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`Overview of Refractive Surgery
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`Overview of Refractive Surgery
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`KRAIG SCOT BOWER, LTC, MC, USA, ERIC D WEICHEL, CPT, MC, USA, AND THOMAS J KIM, CPT, MC,
`USA
`
` Am Fam Physician. 2001;64(7):1183-1191
` A more recent article on LASIK is available (https://www.aafp.org/afp/2017/0515/p637.html).
` See patient information handout on refractive surgery
`(https://www.aafp.org/afp/2001/1001/p1193.html), written by the authors of this article.
`
`Patients with myopia, hyperopia and astigmatism can now reduce or eliminate their dependence on
`contact lenses and eyeglasses through refractive surgery that includes radial keratotomy (RK),
`photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK), laser thermal
`keratoplasty (LTK) and intrastromal corneal rings (ICR). Since the approval of the excimer laser in
`1995, the popularity of RK has declined because of the superior outcomes from PRK and LASIK. In
`patients with low-to-moderate myopia, PRK produces stable and predictable results with an
`excellent safety profile. LASIK is also efficacious, predictable and safe, with the additional
`advantages of rapid vision recovery and minimal pain. LASIK has rapidly become the most widely
`performed refractive surgery, with high patient and surgeon satisfaction. Noncontact Holium: YAG
`LTK provides satisfactory correction in patients with low hyperopia. ICR offers patients with low
`myopia the potential advantage of removal if the vision outcome is unsatisfactory. Despite the
`current widespread advertising and media attention about laser refractive surgery, not all patients
`are good candidates for this surgery. Family physicians should be familiar with the different
`refractive surgeries and their potential complications.
`
`Extensive television, radio and newspaper advertising have promoted laser vision correction, and the
`public has a variety of refractive procedures from which to choose. Because of this media attention,
`many patients will be asking their family physician about these procedures. Family physicians
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`th the ndicat ons and contraind cations of the refractive procedures, as well as
`shou d be familiar
`the results, follo -up courses and potential complications.
`
`Anatomy of the Cornea
`
`The t ansparent cornea s about 0 5 mm thick, w th ve d stinct laye s the epithelium Bowman's
`membrane, stroma, endothelium and Descemet's membrane The epithelium s the most exter or
`layer, prov ding the smooth refract ve s rface and serv g as a barr e aga st infection The function
`of Bowman's membrane wh ch lies beneath the epithelium and its basement membrane is unclear.
`The st oma, ade p of
`tertw ing la ellae of collage br s p ovides st uctu e to the cornea and
`accounts for 90 percent of the corneal thickness. The endothelium and its basement membrane
`(Descemets me brane) form the inne most layers E dothelial cells, via an active sod m
`potassium–adenosine triphosphatase pump, are respons ble for the relative corneal dehydration
`necessary for co neal cla ity
`
`Optics, Refract on and Refractive Error
`
`Refract on is the bending of light rays as they pass from one transparent med um to another medium
`of a different dens ty; t is measured in diopters. The refract ve power of a lens is the reciprocal of ts
`foca ength measured in meters (e g., a one-diopter lens has a foca point of 1 m; a two-diopter lens
`has a focal length of 0.5 m). The cornea and the crysta ne lens refract light that enters the eye. The
`cornea is responsible for two thirds of the eye's total focus ng po er wh e the crystalline lens
`accounts for the remain ng one third The focusing power of the cornea is xed whereas the focusing
`power of the crystalline ens is not Through a process ca ed accommodation, the lens changes its
`shape to br ng ob ects into focus.
`
`In emmetropia (an eye with normal vision , the focusing powers of the cornea and the lens are
`perfectly matched to the length of the g obe. When a person with normal vision acuity v e s an
`object the cornea and the lens focus the para e ght rays emitted from the object precisely on the
`retina (Figure 1a), and a clear mage s perceived. The eye's focal po nt is at nnity.
`
`Refract ve errors occur when the cornea and the ens do not properly focus the ght rays on the
`retina. In myopia (nearsightedness , the most common type of refractive error, the cornea is too
`curved or the ens too powerful for the length of the globe. D stant objects cannot be seen clear y
`because ght rays are focused in front of the retina (Figure 1b) however, near objects appear clear
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`Concave lenses with minus or divergent power correct this refractive error and refocus the light rays
`on the correct point on the retina.
`
`In hyperopia (farsightedness), the cornea is too flat or the lens too weak for the length of the globe.
`As a result, the cornea and lens focus the light rays behind the retina (Figure 1c). The process of
`accommodation may bring a distant object into focus; however, near vision is unclear. Convex lenses
`with plus or convergent power correct this refractive error and refocus the light rays to the correct
`point on the retina.
`
`FIGURE 1A.
`
`The emmetropic eye.
`
`FIGURE 1B.
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`The myopic eye.
`
`FIGURE 1C.
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`The hyperopic eye.
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`Overview of Refractive Surgery
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`In astigmatism, the refractive power of the eye is different in different meridians. The cornea and the
`lens cannot bring the light rays to the precise point on the retina to provide clear vision; thus, objects
`will appear blurry at any distance. Astigmatism may occur with myopia or hyperopia.
`
`Refractive Surgery
`
`Radial keratotomy (RK), photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis
`(LASIK), laser thermal keratoplasty (LTK) and intrastromal corneal rings (ICR) are the most common
`refractive surgeries presently performed in the United States in the treatment of patients with myopia,
`hyperopia and astigmatism. The object of these procedures is to change the refractive state of the
`eye by changing the shape of the cornea.
`
`RADIAL KERATOTOMY
`
`In the past, RK was performed to treat patients with myopia. The surgeon makes a number of
`microscopic corneal incisions in a radial or spoke-like pattern. This allows the outer cornea to relax
`so that the central cornea flattens (Figure 2). The new shape of the cornea is permanently retained as
`the cornea heals.
`
`Potential serious complications include loss of best-corrected vision acuity, perforation of the cornea,
`infection and rupture of the globe. Some of the major concerns with this procedure relate to the
`signicant corneal instability induced by the surgery, including diurnal fluctuation of refractive error,
`overcorrection, hyperopic shift and potential rupture of the globe with blunt trauma. This procedure
`1
`has declined in popularity since 1995, when the U.S. Food and Drug Administration (FDA) approved
`the use of the excimer laser, and because of the superior results of the other commonly performed
`refractive surgeries.
`
`THE EXCIMER LASER
`
`The excimer laser is used to perform PRK and LASIK procedures and works by changing the shape of
`the cornea. The excimer laser emits an ultraviolet beam that has sufcient energy to break
`intermolecular bonds within the cornea (photoablation). Because little or no thermal damage occurs
`to adjacent tissue, this is often referred to as a “cool” laser beam. A computer, programmed with the
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`patient's refraction and corneal topography, controls the laser beam to precisely remove corneal
`tissue. With improving technology, the width of the laser beam has continued to decrease to less
`2
`than 100μ. In addition, laser eye-tracking systems are now available that allow precision corneal
`ablation during eye movements.
`3
`
`In myopia, the laser flattens the central cornea to decrease its focusing power. In hyperopia, the laser
`indirectly steepens the central cornea by removing tissue from the periphery, thus increasing the
`cornea's focusing power. Astigmatism is treated with an elliptic or cylindrical beam that flattens the
`steepest corneal meridian (Figure 3).
`
`FIGURE 2.
`
`
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`
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`Radial keratotomy. (Top) Partial-thickness incisions of the cornea. (Bottom) Compensatory flattening of the central
`cornea.
`
`FIGURE 3.
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`Overview of Refractive Surgery
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`Laser assisted in situ keratomileusis (LASIK) surgery (Top) Treatment for myopia (Bottom) Treatment for
`hyperopia.
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`Not every patient is a candidate for treatment using the excimer laser. Age, high refractive error, and
`ocular and medical disease may prevent a patient from obtaining a predictable refractive outcome.
`Table 1 reviews patient selection criteria for PRK and LASIK procedures.
`
`PHOTOREFRACTIVE KERATECTOMY
`
`PRK effectively treats patients with low-to-moderate myopia, myopia with astigmatism and low-to-
`moderate hyperopia without astigmatism
` It is performed on an outpatient basis using topical
`3–10
`anesthetic. The corneal epithelium in the ablation zone is rst removed or pushed to the side to allow
`a more accurate ablation of corneal tissue The laser treatment is then applied to the exposed corneal
`stroma. Immediately following the laser treatment, the ophthalmologist applies topical antibiotic,
`4
`steroid and nonsteroidal anti inflammatory drugs (NSAIDs) A disposable bandage contact lens is
`then placed over the cornea.
`
`TABLE 1
`
`Patient Selection Criteria for LASIK and PRK*
`
`Age 18 years or older
`
`Stable refraction of at least one year's duration
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`Myopia betwee 0 5 and 12 0 diopters
`
`Overview of Refractive Surgery
`
`Ast gmatism 5 0 diopters
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`Hyperop a < +6 0 diopters
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`Abse ce of oc lar co tra dicat ons
`
`Keratoco us
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`He petic keratitis
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`Progressive myopia
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`Co neal d sease†
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`G aucoma†
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`Cataract†
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`Any other preexisting pathology of the cornea or anterior segment, cluding scarr g,
`agophthalmos d y eye and blepha itis†
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`Abse ce of medical co tra dicat ons
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`U cont olled vasc lar d sease
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`Autoim une disease
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`Immunos ppressed/immu ocompro ised
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`Pregnant or n rsing
`
`H story of keloids
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`D abetes mellitus†
`
`LAS K
`
` laser assisted in situ ke atomile sis PRK
`
` photo ef active keratecto
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`*—Correction with the e cimer laser
`
`†—Determ ned by an ophthalmologist
`
`D ring the ear y postoperat ve period patients ay expe ience sign cant tearing, photophobia,
`blurred vision and d scomfort because of the central corneal abrasion With the use of the bandage
`contact lens a d NSAID eyedrops postope ative pain is usually mild to moderate; however, pat e ts
`occasionally require systemic ana gesia for more severe pa n. The contact ens remains on the eye
`unt the epithelial defect is healed (an ave age of three to fo r days) Antibiot c therapy is usua y
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`cont nued for two to three days after the defect has hea ed, and topical steroid eye drops may be
`cont nued for up to three months postoperatively.
`5
`
`Vision acuity mproves once the epithe al defect heals—usually with n one week postoperatively.
`Vision acuity typically f uctuates follo ng surgery before stabilizing at around three months
`postoperatively Glare, halos and dry-eye symptoms are common dur ng the rst month follo ng
`surgery but usually d minish or disappear entirely by three to six months postoperatively.
`
`LASER-ASSISTED IN SITU KERATOM LEUSIS
`
`LASIK, no the most commonly performed refractive surgery, s an effective treatment in patients
`with low, moderate and high myopia w th or w thout astig at s , as well as hyperop a with o
`without ast gmatism.
` Like PRK, LASIK is an outpatient surgery performed with topical
`11 1
`anesthesia
`
`A microkeratome, hich works ke a carpenter's plane is used to raise a corneal flap about the s ze
`of a contact lens (F gure 4) his flap, which averages 16 μ in thickness, is folded back to expose the
`underlying stroma.
` The excimer aser is used to ab ate a precise amount of corneal stroma and the
`14
`flap s rigated and placed back in its original posit on The cornea flap is stabilized without s tures
`by the relative corneal dehydration created by the endothelia pump. The stability of the corneal f ap
`and adhere ce to the corneal stro a is checked follow g surgery, and patients are usually sent
`home with topical steroid, topical antibiot c and topical NSAID eye drops. In addition the pat ent is
`instr cted to use an eye shield over ight with follow up typically sched led on postoperat ve day 1
`and again at one eek
` The patient is usua y checked again at one, three and six months.
`18
`
`The LASIK procedure has signicant attractions for patients It causes litt e pain, provides qu ck
`recovery of vision and has the potent al for treat ng higher levels of myop a.
` LASIK has a so been
`15
`fou d to be safe a d effective in treating both eyes on the same day; whereas, PRK is usua y
`performed on t o separate days
`19
`
`LASIK enhancements a e mo e easily performed (at least within the rst six to 12 months follow g
`the in tial surgery) by fting the or ginal corneal f ap and re-treating the stromal bed to correct any
`residual refractive error U like PRK LASIK does not produce st omal haze, and pat e ts who have
`undergone this procedure do not usua y require topica steroid eye drops beyond the rst
`postoperative week At one year postoperat ve y, patients who have u dergone the AS K procedure
`are more satised than patients ho have undergone the PRK procedure (90 versus 52 percent,
`respect ve y)
` Despite the different s rgica techniq es of PRK and LASIK, the efractive outcomes
`11
` Tab e 2
`are similar.
` compares outcomes of PRK and LASIK
`6,7,11
`3 7,8,11
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`FIGURE 4.
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`Overview of Refractive Surgery
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`Laser-assisted in situ keratomileusis (LASIK) microkeratome. The microkeratome is used to shave a thin layer of the
`cornea, creating a hinged “flap.” A predetermined amount of cornea is removed from underneath the flap.
`
`LASER THERMAL KERATOPLASTY
`
`LTK is performed using the noncontact Holmium: YAG laser. The ophthalmologist uses the laser to
`symmetrically place radial spots that are outside the visual axis. This heats the cornea, resulting in
`stromal collagen shrinkage, thus modifying the anterior corneal curvature. This procedure is
`performed under topical anesthetic with a variable follow-up schedule.
`
`TABLE 2
`
`Results of PRK vs. LASIK at 12 Months*
`
`Procedure
`
`UCVA efcacy of
`20/20 (%)
`
`UCVA efcacy of
`20/40 (%)
`
`Predictability within one
`diopter (%)
`
`Satisfaction
`(%)
`
`PRK
`
`LASIK
`
`53 to 86
`
`67 to 83
`
`94.4 to 98.1
`
`97 to 100
`
`87 to 94.4
`
`90 to 98.7
`
`52
`
`90
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`UCVA = uncorrected vision acuity; PRK = photorefractive keratectomy; LASIK = laser-assisted in situ keratomileusis.
`
`*—Among many different studies, the refractive outcomes of PRK and LASIK vary greatly for patients with mild myopia (zero
`to −3.00 diopters), moderate myopia (−3.25 to −6.00 diopters), high myopia (>−6.00 diopters) with or without astigmatism,
`low hyperopia (zero to +3.00 diopters), and moderate hyperopia (+3.00 to +6.00 diopters) with and without astigmatism.
`
`Information from references 3, 7, 8 and 11.
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`LTK is used to treat patients with a hyperopic refractive error of up to +4.00 diopters who have
`become presbyopic and have no ocular pathology. The refractive outcomes vary with the amount of
`laser treatment.
` The most common complications include irregular, induced astigmatism and
`20
`hyperopic regression with a need for retreatment using LTK or the excimer laser. LTK is safe, effective
`and provides satisfactory correction of low hyperopia with minimal complications.
`21
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`FIGURE 5.
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`Intrastromal corneal ring (ICR). A polymethylmethacrylate ring is placed into the stroma (the periphery of the
`cornea), causing the cornea to flatten
`
`INTRASTROMAL CORNEAL RING
`
`Results of preliminary studies indicate that ICR is a promising option in correcting myopic refractive
`errors of less than −3.00 diopters.
` During this procedure, the ophthalmologist places a
`22
`polymethylmethacrylate ring into the periphery of the cornea at about two thirds of its depth. The ring
`causes the cornea to flatten, thus correcting the refractive error (Figure 5). The benets of the ICR
`include rapid vision recovery following placement (because surgical manipulation does not occur
`over the central cornea/visual axis). An added benet is that the ophthalmologist can remove the
`device at any time. Risks include infection, abnormal wound healing and irregular astigmatism.
`23
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`POSTOPERATIVE COMPLICATIONS
`
`Complications of refractive surgery may arise intraoperatively or during the postoperative period
`(Tables 3
` and 4). With the LASIK procedure, intraoperative flap complications that occur with
`1,9,24–31
`the use of the microkeratome include incomplete flaps, irregular flaps, small flaps, flap decentration,
`buttonhole flaps or free cap flaps.
` Early postoperative complications include dislodging of the
`18,24
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`flap from the corneal stroma, flap striae, interface debris, epithelia downgrowth nto the flap corneal
`stroma interface or a sterile inflammatory response termed diffuse lame ar keratitis, or “Sands of
`Sahara” syndrome.
`18 25 26
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`TABLE 3
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`Complicat ons of Laser Refract ve Surgery
`
`PRK and LASIK
`
`More common with
`PRK
`
`More common with LASIK
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`Ove correction/unde correction Postoperat ve pain
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`lap co plications
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`Ast gmatism
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`Delayed epithelia
`healing
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`pithe al ingrowth
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`Regression
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`Infection
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`iffuse lamellar keratitis, or Sa ds of
`Sahara” syndrome
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`G are, halo monoc lar
`diplopia
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`D y eye symptoms
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`Reduced co trast sensitivity
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`Scarr g/cornea
`haze
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` photorefractive keratectomy; LASIK
`
` laser-assisted n s tu keratomi eus s
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`formatio from refe e ces 1, 9 and 24
`
`PRK
`31.
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`Intraoperative PRK complications most common y nclude decentration of the laser ablation and
`central islands of higher refractive power.
` Postoperative complications include pain secondary
`4 27 28
`to an epithe al defect and/or de ayed epithelial hea ng, which may increase the r sk of infection, as
`well as late haze formation and corneal scarring.
`27
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`With any procedure using the excimer laser, the refract ve outcome may not always resu t in an
`uncorrected v s on acu ty or best-spectacle corrected vision acuity of 20/20 or better Some pat ents
`may develop a orsening of vision clarity and acuity secondary to scarr ng, glare halos, monocular
`diplopia and reduced contrast sens tivity.
` Patients may have a postoperative overcorrect on,
`4 5,7,29,30
`undercorrection and ast gmatism that may need an enhancement to correct the residual refractive
`error
` Furthermore, follo ng excimer laser refract ve surgery, most patients will compla n of
`6,9 10 17 32
`dry-eye symptoms secondary to disruption of the corneal nerve innervat on. These patients are most
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`effectively treated with nonpreserved articial tears or punctual plugs, as needed; the dry-eye
`symptoms usually resolve within three months.
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`TABLE 4
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`Refractive Surgery: Postoperative Warning Signs
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`Decreased vision acuity
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`Pain secondary to epithelial abrasion or LASIK flap complication
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`Redness, with or without drainage, secondary to infection
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`LASIK = laser-assisted in situ keratomileusis.
`
`Finally, patients should understand that there is a possibility they may still require correction with
`eyeglasses or contact lenses in order to obtain the best-vision acuity and that over time,
`postoperative refractive error regression may require additional laser treatment.
`31
`
`Author Information
`
`KRAIG SCOT BOWER, LTC, MC, USA, is director of refractive surgery, cornea and external disease at
`Walter Reed Army Medical Center, Washington, D.C., and assistant professor of surgery at the
`Uniformed Services University of the Health Sciences F. Edward Hébert School of Medicine, Bethesda,
`Md. Dr. Bower received his medical degree from the University of Texas Southwestern Medical
`School, Dallas. He completed a residency in ophthalmology and a fellowship in cornea and external
`disease at the University of Pittsburgh Medical Center, Pittsburgh, Pa.
`
`ERIC D. WEICHEL, CPT, MC, USA, is the chief resident of ophthalmology at Walter Reed Army Medical
`Center. He received his medical degree from the Northeastern Ohio Universities College of Medicine,
`Akron. Dr. Weichel completed an internship in internal medicine at Tripler Army Medical Center,
`Honolulu, Hawaii. Before beginning his residency in ophthalmology, Dr. Weichel served as a general
`medical ofcer and flight surgeon.
`
`THOMAS J. KIM, CPT, MC, USA, is an ophthalmology resident at Walter Reed Army Medical Center.
`He received his medical degree from the Finch University of Health Sciences/Chicago Medical
`School, North Chicago, Ill. Dr. Kim completed a transitional internship at Walter Reed Army Medical
`Center.
`
`The opinions and assertions contained herein are the private views of the authors and are not to be
`construed as ofcial or as reflecting the views of the U.S. Army Medical Department or the U.S. Army
`Service at large.
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`The authors ndicate that they do not have any co fl cts of i terest So ces of funding none reported
`
`Address correspondence to Er c D Weiche , M.D 2007 Westchester Dr., Silver Spring, MD 20902 (e-
`mail eweichel@hotmai .com (mai to:eweichel@hotmail.com)). Reprints are not a ailable from the authors.
`
`Reference(s)
`
`1. Filatov V, Vidaurr -Leal JS, Talamo JH. Selected complications of radial keratotomy, photorefractive
`keratectomy, and laser in s tu keratom eusis. Int Ophthalmol Clin 997 37: 23-48.
`
`2. A ess o G Boscia F, La Tego a MG, Sborgia C. Topography-driven photorefract ve keratectomy:
`results of corneal interactive programmed topographic ablation soft are. Ophthalmolog .
`2000; 07:1578-87.
`
`3. McDonald MB, Deitz MR, Frantz JM, Kraff MC, Krueger RR, Sa z JJ, et al. Photorefractive
`keratectomy for lo -to-moderate myopia and astigmatism
`th a small-beam, tracker-d rected
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`24 Tham VM Maloney RK Microkeratome co plications of laser in s tu keratom e sis
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