Refractive Surgery — LASIK/PRK/SMILE

Overview

LASIK, PRK, and SMILE are the three laser-based corneal refractive procedures in routine use. All three correct refractive error the same way: an excimer or femtosecond laser removes or repositions tissue in the corneal stroma, changing the cornea's curvature and therefore its refractive power. The procedures differ in how the laser reaches the stroma: through a hinged flap, through a removed epithelial layer, or through a small stromal incision. That access route drives most of the practical differences between them, including healing time, postoperative discomfort, and which patients are good candidates for which procedure.

LASIK

LASIK, laser-assisted in situ keratomileusis, corrects refractive error by reshaping the corneal stroma. A femtosecond laser first cuts a thin, hinged flap in the anterior cornea and lifts it to expose the stroma underneath. An excimer laser then ablates stromal tissue in a pattern calculated to correct the patient's refractive error, and the flap is repositioned. Modern femtosecond flap creation typically targets a thickness of roughly 100 to 120 micrometers, thinner than the mechanical microkeratome flaps used in older LASIK technique. The exact target is planned per patient, based on corneal pachymetry and the refractive correction needed.

Because the flap is repositioned rather than left to re-epithelialize from scratch, LASIK's chief practical advantage over PRK is faster visual recovery and less postoperative discomfort. That same flap, though, is the source of LASIK-specific long-term risks: dislocation from later trauma, and a lifelong, if uncommon, risk of other flap-related complications that flapless procedures simply don't share.

Lifting that flap also cuts across the dense subbasal and anterior stromal nerve plexus, reducing corneal sensation and the reflex drive to tear production. Dry eye is accordingly the most common early LASIK complaint. Sensation and symptoms usually recover over several months to a year as the nerves regrow, which is also why SMILE, a flapless procedure, carries a lower early dry eye burden for an equivalent correction.

PRK

Photorefractive keratectomy skips the flap entirely. Instead, the corneal epithelium is removed outright, mechanically with a rotating brush, with dilute alcohol to loosen epithelial adhesion, or trans-epithelially with the excimer laser itself, to expose the stroma. That stroma is then ablated the same way as in LASIK, and the epithelium regenerates on its own over the following several days.

That epithelial healing process is the main practical trade-off against LASIK. PRK brings more postoperative discomfort and a slower visual recovery while the epithelium regrows, but it avoids creating a permanent structural flap. For that reason PRK is often preferred in patients whose cornea is thin or otherwise poorly suited to flap creation, and in those whose occupation or lifestyle carries a higher risk of eye trauma that could dislodge a LASIK flap.

SMILE

Small incision lenticule extraction is the newest of the three, and it avoids both a surface flap and epithelial removal. A femtosecond laser creates a lens-shaped disc of tissue, the lenticule, entirely within the stroma, sized and shaped to correct the patient's refractive error, along with a small peripheral incision. The surgeon separates and removes the lenticule through that incision without ever lifting a flap or disturbing the epithelium. The refractive correction still comes from the shape of tissue removed, the same underlying principle as LASIK and PRK, just reached a different way.

Candidacy & Contraindications

All three procedures share the same requirement: a cornea with a regular, predictable shape to plan an ablation or lenticule pattern against. Keratoconus is the classic reason a patient is excluded from any of them. It produces irregular astigmatism that spectacles and standard sphero-cylindrical correction cannot fully correct, and a cornea that is already irregular and biomechanically weakened is a poor candidate for further stromal tissue removal, which weakens it further still and can accelerate progression toward corneal ectasia.

For a patient with keratoconus or other irregular astigmatism who still needs functional vision, rigid gas-permeable (RGP) contact lenses are the standard alternative. The rigid lens vaults over the irregular corneal surface, creating a smooth, regular tear-film interface in front of it and correcting the irregular astigmatism in a way no spectacle lens or corneal laser procedure can.

Screening for ectasia risk goes further than simply ruling out overt keratoconus. Preoperative risk-scoring systems, most established by Randleman and colleagues, weigh several factors together: topographic pattern, the calculated residual stromal bed (RSB) after ablation, patient age, preoperative corneal thickness, and the degree of attempted myopic correction. Abnormal topography carries the heaviest weight of any single factor in these systems. An eye with a completely normal RSB can still land in a high-risk category if its topography is irregular, which is why a fixed RSB cutoff such as 250 micrometers was never meant to stand alone as a screening rule.

Keratoconus is not the only relative contraindication worth screening for. Autoimmune and collagen vascular disease, rheumatoid arthritis, lupus, and especially Sjögren syndrome among them, count as a relative contraindication rather than an absolute one. Active disease, ocular surface involvement, or a multidrug immunosuppressive regimen rules a patient out. A candidate instead needs disease quiescent for at least six months, a normal ocular surface exam, rheumatology clearance, and a documented negative Sjögren workup, since that one diagnosis predicts post-LASIK dry eye more reliably than the rest of the group. Pregnancy and breastfeeding exclude a patient too, for a related reason. Rising estrogen softens corneal biomechanics and can steepen the cornea through the second and third trimesters, so refraction stays unstable until several months after delivery and weaning.

Related Topics

Continue exploring Cataract & Refractive Surgery: