Photorefractive keratectomy ( PRK ) and laser assisted sub-epithelial keratectomy (or keratomileusis laser epitel ) ( LASEK ) is an eye laser surgery procedure aimed at improving one's eyesight, reducing dependence on glasses or contact lenses. LASEK and PRK permanently alter the shape of the anterior central cornea using an excimer laser to blur (erase by evaporation) a small amount of tissue from the corneal stroma on the front of the eye, just below the corneal epithelium. The outer layer of the cornea is removed before ablation.
The computer system tracks the patient's eye position 60 to 4,000 times per second, depending on the laser specifications used. The computer system redirects the laser pulses for proper laser placement. Most modern lasers will automatically center on the patient's visual axis and will stop if the eye moves out of range and then continue ablative at that point after the patient's eyes are centered again.
The outer layer of the cornea, or epithelium, is the soft layer, which grows back rapidly and comes into contact with the teardrop that can completely replace itself from the limbal stem cell within days without losing clarity. The deeper layers of the cornea, compared to the outer epithelium, are placed early in life and have very limited regenerative capacity. A deeper layer, if re-shaped by laser or cut by microtom, will remain as permanently with only limited healing or remodeling.
With PRK, the corneal epithelium will be removed and discarded, allowing cells to regenerate after surgery. This procedure is different from LASIK (in situ keratomileusis laser aid), a form of laser eye surgery in which a permanent flap is made in the deeper layers of the cornea.
Video Photorefractive keratectomy
LASEK
LASEK and PRK are two different procedures. While both procedures interact with the epithelium above the cornea, the PRK procedure completely eliminates this, while LASEK bruses the material for the procedure, before being placed back for healing after laser surgery. This procedure can be used to treat astigmatism, farsightedness, and farsightedness. During the procedure, the epithelium is shifted using a dilute alcohol solution.
LASEK has advantages over LASIK because it avoids the additional complications associated with flaps created during operation. This procedure can also reduce changes in dry eye symptoms after surgery. Because the LASEK procedure does not require surgical flap, athletes or individuals concerned with the trauma introduced by the flap may see benefits for LASEK. Patients who wear contact lenses should usually stop wearing this for the prescribed time before the procedure.
LASEK deficiencies include longer recovery time for different vision with LASIK. Postoperative patients are requested to wear "eye-mask" contact lenses over the eyes, which are not necessary for post-surgical LASIK. Another disadvantage is that patients may need to apply steroid eye drops for several weeks longer than LASIK procedures. Vision after LASEK procedure has a longer recovery than LASIK may be between five days and two weeks for blurred vision to properly clean.
When LASEK is compared to LASIK, LASIK can have better results with corneal haze while LASEK has lower rates of flap complications than LASIK.
Maps Photorefractive keratectomy
Feasibility
There are a number of basic criteria that must be fulfilled by a person:
- Normal ocular health
- Age 18 and above
- Stable refractive errors (no real changes in the past year) that can be fixed to 20/40 or better
- Between -1.00 to -12.00 Miopia diopters
- Not pregnant at the time of operation
- A realistic expectation of the final result (with a complete understanding of the benefits, as well as possible risks)
- The size of pupils 6 mm or less in a dark room is ideal (but some newer lasers may be acceptable for larger students)
- Assessment of allergies, (eg, pollen) where allergies can complicate the eyelid margin after surgery that leads to dry eye.
There are also some pre-existing conditions that may complicate or preclude treatment.
- Vascular collagen disease (eg, corneal ulceration or melting)
- Eye diseases (eg, dry eyes, keratoconus, glaucoma)
- Systemic disorders (eg, diabetes, rheumatoid arthritis)
- History of side effects of steroids
- Dystrophy of type II granular cornea
Possible complications
Some complications that may be temporary or permanent include:
- Dry eye
- Repeated Erosion during sleep
- Long healing period
- Pain
- Glare, halos, or starburst deviation
- Straylight enhancement of the eye
- Not good or too much corrected
- Myopia recurrence
- Corneal fog
- Scars
- Reduce the best correction visual acuity
- Reduces sharpness in dim light
- Increased sensitivity
Dry eyes
As with other forms of refractive surgery, keratoconjunctivitis sicca, colloquially referred to as 'dry eye,' is the most common CRP complication, and can be permanent. In the latter case, recurrent erosion occurs during sleep from adherence to the corneal epithelium to the upper eyelid with rapid eye movement. Adjuvant polyunsaturated fatty acids (PUFAs) with high Omega-3 levels before and after surgery improve sicca, possibly because of their anti-inflammatory effects. Foods containing PUFA include hemp and fish oil. The PRK brush to reject epithelials, not alcohol-based techniques, also results in lower quantitative ocular droughts after surgery. The number of corneal hoarses after surgery also decreased with brush technique. LAU-0901 platelet activation factor has shown an effect in reducing dry eye on mouse models. The rabbit model also showed improvement by topical nerve growth factor (NGF) in combination with docosahexaenoic acid (DHA). Mitomycin C worsens dry eyes after surgery.
CRP can be performed on one eye at a time to assess the outcome of the procedure and ensure adequate vision during the healing process. Activities that require good binocular vision may have to be suspended between surgery and during periods of occasional healing.
Halos, starbust, and refractive errors
PRK may be associated with glare, halos, and starburst deviations, which can occur with postoperative corneal fog during the healing process. Halo nights are seen more often in revisions with small ablation zones. With the latest developments in laser technology, this is less common after 6 months although symptoms can persist for more than a year in some cases. An aqueous concentration of a chemotherapy agent, Mitomycin-C, may be applied briefly to the completion of the operation to reduce the risk of hazing, albeit with an increased risk of sicca.
The predictability of refractive correction produced after healing is not entirely appropriate, especially for those with more severe myopia. This can lead to an error correction/excess correction. In the case of overcorrection, premature presbyopia is a possibility. Experienced surgeons use a special-profile algorithm to further increase the predictability in their results.
In 1 to 3% of cases, the best corrected visual acuity loss (BCVA) can occur, due to a decentralized ablation zone or other surgical complications. PRK results in an increase in BCVA approximately twice as often as it causes loss. Decentration is becoming increasingly no problem with more modern lasers using sophisticated eye-centering and tracking methods.
Comparison with LASIK
A systematic review comparing PRK and LASIK concludes that LASIK has a shorter recovery time and less pain. Two techniques after a period of one year have similar results.
A systematic review of 2016 found that it was unclear whether there were differences in efficacy, accuracy, and adverse effects when comparing CRP and LASEK procedures among people with low to moderate myopia. The review stated that no trial was conducted comparing two procedures in people with high myopia.
A systematic review of 2017 uncovered uncertainty in visual acuity, but found that in one study, those who received a PRK were less likely to achieve biased errors, and tended to experience less correction than compared to LASIK.
Type
- LASEK
- Alcohol helps PRK
- Transepithelial PRK (TransPRK)
- ASA (Advanced Surface Ablation) LASEK
Using Amoils Brush and gas cooling to reduce pain
- M-LASEK
Using mitomycin in an effort to reduce postoperative fog but its effectiveness is dubious. The possibility of long-term side effects is unknown.
Pilot
U.S. Federal Aviation Administration will consider applicants with the CRP once fully healed and stabilized, provided that no complications and all other visual standards are met. However, pilots should be aware that prospective employers, such as commercial airlines and private companies, may have policies that regard refraction operations as disqualifying conditions. Also, civilians who want to fly military aircraft must know that there are restrictions on those who have undergone corrective surgery. The Army now allows flight applicants who have undergone a PRK or LASIK. Corneal refraction surgery that is not complicated and successful does not require release and recorded as information only.
Navy and Marine regularly provide exemption for student pilots or naval aviators, as well as naval aviation officers, UAS operators and aircrew, to fly after PRK and LASIK, assuming preoperative bias standards are met, no complications in the healing process encountered. , without symptoms associated with significant halo, glare or dry eye, of all drugs, and by passing their standard vision tests. In one study, 967 of the 968 naval aviators whose PRK returned to tasks involving flying after the procedure. In fact, the US Navy now offers free CRC and LASIK operations at the National Naval Medical Center for the Naval Academy Midshipmen who intend to pursue career paths that require unconfirmed vision, including aviation schools and special operations training.
The US Air Force approved the use of PRK and LASIK. Since 2000, USAF has conducted a CRP for aviators at Wilford Hall Medical Center. More aviators were allowed over the years and in 2004, the USAF approved Lasik for pilots, with restrictions on the type of aircraft they could fly. Then in 2007 the limits were revoked. Recently in 2011, USAF expanded the program, making it easier for more pilots to qualify for operations. The current pilot (Compatible Active Air Components and Air Forms) is the official surgery at the DOD Refractive Surgery Center. Those pilots are not eligible, can still get the operation done at the expense of themselves by a civilian surgeon, but must first be approved (Approval is based on the same USAF-RS program). Others not included in the category (ie applicants seeking a pilot slot) may still choose to perform the operation, but must follow the criteria in accordance with the USAF Liberation Guidelines. The applicants will be evaluated in the ACS during their Medical Aviation Inspection appointment to determine whether they meet the waiver criteria.
In most patients, CRP has proven to be a safe and effective procedure for myopia correction. PRK is still evolving with other countries currently using refined techniques and alternative procedures. Many of these procedures are under investigation in the US Given that the CRP is not reversible, a patient considering a PRK is encouraged to contact an eye care practitioner for assistance in making informed decisions about potential benefits and obligations that may be specific to him or her.
Military
In the US, candidates who have had a CRP may obtain a blank exemption for the Special Forces Qualification, Diving Qualification, and the Military Free Fall Training. PRK and LASIK were both freed for Airborne, Air Assault and Ranger schools. However, those who already have LASIK must enroll in observational studies, if slots are available, to undergo training in a Special Forces qualification. LASIK disqualify/can not be eliminated for some schools of US Army Special Operations Command (USASOC) (HALO, SCUBA, SERE) per Army Regulation 40-501.
History
The first CRP procedure was conducted in 1987 by Drs. Theo Seiler, then at the Free University Medical Center in Berlin, Germany. The first procedure similar to LASEK was performed at the Massachusetts Eye and Ear Infirmary in 1996 by an ophthalmologist, refractive surgeon Dimitri Azar. Dr Massimo Camellin, an Italian surgeon, was the first to write a scientific publication on a new surgical technique in 1998, combining the LASEK term for keratomileusis laser epithelium.
References
External links
- CRP operation (with corneal scars) at St. Lukes International Eye Institute - Quezon City, Philippines
Source of the article : Wikipedia
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