LASIK for Correction of Myopia

Background

One of the most promising and exciting developments in the world of refractive surgery has been the advent of laser in situ keratomileusis (LASIK). The surgical technique involves the creation of a hinged lamellar corneal flap, after which an excimer laser is used to make a refractive cut on the underlying stromal bed. LASIK is a fusion of old and new technologies, with its roots in keratomileusis and automated lamellar keratectomy (ALK). However, as currently practiced, it is perhaps best thought of as photorefractive keratectomy (PRK) performed under a flap instead of on the corneal surface.

LASIK has been available in the United States as an off-label procedure since the mid 1990s. FDA approval of excimer lasers for LASIK dates to about 1999.^[1] Many millions of procedures have been performed worldwide. According to the American Society of Cataract and Refractive Surgery, about 700,000 procedures a year are currently performed in the United States.

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Spherical aberration: a schematic diagram for the human eye.

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History of the Procedure

Jose Barraquer is generally credited with much of the early work leading to corneal lamellar refractive procedures as they are currently practiced. He noted that refractive change could be accomplished in the cornea by tissue addition or subtraction. He subsequently developed the idea of resecting a corneal disc and freezing it, followed by shaping the disc with a cryolathe.^{[2, 3, 4]} However, the technique was limited by complexity of the equipment and tissue damage to the resected corneal disc caused by freezing.

Ruiz and Barraquer performed keratomileusis in situ in the late 1980s. Using principles developed by Krumeich, this technique involved first removing a corneal disc with a microkeratome. Refractive change was accomplished by performing a second plano cut with the microkeratome. The thickness and diameter of this second disc of tissue determined the end refractive result; then, the first disc was sutured back onto the cornea. Problems included complexity, poor predictability, and irregular astigmatism.

Burratto and Pallikaris were the first to combine the use of the excimer laser and microkeratome technology. Burratto's original work involved performing a corrective excimer laser ablation on the back of a resected disc of corneal tissue. This disc was replaced and sutured onto the cornea. Pallikaris developed the technique of performing the excimer laser corrective ablation in the corneal stromal bed under a hinged flap. He first studied the procedure in rabbits, followed by blind human eyes in 1989, and then sighted eyes in 1991.

In 1993, Steve Slade added the refinement of using an automated microkeratome to create the flap and was one of the first US surgeons to perform LASIK.

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Indications

As of 2018, LASIK has been approved by the Food and Drug Administration (FDA) for several different laser platforms, including the VISX STAR S4, Allegretto Wavelight, Technolas, and NIDEK lasers. The approved range for myopic, hyperopic, and custom treatments varies slightly between platforms.

Table 1 summarizes these devices and their FDA status.

Table 1. Device Summary and FDA Status

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See Table

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Relevant Anatomy

The cornea is a thin layer of transparent tissue that protects the intraocular contents and refracts light. Average central corneal thickness is about 550 µm, increasing to about 700 µm in the periphery. The cornea has a diameter (from the front surface) of about 11 mm vertically and 12 mm horizontally. The air-tear interface is the first refractive surface that light encounters and accounts for about 80% of the eye's total refractive power; the average corneal curvature (K readings) in the adult cornea is approximately 44.00 diopters (D).

Anatomically, the cornea consists of 5 layers: epithelium, Bowman layer, stroma, Descemet membrane, and endothelium.

Three types of cells are present in the epithelium: (1) basal columnar cells attached to the epithelial basement membrane via hemidesmosomes, (2) wing cells noted for thin winglike projections, and (3) surface cells joined by connecting bridges and covered by microvilli. Mucin is attached strongly to the surface. Usually, 5-7 layers of cells are present. Unlike stratified squamous epithelium in other areas of the body, the epithelium in the eye has an exceptionally smooth and regular surface, contributing to the transparency and light transmission characteristics of the cornea.

The Bowman layer is not a membrane, but rather an acellular structure consisting of collagen and representing the most superficial layer of the stroma.

The stroma makes up about 90% of the corneal thickness and consists of regularly arrayed flattened bundles of collagen called lamellae. Approximately 200-250 lamellae are present in the human cornea. Each bundle extends the width of the cornea and is about 2 µm thick and up to 260 µm wide. The parallel arrangement of these bundles together with the uniform spacing between collagen fibrils helps explain corneal transparency. Although relatively acellular, stromal fibroblasts called keratocytes can be found scattered throughout the stroma between lamellae, and they are responsible for collagen production and wound healing.

The Descemet membrane is composed of a fine latticework of collagen fibers. It represents a true basement membrane, and it is produced by the corneal endothelium.

The endothelium is a single layer of hexagonal cells whose sole purpose is to act as a barrier to the influx of fluid into the cornea and to pump fluid out of the cornea keeping it deturgesced and clear. These cells are incapable of regeneration.

The cornea is richly innervated; myelin sheaths are present on the nerves as they traverse the superficial layers of the cornea. The nerve endings lose their sheath as they penetrate the epithelium. In terms of density, more nerve endings are present in the corneal epithelium than anywhere else in the human body.^{[5, 6]}

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Contraindications

Contraindications include unstable refractive error, active collagen vascular disease (especially in the presence of iritis or scleritis), pregnancy, presence of a pacemaker, any ongoing active inflammation of the external eye (eg, conjunctivitis, severe dry eye), and a refractive error outside the range of laser correction.

Other contraindications include leaving less than a calculated residual bed of 250 µm of untouched cornea, as well as signs, symptoms, or topographic findings consistent with keratoconus. Residual stromal bed thickness is calculated by subtracting ablation depth plus flap thickness from the corneal thickness as measured by pachymetry.

Patients who are on Accutane (isotretinoin), Cordarone (amiodarone hydrochloride), and Imitrex (sumatriptan) should be treated with caution, and patient counseling should be provided because these medications may adversely affect corneal wound healing.

A history of herpetic keratitis is a relative contraindication. Although patients have been treated safely with a history of herpes simplex keratitis and the appropriate use of prophylactic antivirals, reactivation of the virus following treatment remains a concern.

Patients who cannot cooperate with procedures under a topical anesthetic and cannot accurately fixate or lay flat without difficulty are poor candidates for refractive surgery.

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Other Tests

Ultrasound corneal pachymetry

Pachymetry is an important part of the refractive surgery workup.

The FDA has mandated that 250 µm of untouched cornea remain in the bed following LASIK. This is calculated as follows: initial pachymetry minus calculated (or measured) flap thickness minus ablation depth must be greater than or equal to 250 µm.^[7] Ectasia, which represents a biomechanical weakening of the cornea (see Complications), is risked when the residual bed is less than 250 µm. Note that leaving 250 µm in the residual bed does not guarantee that ectasia will not occur; this is simply the current FDA guideline.

Wavefront analysis

Some basic concepts are useful in understanding wavefront analysis customized corneal ablations.^{[8, 9]}

Wavefront technology is an offshoot of astrophysics and was initially developed to help obtain undistorted telescopic images of the night sky. The current technology used in refractive surgery examines what happens as light interacts with the optical system of the eye.

A wavefront represents a locus of points that connects all the rays of light emanating from a point source that have the same temporal phase and optical path length. The optical path length specifies the number of times a light wave must oscillate in traveling from one point to another point. Light propagation is slower in the refractive media of the eye than in air, so that more oscillations will occur in an optical system, such as an eye, than in air for light to travel the same distance. If the optical system of the eye is perfect, a point source of light emanating from the back of the eye will create a locus of points with the same optical path length exiting the pupillary plane in the form of a flat sheet. This represents an unaberrated wavefront. When the cornea or lens has imperfections, optical aberrations are created, causing the wavefront to exit the eye as curved or bent sheets of light.

Aberrations can be defined as the difference in optical path length (OPL) between any ray passing through a point in the pupillary plane and the chief ray passing through the pupil center. This is called the optical path difference (OPD) and would be 0 for a perfect optical system.

Another way of characterizing the wavefront is to measure the actual slope of light rays exiting the pupil plane at different points in the plane and compare these to the ideal; the direction of propagation of light rays will be perpendicular to the wavefront. This is the basic principle behind the Hartman-Shack devices commonly used to measure the wavefront. Wavefronts exiting the eye are allowed to interact with a microlenslet array. If the wavefront is a perfect flat sheet, it will form a perfect lattice of point images corresponding to the optical axis of each lenslet. If the wavefront is aberrated, the local slope of the wavefront will be different for each lenslet and result in a displaced spot on the grid as compared to the ideal. The displacement in location from the actual spot versus the ideal represents a measure of the shape of the wavefront.

Once the wavefront image is captured, it can be analyzed. One method of wavefront analysis and classification is to consider each wavefront map to be the weighted sum of fundamental shapes. Zernike and Fourier transforms are polynomial equations that have been adapted for this purpose. Zernike polynomials have proven especially useful since they contain radial components and the shape of the wavefront follows that of the pupil, which is circular. Fourier transforms, however, may prove to be more robust and allow mathematical description of the wavefront with less smoothing effect and greater fidelity. Illustrations of the basic Zernike shapes are appended.

The term higher order optical aberration has begun to replace the older term irregular astigmatism as wavefront analysis has become more accepted. This simply refers to the mathematical term used to describe the aberration and its place in the polynomial expansion. Lower order aberrations, such as sphere and cylinder, require lower order mathematical terms within the polynomial expansion to characterize them and are commonly referred to as second order aberrations. The most important higher order terms are spherical aberration (a fourth order term) and coma (a third order term).

Fortunately, spherical aberration is relatively easy to understand. Light rays entering the central area of a lens are bent less and come to a sharp focus at the focal point of a lens system; however, peripheral light rays tend to be bent more by the edge of a given lens system so that in a plus lens, the light rays are focused in front of the normal focal point of the lens and secondary images are created. This is why many lens systems incorporate an aspheric grind, so that the periphery of the lens system gradually tapers and refracts or bends light to a lesser degree than if this optical adaptation was not included. See the image below.

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Spherical aberration: a schematic diagram for the human eye.

Traditional myopic LASIK patterns tend to induce spherical aberration; the higher the degree of correction, the greater the induction of this optical error. During the day, the pupil size tends to limit the effect of spherical aberration, since peripheral light rays are blocked. At night, as the pupil enlarges in dark or scotopic conditions, these light rays enter the eye and can create a blurred focal point and secondary images.

Custom laser treatments incorporate a specific algorithm to help limit the induction of spherical aberration. This algorithm is based on a patient's unique wavefront measurement of their individual eye to some extent. However, the most important aspect of treatment is a blend or tapering of the peripheral treatment zone. Some lasers have incorporated a noncustom approach to this problem and create the transition zone at the edge of the ablation based on an empirical approach that takes into account the patient's prescription glasses and corneal curvature readings instead of using unique patient wavefront data. The best approach to limit this problem is under investigation. Note that coma and other preexisting aberrations would only be corrected by using data from an individual patient's unique wavefront error to plan and determine the shape of the laser ablation pattern. This approach is used in custom treatments.

Wavefront maps are commonly displayed as 2-dimensional maps. Just as interpretation of corneal topography has been greatly aided by the use of color maps, so too has wavefront mapping. The color green indicates minimal wavefront distortion from the ideal, while blue is characteristic of myopic wavefronts and red is characteristic of hyperopic wavefront errors.

Remember that wavefront maps are a 2-dimensional attempt to display 3-dimensional shapes. See the images below.

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Spherical aberration post-LASIK. The original refractive error was -10.00 diopters.

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Coma in a patient with mild ectasia. This higher order optical aberration is also characteristic of decentered ablation zones and ectasia.

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Postoperative ectasia: Orbscan. Note the elevation on anterior and posterior floats and the thinning of the central cornea on the pachymetry map.

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Ectasia post-LASIK: Tracey WaveScan. Note the preponderance of higher order aberrations, including spherical aberration and coma. The Orbscan of this ....

The Root Mean Square (or RMS) value has proven to be a useful way of quantifying the wavefront error and comparing it to normal. This number can be calculated for the wavefront as a whole or as individual components of the wavefront when displayed as Zernike polynomials. See the image below.

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Zernike polynomials: pictorial representation.

Corneal topography

Corneal topography is a method of measuring and quantifying the shape and the curvature of the corneal surface. Most topographers consist of a placido disc made up of multiple circles, which is backlit or projected onto the corneal surface. The resultant circular images are reflected and captured with a video camera and digitized.^[10]

Using the mathematics of convex mirrors and proprietary mathematical algorithms, the image size is measured and quantified. The resulting data are displayed as a corneal curvature map.

The maps consist of colors corresponding to corneal power and curvature; steep contours are displayed as warm colors (eg, red), while flat contours correspond to cool colors (eg, green, blue).

Both absolute and normalized maps can be displayed. Absolute maps always assign the same color to the same power, and normalized maps take into account the range of power over a given cornea, ascribing red and yellow colors to the steepest contours and blue and green colors to the flattest contours for that particular cornea.

Many factors can affect the accuracy and reproducibility of corneal topography maps; these factors include quality of the tear film, the ability of the patient to maintain fixation, and operator experience.

Corneal topography is used primarily as a screening tool to evaluate prospective refractive surgery candidates and a diagnostic aid in evaluating refractive surgery patients with poor outcomes.^{[11, 12]} Irregular corneas are poor candidates for refractive surgery since results with current lasers can be unpredictable. Keratoconus and contact lens warpage are the most common causes of irregular corneas in the screening population. Steep (ie, red) areas isolated to the inferior cornea suggest keratoconus, and many topographers come equipped with programs to alert the clinician when a diagnosis of keratoconus is likely. Postoperative patients with poor vision should have topography; such problems as central islands, irregular ablation profiles, and decentered laser ablations can be assessed with these devices.

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Normal astigmatism pattern with corneal topography.

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Normal corneal topography spherical pattern.

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Keratoconus suspect; inferior and asymmetric corneal astigmatism pattern.

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Keratoconus with elevation map; asymmetric and irregular astigmatism with inferior corneal elevation and steep area of inferior cornea.

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Preoperative Details

To a large extent, patient selection for LASIK often determines the overall success of the procedure; therefore, it is crucial that a thorough preoperative examination be performed, accompanied by appropriate counseling. Contact lens wear should be discontinued prior to the examination, ideally 3-5 days for soft contact lens wear and 2-3 weeks for rigid gas permeable lenses.

A complete eye examination, including manifest and cycloplegic refraction, slit lamp examination, dilated fundus examination, and corneal topography, is recommended. Wavefront measurements can also be taken as part of the initial screening examination and are helpful in determining if the patient is a candidate for custom treatment and as a comparison to the current glasses prescription and refraction. In addition, an estimate of scotopic pupil size is helpful in screening candidates who may be at risk for postoperative glare.

Poor surgical candidates include patients with a refraction out of the recommended correction range, patients with active inflammation of the external eye or iritis, and patients with cataracts or retinal holes or tears. Although LASIK surgery has only rarely been associated with vitreoretinal pathology, retinal detachments following surgery have been reported.^[13] Therefore, screening with indirect ophthalmoscopy is advisable. Dry eye is a relative contraindication as well, and every effort should be made to improve the health of the ocular surface prior to performing any refractive procedure.^[14] Patients with chronic punctate keratitis, meibomitis, and blepharitis are generally poor candidates unless these conditions can be resolved prior to surgery. A short trial of Restasis, artificial tears, and even tetracycline (for meibomitis) often results in a significant improvement of the ocular surface.

The refraction should be stable prior to performing surgery. Stability can be assessed by serial refractions and an evaluation of medical records and old glasses. Any change greater than 0.50 D in sphere or cylinder or an axis change greater than 10° in cylinder correction compared to the above is suspect and suggests that the current refraction is not stable.

Corneal topography is essential to rule out keratoconus and irregular astigmatism. These problems tend to make the surgical outcome unpredictable. In particular, keratoconus patients may be more prone to the development of ectasia or thinning following LASIK; refractive surgery on this group of patients is considered investigational. Several topography units come with built-in screening programs based on criteria developed by Rabinowitz and Klyce to aid in the detection of keratoconus.^[12] Corneal topography also is helpful in evaluating contact lens-induced corneal warping. Patients with irregular corneas and a history of contact lens wear should be observed with serial refractions and topography until both stabilize.

Finally, ultrasonic pachymetry is necessary to determine if enough corneal thickness is present to create a flap, ablate the cornea, and still leave enough tissue behind to prevent structural weakening and ectasia. Current guidelines recommend leaving at least 250 µm of cornea untouched.

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Intraoperative Details

The procedure usually is performed under topical anesthesia, but it can be supplemented by intravenous or oral conscious sedation.

A sterile drape and lid speculum is placed carefully to maximize exposure and to isolate the lashes. The patient is positioned underneath the microscope of the laser so that the flap can be cut under direct visualization.

The cornea is marked. A radial keratotomy marker and optic zone marker (placed eccentrically) dipped in methylene blue or gentian violet can be used. The marks allow replacement and alignment of the flap in the event that a nonhinged free flap is cut by the microkeratome.

Balanced salt solution (BSS) is used to rinse the ocular surface and to moisten the conjunctiva. Excess solution can be removed from the conjunctival fornices with Weck-cel sponges or a suction speculum. This rinsing removes mucus and debris from the ocular surface decreasing the chance that this material will find its way under the flap at the end of the procedure.

Microkeratomes differ in the method of assembly, flap hinge location, method of translation across the cornea (manual or automated), and whether components are disposable. The following technique applies to the use of the Moria One Use Plus microkeratome, a disposable device that creates a nasal hinge. However, the principles are similar no matter which microkeratome is used. The combined suction ring and microkeratome is placed on the eye and centered over the limbus with slight nasal displacement. Unlike the older style of microkeratomes, this microkeratome does not require on-eye assembly; this is particularly advantageous for novice surgeons. Nasal displacement ensures that the hinge of the flap will be clear of the path of the excimer laser ablation, but it increases the risk of a free flap. (This technique does not apply to the Hansatome microkeratome because the hinge is located superiorly with this device.) Suction is turned on by the surgeon or assistant.

The pressure in the eye is checked with a tonometer confirming that the intraocular pressure is at least 60 mm Hg. The pupil often can be seen to dilate, and the patient's vision will black out momentarily. Intraocular pressure with the suction ring applied is between 60-90 mm Hg. High pressure is necessary to hold the suction ring firmly in place and to properly expose the cornea to the cutting mechanism of the microkeratome.

A depth plate in the microkeratome determines the planned thickness for the flap resection (130 µm for the Moria One Use Plus). However, this represents only an estimate of the actual flap thickness; confirmation with on-the-table pachymetry measurements taken immediately before cutting the flap (total corneal thickness) and in the stromal bed after the flap has been lifted (residual stromal bed) is the best way to determine actual flap thickness.

Once good suction is confirmed, a foot pedal is used to simultaneously switch on the motorized vibrating blade that cuts the corneal flap and the mechanism that advances the microkeratome. The microkeratome should not be manipulated during the flap cutting phase, and it is important to remind the patient not to move or attempt to squeeze the eye shut during the cut. The hinge width can be set on this microkeratome by setting a stop on the suction ring. The stop setting is based on a nomogram supplied by the manufacturer and is based on the size of the opening of the suction ring and corneal curvature keratometry readings.

Inspect the flap prior to lifting. In general, thin or irregular flaps are left in place with minimal manipulation. A spatula is placed between the flap and the stromal bed, and the flap is reflected toward the hinge.

The laser is focused and centered, and the planned refractive ablation takes place. Most lasers have a tracking mechanism that tracks eye movements and locks onto the pupil.^[15]  The tracker is engaged prior to performing the planned laser ablation. Upon completion of the ablation, the flap is swept back into position with a spatula and then floated into position with irrigation under the flap. This irrigation also helps keep the interface between the flap and the corneal bed free of debris. A moistened Weck-cel sponge is lightly stroked once or twice over the corneal flap to squeeze out excess moisture in the bed, being careful not to apply so much pressure as to induce wrinkles in the flap.

The previously placed radial keratotomy marks and the eccentric zone mark are used to ensure that the flap is aligned properly. Once the surgeon feels alignment is satisfactory, the flap is allowed to remain undisturbed for several minutes. Endothelial pump pressure is the initial force that holds the flap in place.

The lid speculum and draping is removed carefully from the eye. The patient is allowed to blink to ensure that the flap remains in place. Immediate slit lamp examination is useful in detecting misplacement or wrinkles in the flap. The flap can be refloated and repositioned, if necessary. In some cases, light stroking of a flap immediately at the slit lamp with a Weck-Cel sponge can resolve small displacements or folds in the flap.

Postoperative medications (eg, topical antibiotic drops, topical steroid drops) are administered. Then, a see-through bubble shield is placed over the eye to prevent inadvertent rubbing of the eye.

Femtosecond laser flap creation

The procedure is different if a femtosecond laser is used to create the flap. Femtosecond lasers emit in the infrared range (1053 nm wavelength) and work by creating overlapping microcavitation bubbles, producing a lamellar intrastromal cut. The laser is first programmed to confirm the desired depth, diameter, and hinge location of the flap. The laser then must be "docked" to the patient's eye to hold the eye completely immobile during laser emission. The laser is then fired, creating the potential lamellar space first followed by a side cut to connect the flap to the surface of the cornea. Docking is released and the flap inspected. Unlike the microkeratome, no marks are generally placed on the flap prior to lifting it gently with a specialized instrument designed for this purpose. One end of the spatula is placed under the flap near the hinge and across the flap, exiting the flap near the opposite side. Gentle sweeping motions starting at the hinge and moving opposite of the hinge location toward the flap edge are used to separate the flap from the underlying stromal bed. The flap is then gently reflected toward the hinge, and the refractive laser procedure is performed as noted above. After completion of the ablation, the flap is reposited back into position similar to the method outlined for microkeratome created flaps.

One of the great advantages of the femtosecond laser for flap creation (such as the Intralase, AMO) is the ability to customize diameter and hinge location; this is especially useful for hyperopic ablations and treatments for mixed astigmatism, which require larger ablation zones. A smaller stromal bed would mean that the planned excimer laser treatment would overlap uncut cornea, potentially resulting in an incomplete or partial ablation and correction of the desired refractive error.

Microkeratome-created flaps depend on corneal curvature measurements, that is, the steeper the cornea, the more cornea that is exposed to the microkeratome during the forward pass, resulting in a larger diameter flap. The flatter the cornea curvature, the smaller the flap diameter. Very steep corneas (>48-49 D) are prone to button hole flaps, while very flat corneas (< 41-42 D) are prone to free flaps. Femtosecond flap creation is not corneal curvature dependent and therefore should be considered for these extremes of corneal curvature (see Complications).

Disadvantages of laser created flaps include extra time, expense, and difficulty lifting the flap if the microcavitation bubbles leave tissue bridges between the flap and stromal bed.

Enhancements

One of the great advantages of LASIK over other refractive procedures is the ease and safety of performing enhancement surgery.^[16] Enhancements should be postponed until the refractive error is stable, usually about 3 months postoperatively. It is common to wait longer, up to 6 months, for patients who experience an overcorrection because this will often regress.^[17] The corneal flap created by a microkeratome can usually be lifted easily within the first several years after surgery; beyond this time period, consideration should be given to alternate methods of refractive enhancement such as PRK performed on the flap surface without lifting.

Enhancement surgery is performed by first positioning the patient at the slit lamp. A special shaped spatula is used to gently lift the edge of the flap and to find the corneal plane of the original cut. Then, the patient is positioned under the laser. The cornea is marked as usual. There is no risk of a free flap, but these marks help in realigning the flap. A blunt spatula is passed under the flap and swept gently back and forth, almost to the edge of the flap but avoiding breaking through the edge to the surface. The flap is grasped firmly with non-tooth forceps and peeled back, creating a clean epithelial edge. The laser treatment proceeds as usual, replacing the flap after the procedure is complete. Some practitioners prefer to use a contact bandage lens to protect the flap and for patient comfort after surgery since the epithelial edge tends to be more irregular than if the flap were cut with the microkeratome.

Noting the original depth plate and the hinge location is helpful when primary LASIK is performed; this helps prevent tearing the hinge accidentally. Tearing can occur if the surgeon anticipates a nasal hinge, but a superior hinge was used previously.

Occasionally, a patient will not have enough residual corneal stroma in the flap to allow for an enhancement. In these patients, the correction can be applied to the surface on top of the flap by performing PRK. In general, this requires the use of mitomycin C to prevent corneal scarring and haze after treatment.

In patients whose records may not be available, the use of a device, such as an anterior segment OCT, may aid in determining the thickness of the original flap.

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Postoperative Details

The patient usually is seen within the first 24 hours following surgery to check visual acuity, to inspect flap position, and to ensure that no signs of infection or inflammation in the cornea are present.

A regimen of postoperative antibiotics, given 4 times a day for 1 week, is recommended. Fourth-generation fluoroquinolones are a good choice because of excellent corneal penetration and broad-spectrum coverage. Moxifloxacin 0.5% is the author's current preferred choice because of availability, penetration, and coverage of the most common bacterial pathogens likely to be encountered in the early postoperative period. Consensus on the use of topical steroids does not exist. However, most surgeons prescribe their use for the first week after surgery, discontinuing or tapering rapidly thereafter. A potent and penetrating steroid, such as prednisolone acetate 1%, commonly is used. This helps prevent inflammation under the flap. The role of topical steroids in influencing postoperative healing and regression has not been determined.

Patients who are undercorrected or who appear to be regressing rapidly (increasing myopia), as determined by serial refractions, may benefit from more prolonged treatment with topical steroids and a slower tapering off of these drops. Overcorrected patients may benefit from discontinuing steroids early in the postoperative period and by the use of topical nonsteroidal drops. These pharmaceutical maneuvers have not been studied in any controlled or randomized fashion.

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Follow-up

Follow-up examinations are performed on day 1, week 1-2, 3 months, 6 months, and 1 year after surgery. The examination should include uncorrected and best-corrected visual acuity, slit lamp examination, and tonometry (this examination is crucial if the patient is still on topical steroid drops). The corneal thinning associated with LASIK surgery can result in falsely low tonometry readings. It is important to also note that this is not the same as congenitally thin corneas, and nomogram adjustments for corneal thickness versus pressure have not been worked out for postrefractive patients. The Tono-Pen may be preferred over applanation for pressure measurements, since it seems to be less sensitive to corneal thickness variations.^[18]

Corneal topography is a useful adjunct in assessing postoperative results and planning enhancements and should be performed between week 1 and month 6. Centration and ablation pattern can be assessed best with topography; it is especially useful in patients who have an unexplained decrease in best-corrected visual acuity.

Repeat wavefront analysis prior to performing an enhancement is helpful to confirm the refraction, especially astigmatism and cylinder axis.

For excellent patient education resources, visit eMedicineHealth's Eye and Vision Center. Also, see eMedicineHealth's patient education article Vision Correction Surgery.

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Complications

Complications can be divided into intraoperative (usually microkeratome related) and those that occur postoperatively.^{[19, 20, 21]} The following list outlines the more common complications, the time period in which they are likely to be seen (ie, immediate, early postoperative, late postoperative), and an approximate incidence of occurrence. Each complication will be discussed in more detail in the following section.

Intraoperative microkeratome flap complications include the following:

• Entry into eye (intraoperative; rare)^[22]
• Thin, irregular, or perforated flap (intraoperative; < 0.2%)^[23]
• Free flap (intraoperative; rare; 0.2%)

Femtosecond laser flap complications are as follows:

• Opaque bubble layer
• Vertical gas breakthrough
• Anterior chamber gas bubbles
• Suction loss during flap creation

Laser-related complications include the following:

• Decentration (< 1%)
• Irregular astigmatism (< 1%) and central islands

Other postoperative complications include the following:

• Visually significant wrinkles or striae in the flap (1%)
• Dislocated flap (early postoperative period)
• Infection (early postoperative period; very rare; < 0.02%)^{[24, 25, 26]}
• Diffuse intralamellar keratitis (< 0.1%)^{[27, 28]}
• Epithelial ingrowth (early to late postoperative; 1-2%)^[29]
• Under/overcorrection (see results)
• Ectasia (incidence unknown; < 0.01%)

Intraoperative microkeratome-related complications

Perforation and entry into the eye

Probably the most dreaded complication related to use of the automatic corneal shaper (ACS) is perforation and entry into the eye. Because the eye is pressurized to about 60 mm Hg, entering the eye at this pressure is particularly hazardous. Case reports of iris and lens injury occurring at the time of entry are well documented. The cause is improper assembly of the ACS unit; specifically, leaving the depth plate out. True incidence of this rare complication is unknown. The ACS and similar first-generation microkeratomes are no longer in clinical use, further reducing the chance of this complication. Newer microkeratomes from most manufacturers (eg, Hansatome unit from Bausch & Lomb Surgical, Amadeus unit from AMO, One Use Plus from Moria) have a built-in depth plate to prevent this assembly error.

Thin or perforated (poor) flap

Another feared complication is a thin or perforated flap.

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Thin, perforated flap.

It usually occurs with loss of suction or poor suction when the suction ring is applied. Steep corneas with average K readings of 47.00 D or greater are also at higher risk for perforated buttonhole flaps (see the image below). When suction is turned on, the suction ring presses down around the limbus, causing distortion and an abrupt increase in pressure inside the eye. Characteristically, the pressure will rise to more than 60 mm Hg. A handheld tonometer is used to check the level of pressure in the eye. The pupil dilates, and the patient's vision blacks out as a result of temporary ischemia. Lifting the ring will correspondingly lift the eye.

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Buttonhole in flap.

All of the above are signs of good suction. If the surgeon detects poor suction, the procedure should be aborted and performed another day. In general, no attempt should be made to immediately replace the suction ring since the initial placement often causes slight conjunctival chemosis, precluding the possibility of obtaining good suction. Experienced surgeons may feel comfortable reapplying suction and attempting to cut the flap; however, the risk for a thin or poor flap is probably higher when repositioning the ring is attempted.

Another reason for poor flaps is patient eyelid squeezing during the microkeratome pass. This action pushes the microkeratome suction ring up and results in a thin flap or buttonhole. Squeezing can be prevented by adequately preparing the patient for the sound of the microkeratome and asking the patient to be especially careful about movement and eyelid closure at that moment. In general, lid blocks are not necessary.

The second eye in a bilateral case often has a slightly thinner flap as measured by subtraction pachymetry. Using one blade for both eyes in a given patient is common practice. The blade may dull slightly on the first eye; therefore, in patients with very steep corneas (46-47 D), using a new blade for the second eye may be helpful. This has not been shown to decrease the incidence of a thin or irregular flap.

Treatment of a perforated or buttonhole flap consists of immediately replacing the flap with as little manipulation as possible. If a buttonhole flap is recognized prior to lifting it, it should be smoothed back into position with Weck-cel sponges. Subsequent treatment depends on how the flap heals. If the flap adheres smoothly and heals without complications and if there is a good return of BCVA, then no immediate intervention is necessary. After 6 months, a flap can be recut. An alternative would be to consider subsequent surgery with the femtosecond laser cutting a flap well beneath the plane of the original cut. If the flap appears to have significant central irregularity, transepithelial PRK with adjunctive mitomycin C can be performed relatively early in the postoperative period.^{[30, 31]} The author has used this technique on occasion with good results.

Thin or irregular flap

Other microkeratome-related problems that can result in a thin or irregular flap include binding, jamming, or a jerky pass of the microkeratome over the corneal surface. Such problems often are caused by poor maintenance and failure to inspect the microkeratome by the surgeon or technician. It is the surgeon's responsibility to confirm a smooth pass of the microkeratome while it is engaged in the suction ring prior to making a corneal flap. The blade should be inspected carefully under the surgical microscope by the surgeon or the technician to confirm its proper insertion into the microkeratome and to ensure that edge abnormalities are not present. For instance, a notch in the blade has been shown to cause a divided flap, according to Robert Maloney, MD, in a presentation at the University of Colorado in 1997.

Free or partial flap

Free flaps occur for various reasons. Flat corneas with average K readings of 41.00 D and below are at risk for this complication. Excessive decentration of the suction ring on the cornea also can result in a free or partial flap. The key to managing this complication is composure and planning. First, good precut marks (usually made with a radial keratotomy marker and optical zone marker) on the cornea are essential for helping realign a free flap. Next, an assessment of the quality of the underlying stromal bed needs to be performed. If the bed is of good quality and appropriate size and position, the ablation is performed as usual.

Handling a free flap

The recommended method of handling the flap is that, in general, less manipulation of the flap is required if no attempt is made to remove it from the microkeratome and place it in a desiccation chamber. Instead, the flap is left in the microkeratome. After the ablation is performed, the unit is repositioned in the same orientation as the cut was made. The flap is removed gently from the microkeratome by grasping it with toothless forceps and sliding it onto the stromal bed (premoistened). If performed properly, minimal rotation of the flap is required to align it with the marks made on the corneal surface at the beginning of the procedure. The flap is allowed to settle onto the corneal surface for a few seconds; then, it is smoothed gently into position with very light strokes of a moistened Weck-cel sponge.

Suturing the flap usually is not necessary, although a suture can be placed in the flap following replacement and drying to create a pseudohinge. A bandage contact lens is not necessary.

Femtosecond laser flap complication: Opaque bubble layer

Opaque bubble layer is a complication unique to femtosecond lasers and is related to energy and spot placement; adjusting these parameters can decrease its presence. Although not dangerous in and of itself, it can interfere with eye tracking during the excimer laser ablation part of the procedure.

Vertical gas breakthrough usually occurs in eyes with corneal scars or surface irregularities that may not be recognized by the clinician as significant. Proceeding with a flap lift when this is noted can result in a torn flap. In this instance, it is best to abort the procedure and consider rescheduling the patient.

Anterior chamber gas bubbles can occur since the microcavitation bubbles represent intracorneal gas as the femtosecond laser creates the flap. This gas may not vent properly through the side cut and instead finds its way into the anterior chamber. The significance of this complication is interference with tracking by the laser. The tracking devices used on many excimer laser platforms follow the pupil; a gas bubble may be mistaken for the pupil, depending on size and location. Proceeding with the ablation part of the procedure may result in a decentered ablation (see below).

Laser-related complications

Decentration

Experienced laser surgeons recommend centering the laser ablation pattern over the pupil. All lasers currently approved for use in the United States are able to track the center of the pupil. A brief discussion of tracking technology appears in Future and Controversies. Despite tracking, however, decentration of the ablation (see the image below) can still be a significant problem with all excimer laser systems.^[32]

View Image

Decentered flap and ablation.

Factors that probably contribute to decentration include the following: (1) surgeon experience, (2) degree of myopia to be corrected, and (3) location of the visual axis line of sight versus the center of the pupil. The more myopic a patient is, the greater difficulty the patient may have in seeing fixation lights. Turning down external light sources (eg, oblique and ring lights on the VISX laser) aids patient fixation. Some controversy remains over whether it is better to center laser ablations over the pupil or the patient's line of sight. Normally, little clinical difference exists between the two methods; however, occasionally, patients have a large positive or negative angle kappa, and the decision on where to focus the laser becomes problematic. At present, no clinical studies that compare the two methods of laser alignment exist. See the image below.

View Image

Pupil alignment or visual axis alignment for laser ablation.

Compounding the problem is the way in which decentration is measured clinically. In general, topography maps of the cornea are used to assess alignment. However, topography centers around the line of sight and is based on patient fixation. This alignment is often slightly different than the corneal apex (highest point on the cornea) and the center of the pupil. If the difference between the line of sight and the center of the pupil is relatively large, the ablation pattern will appear decentered on the topographic map.

Actual decentration is characterized clinically by poor uncorrected and best-corrected vision, complaints of glare, "ghosting" around images and haloes, and refractive astigmatism (usually plus cylinder) in the axis of decentration. Light scatter occurring at the edge of the ablation zone causes the above symptoms. Normally, the pupil would mask light scatter; however, if the edge of the ablation pattern is near the center of the pupil, it becomes readily evident to the patient. Wavefront analysis may be helpful in establishing the diagnosis of decentered laser ablation since higher order aberrations, such as coma, may be more prevalent. Customized corneal ablations or topographic linked systems offer the best hope for correcting this problem and have been shown clinically to improve uncorrected and best-corrected vision while decreasing symptoms of glare and haloes associated with decentration.

Irregular astigmatism

Irregular astigmatism can be caused by various intraoperative and postoperative complications. The most common complications include the following: (1) decentration of the ablation pattern, (2) problems with beam homogeneity, (3) irregular healing, and (4) scar formation from flap complications. The symptoms are similar to decentration: poor vision and optical aberrations (eg, glare, haloes).

Beam homogeneity can be assessed best by ablating thin films and looking for hot or cold spots. Subtraction topography (preoperative minus postoperative ablation) also can be useful in assessing this problem. A smooth ablation pattern should be evident after subtraction is performed since it will "subtract" preexisting topographic abnormalities from the postoperative topography. Meticulous laser maintenance with careful attention to the optical system is necessary to prevent this problem.

Central islands are a special case of irregular astigmatism and represent areas of unablated tissue in the central cornea. This problem has largely disappeared with the introduction of newer technology and software on all laser platforms. Patients may complain of poor vision, and undercorrection may be evident on refraction. Topography typically reveals a central area of elevation. Many central islands simply resolve over time and require no treatment. Again, a customized treatment approach or topographic linked lasers may offer the best hope of treating this condition.

Use of a rigid gas permeable contact lens should optically correct irregular astigmatism and can be used as a short-term solution (as well as a diagnostic aid for irregular astigmatism). Corneal transplantation offers good results and can be used, if necessary, but it should be considered a last resort for those patients who are contact lens intolerant, who are significantly visually impaired, and who cannot wait for future technological fixes.

Other postoperative complications

Dislocated flaps

Dislocated flaps usually occur in the early postoperative period (first 48 hours) and can result in poor vision, pain, and permanent striae, if not treated aggressively and appropriately.^[33] Prevention is paramount and is accomplished by meticulous alignment of the flap at the time of surgery and checking the flap again at the slit lamp prior to allowing the patient to leave the laser center, usually within 20 minutes. If flap dislocation is noted, the flap can be refloated and repositioned easily before the patient leaves.

Patients leave the eye center with plastic bubble eye shields and are instructed not to remove them for the first day and evening after surgery, except to instill drops. They also are instructed not to touch or rub the eye.

The flap is inspected again at the slit lamp within 24 hours and any misalignment, significant striae or folds, or dislocation is treated immediately by refloating the flap. See the image below.

View Image

Striae.

Late dislocation is uncommon and usually involves significant eye trauma. Striae in the flap can occur despite the most careful alignment of the flap and vigilant postoperative care. Thicker flaps (180-200 µm) may be less prone to this problem. If outside the visual axis and center of the pupil, they can be ignored. However, if they appear to be central and are associated with a loss of best-corrected visual acuity, they should be treated. Note that no topographic abnormalities may be present despite the slit lamp appearance.

Various methods have been described to remove visually significant striae from the flap. These methods include simply lifting and smoothing the flap with multiple strokes of a spatula over the surface, suturing the flap, and thermal ironing of the flap.^{[34, 33]} Unfortunately, no consensus currently exists on the treatment of striae. Generally, a stepwise approach is used, and suturing or thermal ironing procedures are reserved for long-standing striae or those that do not resolve after a simple lift and smooth technique is tried. No attempt is made to mark the flap since alignment marks made prior to performing this stretching maneuver will not correspond to the actual flap alignment noted after stretching is complete.

Striae may still be present immediately after flap stretching, but they usually will be improved or resolved within 24 hours. Creating an epithelial defect directly over the striae may be helpful in recalcitrant cases. A bandage lens in this situation also may be helpful because it will likely induce flap edema and further stretch the cornea.

Epithelial ingrowth

Epithelial ingrowth (see the image below) under the LASIK flap has been reported to occur in 1-2% of patients. Fortunately, significant epithelial ingrowth requiring treatment is rare. It is more commonly seen after a flap lift is performed for enhancement surgery, probably because lifting the flap creates an uneven tear in the epithelium as opposed to the clean cut edges created by the microkeratome or femtosecond laser.

View Image

Epithelial ingrowth.

Poor technique and adhesion of the flap can be associated with this complication. Grasping the flap with forceps or pinching the flap also may allow an avenue for ingrowth to occur. Mild, stable, ingrowth at the edge of the flap extending no more than 1 mm from the edge does not require treatment. However, sheets of epithelium growing in from the edge or epithelial "nests" involving the central visual axis or inducing topographic abnormalities and irregular astigmatism should be treated as soon as possible. Untreated sheets of epithelium with poor adherence of the flap edge can lead to corneal flap melting and permanent damage to the flap.

Usually, epithelium can be seen easily on slit lamp examination and in retroillumination. Fluorescein staining of the cornea can reveal communication of a pocket or sheet of epithelium with the flap edge.

Treatment involves lifting the flap and mechanically scraping both the stromal surface and the back of the flap. (A Paton spatula or 69 Beaver blade can be used.) Alcohol or cocaine on the stromal surface or the flap usually is not necessary and is not advised due to potential toxicity to the cornea and the endothelium. Sealing the edge of the flap with fibrin glue can be a useful adjunct in recalcitrant cases with multiple recurrences.^[35]

Diffuse lamellar keratitis

Diffuse lamellar keratitis (see the image below), also known as Sands of the Sahara syndrome, represents a sterile inflammation occurring in the flap interface.^[36] Maddox first described this complication, and Hatsis subsequently classified it into 4 grades based on severity.^[37] The etiology is unknown, but it is believed to be due to the introduction of toxins under the flap at the time of surgery. Gram-negative endotoxin from dead bacteria and hydrocarbon contamination from the microkeratome motor or head lead the possible suspects. Milder cases have been associated with epithelial defects that sometimes occur on the surface of the flap following surgery.

View Image

Diffuse intralamellar keratitis (day 5).

Grades I and II are characterized by asymptomatic patients with normal vision on the first postoperative day. Slit lamp examination may reveal a fine, diffuse, powdery infiltrate (sandlike in color and appearance) confined to the interface. Grade I partially covers the interface, while grade II covers the entire interface and is associated with a denser infiltrate. Sometimes, wavy, fine lines with intervening clear areas can be seen. If untreated, the infiltrate typically worsens during the first postoperative week.

Grade III is associated with worsening vision and focal plaquelike infiltrates against a background of diffuse infiltration.

The most aggressive stage, grade IV, can be accompanied by significant visual loss and inflammatory signs (eg, lid edema, profound photophobia, perilimbal injection, flare and cell) in the anterior chamber. Typically, the infiltrate is dense and associated with large focal clumps of cells. Corneal topographic changes reflect the severity of the inflammation and become more marked as enzymatic digestion of the flap and the stromal bed progress. This process can result in permanent corneal changes.

The key to treatment is early recognition and intervention. Topical therapy consists of prednisolone acetate 1% or a steroid drop of equal potency given hourly. Topography and visual acuity are helpful in assessing progression. The trend in treatment has been toward early intervention before progression to grade III or IV. This consists of lifting the flap and thoroughly irrigating the interface. Cultures can be performed to rule out infectious keratitis if suspected.

The usual outcome is gradual resolution and return of best-corrected visual acuity, even in more severe cases. Complete resolution may take weeks to months. As noted, permanent corneal topographic changes due to melting of the cap and the stromal bed are possible and can result in corneal scarring and irregular astigmatism.

Efforts to reduce the incidence of diffuse lamellar keratitis focus on prevention, and they are updated on a continual basis. Particular attention has been given to the cleaning of instruments, especially the microkeratome, and sterilization technique. The author's center uses sterile distilled water in the steam autoclave, which may help prevent the buildup of gram-negative endotoxin. The author's center also uses a filter capable of removing bacteria from any solution used to irrigate under a flap.

Balanced salt solution is used for irrigation under the flap and is placed on a syringe, with the filter interposed between the syringe and the irrigation cannula. Disposable cannulas only are used for irrigation, since endotoxin and biofilm can build up on the inside of reusable cannulas. Finally, using a microkeratome whose head is disposable may also decrease the incidence of this complication, since no cleaning solutions are used and therefore no chemical residue can be left in the stromal bed during flap cutting.

Infectious keratitis

Infectious keratitis after LASIK is exceedingly rare. This finding is despite the fact that LASIK usually is performed in outpatient centers not subject to the rigid sterility protocols in force for the operating room. Surgeons often do not wear gloves during the procedure. The low infection rate may be due in part to the fact that epithelial integrity is relatively well maintained (compared to PRK).

Other factors that may contribute to the low incidence of infection are the limited use of topical steroids (usually 1-2 wk) and the routine use of potent topical antibiotics (eg, fluoroquinolones) during the perioperative period. However, infections have been reported and tend to be serious. This finding is partly due to the fact that when infection does occur, the invading organism has already gained access to the deep corneal stroma.

Organisms that have been reported to cause infectious keratitis following LASIK include Streptococcus pneumoniae, Staphylococcus aureus, Mycobacterium chelonae, and Nocardia asteroides. Atypical mycobacterial infections represent about one half of all reported cases. Mycobacterial infections may be more frequent when cold or chemical sterilization techniques are used for the microkeratome. The actual source of mycobacteria is often contaminated tap water or ice. These organisms seem to have a predilection for the relatively anoxic environment that exists in the flap interface. See the image below.

Methicillin-resistant Staphylococcus aureus (MRSA) is of particular concern in health care workers seeking refractive surgery since they may have become colonized with this organism through patient contact. Prophylaxis for this potentially devastating infection includes the use of polymyxin-trimethoprim drops 4 times per day starting 2-3 days prior to planned surgery.

View Image

Bacterial keratitis following LASIK.

Symptoms of infection include poor vision, pain, and redness. Signs include infiltration under the flap with possible anterior chamber reaction. Patients with diffuse intralamellar keratitis can present with similar findings, but the eye tends to be quiet, eliciting minimal pain and redness. Mycobacterial infections often present 2-4 weeks following surgery and are characterized by multiple discrete interface infiltrates with indistinct and feathery edges.

The principles of diagnosis and treatment remain the same as with any bacterial or fungal corneal infection; identify the organism and treat aggressively with appropriate broad-spectrum antibiotic drops based on Gram stain and culture results. However, management of the flap can be problematic. In general, cultures should be obtained from under the flap. Sometimes, cultures can be performed atraumatically by gently lifting an edge of the flap and inoculating a calginate swab soaked in culture media broth (eg, BHI, thioglycolate). If an infection appears to be progressing despite aggressive antibiotic treatment, the flap should be lifted, cultures should be repeated, and antibiotics should be irrigated in the flap interface. Initial therapy could consist of a fluoroquinolone combined with a fortified cephalosporin drop. This treatment provides adequate coverage for most bacteria. Infections that do not respond may benefit from therapeutic penetrating keratoplasty.

Mycobacterial infections may require prolonged antibiotic treatment over a course of weeks to months. The current antibiotics of choice are fortified amikacin or clarithromycin. They penetrate the flap poorly. Fourth-generation fluoroquinolones have significant activity against mycobacteria and much better penetration; however, there are no reported cases to date treated successfully with these antibiotics as a single agent. Cases of confirmed mycobacterial infection that do not respond to antibiotic treatment may require very aggressive treatment with amputation of the LASIK flap and the addition of systemic antibiotics.^[38]

LASIK has not been reported to cause damage to the corneal endothelium, and, in fact, several studies have shown no decrease in average endothelial cell density following LASIK.

Ectasia

Ectasia refers to the apparent postoperative biomechanical weakening of the cornea following LASIK or, more rarely, PRK. Ectasia is characterized by poor vision and topographic findings resembling that of keratoconus. See the image below.

View Image

Postoperative ectasia: Orbscan. Note the elevation on anterior and posterior floats and the thinning of the central cornea on the pachymetry map.

Although the etiology remains unknown, several important risk factors for the development of ectasia have been outlined by Randleman and colleagues.^{[39, 40, 41, 42]} These risk factors include abnormal topographic findings suggestive of forme fruste keratoconus, a residual corneal stromal bed thickness of less than 250 µm, high myopia, a thin cornea (< 500 µm) preoperatively, and an age of 21 years at the time of surgery. Of these, abnormal topographic findings appear to be the most important, and, in fact, there are many examples of patients with thin corneas who have undergone uncomplicated LASIK procedures.

Treatment modalities range from the use of contact lenses to corneal transplantation. Intacs have been used with some success to mechanically bolster the cornea. More recently, cornea collagen cross-linking has been tried. In this technique, the cornea epithelium is removed, and the stroma is exposed first to riboflavin and then to ultraviolet light. The concentration of riboflavin and the time of exposure to ultraviolet light determine the extent of cross-linking. This treatment results in stiffening and variable flattening of the cornea. As of April 2016, the Avedro system has received FDA approval for use in the treatment of post-LASIK ectasia.

• References

Outcome and Prognosis

Analyzing and comparing outcomes from refractive procedures can be a complicated and frustrating process. Compounding the problem is lack of standardization in the way results are reported.

Clinicians need to be familiar and to look for certain parameters when outcomes are presented in journals or presentations. These parameters include the range of refractive error treated, the percentage of patients achieving 20/20 and 20/40 vision or better (efficacy), the percentage of patients within ±0.50 D and ±1.00 D of the target refraction (predictability), and the percentage of patients losing 2 or more lines of best-corrected visual acuity (safety).

In general, LASIK results are better for patients with low myopia (between 1-6 D) and low astigmatism (< 1 D). Stability has been reported to be good with little or no change noted in most patients between 3 months and 1 year postoperative. Other factors that can affect results include the type of laser and microkeratome used and surgeon experience. Table 2 summarizes LASIK results for conventional myopic treatments; Table 3 summarizes LASIK results for custom myopic treatments. The author has elected to present outcomes from the FDA clinical trials that led to the approval of these procedures; clinical results outside of tightly controlled investigational trials have generally mirrored the outcomes obtained in these trials. Published outcomes are provided in the References section.^{[43, 44, 45, 46, 47, 48, 49]}

Table 2. Myopia: Conventional LASIK Outcomes in FDA Trials

View Table

See Table

Table 3. Myopia: Wavefront-guided LASIK Outcomes in FDA Trials

View Table

See Table

• References

Future and Controversies

.One area of significant controversy revolves around the issue of what method is best to create the corneal flap. Two technologies are available: the femtosecond laser and the more traditional microkeratome technology.^{[50, 51, 52]} Both devices have their advocates and inherent advantages and disadvantages.

The femtosecond laser is a solid-state laser that uses an infrared frequency of 1053 µm to create 3-µm spots adjacent to one another. There are several manufacturers of these lasers, but the most widely use laser in the United States currently is the Intralase (AMO, Inc, Santa Anna, CA). A flap is created by delivering multiple laser shots to a predetermined depth of the cornea. Photodisruption essentially creates microscopic connected perforations in one layer of the cornea. Advantages appear to be a more predictable depth of treatment and an excellent safety profile.

Femtosecond flaps, unlike microkeratome flaps, tend to be uniform in thickness and not meniscus shaped (ie, thinner in the center). The edge of the flap when cut by the laser is more vertical than that achieved with the microkeratome and allows for a good fit of the flap over the stromal bed with minimal leeway for sliding. The laser also allows for precise customization of flap diameter and hinge location. As noted, this may be especially useful for hyperopic or mixed astigmatism treatments, which require larger optical zones than myopic treatments.^{[53, 54]}

Several reports have shown improvement in the predictability of refractive outcome and less induction of higher order optical aberrations when the flap is made by a femtosecond laser.^[55] However, this has not translated to improvement in uncorrected visual acuity. As noted in Complications, there are complications unique to femtosecond laser technology, including vertical gas breakthrough, transient light sensitivity (TLS), opaque bubble layer, and anterior chamber gas bubbles. TLS is a corneal inflammatory problem generally associated with earlier femtosecond laser models and is characterized by patient pain and discomfort in the early postoperative period. It has largely been eliminated by adjusting laser energy density, delivery pattern, and spot settings. When it does occur, it can usually be managed with a longer course of topical steroid drops and tends to improve and diminish with time.

The use of microkeratomes for flap creation is associated with a lower cost, less surgical time, and ease of lifting the flap both at the time of the primary procedure and for enhancement surgery. Microkeratome technology has also advanced considerably, improving the safety, precision, and ease of use of these devices as well. An example of such improvements can be seen with the Amadeus microkeratome (Zeimer, Inc) and the Moria One Use Plus (Moria, Inc, Doylestown, PA). Both microkeratomes can be used with no on-eye assembly, making them easier to use for novice surgeons.

The Amadeus has a very sophisticated computerized interface so that the surgeon can vary the advance speed and the blade rpm frequency; important factors in determining flap thickness. The suction pump for the Amadeus also adjusts automatically for ambient air pressure, which may be important when working at elevation. A secondary pump helps prevent suction loss during the microkeratome pass.

The Moria features a disposable head. This means a technician does not need to insert a blade, eliminating another potential source of error. A disposable head may decrease the risk of DLK since the flap interface will not be exposed to the cleaning chemical solutions used on disposable units prior to sterilization.

A suction break during flap creation has starkly different outcomes with the 2 devices.

With the femtosecond laser (Intralase), a docking ring is attached to the eye with relatively low suction. This docking ring couples the femtosecond laser to the cornea, ensuring proper depth and centration of the flap. If a suction break occurs, the device can usually be reattached and the flap completed with no adverse consequences if the laser pattern has not reached the visual axis. If the laser has reached the visual axis, the patient can be rescheduled; no permanent corneal changes appear to occur.

When a suction break occurs during a microkeratome pass, an incomplete or thin and irregular flap can occur (see the image below). Management usually consists of replacing the flap as best as possible and aborting the procedure. Corneal scarring can result. Depending on how the cornea heals, the flap may be recut in 3-6 months instead of scheduling a transepithelial PRK with mitomycin C within several weeks in the event of irregular surface healing. Back-up suction pumps on the Moria and the Amadeus microkeratome are examples of technological improvements in microkeratomes that help prevent loss of suction when compared to earlier devices.

Both devices are still in common use. Individual surgeon preference and patient characteristics, such as corneal curvature and the diameter of the excimer laser optical zone to be ablated, are helpful in determining which patients may benefit from femtosecond laser–created flaps versus flaps made by the microkeratome. It is interesting to note that a metaanalysis comparing flap complications showed almost no difference in the overall incidence of flap complications between the two methods.^{[51, 52]}

View Image

Incomplete flap.

Another important technological development has been the use of tracking devices to follow eye movements during surgery. Important considerations in tracking include the speed of saccadic and nonvoluntary eye movements that occur during fixation versus the speed of response of the tracking device used. The response time requires 2 components: recognition that the target being tracked has moved (the eye) with a subsequent shift of the targeting mechanism of the laser to follow the movement.

Video tracking is based on real-time video images of the pupil. The VISX, Technolas, and Wavelight Allegretto laser systems use this type of tracking technology. Video tracking of the center of the pupil generally works well; however, the center of the pupil will change as the pupil dilates or constricts, introducing one source of error. There is a lag time between video detection of movement of the pupil, computer processing of the images of the pupil, and movement of the targeting mirrors to adjust for the new target location. This lag time is variable and is dependent also on the repetition rate of the laser; the faster the shot pattern is delivered the faster the response of the system to target movement and acquisition must be.

Surgery may be performed bilaterally or unilaterally. Advantages of unilateral surgery include the potential for increased safety, and, perhaps, better predictability because the surgery algorithm can be adjusted for the second eye based on the results of the first eye. Advantages of bilateral surgery are mostly economic and include convenience for the patient and the surgeon in terms of time off of work, scheduling surgery, and postoperative visits.

To date, several studies addressing this issue have not shown increased risk of serious complications associated with bilateral surgery.^{[56, 57]} In addition, unilateral surgery is associated with a minimal increase in predictability. Most surgeons performing LASIK today offer their patients the option of bilateral surgery.

Long-term effects of LASIK on the cornea may occur. Because this procedure is relatively new, the long-term effects cannot be determined satisfactorily.

Of particular concern is the ability to identify patients at risk of developing progressive ectasia and central corneal thinning (see Complications). Evidence suggests that if the flap and the ablation depth can be limited to the anterior one third of the cornea, improved biomechanical properties of the cornea can be maintained, similar to those seen with PRK.^{[58, 59]} Evidence also suggests that less induction of higher order optical aberrations may occur compared to thick flaps and deeper ablations.^[60] . Several studies have attempted to to identify patients at risk for developing ectasia based on factors such as age, corneal thickness, depth of ablation, flap thickness (percentage of tissue altered), and topographic criteria. However, this has not eliminated the risk of ectasia.

Determination of the better method is under research, and studies of this technique have only just begun. Its disadvantages may include the difficulty in handling thin flaps, the difficulty in lifting these flaps for subsequent enhancement, and, in the author’s opinion, the observation that thin flaps may be more prone to striae. At present, long-term stability (>1 y) of standard LASIK appears to be good. The late development of ectasia is still a concern, and patients who have progressive myopic changes following LASIK must be evaluated for this possibility with serial topography and pachymetry. Topographers capable of mapping the posterior corneal surface and combining topography with tomography (Pentacam) have proven useful in detecting this problem.^[61]

LASIK may affect not only the quantity but also the quality of vision. Wavefront-guided treatments are an attempt to improve postoperative quality of vision by eliminating or decreasing higher-order optical aberrations such as spherical aberration and coma (see Wavefront analysis).

Another method of reducing spherical aberration is an approach known as wavefront optimized. This method relies on empirical algorithms based on refractive error and corneal curvature to apply a sophisticated blend zone between the center of the treatment and the periphery. It is not based on individual patient wavefront measurements. Wavefront-guided treatments have the advantage of using a wavefront unit to measure refractive error and are therefore very accurate, reproducible, and less technician-dependent than a manifest refraction. Since the wavefront measurements are used to directly design the laser ablation pattern, entry errors, especially transposition errors in which the incorrect axis of astigmatism treatment can be entered into the laser by a technician, are minimized.

To date, head-to-head comparisons of the two main platforms comparing uncorrected visual acuity and quality of vision using these treatment strategies (AMO VISX for wavefront guided and Alcon Wavelight for wavefront optimized) have shown very little difference in patient satisfaction, quality of vision, or outcomes.^{[62, 63]}

Newer technology has begun integrating topography data into the ablation patterns. Relying on topography alone without considering the refraction to design an ablation pattern may result in significant error, especially when treating astigmatism, since the topographic and refractive axis of astigmatism often differ significantly. Topography, however, can be used to refine the refraction and improve the accuracy of the treatment.^[64] This approach may have great utility in attempting to correct complications such as a decentered ablation or irregular astigmatism from other causes.^[65] An example of this technology is the Alcon Contoura system used with the Wavelight excimer laser, FDA approved in November 2015.

• References