Small-Incision Lenticule Extraction



Small-Incision Lenticule Extraction


D. Rex Hamilton



Cornea refractive surgery began in the late 1970s and early 1980s with radial keratotomy (RK) and moved into the laser arena with the introduction of excimer laser (193 nm) ablation in the 1990s. The Food and Drug Administration (FDA) approval of photorefractive keratectomy (PRK) in 1995 represented the first excimer laser technique for vision correction. The PRK procedure afforded many advantages over the incisional RK procedure: higher predictability, improved safety, and better longterm stability. While the technique is still used today, recovery time is quite long with PRK as the technique requires removal of the corneal epithelium, which must subsequently heal and stabilize. Patients can expect about a month before their vision stabilizes. Laser in situ keratomileusis (LASIK) was developed in the early 1990s with the FDA approval occurring in 1999. The stromal flap created as the first step of this procedure affords overnight recovery with outstanding vision on postoperative day 1. The second step of the procedure is the same as the PRK excimer ablation. The fast recovery, coupled with predictability, safety, and long-term stability equivalent to PRK, led to the ascension of LASIK as the procedure of choice for laser vision correction. To date, more than 40 million LASIK procedures have been performed worldwide.

Nevertheless, the penetration of laser vision correction amongst myopic patients throughout the world remains very low. In the United States, for example, in 2012, approximately 700,000 eyes were treated, representing only 1.2% of the population pool.1 Despite the outstanding safety record of LASIK, fear remains the major barrier to entry for the majority of potential vision correction candidates. These fears revolve around the existence of the flap, concerns over complications, and simply the concept of having laser surgery on one’s eyes.

The Small-Incision Lenticule Extraction (SMILE) procedure was developed in the late 2000s and utilizes a single femtosecond laser to create a lenticule-shaped piece of corneal stromal tissue, customized to a patient’s refractive correction. The laser also creates a small surface incision through which the lenticule is extracted without the creation of a flap. The SMILE procedure was FDA approved for spherical myopic treatments in October 2016. In March 2018, the procedure was approved for myopic astigmatic treatments. At the time of publication, SMILE is gaining popularity throughout the world with more than 4 million procedures
performed. SMILE may be an attractive alternative to LASIK for those patients fearful of laser surgery: no flap, no sound (excimer laser makes sound), no smell (odor of vaporized corneal tissue from excimer ablation), no pressure (negligible pressure associated with the VisuMax femtosecond laser), and shorter procedure time relative to LASIK. In addition, because there is no flap and the incision is so small, there are virtually no postoperative restrictions on lifestyle. There is also strong evidence that the severity and duration of dry eye symptoms is less with SMILE than LASIK.2,3




INDICATIONS



Key Indications



  • Myopia and myopic astigmatism: −1.00D to −10.00D in spherical power with −0.75D to −3.00D in astigmatic power with manifest spherical equivalent of no more than −10.00D (FDA-approved ranges)


CONTRAINDICATIONS



Key Contraindications



  • Keratoconus or other corneal ectatic disorders: Normal corneal tomography (and regular epithelial thickness maps if available) is essential for clearing patients for any corneal refractive surgery.


  • Previous herpes simplex keratitis: Contraindicated if active disease within 1 year. If more than 1 year, check corneal sensitivity and use oral antiviral medication prophylaxis before and after surgery.


  • Active autoimmune disease: Lupus, rheumatoid arthritis, Sjögren’s syndrome


  • Severe aqueous deficient dry eye


  • Pregnancy or nursing: Vision can fluctuate during pregnancy. In addition, antibiotic eye drops are used for several days after surgery while steroid eye drops are used for 7 to 10 days following surgery. There may be some systemic absorption of these medications that could affect the fetus and/or be present in breast milk.


  • Current isotretinoin (Accutane) use


  • Unrealistic expectations




Relative Contraindications



  • Cataract: Corneal refractive surgery is a good option for patients into their mid-50s with mild, nonprogressive cataract due to risk of retinal complications from refractive lens exchange. Progressive myopia and/or astigmatism changes on manifest that do not match corneal astigmatism usually indicate need for cataract surgery


  • Glaucoma: If intraocular pressure (IOP) is well controlled with minimal visual field loss, SMILE surgery can be safely performed. It is important to take note of change in corneal thickness as this has an impact on IOP measurements: measured IOP will be lower than actual IOP following myopic corneal refractive surgery.4 Intraoperative IOP elevation associated with VisuMax laser docking is the lowest of any femtosecond laser and is not a contraindication for patients with mild glaucoma.


  • Optic nerve head drusen or crowded optic nerve: PRK is not associated with increased IOP as there is no suction ring required and it has been considered a safer option relative to LASIK in this setting. However, the VisuMax affords the lowest increase in IOP of any femtosecond laser or microkeratome at levels that are negligible.5,6


  • Epithelial or anterior stromal dystrophies: PRK may be a more appropriate treatment as it has the therapeutic effect of removing opacities and/or increasing adherence of corneal epithelium in basement membrane disease. LASIK is contraindicated in granular corneal dystrophy type 2 (Avellino corneal dystrophy). There are no reports of SMILE in Avellino corneal dystrophy, but corneal opacities are a relative contraindication (see in the following section).


  • Fuchsendothelial dystrophy: Endothelial cell count should be performed before surgery in patients with corneal guttata. Low endothelial cell count places the patient at risk of poor settling of the tissues on either side of the SMILE interface, leading to increased corneal back scatter and poorquality vision.


  • Corneal opacity: Any opacity can alter the efficacy of femtosecond laser cutting. Because the laser energies for SMILE are significantly lower than those used for a LASIK flap, care must be taken to identify any significant opacity that will fall within the lenticule zone as treatment can lead to an uncut area, making dissection and lenticule removal challenging.


  • Depression or anxiety conditions if not stabilized: This is an important relative contraindication for all refractive surgical procedures.


  • Uncontrolled Diabetes: These patients have a higher risk of infection and slower healing response. Of the corneal refractive procedures, SMILE has the smallest incision and, thus, has the quickest healing time, minimizing the infection risk relative to LASIK or PRK.



INFORMED CONSENT CONSIDERATIONS



Key Informed Consent Adverse Events



  • Infection


  • Glare, halos, starburst from irregular astigmatism (decentered treatment, retained lenticule fragment)


  • Suction break requiring conversion to LASIK, PRK, or postponement of surgery


  • Residual myopia and/or astigmatism


  • Consecutive hyperopia and/or astigmatism


  • Corneal ectasia

Informed consent should include a description of the procedure in plain language. For example, “A laser will be used to create a lens-shaped piece of tissue (lenticule) within the cornea (front window of the eye). This lenticule, customized to your prescription, will then be removed by your surgeon through a small surface incision, also created by the laser. By removing this lenticule, your cornea will be flatter and rounder, thus improving your vision without glasses or contact lenses.”

Potential alternative treatments, such as LASIK, PRK, phakic intraocular lens implantation, glasses, and contact lenses, should be listed.

A note from the counseling physician should be included and phrased similar to, “I have counseled this patient as to the nature of the proposed procedure, the attendant risks involved, and the expected results.” The patient’s and doctor’s name should be printed and signed with the date as well as with a witness (typically a staff member).


PREOPERATIVE CARE



Key Preoperative Considerations



  • Medical history including diagnoses and medications


  • Ophthalmic history including diagnoses, drops, and previous surgery


  • Contact lens history (e.g., type of lens, wearing and cleaning habits, date of last use)


  • Corneal tomography including back surface imaging


  • Ocular dominance


  • Pupils


  • Cover testing at distance and near, with and without glasses, and ocular motility testing


  • Confrontational fields testing


  • Monocular and binocular uncorrected distance visual acuity (UDVA)




Key Preoperative Considerations



  • Monocular and binocular uncorrected near visual acuity


  • Lensometry of current spectacles and corrected distance visual acuity (CDVA)


  • Distance-corrected near vision for myopic patients over 40


  • Distance manifest refraction


  • Slit-lamp examination including fluorescein staining, taking note of location, size, and depth of any corneal opacities


  • Tear break-up time


  • Goldmann applanation tonometry


  • Cycloplegic refraction and CDVA (after dilation with 1.0% tropicamide)


  • Dilated fundus examination using slit-lamp and binocular indirect ophthalmoscopy

As with all refractive surgical techniques, the manifest refraction is the cornerstone to a successful SMILE outcome. Consistency in refraction is critical and, therefore, it is desirable to have the same refractionist performing the measurements on all patients. Variations in refraction technique can account for differences in endpoints, which can lead to variability in outcomes. Binocular balance is important to reduce the possibility of postoperative anisometropia. Patients with spectacle- or contact lens-corrected vision rarely need to deal with anisometropia: the power of the spectacle lens or contact lens can be easily adjusted to bring both eyes to a plano endpoint. Since spectacle-/contact lens-corrected patients are not used to it, anisometropia following refractive surgery can be quite noticeable, particularly in the immediate postoperative period. Cycloplegic refraction is particularly important in younger patients who are susceptible to over-minusing. Care should be taken to identify amblyopia in patients with significant anisometropia, asymmetric astigmatism, and a history of strabismus. It is important to counsel amblyopic patients so that they understand the laser procedure cannot fix the “wiring of the eye to the brain”, which limits the ultimate visual acuity.


PROCEDURE AND SETTINGS

image Video 8.1 shows the key steps of the SMILE procedure. There are two components:



  • 1. Laser treatment


  • 2. Lenticule dissection and removal

The VisuMax laser has two microscopes (laser and operating) one for each of these steps (Figure 8.1).








Laser Treatment


Preparation of the Eye

Surgeon’s may choose to premedicate the patient with an oral anxiolytic agent such as alprazolam 0.5 mg. One drop of proparacaine is placed in each eye of the patient in the preoperative area. Once the patient is positioned on the laser bed, a second drop of proparacaine is placed and the patient is asked if there was any stinging. While it is obviously important to anesthetize the eye, it is also important not to use too much proparacaine as this loosens the corneal epithelium. Because there is some pressure placed on the posterior edge of the surface incision during lenticule dissection, epithelial sloughing can occur. If this occurs, there is some risk of introducing epithelium into the interface, which can lead to epithelial proliferation (much like epithelial ingrowth with LASIK).

The skin surrounding the eye is prepped with betadine swabs. A drop of antibiotic is placed in the eye. The lashes are draped and a lid speculum is placed.

The corneal surface is wiped clean using a Weck-Cel sponge soaked in sterile balanced salt solution (BSS). It is very important to confirm there is no material (e.g., mucous) on the cornea or the patient interface prior to docking. Any material trapped between the interface and the cornea can block the laser shots, resulting in “black spots,” which represent tissue that has not been cut by the laser, leading to inability to dissect the lenticule in that area.


Centration

In LASIK flap creation, the femtosecond laser is not performing the refractive correction. Thus, centration of the flap is important but not as critical as with SMILE where the laser
is performing the refractive correction. Consequently, care must be taken to properly center the suction ring on the visual axis. Certain diagnostic systems (e.g., Galilei G4 Tomography system, Ziemer USA) take pictures of the iris and pupil with the position of the visual axis within the pupil identified with crosshairs (Figure 8.2). This picture can be printed and taken to the laser suite and used as a reference during docking. By noting the position of the visual axis relative to the pupil through the laser microscope, the surgeon can ensure the eye is appropriately centered during the docking procedure.







Docking

The VisuMax patient interface features a curved applanation glass that only touches the cornea (Figure 8.3). The pressure associated with docking is minimal. It feels like putting a contact lens in the eye of the patient. With no conjunctival touch, there is no subconjunctival hemorrhage with the VisuMax that is commonly seen with other femtosecond lasers that grab onto the conjunctiva.


Laser Treatment

Figure 8.4 shows the tissue planes that define the SMILE lenticule and the opening incision. The posterior aspect of the lenticule is cut first with the laser spiraling in from the periphery. The optical zone is 6.5 mm in the United States. This zone can be decreased to 6.0 mm in sphere-only treatments to remove less tissue. The depth of the posterior aspect of the lenticule is defined by the amount of refractive correction. Next, the lenticule side cut is created, followed by the cap cut (anterior

aspect of lenticule and cap zone). The cap cut proceeds from the center and spirals outward. The diameter of the cap is 7.5 mm in the United States or 7.0 mm if the optical zone is decreased to 6.0 mm in a sphere-only treatment. The depth of the cap is set at 120 microns in the United States. Finally, the opening incision is made superiorly, centered on the 12 o’clock meridian, at the peripheral aspect of the cap cut. This cut up to the corneal surface is typically 60° wide in the United States but can be increased to 90°.