Photorefractive keratectomy
In the case of the over-correction, premature presbyopia is a possibility. In 1 to 3% of cases, loss of best corrected visual acuity (BCVA) can result, due to decentered ablative zones or other surgical complications. It also treats the surface of the cornea but unlike other techniques, requires no assistance from manual surgical instruments.The deeper layers of the cornea, as opposed to the outer epithelium, are laid down early in life and have very limited regenerative capacity. Unlike LASIK, PRK does not create the risk of dislocated corneal flaps which may occur (especially with trauma), at any time after LASIK. An evolved form of PRK is called No Touch laser vision correction.
The procedure is distinct from LASIK (Laser-Assisted in-Situ Keratomileusis), a form of laser eye surgery where a permanent flap is created in the deeper layers of the cornea. Because PRK does not create a permanent flap in the deeper corneal layers (the LASIK procedure involves a mechanical microkeratome using a metal blade or a femtosecond laser microkeratome to create a flap out of the outer cornea), the cornea s structural integrity is less altered by PRK. The LASIK process covers the laser treated area with the flap of tissue which is from 100 to 180 micrometres thick. The outer layer of the cornea is removed prior to the ablation.
However, those who have had LASIK must enroll in an observational study, if a slot is available, to undergo training in Special Forces qualification. The deeper layers, if reshaped by a laser or cut by a microkeratome, will remain that way permanently with only limited healing or remodelling.
Activities requiring good binocular vision may have to be suspended between surgeries and during the sometimes extended healing periods. A few post-PRK patients have complained of glare, halos, and starburst aberrations, which may be the result of postoperative corneal haze that may develop during the healing process. Decentration is becoming less and less of a problem with more modern lasers using sophisticated eye centering and tracking methods. Operation of an aircraft is a visually demanding activity performed in an environment that is not always user friendly.
PRK results in improved BCVA about twice as often as it causes loss. PRK and LASIK are both waived for Airborne, Air Assault and Ranger schools.
Given that PRK is not reversible, a patient considering PRK is recommended to contact an eye-care practitioner for assistance in making an informed decision concerning the potential benefits and liabilities that may be specific to him or her. In the USA candidates who have had PRK can get a blanket waiver for the Special Forces Qualification, Combat Diving Qualification and Military Free Fall courses. LASIK is disqualifying/non-waiverable for several United States Army Special Operations Command (USASOC) schools (HALO, SCUBA, SERE).
PRK is still evolving with other countries currently using refined techniques and alternative procedures. In addition, exposure to intense UV radiation has been associated with late-onset corneal haze and recurrence of myopia. The US Federal Aviation Administration will consider applicants with PRK once they are fully healed and stabilized, provided there are no complications and all other visual standards are met.
. In the majority of patients, PRK has proven to be a safe and effective procedure for the correction of myopia. Most modern lasers will automatically center on the patient s visual axis and will pause if the eye moves out of range and then resume ablating at that point after the patient s eye is re-centered. The outer layer of the cornea, or epithelium, is a soft, rapidly regrowing layer in contact with the tear film that can completely replace itself from limbal stem cells within a few days with no loss of clarity.
There are a number of basic criteria which a potential candidate should satisfy: There are also some pre-existing conditions that may complicate or preclude the treatment. Some complications that can be temporary or permanent include: As with other forms of refractive surgery, Lasik, Epi-Lasik, Lasek and so forth, dry eyes is the most common complication of PRK surgery and can be permanent. PRK may be performed on one eye at a time to assess the results of the procedure and ensure adequate vision during the healing process. Photorefractive keratectomy (PRK) and Laser-Assisted Sub-Epithelial Keratectomy (or Laser Epithelial Keratomileusis The procedure was later popularized by Camellin, who coined the term LASEK for laser epithelial keratomileusis.
A computer system tracks the patient s eye position 60 to 4,000 times per second, depending on the brand of laser used, redirecting laser pulses for precise placement. This can lead to under/over-correction of the refractive error.
Army Regulation 40-501 . Using modern lasers as of the year 2005, this is quite rare after 6 months but reportedly, symptoms have occasionally lingered longer than a year in some cases. Predictability of the resulting refractive correction after healing is not totally exact, particularly for those with more severe myopia.
Also, civilians who wish to fly military aircraft should know that there are restrictions on those who have had corrective surgery. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate after the surgery.
Many of these procedures are under investigation in the U.S. Pilots who experience postoperative vision problems could be further compromised by the variations in lighting common to the aviation environment.
The Army now permits flight applicants who have undergone PRK or LASIK, though it still requires a standard waiver. Currently, over 50% of the civilian pilot population uses some form of visual correction.
This flap can mute the nuances of the laser ablation, whereas PRK performs the laser ablation at the outer surface of the cornea. The use of the anti-metabolite mitomycin can minimize the risk of post-operative haze in persons requiring larger PRK corrections. PRK does not involve a knife, microkeratome, or cutting laser as used in LASIK, but there may be more pain and slower visual recovery.
Pilots should be aware, however, that potential employers, such as commercial airlines and private companies, may have policies that consider refractive surgery a disqualifying condition. Aviators considering PRK should know that clinical trials claiming success rates of 90% or higher are based on criteria of patients post-operative refractive errors of 20/40 or better, not 20/20 or better, uncorrected visual acuity. Some PRK patients have reported dissatisfaction with their vision under low ambient lighting (dusk/nighttime) conditions.
LASEK and PRK permanently change the shape of the anterior central cornea using an excimer laser to ablate (remove by vaporization) a small amount of tissue from the corneal stroma at the front of the eye, just under the corneal epithelium. It is the only technique to use exclusively an excimer laser from start to finish. It is estimated that up to 80% of the myopic population may physically qualify as potential PRK candidates.
