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Corneal Lenticule Extraction

21 Nov 2021
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Corneal Lenticule Extraction as Femtolaser Eye Surgery

The introduction of the femtosecond laser into refractive surgery has made it possible to create corneal flaps in a more precise, stable and safer way 1. Since their first clinical use in 2001 in the LASIK flap, femtosecond lasers have consistently taken the place of the dominant flap technology worldwide, i.e. LASIK, due to their reproducibility, safety, accuracy and versatility 2. What about Corneal Lenticule Extraction ?


The process of cutting the cornea with a femtosecond laser by ultrashort pulses (tens of 1/1015 seconds) is also known as photodisruption or photomechanical (photoacoustic) damage 3. Hot plasma is created inside the cornea at the absorption point of the femtosecond laser pulse. Subsequently, this plasma propagates (expands) in the form of a supersonic wave forming a cavitation bubble 4 5. In other words, extremely short pulses lead to surgical separation of the corneal tissue with minimal concomitant damage. Many bubbles, one after the other, create a cut.


Although the origin of this technique goes back to 1995 with the first patents 6, for a long time only one platform was available to perform this kind of procedure 7. Since 2019, other companies have started to enter the market with similar proposals. Corneal lenticular extraction procedures have several commercial names depending on the manufacturer. This is the first femtosecond extraction of the leticule (corneal layer) using a flap, the commercial name given by the manufacturer was ReLEx FLEx® 8. Later, ReLEx SMILE® 8 9 was called Small Incision (cut) Lenticule Extraction. There are now several operations from other manufacturers such as SmartSight® 10, CLEAR 11 (Corneal Lenticule Extraction for Advanced Refraction). Of course, it makes sense to believe that this number of operations will increase.



What is corneal lenticular extraction ?

Small incision lenticule extraction or SMILE is the first corneal lenticular extraction technology that has existed for almost 15 years 12. In theory, it is a flapless refractive corneal operation or vision correction using only a femtosecond laser system 13. Refractive correction is achieved by removing 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) docks on (contacts) an 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 lenticle is removed with tweezers.


Following the introduction in 2007 of the VisuMax® femtosecond laser 14, one of the first operations to remove the lenticule to correct myopia by flap formation (FLEX) was performed. This procedure was followed by the SMILE procedure (removal of the lenticule through a small incision). Because of eliminating the need for a flap on the surface of the cornea, SMILE damages fewer corneal nerves and has the potential to preserve more original corneal biomechanics than (Femto-)LASIK 15. In this technique the lenticule is selected through a lateral incision of 2.0 to 4.0 mm, which is much shorter than the LASIK flap15.



How long does the lenticule extraction surgery last ?

The eye contact with the PI during the surgery is typically less than 5 minutes. Patient lies on a patient
bed. To ensure adequate suction prior to and throughout the surgery the surgeon
shall optimize the eye position along the X and Y axes as the eye is brought closer to the laser contact
element. The duration of this step for lenticule surgery depends on the experience of the surgeon. The necessary suction level on the patient’s eye is achieved in several seconds. Then the start of the lenticule’s creation shall be confirmed.


The max duration of applied vacuum to the patient interface is usually less than 2 minutes. Application
of lower IntraOcular Presuure decreases the probability of intraocular pressure complications.
Thus, it shall be kept as short as possible. The laser portion  takes from 30 seconds to one minute that is roughly twice the duration to complete the flap cuts during Femto-LASIK 16. The duration of the photodisruption procedure shall almost not depend on the refractive error and remain the same if other lenticule and cap parameters remain unchanged. The entire procedure, including the removal of the corneal tissue, generally takes 10 to 20 minutes in total and is strictly dependent on the experience of the surgeon.



See video here.




Benefits of corneal lenticule extraction


    
 	
  • It is a “flapless” (no flap) and painless procedure, which means that there is no risk of loss of vision quality for the patient due to complications with the flap with (Femto-)LASIK 16.
    • 
 	
  • The lifestyle or profession of patients sometimes contraindicates LASIK due to the flap. The type of surgery with extraction of the lenticule has a minimum of restrictions and maximum postoperative comfort. Corneal extraction provides a fast way back to sport. Since surgery is minimal here, patients can return to sports almost immediately after surgery. The risk of complications due to the potential bacterial invasion (for swimmers) is also close to zero.
    • 
 	
  • Makeup after laser vision correction? Because of the small incision and consecutive less chance of infection in the cornea, makeup can be applied soon after corneal lenticule extraction. The access of microbes or bacteria to the structure of the cornea is minimized as much as possible 16.
    • 
 	
  • A minimum incision size of a few millimeters for lenticule extraction ensures corneal integrity and biomechanical stability 17. Review results suggest that SMILE has less impact on corneal viscoelastic properties. However, the putative biomechanical advantage in the cornea after SMILE has not been demonstrated by non-contact air tonometry (intraocular pressure measurement) in nine studies 17.
    • 
 	
  • Vision restoration is almost the same as in LASIK and much faster than after PRK. After a few days almost all patients have high preoperative visual acuity.
    • 
 	
  • Sufficient patient comfort on postoperative days compared to other surgical methods due to minimal corneal response 16.
    • 
 	
  • Since the corneal nerves are partially responsible for the production of tears and lacrimal secretion is minimally impaired, the likelihood of postoperative dryness is less than 16.
    • 
 	
  • When working with a femtosecond laser, there is no evaporation of the cornea, and therefore no smell. This benefit can be appreciated by odor-sensitive patients 16.
    • 




Before SMILE entered the market, PRK was the only solution for military, firefighters, police officers and contact sportsmen, which requires no work or sports for some time after surgery.






Clinical results after corneal lenticule extraction vs risks of laser eye surgery

In a 2019 study, results from 622 eyes demonstrated that SMILE provides effective, predictable, stable and safe vision correction in patients with myopia and astigmatism 18. 6 months after the surgery, 95% of eyes had visual acuity greater than 1 (20/20 according to the Snellen chart or 10 lines according to Sivtsev's table). It is noteworthy that the deterioration in visual acuity after surgery compared with preoperative Corrected Distance Visual Acuity (CDVA) in most eyes was from 20/16 to 20/20, i.e. deterioration is not less than one. At 6 months, all SMILE-treated eyes with astigmatism were within ± 1.00 D, and 92% of the eyes were within ± 0.50 D (achieved refractive target versus target).


From March 2018 to July 2020 a comprehensive search was carried out in international scientific libraries and several non-English language databases. This meta-analysis included twelve studies involving 1400 eyes from 766 patients, of whom 748 underwent SMILE. This analysis showed that postoperative distance visual acuity is 20/20 with 95% probability in the long term 19.


SMILE results show remarkable stability over 3 years 1,15,20–24 and more than 88% of the refractive index results of patients after SMILE remain within ± 0.5 D 22. In theory, stronger vision correction (deeper laser intervention in cornea) is possible with SMILE without the additional risk of ectasia (deformity, bulging of the cornea), but again in theory. However, there are at least several reports in the literature about such cases caused after SMILE 25 26 27. Results of the meta-analysis collected worldwide from different countries, including Germany, Switzerland, France, China, Turkey, India, etc. 1,15,20–23,28, state that temporary dry eye after SMILE can occur 23 29. However, SMILE is relatively safe, which is also associated with fewer dry eye symptoms than after (Femto-)LASIK 23 24.


Therefore, like after LASIK, patients scheduled for lenticule removal should be informed about the risk of dry eye symptom and potential visual symptoms. For example, ghosting, fogging or focusing problems can temporarily impair the quality of vision during the first postoperative week. However, there are no further significant differences in the overall satisfaction score between corneal lenticule extraction and other types of surgery 30.


Extraction of the cornea may offer a much faster recovery of vision than PRK 31. Some study also reported fewer dry eyes in the postoperative period and faster recovery of corneal sensitivity using the corneal extraction technique compared to LASIK 32, as well as better regeneration of the corneal nerves in the postoperative period compared to LASIK 33.



Prospects for corneal lenticule extraction

Studies show that the procedure for corneal lenticule extraction can be reversed. Donor lenticules (layers of the cornea) have been successfully implanted to eliminate myopia, treat presbyopia, hyperopia and keratoconus. The use of stromal lenticules is also described for therapeutic purposes, in which the lenticule is transplanted under the flap in cases of excessive removal of stromal tissue by LASIK 34. Implantation of a refractive lenticule (RL) can be an effective alternative to Presbyopic Corneal Inlays (PCI) 34, providing better diffusion nutrients through the cornea. Synthetic PCI is often associated with subsequent opacification or even fibrosis (scarring). After RL implantation for the treatment of hyperopia, the corneal shape remains more natural 34.

 

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