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Laser Eye Surgery

PRK and LASEK

05 Jan 2021
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The introduction of the excimer laser into the world of keratorefractive surgery in the 1990s changed the treatment of myopia, hyperopia and astigmatism forever 1. Photorefractive keratectomy (PRK) was developed in the year 1983 using an excimer laser emitting ultraviolet light to change the curvature of the cornea. Although The United States Food and Drug Administration approved PRK as a refractive surgery method only in 1996 2. However, in recent years, the use of PRK  has shrunk due to the introduction of laser keratomileusis LASIK and later Femto-LASIK 2. 


Types of eye microsurgery. Treatment of myopia and hyperopia

Corneal laser surgery has become an effective alternative to glasses and contact lenses for correcting refractive errors 3. Since PRK a wide range of surgical techniques have been developed that correct the refractive errors of the eye (ametropia) by removing corneal tissue and thereby changing its curvature.


These operations can be divided into three main types:



    
 	
  1. PRK* (mechanically or alcohol) and LASEK (alcohol only);
    2. 


* also known as MAGEK on the territory of the Russian Federation



    
 	
  1. Ablation of the cornea with an excimer or solid-state laser when creating a flap on its surface with an electric blade or a femtosecond laser: LASIK** and Femto-LASIK;
    2. 


**also known as the REIK on the territory of the Russian Federation



    
 	
  1. Extraction of the corneal lenticule without and with a flap 3.
    2. 




It is worth mentioning that for the first two types of operations, you can find many marketing names, which are often invented by the clinics themselves to attract more attention. These names may be neither confirmed in the scientific literature nor patented by any manufacturer. But no matter how these types of operations are presented, their principle remains unchanged.


The type of surgery with the extraction of the corneal lenticule has several commercial names, which are patented by the manufacturers of the femtosecond lasers. Of course, it should not be ruled out that clinics may also start advertising their services by replacing their proprietary names with their own for marketing purposes.


The exception is laser epithelial keratomileusis (LASEK), which historically was also called Excimer Laser Subepithelial Ablation (ELSA) 4. As already mentioned, in the present LASEK the epithelium is brought back onto the cornea.



How PRK and LASEK are done

The first step in the PRK procedure starts by removing the epithelial layer by physical scraping or peeling. For this, an instrument resembling a miniature hockey stick is used (“hockey knife”). In eye surgery PRK and LASEK, an epithelial flap can also be created by applying an alcohol solution to the cornea for mechanical ablation.


The second step of the procedure is the evaporation of a part of the cornea (ablation) 5. The laser is applied on the surface of the cornea, known as the stroma, and modifies its shape (curvature), therefore correcting visual errors 6. Later, the surface of the cornea without epithelium (the upper layer of the cornea of about 6 hundredths of a millimeter) left for natural healing under a contact lens 6. In the LASEK operation, the epithelium may not be removed, but placed back onto the stroma. In PRK operations, the epithelium is removed, regardless of whether it was "scraped" mechanically or with the use of alcohol.


In SUPER PRK and SUPER LASEK the term "SUPER" usually denotes individual (personalized) laser correction, which takes into account the peculiarities of the eye, that are needed to eliminated more complex refractive errors. In this case, it is not enough to correct only spherical vision error or astigmatism.



See video here.






Features of PRK and LASEK

PRK was the first refractive procedure using a laser and has been widely used for its safety and efficacy 5. In PRK the corneal epithelium is removed to facilitate stromal ablation 7 2.


Today PRK surgery is often used in cases where LASIK is contraindicated, since LASIK is a “flap surgery” type, and another PRK surgery is a “surface treatment” type 6. For example, if the thickness of the cornea does not allow leaving the recommended minimum safe thickness of the cornea (0.25mm - 0.3mm) after changing its curvature by tissue evaporation, apart from the thickness of the flap, PRK is used. PRK corrects mild to moderate myopia, hyperopia and astigmatism with a high level of safety and efficacy 2.


Although LASIK is primarily used in refractive surgery due to minor pain and rapid visual rehabilitation, LASIK is also a popular method for high myopia, thinner cornea or retinal pathology. The advantages of LASEK include the absence of possible complications associated with the flap in LASIK 8. The cornea is known to lead in the number of nerve endings in the human body. There are about 7000 of them per square millimeter. Therefore, removal of the epithelial layer in PRK results in a delayed healing time, slower visual recovery, and increased postoperative pain compared to LASIK 5.


Therefore, LASEK was developed to reduce corneal pain and opacification associated with PRK and to speed up vision recovery. Removal of the epithelium with dilute alcohol in PRK and LASEK has shown a smoother surface suitable for laser ablation 7, 2. Such a surface aids in corneal healing and prevents corneal opacities (haze) 9 that were originally found after PRK. Recently, the problem of stromal opacification after PRK and LASEK has been significantly reduced with the introduction of new profiles of laser ablation and Mitomycin-C (MMC) 10.


On the territory of the Russian Federation, PRK with the use of Mitomycin-C is better known as MAGEK. Mitomycin-C is a solution from the group of antitumor antibiotics for suppressing division (mitosis) of epithelial cells during the healing reaction after PRK and LASEK, also used after keratoplasty (corneal transplantation) 10. Thus, MMS reduces corneal opacity and, as a consequence, improves visual acuity after PRK and LASEK 2. However, new algorithms or energy settings in lasers can reduce the inflammatory response. For example, studies without MMC but using a specific excimer laser profile in Central Europe did not result in clinically significant corneal opacities 11.


Epithelial cells are the first cells involved in the process of corneal regeneration after PRK, LASIK and Transepithelial PRK (see Transepithelial PRK) 12 2. After removal of the epithelium for laser correction, its cells migrate from the edges of the cornea (limb) to restore the removed area 13,14 2. Removal of the epithelial layer contributes to some inflammation and pain, which usually begins shortly after the PRK and LASEK treatments and continues in the first 3-5 days until the end of re-epithelialization (restoration of the removed epithelium at the sites of tissue damage). Nevertheless, the severity of this pain differs widely in patients, whereas chronic pain is much less common and is often classified by doctors and patients as symptoms of dry eye 5. During the healing process, the superficial epithelium and a part of the cornea may change and return to its original state. This process is called "regression". Studies have also shown a decrease in the likelihood of its occurrence after PRK and LASEK when using MMS 15,16 17.


Pain management strategies are very diverse and include techniques like cooling the cornea to reduce inflammation and suppress the perception of pain. The tissue can be cooled by washing with physiological solution (saline) or by cooling with a wet ring. Silicone hydrogel lenses, worn after surgery, allow oxygen to pass through and thus reduce pain while increase patient comfort. Also, local anesthetics (drops) applied locally are good at suppressing corneal pain. This is why this class of medication is used before surgery to numb tissues. For the best results, a contact lens is worn after surgery for mechanical protection and support for re-epithelialization in combination with the use of local non-steroidal anti-inflammatory drugs and / or diluted anesthetics. Other pain relievers can be taken in case of sudden pain 5.



Results after PRK and LASEK

The recent completion of a ten-year study (2008-2019) of PRK and Femto-LASIK treatments has proven their safety and effectiveness in the long term 18.


According to one study 19, LASIK suggests a higher likelihood of postoperative dry eye syndrome than PRK. On the other hand, dry eye is also associated with lower corneal sensitivity and less frequent blinking.


Another recent study 20 of the results after LASEK with patients aged 18 to 37 years old using lasers from two German manufacturers showed that 100% of patients had visual acuity higher than 20/20 according to the Snellen chart, i.e. more than one (more than 10 lines according to Sivtsev's table) in one group and more than 50% in another. After the first month, 92% of patients in the first group and 81% in the second had more than 20/20 vision, i.e. more than 10 lines according to Sivtsev's table.


In the first weeks after LASEK surgery, the side effect of corneal opacity was noticed, but only in one eye in one of the groups. With the use of the prescribed drops, the cloudiness disappeared in the first month. 6 months after surgery, no corneal opacities were observed in any of the groups.

 

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