Chapter 15 – Current Concepts, Classification, and History of Refractive Surgery
JUAN CARLOS ABAD
DIMITRI T. AZAR
• Refractive surgery is an evolving field of ophthalmology focusing on the correction of refractive errors of the eye, including myopia, hyperopia, astigmatism, and presbyopia.
• Realistic patient expectations.
• Stable preoperative refraction.
• Absence of ocular diseases.
• Adequate understanding of potential surgical complications, limitations, and alternatives.
• Avoidance of untested and unstable surgical procedures or procedures with high unpredictability and loss of best-corrected visual acuity.
• Alterations in optical aberrations after surgery.
• Reduced dependence on spectacles and contact lenses.
Owing to the permanent nature of refractive surgery, one of its most important aspects is adequate patient selection and counseling. With increased exposure in the press and continual advertisements about the extreme precision of refractive surgery, it is not surprising that many potential refractive surgery candidates walk into the doctor’s office with great expectations. A patient may meet all the medical and surgical requirements for refractive surgery but not be a good candidate because of unrealistic expectations or because of inadequate knowledge about the procedure, its risks and benefits, or alternatives.
Spectacles and contact lenses are reasonable alternatives to refractive surgical procedures. Not only is the accuracy of these forms of optical correction greater than that of refractive surgery, but they are totally reversible. Additionally, in refractive surgery, the variation in biomechanical properties and in corneal wound healing must be taken into account.
Although operating on a normal eye merely to free a patient of the need for glasses or contact lenses may seem aggressive, patients are generally delighted after successful refractive surgery. To achieve uniform satisfaction with newer refractive surgical procedures, they must be validated continuously through controlled and well-designed scientific investigations to ensure better predictability and reproducibility.
OPTICS FOR THE REFRACTIVE SURGEON
The successful performance of refractive surgery demands a thorough understanding of the optics of the human eye. The eye’s refractive power is determined predominantly by three variables: power of the cornea, power of the lens, and length of the eye.
In emmetropia, these three components combine to produce no refractive error. In an emmetropic eye, a pencil-like ray of light parallel to the optical axis and limited by the pupil focuses on the retina (the secondary focal point of an emmetropic eye; Fig. 15-1 ). The far point in emmetropia (defined as the point conjugate to the retina in the nonaccommodating state) is optical infinity.
Eyes with refractive errors have a mismatch of these variables. For example, an eye with an axial length in the upper range of normal may be myopic if the corneal steepness variable is also in the upper range of normal.
Myopia is the most common visually significant refractive error, with a prevalence of nearly 25% for Caucasians and 13% for African Americans. The myopic eye brings a pencil of parallel rays of light into focus at a point anterior to the retina. This point, the secondary focal point of the eye, is in the vitreous (see Fig. 15-1 ). The far point of a myopic eye is between infinity and the anterior surface of the cornea. Rays that diverge from this point are brought to focus on the retina without the aid of accommodation. For the full correction of myopia, a distance corrective lens placed in front of the eye must have its secondary focal point coincident with the far point. The newly created optical system allows parallel rays that come from infinity to diverge as if they originated from the far point of the eye and thus focus on the retina. In refractive surgical procedures for myopia, the refractive power of the cornea, or the crystalline lens, is reduced so that parallel rays from infinity can also focus on the retina.
The hyperopic eye brings a pencil of parallel rays of light into focus at a point behind the retina. Accommodation of the eye may produce enough additional plus power to allow the light rays to focus on the retina. The far point of a hyperopic eye is behind the eye, or beyond infinity. For the full correction of hyperopia, a corrective lens placed in front of the eye must have its secondary focal point coincident with the far point. Parallel rays from infinity exit the corrective lens, converge toward the far point of the eye, and thus can focus on the retina without the aid of accommodation. Hyperopia affects approximately 40% of the adult population. In the prepresbyopic age group, low to moderate hyperopia is less visually significant than myopia is. The great majority of young hyperopes regard their eyes as being optically normal. They might experience an earlier onset of presbyopia, however. Older hyperopes or patients with high degrees of hyperopia that exceed their accommodative reserve require optical correction for clear distance vision. They can benefit from refractive surgical procedures in which the corneal curvature is steepened or the power of the crystalline lens is increased
Figure 15-1 Emmetropia, myopia, and hyperopia. In emmetropia, the far point is at infinity, and the secondary focal point (F2 ) is at the retina. In myopia, the far point is in front of the eye, and the secondary focal point (F2 ) is in the vitreous. In hyperopia, the secondary focal point (F2 ) is behind the eye. (Modified with permission from Azar DT, Strauss L. Principles of applied clinical optics. In: Albert DM, Jakobie FA, eds. Principles and practice of ophthalmology, vol 6, ed. 2. Philadelphia: WB Saunders; 2000:5329–40.)
to converge rays of light that emanate from distant objects onto the retina without the aid of accommodation.
Astigmatism is caused by a toric cornea or, less often, by astigmatic effects of the crystalline lens. Astigmatism is regular when it is correctable with cylindrical or spherocylindrical lenses. Otherwise, the astigmatism is irregular (see Wave Front Deformation , later). Regular astigmatism is termed “with the rule” when the steepest corneal meridian is close to 90° and “against the rule” when the steepest meridian is close to 180°. When the astigmatism is regular but the principal meridians do not lie close to 90° or 180°, the astigmatism is called oblique. Depending on the spherical ametropia of the particular eye, astigmatism may be classified as simple or compound based on whether one or both meridians, respectively, are focused outside the retina. If one meridian focuses in front of the retina and the other meridian focuses behind it, the astigmatism is called mixed.
Astigmatism can be natural or surgically induced. Natural astigmatism is common; up to 95% of eyes may have some clinically detectable astigmatism. In the general population, 10–20% can be expected to have natural astigmatism greater than 1D, with an uncorrected visual acuity that might be considered unsatisfactory. Binocular spectacle correction of oblique astigmatism tilts each eye’s view and may distort the perceived three-dimensional image. This spatial distortion disappears when one eye is occluded; it is minimized, at the expense of clarity, by rotation of the axis toward 90° or 180° to reduce the tilt, or by reduction of the cylinder power ( Fig. 15-2 ).  When astigmatism is corrected at the corneal plane, such as with contact lenses or keratorefractive surgery, full correction reduces meridional magnification and eliminates the optical distortion.
The mechanism of accommodation, suggested by von Helmholtz in the 1850s, states that as the ciliary muscle contracts circumferentially, it relaxes the zonules and allows the crystalline lens to assume a more spherical configuration by virtue of its own elasticity. As the crystalline lens hardens with age, it is no longer able to attain the more spherical form, leading to the onset of presbyopia. An alternative theory of accommodation contends that the rounding of the central crystalline lens is due to the peripheral pulling of the radial fibers of the ciliary muscle, increasing the tautness of the equatorial zonules. This latter theory states that the cause of presbyopia is the continuous growth of the crystalline lens throughout life with relaxation of the equatorial zonules and decrease of its pulling ability.
The age of onset of presbyopia depends on the refractive error. A myope always has clear vision of objects placed at or near his or her far point. A latent hyperope, in contrast, uses accommodative reserve for clear distance vision; as the amplitude of accommodation decreases with age, reading difficulties arise. The method of optical correction also affects the age of onset of presbyopia. When myopes focus on a near object through the distance prescription of spectacles, less accommodation is needed than with contact lenses or after keratorefractive surgery. Conversely, hyperopes have a decreased accommodative demand with contact lenses or keratorefractive surgery.
Presbyopia is an important aspect of informed consent for keratorefractive surgery patients. Patients older than 40 years who consider refractive surgery for myopia must appreciate the extent to which they exchange dependence on distance spectacles for dependence on near-vision spectacles. Surgically corrected presbyopic myopes will need reading glasses as they age, whereas before surgery, they could remove their spectacles to read.
When an image of an object is formed, the linear magnification may be defined as the quotient of the sizes, measured perpendicular to the optical axis, of image and object. Axial magnification is measured in a similar fashion but parallel to the optical axis; it is the square of the linear magnification. Angular magnification refers to the quotient of the angles subtended by the object’s image when viewed with and without an optical aid.
Different optical aids create different angular magnifications. For viewing through spectacle minus lenses, a minification of roughly 2% per diopter occurs; this decreases dramatically when the same correction occurs at the corneal plane by either contact lenses or keratorefractive surgery. A patient with -20D who undergoes refractive surgery is expected to gain a line of best-corrected visual acuity by the elimination of image minification secondary to high-power spectacles. With high hyperopes, the effect is the opposite.
After the pupil has dilated, spherical aberration is produced as the rays of light hit the peripheral lens and are bent more than
Figure 15-2 Simulation of changes to astigmatic correction to reduce distortion. A, Distorted image from full spectacle correction of oblique astigmatism. B, Decreased distortion obtained by a reduction of the cylinder power. C, Improved direction of distortion (vertical) as well as decreased amount of distortion obtained by rotation of the plus cylinder axis to 180° and reduction of the cylinder power. (Reprinted with permission from Guyton DL. Prescribing cylinders: the problem of distortion. Surv Ophthalmol. 1977;22:177–88.)
the central rays; this produces a myopic shift, which is partially counteracted by the asphericity where the cornea flattens toward the periphery. A cycloplegic refraction is an essential part of the evaluation of a refractive surgery candidate. With cycloplegia, or relaxation of the ciliary body, a hyperopic shift occurs, counteracted by the spherical aberration just described. The intensity of this shift depends on the accommodative tone and is quite pronounced in young individuals. It may be useful to measure the cycloplegic refraction through a 3–4?mm aperture to account only for the accommodative tone; in this way, the effect of the peripheral cornea and the lens on the refraction is negated.
Pupil Size and Centration of Refractive Procedures
Rays of light from a single point source are refracted by the area of the cornea that overlies the entrance pupil. That area is called the corneal optical zone (see Central Cornea , later). The entrance pupil is the virtual image of the anatomical pupil formed by the magnifying effect of the cornea; it is larger and closer to the cornea than is the real pupil. The entrance pupil is conjugate to the anatomical pupil, so light rays refracted by the cornea and directed toward the entrance pupil pass through the anatomical pupil. Even if the pupil is eccentric, the pencil of light rays that reaches the fovea is limited by the entrance pupil ( Fig. 15-3 ). The elusive visual axis is located within this bundle of light rays and does not correspond to the corneal light reflex or the geometrical apex of the cornea. The foveal photoreceptors orient themselves toward the center of the pupil (Stiles-Crawford effect), even if the entrance pupil becomes eccentric.
Another factor to consider when establishing the center for keratorefractive procedures is pupillary dilatation under mesopic or scotopic conditions. The pupil diameter might reach 6–8?mm under decreased light conditions. The optical zone in a keratorefractive procedure is defined as the area of the central cornea that bears the refractive change caused by the surgery. There is a limit to the size of the optical zone in the different keratorefractive procedures: 3.0–5.5?mm in radial keratotomy and 4.5–8.0?mm in photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (LASIK). As the pupil dilates beyond the edge of the optical zone, the rays of light are refracted differently in the midperipheral and the central cornea. This differential causes edge glare and haloes around objects, a phenomenon that is more pronounced at night or in cases of decentration of the optical zone. Some patients with particularly large pupils may have a mismatch between pupil size and optical zone diameter and should be warned before surgery about possible optical distortion under mesopic conditions.
Figure 15-3 Effect of entrance pupil. The optical zone of the cornea and the line of sight are limited by the entrance pupil. If the pupil becomes eccentric, the optical zone and the line of sight are limited by the new entrance pupil. Note that the new optical zone and line of sight are unrelated to the center of the cornea or the corneal light reflex. (Modified with permission from Uozato H, Guyton DL. Centering corneal surgical procedures. Am J Ophthalmol. 1987;103:264–75.)
The active reorientation of the photoreceptors toward the center of the pupil and the possibility of edge glare if the entrance pupil extends beyond the optic zone of the keratorefractive procedure favor the centration of keratorefractive procedures on the pupil instead of on the elusive visual axis. The same optical principles apply to anterior and posterior chamber intraocular lenses (IOLs).
It may be better not to use miotics to center the refractive procedure, because the center of the pupil may move in the nasal direction with miosis, which produces temporal edge glare when the pupil dilates after surgery.
Corneal Topographical Changes After Keratorefractive Procedures
Classic and automated keratometers are used to measure the radii of curvature of the cornea along two major axes of a circle approximately 3.0?mm in diameter. The dioptric power of the cornea is calculated from the radii measurement, using the keratometric index.
To obtain information about the radius of curvature at corneal points other than those at 3.0?mm, several corneal topographical systems have been developed. These provide a complete dioptric map of the central and peripheral cornea. Most available corneal topographical systems are based on Placido’s disc, in which a series of concentric rings is projected onto the anterior corneal surface. The reduced and upright specular image is digitized and used to calculate the radii of curvature of the cornea at different points. From the radii values, the dioptric power of the cornea is calculated and displayed in a color-coded scale.
The use of corneal topography has enhanced the understanding of various keratorefractive procedures and has unveiled crucial information about regular and irregular astigmatism, optical zone sizes, centration, refractive change, regional healing patterns, and many other aspects of the refractive procedure ( Fig. 15-4 ). An irregular corneal surface (intraoperative drift) is more deleterious to visual acuity than is a displaced ablation (shift) after excimer laser surgery.  It has also been useful to screen candidates for keratorefractive surgery to rule out the presence of contact lens warpage ( Fig. 15-5 ) and keratoconus. Notwithstanding these advantages, Placido-based corneal topography systems have multiple sources of error, including the following:
• Variability with poor centration and focusing.
• Use of spherical instead of aspherical reference surfaces.
• Excessive estimation of missing values.
• Inaccurate conversion from radii of curvature to dioptric power.
• Low performance in irregular, poorly reflecting corneas.
New software developments, such as the use of tangential dioptric maps and Fourier analysis, have addressed some of these issues. The Placido-based corneal topographical systems provide useful clinical information rapidly, but the fundamental problem
Figure 15-4 Corneal topography after refractive surgery in a 58-year-old man.
of calculating surface elevation from a projected set of rings seems inherent to all machines. Two new forms of corneal topography are commercially available today—rasterstereo videokeratography and slit-lamp scanning systems. The latter also allows calculation of the shape of the posterior surface of the cornea, the thickness of the cornea, the depth of the anterior chamber, and the anterior and posterior surfaces of the crystalline lens.
Wave Front Deformation
The principles of wave front deformation measurements come from astronomical optics. In a perfect optical system, all the refracted rays are focused on a single plane (wave front). Optical aberrations induce deformations on this plane and can be quantitated. They represent the optical performance of the entire visual system, not only the anterior surface of the cornea, as in most corneal topography machines. The lower-order optical aberrations (sphere and astigmatism) can be corrected with spherocylindrical glasses. The higher-order aberrations (spherical aberration, coma) correspond to what is clinically known as irregular astigmatism ( Fig. 15-6 ). With the use of advanced lasers and wave front deformation measuring devices, the correction of these distortions of the human eye has been attempted. 
Intraocular Lens Calculation After Keratorefractive Surgery
When calculating the power of an IOL, two biological variables are measured—the effective corneal power, and the length of the eye. Because the shape of the cornea is modified by keratorefractive surgery, calculation of the effective corneal power using the traditional method of keratometry before cataract surgery has led to poor estimations of the IOL power required after surgery.
Two problems may be encountered in estimating corneal power after keratorefractive surgery. The first occurs after radial keratotomy (RK) or other peripheral corneal procedures with small optical zones, in which the ring measured by classic and automated keratometers falls in the midperipheral, steepened cornea outside the small, flattened optical zone, overestimating the effective
Figure 15-5 Corneal topography before refractive surgery in a 34-year-old woman who had used daily-wear soft contact lenses for 12 years. A, Despite not wearing contact lenses for 2 weeks prior to the appointment, the patient’s corneal topographies were asymmetrical, with a pseudokeratoconus picture of the left eye. B, After 2 months, the topographical picture assumed a more normal pattern, and the refraction stabilized without using the contact lenses; refractive surgery was then undertaken.
corneal power. The second problem occurs in cases of PRK or LASIK for myopia, in which a decrease occurs in the central thickness of the cornea. The refractive index of the corneal tissue is 1.37. The keratometric index used in most keratometers to convert from radius of curvature to dioptric power is the slightly lower value of 1.3375, to account for the divergent posterior surface of the cornea. If the thickness of the cornea is decreased, the diverging power of the posterior surface of the cornea increases relative to that of the anterior surface, so the keratometric index should be even smaller than 1.3375. Current keratometers and computerized videokeratoscopes that use this value when the cornea has been thinned tend to overestimate the effective corneal power.
Different approaches, such as the use of computerized videokeratography with smaller rings and different software or of hard contact lens overrefraction using a standardized lens, have been suggested to calculate the effective corneal power after keratorefractive procedures. If the preoperative keratometric readings are known, the change in refraction at the corneal plane can be subtracted from those readings to calculate the postoperative effective corneal power.
CLASSIFICATION OF REFRACTIVE PROCEDURES
New refractive techniques are being developed continually and the older techniques refined and simplified. The refractive power of an optical system, such as the eye, can be modified by changing the curvature of the refractive surfaces, the index of refraction of the different media, or the relative location of the different elements of the system.
Several classifications of keratorefractive surgery have been proposed, based on the mechanisms of action of the surgery or on the type of surgery. A simplified classification has been proposed in which the site of action of the surgery on the cornea—either over the optical zone or peripheral to it—is matched against the four different mechanisms of action of corneal surgery: addition, subtraction, relaxation, and coagulation-compression. The procedures that act on the optical zone are further subdivided into superficial or intrastromal ( Table 15-1 ). The use of IOLs to correct the refractive error does not have as many variations as do keratorefractive procedures. The lenses can be inserted into a phakic or aphakic eye or into the anterior or posterior chamber to add or subtract from the refractive status of the eye. The management of presbyopia depends on the theory of its pathogenesis that the technique intends to correct; it may either render the crystalline lens more pliable or increase the tension of the equatorial zonules.
Approximately two thirds of refraction occurs at the air-tear interface, which generally parallels the anterior surface of the cornea. The cornea is readily accessible, and its curvature can be modified as an extraocular procedure. Most keratorefractive procedures to date modify the radius of curvature of the anterior surface of the cornea.
Figure 15-6 Wave front changes after refractive surgery. (From Mrochen M, Kraemmerer M, Seiler T. Wavefront-guided laser in situ keratomileusis: early results in three eyes. J Refract Surg. 2000;16:116–21.)
TABLE 15-1 — PROPOSED CLASSIFICATION OF KERATOREFRACTIVE SURGICAL PROCEDURES
Keratomileusis in situ
Intracorneal ring segments
BKS, Barraquer-Krumeich-Swinger; LASEK, laser subepithelial keratomileusis; LASIK, laser stromal in situ keratomileusis; PRK, photorefractive keratectomy.
Most procedures used to modify the corneal optical zone, or central cornea, change the relationship between its anterior and posterior surfaces; the thickness of the cornea is also modified. The central cornea may be modified either on the surface or intrastromally. If the intrastromal procedure involves either the blunt or sharp dissection of the corneal lamellae, it is called lamellar refractive surgery.
Corneal Surface: Addition
Epikeratophakia (also known as epikeratoplasty and onlay lamellar keratoplasty) was introduced by Werblin et al. It involves removal of the epithelium from the central cornea and preparation of a peripheral annular keratotomy. No microkeratome is used. A lyophilized donor lenticule (consisting of Bowman’s layer and anterior stroma) is reconstituted and sewn into the annular keratotomy site. Theoretical advantages of epikeratophakia are its simplicity and reversibility.
Although this procedure can be used to correct greater degrees of myopia and hyperopia, complications of irregular astigmatism, delayed visual recovery, and prolonged epithelial defects are common. Its use for the general treatment of myopia and hyperopia has been abandoned, largely because of the potential loss of best-corrected visual acuity.
Synthetic materials and improved means of attaching the lenticule to the cornea may allow epikeratoplasty to become a more useful refractive technique in the future.
Corneal Surface: Subtraction
Excimer laser corneal surgery was introduced as a precise tool for linear keratectomies by Trokel et al. in 1983 but was later used for corneal reprofiling or PRK. The 193?nm ultraviolet laser (excimer or solid state) allows the anterior corneal surface to be reprofiled precisely to change its radius of curvature ( Fig. 15-7 ).
Three types of laser delivery are available: wide-area ablation, scanning slit, and flying spot lasers. The trend is toward use of the last, which allows customization of the treatment to each patient. Treatment of myopia, astigmatism, hyperopia, and even presbyopia has been attempted.
The depth of ablation necessary to correct myopia is highly dependent on the size of the treatment zone. To minimize haloes and edge glare in scotopic conditions, it is recommended that the optical zone be larger than the physiologically dilated pupil, which in certain cases may require significant ablation of the anterior stroma.
The excimer laser can be used to flatten or steepen differentially the corneal meridians and hence to treat compound myopic and compound hyperopic astigmatism. Mixed astigmatism can be treated by flattening the refractively more powerful meridian or by steepening the weaker one. Two methods have been used: the bitoric method, where the astigmatism is divided at the circle of least confusion of the conoid of Sturm, thus minimizing the total amount of ablation to the cornea, and the cross-cylinder method, where the astigmatism is divided in equal amounts, adding the spherical equivalent ablation and
Figure 15-7 Photorefractive keratectomy (PRK). After removal of the corneal epithelium, the excimer laser is used to reprofile the anterior curvature of the cornea, which changes its refractive power.
thus increasing the symmetry of the ablation. Further clinical studies are needed to elucidate the best method to correct this form of astigmatism.
After PRK, the corneal epithelium undergoes a hyperplastic phase in which the refractive status of the eye may be modified. If corneal epithelium accumulates centrally after a myopic ablation, it causes the refractive effect to regress ( Fig. 15-8 ). The deposition of new collagen and glycosaminoglycans by activated stromal keratocytes after PRK is a common phenomenon ( Fig. 15-9 ), manifested as corneal haze or subepithelial scarring. The activation of the keratocytes seems to stem from interaction of epithelial cells and raw corneal stroma as the epithelium migrates to cover the defect, or from activation of keratocytes by soluble tear factors that percolate through the initial epithelial defect after PRK. The haze may be associated with regression of the refractive effect or focal topographical abnormalities; it peaks in humans 3–6 months after the operation and disappears after 1 year for most patients.
PRK results for low myopia (up to 6D) and low hyperopia (up to 3D) are encouraging. Highly ametropic patients often regress 6–12 months after surface PRK, presumably because of stromal regeneration or epithelial hyperplasia.
Laser subepithelial keratomileusis.
Laser subepithelial keratomileusis (LASEK) involves cleaving the epithelial sheet at the basement membrane with dilute alcohol, applying the laser as in
Figure 15-8 Epithelial healing. Light microscopic findings of rabbit cornea 1 week after superficial midperipheral annular keratectomy. Note the epithelial hyperplasia on the ablated area toward the right side of the microphotograph. (From Jain S, Chamon W, Stark WJ, et al. Intrastromal epithelial accretion follows deep excimer annular keratectomy. Cornea. 1996;15:248–57.)
Figure 15-9 Subepithelial haze. Fluorescent microscope composite microphotograph 1 month after photorefractive keratectomy in a rabbit. The collagen at the base of the ablation was permanently stained with fluorescein to reveal a dark area of new connective tissue deposition between the stained area and the corneal epithelium.
conventional PRK, and repositioning the epithelium afterward. There is some decrease in pain, quicker visual rehabilitation, and less haze than after classic PRK.
Corneal Stroma: Subtraction
Keratomileusis refers to carving the cornea. Barraquer first reported clinical results in 1964. Krwawicz from Poland reported on the resection of midstromal corneal tissue or stromectomy for the treatment of myopia in 1963, but he did not elaborate or develop the technique as extensively as Barraquer did.
Classic keratomileusis involves the excision of a lamellar button of parallel faces from the cornea with a microkeratome, freezing and reshaping the lamellar button, and replacing it in position with sutures ( Fig. 15-10 ). The procedure was modified by Krumeich and Swinger, who reshaped the disc with a second microkeratome pass and did not have to freeze it, in a procedure known as BKS (Barraquer-Krumeich-Swinger) keratomileusis. Ruiz and Rowsey made further modifications by applying the second microkeratome pass to the stromal bed instead of to the resected disc, in a procedure called in situ keratomileusis. Even though the refractive cut with the microkeratome gave a disc of parallel surfaces with no optical power, a dioptric effect was achieved because of the remodeling corneal tissue, as described by Barraquer in the law of thickness. The development of a mechanized microkeratome, or automatic corneal shaper, provided a more consistent thickness and diameter of the corneal disc and improved the predictability of the procedure. This procedure is known as automated lamellar keratoplasty (ALK). The fact that the corneal cap does not have to be modified led to the use of a hinged flap instead of a free cap. This, in turn, led to sutureless repositioning of the flap, which simplified the procedure further.
Laser-assisted in situ keratomileusis.
The exponential growth of LASIK refractive correction makes it the most commonly performed refractive surgery in the world today. The combination of a lamellar dissection with the microkeratome and a refractive ablation in the bed with the excimer laser was first performed in
Figure 15-10 Freeze keratomileusis. A disc of parallel sides is resected from the cornea with the microkeratome. After freezing the disc, a lenticule of predetermined power is removed from the stromal side with a lathe. The removed cornea is sutured back in place.
rabbits by Pallikaris et al. in a modification of Ruiz’s keratomileusis in situ ( Fig. 15-11 ). Buratto and Ferrari first performed this procedure in humans after inadvertently obtaining a thin resection with the microkeratome while performing a modification of Barraquer’s classic keratomileusis using the excimer laser instead of the cryolathe to modify the corneal cap.
LASIK is similar to PRK in that an excimer or ultraviolet laser is applied to the cornea to modify its radius of curvature. The difference is that in PRK the laser is applied directly to Bowman’s layer, whereas in LASIK the laser is applied to the midstroma after a flap has been lifted from the cornea. The flap is replaced and adheres spontaneously, helped by the endothelial pump. In LASIK there is some degree of epithelial hyperplasia that causes regression of the effect, although to a lesser degree than in PRK