up ranged between 6 and 12 months. The authors have been treating patients with all grades of myopia. They were divided into four groups: 3.00 diopters to 6.00 D, 6.00 D to 10.00 D, 10.00 D to 20.00 D, and over 20.00 D. Eighty percent of the patients with myopia of less than 10.00 D have been corrected to emmetropia. The group with myopia of between 10.00 D and 20.00 D has been reduced by 8.20 D, and the group with over 20.00 D by 13.70 D. These are mean values. Regression, haze, and other side effects are compared in these four groups.

Excimer Laser Photorefractive Keratectomy for Myopia: Clinical Results in Sighted Eyes. B Tengroth, D Epstein, P Fagerholm, et al. St Eriks Hospital, Stockholm, Sweden.

More than 500 normally-sighted (correctable to 20/20 or better) eyes were treated with photorefractive keratectomy for myopia. The preoperative refraction ranged from -1.25 D to -7.50 D(× = -4.56 D ±SD 1.41). At 1 month post-photorefractive keratectomy, refraction ranged from -0.50 D to +4.50 D(× = +1.13 D ±SD 1.30). No wound healing problems were noted. Subepithelial haze at 1 month ranged from 0 to 2 + (clinical grading scale 0 to 4+). Increased IOP (>22 mmHg) was observed in about 7% of cases. These eyes responded promptly to topical medication. There was no case of visual acuity loss. Follow-up data, covering up to 8 months post-photorefractive keratectomy, will also be presented.

Two Years' Experience With Laser Keratomileusis. T Seiler, T Bende. University Eye Clinic, Berlin, FRG.

Excimer Laser Photorefractive Keratectomy for Hyperopia. RJ Klein, D Dausch, E Schroder. Strasbourg, France.

A special technique with a rotating mask enables corneal shaping io hyperopia. A first group of 50 sighted eyes has been treated, the correction range is from 4.00 to 12.00 diopters. Visual acuity, retraction, regression, and haze will be discussed after a follow up of 6 months.

Astigmatic Excimer Laser Keratectomy. RJ Klein, D Dausch, E Schroder. Strasbourg, France.

In this study, 138 operations to correct astigmatism were performed under standardized conditions using an ArF excimer laser. Patients with both congenital and postoperative astigmatism were treated, in each case with four T-excisions. The results will be discussed.

Astigmatism Surgery With the Excimer Laser. K Ditzen. Ophthalmic Laser Institute, Weinheim, FRG.

The results of 70 cases with excimer astigmatism corneal surgery will be presented. The control time was 3 to 6 months. Operations were with four T-cuts and eight T-cuts (KVYZ-method). The T-incision weakens the strongest meridian of the cornea considerably and diminishes the amount of astigmatism. The average change of astigmatism was from 2.50 diopters to 3.50 D.

Model of a Combined Presbyopia With the Excimer Laser and Report of Treated Cases. T Anschuetz. Stadtklinik Baden-Baden, Gaggenau, FRG.

With this model, it should be possible to transpose the principles of bifocal or multifocal IOLs at the human cornea. In combination with corresponding masks, it seems possible to shape multifocal zones at the cornea. This model of combined presbyopia therapy in the excimer laser refractive surgery is demonstrated in some cases.

A New Silicon Cast Method for Quantification of Photoablation. W Bachmann, B Jean, T Seiler, et al. University Eye Hospital, Dept I, Tübingen, FRG.

Topometry and measurement of photoablation patterns are key questions for keratorefractive photoablation. So far, ablation rates have been determined either by tissue perforation or histological micrometry. A 3-diopter cast of irradiated cornea was made by using a two-component silicon, which polymerizes within minutes, thus preserving the corneal topography immediately after photoablation. The silicon surface is extremely smooth and accuracy of the cast is better than 1 µp?. Reproducibility and long-term stability was proved for casts of photoablated PMMA. Thus ablation rates, ablation profile, volumetry, and tomometry can be determined at any time. The method has been applied for 193 nm excimer and 2.94 µm Er: YAG laser in vitro irradiation of the human cornea. Ablation rates in Bowman's layer and stroma could be observed for various radiant energies and distinct pulse numbers. Average ablation rates are in agreement with published data, and an incubation effect of the first pulses could be demonstrated.

Comparative Scanning Electron Microscopy of Ablation by Three Excimer Lasers on PMMA. R Brancate, A Scialdone, G Trabucchi, et al. University of Milan, Italy.

Commercially available ArF lasers employ different lasing cavities, delivery systems, and energy. These could influence the quality of the ablation surface. PMMA is a suitable target for evaluation of the quality of ablation. Ablations produced with Meditec Phoenix Summit UV200 and VisX 20/20 were studied for each laser: photorefractive keratectomy -3.00 D and -8.00 D-PTK 20 to 40 pulses (for Meditec depths corresponding to 0.25 µm per pulse). Samples were 4.5 mm in diameter. Surfaces studied at SEM showed uniform even ground with slight variations between each of them. In many cases, well delimited small irregularities with random distribution were present. In photorefractive keratectomy, the steps were visible but very smooth. The three lasers do not appear significantly different on PMMA.

Photochemical Injury of the Cornea From ?93-nm ArF Laser Radiation. DH Sliney. US Army Env Hygiene Agency, Aberdeen Proving Ground, Md.

Laser photoablation of the cornea at 193 nm is now used in corneal surgery. Of continued interest are the potential side effects of this short- wavelength ultraviolet radiation to corneal tissues. Although the 193-nm wavelength corresponds to high photon energy, the penetration depth is so hmited that the side effects are thought to be small. As one aspect of this safety study, the threshold for photokeratitis at 193 nm was obtained for the rabbit cornea using an ArF excimer laser. From this experiment and from knowledge of environmental exposures, it is concluded the 193 nm photons have such a shallow penetration depth, being limited to the outermost epithelial cells, that classical photokeratitis occurs from the fluorescence emitted at the corneal epithelial absorption site.

Characterization of Acoustic Effects In Corneal Photoablation and its Possible Use for On-Une Control of Cutting Depths. H Lubatschowski, O Kermani, W Ertmer, et al. Institute of Applied Physics, University of Bonn, FRG.

The structure and dynamics of ArF-excimer-laserinduced acoustic Shockwaves in corneal photoablation were investigated using piezoelectric transducers (PVDFfoils). A significant correlation between the fluence and the intensity of the measured Shockwaves was proved. Shockwave amplitudes range between 40 bar at 100 mJ/cmp 2 and 400 bar at 1000 mj/cmp 2. The potential mechanical stress of such acoustic transients is shown and might be one cause of postoperative subepithelial corneal scar formation. By on-line measurement of the running time of the acoustic transients through the cornea, the authors could control the actual excision depth with an accuracy of ≤l0 µm. Finally, the authors present early results of a comparative Shockwave analysis of ArF-excimer laser and Q-switched Er-YSGG laser.

Influence of UV-Absorblng Drugs on Photoablatlon of the Cornea and the Sclera. N Muller-Stolzenburg, HJ Buchwald, A Bergmann, et al. Univ-Augenklinik, Berlin, FRG.

Many topical drugs used in ophthalmology, such as local anesthetics or sulfonamides, are strong UV absorbers. With topical application of these drugs, the UV-B secondary radiation penetrating into the eye during 193 nm excimer laser irradiation of the cornea is drastically reduced in in- vitro experiments on cadaver eyes. Scattered 308 nm radiation reaching the lens during ab-interno sclerostomy via quartz fiber is also suppressed, but the risk of damage to the fiber tip increases. Potential risks due to photochemical decomposition of these drugs at subablative fluences should be considered.

Raman Laser Spectroscopy to Correlate the Molecular Structure of Ocular Tissues With the Pathology. R Caramazza, A Bertoluzza, C Fognano, et al. Inst Clinical Ophthalmology, Bologna, Italy.

The Raman laser spectroscopy in its conventional or multichannel form actually represents a vibrational spectroscopic method highly suitable for correlating molecular structure with pathology. This is due to the characteristic "not invasive-not destructive" method of investigation and to the possibility of realizing the spectroscopic measurements directly "in situ" and "in vivo" (Multichannel Raman laser spectroscopy). The application of this technique, as far as the ocular tissues are concerned, is discussed; particularly the structure of the cornea in relation to alterations due to injuries or pathologies. The results obtained are valued in relation to the structural changes which occur in the main components of the cornea (collagen) and in relation to the mechanical (elasticity modulus) of the corneal tissue.

An Automated In-Surgery Topography Measurement System for Cornea Laser Microsurgery. CT Hsuen, JF Bille, JJ Persoff. Intelligent Surgical Lasers, Ine, San Diego, Calif.

A cornea topography measurement system employing an IR-laser diode and a Hartmann-Shack wave-front sensor has been developed. With the IR laser focused at the nominal center of curvature of the cornea, the reflected fight is directed through a micro-lens array. The positions of the focal spots from this array collectively form a map of the cornea surface. This system has been integrated into the ISL Laser Surgical System Model 4001 for in-surgery measurement. The capabilities of automatic eye setup and tracking in three dimensions provide improved accuracy and measurement repeatability.

Imaging and Measurement of Anterior Segment Morphology With the Laser Tomographic Scanner (LTS). B Jean, A Frohn, T Seiler, et al. Eye Clinic, Free University of Berlin, FRG.

Laser scanning with confocal signal detection of Purkinje reflexes provides a method for obtaining section images which can be used for imaging and measurement of anterior segment morphology. A modified laser tomographic scanner (LTS), formerly manufactured by Heidelberg Instruments, was used to obtain optical 2-D sections in the frontal and horizontal planes as well as 3-D plots, both experimentally in vitro and clinically. The following clinical applications have been tried successfully: frontal and horizontal section images of the cornea; corneal thickness measurement; 3-D topography of corneal photoablation patterns in vitro; chamber angle imaging and precision measurement longitudinally, before/after ECCE + IOL; imaging, measurement, and partial 3-D reconstruction of posterior lens capsule after ECCE and IOL; frontal section images of the cataract lens, including planimetry and densitometry. The underlying principles will be summed up, typical examples will be shown, measuring procedures and their reproducibility will be discussed and actual limitations of the method will be described.

The Design of an Instrument for Evaluation of Vision Before, During, and After Refractive Surgery. JM Enoch. Berkeley School of Optometry, Berkeley, Calif.

Current designs of equipment for refractive surgery do not include fine means for evaluating vision before, during, and after surgery. A device is proposed for inclusion which will allow (1) monitoring of fixation during surgery and visual assessment, (2) focusing, locating, adjusting, and monitoring an aperture imaged in the surgical plane, (3) provision of any desired test of vision, and (4) measurement and correction of refractive error as often as required. It is further suggested that a form of hyperacuity testing be used as reference control. Hyperacuity thresholds are measured in seconds of arc. Also, these tests allow assessment of vision through cloudy, distorted, or absorbing ocular media, and it is possible to separate optical from neural decrements in visual performance. It is also possible to detect and measure finely metamorphopsia.

Laboratory and Clinical Studies of Excimer Laser Corneal Surgery. G Waring, F Fantes, K Hanna, et al. Emory University School of Medicine, Atlanta, Ga.

The wound healing response of the cornea after excimer laser photorefractive keratectomy was studied in 29 Rhesus monkey corneas. Four mm diameter ablations were performed with an effect ranging from -2,00 to -8.00 diopters. Corneas were studied by slit-lamp, light, and transmission electron microscopy, and by immunohistochemistry. Synthesis of type VII collagen was evident by 7 days. Activated stromal fibroblasts expressed a fetal antigen for 6 weeks, when mild to moderate subepithelial haze was present in 93% of corneas. By 6 to 9 months, 92% of corneas were judged to be either clear or minimally hazy. However, even 18 months after ablation, the subepithelial stroma remained abnormal. The authors conclude that the monkey cornea undergoes a typical wound healing response after excimer laser ablation, and remains abnormal even 1 1/2 years after treatment.

Range and Predictability of Photorefractive Keratectomy. S Trokel. Department of Clinical Ophthalmology, Columbia University, New York.

The authors have studied the clinical results with photorefractive keratectomy using no post-treatment corticosteroids and have compared them to clinical series using several different corticosteroid regions. They have also investigated the effect of varying ablation zone diameters and ablation algorithms, both of which affect total ablation depth. The group of patients who had their topical corticosteroids stopped after 4 to 6 weeks, showed no difference in their clinical response compared to patients who had had no topical corticosteroids following photorefractive keratectomy.

Photorefractive Keratectomy and the Corneal Epithelium. G Simon, JM Parel, G Kervick, et al. Bascom Palmer Eye Institute, Miami, Fla.

ArF excimer laser photorefractive keratectomy shows potential for the precise correction of refractive errors in patients. In photorefractive keratectomy, the epithelium is mechanically removed and Bowman's layer and stromal tissue are photoablated to precomputed depths and shapes that are based on known ablation rates for this tissues. Assuming the epithelium to be preoperatively uniform in thickness across the central optical zone and assuming it regrows to the same thickness, a theoretical precision better than ±0.10 diopters is easily achievable with photorefractive keratectomy. Yet, in practice, postoperative errors of - 2.00 D are common. Epithelium/Bowman differential keratometry performed in a series of cadaver eyes demonstrates differences of up to 1.80 D. Bowman's layer was found to be more prolated than the epithelium (ratios: 1.005 to 1.033). In addition, localized changes in astigmatic power (up to 0.50 diopters) and axis (up to 70°) were found between the epithelial and Bowman surfaces, indicating that the epithelium thickness varies nonuniformly across the central cornea. As the preoperative topography of the epithelium cannot he used as a reference surface when computing photoablation depth, intraoperative keratometry of Bowman's surface becomes a necessity in photorefractive keratectomy.

Objective Measurement of Corneal Transparency Before and After Excimer Laser Photorefractive Keratectomy. C Lohmann, D Gartry, F Fitzke, et al. Institute of Ophthalmology, London, UK.

A marginal loss of corneal transparency or "haze" is a phenomenon experienced after photorefractive keratectomy using an excimer laser (193 nm). The authors have developed a new system where a CCD-camera was fixed at 40° to a slit-lamp and connected via a frame-grabber to a computer. Both scattered and reflected light in the slitimage of subject corneas were measured using polarizing technologies and image analysis software. These objective measures of changes in transparency were correlated with changes in visual function. Detailed results will be presented and discussed.

Infrared Laser Corneal Surgery

The Infrared Laser: An Update. JF Bille. Intelligent Surgical Lasers Ine, San Diego, Calif.

A solid-state, ultrashort pulse, high repetition rate laser system is presented. It uses an Nd'.YLF gain medium in a two-stage oscillator/amplifier combination to provide pulses of up to 400 mJ in energy, with pulse widths of 30 psec at a wavelength of 1053 nm. Focusing the laser to a spot size of 10 mm provides extremely high peak power, 1013 watts/cmp 2, which exceeds the requirements for achieving photoablation. For corneal applications, the laser has an automated delivery system which incorporates a closed loop, three-dimensional eye tracking system. First clinical results on intrastromal refractive surgical procedures are reported.

Plasma-Mediated Ablation of Corneal Tissue by Picosecond Infrared Laser Pulses. EG Klancnik, MH Niemz, JF Bille. Intelligent Surgical Lasers, Inc, San Diego, Calif.

A new quantitative model has been developed that describes plasma-mediated ablation due to picosecond laser pulses which are focused onto corneal tissue. The model predicts the ablation depth to be exponentially related to the pulse energy density. Plasma-mediated surface ablations were performed on a series of human donor corneas (SD Eye Bank) with a mode-locked Nd:YLF regenerative laser system, at 1053 nm. The laser had a 60 psee pulse-width, 1 kHz repetition rate, and focal spot size of 15 µm. The ablation depths were determined by either fight or electron microscopy. Experimental results were in very good agreement with the theoretical model.

Intrastromal Relaxing Excisions in Rabbits With a Picosecond IR Laser. BE Frueh, JF Bille, SI Brown. Department of Ophthalmology, University of California at San Diego, Calif.

Using a picosecond pulsed Nd:YLF laser slit-lamp system, 2-millimeter curved laser patterns were delivered straddling the horizontal meridians 5 mm from the center within the stroma of rabbit eyes. Central keratometry, photokeratoscopy, slit-lamp examination, and histologic and electron-microscopy studies were performed preoperatively and postoperatively at regular intervals up to 60 days. Corneal flattening in the desired meridian with a peak at 24 hours was achieved. None of the rabbits showed significant intraocular inflammation or scarring. Advantages of this technique are the preservation of the epithelium and a precise laser delivery of low energy impulses.

Intrastromal Tissue Removal Using an Infrared Picosecond Pulsed Nd:YLF Ophthalmic Laser Operating at 1053 nm. RMRemmel, CMDardenne, JF Bille. University of California, San Diego, Calif.

A new technique for the intrastromal removal of corneal tissue was developed using a 1053-nm infrared picosecond-pulsed Nd:YLF ophthalmic laser. Cavities of different volumes and predictable depths were created, confined within the corneal stroma. The configurations and dimensions of the cavities, which were confirmed with SEM, indicate that a contiguous layer of cornea was removed within the stroma. These cavities collapsed, which should result in a flattening of the cornea and in changes of refractive power. The experiments were performed on fresh human cadaver eyes (SD Eye Bank) using a modified Zeiss 30SL slit-lamp delivery system.

Novel Ophthalmic Applications of Femtosecond and Picosecond Lasers. CA Puliafito. Massachusetts Eye and Ear Infirmary, Boston, Mass.

Laser-induced optical breakdown by nanosecond duration Nd:YAG laser pulses has been used in clinical ophthalmology for posterior capsulotomies and for incision of vitreous membranes. Nanosecond optical breakdown produces a zone of tissue damage ranging up to 2 mm in diameter. Picosecond pulses have the advantage of greater peak intensities for a given pulse energy and, therefore, breakdown can occur at significantly lower energies. Because the radius of tissue damage principally depends on the pulse energy and not the pulse duration, picosecond lasers will allow the desired surgical photodisruption at much lower pulse energies and with better containment of collateral damage. In this study, the authors demonstrate experimental vitreous membrane cutting using a high repetition rate picosecond Nd:YAG laser. A single 100-picosecond pulse was selected from a mode-locked, Q-switched pulse train at repetition rates of up to 1000 Hz. In vivo experiments were performed by directing the selected pulses through a LASAG delivery arm and slit-lamp. At energies of 90 µJ/pulse intravitreal optical breakdown (photodisruption) was induced through a contact lens in rabbit eyes. With the aid of an He-Ne aiming beam and at a pulse repetition rate of 200 Hz, precise and rapid severing of experimental vitreous membranes was achieved. The ability to localize laser-induced breakdown effects was assessed by examining the retinal tissue posterior to severed vitreous membranes. "Using an in vitro model developed to improve quantification of damage radii, cultured fibroblast monolayers were cut as close as 120 μ to corneal endothelium without evidence of endothelial disruption. With its lower pulse energy requirements, this variable repetition rate picosecond Nd:YAG laser should be capable of cutting within the transparent structures of the eye with damage radii greatly reduced compared to current nanosecond duration Nd:YAG laser pulses.

Membrane Cutting and Collateral Damage of Intraocular Ps-Photodisruptfon. R Bimgruber, Y Hefetz, J Roider, et al. H. Wacker Laboratory for Medical Laser Applications, Munich, FRG.

Previous studies have shown the potential of improving the spatial confinement of NdYAG photodisruptive effects by using multiple low-energy-ps-pulses instead of nspulses. Picosecond photodisruptive experiments were performed in vivo using two intraocular structures, the lens capsule, and the neural retina of rabbit eyes, as clinically relevant test structures for membrane cutting and collateral damage, respectively. Light and electron microscopical investigations indicate highly localized disruptive effects with a spatial extent of less than 100 µm, and collateral tissue damage in the µm range.

Intraocular Tissue Effects With Picosecond Nd:YAG Laser Pulses. A Vogel, A Frieser, R Bimgruber. H. Wacker Laboratory for Medical Laser Applications, Munich, FRG.

Tissue effects produced by series of picosecond Nd.YAG laser pulses (30 psec, 10 Hz) in the cornea, lens, and retina of bovine eyes were investigated in vitro. Special emphasis was laid on the determination of the spatial extension and damage range of the effects. With 50 µJ pulse energy, it was, for example, possible to achieve clean cuts in Descemet's membrane or intrastromal effects with a range of about 120 µp? for endothelial damage. The damage range scales were proportional to Qaser pulse energy) 1/3. The results will be analyzed with respect to the feasibility of intrastromal corneal cutting (refractive surgery), phacoemulsification, and vitroretinal laser surgery with picosecond pulses.

Hydrogen Fluoride Laser Keratotomy. JM Parel, G Simon, JA Lowery, et al. Bascom Palmer Eye Institute, Miami, Fla.

A noncontact laser surgical system was designed to perform linear and curvilinear corneal incisions for the correction of astigmatism, myopia, and hyperopia. Using 50 fresh cadaver eyes and an automated digital keratometer (2.0 mm OZ ± 0, 1.00 D, ± 1), the induced change in corneal radius of curvature produced by the laser and by a diamond knife was quantified in function of the incision location, length, and depth. BSS was used to visualize the keratometer's mire. The laser and the diamond knife produced similar results (P ≤ .05, 85% incision depth). However, the induced change was found to be very sensitive to wound hydration (swelling rate: - 1.00 D per minute). When hyperosmotic agems (Dextran B, PVP) were used instead of BSS, the corneal radius of curvature returned to preoperative values. Deepening the incision to 100% produced no effect. Wound hydration prevents accurate prediction of the final corneal dioptric power, regardless of the surgical modality used to perform the keratotomy.

Photothermo Refractive Keratoplasty. JM Parel, G Simon, JA Lowery, et al. Bascom Palmer Eye Institute, Miami, Fla.

Myopic and hyperopic corrections were obtained in cadaver and primate eyes with an optical delivery system coupled to a Tm:Ho:Cr:YAG laser (2.12 µm, 250 µs, 1 Hz). An annulus of energy (5 mm ∅, 250 µm wide, 5.6 J/cmp 2) produced -5.00 D of corneal flattening in human (25 pulses) and - 2.00 D in owl monkeys (10 pulses) corneas. A double annular array (at 5 and 6 mm ∅) of 16 symmetrically distributed points (200 µm ∅, 19.5 J/cmp 2) produced + 2.50 D of steepening. Histology revealed zones of altered collagen tapering from the anterior human corneal surface to a point approximately 2/3 deep. In the primate, vital stain showed a 250-µm wide ring of endothelium damage, while the ring of epithelial damage was of - 500 µn (LM). Damaged keratocytes were found outside the zones of altered collagen (EM). No statistical correlation could be found between the total energy delivered to the cornea, the corneal thickness, and the amount of dioptric change. Nonuniformity in the laser beam energy distribution at target induced corneal astigmatism. Laser thermokeratoplasty mandates real-time intraoperative keratometry and improvements in laser beam characteristics.

Comparison of Biological Effects of Er: YAG and Excimer Lasers: Irradiation on the Corneal Endothelial Cell. KS Wang, ZL Lan, ZH Yao, et al. Shanghai Second Medical University, China.

The authors have previously reported the different biological effects on corneal cells of 308 nm excimer laser and Er:YAG laser irradiation. Obviously, the biological effects on endothelial cells between 193 nm excimer laser and Er.YAG laser irradiation should be compared as well. The results will be presented from observations using electron microscopy, microspectrophotometry, and image analysis.

Corneal Area Ablation by Er:YAG Laser Irradiation. B Jean, R Hibst, T Seiler, et al. Eye Clinic, Free University of Berlin, FRG.

Until now UV excimer lasers have mainly been used for area ablation of the cornea. Since good absorption is also found at 2.94 µm, the Er:YAG laser might be considered a more economical laser source for refractive surgery, further avoiding potential UV hazards. A flashlamp pumped Er:YAG laser in the normal spiking mode (τ 1250 µs, λ = 234 gm) was used to irradiate human donor corneas. Beam profile was nearly rectangular with a spot size of 2.0 mm. Number of laser pulses ranged from 1 to 8, radiant energy per pulse was varied from 90 to 230 mJ. Ablation depth increases linearly with pulse number and radiant energy per pulse. About 120 mJ (3.9 Jcmp 2) are needed by the first pulse(s) before ablation becomes efficient. Threshold energy is then 32 mJ (1.0 Jemp 2), slope efficiency is 0.17 µm/mJ (5.2 µm/Jcmp 2). Local surface roughness is about 15 µm. Ablation rates are considerably higher compared to the 193 nm excimer laser, making the Er: YAG laser a potentially effective tool. Increased surface roughness and thermal damage, however, might be serious obstacles for clinical use.

IR Diode Laser Corneal Suturing: First Attempts. G Timberlake, A Patmore, A Shallal, et al. Eye Research Institute, Boston, Mass.

The authors investigated corneal laser "suturing" because interdigitation of severed collagen fibrils "unwound" by laser-thermal effects may produce laser tissue "welds." Incisions in postmortem human corneas were either (a) infused with ICG, (b) infused with ICG/albumin mixture or (c) not infused. Incisions were irradiated (0.1 to 3.16 kJ/cmp 2) using an 800-nm diode laser/fiberoptic delivery system. Light and electron microscopy showed a variety of changes associated with both the creation of the incision and the subsequent laser irradiation. The infused incisions exhibited collagen fiber shrinkage and incision widening. The sides of the ICG/albumin incision remained opposed, perhaps due to fine fibers seen crossing the incision. Biochemical analysis of collagen, using SDS-PAGE, demonstrated differences in the protein profiles of infused and noninfused corneal stroma.

Laser Therapeutic Corneal Surgery

Excimer Laser Photorefractive Keratectomy tor Diseases of the Cornea and Sclera. D Dausch, RJ Klein, E Schroder. Amberg, FRG.

Three years' experience in the treatment of diseases of the cornea and sclera with the excimer laser will be presented. The authors have been using the excimer laser for the treatment of epithelial diseases after chemical injury (three cases) and herpes cornea (20 cases). A further indication for the use of this kind of laser is the possibility of removing pathologic tissue from the cornea, such as tumors (three cases) and hand-shaped keratopathies (two cases). In addition, the authors performed perforating (three cases) and lamellar (one case) keratoplasties. The healing process and the clinical results will be discussed.

Excimer Laser Photoablation of Corneal Opacities and Irregularities. P Fagerholm, T Fitzsimmons, B Tengroth, et al. St Eriks Hospital, Stockholm, Sweden.

The 193 nm ArFl laser was used to ablate corneal opacities/irregularities in 30 patients between April 1990 and January 1991. Follow-up time varied from 3 weeks to 9 months. The diagnosis varied but fell into six main groups: visual reduction due to corneal dystrophy (6), herpes simplex opacification (5), contact lens induced keratitis (4), opacifications in corneal grafts (3), recurrent erosions (3), and opacifations and/or irregularities for miscellaneous reasons (9). A treatment goal was individualized for each patient and was reached in 25 of the 30 patients. Failure to obtain the goal was due to a too deep opacification.

Laser Therapy of Pterygia. H Paul, R Sihota. Dr R.P. Centre for Ophthalmic Sciences, AIIMS, New Delhi, India.

Surgical procedures described for pterygia are associated with a high and unacceptable rate of recurrence. The authors studied argon laser photocoagulation of pterygia under fluorescein angiographic control in 30 patients with and without the photochemical effect of rose Bengal dye. There was marked symptomatic relief in 90% and cosmetic improvement in 93.3% of patients. They documented regression in 93.3% of cases, although there was no significant change in keratometric readings. They found the best results in patients with small, primary pterygia. However, in recurrent pterygia, decreased vascularity aided subsequent surgery. Patients had no significant problems post-laser and no recurrences were noted.

Laser Coagulation of Corneal Neovascularizations. W Wiegand. Department of Ophthalmology, Philipps University Marburg, FRG.

The formation of corneal neovascularizations after penetrating eye injuries is one of the major reasons for failure of a subsequent keratoplasty. In 12 eyes with central corneal scars and corneal neovascularization, the authors attempted to occlude the corneal vessels prior to keratoplasty by focal dye laser coagulation. Successful coagulations could only be performed after removing the sutures of the primary corneal reconstruction and in cases in which only a few large vessel trunks crossed the limbus. The authors will demonstrate the surgical approach and results.

10.3928/1081-597X-19920101-20

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