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Photorefractive keratectomy (PRK) is a procedure in which the cornea is reshaped using an excimer laser. PRK involves epithelial removal and photoablation of Bowman’s layer and anterior corneal stromal tissue. In contrast to laser-assisted in situ keratomileusis (LASIK), there is no need for flap creation with a microkeratome.
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Laser subepithelial keratomileusis (LASEK) and epi-LASIK are corneal surface ablative refractive procedures.
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LASEK involves creating an epithelial flap with dilute alcohol and repositioning this flap after laser ablation.
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Epi-LASIK involves the use of a motorized epithelial separator to mechanically separate the corneal epithelium from the stroma.
Key Features
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Excimer laser surface ablation results in removal of a precise amount of tissue from the anterior cornea. The central cornea is reprofiled to achieve myopic, hyperopic, or astigmatic correction.
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The refractive result is related to the depth of ablation and the diameter of the optical zone.
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Preoperative assessment plays a key role in determining a safe and effective outcome. It includes corneal pachymetry, topography, and cycloplegic refraction.
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Mitomycin-C is useful to prevent corneal haze and scarring in high myopic corrections.
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Wavefront guided ablations result in higher percentage of patients achieving uncorrected visual acuity of 20/20 or better compared with conventional ablations.
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PRK, LASEK and epi-LASIK are considered in patients with thin, steep, or flat corneas and in patients predisposed to flap trauma in thinner corneas, where creation of LASIK flap may leave less tissue than desired (usually 250 µm of corneal tissue) remaining to the posterior stroma.
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Postoperative complications of PRK, LASEK, and epi-LASIK include epithelial healing, pain, infiltrates and infection, dry eye, and corneal haze.
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LASEK-related intraoperative complications include alcohol leakage, incomplete epithelial detachment, and laser-related complications.
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Epi-LASIK–related intraoperative complications include flap-related complications (when these occur the procedure may be converted to PRK) and laser-related complications.
Introduction
The surgical treatment of myopia, hyperopia, and astigmatism has made great strides over time, with the introduction and advancement of the excimer laser PRK followed by LASIK surgery. Ultraviolet radiation at 193 nm wavelength utilized by the excimer laser can remove precise amounts of tissue from the anterior cornea. Use of the excimer laser for the treatment of myopia, hyperopia, and astigmatism is now well established.
Other corneal surface ablative procedures include laser-assisted subepithelial keratomileusis (LASEK) and epi-LASIK. In principle, they combine the advantages of both LASIK (laser-assisted in situ keratomileusis) and PRK, while at the same time overcoming some of their problems. LASEK was first conceived independently in 1996 by Azar, as well as Cimberle and Camellin. LASEK involves creating an epithelial flap with dilute alcohol solution and repositioning this flap after laser ablation, thus eliminating any inherent flap complications. Epi-LASIK makes use of a motorized epithelial separator to mechanically separate the corneal epithelium in toto from the stroma without the use of alcohol or chemicals. This device makes use of a proprietary oscillating blade that separates the epithelial layer at the layer of Bowman’s membrane without dissecting the corneal stroma.
Development of excimer lasers began in 1975 when Velasco and Setser noted that metastable rare gas atoms such as xenon (Xe) could react under high pressures with halogens such as fluorine (F) to produce unstable compounds such as XeF. These compounds rapidly dissociated to the ground state of the individual molecules associated with the release of an energetic ultraviolet photon and could be made to undergo light amplification by stimulated emission when they were excited by an electron beam, with the argon-fluorine (ArF) molecule emitting light with a wavelength of 193 nm. The ablation thresholds, ablation rates, and healing patterns for different excimer wavelengths were described by Krueger and Trokel. The ablation threshold for the cornea is the fluence at which tissue removal begins, which is approximately 50 mJ/cm 2 for 193 nm. The 193 nm light has very low tissue penetrance, enabling the laser to operate on the surface of the corneal tissue with precision and safety.
Large-area ablation with resculpting of the cornea to correct refractive errors is termed laser keratomileusis. Tissue is removed with great precision, and the corneal epithelium heals over the ablated area to create a smooth surface. About 0.24 µm of tissue is removed with each laser pulse. Corneal epithelium ablates at a slightly faster and more irregular rate than corneal stroma, which is the reason that the epithelium is typically removed mechanically before ablation of the stroma for the PRK procedure. Bowman’s layer ablates about 30% slower than the stroma, and fluorescein decreases the ablation rate by about 40%.
Ablation Profiles
Munnerlyn described the direct relation between the amount of tissue that must be removed to produce a certain refractive result and the optical zone size. The relationship can be simplified to:
Depth of ablation(μm)=[diameter of optical zone(mm)]2×1/3power(D)
As the optical zone is increased, the ablation depth needs to be increased. The optical zone size and depth are optimized to reduce excessive wound healing seen in deep ablations and the excessive halos, edge glare, and irregular astigmatism seen with small optical zones.
In correction of spherical hyperopia, the cornea needs to be reshaped into a steeper convex structure. This can be achieved by a peripheral annular ablation with peripheral flattening creating central corneal steepening. Generally, a larger (9–9.5 mm) ablation diameter is required to achieve permanent steepening of the cornea. It is more challenging to deliver larger-diameter hyperopic ablations than the equivalent myopic correction.
Excimer laser surgical correction of astigmatism requires that the cornea be ablated in a cylindrical or toric pattern. In the early excimer systems, the excimer laser beams were passed through a set of parallel blades that gradually open as directed by the computer algorithm. The orientation and speed of opening depends on the orientation and amount of astigmatism to be corrected. Flattening occurs perpendicular to the long axis of the slits. No change in power occurs along the axis of the slits. Another method of creating a toric ablation utilizes an ablatable mask. The laser first ablates the thinnest areas of the mask, thus allowing greater treatment of the cornea in areas where the mask is thinnest. Any pattern can be created by differential protection of the cornea from treatment.
A computer-controlled scanning beam can be utilized to treat astigmatism. The size of the beam can be varied to create a transition zone, preventing a steep step off at the edges of treatment. With this method, it is possible to steepen, rather than flatten an axis, thus allowing for a more direct and tissue-conserving treatment of hyperopic astigmatism. Wavefront-guided customized corneal ablation profiles have been introduced for correction of irregular astigmatism (higher-order aberrations) as well as spherocylindrical refractive errors.
Indications
PRK, LASEK, and epi-LASIK surface ablation may be performed in patients who are at low risk for subepithelial haze with low to moderate myopia and myopic astigmatism. The ideal candidates for LASEK and epi-LASIK are those with mild to moderate myopia up to −7.00 diopters (D). LASEK has also been shown to be effective for hyperopia up to +4.00 D.
Surgeons should consider these surface ablative refractive procedures for patients whose corneal characteristics render them at greater risk for LASIK, such as those with thin corneas where less than 250 µm of residual stromal bed would be left and those with steep or flat corneas. These would also be the preferred surgical procedures in patients with lifestyles or professions that predispose them to flap trauma. LASEK may also be a better choice for patients with narrow palpebral fissures where the microkeratome cannot be well applied.
Contraindications for these procedures include exposure keratopathy, neuropathic keratopathy, severe dry eye (Sjögren’s syndrome), keratoconus, central or paracentral corneal scars, unstable myopia, and irregular astigmatism.
Preoperative Evaluation
As for any refractive procedure, the preoperative workup for PRK, LASEK, and epi-LASIK includes uncorrected and best-corrected distance and near visual acuities with a manifest and cycloplegic refraction. Ocular dominance testing, anterior segment and posterior segment examinations, keratometry, tonometry, pachymetry, aberrometry, and computerized topographical analysis are other important parts. A careful systemic and ocular history and examination is necessary to look for conditions that may require preoperative management or contraindicate the procedure ( Box 3.3.1 ). For example, mild degrees of dysfunctional tear syndrome or dry eye ( Fig. 3.3.1 ) can be managed preoperatively with lid hygiene, artificial tears, and topical cyclosporin for better early postoperative recovery. Meticulous preoperative counseling is also a very important aspect, as for any refractive procedure.
Potential Preference of PRK Over LASIK
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Thin corneal pachymetry
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Epithelial irregularities/dystrophies
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LASIK complications in the contralateral eye
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Predisposition to trauma
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Low myopia
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Irregular astigmatism
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Dry eyes
Cautions
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Postoperative pain intolerance/concern
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Keratoconus
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Glaucoma
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Pregnancy
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Advanced diabetes
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Collagen vascular disease
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Previous herpes (simplex or zoster) infection
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Severe dry eye
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Untreated blepharitis
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Neurotrophic cornea
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Peripheral ulcerative keratitis
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Patients on isotretinoin (Accutane), amiodarone (Cordarone), or sumatriptan (Imitrex)
PRK Surgical Technique
Patient Preparation and Epithelial Removal
The initial patient preparation is similar to all the three surface ablation procedures. The patient is positioned under the microscope, and the head is carefully aligned to make sure that the iris plane is perpendicular to the laser beam. After topical anesthesia (0.5% proparacaine or tetracaine), the eyelids and periocular skin are prepped with dilute povidone–iodine (Betadine) solution. A lid speculum is placed to provide adequate exposure of the globe. Careful centration with the eye aligned in the x-, y-, and z-planes is crucial.
It is important to perform the treatment as soon as possible after the proparacaine or tetracaine drops are instilled or have the patient close their eyes to prevent exposure keratitis from poor blinking. Drying of the inferior half of the cornea, as often occurs after anesthetic drops have been instilled, can lead to increased thinning inferiorly.
The epithelium is marked with a 7- or 8-mm optical zone marker centered on the pupil for myopia or a 9- or 10-mm marker for hyperopia or wavefront correction. It is helpful to remove a 1-mm larger area of epithelium than the planned ablation. Mechanical epithelial removal involves the use of a Tooke knife, disposable excimer spatula, or rotating brush. Alternatively, alcohol (18%–25% ethanol for 21–30 seconds) can be used to loosen the epithelium or excimer laser for partial or complete removal. The laser is typically set to a depth of approximately 45 µm, and the epithelium being ablated by the laser beam can be visualized under blue fluorescence. The ablation is stopped when a change from a fluorescent pattern to a dark pattern is seen, indicating that the epithelium has been ablated. If fluorescence persists across the whole area after a 50 µm ablation has been performed, an additional depth of 25 µm should be set for the laser. It may be helpful to scrape the remaining epithelium. It is important to remove the epithelium totally. Any residual epithelium will create an uneven ablation and irregular astigmatism. Also, epithelial removal should be quick to avoid corneal hydration changes.
Stromal Ablation
The ablation should promptly follow epithelial removal to prevent drying, which can lead to increased haze and scarring.
Centration is rechecked after epithelial removal. For astigmatic corrections, alignment on the proper axis should be verified by marking the patient’s limbus at the 12 and 6 o’clock positions with gentian violet dye on a Sinskey hook at the slit lamp before the procedure. The ultraviolet excimer lasers used have wavelengths outside the visible spectrum; an auxiliary aiming device that is coaxial to the ablating laser is required to make certain the laser is centered on the eye. Helium–neon lasers, laser diodes, or a coaxial aiming target are commonly used for this. In automated systems with eye trackers, this is used only for initial alignment, but in manually controlled systems, it is used to align the eye during the entire procedure.
The ablation is begun, centered over the pupil with the patient looking at the fixation light. Eye movements should be minimized during the ablation to reduce irregular surfaces. The use of an eye tracker is helpful in maintaining centration. It is important to make certain that the hydration status of the corneal stroma is uniform during the procedure. If excess fluid is detected, the procedure should be paused and the excess fluid removed by using a cellulose sponge to dry the cornea ( Fig. 3.3.2 ). Likewise, if the cornea becomes too dry, a cellulose sponge can be used to evenly hydrate it. Inadequate or excessive tissue hydration leads to more or less tissue ablation per pulse, resulting in an overcorrection or undercorrection, respectively.
LASEK Surgical Technique (see Fig. 3.3.3 )
Patient is prepared and eye aligned as described earlier. Just before surgery, topical broad-spectrum antibiotics (e.g., tobramycin or a fluoroquinolone) may be applied prophylactically. Some surgeons use topical nonsteroidal anti-inflammatory drugs (NSAIDs) for pain relief. Dilute ethanol at a concentration of 18% is prepared by drawing 2 mL of dehydrated alcohol into a 12 mL syringe and diluting it with sterile water to 11 mL.
The cornea is marked with 3-mm circles around the corneal periphery to allow the surgeon to have the precise reference points to realign the flap over the corneal bed. An alcohol dispenser consisting of a customized 7- or 9-mm semisharp marker (ASICO, Westmont, IL) attached to a hollow metal handle serves as a reservoir for the 18% alcohol.
Firm pressure is exerted on the central cornea and a button is pushed on the side of the handle, releasing the alcohol into the well of the marker. Alternatively, a 7-mm optical marker (Storz, St Louis, MO) is used to delineate the area centered on the pupil. Gentle pressure is applied on the cornea while the barrel of the marker is filled with two drops of 18% ethanol. After 25–35 seconds, the ethanol is absorbed using an aspiration hole followed by dry sponges (Weck-cel or Merocel, Xomed, Jacksonville, FL), to prevent alcohol spillage onto the epithelium outside the marker barrel. The ethanol application may be repeated for an additional 10–15 seconds.
One arm of a modified curved Vannas scissors or a jeweler’s forceps is inserted under the epithelium and traced around the delineated margin of the epithelium, leaving 2–3 clock hours of intact margin, preferably at the 12 o’clock position. The loosened epithelium is peeled as a single sheet using a jeweler’s forceps, spatula, or a Merocel sponge, leaving a flap of epithelium with the hinge still attached. The ablation is then initiated immediately using an excimer laser.
After ablation, a 30-gauge anterior chamber cannula is used to hydrate the stroma and epithelial sheet with balanced salt solution. The epithelial sheet is replaced on the stroma using the straight part of the cannula under intermittent irrigation. The epithelial flaps are realigned using the previous marks. The flap is then allowed to dry for 2–5 minutes.
A bandage contact lens is placed on the operated eye at the end of the procedure.
Epi-LASIK Surgical Technique ( Fig. 3.3.4 )
The corneal epithelium is dried with sponges after proper positioning and ocular surface irrigation with balanced salt solution using an anterior chamber cannula. The cornea is marked with a standard LASIK marker.
The subepithelial separator is applied to the eye and suction is activated by a foot pedal. The oscillating blade separates the epithelium leaving a 2–3-mm nasal hinge, the suction is released, and the device is removed from the eye. The epithelial sheet is reflected nasally using a moistened Merocel sponge or a spatula. Laser ablation is then initiated immediately, with use of an excimer laser. The cornea then is irrigated with balanced salt solution. The epithelial sheet is carefully repositioned using the straight part of the cannula. The replaced corneal epithelial sheet is left to dry for 2–3 minutes to allow adhesion to the underlying corneal stroma. Some surgeons may choose to discard the epithelial flap instead. In this situation, the subepithelial separator would then simply remove the epithelium before the laser ablation.
At the end of the procedure, a bandage contact lens is placed on the operated eye.
Surface Ablation With Mitomycin-C
In refractive surgery, patients with high myopia are at a higher risk of haze formation. Early haze formation has been more common with higher attempted corrections, smaller ablation zones (<4.5 mm), male gender, ablations deeper than 80 µm, and discontinuation of topical corticosteroids. Subepithelial stromal tissue reformation following photorefractive ablation is attributable to abnormal activation or proliferation of stromal keratocytes following surgical trauma to Bowman’s layer.
Mitomycin-C (MMC) is an alkylating antibiotic substance with antiproliferative and antifibrotic actions. Topical MMC 0.02% (0.2 mg/mL) application for 2 minutes on the exposed stromal bed has been used successfully to treat central subepithelial fibrosis after radial keratotomy (RK) or PRK. It also has a role in prevention of corneal haze formation after PRK for treatment of myopia and hyperopia. The typical application times range between 1 and 2 minutes, although as little as a 12-second application of 0.02% MMC has been found effective for prevention of corneal haze. Application is performed immediately after the laser ablation. The corneal surface and the entire conjunctiva are then vigorously irrigated with 20 mL of cold normal saline to remove any residual MMC.
PRK with MMC application may offer a good alternative for patients with high refractive error, where the cornea is not thick enough for a safe LASIK procedure or if LASIK is contraindicated for other reasons. It has also been tried in post keratoplasty, post RK, and immediately after the occurrence of the LASIK flap buttonhole with successful results.
Wavefront-Guided Surface Ablation
Conventional laser treatments tend to increase the higher-order aberrations (HOA) in an attempt to treat the lower-order aberrations by altering the corneal shape from its normal prolate to a more oblate configuration. Spherical aberration is the most common of these, causing mainly loss of contrast sensitivity rather than Snellen visual acuity. Wavefront-guided laser utilizes the preoperative wavefront of the patient and compares it to the desired postoperative wavefront. The difference is used to generate a three-dimensional map of the planned ablation. Thus customized, computer-generated, complex patterns of ablation are delivered in an attempt to decrease the pre-existing HOA as well.
Patient Preparation and Epithelial Removal
The initial patient preparation is similar to all the three surface ablation procedures. The patient is positioned under the microscope, and the head is carefully aligned to make sure that the iris plane is perpendicular to the laser beam. After topical anesthesia (0.5% proparacaine or tetracaine), the eyelids and periocular skin are prepped with dilute povidone–iodine (Betadine) solution. A lid speculum is placed to provide adequate exposure of the globe. Careful centration with the eye aligned in the x-, y-, and z-planes is crucial.
It is important to perform the treatment as soon as possible after the proparacaine or tetracaine drops are instilled or have the patient close their eyes to prevent exposure keratitis from poor blinking. Drying of the inferior half of the cornea, as often occurs after anesthetic drops have been instilled, can lead to increased thinning inferiorly.
The epithelium is marked with a 7- or 8-mm optical zone marker centered on the pupil for myopia or a 9- or 10-mm marker for hyperopia or wavefront correction. It is helpful to remove a 1-mm larger area of epithelium than the planned ablation. Mechanical epithelial removal involves the use of a Tooke knife, disposable excimer spatula, or rotating brush. Alternatively, alcohol (18%–25% ethanol for 21–30 seconds) can be used to loosen the epithelium or excimer laser for partial or complete removal. The laser is typically set to a depth of approximately 45 µm, and the epithelium being ablated by the laser beam can be visualized under blue fluorescence. The ablation is stopped when a change from a fluorescent pattern to a dark pattern is seen, indicating that the epithelium has been ablated. If fluorescence persists across the whole area after a 50 µm ablation has been performed, an additional depth of 25 µm should be set for the laser. It may be helpful to scrape the remaining epithelium. It is important to remove the epithelium totally. Any residual epithelium will create an uneven ablation and irregular astigmatism. Also, epithelial removal should be quick to avoid corneal hydration changes.
Stromal Ablation
The ablation should promptly follow epithelial removal to prevent drying, which can lead to increased haze and scarring.
Centration is rechecked after epithelial removal. For astigmatic corrections, alignment on the proper axis should be verified by marking the patient’s limbus at the 12 and 6 o’clock positions with gentian violet dye on a Sinskey hook at the slit lamp before the procedure. The ultraviolet excimer lasers used have wavelengths outside the visible spectrum; an auxiliary aiming device that is coaxial to the ablating laser is required to make certain the laser is centered on the eye. Helium–neon lasers, laser diodes, or a coaxial aiming target are commonly used for this. In automated systems with eye trackers, this is used only for initial alignment, but in manually controlled systems, it is used to align the eye during the entire procedure.
The ablation is begun, centered over the pupil with the patient looking at the fixation light. Eye movements should be minimized during the ablation to reduce irregular surfaces. The use of an eye tracker is helpful in maintaining centration. It is important to make certain that the hydration status of the corneal stroma is uniform during the procedure. If excess fluid is detected, the procedure should be paused and the excess fluid removed by using a cellulose sponge to dry the cornea ( Fig. 3.3.2 ). Likewise, if the cornea becomes too dry, a cellulose sponge can be used to evenly hydrate it. Inadequate or excessive tissue hydration leads to more or less tissue ablation per pulse, resulting in an overcorrection or undercorrection, respectively.
