Journal of Refractive Surgery

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Corneal Lathing Using the Excimer Laser and a Computer-controlled Positioning System

Robert Biowski, MD; Peter Homolka, PhD; Talin Barisani-Asenbauer, MD; Isabella Baumgartner, MD; Wolfgang Husinsky, PhD; Stephan Kaminski, MD; Anton Lametschwandtner, PhD; Wolfgang Muss, PhD; Günther Grabner, MD

Abstract

ABSTRACT

PURPOSE: To present the excimer laser corneal shaping system (ELCS-S), an add-on device to the Keratom, a commercially available 193-nm excimer laser built by Schwind.

METHODS: The system is designed for the preparation of donor corneas under sterile conditions using the ultraviolet laser to offer greatest possible flexibility. Lenticules for planolamellar grafting and refractive epikeratoplasty, as well as donor buttons for penetrating keratoplasty can be computer-designed by the surgeon or technician and lathed with the system.

RESULTS: Using the excimer laser corneal shaping system (ELCS-S) on human donor corneas, the central surface of the epikeratoplasty lenticule exhibited only narrow, flat concentric notches corresponding to the single lathing steps. Transmission electron microscopy revealed a damage zone of less than 0.3 µta in close approximation to the treated surface. The final thickness revealed a difference of less than ±53 µm from the intended, initially programmed value. Ultrastructural studies showed the perpendicular stromal surface of the penetrating keratoplasty buttons to be smooth with minimal protrusion of Descemet's membrane. Endothelial injury was observed in a zone averaging between 40 and 100 µta adjacent to the cutting edge only.

CONCLUSION: The excimer laser corneal shaping system (ELCS-S) allows a computer-controlled, surgeon-designed, sterile preparation of lamellar and penetrating corneal grafts with the use of the excimer laser. This could offer significant advantages in comparison to presently available systems for lamellar dissection and trephination. [J Refract Surg 2000;16:23-31]

Abstract

ABSTRACT

PURPOSE: To present the excimer laser corneal shaping system (ELCS-S), an add-on device to the Keratom, a commercially available 193-nm excimer laser built by Schwind.

METHODS: The system is designed for the preparation of donor corneas under sterile conditions using the ultraviolet laser to offer greatest possible flexibility. Lenticules for planolamellar grafting and refractive epikeratoplasty, as well as donor buttons for penetrating keratoplasty can be computer-designed by the surgeon or technician and lathed with the system.

RESULTS: Using the excimer laser corneal shaping system (ELCS-S) on human donor corneas, the central surface of the epikeratoplasty lenticule exhibited only narrow, flat concentric notches corresponding to the single lathing steps. Transmission electron microscopy revealed a damage zone of less than 0.3 µta in close approximation to the treated surface. The final thickness revealed a difference of less than ±53 µm from the intended, initially programmed value. Ultrastructural studies showed the perpendicular stromal surface of the penetrating keratoplasty buttons to be smooth with minimal protrusion of Descemet's membrane. Endothelial injury was observed in a zone averaging between 40 and 100 µta adjacent to the cutting edge only.

CONCLUSION: The excimer laser corneal shaping system (ELCS-S) allows a computer-controlled, surgeon-designed, sterile preparation of lamellar and penetrating corneal grafts with the use of the excimer laser. This could offer significant advantages in comparison to presently available systems for lamellar dissection and trephination. [J Refract Surg 2000;16:23-31]

For more than a decade, the ArF excimer laser has been used in clinical ophthalmology for photorefractive keratectomy (PRK), phototherapeutic keratectomy (PTK), and more complex techniques such as LASIK, and is well established in refractive surgery. Moreover, it has long been postulated that its precision might offer significant advantages over conventional mechanical methods for the shaping and processing of corneal tissue for transplantation in vivo and in vitro.1'4 In contrast to innumerable clinical reports about the excimer laser, papers about in vitro applications for this device have been comparatively sparse and the methods suggested have been quite variable.

In an animal study, Lieurance and colleagues1 transplanted nonfrozen human lamellar corneal grafts prepared with the excimer laser onto rabbit eyes, aiming at a final refractive change of power of +8.00 diopters (D). The donor tissue was placed in a concave mold and all of the corneal stroma surpassing the rim of the device was tangentially ablated. This animal study showed good clinical results, with clear graft tissue and lack of haze at the graft-host interface. In an identical manner, Gabay and coworkers2 used the excimer laser to cut piano corneal lenticules from fresh human corneal tissue for lamellar keratoplasty in humans. Both studies demonstrated the potential advantages of ablating the non-frozen tissue in a non-contact mode using an excimer laser at a wavelength of 193 nm over conventional mechanical and/or freezing techniques. With regard to the overall shape and thickness variability of the transplant, these two techniques are severely limited by the design of the mold(s).

For penetrating keratoplasty, Lang and Naumann3,4 used grafts cut with the help of circular and elliptical metal masks (with and without orientation teeth) that were placed in the laser beam. Again, overall variability in shape was restricted by the stencils provided.

Loertscher and Thompson5,6 used an axicon beam delivery system for cutting the cornea. This sophisticated conical lens system focuses the laser beam into a circle. The excimer laser used in their study required 90 seconds to trephine a circular button. Tamkivi7 mounted the cornea on a rotating device and performed the trephination with a focused laser beam without use of a slit mask. Their methods allowed free choice of the radius but had limited flexibility in overall shape design and cutting angle.

For several years, a collaborative effort joining the Second Department of Ophthalmology, University of Vienna, (G. Grabner and colleagues) and the Institute for General Physics, Technical University of Vienna, (W. Husinsky and colleagues) focused on the design of a system that would allow preparation of corneal transplants and lenticules of any shape, thickness, and cutting angle using an industrial ArF excimer laser (Lambda Physik). Experimental results were presented in 1988 using the experimental prototype designed for laboratory work.813 Based on this original prototype, a system (ELCS-System) was built by Herbert Schwind Gmbh &Co. KG (Kleinostheim, Germany) that is at present commercially available on request and can be attached to the Keratom excimer laser system that is widely used for phototherapeutic keratectomy (PTK), photorefractive keratectomy (PRK), and laser in situ keratomileusis (LASIK). The ELCSSystem described here is used at the eyebank of the Department of Ophthalmology, University of Vienna.

The authors postulate that, when compared to currently available systems for preparing corneal lenticules- such as the cryolathe, a variety of microkeratomes, the Draeger-rotorkeratome, and other mechanical trephines14- contact-free excimer laser ablation and shaping of donor corneal tissue has the potential to create grafts with higher precision, less tissue damage, and more flexibility in shaping the lenticule. The following potential advantages using a focused beam of an excimer laser for lathing the cornea are expected: 1 ) Variable cutting and lathing with no significant stromal deformation during the procedure. Freezing the donor cornea (on the cryolathe)- which leads to complete loss of stromal cells in the lenticule and to a distortion by volume increase that cannot be precisely taken into account- is avoided; 2) Better surface quality, as the ablation depth of one laser pulse is in the range of about 1 µm?- a precision that cannot be achieved with currently available mechanical means; 3) Maximum flexibility, as the computer-controlled positioning facilitates preparation of transplants with choice of any shape and a variety of cutting angles relative to the optical axis.

This paper describes the laser system for the preparation of donor corneas that is now commercially available and distributed by Schwind under the name, Schwind Excimer Laser Corneal Shaping System (ELCS-S) combined with the recently developed software package, Optimized Surface Laser Ablation (OSLA).

MATERIALS AND METHODS

Design of the ELCS-System

The ELCS-System, currently in a prolonged in vitro and clinical test period at the Eye Bank of the Department of Ophthalmology, University of Vienna, makes use of an industrial ArF excimer laser (LPX 205 SD) produced by Lambda Physik (Göttingen, Germany). The scanning-spot method that we designed 10 years ago makes use of small laser spots that ablate tissue in a perpendicular fashion and is supposed to remove defined volumes; thereby, a large-area ablation can be achieved by repeated scanning of the corneal surface using an algorithm and an optical and positioning system to achieve highest precision. Typical instrument parameters are a laser fluence between 0.5 and 1.5 J/cm2 in the ablation area and a repetition rate between 1 and 40 Hz that can be selected independently. As the initial beam of the excimer laser is rectangular (about 1x2 cm) and also slightly divergent, an optical cylindrical telescope consisting of two lenses near the outlet of the laser source has been added. This lens system delivers a parallel beam with a square cross section and a diameter of approximately 6 cm. An additional spherical telescope focuses the beam to roughly 2 cm in diameter. After this telescope, a slit mask with a total of 30 apertures varying from 0. 1 to 2 cm in diameter is positioned in the beam, then followed by a mirror through which an external video camera captures the image of the cornea and allows continuous observation of the ablation process on a video monitor. The resulting beam of parallel rays is then focused by two spherical lenses onto the ablation zone. The final cross section of the laser beam, in either circular or semicircular form, can be selected; both result in an ablation diameter between 0.1 to 2 mm in the working plane. The individual variation of the spot diameter is achieved by the slit mask that is positioned in the beam path. The experimental laser fluences of a series of semicircular laser beams in the working area are shown in the table. The numbers in the second column represent the corresponding diameter of the aperture at the slit mask. According to the base scale of 1:5, for example, aperture #10 would provide a diameter of 1 mm in the working area. With this new system (as compared to the prototype described previously)8,10, it is possible to automatically select the optimal spot size to achieve better surface quality and minimal ablation time.

Table

Figure 1. Overview of the excimer laser corneal shaping system (ELCS-S). Connected to the biggest part of the system - the excimer laser - is the optical system and the mounting device for the corneas. The user interface is next to the machine, and consists of a standard IBM PC, a computer scanner, and monitor for controlling the procedure.

Figure 1. Overview of the excimer laser corneal shaping system (ELCS-S). Connected to the biggest part of the system - the excimer laser - is the optical system and the mounting device for the corneas. The user interface is next to the machine, and consists of a standard IBM PC, a computer scanner, and monitor for controlling the procedure.

In addition to the optical system, the computercontrolled positioning system- which uses three high-precision elements (Newport, M-495-A Series, Ser. NO. 734, Fountain Valley, CA) for tilt, rotation, and translation of the donor cornea is a feature of the machine. The entire system (scanner, excimer laser, and moving elements) is controlled by one standard IBM PC also used for all calculations (Fig 1).

Figure 2. Device for lamellar and refractive transplants. First the cornea is is placed epithelial side down into the cup (center). It is then covered by the plate (right) and secured with a screw (left).Figure 3. Device for penetrating keratoplasty. The cornea has been placed endothelial side down in the device, resting on viscoelastic fluid, and secured with a screw.

Figure 2. Device for lamellar and refractive transplants. First the cornea is is placed epithelial side down into the cup (center). It is then covered by the plate (right) and secured with a screw (left).

Figure 3. Device for penetrating keratoplasty. The cornea has been placed endothelial side down in the device, resting on viscoelastic fluid, and secured with a screw.

Penetrating Keratoplasty

All working parameters, such as circular and elliptical shapes of arbitrary size and cutting angle (parallel to the optical axis, perpendicular to the front surface, or in different positive and negative inclinations and steps) are entered into the computer by the operator. The maximum values for the diameter of the transplant- in particular the elliptical shape- can be selected between nearly 0 and 12 mm. The cutting angle can be selected in a range of ±30° inclination from the optical axis. For lamellar keratoplasty and epikeratoplasty, calculations for the areal ablation are performed using a workstation.

Lamellar Keratoplasty

Both shape and thickness of the lenticule as well as width and profile of the peripheral "wing zone" can be selected according to a surgeon's requirements. For example, typical parameters for the preparation of a piano lamellar graft for keratoconus/epikeratoplasty would be a constant thickness of 300 µ?? over a central region with an optical diameter of 7 mm and a wing zone of 1 mm, where the thickness is continuously reduced to 150 µp?, resulting in a total transplant diameter of 9 mm.

Refractive Epikeratoplasty

Different refractive powers (both plus and minus) can be selected for the optical zone; the wing zone can be chosen as described above. The limiting values for the refractive power are the parameters of the donor cornea and the diameter of the optical zone. For example, when selecting a diameter of 7 mm, the value for the theoretically possible refractive power will be in the range of about ±20.00 D.15,16

With this system the laser fluence is intentionally set at a constant level considerably higher (0.5 to 1.5 J/cmp 2) than the one widely used for ablation in vivo, be it for PTK, PRK, or LASIK (about 0.2 J/cmp 2). This enables a constant ablation rate because of plateauing in corneal ablation rate that occurs in a fluence range between 0.5 and 1.5 J/cm2 and results in a constant ablation depth of 1.5 µm. per pulse, as previously demonstrated. Therefore, slight, unavoidable fluctuations in laser fluence do not significantly change the total quantity of tissue ablated per pulse in this fluence range.17-22

Ablation rate is not only dependent on laser fluence but is also influenced by water content of the corneal stroma. Tissue that has been stored for a longer period (>12 hours) in nutrient solution shows a higher ablation rate than corneas of lower thickness and lower hydration. Therefore, prior to preparation, donor corneas must be "deswollen" in special tissue culture media, as published.23,24

For mounting donor corneas, two devices have been developed: 1) For epikeratoplasty or lamellar keratoplasty, the donor cornea is placed epithelial side down into a cup with a constant radius of curvature (Fig 2). The radius chosen is 7.5 mm, somewhat smaller than the average radius of curvature of the anterior corneal surface, to avoid aspiration of air under the graft during preparation; this would lead to unpredictable shrinkage of the tissue; 2) For penetrating keratoplasty, the donor cornea is mounted epithelial side up (Fig 3). The endothelium rests on a cushion of viscoelastic, Healonid GV (Sodium hyaluronate, Kabi Pharmacia, Bad Homburg, Germany) during the cutting procedure. This provides protection from mechanical damage and ensures stable positioning during preparation.

Figure 4. Design of the shape of a peripheral corneal transplant for a limbal dermoid by means of digital image processing.Figure 5. Principle of areal ablation of the cornea with circular overlapping laser spots, as used by Optimized Scanning Laser Ablation (OSLA). Two ablation rings are partially shown.

Figure 4. Design of the shape of a peripheral corneal transplant for a limbal dermoid by means of digital image processing.

Figure 5. Principle of areal ablation of the cornea with circular overlapping laser spots, as used by Optimized Scanning Laser Ablation (OSLA). Two ablation rings are partially shown.

The most important data set needed for calculations and computer-controlled positioning is corneal thickness in different areas- currently measured with an ultrasonic pachymeter (Omega, Storz, Heidelberg, Germany) at a minimum of nine different points (1 central, 8 at a radius of 4 mm). The 8 peripheral measurements are then averaged. The inner radius of curvature is calculated using the resulting thickness data.

The user-friendly computer software facilitates quick entry of the different parameters, such as the diameter of the optical zone, width of the wing zone, planned refractive power, central and minimal thickness of the lamellar graft, as well as overall shape.

Entry of the individual contour, needed for pterygium surgery or repair of peripheral corneal thinning disorders, is achieved by using a scanned image of the patient's cornea. By means of digital image processing the optimized shape of the transplant is "designed" on the monitor (Fig 4) by either the surgeon or the technician.

Preparation of a lenticule starts in the center of the cornea from the endothelial side using circular laser spots. During this first stage, the ablation in the central area is performed to the ultimate depth. Up to that point, calculations are straightforward, eg, to remove 400 µm of corneal tissue a total of 267 shots (400 divided by the ablation rate) are needed. The cornea is then laterally translated, rotated, and tilted under the spacially fixed laser beam. Large area ablation is then achieved by overlapping laser spots corresponding to concentric orbits of ascending radii (Fig 5). In recent years, the complex OSLA algorithm (Optimized Scanning Laser Ablation) was developed; it calculates the suitable repetition rate for the excimer laser at the highest velocity of the rotation element (10° per second), moving the cornea whenever possible.25 The step width (radial distance between adjacent ablation rings) is optimized with respect to achieving a short preparation time and best possible surface quality. The diameter of the circular laser spot is always set at 1 mm. Depending on the parameters selected, preparation requires between 30 and 60 minutes during which time the cornea is moistened from the side by the technician at regular intervals.

For penetrating keratoplasty, complete trephination ranges between 5 and 15 minutes depending on the corneal thickness at the radius of the working area and the preselected cutting angle.

Histological Studies

After lathing, corneas were fixed by immersion in phosphate buffered (0.1 M) 4% glutaraldehyde, washed in 0.13 M phosphate buffer (four times), postfixed with 1% osmiumtetroxid (in Millonigs buffer), washed in 0.1 M phosphate buffer, dehydrated in a graded series of ethanol, critical point dried via carbon dioxide, mounted onto specimen stubs, vapor coated with carbon and gold, sputtered with gold, and examined in the scanning electron microscope (Stereoscan 250; Cambridge Ltd, Cambridge, UK) with an accelerating voltage of 20 kV. Photographic documentation was made with Agfapan APX 25 (Agfa, Germany).

Figure 6. Margin (M) of the ablated cornea. Note the loss of endothelial celts (e) close to the margin. In these areas, underlying Descemets membrane (DM) is exposed.Figure 7. Endothelium stained with alizarin red and trypan blue. On the lower left side, the colored beveled cut margin supersedes the display of the endothelial cells on the photograph. The boundary of the endothelium is marked by the red line.Figure 8. Lamellar graft, 150-pm thick. The sample has been cut centrally to allow better evaluation of its thickness (scanning electron microscopy).Figure 9. Detailed view of Figure 8. View of the central cut shows small irregularities resulting from scanning ablation. Right: toward the periphery of the cornea; Left: toward the center of the cornea (scanning electron microscopy).

Figure 6. Margin (M) of the ablated cornea. Note the loss of endothelial celts (e) close to the margin. In these areas, underlying Descemets membrane (DM) is exposed.

Figure 7. Endothelium stained with alizarin red and trypan blue. On the lower left side, the colored beveled cut margin supersedes the display of the endothelial cells on the photograph. The boundary of the endothelium is marked by the red line.

Figure 8. Lamellar graft, 150-pm thick. The sample has been cut centrally to allow better evaluation of its thickness (scanning electron microscopy).

Figure 9. Detailed view of Figure 8. View of the central cut shows small irregularities resulting from scanning ablation. Right: toward the periphery of the cornea; Left: toward the center of the cornea (scanning electron microscopy).

RESULTS

Previous in vitro experiments using ultrasonic pachymetry, light microscopy, confocal microscopy, videokeratoscopy, and scanning electron microscopy showed good quality of the ablated surface.26

Penetrating Corneal Grafts

The scanning electron microscopy study of the vertical face of the penetrating cut showed a smooth surface. However, loss of endothelial cells in a zone averaging between 40 and 100 µt? from the graft margin was seen, leaving a bare Descemet's membrane. Figure 6 shows that the characteristic narrow junctions of the endothelium cannot be distinguished. Whether this observation is a result of fixation or a side effect of the excimer laser or the positioning method remains to be determined. Ultrastructural studies of the surface of the incision showed a smooth stromal cut and minimal protrusion of Descemet's membrane.27

Using trypan blue and alizarin red staining, loss of the endothelial cells was observed in an average zone between 20 and 30 µt? adjacent to the incision (Fig 7).

Further ongoing investigations (using a larger number of corneas) will lead to more detailed results and hopefully explain the discrepancy between the two results mentioned. Even with a damage zone of up to 100 /un, this would favorably compare with that observed using mechanical methods.28'31

Lamellar Corneal Grafts

Piano Lamellar Grafts- Analysis by scanning electron microscopy (Figs 8, 9) showed that the resulting corneal transplant had concentric notches that correspond to the lathing steps that start close to the central area. The thickness of the transplant (150 ± 10 µta) was constant over the whole diameter and the dimensions of the steps were homogeneous over the whole zone. Figure 10 shows the theoretically calculated surface quality as designed by the workstation using optimized scanning laser ablation (OSLA). Higher magnification revealed a stromal damage zone of less than 0.3 ¿m, which leaves the keratocytes viable in the lenticule in close approximation to the treated surface.2,32-34

Figure 10. Ablation depth and calculated profile vs. radius of a graft corresponding to Figs 8 and 9.Figure 1 1 . Videokeratograph (TMS) (sample slightly off center) of a refractive lenticule (-10.00 D with optical zone 7 mm and overall diameter 9 mm).Figure 12. Central cut of a lenticule. Optical zone has a refractive power of +10.00 D. The peripheral wing zone has a constant thickness.Figure 13. Diagram of the refractive power that results from changes in central thickness (5-mm optical zone, 7.8-mm radius of curvature, and 500-Mm central corneal thickness of the recipient).

Figure 10. Ablation depth and calculated profile vs. radius of a graft corresponding to Figs 8 and 9.

Figure 1 1 . Videokeratograph (TMS) (sample slightly off center) of a refractive lenticule (-10.00 D with optical zone 7 mm and overall diameter 9 mm).

Figure 12. Central cut of a lenticule. Optical zone has a refractive power of +10.00 D. The peripheral wing zone has a constant thickness.

Figure 13. Diagram of the refractive power that results from changes in central thickness (5-mm optical zone, 7.8-mm radius of curvature, and 500-Mm central corneal thickness of the recipient).

Lenticules for Refractive Epikeratoplasty- For evaluation with a videokeratoscope (TMS-I, Tomey, Erlangen, BRD), lenticules with selected refractive powers were placed on a blackened steel ball with a defined radius of 7.8 mm. This allowed for evaluation of the anterior curvature of the refractive lenticules. First results showed that the intended change of refractive power of -5.00 and -10.00 D, respectively, could be achieved with a standard deviation of ±3.00 D (Fig H).35 Using ultrasound pachymetry, the final thickness revealed a variation of less than ±53 µta from the intended value. Central islands, as observed in PRK, did not occur (Fig 12).

Recent examinations using in vitro laser scanning microscopy showed that the required precision is now within a range of ±10 µm. Therefore, mathematically, the deviation from the resulting refractive power should be smaller than ±1.50 D (Fig 13).

DISCUSSION

The use of an excimer laser in the ELCS-System for the preparation of grafts facilitates minimal damage of corneal tissue, compared to other methods presently available. It is well established that the ablation process is primarily achieved via a photochemical pathway. This new commercial device therefore combines these advantages, clinically proven in PRK/PTK and LASIK, with a high flexibility that can only be achieved by using a small laser spot with a high laser fluence.1,2,5'36"40 Although this assures more flexibility in the choice of desired parameters, the current drawback remains the time range needed for the preparation. Apparently other systems such as the axicon (to our knowledge not clinically tested) required only a few seconds.5,6 The ELCS-System, combined with OSLA, can prepare lenticules with a regularity approaching that of contact lenses with a choice of refractive power. We think this precision can only be achieved using small spot ablation and is therefore theoretically superior to other systems36 that make use of larger laser spots.

Any desired contour of the donor button for penetrating keratoplasty can be designed in a computercontrolled way, thus achieving precision and symmetry or asymmetry. Grafts can be circular or elliptical, and- if required by the surgeon- have a varying number of orientation teeth. In comparison to the slit mask system4, the choice of diameters is limited only by the size of the donor cornea. Local heating effects, as induced by metal masks, can also be avoided.

In the case of lamellar keratoplasty, the flexibility of the ELCS-System was also demonstrated. The operator has a choice of thickness, and higher precision can be achieved than with a mold-tissue interface-fixing the cornea by suction- whereby the laser beam interacts with the cornea from the side.2 To achieve such accuracy the ELCS-System uses a complex algorithm that uses both the outer and inner radius of curvature for calculations. In addition, this system is not restricted by the diameter of a preformed mold and therefore the diameter of the transplant and the overall contour can be chosen as individually required.

The ELCS-system, although complex to handle, is theoretically able to give results superior to conventional mechanical devices such as the cryolathe, the microkeratome, or the rotorkeratome41 under clinical conditions, as it allows preparation of grafts without mechanical contact. It also may provide higher precision with real-time pachymetry and additionally offers the option to prepare individually shaped transplants for new surgical techniques.42

For penetrating keratoplasty transplants, a smaller endothelial damage zone can be facilitated.

With the Schwind ELCS-System, some of the problems with mechanical preparation of donor corneas and lenticules can be solved, with the option that a few specialized centers might offer their services to produce specially shaped transplants according to individual designs and needs of corneal surgeons worldwide. Ongoing investigations using real time non-contact Doppler laser interferometry will attempt to achieve better precision for refractive epikeratoplasty.43·44 Early clinical results of the ELCS-System have been reported.45

REFERENCES

1. Lieurance RC, Patel AC, Lee Wan W, Beatty RF, Kash RL, Schanzlin DJ. Excimer laser cut lenticules for epikeratophakia. Am J Ophthalmol 1987;103:475-476.

2. Gabay S, Slomovic A, Jares T. Excimer laser-processed donor corneal lenticules for lamellar keratoplasty. Am J Ophthalmol 1989;107:47-51.

3. Lang G, Koch J, Schröder E, Yanoff M, Naumann G. Korneale Schnittkonfigurationen mit dem Excimer Laser: Eine experimentelle Studie. Fortschr Ophthalmol 1989;86:437-442.

4. Naumann G, Seitz B, Lang G, Langenbucher A, Kus M. Excimer-Laser-193 nm Trepanation bei der perforierenden Keratoplastik. Bericht über die ersten 70 Patienten. Klin Monatsbl Augenheilkd 1993;203:252-261.

5. Loertscher H, Mandelbaum S, Parel JM, Parrish RK. Noncontact trephination of the cornea using a pulsed hydrogen fluoride laser. Am J Ophthalmol 1987;104:471-475.

6. Thompson KP, Barraquer E, Parel JM, Loertscher H, Pflugfelder S, Roussel T, Holland S, Hanna K. Potential use of lasers for penetrating keratoplasty. J Cataract Refract Surg 1989;15:397-403.

7. Tamkivi R, Schotter L, Pakhomova TA. Excimer Laser cutting of corneal transplants. Excimer Lasers and Applications III. 1991:375-378.

8. Altmann J, Grabner G, Husinsky W, Mitterer S, Baumgartner I, Skorpik F, Asenbauer T. Corneal lathing using the excimer laser and a computer-controlled positioning system: Part I-lathing of epikeratoplasty lenticules. Refract Corneal Surg 1991;7:377-384.

9. Husinsky W, Grabner G, Mitterer S, Altmann J, Baumgartner I. Die Herstellung von hochpräzisen Transplantaten für die lamellierende Keratoplastik und von Epikeratophakie-Lentikeln mit Hilfe des Excimer-Lasers bei 193 nm und eines computergesteuerten Positioniersystems. Spektrum Augenheilkd 1992;6:176-179.

10. Husinsky W, Mitterer S, Altmann J, Grabner G, Baumgartner I, Skorpik F, Asenbauer T. Corneal lathing using the excimer laser and a computer-controlled positioning system: Part II-Variable trephination of corneal buttons. Refract Corneal Surg 1991;7:385-389.

11. Mitterer S. Excimer-Laser Corneal Shaping System Grundlagen, Realisierung und erste klinische Anwendung. Dissertation TU Wien. 1993.

12. Mitterer S, Grabner G, Husinsky W, Altmann J, Baumgartner I. Über die Präparation von Hornhauttransplantaten und Epikeratophakie-Lentikeln mittels eines computerisierten Excimer-Laser-Systems bei 193 nm. 5. Kongreß der Deutschsprachigen Gesellschaft für Intraokularlinsen Implantation. (Hrsg. Wenzel M., Reim M., Freyler H., Hartmann Ch.). Heidelberg, Germany: Springer Verlag; 1991:63-71.

13. Nagel G. In vitro Untersuchung von lameliierenden Hornhauttransplantaten geschnitten am Excimer Laser Corneal Shaping System. Dissertation, Universität Wien. 1992.

14. van Rij G, Waring GO HI, Configuration of corneal trephine openings using five différent trephines m human donor eyes. Arch Ophthalmol 1988;106:1228-1233.

15. Arffa C, Barron A, McDonald M. Optics of lamellar refractive keratoplasty. CUn Ophth 1988;64:1-9.

16. Aron-Rosa D. Lasers for refractive surgery. South Med J 1994;87:1281-1282.

17. Aron-Rosa D, Gross M, Maden A, Ramirez S, Timsit J. Analyse quantitative des excisions corneennes au laser a excimere argon fluor (193 nanometres). Bull Soc Opht 1989;8-9:1051-1055.

18. Campos M, Wang XW, Hertzog L, Lee M, Clapham T, Trokel SL, McDonnell PJ. Ablation rate and surface ultrastructure of 193 nm excimer laser keratectomies. Invest Ophthalmol Vis Sci 1993;34:2493-2500.

19. Dougherty P, Wellish K, Maloney R. Excimer laser ablation rate and corneal hydration. Am J Ophthalmol 1994;118: 169-176.

20. Huebscher HJ, Genth U, Seiler T. Determination of excimer laser ablation rate of the human cornea using in vivo Scheimpflug videography. Invest Ophthalmol Vis Sci 1996;37:42-46.

21. Kubota T, Seitz B, Tetsumoto K, Naumann G. Lamellar excimer laser keratoplasty: reproducible photoablation of corneal tissue. A laboratory study. Doc Ophthalmol 1992;82:193-200.

22. Puliafito CA, Wong K, Steinert RF. Quantitative and ultrastructural studies of excimer laser ablation of the cornea at 193 and 248 nanometers. Lasers Surg Med 1987;7:155-159.

23. Husinsky W, Grabner G, Baumgartner I, Skorpik F, Mitterer S, Timmel T. Mechanisms of laser ablation of biological tissue. Springer Series in Surface Sciences. 1990;19:362-367.

24. Kautek W, Mitterer S, Krüger J, Husinsky W, Grabner G. Femtosecond-pulse laser ablation of human corneas. Appi Phys 1994^58:513-518.

25. Homolka P, Biowski R, Kaminski S, Barisani T, Husinsky W, Bergmann H, Grabner G. Laser shaping of corneal transplants in vitro: area ablation with small overlapping laser spots produced by a pulsed scanning laser beam using an optimized ablation algorithm. Phys Med Biol 1999;44: 1169-1180.

26. Campos M, Trokel SL, McDonnell PJ. Surface morphology following photorefractive keratectomy. Ophthalmic Surg 1993;24:822-825.

27. Serdarevic ON, Hanna K, Gribomont AC, Savoldelli M, Renard G, Pouliquen Y. Excimer laser trephination in penetrating keratoplasty, morphologic features and wound healing Ophthalmology 1988;95:493-505.

28. Bull H, Deutschmann S, Schlote HW. Doppelt geführtes Vakuumtrepansystem Asmotom. Fortschr Ophthalmol 1991;88:574-576.

29. Clinch TE, Fung KL, Laibson PL. Corneal endothelial cell loss following trephination. Ophthalmie Surg 1988;19: 703-705.

30. Koch J, Lang G, Naumann G. Endothelial reaction to perforating and non-perforating excimer laser excisions in rabbits. Refract Corneal Surg 1991;7:214-222.

31. Sanchez-Thorin JC, Rocha G, Bowyet BL, Balisky L, Duplessie MD, Stevens SX, Rowsey JJ. The tampa trephine technique with cat corneal endothelium. Cornea 1997;16: 79-87.

32. Beuerman R, McDonald M, Shofner R, Munnerlyn C, Clapham T, Salmerón B, Kaufmann H. Quantitative histological studies of primate corneas after excimer laser photorefractive keratectomy. Arch Ophthalmol 1994; 112: 1103-1110.

33. Marshall J, Trokel S, Rothery S, Schubert H. An ultrastructural study of corneal incisions induced by an excimer laser at 193 nm. Ophthalmology 1985;92:749-758.

34. Stern D, Schoenlein RW, Puliafito CA1 Dobi ET, Birngruber R, Fujimoto JG. Corneal ablation by nanosecond, picosecond and femtosecond lasers at 532 and 625 nm. Arch Ophthalmol 1989;107:587-592.

35. Boehnke M, Grabner G, Asenbauer T, Baumgartner I1 Draeger J. In-vitro-Videokeratoskopie zur Vermessung refraktiver Hornhautlentikel, präpariert aus organkultiviertem Gewebe. 5. Deutschsprachige Gesellschaft für Intraokularlinsenimplantation. (Hrsg.M.Wenzel.N.Reim, H.Freyler, C.Hartmann). Heidelberg, Germany: Springer Verlag; 1991:78-82.

36. Ganem S, Aron-Rosa D, Gross M, Rosolen S. Myopie keratomileusis by excimer laser on a lathe. J Refract Corneal Surg 1994;10:575-581.

37. Husinsky W, Mitterer S, Grabner G, Baumgartner I. Photoablation by UV and visible laser radiation of native and doped biological tissue. Appi Phys 1989;B49:463-467.

38. Krueger RR, Trokel SL, Schubert HD. Interaction of ultraviolet laser light with cornea. Invest Ophthalmol Vis Sci 1985;26:1455-1464.

39. Loertscher H, Parel JM, Parrish T, Mandelbaum S. Laser trephination of the cornea. In: Marshall J, ed. Laser Technology in Ophthalmology. Amsterdam, the Netherlands: Kugler and Ghedini; 1988:195-204.

40. Srinivasan R. Ablation of polymers and biological tissue by ultraviolet lasers. Science 1986;234:559-565.

41. Draeger J, Boehnke M, Klein L, Kohlhaas M. Experimentelle Untersuchungen zur Präzision iamellärer Hornhautschnitte. Fortschr Ophthalmol 1989;86:272-275.

42. Grabner G, Baumgartner I, Husinsky W. Über die Zukunft der refraktiven Hornhautchirurgie mit dem Laser. Spektrum Augenheilkd 1989;3-6:239-244.

43. Hitzenberger C, Drexler W, Fercher A. Measurement of corneal thickness by laser Doppler interferometry. Invest Ophthalmol Vis Sci 1992;33:98-103.

44. Hitzenberger C1 Baumgartner A, Drexler W, Fercher A. Interferometric measurement of corneal thickness with micrometer precision. Am J Ophthalmol 1994;118:468-476.

45. Biowski RK, Homolka P, Simader Ch, Barisani T, Baumgartner I, Kaminski St, Grabner G. Lathing corneal transplants using the upgraded ELCS-system employing advanced measurement techniques. Invest Ophthalmol Vis Sci 1998;39(auppl):S72.

Figure 1. Overview of the excimer laser corneal shaping system (ELCS-S). Connected to the biggest part of the system - the excimer laser - is the optical system and the mounting device for the corneas. The user interface is next to the machine, and consists of a standard IBM PC, a computer scanner, and monitor for controlling the procedure.

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