What is the smile technique?
The techniques referred to by ReLex – Flex – Smile variants are based on the semi-manual "bulk" extraction of a volume of corneal tissue (duckweed) for the correction of myopia. This duckweed is pre-cut to the femtosecond laser. "Flex" corresponds to an acronym for Femtosecond Laser Assisted duckweed extraction: This variant is today neglected for the "Smile" (acronym for "Small Incision duckweed extraction").
Also read: Smile or LASIK?
Introduced about ten years ago, these techniques have not supplanted the LASIK technique because of certain limitations, but can be proposed as an alternative to it. The number of smiles achieved this day remains much lower than that of LASIK.
The principles put into play smile are based on an approach that was implemented Before the onset of LASIK and was to achieve a «» Keratomileusis in situ To correct myopia, i.e. a technique where a duckweed of corneal stroma was removed within the cornea, after cutting a superficial flap.
With the keratomileusis in situ, the flap and the Duckweed were cut by a manual or automated microkeratome (technique proposed by Ruiz at the end of the years 80 see: history of LASIK). Due to the relatively imprecise results (centering, and variations in the thickness of the lenticule cut under the flap), this technique has been supplanted by LASIK using an excimer laser to the photoablation of the lenticule under the flap.
The introduction of the femtosecond laser in corneal surgery helped resurrect the principle of the keratomileusis in situ, thanks to the extra precision provided by this technology.
The ReLEX (Refractive duckweed Extraction) techniques overlap The Variatnes Flex and smile, where the femtosecond laser (Zeiss sumas) performs the cutting of the duckweed to be removed in the thickness of the cornea. For the correction of myopia, this duckweed to the appearance of a lunula. The Flex and smile variants differ in the way this duckweed is removed: with or without flap.
The variant FLex (Femtosecond laser assisted Extraction duckweed) is to cut a flap at the top, always by the delivery of femtoseconds laser impacts (aggrandissant the cleavage plane corresponding to the most superficial part of the duckweed). The technique shares with LASIK the realization of a superficial flap: the laser Femtosconde (precision: 5 to 10 microns) is used to cut a duckweed that is removed in bulk, while the Excimer laser "vaporizes" the cornea layer by layer ( Accuracy: approx. 0.25 microns. In Relex, the femtosecond laser is used as well. This variant is very little used because it consists in making a LASIK, but by depriving itself of the precision of the excimer laser for the corneal sculpture.
The variant SMILE (Small incision duckweed extraction) Consists of a limited superficial lateral cut, in order to remove the duckweed through an incision of some millimetres (there is no flap cut out strictly speaking). The lack of cutting of flap offers a psychological advantage, as for PKR, but unlike this one, the intrastromal extraction of the duckweed refractive, without removing the epithelium allows patients not to feel a sensation of Discomfort or irritation marked.
The Excimer laser is therefore not used for the correction of myopia with these techniques (which can lead to reducing the cost to LASIK); and This is paradoxically one of the weak points of the smile technique.
The performance of the Relex technique is more "manual". than that of LASIK, especially in the Smile variant, which oBlige to dissect with the spatula surfaces anterior and posterior of the lenticule, before you extract it by a small lateral incision through a microphone clip, without that the lenticule disintegrates (the edge of the lenticule is at least 10 microns by default, regardless of the fixed programmed, to avoid that it crumbles during maneuvers of extraction). Thus, the realization of the Smile imposes ablation of a disc of tissue of about 15 microns of thickness... for reasons easier gripping but not for sight correction. If the edge of the lens was prafaitement end (like the one that vaporizes the excimer laser), it is desagregerait during the manoeuvres of extraction.
Benefits advanced for this FLEX technique are (for the Smile variant only) one any superficial corneal cutting with preservation of corneal innervation. The disadvantages in the need of a perfect centering (and keeps it during laser cutting), a technique of manual dissection effective (in order to leave no fragments of lens in the stromal interface and/or generate superficial microplis). The Visual effects associated with the coalescence of two surfaces (increased light diffusion?) femtosecond laser cut are not yet well documented. However,. the Visual recovery after SMILE is significantly longer than in LASIK; in the postoperative period, the presence of a 'fuzzy' Visual light is reported by the majority of patients. This vagueness fades in 1 to 6 months, and is probably linked to inflammation and necessary regulation related to the "stress" induced by the mechanical dissection of the faces of the lenses, and its extraction (see surgical videos of the techniques). The Visual results after this transitional phase of recovery are comparable to those of LASIK. Dissection of the lenticule, manual and sometimes laborious, imposes a sometimes excessive distention of the fabric the more anterior stromal, with risk of at least temporary impairment of the Visual quality (optical diffusion).
Theoretical studies suggest that the biomechanical impairment of the cornea would be less important in smile than in LASIK, and that paradoxically, it would be even better to achieve a smile than a PKR in case of "fine" or "atypical" cornea. These considerations, based on biomechanical models more than actual observations, are theoretical and must be carefully adapted to the clinic. Theoretical predictions are derived from models where the corneas are "healthy", but in practice, there is no evidence that the creation of a smile on a "fragile cornea" is less invasive than a PKR (the latter having demonstrated its effectiveness and safety For low and medium myopia). It is even possible that the biomechanical invasivity of the smile is more important than that of the PKR for fragile corneas because the removal of the duckweed is carried out in a deeper plane (it remains to be determined what carries it between the disadvantage of a wall Later Résiudel and the advantage provided by the respect of the very anterior stroma). case of Ectasia were reported after realization of a smile.
The major drawback of the Smile technique is theUnable to retreat in case of dollars or sur-correction, less with the same technique. Indeed, it is not possible to remove a second lens of the cornea; then realize a PKR, who run a risk of superficial inflammation potentially important in this context (the PKR issued on the deep corneal stroma exposed to the risk of haze). You can also try to cut out in full a flap above the interface created by the Smile, and retreat to the excimer laser: we get in a situation equivalent to that of a LASIK.
Initial clinical results show a close precision than LASIK for the correction of myopia, but one significantly slower Visual recovery (up to a few months). The absence of shallow circular cut (Smile) would be an interesting alternative in patients with dry eye pre existing, but this is not demonstrated a frank in the postoperative follow-up studies. case risk of severe eye injury, SMILE technique can also have an interest with respect to LASIK for high myopia surgery. The use of the techniques Smile seems less interesting for the small misuse because the fineness of the lenticule has remove makes its more difficult extraction, and the Visual recovery is longer than in LASIK (and in PKR).
Above all, LASIK and the PKR allow a higher refractive accuracy when there is a astigmatism Pronounced, and FLEX techniques (Relex and smile) do not provide compensation for the cyclorotation of the eyeball, which may have negative effects as part of the correction of astigmatism in case of marked Cyclorotation. So far it is not possible to make custom duckweed (guided by the wavefront), and the correction of the hyperopia, while the smile for myopia has been proposed for about ten years, seems difficult due to the geometry of the duckweed to be removed (topologically analogous to a torus, with a zero thickness in the center: the correction Of the hyperopia in smile requires to increase the volume removed with a constant thickness of 30 microns. Clinical studies are underway to evaluate the results of the smile for the correction of the hyperopia. The duckweed removed, theoretically of zero thickness in the center (in LASIK or PKR), must in smile be equipped with a thickness of 30 microns. This does not bode well for a good ability to effectively arch the cornea, as experience shows that the removal of a duckweed of constant thickness tends irreparably to flatten the cornea.
Even if their supporters put forward some theoretical advantages, the number of procedures in FLEX-smile remains relatively confidential to that of LASIK, whose precision of correction and the speed of obtaining the final visual result remain advantageous in the context of elective surgery. The rate of complications associated with the realization of flap in LASIK is very low, and the benefit of the absence of flap in smile does not seem to have really altered the lines in terms of postoperative satisfaction (recovery Longer in Smile) and visual (no difference in term between smile and LASIK). The eyes where there is a marked dry eye and where the risk of secondary displacement of a LASIK flap are very important are potentially good candidates for surgery by smile. Most surgeons who routinely perform the smile technique continue to perform LASIK and suface techniques (PKR). Some even abandoned the smile, judging that any patient operated in smile could benefit from LASIK, without the result being lower, but with fewer complications and faster recovery.