Key Features
- •
SMILE preserves the corneal integrity better than laser-assisted in situ keratomileusis (LASIK) with its flap-free design eliminating risk of microfolds and flap dislocation.
- •
Perioperative complications include epithelial abrasions, suction loss during femtosecond laser application, and minor tears of the lenticule or incision edges during lenticule removal.
- •
Corneal haze is the most common early postoperative complication.
- •
Early dry eye symptoms can occur, although it spares the subbasal nerve density better than flap-based LASIK, with faster sensitivity recovery.
- •
Enhancements can be performed with the CIRCLE procedure, where the cap is converted to a flap and followed by excimer ablation of the stromal bed or with surface ablation.
Introduction
Over the past decade, femtosecond laser-assisted in situ keratomileusis (FS-LASIK) has become a well-established technique for correcting myopic refractive errors. Femtosecond laser technology allows a more reproducible corneal flap of predetermined thickness, compared with microkeratome. Femtosecond lenticule extraction (FLEX) was later introduced as an alternative to FS-LASIK after development of the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany). FLEX still required a corneal flap to enter the corneal stroma, as with LASIK. However, FLEX allowed corneal tissue removal by creating a stromal lenticule instead of laser ablation. It proved beneficial to further improve on this and develop a laser refractive technique that did not require use of a corneal flap, minimizing the trauma on the corneal surface and removing the risk of microfolds or flap dislocation.
Small incision lenticule extraction (SMILE) was introduced as a next-generation stromal lenticule refractive procedure, further optimizing FLEX. As a flap-free technique, an instrastromal lenticule was cut by a femtosecond laser and removed through a small corneal incision. With use of only a small incision (2–4 mm in width) for removal of the lenticule, the corneal integrity was left almost intact.
Treatment Range for SMILE
SMILE with VisuMax is approved outside the United States for myopia up to −10.00 diopters (D) in sphere and up to 5.00 D of cylindrical component. SMILE for myopia and astigmatism with VisuMax was Conformité Européenne (CE) marked in 2009. In 2016, the US Food and Drug Administration (FDA) approved VisuMax for myopia from −1.00 D and up to −8.00 D in sphere and astigmatism of 0.50 D or less. Patients with more than −10.00 D in sphere still remain a surgical challenge. For now, VisuMax does not have an approved algorithm for treating hyperopia. Initial studies have tested an improved lenticule shape that removes more tissue in the midperiphery than in the center. Although the technique is promising, it still needs refinement to be on par with the state-of-the-art excimer laser treatments for low to moderate hyperopia.
Patient Evaluation
Patients referred to SMILE should preferably be 18 years or older with a stable refraction for more than 2 years. Standard preoperative evaluation includes uncorrected and corrected distance visual acuity, pupil size, tonometry, pachymetry, tomography, slit-lamp examination, and dilated funduscopy. Tomographic measurements of the corneal front and back curvature should be carefully examined to exclude irregular tomographic patterns or subclinical keratoconus. The patient should not wear contact lenses 2 days (soft lenses) or 2 weeks (hard lenses) before tomographic evaluation. Contraindications for SMILE include corneal scars, corneal dystrophies, and severe dry eyes. Very anxious patients may not be candidates for SMILE, due to the increased risk of perioperative suction loss. LASIK is still the chosen technique to achieve monovision in presbyopic patients, as they often require surgical enhancements, which are done more easily after a flap-based procedure. SMILE may be preferred over LASIK in patients who are involved in contact sports or perform jobs with increased risk of eye trauma, because there is no risk of traumatic flap displacement or dislodgement. Studies suggest that SMILE may also be the best option for moderate to high myopic correction due to higher predictability than after LASIK.
Surgical Procedure
SMILE can be performed under topical anesthesia. Bilateral sequential treatment is usually performed. The patient is positioned supine under the femtosecond laser, and the untreated eye is covered and taped to prevent corneal dehydration. Two drops of 0.8% oxybuprocaine tetrachloride are applied 5 minutes before operation and again just before the lid speculum is installed. A contact glass interface that consists of a peripheral ring of small suction ports is attached to the femtosecond laser. The curved contact glass ensures precise contact to the corneal surface during laser application and is available in various sizes (S, M, L, and type KP). The size of the contact glass should correspond to the white-to-white distance of the patient. A small (S)-sized contact glass is generally preferred, especially in Asian patients with small white-to-white distances. The patient is positioned under the laser head. It is important to dry the ocular surface, especially the inferior fornix, with a sponge to remove any excess tear fluid or ocular surface secretions. This can be performed by placing a disposable sponge in the inferior fornix while the patient is being aligned to the interface cone. Some surgeons advocate the use of a speculum with a suction device to remove excess tear fluid. Once the patient is under the laser head, the bed is elevated to allow contact with the interface and the anterior corneal surface. Before contact the patient is asked to fixate on a green light from the femtosecond laser for accurate centration. The bed then is elevated further to allow complete contact with the corneal surface. The degree of contact with the ocular surface can be ascertained by the spread of the tear fluid meniscus. In most cases when the tear fluid has spread to three quarters of the width of the cornea, suction is applied. Following suction, the patient is still able to fixate on the green light due to a low intraocular pressure rise. Good centration is important because no built-in tracker exists. Patients who have high astigmatism treatments should be marked in the horizontal axis on the slit lamp before the laser procedure. Once under the interface and suction applied, the interface can be rotated to ensure that the marked horizontal axis on the eye is in alignment with that of the horizontal meridian through the right eyepiece in the microscope of the laser, to counter any cyclotorsion.
Femtosecond Laser Application
SMILE is performed with four sequential laser cuts to create a corneal lenticule and a tunnel incision : (1) a posterior lenticule surface cut in a spiral-in pattern (refractive cut), (2) a vertical cut along the circumference of the lenticule, (3) an anterior lenticule surface cut in a spiral-out pattern (corneal cap), and (4) a superiorly placed 2–4 mm tunnel vertical incision cut that gives access to the lenticule from the corneal surface ( Fig. 3.5.1 ). We prefer to rotate the incision to the superior temporal side in the right eye and superior nasal in the left eye to ease the access for a right-handed surgeon and to avoid any superior pannus and intraoperative bleeding, which maybe be common in patients who wear contact lenses.
The spiral-in pattern of the posterior lenticule cut maximizes the time the patient can focus on the fixation target and minimizes the risk of suction loss due to eye movements. Cutting the posterior surface first ensures that the gas bubbles do not block the laser application of the anterior surface cut. The spiral-in and spiral-out laser firing sequence has also been shown to cause minimal disruption to the collagen lamellae. The laser refractive application takes approximately 20–25 seconds depending on the laser settings. Suction is released automatically after treatment.
The following laser settings can be altered by the surgeon and to determine the lenticule thickness and treatment zone. For the lenticule: lenticule diameter, minimum lenticule thickness, and lenticule side cut angle. For the corneal cap: cap thickness, cap diameter, incision position, incision width, and incision side cut angle. The laser settings are determined by the preoperative refractive status, together with the preference and experience of the surgeon. Surgeons starting with SMILE should perform cases with lenticule thickness of above 70 µm (minimum lenticule thickness of 15 µm) because it will be easier to perform the removal. Surgeons with more experience with the procedure may perform treatments of −1 D.
Lenticule Removal
Lenticule removal includes several key stages ( Fig. 3.5.2 ) ( ). The eye can be fixated with a pair of forceps to avoid sudden eye movements during the intrastromal maneuvers. The incision is opened with a Sinskey hook. The two lenticule planes are identified in each corner of the incision. The remaining tissue bridges of the upper surface are broken with a blunt spatula and the lenticule is separated from the cap. The blunt spatula should be gently maneuvered over the lenticule with no major resistance from the remaining tissue bridges. A gentle sweeping movement is advocated ensuring that the dissection passes over the complete area of the anterior surface of the lenticule. The same maneuver is performed on the posterior surface of the lenticule. The lenticule then can be removed through the incision using a pair of forceps. Some surgeons flush the intrastromal pocket with balanced salt solution to remove remaining debris and minimize the risk of epithelial ingrowth. However, the fluid may possibly induce small fluid pockets in the interface and can delay the immediate visual recovery. After lenticule removal, the cap can be massaged with a sponge to remove residual tension folds to the periphery, to minimize irregularities and microfolds on the visual axis when correcting highly myopic patients. One drop of fluoroquinolone and corticosteriod are then applied at the end of the procedure.
Postoperative Management
A postoperative regimen may include a combination of topical dexamethasone and antibiotics, typically 4 times a day for 2 weeks, then tapered to twice a day for 2 weeks. The patient should use lubricating drops hourly for the first week to ease the discomfort in the postoperative period. Daily activities can be performed, but the patient should avoid swimming pools and extensive eye rubbing during the first 2 weeks. Slit-lamp examination should be performed 1 day, 1 week, and 1 and 3 months after the operation. We normally assess refractive outcome with formal refraction at 1 and 3 months.
Femtosecond Laser Application
SMILE is performed with four sequential laser cuts to create a corneal lenticule and a tunnel incision : (1) a posterior lenticule surface cut in a spiral-in pattern (refractive cut), (2) a vertical cut along the circumference of the lenticule, (3) an anterior lenticule surface cut in a spiral-out pattern (corneal cap), and (4) a superiorly placed 2–4 mm tunnel vertical incision cut that gives access to the lenticule from the corneal surface ( Fig. 3.5.1 ). We prefer to rotate the incision to the superior temporal side in the right eye and superior nasal in the left eye to ease the access for a right-handed surgeon and to avoid any superior pannus and intraoperative bleeding, which maybe be common in patients who wear contact lenses.
The spiral-in pattern of the posterior lenticule cut maximizes the time the patient can focus on the fixation target and minimizes the risk of suction loss due to eye movements. Cutting the posterior surface first ensures that the gas bubbles do not block the laser application of the anterior surface cut. The spiral-in and spiral-out laser firing sequence has also been shown to cause minimal disruption to the collagen lamellae. The laser refractive application takes approximately 20–25 seconds depending on the laser settings. Suction is released automatically after treatment.
The following laser settings can be altered by the surgeon and to determine the lenticule thickness and treatment zone. For the lenticule: lenticule diameter, minimum lenticule thickness, and lenticule side cut angle. For the corneal cap: cap thickness, cap diameter, incision position, incision width, and incision side cut angle. The laser settings are determined by the preoperative refractive status, together with the preference and experience of the surgeon. Surgeons starting with SMILE should perform cases with lenticule thickness of above 70 µm (minimum lenticule thickness of 15 µm) because it will be easier to perform the removal. Surgeons with more experience with the procedure may perform treatments of −1 D.
Lenticule Removal
Lenticule removal includes several key stages ( Fig. 3.5.2 ) ( ). The eye can be fixated with a pair of forceps to avoid sudden eye movements during the intrastromal maneuvers. The incision is opened with a Sinskey hook. The two lenticule planes are identified in each corner of the incision. The remaining tissue bridges of the upper surface are broken with a blunt spatula and the lenticule is separated from the cap. The blunt spatula should be gently maneuvered over the lenticule with no major resistance from the remaining tissue bridges. A gentle sweeping movement is advocated ensuring that the dissection passes over the complete area of the anterior surface of the lenticule. The same maneuver is performed on the posterior surface of the lenticule. The lenticule then can be removed through the incision using a pair of forceps. Some surgeons flush the intrastromal pocket with balanced salt solution to remove remaining debris and minimize the risk of epithelial ingrowth. However, the fluid may possibly induce small fluid pockets in the interface and can delay the immediate visual recovery. After lenticule removal, the cap can be massaged with a sponge to remove residual tension folds to the periphery, to minimize irregularities and microfolds on the visual axis when correcting highly myopic patients. One drop of fluoroquinolone and corticosteriod are then applied at the end of the procedure.
Postoperative Management
A postoperative regimen may include a combination of topical dexamethasone and antibiotics, typically 4 times a day for 2 weeks, then tapered to twice a day for 2 weeks. The patient should use lubricating drops hourly for the first week to ease the discomfort in the postoperative period. Daily activities can be performed, but the patient should avoid swimming pools and extensive eye rubbing during the first 2 weeks. Slit-lamp examination should be performed 1 day, 1 week, and 1 and 3 months after the operation. We normally assess refractive outcome with formal refraction at 1 and 3 months.
Complications
SMILE has an advantage over LASIK; flap dislocation and detachment are not seen because of the flap-free approach. Nevertheless, specific complications related to the lenticule cutting and removal do occur. Before operation, the surgeon should be aware of how to avoid and manage the most frequent peri- and postoperative complications ( Table 3.5.1 ).
| Perioperative Complications | Postoperative Complications |
|---|---|
| Peripheral (5.2%) and central (0.2%) epithelial abrasions | Corneal haze/nontransparency (5.6%) |
| Minor and major tears (1.8%) | Dry corneal surface, day 1 (3.2%) |
| Lenticule extraction difficulties (1.6%) | Epithelial ingrowth (0.5%) |
| Suction loss (1%) | Irregular corneal topography (0.5%) |
| Fiber in interface (0.2%) | Visually insignificant microstriae (0.4%) |
| Remaining lenticule remnant (0.04%) | Diffuse lamellar keratitis (0.2%) |
| Monocular ghost images (0.2%) |
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
