|Year : 2019 | Volume
| Issue : 3 | Page : 176-181
Small incision lenticule extraction – Current perspective
Gitansha Shreyas Sachdev, Shreyas Ramamurthy
Department of Cornea and Refractive Services, The Eye Foundation, Coimbatore, Tamil Nadu, India
|Date of Web Publication||31-Dec-2019|
Department of Cornea and Refractive Services, The Eye Foundation, Coimbatore, Tamil Nadu
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Sachdev GS, Ramamurthy S. Small incision lenticule extraction – Current perspective. Kerala J Ophthalmol 2019;31:176-81
Femtosecond lenticule extraction (FLEx) (also known as ReLEx ®, small incision lenticule extraction [SMILE], or FLEx) has emerged in recent years as an alternative paradigm to femtosecond laser in situ keratomileusis (FS-LASIK). The early outcomes of FLEx surgery, which is considered a step toward transition to SMILE, were first published in 2008, whereas the early outcomes of SMILE were first reported in 2011. SMILE became commercially available in 2012. ReLEx ® or SMILE requires only a femtosecond laser to perform the entire refractive procedure, and it has various clinical, practical, and scientific advantages over the more traditional and well-known two-laser solution of LASIK.
SMILE has become a preferred alternative to LASIK for myopic correction within Europe and Asia. This procedure is CE approved in Europe, and it got the U.S. Food and Drug Administration (FDA) approval in 2016. SMILE surgery is currently available only on a single femtosecond laser platform which is VisuMax (Carl Zeiss Meditec AG, Jena, Germany). This platform enables us to perform two different procedures: FLEx and SMILE.
| Selection Criteria|| |
Currently, ReLEx SMILE is available as a treatment modality for myopic correction of up to −12.50 D spherical equivalent, with a maximum astigmatic error of −5.0 D. The procedure is currently not available for hyperopic correction.
Patient selection criteria are similar to LASIK. The patient should be 18 years or older with a stable refraction for at least a year. Preoperative corneal topography to rule out forme fruste keratoconus, posterior keratoconus, and pellucid marginal degeneration is mandatory. A minimal corneal thickness of 490–500 μ and a residual stromal bed thickness of 290 microns are required in accordance with precautionary measures to reduce the incidence of post-LASIK ectasia. The procedure is contraindicated in patients with lenticular changes, glaucoma, or other preexisting ocular diseases. Retinal breaks or holes, if any, should be treated prior to the procedure.
| Visumax Laser System|| |
SMILE procedure is performed using the state of the art VisuMax femtosecond laser [Figure 1]. The VisuMax software (Carl Zeiss Meditec AG, Jena, Germany) calculates the thickness of lenticule required for refractive correction, and the femtosecond laser creates a refractive lenticule with a high degree of precision. The femtosecond laser system in the VisuMax laser system remains fixed.
The femtosecond laser is delivered through a curved contact glass attached to the laser system optical aperture. A green blinking light serves as the fixation target. The contact glass is available in three sizes S, M, and L.
An infrared illumination mode is incorporated into the laser system, which allows the surgeon to ascertain better the centration following initiation of suction. During the illumination mode, pupillary dilatation occurs. The surgeon can thus also verify that the pupil dilates around the intended center.
| Procedure|| |
The procedure is carried out under topical anesthesia. The patient's eyes are painted and draped using all aseptic precautions. A standard eye speculum is used to keep the eye open. There are three discrete phases involved in ReLEx SMILE.
Initial docking with precise centration
The joystick attached to the movable bed is used to align the patient's eye to the curved contact glass. A proper head position is achieved by tilting the patient's head medially to avoid nasal contact with the cone of the contact glass interface. The patient is asked to focus on the green blinking light and maintain fixation. Precise centration should be verified before corneal contact with the applanation interface.
Verification and maintenance of suction during femtosecond laser delivery
Following proper centration and adequate placement of the contact glass on the patient's eye, suction is initiated to hold the cornea against the contact glass interface. Femtosecond laser pulses with a typical pulse energy of 120–170 nJ are delivered with a pulse repetition rate of 500 KHz. Typical spot distance between each pulse is 2–5 μ. The femtosecond laser spots are fired in a spiral track, with either decreasing (posterior lenticule surface) or increasing spirals (anterior lenticule surface).
Femtosecond laser creates photodisruption by inducing a state of plasma that is generated by vaporization of target tissue. The force of cavitation bubble creates a cleavage plane within the corneal stroma as each gas bubble disrupts the corneal tissue at its respective position. Hence, the femtosecond laser is capable of creating cleavage planes at a predetermined depth within the corneal stroma with a high degree of precision.
The various tissue disruption planes created by the laser are as follows [Figure 2]:
|Figure 2: Cross section of the various cleavage planes created during femtosecond laser lenticule extraction|
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- Posterior lenticule surface (from periphery to center) followed by transition zone at the edge of the refractive zone (for spherocylindrical correction). The optical zone selected determines the diameter of the posterior lenticule surface
- Vertical edge incision along the perimeter of the lenticule
- Anterior lenticule surface (from center to periphery), which extends about 0.5–1 mm beyond the posterior lenticule surface
- Peripheral corneal incision for lenticule access and extraction. The incision is generally created superiorly or superotemporally to preserve the nasal and temporal nerve arcades  and to provide surgical convenience. The corneal incision varies from 250° to 300° in Chord length for FLEx and 30°–50° in cordal length for SMILE.
The entire laser procedure takes around 23 s irrespective of the refractive error to be corrected.
| Performing Manual Lenticule Extraction|| |
Following completion of the femtosecond laser (treatment mode), the suction automatically turns off. The patient's eye is repositioned under the microscope (observation mode).
Femtosecond lenticule extraction
A sharp-tipped instrument is used to open a small portion of the side-cut incision on the temporal side. A blunt spatula is used to separate the flap from the underlying lenticule, the lenticule is then peeled off and removed completely, and a drop of balanced salt solution is placed over the remaining stroma. The flap is then refloated back in place. A polyvinyl alcohol (PVA) spear is used to wick excess fluid from the edges of the flap, so as to allow surface tension forces to keep the flap in place. After 30 s, the speculum is removed, and antibiotic drops instilled. Both eyes can be treated at the same time.
Small incision lenticule extraction
A small sharp-tipped instrument is used to open a small portion of the side-cut incision. A small blunt spatula is inserted into the side-cut incision, and the anterior surface of the lenticule separated from the overlying cornea. A small sharp instrument is then used to enter into the cleavage plane on the posterior side of the lenticule to separate the edge of the lenticule. A blunt spatula is then inserted through this edge below the lenticule and used to separate the posterior part of the lenticule from the underlying stroma [Figure 3]. Once the lenticule was free from both surfaces, small micro forceps are inserted to grasp the lenticule and extract it from the corneal stroma. Once the lenticule is removed, the side-cut incision site is patted down with a slightly wet PVA spear. After 30 s, the speculum is removed. Both eyes can be treated at the same time.
|Figure 3: Surgical steps of small incision lenticule extraction procedure: Femto second laser assisted, (a) posterior tissue disruption plane or lenticule cut, (b) anterior tissue disruption plane or flap cut, (c) superior flap side-cut incision manual, (d) delineation of planes, (e) dissection of planes, (f) lenticule removal|
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Postoperatively, with both procedures, patients may be prescribed mild steroids and antibiotics for a week and artificial tear supplements for a period of 4–8 weeks after the procedure.
| Complications|| |
- Minor epithelial abrasions at incision site
- Small tears at the incision
- Difficulty in lenticule removal: During attempted lenticule delineation, incorrect tissue plane identification can result in primary separation of the posterior lenticule surface, resulting in its adherence to the stromal surface of the cap. In this situation, it is still possible to achieve lenticule separation, but it is more difficult
- Suction loss: Predisposing factors are longer duration of suction required in SMILE as compared to femto-assisted LASIK; loss of contact between glass interface and cornea due to sudden eye or head movement; ocular factors including a small palpebral aperture, loose corneal epithelium, excessive reflex tearing, and poor fixation; fluid entry through suction ports; or compressive forces against the contact glass resulting from intraocular gas-bubble transposition.
If the suction loss occurs after >10% of the posterior lenticule cut is complete, one has to convert to a FS-LASIK procedure. At all other stages, SMILE procedure can be repeated in the same sitting, with certain alterations in the parameters.
Symptoms of dry eye may be observed postoperatively; however, the occurrence is less as compared to that with conventional LASIK.
Another complication arises due to unintended abandonment of residual intrastromal lenticule fragments that may induce irregular astigmatism. Cases of epithelial ingrowth, microstriae, and interface inflammation have been reported. A case of unilateral corneal ectasia following small incision lenticule has been reported in a patient with a normal preoperative corneal topography.
| Advantages over Femtosecond Laser-Assisted In Situ Keratomileusis|| |
- Significantly shortened procedural time due to the use of a single-laser platform instead of the two-platform procedure
- Photo disruptive mechanism in SMILE, unlike ablative mechanism of excimer laser is independent of factors such as corneal hydration, temperature, atmospheric humidity, and depth of stromal ablation
- Increased refractive predictability over excimer laser particularly for higher refractive errors
- Significantly fewer total higher-order aberrations particularly spherical aberration
- With femtosecond laser, the peripheral loss of fluence is not a factor at all, and no compensation needs to be carried out. Hence, the amount of tissue required per diopter of treatment is smaller than that required with an excimer laser which compensates for the peripheral energy loss
- Reduced amount of energy is applied on to the cornea. Moreover, the heat generated by an excimer laser is in a relatively shorter period resulting in adverse effects on corneal healing
- Reduced number of corneal nerves is severed due to smaller flap diameter and side-cut incision, thereby reducing the incidence of postoperative dry eye
- No flap-related complications
- The small side-cut incision heals relatively faster causing less patient discomfort.
| Additional Uses of Smile Lenticule|| |
Tailored stromal expansion with refractive lenticule for crosslinking the ultrathin cornea – Sachdev technique
Traditional corneal collagen crosslinking (CXL) requires a minimal stromal thickness of 400 μ for the procedure to be carried out safely and effectively in case of keratoconus. However, patients with advanced forms of keratoconus often have thinner corneas, thus making the disease not amenable to traditional CXL.
We described a technique recently where using refractive lenticule, the thickness of the cornea is increased in the most physiological manner by adding stromal tissue whose biological and absorptive properties are the same as that of the cornea to be treated. Refractive lenticules of variable thickness can be obtained. Placement of the central lenticule over the apex of the cone enables us to augment the corneal thickness where required while sparing the remaining stroma to be crosslinked normally.
Femtosecond laser intrastromal lenticular implantation for hyperopia
Cryopreserved stromal lenticules obtained following SMILE for myopic correction are placed in a femtosecond-created intrastromal pocket for the treatment of hyperopia.
It may potentially be a safe and effective alternative to excimer laser ablation for hyperopia because of the low risks of regression, haze, flap-related complications, postoperative dry eye, and higher-order aberrations.
| Latest Advances: Circle Software|| |
A recent adaptation of SMILE software (Circle, Carl Zeiss Meditec AG) enables revision of the previously created cap by remodeling it into a larger diameter flap (with hinge) followed by excimer laser ablation.
In the circle procedure, the femtosecond laser is used to create (a) an incision plane encircling the original “cap” cut as a lamellar ring and (b) a side cut with hinge around the new incision plane and (c) a “junction cut” which allows the original “cap” and the new incision plane to be part of one larger surface [Figure 4].
|Figure 4: A lamellar ring is created at the same depth as the existing small incision lenticule extraction incision allowing the conversion of the small incision lenticule extraction cap into a hinged flap. The flap is then raised followed by excimer laser ablation|
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| Discussion|| |
An analysis of the results of SMILE procedure indicates safety, efficacy, and predictability of the FLEx procedure, with results for low-to-moderate myopia being more predictable than those for high myopia. Several groups have now published results of the procedure, including comparison of the results with results obtained for FS-LASIK., In general, the results seem to be similar to those obtained after FS-LASIK.,
Sekundo et al. published their initial results on ten eyes, which indicated that the procedure was effective, with over 90% of the eyes within one diopter of the intended correction. No eye lost two or more lines of best-corrected visual acuity at 6 months. The study also revealed a high level of patient satisfaction.
Shah et al. published results on SMILE with a single incision, where no flap was lifted, and the lenticule was extracted from a single small incision. In their study of over 50 eyes, they showed that at 6 months, 79% of the eyes had a uncorrected distance visual acuity (UDVA) of 20/25 or better. The study also revealed a high safety profile with only 5% of the eye losing a line of best-corrected visual acuity.
While initial studies revealed a significantly slower visual recovery after ReLEx ®, relative to FS-LASIK, Shah et al. demonstrated that the slower visual recovery was at least partly due to the laser scanning patterns. Subsequently, the laser manufacturer made scan pattern B into the standard method of treatment.
Hjordtal et al. studied the results of ReLEx ® in a relatively large sample (670 eyes of 335 patients) of patients with relatively high myopia (preoperative mean spherical equivalent of − 7.19 ± 1.03D). At 3 months, 80% and 94% of the eyes were within ± 0.5D and ± 1.0D of the intended correction, respectively. About 84% of the eyes had a UDVA of 20.25 or better. In this study, as many as 2.4% of the eyes lost two or more lines of corrected distance visual acuity (CDVA).
Vestergaard et al. published results of FLEx on a group (127 eyes) of moderate-to-high myopic eyes (preoperative mean spherical equivalent of −7.18 ± 1.57 D). In this study, refractive outcomes were similar to those above, with 73% of the eyes having a UDVA of 20/25 or better at 3 months.
Kamiya et al. attempted to treat a group which comprised mainly of low-to-moderate myopia (preoperative mean spherical equivalent of − 4.26 ± 1.39D) with femtosecond lenticule extraction. In this study, at 6 months, all eyes were within ± 0.5D.
Kunert et al. attempted to perform vector analysis for a group of eyes with myopic astigmatism to analyze the results on astigmatism. They concluded that in terms of safety, predictability, and efficacy, FLEx was similar to excimer lasers for the correction of astigmatism. There was a slight regression of the astigmatism correction (10%) with time. At 6 months, the mean error ratio was 0.68 ± 0.75 (standard deviation) and the mean correction ratio was 1.11 ± 0.69.
Kamiya et al. compared a group of eyes treated with FLEx with eyes treated with wavefront-guided (WFG) LASIK in terms of asphericity and wavefront aberrations. They found that in myopic eyes, FLEx induces significantly fewer ocular fourth-order aberrations than WFG-LASIK, possibly because it causes less oblation in the corneal shape, but there was no statistically significant difference in visual acuity or in the induction of third-order aberrations and total higher-order aberrations. It was suggested by Kimaya et al. that FLEx is essentially equivalent to WFG LASIK in terms of visual acuity and total higher-order aberration induction, although the characteristics of higher-order aberration induction are different.
The ReLEx ® procedure has been shown by several studies to induce fewer spherical aberrations, though most of the same studies also show that the procedure induces higher third-order aberrations when compared to FS-LASIK. As a sum total, in terms of higher-order aberrations, the results of ReLEx ® seem to be in line with those obtained after FS-LASIK.
In the near term, a review by the FDA of clinical data on SMILE in myopic astigmatism is anticipated, and ongoing research is exploring SMILE techniques for correction of hyperopia.
In conclusion, FLEx surgery offers significant clinical, practical, and economic benefits in comparison to conventional or FS-LASIK using the excimer laser.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]