Photorefractive keratectomy

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Photorefractive keratectomy
US Navy 100217-N-7032B-023 Capt. David J. Tanzer, specialty leader of Navy Refractive Surgery and director of Refractive Surgery Program at Naval Medical Center San Diego, performs a photorefractive keratectomy (PRK) at Naval T.jpg
Photorefractive keratectomy at U.S. Naval Medical Center San Diego.
ICD-9-CM 11

Photorefractive keratectomy (PRK) and laser-assisted sub-epithelial keratectomy (or laser epithelial keratomileusis [1] ) (LASEK) are laser eye surgery procedures intended to correct a person's vision, reducing dependency on glasses or contact lenses. 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.

Contents

A computer system tracks the patient's eye position 60 to 4,000 times per second, depending on the specifications of the laser that is used. The computer system redirects laser pulses for precise laser 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.

LASEK

Preliminary investigation of the corneal topography. The test maps a patient's cornea for raised areas and surface inconsistencies. Augenlasern-Voruntersuchung-der-Hornhaut-Topographie-vor-LASIK-OP.jpg
Preliminary investigation of the corneal topography. The test maps a patient's cornea for raised areas and surface inconsistencies.

LASEK and PRK are two different procedures. While both procedures interact with the epithelium atop the cornea, the PRK procedure removes this entirely, while LASEK brushes the material away for the procedure, before being placed back for healing after laser surgery. [2] The procedure can be used to treat astigmatism, nearsightedness, and farsightedness. [3] During the procedure, the epithelium is displaced using a diluted alcohol solution. [4]

PRK has advantages over LASIK in that it avoids added complications associated with the flap created during surgery. The procedure may also reduce the chances of dry eye symptoms after surgery. Due to the PRK procedure not requiring a surgical flap, athletes or individuals concerned with trauma introduced by the flap may see benefits to LASEK. Patients that wear contact lenses will typically need to stop wearing these for a specified time before the procedure. [5]

PRK disadvantages include a longer recovery time for vision in contrast to LASIK. Another disadvantage is that patient may be required to apply steroid eye drops for a few weeks longer than that of a LASIK procedure. [3] Vision after the PRK procedure has a longer recovery than LASIK which may be between five days and two weeks for blurred vision to properly clear. [4] [6]

When LASEK is compared to LASIK, LASIK can have better outcomes with corneal haze while LASEK has a lower rate of flap complications than LASIK. [7]

Eligibility

There are a number of basic criteria which a person should satisfy:[ citation needed ]

There are also some pre-existing conditions that may complicate or preclude the treatment. [8]

Possible complications

Some complications that can be temporary or permanent include:

Dry eyes

As with other forms of refractive surgery, keratoconjunctivitis sicca, colloquially referred to as 'dry eye,' is the most common complication of PRK, and can be permanent. In more advanced cases, recurrent erosions occur during sleeping from adherence of the corneal epithelium to the upper eyelid with rapid eye movement. Adjuvant polyunsaturated fatty acids (PUFAs) with high Omega-3 content before and after surgery improves sicca, possibly due to their anti-inflammatory effects. Foods containing PUFAs include flax and fish oil. [11] Brush PRK to denude the epithelium, instead of alcohol based techniques, also result in quantitatively lower ocular dryness after surgery. The amount of corneal hazing after surgery is also decreased with brush technique. [12] The platelet activating factor LAU-0901 has shown effect in mitigating dry eye in mouse models. [13] Rabbit models have also shown improvement with topical nerve growth factor (NGF) in combination with docosahexaenoic acid (DHA). [14] Mitomycin C worsens post-surgical dry eye. [15]

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. Activities requiring good binocular vision may have to be suspended between surgeries and during the sometimes extended healing periods.[ citation needed ]

Halos, starbursts and refractive errors

PRK can be associated with glare, halos, and starburst aberrations, which can occur with postoperative corneal haze during the healing process. Night halos are seen more often in revisions with small ablation zone size. [16] With more recent developments in laser technology, this is less common after 6 months though symptoms can persist beyond a year in some cases.[ citation needed ] A dilute concentration of the chemotherapeutic agent, Mitomycin-C, can be applied briefly at the completion of surgery to reduce risk of hazing, although with increased risk of sicca. [15]

Predictability of the resulting refractive correction after healing is not totally exact, particularly for those with more severe myopia. This can lead to under/overcorrection of the refractive error. In the case of the overcorrection, premature consequences of presbyopia is a possibility. Experienced surgeons employ a custom-profile algorithm to further enhance predictability in their results.[ citation needed ]

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.

Comparison to LASIK

A systematic review that compared PRK and LASIK concluded that LASIK has shorter recovery time and less pain. [17] The two techniques after a period of one year have similar results. [17]

A 2016 systematic review found that it was unclear whether there were any differences in efficacy, accuracy, and adverse effects when comparing PRK and LASEK procedures among people with low to moderate myopia. [18] The review stated that no trials have been conducted comparing the two procedures on people with high myopia. [18]

A 2017 systematic review found uncertainty in visual acuity, but found that in one study, those receiving PRK were less likely to achieve a refractive error, and were less likely to have an over-correction than compared to LASIK. [19]

Types

US Military

In the U.S.A. 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 Command (USASOC) schools (HALO, SCUBA, SERE) per Army Regulation 40-501. [22]

The U.S. 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. Uncomplicated, successful corneal refractive surgery does not require a waiver and is noted as information only. [23]

The Navy and Marines will routinely grant a waiver for pilots or student naval aviators, as well as naval flight officers, UAS operators and aircrew, to fly after PRK and LASIK, assuming preoperative refractive standards are met, no complications in the healing process were encountered, asymptomatic with regard to significant halos, glare or dry eye, off all medications, and passing their standard vision tests. 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 and LASIK 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 operations training.[ citation needed ]

The U.S. Air Force approves the use of PRK and LASIK. [24] 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. [25] 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 [26] ). 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. [27] Those applicants will be evaluated at the ACS during their Medical Flight Screening appointment to determine if they meet waiver criteria.

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 them.[ citation needed ]

History

The first PRK procedure was performed in 1987 by Dr. Theo Seiler, then at the Free University Medical Center in Berlin, Germany. [28] The first procedure similar to LASEK was performed at Massachusetts Eye and Ear Infirmary in 1996 by ophthalmologist and refractive surgeon Dimitri Azar. [29] Dr. Massimo Camellin, an Italian surgeon, was the first to write a scientific publication about the new surgical technique in 1998, coining the term LASEK for laser epithelial keratomileusis. [30]

Related Research Articles

<span class="mw-page-title-main">Farsightedness</span> Eye condition in which light is focused behind instead of on the retina

Far-sightedness, also known as long-sightedness, hypermetropia, and hyperopia, is a condition of the eye where distant objects are seen clearly but near objects appear blurred. This blur is due to incoming light being focused behind, instead of on, the retina due to insufficient accommodation by the lens. Minor hypermetropia in young patients is usually corrected by their accommodation, without any defects in vision. But, due to this accommodative effort for distant vision, people may complain of eye strain during prolonged reading. If the hypermetropia is high, there will be defective vision for both distance and near. People may also experience accommodative dysfunction, binocular dysfunction, amblyopia, and strabismus. Newborns are almost invariably hypermetropic, but it gradually decreases as the newborn gets older.

<span class="mw-page-title-main">LASIK</span> Corrective ophthalmological surgery

LASIK or Lasik, commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and an actual cure for astigmatism, since it is in the cornea. LASIK surgery is performed by an ophthalmologist who uses a laser or microkeratome to reshape the eye's cornea in order to improve visual acuity.

A microkeratome is a precision surgical instrument with an oscillating blade designed for creating the corneal flap in LASIK or ALK surgery. The normal human cornea varies from around 500 to 600 μm in thickness; and in the LASIK procedure, the microkeratome creates an 83 to 200 μm thick flap. The microkeratome uses an oscillating blade system, which has a blade that oscillates horizontally as the blade travels vertically for a precise cut. This piece of equipment is used all around the world to cut the cornea flap. The microkeratome is also used in Descemet's stripping automated endothelial keratoplasty (DSAEK), where it is used to slice a thin layer from the back of the donor cornea, which is then transplanted into the posterior cornea of the recipient. It was invented by Jose Barraquer and Cesar Carlos Carriazo in the 1950s in Colombia.

<span class="mw-page-title-main">Eye surgery</span> Surgery performed on the eye or its adnexa

Eye surgery, also known as ophthalmic surgery or ocular surgery, is surgery performed on the eye or its adnexa. Eye surgery is part of ophthalmology and is performed by an ophthalmologist or eye surgeon. The eye is a fragile organ, and requires due care before, during, and after a surgical procedure to minimize or prevent further damage. An eye surgeon is responsible for selecting the appropriate surgical procedure for the patient, and for taking the necessary safety precautions. Mentions of eye surgery can be found in several ancient texts dating back as early as 1800 BC, with cataract treatment starting in the fifth century BC. It continues to be a widely practiced class of surgery, with various techniques having been developed for treating eye problems.

<span class="mw-page-title-main">Radial keratotomy</span> Refractive surgical procedure to correct myopia (nearsightedness

Radial keratotomy (RK) is a refractive surgical procedure to correct myopia (nearsightedness). It was developed in 1974 by Svyatoslav Fyodorov, a Russian ophthalmologist. It has been largely supplanted by newer, more accurate operations, such as photorefractive keratectomy, LASIK, Epi-LASIK and the phakic intraocular lens.

<span class="mw-page-title-main">Refractive surgery</span> Surgery to treat common vision disorders

Refractive surgery is optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses. This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement. The most common methods today use excimer lasers to reshape the curvature of the cornea. Refractive eye surgeries are used to treat common vision disorders such as myopia, hyperopia, presbyopia and astigmatism.

<span class="mw-page-title-main">Bowman's layer</span> Layer in the cornea of the eye

The Bowman's layer is a smooth, acellular, nonregenerating layer, located between the superficial epithelium and the stroma in the cornea of the eye. It is composed of strong, randomly oriented collagen fibrils in which the smooth anterior surface faces the epithelial basement membrane and the posterior surface merges with the collagen lamellae of the corneal stroma proper.

<span class="mw-page-title-main">Refractive error</span> Problem with focusing light accurately on the retina due to the shape of the eye

Refractive error is a problem with focusing light accurately on the retina due to the shape of the eye and/or cornea. The most common types of refractive error are near-sightedness, far-sightedness, astigmatism, and presbyopia. Near-sightedness results in far away objects being blurry, far-sightedness and presbyopia result in close objects being blurry, and astigmatism causes objects to appear stretched out or blurry. Other symptoms may include double vision, headaches, and eye strain.

<span class="mw-page-title-main">Phakic intraocular lens</span> Lens implanted in eye in addition to the natural lens

A phakic intraocular lens (PIOL) is an intraocular lens that is implanted surgically into the eye to correct refractive errors without removing the natural lens. Intraocular lenses that are implanted into eyes after the eye's natural lens has been removed during cataract surgery are known as pseudophakic.

Epi-LASIK is a refractive surgery technique designed to reduce a person's dependency on eyeglasses and contact lenses. Invented by Dr. Ioannis Pallikaris, the technique is basically an automatic LASEK without alcohol; it can be better considered as superficial LASIK. The stromal bed is smoother than that obtained by mechanical methods or brush. Unlike alcohol (LASEK), there is no chance of damaging the limbal stem cells. It is also relatively less painful than LASEK.

  1. A device similar to a microkeratome slides over the surface of the cornea, just underneath the epithelial layer of cells while suction is applied.
  2. The result is a hinged sheet of epithelium that is at least partially viable.
  3. It is reflected out of the way so that the ablation can take place.
  4. The sheet is repositioned and a bandage soft contact lens is placed on the eye.

Automated lamellar keratoplasty (ALK), also known as keratomileusis in situ, is a non-laser lamellar refractive procedure used to correct high degree refractive errors. This procedure can correct large amounts of myopia and hyperopia. However, the resultant change is not as predictable as with other procedures.

LCA-Vision is a provider of photorefractive keratectomy in the United States under the LasikPlus brand. The company performs Custom LASIK, PRK and monovision treatment to correct nearsightedness, farsightedness, astigmatism and reduce the effects of presbyopia. According to the company, one million laser vision correction procedures have been performed at its LasikPlus vision centers since 1991.

Vision of humans and other organisms depends on several organs such as the lens of the eye, and any vision correcting devices, which use optics to focus the image.

Diffuse lamellar keratitis (DLK) is a sterile inflammation of the cornea which may occur after refractive surgery, such as LASIK. Its incidence has been estimated to be 1 in 500 patients, though this may be as high as 32% in some cases.

Jeff Machat MD, FRCSC, DABO is an ophthalmologist in the United States and Canada specializing in surgical vision correction better known as refractive eye surgery.

Peter S. Hersh is an American ophthalmologist and specialist in LASIK eye surgery, keratoconus, and diseases of the cornea. He co-authored the article in the journal Ophthalmology that presented the results of the study that led to the first approval by the U.S. Food and Drug Administration (FDA) of the excimer laser for the correction of nearsightedness in the United States. Hersh was also medical monitor of the study that led to approval of corneal collagen crosslinking for the treatment of keratoconus.

The eye, like any other optical system, suffers from a number of specific optical aberrations. The optical quality of the eye is limited by optical aberrations, diffraction and scatter. Correction of spherocylindrical refractive errors has been possible for nearly two centuries following Airy's development of methods to measure and correct ocular astigmatism. It has only recently become possible to measure the aberrations of the eye and with the advent of refractive surgery it might be possible to correct certain types of irregular astigmatism.

Post-LASIK ectasia is a condition similar to keratoconus where the cornea starts to bulge forwards at a variable time after LASIK, PRK, or SMILE corneal laser eye surgery. However, the physiological processes of post-LASIK ectasia seem to be different from keratoconus. The visible changes in the basal epithelial cell and anterior and posterior keratocytes linked with keratoconus were not observed in post-LASIK ectasia.

PiXL is a modern non-invasive non-surgical vision correction procedure.

Clear lens extraction (CLE), also known as refractive lensectomy or refractive lens exchange (RLE) is a surgical procedure in which clear lens of the human eye is removed. Unlike cataract surgery, where cloudy lens is removed to treat cataract, clear lens extraction is done to surgically correct refractive errors such as high myopia. It can also be done in hyperopic or presbyopic patients who wish to have a multifocal IOL implanted to avoid wearing glasses. It is also used as a treatment for diseases such as angle closure glaucoma.

References

  1. ↑ "Laser epithelial keratomileusis (LASEK)" Accessed June 30, 2008.
  2. ↑ "LASEK Eye Surgery: How It Works". All About Vision. Retrieved 2018-01-24.
  3. 1 2 "LASEK Eye Surgery". WebMD. Retrieved 2018-01-24.
  4. 1 2 "LASEK". lasik.wustl.edu. Retrieved 2018-01-29.
  5. ↑ "LASEK Eye Surgery". WebMD. Retrieved 2018-01-24.
  6. ↑ "LASIK Alternatives | UCLA Laser Refractive Center". UCLA Laser Refractive Center. Retrieved 2018-01-29.
  7. ↑ Zhao, LQ; Zhu, H; Li, LM (2014). "Laser-Assisted Subepithelial Keratectomy versus Laser In Situ Keratomileusis in Myopia: A Systematic Review and Meta-Analysis". ISRN Ophthalmology. 2014: 672146. doi: 10.1155/2014/672146 . PMC   4058142 . PMID   24977054.
  8. ↑ content team, content team (July 23, 2019). "LASEK: A Magic Helping You Do Without Eyeglasses". sinahealthtour.com. Archived from the original on July 25, 2019. Retrieved July 25, 2019.
  9. ↑ "Side Effects of Laser Eye Surgery". Eyesurgerycosts.net. December 18, 2011. Archived from the original on April 26, 2012. Retrieved December 19, 2011.
  10. ↑ Stein R, Stein H, Cheskes A, Symons S. Photorefractive keratectomy and postoperative pain. American Journal of Ophthalmology. 1994 March 15; 117(3):403-405.
  11. ↑ Ong, NH; Purcell, TL; Roch-Levecq, AC; Wang, D; Isidro, MA; Bottos, KM; Heichel, CW; Schanzlin, DJ (2013). "Epithelial healing and visual outcomes of patients using omega-3 oral nutritional supplements before and after photorefractive keratectomy: A pilot study". Cornea. 32 (6): 761–5. doi:10.1097/ICO.0b013e31826905b3. PMID   23132445. S2CID   37848356.
  12. ↑ Sia, RK; Ryan, DS; Stutzman, RD; Psolka, M; Mines, MJ; Wagner, ME; Weber, ED; Wroblewski, KJ; Bower, KS (2012). "Alcohol versus brush PRK: Visual outcomes and adverse effects". Lasers in Surgery and Medicine. 44 (6): 475–81. doi:10.1002/lsm.22036. PMID   22674627. S2CID   40461033.
  13. ↑ Esquenazi, S; He, J; Li, N; Bazan, NG; Esquenazi, I; Bazan, HE (2009). "A novel platelet activating factor receptor antagonist reduces cell infiltration and expression of inflammatory mediators in mice exposed to desiccating conditions after PRK". Clinical & Developmental Immunology. 2009: 138513. doi: 10.1155/2009/138513 . PMC   2798082 . PMID   20049336.
  14. ↑ Esquenazi, S; Bazan, HE; Bui, V; He, J; Kim, DB; Bazan, NG (2005). "Topical combination of NGF and DHA increases rabbit corneal nerve regeneration after photorefractive keratectomy". Investigative Ophthalmology & Visual Science. 46 (9): 3121–7. doi:10.1167/iovs.05-0241. PMID   16123410.
  15. 1 2 Kymionis, GD; Tsiklis, NS; Ginis, H; Diakonis, VF; Pallikaris, I (2006). "Dry eye after photorefractive keratectomy with adjuvant mitomycin C". Journal of Refractive Surgery. 22 (5): 511–3. doi:10.3928/1081-597X-20060501-16. PMID   16722493.
  16. ↑ Rajan, Madhavan S.; Jaycock, Philip; O'Brart, David; Nystrom, Helene Hamberg; Marshall, John (2004). "A long-term study of photorefractive keratectomy". Ophthalmology. 111 (10): 1813–24. doi:10.1016/j.ophtha.2004.05.019. PMID   15465541.
  17. 1 2 Shortt, AJ; Allan, BD; Evans, JR (31 January 2013). "Laser-assisted in-situ keratomileusis (LASIK) versus photorefractive keratectomy (PRK) for myopia". The Cochrane Database of Systematic Reviews. 1 (1): CD005135. doi:10.1002/14651858.CD005135.pub3. PMID   23440799.
  18. 1 2 Li SM, Zhan S, Li SY, Peng XX, Hu J, Law HA, Wang NL (2016). "Laser-assisted subepithelial keratectomy (LASEK) versus photorefractive keratectomy (PRK) for correction of myopia". Cochrane Database Syst Rev. 2016 (2): CD009799. doi:10.1002/14651858.CD009799.pub2. PMC   5032141 . PMID   26899152.
  19. ↑ Kuryan J, Cheema A, Chuck RS (2017). "Laser-assisted subepithelial keratectomy (LASEK) versus laser-assisted in-situ keratomileusis (LASIK) for correcting myopia". Cochrane Database Syst Rev. 2017 (2): CD011080. doi:10.1002/14651858.CD011080.pub2. PMC   5408355 . PMID   28197998.
  20. ↑ De Benito-Llopis, L; Teus, MA; Sánchez-Pina, JM (2008). "Comparison between LASEK with mitomycin C and LASIK for the correction of myopia of -7.00 to -13.75 D". Journal of Refractive Surgery. 24 (5): 516–23. doi:10.3928/1081597X-20080501-10. PMID   18494345.
  21. ↑ "Long-term concerns linger on safety of Mitomycin-C". Archived from the original on November 3, 2013. Retrieved April 22, 2013.
  22. ↑ "Standards of Medical Fitness" (PDF). United States Army Medical Services. United States Department of the Army. 4 August 2011. Retrieved 15 August 2012.
  23. ↑ "CORNEAL REFRACTIVE SURGERY (ICD9 V802A/V802B)" (PDF). January 2006. Retrieved August 16, 2013. Archive index at the Wayback Machine
  24. ↑ Tan, Michelle (February 28, 2011). "Eye surgery opens pilot training to wider pool". Air Force Times .
  25. ↑ "Flying jobs now open to LASIK patients". Air Force Times . May 24, 2007.
  26. ↑ "US Air Force Refractive Surgery Program Site" Accessed Aug 9, 2011 Archived July 27, 2011, at the Wayback Machine
  27. ↑ "USAF Waiver Guide" Accessed Aug 9, 2011
  28. ↑ "PRK – Photorefractive Keratectomy" Accessed March 17, 2010 Archived February 23, 2011, at the Wayback Machine
  29. ↑ Taneri, Suphi; Zieske, James D.; Azar, Dimitri T. (2004). "Evolution, techniques, clinical outcomes, and pathophysiology of LASEK: Review of the literature". Survey of Ophthalmology. 49 (6): 576–602. doi:10.1016/j.survophthal.2004.08.003. PMID   15530945.
  30. ↑ Camellin M. LASEK: nuova tecnica di chirurgia rifrattiva mediante laser ad eccimeri. Viscochirurgia 1998;39-43
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