Photorefractive keratectomy (PRK) laser reshaping of the cornea to correct vision defects

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Photorefractive Keratectomy (PRK)

Photorefractive Keratectomy (PRK) Introduction

Photorefractive keratectomy is also often known as its acronym -- PRK. This is a surgical procedure that uses an excimer laser to alter the shape of the cornea and consequently correct vision. It is the operation of choice for those who are looking to throw away their contact lenses or spectacles forever.

This surgery would not be possible if it were not for the discovery of the excimer laser. The excimer laser is the tool that has made the surgical correction of myopia more accessible to the general public for the last fifteen years. Nowadays these excimer lasers are even more advanced then they were when they were first employed in 1975 at Kansas State University.

The procedure has been shown to effectively correct myopia in the regions of minus 1.5 to minus 8.0 dioptors. Any further increase has sometimes resulted in an effect called corneal haze.

There are other things that this procedure can be used for including laser scar removal. This procedure can also substitute for penetrating keratoplasty in patients with anterior corneal scars who are not able to see using contact lenses or glasses. PRK is also treats the myopia that sometimes results after a radial keratotomy.

Excimer lasers burn off the top layer of cells. Exact amounts are removed using ultraviolet radiontion at a wavelength of 193 nm. This wavelength allows the surgeon to remove precise amounts of tissue. This process is known as ablative photodecomposition. The laser that is the most commonly used is called the 193 nm laser.

Protocol of Laser Ablation Procedures

No practicitoner can use an excimer laser without following the rules that The Food and Drug Administration has developed to guide the use of the apparatus during the ablative photodecomposition process.

An inbuilt computer that helps guide the practitioner to use the excimer laser system properly. Excimer laser systems also have a built in microscope to help make sure the beam is aligned properly with the axises of the eye. Some models also have laser tracking mechanisms that can help ensure accuracy.

Some excimer laser systems also come with an apparatus that helps remove residues from the ablation process as well as a foot pedal and fingertip regulation so surgeons can manage the ray manually.

Pre-Operative Safety Measures

To ensure that this operation is as safe as possible several safety measures are employed. First of the entire practitioner warns and counsels the patients regarding all of the risks involved. This is accompanied by pres surgical screenings that also include visual tests for cycloplegic refractions, far and near vision and ocular dominance. A computerized topographical analysis is sometimes performed to detect keratoconus or other diseases of the cornea. The anterior and posterior segments of the eyes are examined to make sure that there are not any secondary causes that could be causing vision problems and that could potentially undermine the success of the procedure.

Contraindications

This type of eye surgery is not for everyone. People with diabetes, collagen vascular disease, a history of herpes infections (either simplex or zoster), dry eye syndrome, uncorrected lepharitis, keratitis and neurotrophic cornea are not suitable candidates for this type of surgery.

Certain medications like Accutane, cordarone or Imitrex can also affect the outcome of this surgical procedure.

Surgical technique

There are various ways that surgeons perform PRK and no one way is correct as the procedure is formatted to suit the needs of the individual patient.

The environment in the surgical room is of some importance, especially control of the humidity and temperature. It is crucial for the cornea to stay hydrated during the surgery.

Safety glasses need to be worn to shield eyes from the wavelength of the 193 nm excimer laser. It is also important that the operating room be well ventilated to minimize the toxic effects of the fluorine gas that is inside the laser and that could also possibly leak from the laser.

To make sure that the laser is always operating safely and that its effects are minimal on the person being operated on the laser operator will also constantly be checking the duration of the laser pulses, the wavelength of the laser and the exposure time of the laser light.

At this point in the history of PRK, the two most used surgical techniques are laser radial Keratectomy and Excimer Laser Keratomileusis.

Laser Radial Keratectomy

In this procedure the excimer laser is used to make incisions in the cornea. It is considered to be the most precise of the two operations.

Excimer Laser Keratomileusis

In this procedure the ablation takes place over a larger surface of the eye, which allows it to be resculpted. This is highly accurate procedure ensures a fast recovery and does not trigger hyperplasia.

Post Surgical Care

After the PRK surgery is complete a soft contact lens is prescribed to be worn as a bandage until the epithelium heals. Pressure patching may also be employed in lieu of the bandage contact lense if necessary.

Non-steroidal drops might be prescribed for pain and antibiotic medications might be prescribed for infections. Steroidal drops are prescribed for extreme inflammation and pain.

Complications

Complications include the undercorrection or overcorrection of vision, scarring, unexplained pain, corneal islands, infectious keratitis, decentration, visual aberrations, recurrent erosion of the epithelial cells and endothelial cell loss.