Reshaping a cornea permanently, in minutes, sounds dramatic — and it is. This article looks at what LASIK, PRK, and SMILE actually do to the eye, how they differ, and who genuinely is, and isn't, a good candidate.
Glasses and contact lenses sit on or in front of the eye, correcting light on its way in. Refractive surgery takes a different approach entirely: it permanently reshapes the cornea itself, so the eye's own focusing power is changed at the source, and no external lens is needed at all.
Nearly all common refractive surgery works on the same underlying principle established in earlier articles in this series: the cornea does the majority of the eye's focusing work, so precisely reshaping its curvature changes exactly where light converges. A laser removes microscopic amounts of corneal tissue in a carefully calculated pattern, flattening the cornea to correct myopia, steepening it to correct hyperopia, or smoothing an uneven curve to correct astigmatism. The change is permanent because the reshaped tissue does not grow back to its original form.
LASIK (Laser-Assisted In Situ Keratomileusis) is the most widely performed refractive procedure worldwide. A surgeon first creates a thin, hinged flap in the outer corneal layer, using either a specialised blade or a femtosecond laser. The flap is folded back, an excimer laser reshapes the exposed corneal tissue beneath according to the patient's exact prescription, and the flap is then folded back into place, where it adheres naturally without stitches. Because the flap protects the surface during healing, LASIK typically causes relatively little pain and offers rapid visual recovery, often within a day or two.
The three core steps of LASIK: creating and lifting a thin corneal flap, reshaping the tissue beneath with a laser, then repositioning the flap.
PRK (Photorefractive Keratectomy) predates LASIK and works on the same reshaping principle, but without creating a flap. Instead, the thin outer layer of cells covering the cornea, called the epithelium, is removed entirely, the laser reshapes the exposed surface directly, and the epithelium is left to regrow naturally over several days, protected by a temporary bandage contact lens. Because there is no flap to heal, PRK avoids certain flap-specific complications altogether, but the recovery is noticeably slower and more uncomfortable than LASIK, with clear vision typically taking days to weeks rather than hours to days.
PRK is often specifically recommended over LASIK for people with thinner corneas, since it does not require enough tissue depth to safely cut a flap, and for people in professions with a higher risk of eye trauma, such as certain military roles or contact sports, since there is no flap that could later be dislodged by a direct blow to the eye.
SMILE (Small Incision Lenticule Extraction) is a more recent technique that reshapes the cornea without creating a large flap at all. A femtosecond laser creates a small, precisely shaped disc of tissue, called a lenticule, within the cornea, then the surgeon removes it through a single small incision just a few millimetres wide. Because SMILE avoids a large flap entirely, it may offer some advantages around corneal biomechanical strength and post-procedure dry eye compared to LASIK, though it is currently primarily approved and studied for correcting myopia and myopic astigmatism rather than the full range of refractive errors LASIK addresses.
| Procedure | Corneal Access | Typical Recovery | Best Suited For |
|---|---|---|---|
| LASIK | Hinged flap, folded back and replaced | Fast, often clear vision within 1-2 days | Most refractive errors, adequate corneal thickness |
| PRK | No flap; surface epithelium removed and regrown | Slower, days to weeks, more initial discomfort | Thinner corneas, higher-impact professions/sports |
| SMILE | Small incision only, no large flap | Fast, generally comparable to LASIK | Myopia and myopic astigmatism specifically |
Refractive surgery is not appropriate for everyone, and a thorough pre-operative assessment exists specifically to identify who should not proceed. Good candidates are generally over 18, ideally with a prescription that has been stable for at least a year, with corneas of adequate thickness and normal shape, and without significant dry eye or active eye disease. Candidates are generally advised against surgery if they have keratoconus or other corneal thinning conditions, since removing further tissue from an already thin or irregular cornea risks serious structural weakening, or if they have uncontrolled diabetes, certain autoimmune conditions, or are pregnant or breastfeeding, since hormonal changes can temporarily alter corneal shape and healing.
For people with very high prescriptions beyond what corneal laser surgery can safely correct, or corneas too thin for reshaping, lens-based alternatives exist. An implantable collamer lens (ICL) is a corrective lens surgically placed inside the eye, in front of the natural lens, without removing any corneal tissue at all — sometimes described as a permanent, surgically implanted contact lens. Refractive lens exchange removes the eye's natural lens entirely and replaces it with an artificial one, similar in principle to cataract surgery, and is generally reserved for older patients, particularly those with presbyopia or early cataract changes, since it eliminates natural accommodation entirely.
Someone with a very high myopic prescription and a cornea too thin for LASIK or PRK might be an excellent ICL candidate instead, since the procedure corrects vision without removing any corneal tissue. This is precisely why a single "best" refractive surgery does not exist — the right procedure depends entirely on an individual's cornea, prescription, and eye health.
Modern refractive surgery has a strong overall safety profile, but it is still surgery, and it carries real risks that a thorough consultation should cover honestly. Dry eye is the most common temporary side effect, often improving over months but occasionally persisting longer. Some patients experience temporary glare, halos, or difficulty with night vision during healing. Rarely, undercorrection, overcorrection, or a need for a touch-up procedure can occur. Results are excellent for the great majority of well-selected candidates, but "20/20 vision guaranteed" is a marketing simplification rather than a medical certainty — a proper consultation should always include a realistic, individualised discussion of expected outcomes, not just success statistics.
LASIK, PRK, SMILE, and lens-based procedures all aim at the same destination — clear vision without daily reliance on glasses or contacts — but they take genuinely different routes to get there, each suited to a different starting cornea. Choosing between them is not about finding the "best" procedure in the abstract; it is about matching a specific eye's thickness, shape, and prescription to the technique built to work safely with exactly that combination.
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