Photorefractive keratectomy

from Wikipedia, the free encyclopedia

The term photorefractive keratectomy ( PRK ) refers to a keratomy procedure in refractive surgery , i.e. an eye operation that is intended to replace glasses or contact lenses to correct ametropia .

Basics

Excimer laser for photorefractive keratectomy

This laser procedure , which has been used since 1987, changes the curvature of the cornea by removing tissue from the corneal surface. Slightly modified PRK techniques are called LASEK (laser epithelial keratomileusis), Epi-LASIK (epithelial laser in situ keratomileusis) and Trans-PRK (transepithelial photorefractive keratectomy).

The aim of photorefractive keratectomy is to correct optical ametropia. In the optimal case, the remaining refraction is less than ± 0.5 diopters , and the patient should generally be able to achieve acceptable visual acuity without visual aids (glasses, contact lenses). The uncorrected visual acuity , i.e. That is, the visual acuity without corrective aids, often improves very significantly depending on the initial situation and can ideally reach a value of 1.0 or even more. The best corrected visual acuity as visual acuity with optimal eyeglass correction, however, remains mostly unchanged or changes only slightly (depending on the treatment method). All superficial excimer laser-based treatments can also be carried out with wave font-optimized laser profiles, depending on the equipment of the laser used, in order to eliminate or improve optical deviations (aberrations) of higher order. These higher-order aberrations cannot usually be eliminated by eyeglass correction and usually arise after irregular corneal injuries, caused by genetically determined corneal diseases or previous less successful refractive laser eye operations.

Pioneer patient Alberta H. Cassady

The first PRK on humans was carried out on the seeing eye of a patient on March 25, 1988 by Marguerite McDonald at Louisiana State University / New Orleans after approval by the FDA (Federal Drug Administration, USA). This eye had to be removed because of a malignant melanoma, although it achieved 100% visual acuity with a glasses correction of −4.5 D. A team of rehabilitation engineers, theoretical opticians and doctors, consisting of Marguerite McDonald, Stephen I. Trokel, Charles R. Munnerly and Stephen Klyce, worked on this project for years in animal experiments and laid the theoretical and practical bases for this first experiment on living humans placed. Refractive surgery and millions of happy patients worldwide owe the courage and bravery of pioneering patient Alberta H. Cassady the convincing first proof that PRK is painful, but can be implemented in practice and produces the best quality of vision. The data from this successful experiment on the living eye, which had to be removed after 11 days because of the malignant tumor in the eye and which had to be examined histologically anyway, helped convince the FDA to approve the PRK for clinical use in humans.

Methods

In the laser procedures PRK ( photorefractive keratectomy ), LASEK ( laser epithelial keratomileusis ), Epi-LASIK ( epithelial laser in situ keratomileusis ) and Trans-PRK ( transepithelial photorefractive keratectomy ) tissue is ablated from the corneal surface. They are therefore also referred to as surface ablation . PRK is the oldest laser procedure for the treatment of ametropia and has been used since 1987.

A study published in 2010 compared the results of PRK and LASEK based on around 500 treated eyes each. Thereafter, there was no statistically significant difference between the procedures for any relevant treatment outcome. Only the frequency of temporary corneal opacity (haze) in the first 3 months after the operation was somewhat lower in the LASEK group.

PRK

Although the term photorefractive keratectomy was originally intended for refractive laser treatments in general (photorefractive (from ancient Greek φῶς , phos , φωτός , photos , "light (the heavenly body)", "brightness" and Latin re = back, frangere = to break); keratectomy (from Greek κέρας ( kéras , "horn") and Greek εκτομή , "cutting out")), it is only used for a special method today. In PRK, the epithelial layer on the cornea is removed with a simple instrument (known as a hockey knife) and is not used again. As with all the methods described here, laser ablation takes place on the corneal surface.

LASEK

Before the laser ablation, the epithelium is loosened with an alcohol solution and then pushed aside with a simple surgical instrument. In contrast to the PRK, the epithelium is reused after the laser treatment and pushed back over the treated corneal area.

Epi-LASIK

The epithelium is also reused in Epi-LASIK. However, the epithelium is removed with a special device called an epi-keratome. This instrument is very similar to the microkeratome, but uses a blunt “ plastic blade” instead of a sharp metal blade . This "blade" lifts the epithelium in a circular shape in the desired treatment area without damaging the cornea. The resulting epithelial flap is then simply folded to the side and the treatment can be carried out on the surface of the cornea. After the treatment, this flap is folded back again.

Trans-PRK

The epithelium is ablated with the excimer laser, either together with the refractive ablation in one step or separately in two steps.

LASIK

For comparison: With laser in situ keratomileusis ( LASIK ), a thin lamella (diameter approx. 8 to 9.5 mm and thickness between 100 and 160 µm) cut into the cornea. This flap is not completely separated, but maintains a connection to the rest of the cornea, which serves as a "hinge". After the cut, the flap is then opened. The excimer laser treatment itself is hardly noticeable because the excimer laser light with its wavelength of 193 nm is invisible. You only see slight changes in shape on the treated corneal surface.

Following the laser ablation, the treated area is rinsed and the flap is folded back with further rinsing. Thorough rinsing is important in order to remove any foreign bodies (debris, epithelial cells) from the area between the flap and the cornea (the so-called interface). The excess liquid is sucked out of the interface with a small sponge and the flap is smoothed out. The treatment is now over and the eyelid retractor can be removed.

In the meantime, the flap cut with the femtosecond laser is becoming more and more popular, as it offers various advantages over the mechanical microkeratome.

Treatment area and contraindications

The Commission for Refractive Surgery ( KRC ) distinguishes between areas of application , i.e. H. the area in which the method is to be regarded as suitable and complications are rare and the limit area in which the method can still be used, side effects and complication rate are expected to be higher. Stricter information to the patient applies to the border area.

scope of application

Myopia up to −6.0 dpt, farsightedness up to +3.0 and astigmatism up to 5 diopters, whereby the two values ​​added together must not exceed −6.0.

Border area

Myopia up to −8.0 dpt, farsightedness up to +3.0 and astigmatism up to 6 dioptres, whereby the two values ​​added together must not exceed −8.0

These guide values ​​can be one to two diopters higher or lower depending on the country, clinic and laser system used. The German Ophthalmological Society (DOG), for example, only classifies PRK as a scientifically validated treatment method for myopia up to −6.00 diopters.

Contraindications are circumstances that prohibit treatment or allow treatment only after careful consideration of the particular risks. A sufficiently thick cornea is an essential prerequisite for performing a photorefractive keratectomy. Too thin a cornea is definitely a contraindication. A remaining residual thickness of at least 250 µm after treatment is the limit. This remaining thickness is calculated from the corneal thickness minus the maximum depth of removal. Photorefractive keratectomy should also not be performed in chronic progressive corneal disease. Treatment is prohibited, especially for keratoconus , as the cornea is weakened further and the clinical picture would worsen dramatically. If the patient's refraction is not stable, i.e. if the measured refraction values ​​deviate significantly from one another in relatively short time intervals, photorefractive keratectomy should not be performed. The eye diseases glaucoma and symptomatic cataract continue to be considered contraindications . General diseases that rule out LASIK treatment are collagenoses , autoimmune diseases and wound healing disorders . Finally, patients eligible for photorefractive keratectomy should not be pregnant and should also be of legal age.

Preliminary examinations

The expectations of the treatment result vary greatly from patient to patient and should be discussed extensively with the attending physician in advance. The preliminary examinations serve to record the correct treatment data in order to rule out contraindications and to compare the patient's expectations with the outcome prognosis. The patient should not wear contact lenses for at least two weeks prior to the preliminary examinations.

An essential part of the preliminary examinations is the determination of the patient's exact subjective refraction by a qualified optometrist or ophthalmologist. The refraction should be determined at least twice with an interval of at least two weeks. The measurement of only the objective refraction values, for example by means of an automatic refractometer , is in any case insufficient.

Other important preliminary examinations are:

Treatment process

In all four methods, the epithelium of the locally anesthetized eye is first removed in a sufficiently large (8-10 mm diameter), central corneal area and then the corneal surface is treated with the laser. The procedures differ in how the epithelium is removed and what happens to it after treatment. In PRK, the epithelium is scraped off with the help of a surgical instrument and not used again. Scraping can be simplified by dissolving it with an alcohol solution.

With LASEK, the epithelium is loosened with alcohol and pushed aside with a suitable instrument, with Epi-LASIK, however, it is lifted off with a blunt corneal plane similar to a microkeratome and forms a kind of epithelial flap. The laser treatment then takes place on the corneal surface and the epithelium is placed back on the treatment zone with LASEK / Epi-LASIK. In Trans-PRK, the epithelium is ablated with the excimer laser, after which the cornea is processed further with the laser. In all four procedures, a therapeutic contact lens is put on after the treatment, which protects the sensitive corneal surface with the defective epithelium until it is completely healed. Epithelial healing takes the longest in PRK because the epithelium has to grow back over the entire treatment area (this happens from the outside in). Depending on the patient and the size of the treatment area, this takes anywhere from two days to a week.

The healing process is significantly faster, especially with Epi-LASIK, as the epithelial flap already covers most of the treatment zone. With LASEK, too, healing is usually faster and more painless, but some of the epithelial cells die when they are detached and have to be replaced by new ones. The average healing after Trans-PRK compared to LASEK is faster, less painful, and associated with less haze.

Advantages and disadvantages

Advantages of surface treatments are:

  • Less destabilization of the cornea compared to LASIK
  • No flap-related complications
  • Low risk of infection compared to implants.
  • We have years of experience from millions of operations worldwide.

Disadvantage:

  • Pain during the first few days after the operation
  • Slow vision recovery
  • Temporary clouding of the cornea (haze) possible
  • The structure of the cornea is destabilized
  • Maximum possible correction depending on corneal thickness and pupil size

In several studies on LASEK, a successful correction with ± 0.5 diopters was achieved in 75–87% of cases after one year. Between 84% and 100% of the eyes had an uncorrected visual acuity equal to or better than the corrected visual acuity before surgery. Individual publications speak of better contrast sensitivity and night vision compared to LASIK.

In a twelve-year long-term study of PRK, 94% of the eyes had the best-corrected visual acuity that was the same as or better than before the operation. After an initial overcorrection in the first four weeks, regression occurred within 3–6 months. Depending on the diopter group, the desired correction was achieved in up to 79% of the cases. After that, the refraction was stable for twelve years, that is, there was no statistically significant change. 12% of the patients complained of night vision problems, 3% of dry eyes. 50% of the patients were " extremely happy " with the result, especially those who were close to normal vision. The others had night vision problems, severe regression, or decentered ablation.

The American Food and Drug Administration (FDA) presents very detailed study results on its website. These studies are carried out as part of the approval process for refractive laser devices and are considered to be very reliable. They are strictly monitored and carried out in parallel at several clinics.

Risks

As with any surgical procedure, there are a number of risks associated with refractive surgery. The type and frequency of complications generally depend on the treatment method. However, the surgeon's experience, the level of correction, the technique used and individual influencing factors also play an important role. It should also be borne in mind that refractive surgical interventions usually represent an operative intervention on a principally healthy organ.

General risks associated with any type of refractive surgery are limitations in twilight and night vision due to reduced contrast sensitivity, glare (glossy effects) and halogons (halos). Short-term to long-term over- or undercorrections can also occur, as well as a reduction in visual acuity with optimal glasses correction (so-called best-corrected visual acuity). Infections of the eye are possible with any type of treatment.

The risk of visual impairment after laser treatment also depends on individual risk factors (such as the number of diopters, flat cornea, pupil size). In addition, the surgeon's experience has a serious influence on the complication rate. A study from 1998 compared the intraoperative complication rate of the first 200 LASIK treatments by a surgeon with that of the following 4,800 treatments. The rate for the first 200 LASIK treatments is 4.5%, and only 0.87% for the subsequent treatments.

There is a very certain risk in the structural weakening of the cornea after tissue removal. This weakening and the constant intraocular pressure acting on the cornea can lead to a bulging of the cornea ( keratectasia ). The risk of this increases as the remaining thickness of the cornea decreases after the treatment. The minimum value for the remaining thickness is 250 µm. The KRC (Commission for Refractive Surgery) recommends planning a reserve of 30 µm for the first procedure in order to be able to carry out any necessary corrections. The planning and calculation of the corneal removal is the responsibility of the certified ophthalmologist in Germany. The remaining thickness is calculated from the central corneal thickness minus the central tissue removal. Keratectasia can also occur due to genetic predisposition, even in the case of an unsuspiciously normal cornea.

Side effects of all superficial excimer laser corneal treatments

In the first few days after PRK, LASEK or Epi-LASIK and transPRK (notouchPRK), eyesight is reduced and there are regular moderate symptoms of varying degrees up to severe pain that can only be controlled by strong painkillers, comparable to the pain that occurs with a "Flashing" occurs when welding without protective goggles. As a rule, a so-called bandage contact lens is worn until the cover membrane (epithelium) has healed. Basically, the complication rate increases with the extent of the Dpt correction made. Possible side effects can be superficial scarring of the cornea (haze). As a result of the formation of haze, a partial regression of the success of the operation can occur within the first weeks and months and a deterioration in vision at dusk, as well as rings of light ("halos") and shadow images to be perceived at night, especially in patients with wide pupils . A temporary increased dryness of the eyes is common and does not represent an abnormal healing process. Extremely rare side effects are infection and severe scarring with considerable permanent impairment of vision. Depending on the severity, the cloudiness disappears after a maximum of 3 months. The use of highly diluted Mitomycin-C has significantly reduced the formation of haze in PRK. However, the scientific data is not conclusively clear. The KRC does not recommend the prophylactic use of mitomycin for initial treatment, but considers it to be justifiable for previously operated corneas (status 12/2018).

In principle, there is always the risk of the treated cornea bulging (ectasia). This is a very serious complication that may require corneal transplantation , or at least " crosslinking ". Modern corneal examination methods ( corneal topographies / corneal microscopes) have been able to reduce this risk considerably, as many candidates with hidden corneal anomalies (e.g. subclinical keratoconus ), i.e. corneal changes with still normal, best-corrected visual acuity, are no longer operated on and primarily the same the so-called "crosslinking" method to consolidate the corneal structure.

Proof of quality

The objective proof of quality is very important for patients. There are various certificates that are used in healthcare.

QM certificate according to ISO 9001: 2008

The ISO 9001: 2000 certificate is a purely process-oriented quality management seal that is awarded across all industries. It reflects process quality and says nothing about the quality of the medical treatment or the technical status of the instruments used.

LASIK TÜV

The so-called LASIK-TÜV, which is based on the ISO-9001: 2000 certificate, has existed especially for laser eye centers since 2006. It is offered by TÜV SÜD and was developed in cooperation with the Refractive Surgery Commission (KRC), the Association of Special Clinics for Laser Eye and Refractive Surgery (VSDAR eV) and the Professional Association of Ophthalmic Surgeons (BDOC). In contrast to the ISO 9001: 2000 certificate, the LASIK TÜV checks not only the process quality, but also the quality of service and results. The following aspects are specifically examined:

  1. Qualifications and experience of employees and doctors,
  2. technical equipment of the facility,
  3. Facility hygiene standards,
  4. Treatment results,
  5. Patient satisfaction.

literature

  • Theo Seiler (Ed.): Refractive surgery of the cornea. Enke im Thieme Verlag, Stuttgart et al. 2000, ISBN 3-13-118071-4 .
  • Berthold Graf: A life without glasses and contact lenses - eye lasers and other alternatives. Baltic Sea Press, Rostock 2009, ISBN 978-3-942129-14-5 .
  • Thomas Kohnen (Ed.): Refractive Surgery. Springer, Berlin 2011, ISBN 978-3-642-05405-1 .
  • Irmgard Huber, Wolfgang Lackner, Wolfgang Pfäffl: Augenlaser - The successful therapy for ametropia Schlütersche Verlagsgesellschaft mbH & Co KG Hannover 2nd edition 2005

Web links

Individual evidence

  1. ^ FDA: FDA-Approved Lasers for PRK and Other Refractive Surgeries. 2019, accessed on April 17, 2017 .
  2. ^ Marguerite McDonald, Howard Larkin: Laser vision correction at 30 . Ed .: Eurotimes. tape 24 , no. 4 . European Society of Cataract & Refractive Surgery, 2019, p. 14-15 .
  3. ↑ Li-Quan Zhao, Rui-Li Wei, Jin-Wei Cheng, You Li, Ji-Ping Cai, Xiao-Ye Ma: Meta-analysis: Clinical Outcomes of Laser-Assisted Subepithelial Keratectomy and Photorefractive Keratectomy in Myopia . In: Ophthalmology . tape 117 , no. 10 , September 2010, p. 1912-1922 , doi : 10.1016 / j.ophtha.2010.02.004 , PMID 20709406 .
  4. ↑ a b Thomas Kohnen, Anja Strenger, Oliver K. Klaproth: Basic knowledge of refractive surgery. Correction of refractive errors with modern surgical procedures. ( Memento of January 24, 2013 in the Internet Archive ) (PDF). In: Deutsches Ärzteblatt. Vol. 1051, No. 9129, 2008, pp. 163-177.
  5. ↑ Brief overview of the methods of refractive surgery. Refractive Surgery Commission (KRC), 2010, archived from the original on March 4, 2016 ; Retrieved November 3, 2010 .
  6. ↑ "Refractive Surgery Commission - KRC"
  7. ↑ Patient information on laser in situ keratomileusis (LASIK). Refractive Surgery Commission (KRC), 2018, archived from the original on November 27, 2010 ; accessed on December 1, 2018 .
  8. ↑ Necessary examinations before the ametropia can be corrected ( Memento from January 6, 2013 in the web archive archive.today ). Carl Gustav Carus University Hospital Dresden at the Technical University of Dresden.
  9. ↑ A. Fadlallah, D. Fahed, K. Khalil, I. Dunia, J. Menassa, H. El Rami, E. Chlela, S. Fahed: Transepithelial photorefracitve keratectomy: Clinical results . In: Journal of Cataract and Refractive Surgery . tape 37 , October 2011, p. 1852-1857 .
  10. ↑ Sunil Shah, Vinod Kumar: Has LASEK superseded LASIK? In: Optometry Today. 6, 2003, pp. 22-25 (PDF)
  11. ↑ Rachel Feit et al: LASEK results. In: Ophtamol Clin N Am. 16, 2003, pp. 127-135. (PDF) ( Memento from October 26, 2005 in the Internet Archive )
  12. ↑ T. van Dorselaer and others: LASEK FOR MYOPIA: FIRST RESULTS. In: Bull Soc belge Ophtalmol. 290, 2003, pp. 59-68 (PDF)
  13. ↑ Madhavan Rajan et al: A Long-term Study of Photorefractive Keratectomy. In: Ophthalmology. 111, No. 10, 2004, pp. 1813–1824 (PDF)
  14. ^ Photorefractive keratectomy. to: augenlaserinfo.at
  15. ↑ FDA-Approved Lasers for LASIK . Food and Drug Administration, November 27, 2009.
  16. ↑ Simulator for twilight and night vision according to LASIK depending on the number of diopters and pupil diameter .
  17. ^ Mihai Pop, Yves Payette: Risk Factors for Night Vision Complaints after LASIK for Myopia. In: Ophthalmology. 111, 2004, pp. 3–10 (PDF)
  18. ↑ Individual risk factors for halos, loss of contrast, glare, starburst after LASIK . In: operationauge.de, March 11, 2010.
  19. ↑ JS Vidaurri-Leal: Complications in 5000 LASIK procedures . In: Group RSSI, ed. Refractive Surgery . 1998, p. 61-64 .
  20. ↑ F. Carones, L. Vigo, E. Scandola, L. Vacchini: Evaluation of the prophylactic use of mitomycin-C to inhibit haze formation after photorefractive keratectomy . In: J Cataract Refract Surg . tape December 28 , 2002, pp. 2088-2095 .
  21. ↑ http://www.tuev-sued.de/gesundheit-lebensmittelicherheit/lasik
  22. ↑ The LASIK TÜV seal of approval. Archived from the original on March 6, 2010 ; Retrieved March 3, 2010 .