Laser Eye Surgery

LASIK eye surgery and Femto-LASIK

09 Dec 2021

How is LASIK procedure and Femto-LASIK done?

  The different types of refractive surgery have a number of individual advantages and disadvantages. Types of stromal ablation (flap surgery where the base layer of the cornea is evaporated) are usually less painful and provide faster visual recovery than superficial ablation techniques.

 

Flap eye surgery uses a blade or femtosecond laser to create a thin flap on the surface of the cornea. The two main surgical techniques used are laser keratomileusis (LASIK) and photorefractive keratectomy (PRK) 1. The flap treatment is called LASer In situ Keratomileusis (LASIK). If an incision in the corneal stroma is created with a femtosecond laser instead of a mechanical blade of a microkeratome, then LASIK eye surgery becomes Femto-LASIK eye surgery. 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 a 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 ablation - evaporation of the corneal tissue with an ultraviolet laser (excimer laser vision correction). The laser beam vaporizes (ablates) the corneal tissue without damaging the stroma. At the end, the flap is returned to its original position and remains for healing in the postoperative period. The flap remains in place by natural fusion until the epithelium is completely healed.

Other types of laser vision correction with flap

  Another variant of LASIK is Sub-Bowman’s Keratomileusis (SBK), which differs from LASIK eye surgery only in that the flap is significantly thinner. Therefore, SBR is also called “LASIK with a thin flap” 2–4, more often just LASIK. Femtosecond laser flap procedures include femtosecond extraction of the lenticule Relex® FLEx 5–7. There are also names SUPER LASIK and SUPER Femto-LASIK. Here "SUPER" or “plus” means individual (personalized or customized) laser correction, which takes into account the irregularities of the eye, necessary to eliminate more complex refractive errors. In this case, it is not enough to correct spherical vision error (defocus) or astigmatism.

LASIK eye surgery and Femto-LASIK
Fig. 1. Schematic representation of LASIK eye surgery and Femto-LASIK procedures, in which the flap is created for laser correction of the cornea under this flap. Other procedures are ablation methods on the surface of the cornea such as (Trans) PRK or LASEK.

Pros and cons of Femto-LASIK and LASIK recovery. Is LASIK safe ?

  Desensitization of the cornea and dry eyes symptoms are the most common temporary complications after LASIK 8. These postoperative consequences occur for reasons such as damage of the corneal nerves and epithelial cells, impairment of the blinking reflex, and increased tear evaporation 9. Since Femto-LASIK creates a more accurate and predictable flap thickness than after mechanical microkeratomes (see description above), the likelihood of dry eye is reduced, and the recovery of corneal sensitivity is faster than after LASIK 10. Femtosecond-laser-LASIK-made flap creation is more predictable than mechanical microkeratome 11. On the other hand, some of the advantages of a mechanical blade microkeratome include reduced procedure duration 12 and cost.

A 2020 meta-analysis using a mechanical microkeratome for LASIK and a femtosecond laser for Femto-LASIK showed that dry eyes as an adverse event may be more common with a mechanical microkeratome (457 per 1000) than with a femtosecond laser (80 per 1000 people) 13. Temporary dry eyes and an inflammatory reaction after several days (diffuse lamellar keratitis) can be side effects after mechanical microkeratome and femtosecond laser, which are treated with tear solution or anti-inflammatory therapy for a week or more, respectively.

It is important to mention that one of the factors for successful laser surgery is the achieved laser refraction and visual acuity after 3-6 months. Dissatisfied patients may experience symptoms related to residual refraction. Improved patient-reported postoperative outcomes and high levels of satisfaction are attributed to modern lasers with improved ablation profiles, combined with experienced surgeons and rigorous preoperative examinations. This makes LASIK one of the safest and most effective procedures in laser surgery 14.

According to authoritative sources 15 16 17 18, the clinically significant refractive error in optometric practice ranges up to 0.75 D, and the spherical equivalent of refraction (SEQ) around ± 0.50 D is considered for the last follow-up visit to an ophthalmologist after a laser vision correction. However, the values of refractive errors can be assessed only taking into account the patient's visual acuity (for example, a row number according to the Snellen’s or Sivtsev’s table for the countries of the former Commonwealth of Independent States (CIS), etc.).

Ultimately, the success of LASIK depends on how satisfied patients are with their vision. A systematic review of the world literature of results after LASIK shows that more than 95% of patients are satisfied with their results 14 19, or have a best vision of more than 100% (e.g. visual acuity 20/20 according to the Snellen’s table or 10 lines according to the Sivtsev’s table for the CIS countries).

One of the other recent scientific studies (2020) presents the results of LASIK patients who have been operated since 2003 at the age of 17 to 20 years. The results show that more than 64% of patients retained more than one (e.g. 100%) visual acuity. Also, there were no cases of degenerative diseases of the eye of the cornea, ectasia (e.g. keratoconus) 20. The recent completion of a ten-year study (2008-2019) of PRK and Femto-LASIK operations proved their safety and efficacy in the long term. However, the Femto-LASIK operation showed a slight superiority in safety and efficacy over the PRK 21.

Femto-LASIK or LASIK ? PRK vs LASIK

  Another of the meta-analyzes showed that there were no statistically significant differences in efficacy, outcomes, or safety between Femto-LASIK and other treatments used. However, Femto-LASIK is better in predictability than any other type of eye microsurgery. Femto-LASIK tends to lead over other refractive eye surgeries in the so-called “SUCRA” rating and predictability score 22. LASIK has been shown to provide faster visual recovery than PRK and is less painful, although one year visual results after surgery are comparable 1. In another type of laser surgery Epi-LASIK, the thickness of the flap corresponds to the thickness of the epithelium (less than 60 µm) when using a mechanical microkeratome 23. Here, the epithelium is folded back after laser ablation. Since Epi-LASIK does not cut the corneal stroma, it avoids the complications associated with superficial laser vision correction including epithelial ingrowth, infections, etc. 24

High results are published after Femto-LASIK 25 and remain unchanged after 3, 5 and more years 26 27. Thus, in one study with a five-year follow-up none of the patients had postoperative keratectasia, dry eye symptoms, infectious keratitis or other complications. Other follow-ups of patients over 10 years till 2020 have demonstrated excellent safety and efficacy indicators for both types of laser surgery Femto-LASIK and PRK, where Femto-LASIK had higher efficacy rates than PRK 21.

Femto-LASIK or lenticule extraction ?

  According to one of the latest international studies, the overall satisfaction between LASIK and SMILE lenticule extraction did not differ between groups throughout the study 28. Targeted analysis during the first postoperative week showed that compared with SMILE the recovery of corneal sensitivity after LASIK was faster, and the number of symptoms reported by the patient was less. However, already one month after surgery, there were no such differences between groups in visual symptoms and satisfaction. The results of one review indicate that the purported biomechanical advantages of a small incision after lenticule extraction over LASIK cannot be demonstrated in existing studies 6.

Is it possible to do a second vision correction after LASIK?

  First of all, it is necessary to undergo a diagnostic examination as well as to gain eye consultation at the clinic in order to answer this question. One of the latest studies showed that after a repeated LASIK procedure for myopia and hyperopia in the period 2012-2018 (originally treated 1997-2012) 88% and 74% of patients acquired visual acuity of 20/20 or more according to the Snellen’s table (10 lines according to the Sivtsev’s table) 29, respectively.
 
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Источники

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