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Medical Laser Application 25 (2010) 250–257www.elsevier.de/mla

pplication principles of excimer lasers in ophthalmologyrs Vossmerbaeumer∗

epartment of Ophthalmology, University of Mainz Medical Center, Langenbeckstr. 1, 55131 Mainz, Germany

eceived 21 June 2010; accepted 6 August 2010

bstract

Excimer lasers (193 nm) can be used for photoablation of the human cornea due to their specific physical characteristics. This

aser is applied in corneal refractive surgery for the targeted removal of corneal tissue for the purpose of correcting refractiverrors. This article provides a short overview of the scientific development of excimer lasers for use in ophthalmic surgery andf the range of applications of these tools.

2010 Published by Elsevier GmbH.

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eywords: Excimer laser; Photoablation; PRK; LASIK; LASEK; R

ackground

The definition of ametropia as a disease or simply as anncongruence with a presumed optimal anatomy of the humanye has been a matter of discussion, both medically and philo-ophically, for a long time. As a consequence it is debatablehether surgical correction of ametropia is purely functionalr should be considered as cosmetic surgery.

The goal of refractive surgery is to remodel the eye’sicroanatomy in such a way that the optical properties will be

ligned to the ideal focus. A successful procedure will elim-nate optical errors and imperfections and enable the patiento enjoy optimal vision without the need for further externalupport devices.

The principle of any surgical approach to move the focusoint of the eye exactly into the fovea is to alter the refrac-ive power of either the cornea or the lens which may beoo high (myopia) or too low (hyperopia) or too inconsis-

ent (astigmatism), depending on the nature of the refractiverror. Refractive surgery using the excimer laser, also termeds laser vision correction (LVC) is a surgical intervention that

∗Tel.: +49 6131 174061; fax: +49 6131 176620.E-mail address: urs.vossmerbaeumer@unimedizin-mainz.de.

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akes place on the frontal transparent part of the human eye,he corneal surface and within the corneal stroma.

The concept of remodeling the corneal surface for refrac-ive purposes by lamellar ablation was first applied byarraquer [1] who performed early experimental proce-ures by removing or adding thin slices of corneal tissue tometropic eyes. However, the quality of the results was veryimited due to the instruments used. A number of years wereo elapse before instruments matching the concept becamevailable and these were excimer lasers.

he excimer laser

In 1970, Nikolai Basov and co-workers developed the firstxcimer laser at the Lebedev Physical Institute in Moscow. Aecade later, observation of the high sensitivity of the cornealpithelium to 193 nm laser pulses emitted by an excimer lasery Taboada and Archibald [2] determined the direction these would take in tissue ablation of the cornea. Trokel et al.3] were the first to explore this potential and transfer the

ethod to clinical application.The excimer laser derives its name from an artificial combi-

ation of two descriptive terms that characterize the principaleatures of the laser, “exited dimer”. Dimer refers to the

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edium of the laser source which is in this case a halogennd a noble gas which are mixed in the cavity of the laserource. The wavelength of the emitted light is determined byhe specific combination of gases. Ophthalmic excimer lasersse argon and fluoride as laser media, emitting photons in thear UV range at 193 nm. The corneal tissue has an absorptionaximum at this wavelength.The basic layout of excimer lasers used today still follow

he original principle as was given in the patent documentsrom 1981 (patent granted in 1986) co-authored by Blum etl. [4]. The invention however was not primarily describedor an ophthalmic application but the authors suggested itsse for removal of tissue from bones and for drilling teethn dentistry. The principal idea of the surgical potential ofxcimer lasers was laid down in a landmark paper publishedn 1983 [5] describing the excimer laser etching of bovineorneal tissue. This paper played a pivotal role in inspiringphthalmologists to develop procedures to be coined later ashotorefractive keratectomy (PRK) and laser-assisted in situeratomileusis (LASIK). As early as 1987, the first PRK waserformed in a human eye in vivo [6–8]. Until now, over 20illion people worldwide have undergone such treatments.

hysical basics

Laser effects on biological tissue can be grouped intohree major categories: thermal, ionizing and photochemi-al. Excimer lasers affect changes in tissue by virtue of theirhotochemical interaction. When pulsed ultraviolet light, atavelengths below 350 nm, is applied in the nanosecond (ns)

ange it causes breaks in the intermolecular bonds of theolymer chains of corneal collagens, disintegrating the tar-et tissue. This process is called photoablation. The resultingmall volatile fragments are propelled at high speed above theissue surface, where they can be evacuated. This process doesot involve a significant increase in the tissue temperatures the diffusion time for heat conduction into the surround-ng tissue is much longer than the millisecond (ms) rangeulse duration. The energy of the laser light is almost entirelybsorbed at the surface of the treated tissue without producingelevant collateral effects within the stroma.

The depth of ablation depends both on the excimer laseravelength and the energy density of the pulse [9]. The

bsorption maximum of the cornea at 193 nm makes the usef the excimer laser particularly suitable for corneal surgery.rradiance at 193 nm leaves the immediate vicinity of theblated stroma unchanged. The wavelength is crucial as the93 nm pattern ablates creating smooth edges whereas a com-arable 249 nm pulse pattern leaves jagged tissue edges [5].ith increasing wavelengths the thermal effects and collat-

ral damage of an excimer laser pulse increase. Ablation

tarts at irradiance >50 mJ/cm2, with the ablation thresholdeing independent of the laser firing rate. For energy levelsf 200–2000 mJ/cm2/pulse, the ablation depth ranges for a93 nm wavelength from approximately 0.25–1.2 �m/pulse

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pplication 25 (2010) 250–257 251

9]. For comparison, a human hair has an average diame-er of 50 �m and an excimer laser pulse would be capablef removing only 0.5% of its thickness. The amount of tis-ue that is ablated per applied unit of laser pulse energyablation efficiency) has a maximum for a laser fluence of80–600 mJ/cm2 with an absolute maximum at 440 mJ/cm2

10].

echnical aspects

Excimer lasers for refractive surgery can be subdividednto the two major categories, broad beam and scanning lasers11,12].

Broad beam lasers operate a beam with a large totaldiameter of approximately 6–8 mm, which is deformedappropriately during ablation. The “raw laser beam” isguided through an optical system consisting of a seriesof lenses and mirrors to ensure a square cross section ofthe homogenized beam at the level of treatment. Earliermachines of this type had an increased risk of leavingcentral islands of the cornea sub totally treated when theplume from ejected particles would shade the tissue. Thelarge area of simultaneously treated tissue results in shortprocedure times.Scanning lasers apply a beam of smaller diameter to thecorneal tissue which consequently has to be scanned acrossthe ablation zone. The size of the laser spot is modulatedby rotational mask devices with slit holes of variable size,the laser spot being scanned across the apertures duringthe procedure. The ablated surface tends to be smootherwith this technique than with broad beam lasers.Spot scanning lasers operate a pixel-by-pixel computerizedcontrol of the scanning process across the cornea whichallows for a high degree of precision and versatility fortheoretically any possible pattern of the corneal surface. Inthe earlier days of excimer laser refractive surgery, thesedevices had a longer treatment time compared to broadbeam lasers. However with current high frequency laserpulse application, this is no more relevant.

Nowadays, excimer laser surgery is to a great extent roboticurgery, i.e. the individual photoablation pattern is calculatedy software, and the laser beam is being guided by digitalontrol units. Usually, excimer laser devices contain the fol-owing components: reservoirs for argon and fluoride gases,he power source and the laser cavity as the “heart” of the

achine, optical pathways to form and steer the beam, ahutter and the beam delivery unit, aiming laser systems, aurgical microscope and computer systems for digital con-rol of the machine and the procedures. Typically, the lasertself is combined with a specially designed patient bed that

ot only supports the patient during the procedure but alsollows highly precise three-dimensional micro-movements tonsure optimized positioning of the eye with respect to theaser.

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orneal refractive surgery

io-physiological aspects

The human cornea is composed of five distinguishableayers, each one with very particular functions and needs13]. Physiologically, of the total 59 diopters (dpt) of refrac-ive power of the ideal human eye, the cornea accounts forbout 75%, i.e. approximately 43 dpt, the lens contributinghe remaining 16 dpt. The corneal refractive power is dis-ributed between the anterior and the posterior surface. Thenterior surface with the tear film, representing the greatestifference of refractive indices between air and water/cornea,lays a major role for light travelling into the eye. Physiologi-ally, the corneal curvature follows an aspherical design withprolate anterior surface. Spherical aberrations are henceinimized by nature and ideally few higher order aberra-

ions exist in the native cornea. Astigmatism of the eye resultsostly from differing corneal curvature radii in the main axes.As with any curved optical surface, the radius of corneal

urvature is the one major determining factor of the refractiveower of the structure. The refractive index gradient is highestt the anterior surface where light rays travelling throughtmosphere meet the tear film and the cornea; alterations ofhe curvature at this level are most efficient.

ndications

Myopia correction: Myopia results when incoming lightis focused by cornea and lens to a point in front of theretina. Hence, the refractive power needs to be diminishedto create a sharp image on the retina by shifting the focusinto the fovea. Excimer laser surgery for the correction ofmyopia has the task to flatten the anterior corneal curva-ture by photoablation of a tissue lenticule. For practicalpurposes, such ablation should not include the entire sur-face of the corneal dome but is limited to a central areadesignated as the optical zone. The true optical zone istypically surrounded by a transition zone where the abla-tion pattern fades out into the virgin cornea. The necessarysize of the optical zone is individually determined mostlyby the diameter of the mesopic pupil of the patient. A min-imum overlap of the treatment zone with the pupil marginis desirable. The earlier belief that entry of light rays frombeyond the treated zone would cause optical disturbancessuch as halos and glare for the patient has been replacedby a better understanding of the optical properties of thetreated cornea. It seems that an increase in optical aber-rations towards the periphery plays a major role, causingvisual disturbances particularly for night vision. Of course,the posterior surface of the cornea remains unaltered by

photoablation procedures.Hyperopia correction: By the same logic, in order to treathyperopia the anterior corneal surface must be steepenedin order to increase the refractive power of the eye. This

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pplication 25 (2010) 250–257

can be achieved for the center of the corneal “dome” bycircular tissue ablation in the periphery. Incoming lightrays are then stronger refracted when meeting the steeperflanks of the dome. This shifts the focus of the eye from avirtual point behind the retina into the fovea, again creatingthe sensation of a sharp image for the treated patient.Astigmatism correction: Astigmatism correction followsthe same basic principles by altering the shape of the ante-rior corneal surface over a limited treatment zone. In thiscase differences of corneal curvature radii should be equal-ized.

reatment risks

One of the major challenges with the development ofhe above mentioned procedures lies in the risk of inducingpherical aberration in the cornea that can severely impedehe quality of vision for the patient. Naturally, the corneaas a prolate aspheric shape resulting from the evolutiono minimize spherical aberration. By decreasing the cen-ral anterior curvature, this profile may be reversed into anblate shape. Original ablation profiles were based on theo-etical calculations for spherocylindrical ablation. The mostidely accepted formula was proposed by Munnerlyn et al.

14,15]:

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(in diopters) to be modified and the diameter d of theaser-ablation zone. Older treatment algorithms induce spher-cal aberration; they ablate less efficiently in the peripheryhereby worsening the preexisting physiological sphericalberration. Peripheral shots strike obliquely, are more oval,emove less tissue, and contribute to the oblate shape in stan-ard treatments. Formulas were later consecutively modifiednd optimized through experimental iteration, also termed asomogram adjustment, to achieve the correct effect on thecular defocus. However, ablation along these profiles maynduce higher order aberrations (HOA), mainly fourth-orderpherical aberrations and third-order coma. The patient mayventually perceive this as disturbances in vision under dimight and night conditions such as the glare or halo phenom-na.

nfluencing factors

The definition of the amount of tissue ablated by a sin-le laser pulse depends on a variety of factors, which ift is not properly taken into account, will negatively influ-

nce the refractive effect. Ideally the effective laser fluencen the corneal surface should be known and calculated forvery single spot to optimize predictability of the surgicalutcome.

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Major factors influencing photoablation of the cornea are:

Plume evacuation: From the output optics the laser lighttravels a certain distance through the atmosphere beforereaching the ocular surface. It is obvious that the atmo-spheric conditions in this path will affect the laser fluenceon the target. Not only the humidity but also the eventualamounts of ablated molecules play a role. As a result notonly close control of the climate in the operating theatre isrequired but also effective evacuation mechanisms for the“smoke plume” arising during ablation.Projection correction: Scanning a laser spot (Ø �m range)over a curved surface, results in a variable spot size,depending on the localization of the projection spot. Fol-lowing the laws of geometry, only exact perpendicularprojection gives a strictly circular spot, with image distor-tion to an increasingly elliptical shape of the beam “end”towards the periphery. A larger area of illumination meansa smaller amount of laser fluence per area unit, resultingin less tissue ablation. Geometric modeling shows that thisdrop in laser fluence, with increasing distance from the cen-ter, amounts to a substantial 15% at 4 mm for a simulatedcurved surface.Reflection correction: The smoothness of the flap bed andthe water content of the corneal stroma affect the reflectiv-ity of the exposed surface. This is even more so the case forBowman’s membrane when it is laid bare after removal ofthe epithelium for surface ablation procedures. In the sameway as visible light, UV light from the excimer laser is alsopartially (approximately 2.5%) reflected, instead of beingabsorbed by the corneal stroma. The incident light angleis a determining factor for this loss in ablation efficiency.Digital compensation mechanisms are required to ensurethat these effects are adequately taken into account for thetotal ablation profile.Tissue hydration: The standard 193 nm wavelength emit-ted by excimer laser systems used in refractive surgery issignificantly attenuated by 0.9% sodium chloride (NaCl)solution (physiological saline). This means that pre-dictability of tissue ablation is dependent on the hydrationof the treated tissue. Dried out tissue e.g. from prolongedpreparative work during flap lift, has a detrimental effecton ablation precision as does also saline solution used torinse the flap bed.

ound healing

The key characteristic of excimer laser photoablation oforneal tissue is the fact that within certain limits, the corneaolerates the removal of parts of its substance without majorissue re-arrangements. The optical property of clear trans-arency, unique to this tissue, is due to the perfect regular

lignment of the collagen lamellae forming the stroma. Aealing response to regenerate ablated tissue results frombroblast activation and forms a scar, as it does in other

issues.

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pplication 25 (2010) 250–257 253

The structure of a scar however cannot remodel the origi-al regular structure and hence results in an opaque focusn the cornea. Scarring, following refractive tissue removaln the cornea, would therefore be detrimental to the opticalroperties. Excimer laser photoablation of the corneal stromaan be performed without eliciting relevant tissue remodel-ng. Experimental and clinical evidence has shown that thetroma is less prone to scarring when the Bowman layer isntact. Antiproliferative agents such as mitomycin C haveroved useful to prevent fibroblast activation known as “haze”16], following photoablation in surface procedures [17–19].

However, the use of a cytostatic agent is subject toontroversial discussion in corneal refractive surgery as con-erns about endothelial cell toxicity and reduced keratocyteepopulation have been raised [19,20].

evelopment of laser vision correctionrocedures

Two major categories of excimer LVC procedures can beistinguished by the level of photoablation: surface ablationPRK) and intrastromal ablation (LASIK). In the first proce-ure, the corneal epithelium is removed and ablation startsith removal of Bowman’s layer, and the second procedure

akes place within the stroma, under a stromal tissue flap.

hotorefractive keratectomy

In conventional PRK a sharp blade, typically a hockey-nife is used to shave away the epithelium. This is performedver an area just above the size of the planned treatmentone, leaving the limbal area intact. The ablation is performedith the eye tracker activated and the patient gazing at an

nternal fixation light. The process of ablation itself is a pre-rogrammed individual pattern of a large number (typicallyn the thousands) of laser pulses that are scanned in a fastequence over the treatment area.

The resulting large or subtotal epithelial wound area isovered with a bandage contact lens to allow healing ofhe epithelium underneath. The extensive epithelial defectogether with the irritation of corneal nerves during ablationccount for substantial postoperative pain until the wound islosed, usually after 3–4 days.

After PRK, patients have to wait a period of approximatelyne month before final and stable refraction and visual acuityre reached. This is partly due to the epithelial healing pro-ess which in the first instance creates surface irregularitieshen the epithelial lining above the ablation area regenerates.tudies have demonstrated good long-term stability of theesults, with an advantage for myopic over hyperopic treat-

ents [21,22]. Treatment of hyperopia is, in general, more

hallenging in terms of defining the ideal individual abla-ion nomogram. The broad circular pericentral ablation zoneorming a shallow groove in the mid-periphery tends to be

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artially equalized by a thickened cover of epithelium. Thisan reduce the refractive effect, i.e. reduce the steepening ofhe central cornea. Such an evolution is termed as regression.

The major discomfort of the early postoperative period,ncluding burning, tear flow, pain and low vision, have inspred

any ideas to reduce these problems. Several modificationsf the procedure, such as the removal of the epithelium, haveonsequently been developed; microkeratomes with a speciallunt blade were designed to cleave the epithelial cells fromowman’s membrane and to detach the epithelium as a sheet

hat may be replaced at the end of surgery. Such a procedures called epiLASIK [23], or if the epithelium is detached bypplication of a 20% ethanol solution for 30 s, laser-assistedubepithelial keratomileusis (LASEK) [24]. These modifica-ions of the original procedure type have some advantages inerms of visual rehabilitation and process comfort comparedo PRK; however the principle remains the same, i.e. to ablateorneal tissue from the most anterior strata of the cornea withhe goal of targeted flattening of the corneal curvature.

aser-assisted in situ keratomileusis

The perceived imperfections of the PRK procedure,amely the risk of haze formation and the painful periodf postoperative visual recovery has fostered the quest fortechnique that would use the benefits of the excimer laserithout incurring the above named disadvantages. A combi-ation of Barraquer’s [1] original concept of keratomileusis,nvolving reshaping of the anterior surface by lamellar kera-ectomy and excimer laser photoablation has proved suitableor this purpose. Pallikaris et al. [25–28] introduced the novelrocedure and suggested the name ‘laser in situ keratomileu-is’, which was soon to stand in its abbreviated form ‘LASIK’s synonymous to LVC procedures. LASIK is now by far theost frequent application of excimer lasers in medicine. It

asically is a three step procedure: (1) the creation of a flap,2) the ablation process and (3) the re-positioning of the flap.bviously the flap merely has an adjunctive role, exposing

he corneal stroma to the excimer laser pulses. A peculiarityf such corneal flaps is that they do not fully heal followingurgery, as would usually be expected of any other surgi-al wound. Keratocytes do not bridge the tissue gap, exceptor the outer rim under certain geometrical circumstances.he flap is held in place by cohesion forces and coveredy the epithelium. Both factors, under physiological condi-ions, are sufficient to create an equivalent of integrity thatnsures stability of the cornea. However in the absence of truengraftment into its bed, the flap does not contribute to theechanical firmness of the corneal architecture. Originally

he thickness of the flap was about 160–180 �m. The primaryrucial dimension however is the residual stromal bed thick-

ess (RST) after the ablation process. A minimum of 250 �mas been determined to be the minimum RST [29,30]. Withthickness of about 530 �m for the native cornea and tis-

ue ablation dimensions of up to >100 �m, depending on

pplication 25 (2010) 250–257

he intended correction, the flap tissue may appear as “losterrain” under the consideration of postoperative mechani-al stability of the cornea. The residual stromal thickness inact limits the degree of possible myopic correction to theurrently accepted −9 dpt.

Corneal flaps for LASIK procedures traditionally wereut using mechanical microkeratomes. These devices areounted upon a metal suction ring used to hold the eye in

osition and slice a flap using an ultra sharp oscillating bladeassing over an aplanation plate. More recently, femtosecondfs) lasers have become the technical gold standard for thereation of more sophisticated corneal flaps. Such devices,sing ultra short near-infrared laser pulses, may be used toorm extremely thin corneal flaps that cleave the stroma justnder Bowman’s layer and create a rim of the flap with a sharpdged profile, thus anchored in its bed. Minimization of theap thickness allows for a greater flexibility in the compo-ition of the ablation profile, namely the ablation depth. Therocedure of intrastromal excimer laser ablation under a flapreated using an fs laser is usually called FemtoLASIK. Thisrocedure has emerged as the state-of-the-art standard of caren the past few years [31–33].

asic requirements for laser-assisted cornealefractive surgery

As stated before, refractive surgery by definition is surgeryn a healthy eye which has the only fault of not having under-one the physiological process of emmetropization duringevelopment in a perfect manner. This incurs requirementsf especially high standards for any procedure in the field.

Predictability: With proper preoperative diagnostics andsurgical planning, the result of the photoablation has to beprecisely predictable. The patient would not benefit from“some reduction” of ametropia but wants exact, usuallyfull correction. Current standards for predictability are inthe range of >94% for ±0.5 dpt of target refraction [34,35].This sets excimer laser corneal refractive surgery amongthe most precisely effective procedures in surgery.Efficacy: It must be guaranteed that the desired alterationof the refractive power of the cornea is achieved with thegiven means of technology. Efficacy reaches similar levelsas predictability today [22].Safety: For healthy eyes, a close to zero tolerance con-cerning complications is appropriate. In the early days ofthe development of techniques, both undesired outcomesand manifest complications were seen more frequently,however present day techniques have eliminated many ofthe risks. Typical adverse events include under- or over-correction, night vision disturbances, flap striae and dry

eye problems. Major complications with a potential forrelevant long-term reduction of best corrected visual acu-ity are flap dislocation, corneal infection or keratectasia[21,36]. The incidence has been considerably reduced with

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the advancement of preoperative diagnostic assessmentand narrower indications for the procedures. Preexistingcorneal pathologies such as various, including subclini-cal, forms of keratoconus are today recognized to be acontraindication against excimer laser ablative procedures.Stability of the outcome: As stated above, the peculiar modeof action of the photoablative process together with thetissue-inherent characteristics of the cornea allow a veryhigh degree of long-term stability [37–40].Process comfort: Healthy patients are not prepared toaccept medical tortures or prolonged visual rehabilitation.With topical anesthesia, short procedure times in the rangeof seconds and almost immediate visual recovery allowsLVC patients to positively evaluate these procedures.

reoperative diagnostics and patient selectionor excimer laser refractive surgery

As for any procedure designed to optimize a physiologicalorm variant, excimer LVC surgery requires a strict code foratient selection and preoperative diagnostics. The conceptf customer orientation as established in marketing scienceanks first when considering excimer laser refractive surgery.eyond the seemingly obvious desire of a patient to gain

pectacle independency, the individual visual priorities muste determined (e.g. near-vision profession, distance-visioneisure activities, work environment) and set in correlation toarameters such as refraction, age, general and eye health.oth surgeon and patient must share a common understand-

ng of success criteria of the procedure to ensure patientatisfaction.

The range of preoperative diagnostics e.g. for a LASIK orFemtoLASIK procedure may vary depending on the com-lexity of the intended ablation profile and the individualefractive error. The standard set of examinations encom-asses, beyond a general examination of anterior and poste-ior segments of the eye, measurements of objective and sub-ective refraction, corneal topography and multipoint cornealachymetry. Additionally, determination of the mesopic orcotopic pupil diameter is crucial to define the diameter of theblation zone. Also, mesopic and contrast vision tests shoulde performed to assess the optimal diameter of the requiredblation zone. The individual ablation profile is based on mea-urements of objective and subjective refraction. Measure-ents of the ocular wave front can also be integrated into the

omposition of the ablation profile which is advantageous forhe visual result especially in eyes with a substantial degreef wave front errors with higher order aberrations [41–45].

lternative applications of excimer lasers

Although LVC procedures constitute the large majority ofxcimer laser procedures in ophthalmology, few other appli-ations of the technology exist in the subspecialty.

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pplication 25 (2010) 250–257 255

Phototherapeutic keratectomy (PTK) is similar to PRK.his procedure is applied to treat dystrophies and abnormali-

ies of the epithelial basement membrane. The typical clinicalresentation of patients is a history of recurrent spontaneousorneal erosions incurring severely painful sensations andlurred vision. The common explanation for this conditions that the epithelial cells cannot properly adhere to theirasement membrane. The goal of the treatment is thereforeo create a new surface underneath the epithelium to ensurerm adhesion. A typical finding during surgical removal of

he epithelium is that the epithelium slides on the surface andan be easily detached without prior disruption of its adhe-ion to the basement membrane. Treatment with the excimeraser is limited to an ablation of <10 �m, which just into Bow-

an’s membrane. PTK has been established as being a safend effective treatment for corneal subsurface disorders [46].

Another potential application of excimer lasers in oph-halmic surgery is the intraocular procedure for the treatmentf glaucomas. In excimer laser trabeculoplasty (ELT) abnterno, the laser energy is guided into the eye by a fiberptic with the purpose of ablating tissue around the trabecu-ar meshwork and thus to increase the outflow facility for thequeous humor [47,48].

The potential for precise tissue ablation with sharp edgesas inspired researchers to use excimer lasers for trephinationf the cornea in corneal transplant surgery. Seitz et al. [49]roposed a technique of penetrating ablation around the edgef corneal masks in both donor and recipient tissue to createrecisely matching grafts. This was further refined by addingositional spikes into the contour of the graft to ensure opti-ized postoperative rotational stability. Despite a promising

otential of the concept, this has not been further developedn depth, partly due to the advent of fs lasers.

onclusion

The discovery that 193 nm excimer laser light ablatesorneal tissue very precisely by photochemical decomposi-ion has allowed the development of an entire subspecialty inphthalmology. Over the last 25 years LVC procedures usingxcimer lasers have become the most precise applicationsf such tools in ophthalmology, allowing highly effectiveurgery with almost total predictability and safety. This makeshem an example of nearly ideal surgical instruments for

icrosurgery.

usammenfassung

nwendungsprinzipien des Excimerlasers in derphthalmologie

xcimerlaser mit 193 nm Wellenlänge können zum photo-hemischen Abtrag von Hornhautgewebe genutzt werden.ies ermöglicht ihren Einsatz als hochpräzise Werkzeuge

n der refraktiven Chirurgie zur Korrektur von Refrak-

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ionsfehlern des menschlichen Auges. Der Artikel gibtinen Überblick über die wissenschaftliche Entwicklung vonxcimerlasern in der Ophthalmochirurgie und über die Band-reite ihrer Anwendung.

chlüsselwörter: Excimerlaser; Photoablation; PRK; LASIK;ASEK; Refraktive Chirurgie

eferences

[1] Barraquer JI. Autokeratoplasty with optical carving forthe correction of myopia (Keratomileusis). An Med Espec1965;51(1):66–82.

[2] Taboada J, Archibald CJ. An extreme sensitivity in the cornealepithelium to far UV ArF excimer laser pulses. In: AerospaceMedical Association. Proceedings of the Scientific Program,San Antonio, Texas. 1981. p. 98–9.

[3] Trokel SL, Srinivasan R, Braren B. Excimer laser surgery ofthe cornea. Am J Ophthalmol 1983;96(6):710–5.

[4] Blum SE, Srinivasan R, Wynne JJ. Far ultraviolet surgical anddental procedures. U.S. Patent 4784135. Nov 1988.

[5] Krueger RR, Trokel SL, Schubert HD. Interaction of ultra-violet laser light with the cornea. Invest Ophthalmol Vis Sci1985;26(11):1455–64.

[6] McDonald MB, Frantz JM, Klyce SD, Beuerman RW, VarnellR, Munnerlyn CR, et al. Central photorefractive keratec-tomy for myopia. The blind eye study. Arch Ophthalmol1990;108(6):799–808.

[7] McDonald MB, Kaufman HE, Frantz JM, Shofner S, SalmeronB, Klyce SD. Excimer laser ablation in a human eye. Casereport. Arch Ophthalmol 1989;107(5):641–2.

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