Laser eye surgery treatments to improve visual acuity

Laser eye surgery treatments may be divided into therapeutic and refractive groups depending on whether the goal is to improve or safe visual acuity. The main goal of refractive surgeries is to improve the refractive effect without creating new optical problems in the eye. For example, in order to correct myopia it is necessary to reduce the refractive power of the eye either by reducing the curvature of the corneal surface, or by introducing a silicone intraocular lens (IOL) of the required power. The second option, as a rule, is used for high diopters or for diseases of the lens (cataract). There are several surgical methods available to treat ametropia (e.g. myopia, hyperopia). These techniques are broadly divided into two groups 1: those related to surgery on the cornea (refractive surgery of the cornea) and those related to surgery of the eye lens (lens refractive surgery).

Eye surgery types for vision correction

Fig. 1. Refractive and therapeutic eye surgery types

  • Eye flap surgeries use a blade or a femtosecond laser to create a thin flap on the surface of the cornea. The two main surgical methods used are LASer In-situ Keratomileisis (LASIK) (read more about LASIK) and PhotoRefractive Keratectomy (PRK)2 (read more about PRK). The flap treatment is called LASIK. If an incision (the cut) in the corneal stroma is created with a femtosecond laser instead of a mechanical blade a microkeratome, then LASIK is called Femto-LASIK. It is worth mentioning that a mechanical microkeratome is a thin scalpel driven by a mini electric motor. It is put on a ring, which is fixed on the eye by means of a vacuum and is controlled by the surgeon by pressing the pedal. A hinge is left on one side of the flap to fold the flap back and open the eye stroma. After that, the refractive procedure is performed by evaporation (ablation) of the corneal tissue with an ultraviolet laser source (excimer laser visual acuity correction). The laser beam vaporizes the corneal tissue without damaging the stroma. At the end of the surgery, the flap is returned to its original position to remain for healing in the postoperative period. The flap remains in place by natural fusion until the epithelium is completely healed.

  • In another type of laser eye surgery (Epi-LASIK), when using a mechanical microkeratome, the thickness of the flap corresponds to the epithelium thickness (less than 60 µm) 3. Here the epithelium is folded back after laser ablation. Since Epi-LASIK does not make a section in the corneal stroma, it is possible to avoid complications associated with superficial laser visual acuity correction including epithelial ingrowth, infections, etc. 4

  • The first step in the PRK procedure is to remove the epithelial layer by physical scraping or peeling. For this, an instrument resembling a miniature hockey stick is used. The second stage of the procedure is the evaporation of a cornea part (ablation) 5. The laser is applied on the surface of the cornea, known as the stroma, and changes its shape (curvature), therefore correcting visual errors 2. Later, the surface of the cornea without epithelium (the upper layer of the cornea is about 6 hundredths of a millimeter) is left to heal naturally with a contact lens 2.

  • In the LASEK operation, the necessary part of the epithelium may be not entirely removed, but folded back onto the stroma. In PRK operations, the epithelium is removed, regardless of whether it was “scraped” mechanically or with the help of alcohol. In both PRK and LASEK operations the separation of the epithelial “surface” can be carried out by applying an alcohol solution to the cornea for mechanical ablation.

  • Transepithelial PhotoRefractive Keratectomy (TransPRK) is a simplified one-stage PRK (read more about TransPRK). Here, the epithelium as well as a part of the stroma (the main layer under the epithelium), is removed by a laser without mechanical scraping of the epithelium with an instrument resembling a hockey stick or a rotating mechanical brush. Chemical agents such as dilute ethanol solution 6,7 8 can be used to remove corneal epithelium . Total treatment usually takes less than 30 seconds depending on the number of diopters. Removing the epithelium with a laser smoothes the surface of the cornea, which can affect the clinical results 9. Here you need to understand that the epithelium, covering the cornea, masks its microroughness.

  • The aim of all laser refractive operations for presbyopia (Presby-LASIK and Femto-Presby-LASIK) (read more about Presby-LASIK) is multifocal ablation, i.e. based on the creation of two or more optical areas in which the best visual acuity is achieved. This can be achieved by creating different curvatures of the cornea for distance acuity and near reading in different eyes (monovision) or in one eye. In monovision, the dominant eye is usually corrected for distance. In the second case (one eye), an optical region is created with a steeper curvature for reading and a less steep region for distance vision. The number of areas can be more than 2. Here visual acuity depends on the size of the pupil 10,11. It should be understood that each person has a dominant eye, which is treated so that the visual acuity in the distance is achieved on it by the best approach.

  • SMILE® corneal lenticule extraction is the first of the corneal lenticule extraction technologies, which has existed for almost 15 years 12 13. In theory, it is a flapless refractive corneal operation or vision correction using only a femtosecond laser system 14. Here refractive correction is achieved by removing part of the corneal tissue. In the first step a femtosecond laser creates an intrastromal lenticule (a detached layer) inside the cornea. For this, a contact element (patient interface) touches the eye using a vacuum for stability during laser operation. In the second stage, two “pockets” are prepared with a special instrument (dissector) above and below the lenticule. After that, the lenticule is removed with tweezers in common (read more about lenticule extraction).

  • Methods where synthetic materials (implants) are inserted into the cornea to reshape the cornea include intracorneal inlays and rings for treating presbyopia and segments for corneal diseases (e.g. keratoconus) 2 15. Unlike Implantable Collamer (also Contact) Lenses (ICL) and Intraocular Lenses (IOL), which are implanted behind the cornea in the posterior chamber of the eye, the implants are inserted into the incision made in the cornea.

  • It shall be mentioned, that in general therapeutic surgeries are applied in case of preventing progression or formation of keratoconus (type of ectasia or bulging out cornea), scars or need of corneal transplantation. The name CorssLinking means a minimally invasive treatment, where a combination of ultraviolet light and a solution containing vitamin B2 (Riboflavin) forms additional bonds between molecules of the cornea, i.e. strengthens it. In case of following corneal abrasions after trauma such strategy as excimer laser PhotoTherapeutic Keratectomy (PTK) is applied 16. In simple words the excimer laser deletes the part of epithelium and serves for better errosion (epithelium cells healing).

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  2. Shortt AJ ABDS, Evans JR. Laser assisted in situ keratomileusis (LASIK) versus photorefractive keratectomy (PRK) for myopia. Cochrane Database Syst Rev. 2013;(1). doi:10.1002/14651858.CD005135.pub3
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  15. Sakellaris D, Balidis M, Gorou O, et al. Intracorneal Ring Segment Implantation in the Management of Keratoconus: An Evidence-Based Approach. Ophthalmol Ther. 2019;8(s1):5-14. doi:10.1007/s40123-019-00211-2
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