Residency · Residency · Ophthalmology

Refractive Surgery: LASIK, PRK, SMILE, and ICL

Overview

Refractive surgery encompasses a family of procedures designed to correct myopia, hyperopia, and astigmatism with the goal of reducing or eliminating dependence on spectacles and contact lenses. The two broad categories are corneal-based procedures, which reshape the cornea itself (LASIK, PRK, and SMILE), and lens-based procedures, which either add a phakic intraocular lens (ICL) or replace the crystalline lens entirely (refractive lens exchange). Regardless of which technique is chosen, rigorous candidacy screening is the foundation of safe refractive surgery -- the surgeon must confirm a stable refraction, adequate corneal thickness, and the absence of ectasia risk before proceeding.

Preoperative Evaluation

A thorough preoperative evaluation begins with documenting refractive stability over at least 12 months, confirmed by two consistent refractions. Although FDA approval permits surgery at age 18, many surgeons prefer to wait until age 21 or older to ensure refractive stability. Corneal topography and tomography, typically with the Pentacam, are essential to rule out forme fruste keratoconus and pellucid marginal degeneration. Central corneal thickness is measured with pachymetry, and the surgeon calculates whether the planned ablation will leave a minimum residual stromal bed (RSB) of 250 to 300 micrometers after LASIK -- most surgeons target at least 300 micrometers. The percent tissue altered (PTA), which integrates flap thickness and ablation depth relative to total corneal thickness, should remain below 40% to minimize ectasia risk.

Scotopic pupil size is measured because large pupils may increase the risk of dysphotopsias, particularly glare and halos, when combined with small ablation zones. The tear film is assessed and any dry eye treated preoperatively, as an unstable ocular surface degrades both measurements and surgical outcomes. The anterior segment is examined to rule out cataract and narrow angles, and fundoscopy evaluates for lattice degeneration or retinal holes that should be treated before refractive surgery. Contact lenses must be discontinued before evaluation -- soft lenses for one to two weeks, and rigid gas permeable or toric lenses for three to four weeks -- to allow the cornea to return to its natural shape. Surgery is deferred during pregnancy and nursing because hormonal changes can alter refraction and corneal contour.

LASIK (Laser In Situ Keratomileusis)

Technique

LASIK involves creating a thin corneal flap, lifting it to expose the stromal bed, reshaping that bed with an excimer laser, and replacing the flap. The flap is created either mechanically with a microkeratome or, more commonly today, with a femtosecond laser. Typical flap thickness is 90 to 120 micrometers (approximately 110 micrometers with a femtosecond laser), and the flap diameter is 8.5 to 9.5 mm with a superior or nasal hinge. Once the flap is lifted, the excimer laser (193 nm argon-fluoride) performs the ablation. For myopia, tissue is removed centrally, flattening the cornea and reducing its refractive power. For hyperopia, tissue is removed in a peripheral annular pattern, steepening the central cornea. For astigmatism, an elliptical ablation pattern removes more tissue along the steep meridian. After ablation, the flap is replaced, where it adheres through endothelial pump action and stromal dehydration without the need for sutures.

Ablation Profiles

Several ablation profiles are available. Conventional or standard ablations apply a simple spherocylindrical correction based on the manifest refraction. Wavefront-guided ablations use a customized pattern derived from aberrometry data (typically from a Hartmann-Shack sensor), correcting both lower-order and higher-order aberrations for a potentially sharper result. Wavefront-optimized ablations do not correct pre-existing higher-order aberrations but compensate for the spherical aberration that corneal procedures tend to induce by adjusting the peripheral ablation profile. Topography-guided ablations (such as Contoura) base the treatment on corneal topographic data rather than wavefront data, regularizing the corneal shape to improve optical quality.

Treatment Range

LASIK is FDA-approved for myopia up to -12.0 D, though many surgeons limit treatment to -8 to -10 D to maintain adequate corneal biomechanics. Hyperopia can be treated up to +6.0 D, though results are less predictable than for myopia. Astigmatism up to 6.0 D can be corrected.

Outcomes

LASIK outcomes are excellent: more than 95% of patients achieve a refraction within 1.0 D of the target, and over 90% achieve 20/20 uncorrected visual acuity. Enhancement rates are approximately 5 to 10%. Stability is excellent for low-to-moderate myopia, though slight regression can occur with higher corrections.

Complications

Post-LASIK ectasia is the most feared complication -- a progressive thinning and steepening of the cornea that can be visually devastating. Risk factors include a thin residual stromal bed, high PTA, and undetected subclinical keratoconus. Flap complications encompass a range of problems including buttonholes, free caps, incomplete flaps, flap striae, epithelial ingrowth beneath the flap, and flap dislocation, which can occur even years after surgery from blunt trauma. Dry eye is very common due to corneal nerve transection during flap creation; it is usually transient, resolving over three to six months. Diffuse lamellar keratitis (DLK), known as the "Sands of Sahara," is a sterile interface inflammation treated with topical corticosteroids. Infectious keratitis is rare (approximately 0.02%) but when it occurs, atypical mycobacteria are a particular concern under the flap. Regression, a gradual return toward the original refraction, is more common with higher corrections. Night vision disturbances including glare, halos, and starbursts can be problematic, especially in patients with large scotopic pupils and small optical zones. Central toxic keratopathy (CTK) is a rare condition involving central corneal flattening from thermal damage.

PRK (Photorefractive Keratectomy)

Technique

PRK is a surface ablation procedure that does not create a flap. The corneal epithelium is removed -- mechanically, with dilute alcohol, with a brush, or by the excimer laser itself in the transepithelial (transPRK) approach. The excimer laser then ablates Bowman layer and the anterior stroma directly. Mitomycin C at 0.02% is applied to the ablation bed for 20 to 60 seconds to prevent corneal haze by inhibiting keratocyte activation and fibrosis. A bandage contact lens is placed at the conclusion of the procedure and remains until the epithelium regenerates, typically in three to five days.

Advantages Over LASIK

Because PRK creates no flap, it eliminates all flap-related complications entirely. More corneal tissue is preserved since no flap thickness is subtracted from the available stroma, making PRK better suited for thinner corneas. The biomechanical profile is safer, with a lower risk of ectasia. PRK is the preferred procedure for patients at risk of ocular trauma, including military personnel, contact sport athletes, and law enforcement officers, because there is no flap to dislodge. It can treat moderate refractive errors in corneas too thin for safe LASIK.

Disadvantages

The primary drawback of PRK is slower visual recovery, with best vision typically taking one to three months to achieve compared with one to two days after LASIK. Postoperative pain is greater during the three-to-five-day epithelial healing period. There is a risk of corneal haze, which is mitigated but not entirely eliminated by mitomycin C, particularly with high corrections. PRK is not ideal for high hyperopia, and regression is more common than with LASIK at higher correction levels.

Treatment Range

For myopia, PRK treats a similar range as LASIK, typically up to -8.0 D, though higher corrections carry an increased risk of haze. Results for hyperopia are less predictable.

Variants

LASEK involves creating an epithelial flap with dilute alcohol (20% for 20 to 30 seconds), which is repositioned after ablation; this technique has been largely abandoned. Epi-LASIK uses a mechanical epithelial separator to create an epithelial sheet but has also fallen out of favor. TransPRK (SmartSurfACE) is a single-step no-touch technique in which the excimer laser removes the epithelium and performs the refractive ablation in one continuous treatment, simplifying the procedure.

SMILE (Small Incision Lenticule Extraction)

Technique

SMILE uses a femtosecond laser (VisuMax, Zeiss) to create a disc-shaped refractive lenticule within the corneal stroma. The shape and thickness of the lenticule determine the refractive correction. The surgeon then manually dissects the lenticule from the surrounding tissue and extracts it through a small peripheral incision of 2 to 4 mm. No excimer laser is used, and no flap is created. The overlying corneal cap, typically 120 to 140 micrometers thick, remains intact.

Advantages

SMILE is a flapless procedure, eliminating flap-related complications. The small incision transects fewer corneal nerves than the LASIK flap, potentially resulting in less postoperative dry eye. Biomechanically, SMILE preserves the strongest anterior stromal lamellae better than LASIK because tissue is removed from deeper within the stroma rather than from the anterior surface beneath a flap. This preservation theoretically reduces the risk of ectasia, though this advantage is based primarily on biomechanical modeling rather than long-term comparative clinical data.

Disadvantages

SMILE currently has FDA approval only for myopia and myopic astigmatism; it cannot treat hyperopia. No wavefront-guided or topography-guided customization is available -- the treatment profile is based solely on the manifest refraction. Enhancements after SMILE are more complex than after LASIK because there is no flap to re-lift; a touch-up typically requires PRK over the cap or conversion to LASIK. The original VisuMax platform lacked an active eye tracker during lenticule creation, making centration dependent on patient fixation. There is a meaningful learning curve for the manual lenticule dissection and extraction, and complications specific to the technique include difficult extraction, incomplete lenticule removal, and cap perforation. The treatment range for astigmatism is more limited than with LASIK.

SMILE Pro (VISUMAX 800)

The newer SMILE Pro platform features a faster femtosecond laser with improved centration capability including cyclotorsion compensation, enhanced suction and docking, and potentially better visual outcomes compared with the original SMILE system.

Treatment Range

SMILE is FDA-approved for myopia from -1.0 to -10.0 D and for astigmatism up to -5.0 D in combination with myopia. Hyperopia treatment is not yet FDA-approved but is under investigation using a technique that involves peripheral lenticule extraction.

Outcomes

SMILE achieves outcomes comparable to LASIK for myopia, with more than 95% of patients within 1.0 D of the target and over 85% achieving 20/20 uncorrected visual acuity. The safety profile is similar, and several comparative studies have demonstrated a lower incidence of dry eye after SMILE than after LASIK.

Corneal Refractive Procedures Comparison

ICL (Implantable Collamer Lens / Phakic IOL)

Concept

The implantable collamer lens is a posterior chamber phakic intraocular lens that is placed in the ciliary sulcus, sitting behind the iris and in front of the natural crystalline lens. The Visian ICL (STAAR Surgical) is the most widely used phakic IOL and is made of Collamer, a biocompatible copolymer of collagen and hydroxyethylmethacrylate. The current generation -- the EVO ICL and EVO+ ICL -- features a central port called the KS-AquaPORT that allows aqueous humor to flow through the lens, eliminating the need for a preoperative peripheral iridotomy.

Indications

The ICL is primarily indicated for high myopia in the range of -6.0 to -20.0 D, which exceeds the safe treatment range for corneal laser procedures. It is also appropriate for patients with thin corneas that are unsuitable for ablation, patients with stable mild keratoconus (since the ICL does not alter corneal biomechanics), and patients with significant dry eye (since the ICL does not worsen dry eye the way corneal procedures do). Young patients with high myopia who are not candidates for refractive lens exchange represent another important indication.

Surgical Technique

The procedure is performed through a small clear corneal incision of 2.8 to 3.2 mm. The ICL is folded, injected into the anterior chamber, and tucked behind the iris into the ciliary sulcus, where it self-unfolds in the posterior chamber. No sutures are required because the wound is self-sealing. Thorough removal of the ophthalmic viscosurgical device at the conclusion of surgery is critical to prevent a postoperative IOP spike.

Sizing

Proper ICL sizing is essential for safe outcomes. Size selection is based on the white-to-white (WTW) corneal diameter measurement and anterior chamber depth. Sulcus-to-sulcus measurement by UBM or AS-OCT is more accurate but is not universally available. Incorrect sizing leads to vault problems: an oversized ICL results in excessive vault, risking angle closure, pupillary block, and pigment dispersion, while an undersized ICL produces inadequate vault, allowing the ICL to contact the crystalline lens and cause anterior subcapsular cataract formation. The ideal vault, measured by AS-OCT as the distance between the posterior ICL surface and the anterior lens capsule, is 250 to 750 micrometers.

Outcomes

ICL outcomes are excellent, with more than 98% of patients achieving a refraction within 1.0 D of the target and over 95% attaining 20/20 or better uncorrected visual acuity. Optical quality is superior to LASIK for high myopia because the ICL provides a larger effective optical zone and does not alter corneal asphericity. The procedure is also reversible -- the ICL can be explanted if needed.

Complications

Cataract formation occurs in approximately 1 to 2% of patients with the EVO ICL, a rate lower than with earlier models that lacked a central port. Inadequate vault is the most common underlying cause. Acute angle closure and pupillary block are very rare with the current central port design, though older ICL models without the port required a preoperative LPI. Pigment dispersion can occur from ICL rubbing against the posterior iris surface. Progressive endothelial cell loss is a concern if the ICL is too large or anteriorly displaced, necessitating monitoring with specular microscopy. Endophthalmitis is extremely rare. Glaucoma can result from steroid response to postoperative drops or from vault-related mechanisms.

EVO ICL Central Port (AquaPORT)

The AquaPORT is a 360-micrometer central hole in the ICL optic that permits continuous aqueous flow from the posterior chamber to the anterior chamber. This innovation eliminates the need for a laser peripheral iridotomy, reduces the risk of pupillary block, and decreases cataract risk by improving aqueous circulation around the natural crystalline lens.

<image>Comparative cross-sectional diagrams of four refractive procedures. Panel A: LASIK — showing the corneal flap (hinged, lifted), excimer laser ablation of the stromal bed (central tissue removal for myopia), and flap replacement. Panel B: PRK — showing epithelial removal, direct excimer ablation of Bowman layer/anterior stroma, and bandage contact lens placement. Panel C: SMILE — showing the femtosecond-created intrastromal lenticule within the cornea and its extraction through a small peripheral incision (no flap). Panel D: ICL — showing the posterior chamber phakic IOL positioned in the ciliary sulcus behind the iris and in front of the crystalline lens with the central AquaPORT visible. Each panel labeled with the tissue layers and the key structural change.</image>

<image>Anterior segment OCT image of an ICL showing proper vault measurement. The ICL is positioned behind the iris in the ciliary sulcus with the anterior chamber, iris, ICL, and crystalline lens clearly labeled. A measurement line shows the vault (distance between the posterior ICL surface and the anterior crystalline lens capsule) measuring 450 um (ideal range). A second panel shows an over-vaulted ICL (>1000 um) with the ICL pushing the iris forward and narrowing the angle. A third panel shows an under-vaulted ICL (<100 um) with the ICL nearly touching the crystalline lens. Include annotations explaining the clinical significance of each vault scenario.</image>

<image>Excimer laser ablation profiles for different refractive errors. Three diagrams showing the corneal cross-section before and after ablation: (1) Myopia correction — central tissue removal flattening the cornea, reducing its refractive power; (2) Hyperopia correction — peripheral annular tissue removal steepening the central cornea; (3) Astigmatism correction — elliptical ablation removing more tissue along the steep meridian. Each diagram shows the original corneal curvature as a dotted line and the post-ablation curvature as a solid line, with the ablation zone depth indicated.</image>

Key Clinical Pearls

Ectasia screening is the most important aspect of preoperative evaluation -- Pentacam BAD-D, posterior elevation, and corneal biomechanics (Corvis ST TBI) should all be assessed before any corneal refractive procedure. LASIK offers the fastest visual recovery, but PRK is the safer choice for thin corneas, trauma-prone patients, and cases with borderline topography. SMILE preserves more anterior stromal biomechanical strength than LASIK and causes less dry eye, but it lacks the customization options of wavefront-guided and topography-guided LASIK. The ICL is the procedure of choice for high myopia beyond -8 D and for thin corneas, providing superior optical quality to corneal procedures in this range. Post-LASIK ectasia is preventable -- surgery should never be performed on patients with subclinical keratoconus or an inadequate residual stromal bed (the absolute minimum is 250 micrometers, with 300 micrometers or more preferred). Mitomycin C application during PRK is essential for corrections greater than -4 D to prevent corneal haze. ICL vault must be monitored with annual AS-OCT to ensure it remains within the 250 to 750 micrometer range. Enhancement after SMILE is challenging, so accurate planning on the initial procedure is critical; if enhancement is needed, PRK can be performed over the SMILE cap.

References

• Reinstein DZ, Archer TJ, Gobbe M. The history of LASIK. J Refract Surg. 2012;28(4):291-298.
• Sekundo W, et al. SMILE vs. LASIK: systematic review and meta-analysis. J Cataract Refract Surg. 2014;40(1):163-174.
• Packer M. The Implantable Collamer Lens with a central port: review of the literature. Clin Ophthalmol. 2018;12:2427-2438.
• Randleman JB, et al. Risk assessment for ectasia after corneal refractive surgery. Ophthalmology. 2008;115(1):37-50.
• AAO BCSC Section 13: Refractive Surgery. 2023-2024.

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