Laser In Situ Keratomileusis

Louis E. Probst and John F. Doane

CHAPTER CONTENTS

The Evolution of Lamellar Refractive Surgery

Laser In Situ Keratomileusis as an Enhancement Procedure

Lamellar corneal surgery for the correction of refractive errors has been evolving for 50 years since Professor Jose I. Barraquer of Bogota, Colombia, invented the field in 1948. Many of his concepts continue to guide this particular field of ophthalmic surgery, and much of today’s instrumentation originated from his ideas.

Refractive lamellar corneal surgery attempts to remove, add, or modify the corneal stroma to alter the tear film/anterior corneal interface radius of curvature. For the treatment of myopia, tissue may be removed centrally [i.e., via myopic laser in situ keratomileusis (LASIK)] or added peripherally [i.e., by insertion of an intracorneal ring (ICR)] to induce central corneal flattening and reduction of myopia. For hyperopia, tissue may be added centrally (i.e., through keratophakia), subtracted peripherally (i.e., via hyperopic LASIK), or affected by a deep keratectomy with controlled ectasia [i.e., automated lamellar keratoplasty (ALK) for hyperopia], which may be performed to induce central corneal steepening with a reduction of spherical plus refractive power. Treatment of astigmatism is accomplished by flattening the steep axis or steepening the flat axis (to remove tissue) to make the overall cornea spherical.

THE EVOLUTION OF LAMELLAR REFRACTIVE SURGERY

The surgical techniques used in lamellar refractive surgery have evolved toward increased simplicity and greater patient safety. The end results have been improvement in refractive results and wider acceptance of the procedures by surgeons. Several distinct techniques for performing lamellar surgery exist today.

General Advantages

ability to treat a larger range of refractive errors

shorter visual rehabilitation time

minimal postoperative discomfort

reduced healing response compared with photorefractive keratectomy (PRK)
relative ease of enhancement compared with PRK

General Disadvantages

need for advanced technical expertise

large ongoing capital expenditures

potential for serious intraoperative complications (see pp. 121–122)

Instrumentation

microkeratome (Figs. 11-1 and Fig. 11-2)
- functions like a carpenter’s plane to perform a planar (no refractive power) resection of anterior cornea tissue
- contains an oscillating blade that incises the tissue at an even depth [if intraocular pressure (IOP) is constant and the microkeratome moves across the cornea at a constant velocity]

Figure 11-1 Keratome creating a lamellar dissection by applanating the cornea like a carpenter’s plane. (Courtesy of Stephen G. Slade, M.D.)

Figure 11-2 Manual Barraquer microkeratome and suction ring. (Courtesy of Stephen G. Slade, M.D.)

Freeze Keratomileusis

The term “keratomileusis” derives from the Greek roots keras (horn-like: cornea) and mileusis (carving). In 1949, Barraquer developed freeze keratomileusis, which alters the cornea’s refractive power by shaping the cornea itself. However, because of its many disadvantages, freeze myopic keratomileusis is no longer performed.

Disadvantages

steep learning curve for performing the keratectomy and using the cryolathe
dependence on the constant velocity of the manual microkeratome to maintain the thickness and evenness of the keratectomy
possibility of irregular astigmatism and a loss of best-corrected visual acuity (BCVA) from any keratectomy irregularity
difficulty mastering and maintaining skill with the cryolathe
lack of ability to transfer skills to many other procedures
availability of better options (e.g., LASIK)
difficulty placing antitorque sutures

Indications and Inclusion Criteria

myopia (≤ −15 to −18 D)
hyperopia (≤ +6 D)
not for treatment of astigmatism

Methods (Fig. 11-3)

Harvest 300 μm of the corneal cap.
Stain the corneal cap with Chiton-green stain.
Invert the cap epithelial-side down on a freezing block.
Sculpt the back (stromal side) surface with a cryolathe (Figs. 11-4 and Fig. 11-5).
Reposition the cap on the stromal bed.
Suture the cap in place.
Remove sutures 2 to 3 weeks after complete epithelialization.

Figure 11-3 Freeze Myopic Keratomileusis. (A) 300-μm corneal cap created with microkeratome. (B) Central stroma from posterior aspect of cap removed with cryolathe. (C) Cap replaced and sutured in place with antitorque suture. Central cornea has been flattened to reduce myopia.

Keratomileusis In Situ for Myopia

Barraquer also conceived the process of raising a corneal cap and removing central tissue from the bed. Krwawicz and Pureskin described similar techniques as “central stromectomy.” Because of several disadvantages and the frequency of certain complications, this technique is no longer practiced.

Disadvantages

availability of a better option (i.e., LASIK)
difficulty placing antitorque sutures

Indications and Inclusion Criteria

myopia (≤ –18 D)
not for treatment of astigmatism

Figure 11-4 Surgical team member operating the cryolathe. (Courtesy of Stephen G. Slade, M.D.)

Figure 11-5 Cryolathing of a lamellar corneal disk. (Photo courtesy of Stephen G. Slade, M.D.)

Methods (Figs. 11-6)

Create a free cap to gain access to the corneal stroma using a manual pass of the microkeratome.
Remove a smaller-diameter disc of tissue with a second pass of the microkeratome across the stromal bed.
Determine the size of the refractive correction with the thickness of the second “power cut.”
Replace the cap to reduce corneal curvature and myopia.
Suture the cap in place using an antitorque technique.

Complications

infection
irregular astigmatism (frequent)
loss of cap
lamellar complications (epithelial ingrowth and flap melting)

Keratophakia

Keratophakia means “corneal lens.” Barraquer first developed the procedure keratophakia in the 1960s as a treatment for aphakia after intracapsular cataract extraction. With the advent of extracapsular and phacoemulsification cataract surgery techniques along with improved intraocular lens (IOL) technology, keratophakia has been relegated to a lesser role; surgical aphakia is no longer a prevalent problem. The future role of keratophakia or corneal inlay techniques in refractive surgery may involve the addition of synthetic tissue or lens material under a flap or placement of a transparent material of a different (or same) index of refraction from the corneal stroma in a stromal pocket to correct myopia, hyperopia, astigmatism, or presbyopia.

Figure 11-6 Keratomileusis in situ for myopia [manual or automated lamellar keratoplasty]. (A) Creation of corneal flap with first pass of a microkeratome. (B) Removal of central stromal tissue from bed (refractive cut) with the second pass of the microkeratome. (C) Reposit flap. Central cornea has been flattened for treatment of myopia.

Indications and Inclusion Criteria

hyperopia (typically ≤ ∼6D)
aphakia (≤ 14 D)
not for treatment of astigmatism

Methods (Fig. 11-7)

Create a flap and/or cap.
Place a disc of tissue underneath the flap and/or cap to increase the central corneal curvature.
Typically, harvest the tissue to be added from donor corneal tissue using a microkeratome.
Select a specific disc diameter and thickness of tissue based on the patient’s refractive error.

Complications

infection
irregular astigmatism
loss of cap
lamellar complications(epithelial ingrowth and flap melting)
decentration of lens inlay
loss of BCVA

Barraquer-Krumeich-Swinger Nonfreeze Keratomileusis Technique

The goal of the Barraquer-Krumeich-Swinger (BKS) technique was the improvement of refractive results and the simplification of lamellar surgery. The procedure is no longer performed.

Figure 11-7 Keratophakia (homoplastic allograft or synthetic inlay). (A) Creation of flap or cap. (B) Graft inlay positioned over central stromal bed. (C) Flap or cap repositioned. Central corneal power increased effectively treating hyperopia or aphakia.

Advantages

avoidance of adverse tissue effects (loss of fibroblasts) associated with freezing of the corneal cap
preservation of the corneal epithelium with a more comfortable postoperative course of healing
light processing of the tissue
maintenance of the intact epithelium

Disadvantages

availability of a better option (i.e., LASIK)
technical difficulty performing refractive keratectomy
difficulty placing antitorque sutures
frequency of postoperative irregular astigmatism

Indications and Inclusion Criteria

myopia (≤ −18 D)
hyperopia (≤ +6 D)
not for treatment of astigmatism

Methods (Figs. 11-8 and Fig. 11-9)

See the initial methods for freeze keratomileusis.

Figure 11-8 Barraquer-Krumeich-Swinger microkeratome and suction platform. (Courtesy of Stephen G. Slade, M.D.)

Figure 11–9 Barraquer-Krumeich-Swinger keratomileusis technique. (A) 300-μm cap created with microkeratome. (B) Cap placed epithelial-side down on suction dye. Stroma removed from central aspect of the posterior aspect cap with manual microkeratome. (C) Cap reposited with antitorque sutures. Central cornea flattened for treatment of myopia. (D) Cap placed epithelial-side down on suction dye. Tissue removed from paracentral stroma of posterior cap with manual microkeratome. (E) Cap reposited. Corneal curvature has been steepened for treatment of hyperopia.

Harvest a 300-μm corneal cap using the BKS microkeratome.

Place the cap epithelial-side down on one of several specific suction dyes.

Choose the suction dye based on the planned diopter correction of myopia or hyperopia (Fig. 11-10).
Make the refractive cut with the microkeratome.
Suture the cap in place with the antitorque technique.

Complications

infection
irregular astigmatism (frequent and major drawback)
loss of cap
lamellar complications (epithelial ingrowth and flap melting)

Figure 11-10 Barraquer-Krumeich-Swinger suction dyes. (Courtesy of Stephen G. Slade, M.D.)

Epikeratophakia

The first “Americanized” version of a lamellar refractive surgery technique was epikeratophakia/epikeratoplasty as described by Kaufman and Werblin. What seemed to be a modernization of lamellar surgery ultimately was not a viable alternative.

Advantages

automation of the difficult task of using the cryolathe

provision of a high-quality, preprocessed refractive corneal lenticule from a centralized manufacturer that freezes and cryolathes specific corrective powers, lyophilizes the lenticule in a sterile bottle (Fig. 11-11), and then ships it to the surgeon.

Disadvantages

variability of surgical results
poor profitability for the manufacturer of the lenticules
emergence of better alternative therapies (e.g., IOLs and laser refractive techniques)

Indications and Inclusion Criteria

treatment of the following for children and adults
- aphakia
- myopia
- hyperopia
- keratoconus
- keratoglobus
not for treatment of astigmatism

Methods

Reconstitute the lenticule with balanced salt solution (BSS).
Suture the lenticule onto the eye after host-bed preparation (Fig. 11-12).

Complications

those associated with re-epithelialization over the donor lenticule
infection
irregular astigmatism
loss of cap

Figure 11-11 Lyophilized epikeratophakia lenticule in transport bottle labeled with refractive power.

Automated Keratomileusis In Situ

The next major advance in the field of lamellar refractive surgery was development of the motorized Ruiz microkeratome, which was invented by another Colombian surgeon and protégé of Barraquer, Dr. Luis Antonio Ruiz. ALK is no longer performed.

Advantages

gears that advance the microkeratome across a geared track

an adjustable-height suction ring (defined the technique now known as ALK; Fig. 11-13)
a constant passage velocity
a lamellar tissue disc of even thickness
a smoother corneal stromal bed
increased appeal for a larger number of surgeons who found manual microkeratomes difficult to master

Figure 11–12 Epikeratophakia. (A) Preparation of the recipient bed by removing the epithelium and creating a 360-degree circumferential flange after epithelial removal. (B) Prefabricated onlay lenticule is positioned under flange edge and sutured in place. Depending on the lenticule power, myopic, hyperopic, or aphakic refractive errors may be treated.

Figure 11–13 Automated geared microkeratome and geared-track of suction ring.

Disadvantages

availability of a better option (i.e., LASIK)
abandonment of the hyperopic technique because of concern about progressive ectasia
technical demands of procedure

Indications and Inclusion Criteria

myopia (≤ – 20 D)
hyperopia (≤ +6 D)
not for treatment of astigmatism

Methods

Perform ALK in an outpatient setting with topical anesthesia (oral sedation is optional).

Irrigate the conjunctival cul-de-sacs to remove debris and eyelid glandular secretions.

Clean eyelids in the standard manner with povidone-iodine solution (Betadine).

Drape the eyelids.

creates a clear path for microkeratome passage
removes the lashes and extraneous periocular tissue from the surgical field

Instruct the patient to look at the microscope light.

With an inked marker, delineate the optical center, a pararadial line for orientation of the flap, and a 9-mm optical-zone (OZ) circle to center the circular suction ring (Fig. 11-14).
Place a spacer device (or depth plate) in the microkeratome to determine the thickness of the cut. (The first keratectomy in ALK typically utilizes a 160-μm depth plate.)
Place an adjustable suction ring on the eye and engage the suction to do the following.
- fixate the globe
- provide a geared path for the microkeratome
- raise the IOP to allow presentation of adequate corneal tissue (enables a smooth keratectomy of even thickness)
Create a hinge in the corneal flap with a stopper device.
Just prior to passage of the microkeratome, use a hand-held Barraquer tonometer to check the IOP to ensure that it is greater than 65 mmHg.
Remove the suction ring after the first cut, which has a planned diameter of 7.5 to 8.0 mm.
Reset the adjustable suction ring to resect a 4.2-mm-diameter piece of stromal tissue.
Place a second depth plate, which corresponds to the planned myopic correction.
Replace the suction ring and make a second cut (the “power cut”) after checking the diameter and the IOP.
Replace the flap and position it without sutures.
Protect the patient’s eye with a clear, plastic orbital shield (Fig. 11-15) and discharge the patient with broad-spectrum topical antibiotics
Examine the eye the next day.

Figure 11-14 (A and B) A Ruiz marker is impregnated with ink and used to make a pararadial mark for orientation of the flap and 9-mm OZ for proper placement of the suction ring. (Courtesy of Stephen G. Slade, M.D.)

Complications

infection
irregular astigmatism
loss of cap
lamellar complications (epithelial ingrowth and flap melting)
significant loss of BCVA (5-10% in some studies)

Figure 11-15 A plastic orbital shield is used to protect the globe from direct physical contact for three to four nights while asleep.

LASER IN SITU KERATOMILEUSIS

The postoperative complications of central corneal haze, regression of effect, and diminished efficacy for myopia over – 6 D with excimer laser PRK and the predictable shortcomings of ALK led to the development of LASIK. Researchers substituted lasers for the keratome in the refractive correction step of the myopic ALK procedure and LASIK was born. Some researchers postulated that the submicron ablation precision of the excimer laser allows for much more predictable refractive results that do not depend as heavily on individual patient healing responses, as had been noted with PRK.

Preoperative Considerations

Advantages

incredible precision of tissue removal
minimal postoperative discomfort
early recovery of visual function
immediate return to preoperative lifestyle
lack of topical steroid usage
lack of concern for undesirable healing phenomena (e.g., haze formation)
a short postoperative antibiotic regimen
less-intensive follow-up than that for PRK
increased range of efficacy for myopia, hyperopia, and astigmatism (compared with PRK)

Indications and Inclusion Criteria^*

myopia (0.5-15.0 D)
hyperopia (0.5-6.0 D)
regular astigmatism (≤8D)
patient at least 18 years of age (as for refractive surgery in general)^†
refractive stability (≥ 12 months)
clarity of the crystalline lens
education about the alternative refractive surgical procedures (e.g., radial keratotomy, PRK, ICR segments, clear lens extraction, or phakic IOL placement)
evaluation of patient expectations
completion of informed consent form

Preoperative Patient Evaluation

visual acuity testing
refraction
- manifest refraction (duochrome test and fogging techniques)
- cycloplegic refraction (to check excessive accommodation during manifest examination)
- comparison of refraction with prior spectacle corrections to assess the stability of refraction
- discontinuation of soft contact lens wear at least 3 days to preferably 2 weeks prior to examination
- discontinuation of rigid gas-permeable (RGP) lens wear at least 3 weeks before first of at least two examinations performed 3 days apart
- at least 0.25 D difference in refraction (validates stability)
evaluation of central corneal thickness using pachymetry (assesses safe limits of corneal stromal removal with laser ablation by measuring flap thickness, depth of ablation, and residual thickness of the stromal bed after ablation)
- A thin flap more likely leads to irregular astigmatism and lost lines of BCVA.
- A flap at least 160 μm thick minimizes induced irregular astigmatism.
- A thin stromal bed (<250 μm) may increase the likelihood of central ectasia with long-term unstable refractive status.
- For example, central pachymetry (560 μm) – flap thickness (160 μm) = preablation stromal bed (400 μm) so in this case ablate no more that 200 μm of stromal bed.
- Based on the Munnerlynn formula (ablation depth = optical zone² X diopters treated/3), no more than 15 D of myopia could be treated if using a 6-mm OZ.
- If preoperative central corneal thickness is thinner and ablation still leaves a 200-μm-thick stromal bed, less refractive error may be treated safely.
- Multizone ablations can correct larger amounts of refractive error because less tissue is removed than with single-zone ablations (overall OZ is smaller and night vision may be compromised).
computerized videokeratography
slit-lamp examination
retinal evaluation
eye-dominance testing
evaluation for monovision (when appropriate) (see p. 66)
evaluation for astigmatism
- refraction
- keratometry
- videokeratography
formulation of nomogram
- A worksheet in the patient’s chart prevents calculation mishaps (Table 11.1).

^* These ranges are not absolute; many surgeons have successfully treated patients with myopia, hyperopia, or astigmatism beyond these ranges. Other surgeons may work at lower ranges based on their personal experience with the procedure.

^† The use of LASIK for equalization of refractive error between eyes of pediatric patients has been done outside the United States but remains controversial.

Absolute Contraindications

unstable refraction
keratoconus
irregular astigmatism
inadequate corneal thickness
active corneal or ocular disease

Relative Contraindications

thin corneas
active collagen vascular disease
systemic vasculitis
low endothelial cell counts (<500-1500 cells/mm²)
anterior basement dystrophy
history of recurrent erosions (an epithelial defect can occur during the keratectomy and lead to poor flap adhesion with subsequent epithelial ingrowth and flap melting; surgeon experience helps to avoid epithelial ingrowth)
pregnancy or lactation (may be considered if stable through prior pregnancies and have no refractive change from current eyewear; J.F.D. has treated several pregnant and lactating women after careful consent and thorough evaluation of their previous refraction history)
history of herpes simplex or herpes zoster keratitis (use prophylactic oral and/or topical antiviral agents perioperatively because of risk for stromal melting; proceed very carefully with these patients)
patients with monocular vision

TABLE 11-1
**Sample Items on Worksheet for LASIK Nomogram**

Line 1 (preoperative refraction)	−6.0 D sphere
Line 2 (postoperative target)	piano
Line 3 (line 1-line 2)	−6.0 D sphere
Line 4 (% reduction or addition; presume 90% of sphere)	−5.4 D sphere
Line 5 (treatment entered into laser)	−5.4 D sphere

Figure 11-16 Laser in situ keratomileusis for myopia. (A) Creation of flap. (B) Laser ablation of stromal bed and tissue removal (shaded area). (C) Shaded area represents zone of tissue removal (top view). (D) Flap replaced and central cornea flattened.

Surgical Considerations

At least two different techniques have been used for laser myopic keratomileusis: LASIK and the Buratto technique. LASIK is now the predominant technique because the sutureless hinged flap is significantly easier to deal with compared with correctly orientating a free cap onto the stromal bed (Fig. 11-16 and Fig. 11-17). The Buratto technique is analogous to freeze myopic keratomileusis (MKM) except for substitution of the excimer laser for the cryolathe in the refractive step of the procedure.

LASIK

Create a hinged flap.

Remove corneal stroma from the bed with the laser.

Treat sphere and cylinder depending on the capabilities of the laser (Figs. 11-18 to Fig. 11-20).
The Buratto technique
- Remove a thick disc of tissue (300-350 μm) completely from the eye.

Figure 11-17 Laser in situ keratomileusis for hyperopia. (A) Creation of flap. (B) Laser ablation of stromal bed and paracentral tissue removal (shaded area). (C) Shaded area represents zone of tissue removal (top view). (D) Flap replaced and central cornea steepened.

Figure 11-18 VISX Star astigmatic correction. Parallel blades allow preferential ablation parallel to the minus cylinder axis or the flat corneal meridian, which in effect flattens the steep corneal axis.

Figure 11–19 Schematic of minus cylinder ablation with scanning spot laser to flatten the steep corneal meridian.

Figure 11–20 Schematic of plus cylinder ablation with scanning spot laser to steepen the flat axis by removing tissue paracentrally parallel to the minus cylinder axis or the flat axis on keratometry.

Place the disc beneath the laser stroma side up and ablate with the excimer laser (Fig. 11-21)
Replace the disc of tissue on the eye.

Figure 11-21 Buratto style laser keratomileusis. (A) Creation of thick (300-μm) “free” cap. (B) Inversion of cap and laser used to remove central cap stroma (shaded area). (C) Reposition of cap in keratectomy bed. Central cornea has been flattened to treat myopia.

Instrument Setup and Surgical Team Mentality

Select a laser depending on desired ablation depth. 0 most lasers: 12 to 15 μm/D (in 6-mm single zone algorithms)

VISX Star (VISX, Inc., Santa Clara, CA): 15 μm/SE diopter (6-mm single-zone algorithm)
VISX Smoothscan: 12 μm/SE diopter (6-mm single-zone algorithm)

Consider the amount of residual stroma.

160-μm plate leaves a 150 ± 20 μm flap
200 to 250 μm should be left in stromal bed (U.S. Federal Department of Agriculture now recommends 250 μm)
ideal total corneal thickness = 150 + 250 μm

Select a keratome based on keratometry values (Ks).

Ks <42.0 D [200-μm plate with the Automated Corneal Shaper (ACS) or Hansatome (Bausch & Lomb, Rochester, NY)]

Ks = 42-46 D (160-180 μm plate with ACS or Hansatome)

Ks> 46 D (160-μm plate or Hansatome)
Properly clean, assemble, and test the microkeratome.
Calibrate and set up the laser.
- Check fluence (proper energy setting in mJ/cm²).
- Check beam homogeneity (evenness of energy distribution across surface area).
- Each team member is responsible for every aspect of the procedure and should complete multiple checks as the procedure progresses.
Ensure appropriate humidity, temperature, and air purification in the laser room at all times to preserve laser optics and establish standardized treatment conditions.

Patient Preparation

Administer 5 to 10 mg of diazepam orally approximately 45 min prior to the procedure.
Do not use a preoperative miotic.
Irrigate the conjunctival fornices with eyewash to remove any meibomian secretions or tear-film debris.
Clean eyelids with Betadine.
Position the patient under the laser in the supine position with the frontal surface of the cornea perpendicular to the laserbeam aperture.
Instill 1 to 2 drops of a mild topical anesthetic (e.g., proparacaine).
Dry the eyelashes with a gauze sponge and place a fenestrated drape over eyelashes to keep them and redundant periocular tissues out of the surgical field.
Insert a wire-locking eyelid speculum (allows adequate exposure).

The Keratectomy

Open the speculum another 1 to 2 mm after waiting 1 min after initial insertion.

Mark the corneal epithelium with a lamellar corneal surgery marker (useful for proper positioning of a free cap; see Fig. 11-14).
Place the LASIK pneumatic suction ring onto the eye to provide globe fixation, elevation of IOP to create an even-thickness keratectomy, and a track for the advancement of the microkeratome.
Press down suction ring.
After activating the vacuum to the pneumatic suction ring, directly check the IOP using a Barraquer tonometer, pneumotonometer, or digital palpation.
Ensure that the path of the microkeratome is clear.
Pull down the lower lid with the Hansatome.
After obtaining sufficient IOP, lubricate the corneal surface with BSS or proparicaine to minimize epithelium disruption while making a pass with the microkeratome.
Load the microkeratome into the dovetailed groove on the suction ring and then advance and reverse it.
Double depress at the end of the cut to ensure completion.
Discontinue vacuuming and remove the LASIK suction ring or hold the eye with the suction ring while suction is off during the laser ablation.

Laser Ablation

Before lifting the flap, the surgeon and staff should confirm proper laser settings and position the patient’s head so that the corneal surface is perpendicular to the ablation beam.
Lift the flap and fold it out of the ablation field.
Center the ablation over the entrance pupil (critical for hyperopes).
- Monitor the centration cautiously.
- If significant movement occurs, stop the ablation, reorient the patient (center eye by raising patient’s chin and turn head nasally for small palpebral apertures), and proceed when the centration is aligned.
- Use an eye tracker or eye fixation for nystagmus, mobile eyes, or hyperopic eyes.
Turn down the microscope light.
Wipe dry any ablation fluid that accumulates on the corneal surface with a single pass of a nonfragmenting cellulose sponge or blunt spatula.
As the ablation proceeds to the largest-diameter treatment zones, cover the hinge with a blunt instrument, if necessary, to prevent ablation of the back surface of the corneal flap.
If treating concurrent astigmatism, ensure the appropriate axis of treatment by marking a reference point on the patient’s limbus prior to initiating the treatment session.
If significant cyclotorsion occurs when placing the patient in the supine position, adjust the axis that is programmed into the laser.
Correct astigmatism by differentially removing tissue in the frontal plane of the cornea in one of the two major meridians (flatten the steep axis, steepen the flat axis, or perform crossed-cylinder ablation).
- minus cylinder format [Flatten the steep axis by using large-area ablation lasers with closing blades (see Fig. 11-18), masking systems, or flying-spot lasers by passing the spot parallel to the minus cylinder axis or flat axis on keratometry (see Fig. 11-19). It is best used for myopic astigmatism because it minimizes tissue removal.]
- plus cylinder format [Steepen the flat axis best with scanning-spot lasers by removing tissue paracentrally parallel to the minus cylinder axis or flatter axis on keratometry (see Fig. 11-20). It is best used for hyperopic astigmatism because it minimizes tissue removal.]
- crossed cylinder format [Use this method for mixed astigmatism in which each major meridian receives a combination of steepening or flattening, respectively (Fig. 11-22).]

Repositioning the Flap

Gently float the flap into the stromal bed at the completion of the laser ablation.
Place a small-gauge cannula underneath the flap and irrigate for 5 to 10 sec with BSS in a closed system to clear any remaining debris from the interface (increased irrigation time decreases adhesion of the flap and leads to flap edema with greater flap-edge gape; Fig. 11-23).
Inspect the flap to ensure proper positioning by verifying that an equal gutter (keratectomy edge) distance is present throughout the circumference (Fig. 11–24).
Check corneal alignment marks.
Allow the interface to dry for several minutes.
- During the drying phase we recommend that you administer a microdrop of BSS over the central corneal epithelium to maintain its integrity.
Test flap adhesion by performing a striae test.
- Depress the peripheral cornea with closed blunt-tipped forceps.
- When striae pass well into the flap 360 degrees, you have achieved appropriate adhesion.
Remove the speculum and drape.
Reexamine the eye to ensure that the flap is properly positioned.
Instill a topical broad spectrum antibiotic (Ciloxan; Alcon) and nonsteroidal antiinflammatory medication drop (Voltaren; Ciba Vision).
Place a shield over the orbit but do not apply a pressure patch.

Figure 11-22 Crossed-cylinder ablation versus plus and minus cylinder ablation profiles for the treatment of mixed astigmatism. Note less depth is required to achieve identical refractive correction in both the major and minor axes for crossed-cylinder profile (red curve) compared with minus cylinder profile (green) and plus cylinder profile (blue).

Figure 11-23 Doane-Slade superior hinge irrigating cannula under closed flap. Note the “closed system” irrigation of BSS to clear the interface of debris.

Figure 11-24 The reposited flap should be inspected for equal gutter/keratectomy edge distance throughout its circumference. (Courtesy of Stephen G. Slade, M.D.)

Postoperative Considerations

Typical postoperative care of a LASIK patient is relatively simple.

Results

generally no pain
foreign body sensation, light sensitivity, and tearing (typically present for the first few hours after surgery)
uncorrected visual acuity (UCVA) of 20/40 or better in the low and moderate myopia treatment ranges
a clear cornea with intact epithelium on slit-lamp examination
proper flap apposition (check)
stromal edema or interface debris (document)

Patient Management

Prescribe topical prophylactic antibiotics 4 times a day for the first week.
Prescribe topical corticosteroids for the first 4 to 7 days postoperatively.
Topical preservative-free lubricating drops may help select patients (as needed) for several weeks after surgery.
Use an orbital shield to protect the globe from direct pressure during the first 3 to 4 nights.
Systemic analgesics are rarely necessary.
Instruct the patient to resume normal activities after the 1 day postoperative examination (if results are normal), but caution the patient to avoid rubbing the cornea and participating in contact sports without proper eye protection.
Instruct the patient to avoid exposure to potentially contaminated moisture sources such as hot tubs, pools, and fresh-water lakes for at least 2 weeks.
Tell the patient that full visual recovery may take several weeks or more and may be delayed if significant irregular astigmatism exists.

FLAP COMPLICATIONS Flaps may be too short, too thin, irregular, or free (free cap) (Fig. 11-25 to Fig. 11-27). An alteration of the various forces during the LASIK procedure (Table 11-2) may explain their occurrence.

Prevention

Properly assemble and prepare the microkeratome.
Training the surgeon and staff is the cornerstone for achieving reproducible high-quality lamellar incisions.
Confirm adequate (> 65 mmHg) suction levels. [Use a Barraquer tonometer or pneumotonometer to check dilation of the pupil, dimming of the patient’s vision, adherence of the suction ring, level of the suction pressure meter of the base unit, or pressure on palpation with fingertip (i.e., digital pressure).]

Figure 11-25 A centrally perforated flap immediately after LASIK.

Figure 11–26 A horizontal linear flap defect because of a defective blade 4 months after LASIK.

Figure 11-27 A central flap perforation with skip marks 2 months after LASIK.

TABLE 11-2
**Causes of Flap Complications after LASIK**
Flap Complication	Mechanism

Perforated cornea	Absence or incomplete insertion of depth plate

Short flap	Obstruction of microkeratome path

Thin flap	Inadequate or lost suction

Irregular flap	Inadequate or lost suction

Button-hole flap	Inadequate or lost suction

	Ks> 48.0 D

	Mechanical theory of conservation of length leading to central corneal buckling and button-hole flap formation

Free flap	Ks <41.0

	No microkeratome stop screw

Figure 11-28 Interface blood immediately after LASIK.

Management

Abort the excimer laser correction (may further alter the irregular stromal bed and affect the final result).
Leave the flap in place or replace it as precisely as possible into the stromal bed and leave it for at least 5 min.
Monitor the eye weekly for epithelial ingrowth (more common after thin-flap complications).
Consider a repeat LASIK procedure after 3 months using a deeper depth plate (180 or 200 μm).
A free cap may still achieve an excellent outcome with laser treatment in the stromal bed and replacement of the cap.

CORNEAL NEOVASCULARIZATION Prolonged contact-lens use can cause neovascularization of the peripheral cornea. Vessels may be cut during the keratectomy step of LASIK and result in bleeding onto the stromal surface of the ablation bed (Fig. 11-28).Other etiological factors may be at work.

large 9.5-mm keratectomy with Hansatome or other hyperopic rings
a corneal scar or phlyctenule
pterygium
decentered keratectomy
small cornea or microcornea

Prevention

To avoid neovascularization, decenter the flap slightly inferiorly.
Avoid using the large 9.5-mm ring of the Hansatome on small steep corneas.
Anticipate bleeding in eyes with corneal scars that are associated with patent blood vessels.

Management

Corneal neovascularization usually stops without treatment in a few minutes.
Monitor any blood that extends onto the stromal bed to ensure that it does not move into the ablation zone of the laser.
Absorb blood with a dry Murocel sponge as it progresses toward the ablation area.
Temporarily arrest bleeding using direct pressure from the suction ring during the ablation process.
Once ablation is complete, replacement of the corneal flap often tamponades further bleeding.
Persistent corneal bleeds require additional treatment.
To treat persistent corneal bleeding, locally apply a Murocel sponge soaked with Tobradex, 0.1% dexamethasone, 2.5% phenylephrine, or 0.5% aproclo-nidine.
Apply filtered compressed air.
Dry gently with a dry Murocel sponge.

PERFORATION OF THE CORNEA This extremely rare event may occur when LASIK is performed with the ACS (but without proper insertion of the depth plate) or any other microkeratome that allows insertion and removal of depth plates. The perforation becomes obvious when the microkeratome advances. Signs and symptoms of corneal perforation include:

patient complaints of sudden pain
the sudden appearance of intraocular fluid in the operative field
a sudden drop in suction pressure of the suction ring
the presence of extruded intraocular contents with a flat anterior chamber, peaking of the iris, or extrusion of the crystalline lens when reversing the microkeratome

Prevention

This problem is best prevented by carefully inspecting the equipment before each case of LASIK.
Use the “pre-flight checklist” advocated in training programs for technicians and surgeons to check the depth of keratome plate insertion before each use.
Use the newer Hansatome with its fixed unremovable depth plate.

Management

As soon as penetration is suspected, release the suction to minimize extrusion of the ocular contents.
Gently remove the microkeratome from the eye.
Place sutures across the corneal incision to stabilize the eye and then cover the cornea with a contact lens.
Arrange for emergency referral to a surgeon who is comfortable with repair of the anterior segment.

POSTOPERATIVE PAIN Most patients do not experience pain following LASIK because of minimal epithelial disruption and the regularity of the post-LASIK corneal surface. Postoperative pain has many causes:

epithelial defect
dislodged flap
toxic epithelial keratopathy
residual pressure from suction ring/lid speculum
infection (rarely)

Prevention

Ensure that the procedure is as atraumatic as possible.
Minimize the use of preoperative topical anesthesia to reduce postoperative epithelial toxicity.
Administer 1 drop of a topical nonsteroidal medication to reduce the immediate foreign-body sensation.
Advise patients to avoid rubbing or squeezing the eye because it may disturb flap position and cause significant pain.

Management

Address any complaint of significant pain by performing an eye examination to ensure that the flap is in place and the epithelium is intact. (You will usually identify no major problems.)
Administer a mild narcotic to assist with sleeping to reassure the patient.
Provide topical nonsteroidal drops and lubrication drops to help control pain related to epithelial defects.
Immediately treat the superficial punctate keratitis identified on the flap epithelium with generous lubrication.

EPITHELIAL DEFECTS Epithelial defects, which cause postoperative pain and a foreign-body sensation, are generally evident immediately after performing the keratectomy and most commonly form along the superior aspect of the keratectomy edge when using the Bausch and Lomb ACS (Rochester, NY) or the Hansatome (Fig. 11-29). The risk for epithelial ingrowth increases after stimulation of epithelial healing and growth begins. Etiology includes:

anterior basement membrane dystrophy (ABMD) (Patients with ABMD commonly develop these defects; there is an even higher incidence for enhancements.) (Fig. 11-30)
a history of recurrent corneal erosions (Patients with loose epithelium such as those with ABMD or a history of recurrent corneal erosions are best treated with PRK.)
epithelial toxicity from topical anesthesia
operative trauma/manipulation
history of dry eye
insufficient wetting of the corneal surface prior to passage of the microkeratome
advanced age (Patients >45 years old tend to have a less adhesive epithelium to Bowman’s membrane and frequently the epithelium is sloughed when the micro -keratome is reversed.)

Figure 11–29 Loose epithelium with a defect along the flap edge immediately after LASIK.

Figure 11–30 Typical appearance of the map lines of ABMD.

Signs and Symptoms

significant pain
prolonged visual recovery
epithelial ingrowth
flap melt
infection
LASIK interface inflammation (the “sands of Sahara”)

Prevention

Use topical anesthesia sparingly (once under the laser) to avoid any epithelial toxicity.
Keep the corneal surface well lubricated to prevent epithelial irritation.
Perform all surgical maneuvers with great delicacy to avoid any epithelial trauma.
Use a modified technique if the patient has known ABMD.
Do not mark the cornea.
Perform a very wet (copious corneal irrigation prior to passage of the microkeratome) keratectomy to minimize friction between the depth plate and epithelium.
To minimize sloughing of the epithelium, discontinue the vacuum at the completion of the forward pass of the microkeratome, and lift off the microkeratome and suction ring manually from the eye with the flap sliding out of the corneal shoot with minimal to no friction created over the epithelium.

Management

Copiously lubricate the patient’s eye and administer a topical prophylactic antibiotic drop to guard against infection daily until the defect heals.
Small epithelial defects (<3 mm) generally heal quickly in 24 hr, so a postoperative nonsteroidal drop and a good sleep restore eye comfort.
Treat larger epithelial defects with a contact lens, which reduces postoperative pain, lacrimation, and eyelid squeezing and generally remove it after 1 to 2 days.
Alternatively, apply tincture of benzoine to the forehead and cheek skin and pull the lids together with paper tape to prevent blinking but do not apply gauze or a dressing beneath the tape. (This technique achieves minimal to no anterior-posterior pressure, only inferior-superior suspension.)
- Follow patients with epithelial defects weekly for 1 month to monitor for epithelial ingrowth.

Figure 11-31 Grade 3 LIK with dense clumping of cellular material in the interface 3 days after LASIK. UCVA had dropped to 20/200 and BCVA was 20/50.

LASIK INTERFACE KERATITIS Laser interface keratitis (LIK), or the “Sands of Sahara” syndrome, presents as interface inflammation in the 24- to 72-hour postoperative period. The interface inflammation has a “sifted sand” appearance with a dusting of a powder-like material in the interface that generally is more dense centrally (Figs. 11-31 and Fig. 11-32). The rest of the eye is usually unaffected with no conjunctival injection, anterior chamber inflammation, epithelial defects, or corneal infection (unilateral in 70% of cases). There are four grades of LIK (Table 11-3). Likely causes include:

betadine
benzene
BSS
contaminants from eyelids
contaminants on instruments (consecutive serial cases on the same day with same instruments and surgeon)
corneal abrasions
laser thermal effect
lubricant
meibomian gland secretions
metallic debris
talc
topical medications
bacterial endotoxin or exotoxin
- endotoxins released from the cell walls of dead bacteria upon sterilization of improperly cleaned or stored instruments
- exotoxins from patients with bacterial antigen loads from un- or undertreated blepharitis (isolated cases)

TABLE 11-3
**Hatsis Classification of LIK**
Grade	Interface	Topography	Vision

I	Partial	No change	Excellent

II	Complete	Irregular/shifts	Excellent

III	Complete	Irregular/shifts	Foggy

IV^*	Complete	Irregular/shifts	Foggy

^* And inflamed eye

Figure 11-32 Grade 3 to 4 LIK. The circular inflammatory cellular material has formed along the rings of the broad-beam excimer ablation.

Prevention

Because causes of LIK remain unproven, prevention is difficult.
Clean and autoclave instruments and, in particular, the keratome between cases.
At the end of the operating day, clean, sterilize, and dry the instruments to remove all moisture (prevents an environment for bacterial colonization and active division).
Analyze all loaner units or traveling microkeratomes for cleanliness.
Remove potential endotoxins using a proper cleaning regimen (e.g., an absolute ethanol soak).
Gently clean the packaged microkeratome blade before use to remove any surface debris that may stimulate inflammation.
Immediately use topical steroid drops for the first few postoperative days to suppress any inflammation early.

Management

Ensure sterility of cornea (no epithelial defects).
Early identification of LIK allows the prompt initiation of topical steroid treatment, which may rapidly resolve the condition.
More severe cases may take 4 to 8 weeks to resolve.
Start topical steroids every hour and then slowly taper over several weeks.
Treat severe cases with interface irrigation (reported as successful by some surgeons).
Antibiotics have no obvious role.

INFECTION Infections or corneal ulcers are extremely uncommon after LASIK with an incidence of less than 1/5000 cases. Because the corneal epithelium protects against infection, the risk of infection is minimal once the epithelium is completely healed. The short time of exposure of the stroma and the lack of epithelial disruption minimize this risk. Infections begin as infiltrates in the area of an epithelial defect and, if left: untreated, may progress into a typical corneal ulceration with infiltration, ocular inflammation, and pain.

Prevention

Follow any patient with a postoperative epithelial defect daily to ensure that no infection occurs until the defect has healed.
Use prophylactic antibiotics.
Sterilize the microkeratome.
Ensure good lid hygiene.
Use antiseptic eyewash preoperatively.
Wear talc-free gloves.

Management

Assume that any corneal infiltrate associated with an epithelial defect after LASIK is infective and treat it immediately and aggressively.
Perform corneal cultures (utility at this early stage of infection is controversial).
Stop administering nonsteroidal medications and topical steroid drops.
Instill topical fluoroquinolone every hour until improvement is noted.
Lubricate the eye.
Follow up with patient daily.
If not resolved, switch to fortified topical antibiotics.

LASIK FLAP STRIAE Normally the striae are oriented horizontally with a nasal hinge and vertically with the superior hinge. Most flap striae occur within the first postoperative hour. Flap striae become more difficult to remove as the length of the postoperative course increases, and just 1 week postoperatively flap striae begin to become embedded in the corneal flap tissue (Figs. 11-33 and Fig. 11-34). Striae may be caused by

misalignment of the corneal flap after flap replacement
movement of the corneal flap during the first postoperative day
the “tenting effect” of the corneal flap over the ablated stromal bed

Figure 11-33 Retroillumination clearly demonstrates these severe striae caused by flap displacement 3 days after LASIK.

Prevention

Reposition the flap with minimal manipulation once it has been replaced into the correct position.
Instruct patients to avoid rubbing or squeezing the eye postoperatively.
Instruct patients to wear eye protection 24 hours a day for the first week to prevent any eye trauma while the flaps heal.

Management

Identify flap striae on the first postoperative day (imperative) because striae become more difficult to remove as the length of the postoperative course increases.
Retroilluminate the fixation light and aiming beam through the dilated pupil to accurately localize the flap striae and identify microstriae in cases of unexplained reduction of BCVA.
Treat flap striae that extend through the visual axis or induce regular or irregular astigmatism.
Administer phenylephrine 2.5% (allows intraoperative retroillumination).
- Mark the flap edge while viewing under the slit lamp.
- Make no flap alignment markings.
- Reflect back the flap.
- Hydrate the stromal surface of the flap with BSS for 30 sec.
- Replace the flap in the stromal bed and float it into position.
- Leave the flap for 5 min to attach the stromal bed and dry the epithelial surface.
Use the side of blunt forceps or a dry Murocel sponge to stretch the flap perpendicular to the striae.

Figure 11-34 Severe striae associated with residual interface haze after LIK 3 weeks after LASIK.

DISLODGED FLAP Flap dislocation occurs in only 1/500 to 1/1000 of LASIK patients, usually because of some type of eye trauma or vigorous eyelid squeeze during the first 12 hr postoperatively. Vision becomes blurred and the eye becomes painful with an acute foreign-body sensation similar to the pain of a corneal abrasion. The incidence of epithelial ingrowth increases with both displacement and free caps.

Prevention

Check the stability of the flap by ensuring that it is secure at the conclusion of the procedure by using the striae and blink tests (see p. 119).
Wait 1 to 3 min after properly aligning the corneal flap and avoid excessive interface irrigation so that a strong seal may form.
Advise patients to not rub or squeeze the eye.
Provide eye protection for both day and night-time use.

Management

Until flap replacement, lubricate the eye every 30 min with sterile preservative-free artificial tears to hydrate the flap and improve the discomfort.
Patients usually keep the eye closed because of discomfort, or you may tape the eye shut until treated.
Replace the corneal flap as soon as possible to avoid infection, reduce pain, and avoid permanent striae and damage to the flap (same procedure as for treating flap striae).

INTERFACE DEBRIS Interface debris often is present to some extent in most eyes following LASIK. Visual outcome is generally not affected and removal is unnecessary if the debris is inert. LASIK interface inflammation, however, does affect the visual outcome and should be differentiated from focal, noninflammatory interface debris. Following are examples of debris.

lint
red blood cells
meibomian gland secretions
metallic debris from blade
dust
rust particles from instruments

Prevention

Interface irrigation during primary LASIK is the most effective method for reducing and potentially eliminating interface debris (removes > 90% of the minute dust particles that are invisible under the laser microscope but visible postoperatively by slit lamp).
Several other preventive measures help to avoid interface debris.
- preoperative lid hygiene
- washing the fornices
- draping the lids
- air filters in surgical suite
- cleaning of all instruments between use to remove rust

Management

If interface inflammation is suspected, follow the patient closely and begin administration of prophylactic topical steroids.
Remove an interface contaminant that affects vision.
Remove a large strand of lint that extends beyond the edge of the flap by pulling on it laterally with forceps.
Remove debris that is completely under the flap using interface irrigation and flap repositioning (as in primary LASIK).

EPITHELIAL INGROWTH Reports indicate that 1.7% of eyes that undergo LASIK develop significant epithelial ingrowth that requires removal. However, refinements in the technique that delicately preserve the corneal epithelium have reduced this incidence to less than 1%. Epithelial ingrowth can occur in a number of patterns including lines, speckles, nest or pearls, strands, or sheets. Significant and persistent epithelial ingrowth often results in peripheral flap edge melt. Interface epithelial ingrowth may originate at three potential sites.

stem cells peripheral to keratectomy edge (most common)
isolated nest within the interface with no direct connection to the periphery (infrequent)
passage through a defect in Bowman’s membrane and flap stroma (rare; caused by trauma to these structures with direct communication of surface epithelium to the interface

Risk Factors

preoperative risk factors
- °ABMD
- a history of recurrent corneal erosions
- advanced age
- a history of ingrowth in the other eye
postoperative risk factors
- epithelial defects
- flap edema
- inflammation
- repeat LASIK
- flap slippage

Prevention

Eliminate epithelial defects with LASIK (see Epithelial Defects section).

Management

Identify the epithelial ingrowth by using one of several tests (Table 11-4).
Consult Machat’s practical grading system for epithelial ingrowth to guide the course of management (Table 11-5 and Fig. 11-35 to Fig. 11-37).
Indications for treatment include the following.
- greater than 2 mm of ingrowth from the flap edge
- documented progression
- associated flap melting
- a disturbance of BCVA attributable to the ingrowth
Mark the edge of the corneal flap using the slit lamp.
Mark peripheral cornea alignment.
Slowly dissect free the flap at its temporal margin.
Gently fold the flap over its nasal hinge.
Remove the epithelium from underneath the flap by peeling and scraping (use a dry Murocel sponge or blunt instrument).
Remove the epithelium from the stromal side of the Gently push back any epithelium hanging in the peripheral stromal bed.
Replace the flap as for primary LASIK.
Thoroughly irrigate underneath the cap to eliminate residual epithelial cells.

Figure 11-35 Grade 1 epithelial ingrowth.

TABLE 11-4
**Identification of Epithelial Ingrowth**
Test	Finding

Tangential slit-lamp illumination	White to gray material under flap
Fluorescein staining	Pooling at the flap edge
Corneal topography	Surface irregularities
Retroillumination	Margins of sheets of ingrowth

Figure 11-36 Grade 2 epithelial ingrowth.

Figure 11-37 Grade 3 epithelial ingrowth.

ERRORS IN REFRACTIVE OUTCOME Patients are disappointed if any significant refractive error remains after LASIK because they still depend on refractive correction (Table 11.6). Even a −12.00 D myopic will return for a −0.50 D enhancement, despite remarkable improvement in uncorrected vision. Although we are able to accurately program excimer laser ablation and perfect keratectomy techniques, the healing response of the cornea is the unknown variable with LASIK. Some patients may regress up to 3.0 D whereas others do not regress at all. Younger patients (<25 years) tend to regress and experience undercorrections whereas older patients (>45 years) regress less and experience overcorrections.

TABLE 11-5
**Machat’s Epithelial Ingrowth Classification**
Grade	Description	Treatment
1	Thin ingrowth, 1-2 cells thick, limited to≤2 mm of flap edge, transparent, difficult to detect, well delineated white line along advancing edge, no associated flap changes, nonprogressive	No retreatment required
2	Thicker ingrowth, discreet cells evident within nest, ≥2 mm from flap edge, individual cells translucent, easily seen with slit lamp, no demarcation line along nest, corneal flap edge rolled or gray, no flap edge melting or erosion, usually progressive	Requires nonurgent treatment within 2-3 weeks
3	Pronounced ingrowth, several cells thick,>2 mm from flap edge, ingrowth areas opaque, obvious with slit lamp, white geographic areas of necrotic epithelial cells with no demarcation line, corneal flap margins rolled with thickened whitish-gray appearance, progression results in large areas of flap melting from collagenase release from the necrotic epithelium, confluent haze develops peripheral to the flap edge as flap pulls away leaving exposed stromal bed in contact with surface epithelium	Urgent treatment required with close follow-up Recurrences are more common because of the altered flap edges

Prevention

Ensure the accuracy and the stability of the preoperative refraction.

Use a consistent LASIK technique with controlled stromal hydration and drying.

Consult nomograms developed for LASIK that account for the degree of correction and age of the patient to minimize errors in the refractive predictability (Table 11-7).
- Most myopia nomograms err toward undercorrection because myopia is generally less difficult to treat. 0 Overcorrect hyperopes by 10 to 20% because they tend to regress to emmetropia over 1 to 3 months.
- Carefully evaluate the technique that you choose, the geographic location of the laser center, the staffs attitude, and results to determine which nomogram is best for you.

Management

A contact lens may manage any refraction as early as 1 week after LASIK, so that patients may return to full activities almost immediately.
Topical steroids have little effect on refractive results.
After achieving refractive stability, you may perform an enhancement procedure.
- The greater the primary LASIK correction, the more postoperative time that is required to achieve full refractive stability.
- Do not perform PRK over a LASIK flap because PRK may induce corneal haze.

TABLE 11-6
**Errors in Refractive Result Following LASIK and Potential Treatments**
Error	Comment	Enhancement

Hyperopia	Normal cornea Thin cornea	Hyperopic LASIK at 4 months Laser thermokeratoplasty at 6 months

Myopia	1-3 D preoperatively 3-6 D preoperatively 6 D preoperatively	Myopic LASIK at 1 month Myopic LASIK at 2 months Myopic LASIK at 3-4 months

Regression	Wait for stability	Myopic LASIK

Astigmatism	Wait for stability	LASIK at 3-4 months

CENTRAL ISLANDS Several situations lead to the formation of central islands.

central accumulation of fluid
inhomogeneous beam
vortex plume with central shielding of successive pulses when using a broad-beam excimer laser

The presentation of central islands includes the following.

loss of BCVA and UCVA
irregular astigmatism
monocular diplopia
undercorrection

Although most central islands resolve by 1 month with no treatment or PRK.

Prevention

Central island facto (CIF) software has been incorporated into the VISX laser to allow extra treatment to the central cornea to compensate for the central island effect of the broad beam laser.
Scanning excimer lasers eliminate the risk of central islands.
- Fluid does not accumulate centrally.
- The vortex plume is not stationary.
- Homogeneity of the beam with scanning lasers is not critical because of the smoothing or polishing effect of overlapping spots or slits.
Remove any central accumulation of fluid by wiping the stromal bed during LASIK (alters overall stromal hydration and therefore affects the final refractive outcome).

Management

Identify and monitor central islands with topography until they are stable in size and severity (Fig. 11-38).
Do not use refraction as an indicator of stability because it often varies considerably with each refraction.
Treat a central island only if it causes subjective visual complaints or loss of BCVA.
Topographic central island without patient complaints should not be treated just because the practitioner thinks it should be treated; treat the patient.
Measure the size and height of the island with corneal topography (use the Munnerlyn formula to calculate the height of the island).
height in microns = diameter² X diopter height/3
Then correct the exact number of microns and diameter size of the island using the phototherapeutic keratectomy (PTK) mode on the VISX (Star or Smoothscan) or the Chiron 116 excimer laser.
Alternatively, treat the refractive myopia in the PRK mode of the VISX laser but only for the first 50% of the ablation.
- The Chiron 217 does not treat the central island well because the blending effect of the scanning laser does not allow the beam to be focused into the central area alone.
Use a conservative approach because overtreatment results in a central divot, which creates irregular astigmatism and abrupt topographic changes over the pupillary axis (leads to multiple image formation or ghosting, which is difficult to correct surgically but may be treated with a rigid contact lens).

TABLE 11-7
**Probst LASIK Nomogram for VISX SmoothScan Laser**
	Patient Age
Amount of Myopia	<25 years	25-45 years	>45 years

<2.0 D	100%	100%	100%
>2.0 D	90%	86%	84%

Figure 11-38 Corneal topography demonstrating a central island.

IRREGULAR ASTIGMATISM All patients have some degree of postoperative irregular astigmatism immediately, which gradually resolves a few weeks after the procedure. Persistent irregular astigmatism is now relatively uncommon but results in several undesired outcomes.

reduced UCVA and BCVA
visual distortion
ghosting
monocular diplopia

Prevention

Proper surgical technique minimizes flap-related complications, such as striae.
Check the homogeneity of the excimer laser beam regularly to ensure that the beam is smooth and symmetrical (irregular patterns are amplified during large corrections).
Monitor hydration of the stromal bed (uneven wetting of the stromal bed can result in an irregular ablation pattern).

Management

Postoperative topography, photokeratoscopy, manual keratometry, or even retinoscopy often uncovers some degree of irregularity that is related to some variation in corneal healing.
Slit-lamp examination may reveal flap striae, LASIK interface inflammation, or corneal scarring which may result in irregular topographic patterns.
Treatment of the flap striae and LIK (as described previously) dramatically improves both the symptoms and the topographic pattern.
Corneal topography may demonstrate other irregularities of an otherwise normal cornea (e.g., central islands).
Other irregular patterns are more difficult to treat because they require a customized ablation pattern.
Topography-linked scanning excimer lasers offer the most promise for the treatment of irregular patterns.
If all else fails, RGP contact lenses, if tolerated, are usually very helpful in reducing (if not eliminating) unwanted visual symptoms.

NIGHT GLARE Night glare occurs when the pupil dilates greater than the OZ of the refractive correction (Fig. 11-39). Preoperative and postoperative night glare are more common in high myopes. Patients experience halos, starbursts, and reduced quality of vision. Although manageable in most patients, night glare may disable others.

Figure 11-39 Small OZ on topography 3 months after LASIK for a −10.0 D correction using a multizone algorithm on a broad beam laser.

Prevention

Screen patient preoperatively for pupil size.
Counsel patients with large pupils (>6 mm) in dim illumination about the potential risk of postoperative night glare.
Treat patients with large pupils with larger-diameter ablation zones (≤9 mm); however, these lasers ablations remove significantly greater amounts of stroma so stromal thickness is the critical data point for determining candidacy.

Management

Fortunately, most symptoms of night glare abate within the first year after the operation.

Refinements in ablation profiles have resulted in significant reduction in the number of complaints of night glare.

Perform LASIK on the other eye.
Eliminate monovision.
Prescribe night-driving glasses (1.00 D).
Prescribe polarized lenses.
Use spectacle lens filters (shooter’s yellow, lambda 540).
Instruct patient to use a night light in the car while night driving.
Initiate miotic therapy (pilocarpine 1/8%) to constrict pupil when night driving

Surgical.

Retreat the eye by using a larger OZ and ablation zone.

Perform topography-assisted LASIK to same eye.

DECENTERED ABLATIONS Decentration of the ablation results in a residual refractive error (irregular astigmatism), which is associated with visual distortions and ghosting, particularly at night, and the patient may lose some BCVA. The patient’s concerns are often more significant than expected for such a small refractive error. Decentered ablations have many causes.

pharmacological pupillary dilation
pharmacological constriction
large angle kappa
poor patient fixation
wide flap hinge encroaching on ablation zone
eccentric fixation
treatment beam not coaxial with patient fixation beam (validate during instrument setup)

Prevention

Always center excimer ablations on the pupil to minimize decentrations.
Avoid pharmacological alteration several days before performing LASIK.
To ensure centration on the pupil leave the suction ring on the eye but without active suction to control the position of the ablation completely.
- Patients often have difficulty fixating on the fixation beam once the flap has been cut because the stroma does not provide a clear interface.

Figure 11–40 Decentered hyperopic ablation 4 months after hyperopic LASIK for a +5.0 D correction.

Management

Detect decentration (obvious with postoperative topography; Fig. 11-40).
Because the decentered ablation creates an irregular ablation pattern, treatment is difficult with a broad-beam laser.
The refractive error is treatable and the flap is replaceable, these treatment options do not treat the edge glare effect from the sort edge of the ablation zone.
A second ablation in the opposite direction of the original decentration has also been suggested but may result in a more irregular pattern.
Topography-assisted LASIK (TA-LASIK) holds the most promise for decentered ablation because it treats the untreated area of the ablation zone.

LASER IN SITU KERATOMILEUSIS AS AN ENHANCEMENT PROCEDURE

Indications and Inclusion Criteria

at least 1.00 D of myopia or hyperopia
at least 1.00 D of astigmatism
UCVA of 20/40 or worse

Risks

overcorrection
epithelial ingrowth
flap striae
infection
pain

Preoperative Evaluation

Ensure stable refraction.
Ensure adequate corneal thickness using pachymetry.
Perform regular topography.

Methods (Fig. 11-41)

Mark flap edge at slit lamp.
Place alignment marks on cornea.
Lift flap before 6 months postoperatively.
Recut flap at a deeper depth after 6 months.
Peel back flap edge to minimize epithelial disruption.
Perform excimer enhancement ablation.
Refloat flap into position.
Irrigate the stroma interface.
To ensure centration on the pupil, leave that suction ring on the eye with the suction off to control the position of the ablation completely.
Check alignment marks and gutter for alignment.
Wait 1 to 5 min for adhesion to occur.
Perform a blink and/or striae test.

Postoperative Management

Monitor for ingrowth.
Check refractive outcome.