Hyperguides

Ophthalmology / Refractive surgery

Excimer Laser Systems

Ashley Behrens, MD Jonathan R. Pirnazar, MD Francia Torres, MD Enrique Suárez, MD Peter J. McDonnell, MD
Article


Introduction

The variety of commercially available excimer laser systems has grown in proportion to the popularity of laser in situ keratomileusis (LASIK). Millions of these surgeries are expected to be performed worldwide in the next year. This popularity has promoted the research and development of newer, safer, and more accurate laser systems.1 In light of these events and developments, it is worth reviewing the current and forthcoming excimer lasers, and the technical specifications and advantages of these systems.

Currently, all clinical excimer lasers use argon-fluorine (ArF) gas. Their wavelength is in the UV region (193 nm), which has been determined to be safe for human ocular use. However, a common drawback of these lasers is the use of a toxic gas (fluorine) that may pose a potential risk for the surgical personnel in case of leakage. Another disadvantage is the high operating costs due to the periodic gas expenses. Also, sheer bulk of the lasers often requires a specially equipped suite with regulated room temperature and humidity.

The list of available lasers approved by the U.S. Food and Drug Administration (FDA) for refractive surgery is limited. As of early 2001, only three laser systems — the Visx S2 and S3 (Visx, Santa Clara, Calif.), Summit Apex Plus (Summit, Waltham, Mass.), and Alcon Summit Autonomous LADARVision (Alcon, Ft. Worth, Tex.) — were approved for the treatment of myopia, astigmatism, and hyperopia. Four other lasers — the Keracor 116 (Bausch & Lomb, Claremont, Calif.), Technolas 217 (Bausch & Lomb), Nidek EC-5000 (Nidek, Pasadena, Calif.), and LaserSight LaserScan LSX (LaserSight Technologies, Winter Park, Fla.) — were approved for the treatment of myopia and astigmatism. Lasers available outside the United States include the Asclepion-Meditec MEL 70 G-Scan (Asclepion-Meditec AG, Jena, Germany), the Schwind Esiris (Herbert Schwind GmbH & Co.KG., Kleinostheim, Germany), and the Allegretto (WaveLight Laser Technologie AG, Erlangen, Germany).

Visx Star S2 Excimer Laser

The Visx Star S2 is the fourth generation of Visx lasers. It is a modified broad-beam laser with a seven beam scanning system. The laser beam consists of a beam rotator for temporal integration, a spatial integrator, and a beam-shaping device. The Visx Star S2 includes the SmoothScan technology for a more regular ablation profile.

The laser has capabilities for customized ablations using Contoured Ablation Patterns (CAP).2 The more recent Visx S3 version features a 300 Hz Active Track video eye-tracking device and offers the possibility of adapting wavefront-guided technology with its WavePrint system.

The Visx device uses a vision key card — WORM: write once read many — which is necessary to use with every procedure. The card allows compilation and storage of patient data.3 The Windows-based software controls the different laser parameters (Table). For focusing and centering the patient’s eye, a joystick located besides the microscope allows the straightforward movement of the patient’s chair in an X-Y-Z pattern. On each side of the joystick, control panel buttons are located to control lighting conditions at the microscope. The ablation area can be visualized under a direct microscopic view by a set of mires projected into the operating field by an alignment reticule. A Leica MS microscope is standard to the device, with three different magnification positions.

The fixation light is created by a bright light-emitting diode (LED) light below the laser aperture. An aspirator tube is also provided, which can be located close to the patient’s eye during the ablation to absorb the aerosolized materials in the laser plume.

The laser requires a standard calibration at the start of each day. A secondary –4 D calibration is required after three cases. Once the laser parameters are set in the software, the laser ablation is activated by a foot pedal. It can be released any time with immediate stopping of the laser emission. An emergency button is also provided to disconnect the unit.

The FDA-approved parameters for the Visx Star S3 at the time of writing this tutorial are:

  • Myopia less than –14 D -with or without astigmatism between –0.50 and –5 D;
  • Hyperopia between +0.50 D and +5 D, with or without astigmatism between +0.50 D and +4 D.

Summit Apex Plus2

The Summit Apex Plus is another type of broad-beam laser, with a combination of expanding iris diaphragm and erodible discs.4 The Emphasis discs allow a smooth and precise ablation. A simultaneous correction of myopia and astigmatism can be achieved with the “M” discs. The “K” and “L” discs allow a large optical zone with a blend zone in hyperopic corrections. This provides a more regular transition between the treatment zone and the adjacent periphery, reducing postoperative glare. The technical laser specifications are listed in the Table.

The Summit Apex Plus lacks an eye-tracking system. An upgrade is available to the Infinity Package, which includes a new-generation Zeiss microscope with foot pedals to control the zoom electronically (0.4X to 2.4X). In addition to the He-Ne laser-aiming beam, there is now a reticule as part of the microscope to aid in centration, with three concentric rings (3.5 mm, 6.5 mm, and 9.5 mm). Another upgrade is an “ablation by-product removal system” to aspirate the laser plume after each ablative pulse. There is a joystick to control the patient’s position in an X-Y-Z pattern from an arm of the surgeon’s chair.

Windows-based software keeps records of patients and treatment parameters. The software detects changes in calibration and permits recalibration in real time.

The FDA parameters allowed for this laser at the time of writing this tutorial are:

  • Myopia less than –14 D with or without astigmatism between –0.5 D and –5 D;
  • Hyperopia between +1.5 D and +4 D, with or without astigmatism less than –1 D.

Alcon Summit Autonomous LADARVision

The Alcon Summit Autonomous LADARVision is a flying-spot laser, with a beam size of less than 1 mm. The major feature in this laser is the laser radar eye-tracking system, capable of sampling up to 4000 Hz per second, more than 10 times faster than the best video-tracking device available. The system guarantees detection of the eye’s micro-movements during the ablation procedure and assures that the laser shot will not fall in an erroneous point of the cornea. This laser offers an upgrade to wavefront aberration ablation using a Hartman-Shack type sensor called the CustomCornea Measuring Device (CCMD).5

One of the disadvantages of the Alcon Summit Autonomous LADARVision is the need for a preoperative image of the eye with a dilated pupil. This is required for the tracking device to work properly. Obtaining mydriasis is a time-consuming factor that delays the surgical flow in high-volume centers. Likewise, the laser configuration itself is relatively uncomfortable for both the patient and surgeon. The patient’s head centration is performed manually, because the excimer laser lies underneath the patient’s bed, and the latter is fixed to it. The microscope’s location is unusual, separating the patient’s head from the surgeon by a vertical metal structure at the point where the laser and the eye-tracking system reach the eye.

The technical specifications are shown in the Table. The system requires calibration between patients. The eye-tracking system will not work properly in pupils smaller than 7 mm or larger than 11 mm in diameter.

The FDA parameters allowed for this laser at the time of writing this tutorial are:

  • Myopia of less than –10 D with or without astigmatism between –0.5 and –4 D;
  • Hyperopia less than +6 D with or without astigmatism of less than –6 D.

Bausch & Lomb Technolas 217

The Technolas 217 is a third-generation scanning/flying-spot laser with a 2-mm diameter beam. To guide the ablation pattern, the laser uses Windows-based PlanoScan software. Topography-guided ablations may be available with special Topolink software. Bausch & Lomb has also developed software called Zyoptix that allows wavefront-guided ablations.

The Technolas 217 is provided with an eye tracker that must be initialized before every use. The high-resolution infrared video camera must present an image to the computer software to recognize and track the eye. Previous calibration is required. The tracking range is 1.5 mm (3.0 mm total). The technical specifications are shown in the Table.

The laser requires calibration at the beginning of each session, and ideally between patients, to check the beam’s homogeneity and energy. These two variables are essential for an effective ablation.

The FDA approved the use of the Technolas 217 for myopia between –1 D and –7 D, with or without astigmatism less than –3 D.

Nidek EC-5000

The Nidek EC-5000 is a scanning-slit type ofexcimer laser. Customized corneal ablation software will be available in 2001. The laser has an ergonomic design, with a control panel within the reach of the surgeon’s hand to center the patient in the laser aperture by means of a joystick with X-Y-Z control. The control panel also has settings for focusing the laser and to adjust the illumination.

An eye-tracker is not available, but a diode laser that acts as an aiming beam (red light) is used for patient fixation. The technical specifications are shown in the Table.

The FDA approved the use of the Nidek EC-5000 for the correction of myopia from –1 D to –14 D with or without astigmatism of less than –4 D.

LaserSight LaserScan LSX

The LaserScan is a flying-spot laser with an 0.8-mm beam, and is one of the smallest devices on the market. The joystick for patient centration and focusing is attached to the right resting arm of the surgeon’s chair. It has topographic-link capabilities for decentered ablations. A microscope reticule for better centration will be available as an upgrade. The laser comes with a video eye tracker with a maximum range of 3 mm.

The LaserScan treats astigmatism in a cross-cylinder format. It produces a spherical ablation in astigmatic patients by treating a 6-mm optical zone in both the major and minor axes.6

The FDA approved the use of the LaserScan correction for myopia of less than –6 D with or without astigmatism of less than –1 D.

Asclepion-Meditec MEL 70 G-Scan

The MEL 70 G-Scan is flying-spot laser, with abeam of 1.8 mm. This is latest generation of this company, which promises integration with topography (Topography Supported Customized Ablation, TOSCA),7 and wavefront technology (WASCA). Its predecessor (MEL 60) was one of the first lasers used for topography-guided ablations.8 The laser comes with joystick for centering the ablation area and standard Moeller-Wedel operating microscope with free magnifications.

The MEL 70 G-Scan is not FDA approved at the time of writing this tutorial.

Schwind Esiris

The Schwind Esiris, the newest laser made by Schwind, is a scanning-spot laser with a 1-mm diameter beam. An active eye tracker of 300 Hz sampling is included. The energy level is monitored continuously by an integrated measuring system. The unit has a more compact design compared to its predecessor (Keratom Multiscan), because the body of the laser is located underneath the patient’s bed.

The system has wavefront- and topography-guided capabilities with the ORK-link (Optimized Refractive Keratectomy for customized ablation). The laser must be linked to specific wavefront and topography instruments suggested by the manufacturer. It comes with a Moeller-Wedel microscope and integrated joystick attached to the patient’s bed.

The Schwind Esiris device is not FDA-approved at the time of writing this tutorial.

WaveLight Allegretto

One of the more recent companies in the international laser market, WaveLight Laser Technologie AG offers the excimer laser system called Allegretto. This is a scanning-spot laser of 0.9-mm diameter. It is compact in size and self-calibrating. It has topography link capabilities as well as wavefront-aberrometry capabilities through the Dresden Wavefront Analyzer, which is based on the Tscherning aberrometer principle.

The system comes with a notebook computer for data analysis and storage. The system’s compact design is also ergonomical. It includes active eye-tracking and a Gaussian beam profile.

At the time of writing, the Allegretto is not FDA approved.

Conclusion

The laser systems described in this article are part of the latest technology in laser refractive surgery. Each has its advantages and drawbacks. The general trend among manufacturers has been to abandon broad beam laser delivery systems in favor of small spots that can be scanned across the surface. One goal of this technology is to link laser ablation to precise measurement of corneal surface height and aberration of the ophthalmic optics, so as to optimize the outcome of the procedure. Factors that may be considered by the surgeon when selecting a laser for generaluse, or use with a specific patient, include ergonomics, patient comfort, and the financial resources for acquisition. An important point to consider in this rapidly evolving technology is the possibility of upgrades. In the future, visual results will depend on the latest technology as well as the surgeon’s ability.

References

  1. McDonnell PJ. Emergence of refractive surgery. Arch Ophthalmol. 2000;118:1119-1120.

  2. Tamayo Fernandez GE, Serrano MG. Early climical experience using custom excimer laser ablations to treat irreqular astigmatism.J Cataract Refract Surg. 2000:26:1442-1450.

  3. Novick LH, Behrens A, Chao L, Chuck RS. VISX Star S2 excimer laser. In: Gimbel HV, Anderson Penno EE, eds. LASIK Complications, Prevention and Management, 2nd ed. 2001:38-40.

  4. Fraenkel GE, Webber SK, Sutton GL, Lawless MA, Rogers CM. Toric laser in situ keratomileusis for myopic astigmatism using an ablatable mask. J Refract Surg. 1999;15:111-117.

  5. McDonald MB. The Autonomous laser. In: Gimbel HV, Anderson Penno EE, eds. LASIK Complications, Prevention and Management, 2nd ed. 2001:41-45.

  6. King AQ. LaserScan LSX. In: Gimbel HV, Anderson Penno EE, eds. LASIK Complications, Prevention and Management, 2nd ed. 2001:47-49.

  7. Dausch D, Schroeder E, Dausch S. Topography-controlled excimer laser photorefractive keratectomy. J Refract Surg. 2000;16:13-22.

  8. Langenbucher A, Seitz B, Kus MM, van der Heyd G. Topographiegestützte Korrektur von Oberflächen-irregularitäten der Hornhaut mit dem Excimerlaser. Klin Monatsbl Augenheilkd. 1998;213:132-140.