AUTHOR AND EDITOR INFORMATION

Section 1 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

INTRODUCTION

Section 2 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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. About 5 million procedures have been performed in the United States since the approval of the excimer laser for refractive surgery in late 1995. About 1.5 million procedures were performed in 2005 alone.

History of the Procedure

The era of keratomileusis began in 1966 with Pureskin, who demonstrated that refractive changes could be achieved by creating a corneal flap and removing central tissue in a lamellar fashion under the flap. He found that the smaller the diameter of the resected disc, the greater the refractive change.

Jose Barraquer developed the idea of resecting a corneal disc and freezing it, followed by shaping the disc with a cryolathe. 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.

INDICATIONS

Section 3 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

As of December 2005, LASIK has been approved by the Food and Drug Administration (FDA) for several different laser platforms, including the VISX STAR S4, Allegretto Wavelight, Alcon LADARVision 4000, and 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

Source: http://www.fda.gov/cdrh/LASIK/lasers.htm 12/27/05.

RELEVANT ANATOMY

Section 4 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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.

CONTRAINDICATIONS

Section 5 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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 (it is common to have patients treated slightly outside the approved range, but they must understand that it is an off-label use of the excimer laser).

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.

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.

WORKUP

Section 6 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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. 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: A complete discussion of wavefront analysis and customized LASIK treatments is beyond the scope of this article. However, some concepts useful in understanding wavefront analysis are provided below.
• • 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 pupil plane 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 Image 10.
  • 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. Illustrations of myopia, hyperopia, astigmatism, spherical aberration, and coma are included in Pictures.
  • 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.
• 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.
  • 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. 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.

TREATMENT

Section 7 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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; 3 days for soft contact lens wear and 2 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. 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. 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 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. 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.

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.

The following technique applies to the use of the Amadeus microkeratome. 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, the Amadeus microkeratome does not require an 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. Note that pupil dilation cannot be used as a sign of good suction for eyes treated with the Alcon LADARVision system, as dilation is performed prior to treatment. 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 (140 µm and 160 µm depth plates are the most common for the Amadeus microkeratome). Factors that affect the selection of the depth plate include degree of myopia to be corrected and preoperative corneal thickness as measured by ultrasound pachymetry. A residual (untouched) corneal thickness of about 250 µm is desirable to prevent possible structural weakening of the cornea and progressive ectasia. Therefore, a patient with high myopia and thinner corneas would require a thinner depth plate.

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 programmed on the microkeratome computerized interface and is set based on the corneal curvature and the inner diameter of the chosen ring size. The microkeratome automatically reverses, suction is turned off, and the microkeratome assembly is removed.

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 nasally.

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. 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.

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.

Enhancements: One of the great advantages of LASIK over other refractive procedures is the ease and safety of performing enhancement surgery. 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. The corneal flap can usually be lifted easily within the first several years after surgery; beyond this time period, consideration should be given to cutting a new flap with the microkeratome.

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; noting 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.

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. Currently, there are 2 commercially available preparations: moxifloxacin 0.5% and gatifloxacin 0.3%. There is much debate in the ophthalmic community as to which of these topical antibiotics is best for this setting. Similarly, 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.

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). Note that 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.

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 eMedicine's Eye and Vision Center. Also, see eMedicine's patient education article Vision Correction Surgery.

COMPLICATIONS

Section 8 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Complications can be divided into intraoperative (usually microkeratome related) and those that occur postoperatively. 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-related complications include the following:

• Entry into eye (intraoperative; rare)
• Thin, irregular, or perforated flap (intraoperative; <0.2%)
• Free flap (intraoperative; rare; 0.2%)

Laser-related complications include the following:

• Decentration ( <1%)
• Irregular astigmatism ( <1%)

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%)
• Diffuse intralamellar keratitis ( <0.1%)
• Epithelial ingrowth (early to late postoperative; 1-2%)
• Under/overcorrection (see results)

Intraoperative Microkeratome-Related Complications

Perforation and entry into the eye

Probably the most dreaded complication related to use of the 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. Newer microkeratomes from some manufacturers (eg, Hansatome unit from Bausch & Lomb Surgical, Amadeus unit from AMO) 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. It usually occurs with loss of suction or poor suction when the suction ring is applied. Steep corneas with average K readings of 46.00 D or greater are also at higher risk for perforated buttonhole flaps. 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.

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 but may be useful in rare instances.

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.

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 inspection of 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 at the slit lamp 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.

Rippled flap

Unconsciously attempting to push the microkeratome across the cornea can result in a rippled flap (especially with an older ACS unit). If the resulting stromal bed is irregular, the flap should be replaced without performing the laser ablation, and, then, it should be recut no sooner than 6 months later.

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 the 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. With the ACS unit, the footplate is removed gently.

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.

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 can still be a significant problem with all excimer laser systems.

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.

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. 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.

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. 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 under the LASIK flap has been reported to occur in 1-2% of patients. Fortunately, significant epithelial ingrowth requiring treatment is rare.

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.

Diffuse lamellar keratitis

Diffuse lamellar keratitis, also known as Sands of the Sahara syndrome, represents a sterile inflammation occurring in the flap interface. Robert Maddox, MD, first described this complication, and Alexandar Hatsis, MD, subsequently classified it into 4 grades based on severity. 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.

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.

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.

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.

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.

OUTCOME AND PROGNOSIS

Section 9 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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.

Table 2. Myopia: Conventional LASIK Outcomes in FDA Trials

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

*: Only includes patients whose preoperative BSCVA was 20/20 or better

FUTURE AND CONTROVERSIES

Section 10 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Currently, 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.

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. 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. The laser also allows for precise customization of flap diameter and hinge location.
 
Several reports have shown improvement in the predictability of refractive outcome and less induction of higher order optical aberrations. However, this has not translated to improvement in uncorrected visual acuity. The flaps made with the laser are more difficult to lift initially, and occasional moderate inflammation and pain in the cornea were reported when the device was first introduced. This latter problem seems to have been addressed with changes in the amount of total energy delivered to the cornea and the pattern in which it is delivered (Raster pattern).

Microkeratome technology has also advanced considerably, improving the safety, precision, and ease of use of these devices as well.

The differences in LASIK outcomes between the 2 types of devices appear to be small. The laser is expensive, and cost may ultimately determine whether or not this technology becomes widely adapted.

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 with a subsequent shift of the targeting mechanism of the laser to follow the movement. There are 2 types of trackers available for current laser platforms: video tracking and laser radar. Video tracking is based on real-time video images of the pupil.

The VISX, Technolas, and Wavelight Allegretto system use this type of tracking technology. Laser radar is used exclusively by the Alcon LADARVision excimer laser platform and requires pupil dilation to work properly. This method relies on tracking the edge of a dilated pupil. Four beams of light are projected onto the eye at the pupil margin. Eye movement is detected from changes in reflected light.

Both methods have limitations. 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.
 
Tracking the dilated pupil margin also has some limitation. Occasionally, pupils will not dilate enough to allow for the tracker to engage, and the other circular objects, such as the edge of an intraocular lens, can be mistaken for the boundary of the iris by the tracker. Custom wavefront guided procedures require even greater precision of tracking eye movements and must include monitoring of cyclotorsion.

At present, both systems appear to work well clinically for conventional and wavefront guided laser ablations.

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. 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. Several risk factors are thought to be important, such as initial high myopia, forme fruste or a pattern like keratoconus noted on preoperative topography, and subsequent enhancement surgery. The current standard of care is 250 µm of untouched central corneal tissue; however, it is unclear whether some patients may develop ectasia despite having this residual corneal thickness.

At present, long-term stability of LASIK (>1 y) 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 (Orbscan) have proven useful for detecting this problem.

LASIK may affect not only the quantity but also the quality of vision. Contrast sensitivity and glare testing studies comparing preoperative refraction, postoperative corneal curvature, and scotopic pupil size will be helpful in defining patient selection criteria and improving outcomes and patient satisfaction.

MULTIMEDIA

Section 11 of 12  

• Authors and Editors
• Introduction
• Indications
• Relevant Anatomy
• Contraindications
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Media file 1:  Diffuse intralamellar keratitis (day 5).
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Media file 2:  Bacterial keratitis following LASIK.
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Media file 3:  Epithelial ingrowth.
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Media file 4:  Striae.
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Media file 5:  Thin, perforated flap.
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Media file 6:  Incomplete flap.
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Media file 7:  Buttonhole in flap.
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Media file 8:  Decentered flap and ablation.
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Media file 9:  Pupil alignment or visual axis alignment for laser ablation.
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Media file 10:  Spherical aberration: a schematic diagram for the human eye.
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Media file 11:  Zernike polynomials: pictorial representation.
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Media file 12:  Spherical aberration post-LASIK. The original refractive error was -10.00 diopters.
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Media file 13:  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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Media file 14:  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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Media file 15:  Ectasia post-LASIK: Tracey WaveScan. Note the preponderance of higher order aberrations, including spherical aberration and coma. The Orbscan of this same patient appears in Image 14.
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REFERENCES

Section 12 of 12