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Photorefractive Keratectomy (PRK) for Astigmatism Correction

Background

Because of the numerous refractive surgery terms that are used almost exclusively in the successful understanding of refractive disorders, first presenting a glossary of terms to assist the reader is essential.

Glossary of terms

Ablation – Removal. In excimer laser surgery, a frequency of energy causes corneal molecules to detach from one another from their points of attachment.

AK – Abbreviation for astigmatic keratotomy

ALK – Abbreviation for automated lamellar keratectomy

ArF – Abbreviation for argon fluoride

ASA – Abbreviation for advanced surface ablation. A collective term sometimes used to refer to the current techniques for surface treatments.

Astigmatic keratotomy – Surgical procedure in which microscopic incisions are positioned in the peripheral cornea to create a more spherical shape

Astigmatism – A refractive condition where the surface of the cornea is not spherical. A distorted image is formed because light images focus on 2 separate points in the eye.

Automated lamellar keratoplasty – An incisional refractive surgery method used in low-to-moderate myopia. An automated microkeratome placed on the eye removes, in an oscillating shaving motion, a thin layer of cornea that is microns thick. Subsequent removal of a thinner underlying corneal stroma is performed with stitchless replacement of the initial superficial cap.

Excimer – Abbreviation for excited dimer

LASEK – Abbreviation for laser epithelium keratomileusis

Laser – Abbreviation for light amplification by the stimulated emission of radiation

Laser in situ keratomileusis – A refractive laser procedure combining the use of ALK and photorefractive keratectomy in reshaping the central cornea to treat refractive errors. An automated microkeratome, similar to that used in ALK, is used to fashion a flap with a hinge. Subsequent ablation is performed on the corneal stromal bed with stitchless replacement of the corneal flap.

Laser epithelium keratomileusis – A modification of the epithelial debridement performed in photorefractive keratectomy. In this procedure, the epithelium is detached with the use of an alcohol solution (usually 20%) to lyse DNA bonds (cleave basement membrane) and to soften the epithelium to facilitate rolling over into a flap. Ablation is performed subsequently with replacement of the epithelial flap.

LASIK – Abbreviation for laser in situ keratomileusis

Light amplification by stimulated emission of radiation – Laser light is composed of one color (wavelength) traveling in one direction, and each light wave is traveling in step with the adjacent wave, making the laser light more powerful by a factor of millions.

Oblate – Shape of the cornea after conventional laser ablation profile where it is steeper in the periphery

PARK – Abbreviation for photoastigmatic refractive keratectomy

Pachymetry – Optical or ultrasonic procedure to measure the corneal thickness

Photoastigmatic refractive keratectomy – Although similar to photorefractive keratectomy, the ablation profile is specific for astigmatism.

Photorefractive keratectomy – Surgical procedure using an excimer laser to fashion the central cornea to treat refractive errors

Prolate – Normal corneal shape; steeper in the center

PRK – Abbreviation for photorefractive keratectomy

Topography – Used to measure the low or high areas of a plane

Wavefront – Describes the surface that connects all the points on a propagating light wave that are of equal phase

Wavelength – The distance between the top of one wave and the top of the next wave. In the case of an excimer laser, this is measured in nm (eg, argon fluoride has a wavelength of 193 nm).

Astigmatism and the advent of photoastigmatic refractive keratotomy

Astigmatism, a refractive condition where the surface of the cornea is not spherical, can decrease visual acuity by forming a distorted image because light images focus on 2 separate points in the eye. Clinicians and surgeons have searched constantly for the most successful device or procedure to treat this refractive error. Nonsurgical devices include spectacles and contact lenses. To date, these devices are being improved continuously to address the complex problem of astigmatism. Initial surgical approaches include astigmatic keratotomy, compression sutures, and wedge resection. Recent surgical procedures involve the use of the excimer laser in PARK and LASIK with or without wavefront-guided technology.

PRK is the application of ultraviolet high-energy photons (193-nm wavelength) of the ultraviolet range generated by an argon fluoride excimer laser to the anterior corneal stroma to change its curvature and, thus, to correct a refractive error.
The physical process of remodeling by PRK is called photoablation. This surgical procedure reshapes the central cornea to a flatter shape for people who are nearsighted and a more curved surface for people who are farsighted. Several techniques are being used to correct for astigmatism.

Device description

Two different methods of energy delivery are available by the excimer laser device, a large circular beam and a scanning slit or spot.

The earlier devices initially used large area ablation. To date, some manufacturers still use large area ablation in their modern devices. The circular laser beam passes through a diaphragm that slowly enlarges to deliver more cumulative energy in the center and less in the periphery.
Some laser-induced irregularities (central islands) have been reported in these large area systems. This method results in a shorter operating time to deliver the necessary laser pulses versus a system that uses a scanning slit system. The following manufacturers use circular beam lasers: Schwind (Coherent Medical Inc, Palo Alto, Calif), Summit (Waltham, Mass),
and VISX (Santa Clara, Calif).

The scanning slit or spot is an alternative method of energy delivery by the excimer laser. A smaller beam passes through a beam-shaping aperture, delivering a pattern of more pulses centrally than peripherally and resulting in greater corneal tissue ablation centrally. Less total energy is delivered at the corneal surface; therefore, a less powerful laser device may be used. In principle, this system is more effective in providing different ablation patterns in the treatment of astigmatism, irregular astigmatism, and hyperopia.

The use of scanning laser technology with its small moving beam has resulted in reduced thermal heating. This is visualized in a study that showed the different areas of plume production after each area of ablation following movement of the scanning beam. Central islands have not been reported in these systems. The smaller ablation size of the scanning laser consequently results in a longer operating time. Maintaining fixation has always been a problem for these scanning lasers, especially with the longer operating time, which results from more ablations by the smaller beams. Moreover, precise pulse-to-pulse registration of the scan is necessary to achieve a smooth and accurate final pattern. Automatic tracking devices are provided standard in these devices.

Manufacturers of scanning slit systems include the following: Autonomous (Orlando, Fla), LaserSite (Orlando, Fla), Meditec (Aesculap-Meditec, Heroldsberg, Germany)
, Nidek (Fremont, Calif)
, Novatec (Carlsbad, Calif), and Technolas (Chiron Vision Corp, Irvine, Calif). Novatec’s claim to fame is its use of solid-state laser crystals that obviates the need for argon fluoride gas to create its shorter ultraviolet beam. Presently, the Food and Drug Administration (FDA) has approved only the Summit and the VISX laser systems for commercial use within the United States. All the other systems currently are being used in other countries.

PRK variations

The current techniques for surface treatments (eg, PRK, PARK, LASEK, epithelial scrubber assisted PRK, epi-LASIK) are sometimes collectively termed advanced surface ablation (ASA).

LASEK

Surface ablation resurgence was primarily assisted by the introduction of LASEK. The authors believe that LASEK is an improvement of the existing PARK technique. LASEK uses a sterile diluted alcohol solution to separate a viable sheet of corneal epithelium, fashioning a flap that can be rolled back after excimer laser ablation. Several advantages have been identified upon performing this procedure, as follows:

No blade technique for refractive surgery

Corneal tissue sparing

Less postoperative pain as compared to PARK

Less postoperative corneal haze as compared to PARK

Epithelial scrubber assisted photorefractive keratotomy

The use of an Amoils epithelial scrubber allows fast and accurate epithelial removal, leaving a smoother anterior stromal surface.
The quick and efficient epithelial removal in less time decreases the potential for corneal dehydration to occur. It produces an easily seen bull’s eye that aids in centration of the laser application. Less epithelium is removed, resulting in faster healing time.

Epi-LASIK

The use of an epikeratome (plastic blade) mechanically sliding over the surface of the cornea, just underneath the epithelial layer of cells, while suction is applied, has resulted in another technique, which is basically automated LASEK without alcohol.

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