Photorefractive keratectomy
Encyclopedia
Photorefractive keratectomy (PRK) and Laser-Assisted Sub-Epithelial Keratectomy (or Laser Epithelial Keratomileusis) (LASEK) are laser
eye surgery
procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University Medical Center in Berlin, Germany. The first LASEK procedure was performed at Massachusetts Eye and Ear Infirmary
in 1996 by ophthalmologist, refractive surgeon, Dimitri Azar. The procedure was later popularized by Camellin, who coined the term LASEK for laser epithelial keratomileusis. 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
. The outer layer of the cornea is removed prior to the ablation. 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. 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. The deeper layers of the cornea, as opposed to the outer epithelium, are laid down early in life and have very limited regenerative capacity. The deeper layers, if reshaped by a laser or cut by a microtome
, will remain that way permanently with only limited healing or remodelling. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate after the surgery. 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.
procedure involves a mechanical microtome using a metal blade or a femtosecond laser microtome 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 micrometre
s thick. 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
, which is referred as M-LASEK, can minimize the risk of post-operative haze in persons requiring larger PRK corrections.
PRK does not involve a knife, microtome, or cutting laser as used in LASIK, but there may be more pain and slower visual recovery. Unlike LASIK, PRK does not create the risk of dislocated corneal flaps which may occur (especially with trauma), at any time after LASIK.
Uses Amoil Brush and gas cooling to reduce the pain
Uses mitomycin to reduce the haze
There are also some pre-existing conditions that may complicate or preclude the treatment.
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. Recurrent erosions occur while sleeping when the dry corneal epithelium becomes adherent to the upper eyelid, causing pain during REM sleep.
PRK may be rarely performed on one eye at a time to assess the results of the procedure and ensure adequate vision during the healing process. 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. 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. A dilute concentration of the chemotherapeutic agent, Mitomycin-C, may be applied briefly at the end of the procedure to reduce the incidence of haze formation.
Predictability of the resulting refractive correction after healing is not totally exact, particularly for those with more severe myopia. This can lead to under/over-correction of the refractive error. In the case of the over-correction, premature presbyopia
is a possibility. Experienced surgeons employ a custom-profile algorithm to further enhance predictability in their results.
In 1 to 3% of cases, loss of best corrected visual acuity (BCVA) can result, due to decentered ablative zones or other surgical complications. PRK results in improved BCVA about twice as often as it causes loss. Decentration is becoming less and less of a problem with more modern lasers using sophisticated eye centering and tracking methods.
Some PRK patients have reported dissatisfaction with their vision under low ambient lighting (dusk/nighttime) conditions. Pilots who experience postoperative vision problems could be further compromised by the variations in lighting common to the aviation environment. 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. 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. Also, civilians who wish to fly military aircraft should know that there are restrictions on those who have had corrective surgery.
The Army now permits flight applicants who have undergone PRK or LASIK, though it still requires a standard waiver.
The Navy and Marines will routinely grant a waiver for pilots or student naval aviators to fly after PRK, assuming preoperative standards are met, no complications in the healing process were encountered and passing their standard vision tests. LASIK is currently under study for the Navy, however only current Naval Aviators can be admitted into the study. In one study, 967 of 968 naval aviators having PRK returned to duty involving flying after the procedure. In fact, the U.S. Navy now offers free PRK surgery at the National Naval Medical Center to Naval Academy Midshipmen who intend to pursue career paths requiring good uncorrected vision, including flight school and special forces training.
The U.S. Air Force approves the use of PRK and LASIK. Since 2000 the USAF has conducted PRK for aviators at the Wilford Hall Medical Center. More airmen were allowed over the years and in 2004 the USAF approved Lasik for aviators, with limits on the type of aircraft they could fly. Then in 2007 those limits were lifted. Most recently in 2011 the USAF expanded the program, making it easier for more airmen to qualify for the surgery. Current airmen (Active Duty and Air Reserve Components who are eligible) are authorized surgery at any DOD Refractive Surgery Center. Those airmen not eligible, are still able to get the surgery done at their own expense by a civilian surgeon, but must first be approved (Approval is based on the same USAF-RS program). Others that do not fall into those categories (i.e. applicants who are seeking a pilot slot) can still elect to have the surgery done, but must follow the criteria in accordance with the USAF Waiver Guide. Those applicants will be evaluated at the ACS during their Medical Flight Screening appointment to determine if they meet waiver criteria. Applicants are strongly encouraged to read the references contained within this article as information obtained from sources other than those listed could lead to disqualification for a pilot candidate.
In the majority of patients, PRK has proven to be a safe and effective procedure for the correction of myopia. PRK is still evolving with other countries currently using refined techniques and alternative procedures. Many of these procedures are under investigation in the U.S. 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.
(USASOC) schools (HALO, SCUBA, SERE). Army Regulation 40-501
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
eye surgery
Eye surgery
Eye surgery, also known as orogolomistician surgery or ocular surgery, is surgery performed on the eye or its adnexa, typically by an ophthalmologist.-Preparation and precautions:...
procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University Medical Center in Berlin, Germany. The first LASEK procedure was performed at Massachusetts Eye and Ear Infirmary
Massachusetts Eye and Ear Infirmary
Massachusetts Eye and Ear Infirmary, known locally as Mass. Eye and Ear, is a specialty hospital providing patient care for disorders of the eye, ear, nose, throat, head and neck. Founded in 1824, MEEI is an international leader in Ophthalmology and Otolaryngology research and a teaching partner of...
in 1996 by ophthalmologist, refractive surgeon, Dimitri Azar. The procedure was later popularized by Camellin, who coined the term LASEK for laser epithelial keratomileusis. LASEK and PRK permanently change the shape of the anterior central cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
using an excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
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
Corneal epithelium
The corneal epithelium is made up of epithelial tissue and covers the front of the cornea. It acts as a barrier to protect the cornea, resisting the free flow of fluids from the tears, and prevents bacteria from entering the epithelium and corneal stroma.The corneal epithelium consists of several...
. The outer layer of the cornea is removed prior to the ablation. 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. 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. The deeper layers of the cornea, as opposed to the outer epithelium, are laid down early in life and have very limited regenerative capacity. The deeper layers, if reshaped by a laser or cut by a microtome
Microtome
A microtome is a sectioning instrument that allows for the cutting of extremely thin slices of material, known as sections. Microtomes are an important device in microscopy preparation, allowing for the preparation of samples for observation under transmitted light or electron radiation...
, will remain that way permanently with only limited healing or remodelling. With PRK, the corneal epithelium is removed and discarded, allowing the cells to regenerate after the surgery. The procedure is distinct from LASIK
LASIK
LASIK or Lasik , commonly referred to simply as laser eye surgery, is a type of refractive surgery for correcting myopia, hyperopia, and astigmatism...
(Laser-Assisted in-Situ Keratomileusis), a form of laser eye surgery where a permanent flap is created in the deeper layers of the cornea.
PRK versus LASIK
Because PRK does not create a permanent flap in the deeper corneal layers (the LASIKLASIK
LASIK or Lasik , commonly referred to simply as laser eye surgery, is a type of refractive surgery for correcting myopia, hyperopia, and astigmatism...
procedure involves a mechanical microtome using a metal blade or a femtosecond laser microtome 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 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s thick. 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
Mitomycin
The mitomycins are a family of aziridine-containing natural products isolated from Streptomyces caespitosus or Streptomyces lavendulae. One of these compounds, mitomycin C, finds use as a chemotherapeutic agent by virtue of its antitumour antibiotic activity. It is given intravenously to treat...
, which is referred as M-LASEK, can minimize the risk of post-operative haze in persons requiring larger PRK corrections.
PRK does not involve a knife, microtome, or cutting laser as used in LASIK, but there may be more pain and slower visual recovery. Unlike LASIK, PRK does not create the risk of dislocated corneal flaps which may occur (especially with trauma), at any time after LASIK.
PRK versus LASEK
Although PRK and LASEK basically use the same technique, there are minor differences between them. In PRK, epithelium is removed and the outermost layer below the epithelium is treated with laser. In LASEK, epithelium is not removed, but an alcoholic solution is used to cause the epithelial cells to weaken; the surgeon will fold the epithelial layer out of the laser treatment field, and fold it back in its original place after cornea has been reshaped by laser. If the epithelial flap is not strong enough to be laid back in its original place, it will be removed, and the LASEK procedure becomes a PRK procedure.Types of LASEK
- LASEK
- ASA (Advanced Surface Ablation) LASEK
Uses Amoil Brush and gas cooling to reduce the pain
- M-LASEK
Uses mitomycin to reduce the haze
PRK eligibility
Up to 80% of the myopic population may physically qualify as potential PRK candidates. There are a number of basic criteria which a potential candidate should satisfy:- Normal ocular health
- Age 18 years or older
- Stable refractionRefractionRefraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
error (no noticeable change in the last year) correctable to 20/40 or better - Between -1.00 to -12.00 diopters of Myopia
- Not pregnant at the time of surgery
- Realistic expectations of the final results (with a complete understanding of the benefits, as well as the possible risks)
- Pupil size 6 mm or less in a dark room is ideal (but some newer lasers may be acceptable for larger pupils)
- Assessment of allergies, (e.g., pollen) where allergy may complicate the eyelid margins following surgery leading to dry eye.
There are also some pre-existing conditions that may complicate or preclude the treatment.
- CollagenCollagenCollagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
vascular diseaseVascular diseaseVascular disease is a form of cardiovascular disease primarily affecting the blood vessels.Some conditions, such as angina and myocardial ischemia, can be considered both vascular diseases and heart diseases .Cigarette smoking is the major risk factor....
(e.g., corneal ulceration or melting) - Ocular disease (e.g., dry eye, keratoconusKeratoconusKeratoconus , is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve....
, glaucomaGlaucomaGlaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...
) - Systemic disorders (e.g., diabetes, rheumatoid arthritisRheumatoid arthritisRheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...
) - History of side effects from steroidSteroidA steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
s - Granular corneal dystrophy type IIGranular corneal dystrophy type IIGranular corneal dystrophy type II , also called Avellino corneal dystrophy or combined granular-lattice corneal dystrophy, is a rare form of human corneal dystrophy. It is caused by mutations in the TGFBI gene encoding the protein keratoepithelin, and is inherited in an autosomal dominant...
Possible complications
Some complications that can be temporary or permanent include:- Dry eyes
- Recurrent erosions during sleep
- Long healing period
- Pain
- Glare, halos, or starburst Aberrations
- Increased ocular straylightOcular straylightOcular straylight is the unwanted effect of light spreading as seen by human eyes, and caused by the eye itself, not by external factors. It is most easily visualized as the radiation seen spreading from a bright light source against a dark background. The typical practical example is an opposing...
- Under- or over-correction
- Recurrence of myopia
- Corneal haze
- Scarring
- Reduced best corrected visual acuity
- Reduced acuity in low light
- Increased sensitivity
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. Recurrent erosions occur while sleeping when the dry corneal epithelium becomes adherent to the upper eyelid, causing pain during REM sleep.
PRK may be rarely performed on one eye at a time to assess the results of the procedure and ensure adequate vision during the healing process. 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. 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. A dilute concentration of the chemotherapeutic agent, Mitomycin-C, may be applied briefly at the end of the procedure to reduce the incidence of haze formation.
Predictability of the resulting refractive correction after healing is not totally exact, particularly for those with more severe myopia. This can lead to under/over-correction of the refractive error. In the case of the over-correction, premature presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...
is a possibility. Experienced surgeons employ a custom-profile algorithm to further enhance predictability in their results.
In 1 to 3% of cases, loss of best corrected visual acuity (BCVA) can result, due to decentered ablative zones or other surgical complications. PRK results in improved BCVA about twice as often as it causes loss. Decentration is becoming less and less of a problem with more modern lasers using sophisticated eye centering and tracking methods.
Aviator usage
Operation of an aircraft is a visually demanding activity performed in an environment that is not always user friendly. Currently, over 50% of the civilian pilot population uses some form of visual correction. 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. Pilots who experience postoperative vision problems could be further compromised by the variations in lighting common to the aviation environment. In addition, exposure to intense UV radiation has been associated with late-onset corneal haze and recurrence of myopia.
The US Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...
will consider applicants with PRK once they are fully healed and stabilized, provided there are no complications and all other visual standards are met. 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. Also, civilians who wish to fly military aircraft should know that there are restrictions on those who have had corrective surgery.
The Army now permits flight applicants who have undergone PRK or LASIK, though it still requires a standard waiver.
The Navy and Marines will routinely grant a waiver for pilots or student naval aviators to fly after PRK, assuming preoperative standards are met, no complications in the healing process were encountered and passing their standard vision tests. LASIK is currently under study for the Navy, however only current Naval Aviators can be admitted into the study. In one study, 967 of 968 naval aviators having PRK returned to duty involving flying after the procedure. In fact, the U.S. Navy now offers free PRK surgery at the National Naval Medical Center to Naval Academy Midshipmen who intend to pursue career paths requiring good uncorrected vision, including flight school and special forces training.
The U.S. Air Force approves the use of PRK and LASIK. Since 2000 the USAF has conducted PRK for aviators at the Wilford Hall Medical Center. More airmen were allowed over the years and in 2004 the USAF approved Lasik for aviators, with limits on the type of aircraft they could fly. Then in 2007 those limits were lifted. Most recently in 2011 the USAF expanded the program, making it easier for more airmen to qualify for the surgery. Current airmen (Active Duty and Air Reserve Components who are eligible) are authorized surgery at any DOD Refractive Surgery Center. Those airmen not eligible, are still able to get the surgery done at their own expense by a civilian surgeon, but must first be approved (Approval is based on the same USAF-RS program). Others that do not fall into those categories (i.e. applicants who are seeking a pilot slot) can still elect to have the surgery done, but must follow the criteria in accordance with the USAF Waiver Guide. Those applicants will be evaluated at the ACS during their Medical Flight Screening appointment to determine if they meet waiver criteria. Applicants are strongly encouraged to read the references contained within this article as information obtained from sources other than those listed could lead to disqualification for a pilot candidate.
In the majority of patients, PRK has proven to be a safe and effective procedure for the correction of myopia. PRK is still evolving with other countries currently using refined techniques and alternative procedures. Many of these procedures are under investigation in the U.S. 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.
US Army Special Operations
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. PRK and LASIK are both waived for Airborne, Air Assault and Ranger schools. However, those who have had LASIK must enroll in an observational study, if a slot is available, to undergo training in Special Forces qualification. LASIK is disqualifying/non-waiverable for several United States Army Special Operations CommandUnited States Army Special Operations Command
The United States Army Special Operations Command is the command charged with overseeing the various Special Operations Forces...
(USASOC) schools (HALO, SCUBA, SERE). Army Regulation 40-501
See also
- Automated lamellar keratoplastyAutomated lamellar keratoplastyAutomated Lamellar Keratoplasty, commonly abbreviated to ALK uses a device called a microkeratome to separate a thin layer of the cornea and create a flap. The flap is then folded back, and the microkeratome removes a thin disc of corneal stroma below. The thickness and diameter of this disc...
- Phototherapeutic keratectomyPhototherapeutic keratectomyPhototherapeutic keratectomy is a type of eye surgery that uses a laser to treat various ocular disorders by removing tissue from the cornea. PTK allows the removal of superficial corneal opacities and surface irregularities...
- Femtosecond laser intrastromal vision correction
External links
- LASIK, Epi-LASIK & LASEK: Eye Digest Reviews(University of Illinois)
- DA Medical Waiver Memorandum