Overview of Refractive Surgery

Author: Kraig Scot Bower, Thomas J. Kim
Date: Oct 1, 2001

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. (Am Fam Physician 2001;64:1183-90,1193-4.)

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 should be familiar with the indications and contraindications of the refractive procedures, as well as the results, follow-up courses and potential complications.

Anatomy of the Cornea

The transparent cornea is about 0.5 mm thick, with five distinct layers: the epithelium, Bowman's membrane, stroma, endothelium and Descemet's membrane. The epithelium is the most exterior layer, providing the smooth refractive surface and serving as a barrier against infection. The function of Bowman's membrane, which lies beneath the epithelium and its basement membrane, is unclear. The stroma, made up of intertwining lamellae of collagen fibrils, provides structure to the cornea and accounts for 90 percent of the corneal thickness. The endothelium and its basement membrane (Descemet's membrane) form the innermost layers. Endothelial cells, via an active sodium potassium-adenosine triphosphatase pump, are responsible for the relative corneal dehydration necessary for corneal clarity.

Optics, Refraction and Refractive Error

Refraction is the bending of light rays as they pass from one transparent medium to another medium of a different density; it is measured in diopters. The refractive power of a lens is the reciprocal of its focal length measured in meters (e.g., a one-diopter lens has a focal point of 1 m; a two-diopter lens has a focal length of 0.5 m). The cornea and the crystalline lens refract light that enters the eye. The cornea is responsible for two thirds of the eye's total focusing power, while the crystalline lens accounts for the remaining one third. The focusing power of the cornea is fixed, whereas the focusing power of the crystalline lens is not. Through a process called accommodation, the lens changes its shape to bring objects 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 globe. When a person with normal vision acuity views an object, the cornea and the lens focus the parallel light rays emitted from the object precisely on the retina (Figure 1a), and a clear image is perceived. The eye's focal point is at infinity.

Refractive errors occur when the cornea and the lens do not properly focus the light rays on the retina. In myopia (nearsightedness), the most common type of refractive error, the cornea is too curved or the lens too powerful for the length of the globe. Distant objects cannot be seen clearly because light rays are focused in front of the retina (Figure 1b); however, near objects appear clear. 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.

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 significant corneal instability induced by the surgery, including diurnal fluctuation of refractive error, overcorrection, hyperopic shift and potential rupture of the globe with blunt trauma.(1) This procedure 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 sufficient 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 patient's refraction and corneal topography, controls the laser beam to precisely remove corneal tissue.(2) With improving technology, the width of the laser beam has continued to decrease to less than 100 m. 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).

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.

TABLE 1Patient Selection Criteriafor LASIK and PRK[*]Age 18 years or olderStable refraction of at least one year's durationMyopia between -0.50 and -12.00 dioptersAstigmatism [less than or equal] 5.00 dioptersHyperopia [less than] +6.00 dioptersAbsence of ocular contraindications: Keratoconus Herpetic keratitis Progressive myopia Corneal disease[] Glaucoma[] Cataract[] Any other preexisting pathology of the cornea or anterior segment, including scarring, agophthalmos, dry eye and blepharitis[]Absence of medical contraindications: Uncontrolled vascular disease Autoimmune disease Immunosuppressed/immunocompromised Pregnant or nursing History of keloids Diabetes mellitus[]LASIK = laser-assisted in situ keratomileusis;PRK = photorefractive keratectomy.[*]--Correction with the excimer laser.[]--Determined by an ophthalmologist.

PHOTOREFRACTIVE KERATECTOMY

With any procedure using the excimer laser, the refractive outcome may not always result in an uncorrected vision acuity or best-spectacle corrected vision acuity of 20/20 or better. Some patients may develop a worsening of vision clarity and acuity secondary to scarring, glare, halos, monocular diplopia and reduced contrast sensitivity.(4,5,7,29,30) Patients may have a postoperative overcorrection, undercorrection and astigmatism that may need an enhancement to correct the residual refractive error.(6,9,10,17,32) Furthermore, following excimer laser refractive surgery, most patients will complain of dry-eye symptoms secondary to disruption of the corneal nerve innervation. These patients are most effectively treated with nonpreserved artificial tears or punctual plugs, as needed; the dry-eye symptoms usually resolve within three months.

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)

The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the U.S. Army Medical Department or the U.S. Army Service at large.

The authors indicate that they do not have any conflicts of interest. Sources of funding: none reported.

REFERENCES

(1.) Filatov V, Vidaurri-Leal JS, Talamo JH. Selected complications of radial keratotomy, photorefractive keratectomy, and laser in situ keratomileusis. Int Ophthalmol Clin 1997;37:123-48.

(2.) Alessio G, Boscia F, La Tegola MG, Sborgia C. Topography-driven photorefractive keratectomy: results of corneal interactive programmed topographic ablation software. Ophthalmology 2000;107:1578-87.

(3.) McDonald MB, Deitz MR, Frantz JM, Kraff MC, Krueger RR, Salz JJ, et al. Photorefractive keratectomy for low-to-moderate myopia and astigmatism with a small-beam, tracker-directed excimer laser. Ophthalmology 1999;106:1481-8.

(4.) Seiler T, McDonnell PJ. Excimer laser photorefractive keratectomy. Surv Ophthalmol 1995;40:89-118.

(5.) Hersch PS, Stulting RD, Steiner RF, Waring GO 3rd, Thompson KP, O'Connell M, et al. Results of phase III excimer laser photorefractive keratectomy for myopia. The Summit PRK Study Group. Ophthalmology 1997;104:1535-53.

(6.) Hersch PS, Brint, SF, Maloney RK, Durrie DS, Gordon M, Michelson MA, et al. Photorefractive keratectomy versus laser in situ keratomileusis for moderate to high myopia. A randomized prospective study. Ophthalmology 1998;105:1512-23.

(7.) Pop M, Payette Y. Photorefractive keratectomy versus laser in situ keratomileusis: a control-matched study. Ophthalmology 2000;107:251-7.

(8.) Shah S, Chatterjee A, Smith RJ. Predictability of spherical photorefractive keratectomy for myopia. Ophthalmology 1998;105:2178-85.

(9.) Carones F, Gobbi PG, Vigo L, Brancato R. Photorefractive keratectomy for hyperopia: long-term nonlinear and vector analysis of refractive outcome. Ophthalmology 1999;106:1976-83.

(10.) Haw WW, Manche EE. One-year evaluation of myopic laser photoastigmatic refractive keratectomy using the summit apex plus: phase III of a Food and Drug Administration clinical trial. Ophthalmology 2000;107:1572-7.

(11.) El-Maghraby A, Salah T, Waring GO 3rd, Klyce S, Ibrahim O. Randomized bilateral comparison of excimer laser in situ keratomileusis and photorefractive keratectomy for 2.50 to 8.00 diopters of myopia. Ophthalmology 1999;106:447-57.

(12.) Dulaney DD, Barnet RW, Perkins SA, Kezirian GM. Laser in situ keratomileusis for myopia and astigmatism: 6 month results. J Cataract Refract Surg 1998;24:758-64.

(13.) Buzzard KA, Fundingsland BR. Excimer laser assisted in situ keratomileusis for hyperopia. J Cataract Refract Surg 1999;25:197-204.

(14.) Yoo SH, Azar DT. Laser in situ keratomileusis for the treatment of myopia. Int Ophthalmol Clin 1999; 39:37-44.

(15.) Salah T, Waring GO 3rd, El-Maghraby A, Moadel K, Grimm SB. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol 1996;121:143-55.

(16.) Zadok D, Maskaleris G, Montes M, Shah S, Garcia V, Chayet A. Hyperopic laser in situ keratomileusis with the Nidek EC-5000 excimer laser. Ophthalmology 2000;107:1132-7.

(17.) Lindstrom RL, Linebarger EJ, Hardten DR, Houtman DM, Samuelson TW. Early results of hyperopic and astigmatic laser in situ keratomileusis in eyes with secondary hyperopia. Ophthalmology 2000;107: 1858-63.

(18.) Gimbel HV, Penno EE, van Westenbrugge JA, Ferensowicz M, Furlong MT. Incidence and management of intraoperative and early postoperative complications in 1000 consecutive laser in situ keratomileusis cases. Ophthalmology 1998;105:1839-48.

(19.) Gimbel HV, van Westenbrugge JA, Penno EE, Ferensowicz M, Feinerman GA, Chen R. Simultaneous bilateral laser in situ keratomileusis. Ophthalmology 1998;106:1461-7.

(20.) Koch DD, Kohnen T, McDonnell PJ, Menefee RF, Berry MJ. Hyperopia correction by noncontact holmium: YAG laser thermal keratoplasty. United States phase IIA clinical study with a 1-year follow-up. Ophthalmology 1996;103:1525-35.

(21.) Nano HD, Muzzin S. Noncontact holmium: YAG laser thermal keratoplasty for hyperopia. J Cataract Refract Surg 1998;24:751-7.

(22.) Nose W, Neves RA, Burris TE, Schanzlin DJ, Belfort R Jr. intrastromal corneal ring: 12-month sighted myopic eyes. J Refract Surg 1996;12:20-8.

(23.) Cochener B, Savary-LeFloch G, Colin J. Effect of intrastromal corneal ring segment shift on clinical outcome: one year results for low myopia. J Cataract Refract Surg 2000;26:978-86.

(24.) Tham VM, Maloney RK. Microkeratome complications of laser in situ keratomileusis. Ophthalmology 2000;107:920-4.

(25.) Stulting RD, Carr JD, Thompson KP, Waring GO 3rd, Wiley WM, Walker JG. Complications of laser in situ keratomileusis for the correction of myopia. Ophthalmology 1999;106:13-20.

(26.) Linebarger EJ, Hardten DR, Lindstrom RL. Diffuse lamellar keratitis: Diagnosis and management. J Cataract Refract Surg 2000;26;1072-7.

(27.) Alio JL, Artola A, Claramonte PJ, Ayala MJ, Sanchez SP. Complications of photorefractive keratectomy for myopia: two year follow-up of 3000 cases. J Cataract Refract Surg 1998;24:619-26.

(28.) Krueger RR, Saedy NF, McDonnell PJ. Clinical analysis of steep central islands after excimer laser photorefractive keratectomy. Arch Ophthalmol 1996; 114:377-81.

(29.) Hersh PS, Steinert RF, Brint SF. Photorefractive keratectomy versus laser in situ keratomileusis: comparison of optical side effects. Summit PRK-LASIK Study Group. Ophthalmology 2000;107:925-33.

(30.) Bullimore MA, Olson MD, Maloney RK. Visual performance after photorefractive keratectomy with a 6-mm ablation zone. Am J Ophthalmol 1999; 128:1-7.

(31.) Chayet AS, Assil KK, Montes M, Espinosa-Lagana M, Castellanos A, Tsioulias G. Regression and its mechanisms after laser in situ keratomileusis in moderate and high myopia. Ophthalmology 1998;105:1194-9.

(32.) Durrie DS, Vande Garde TL. LASIK enhancements. Int Ophthalmol Clin 2000;40:103-10.

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 Hebert 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 officer 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.

Address correspondence to Eric D. Weichel, M.D., 2007 Westchester Dr., Silver Spring, MD 20902 (e-mail: eweichel@hotmail.com). Reprints are not available from the authors.

COPYRIGHT 2001 American Academy of Family PhysiciansCOPYRIGHT 2001 Gale Group

 
© 2006, DrPlace.com, All Rights Reserved.