Surface photoablation (PRK & variants)

Photorefractive keratectomy was the first clinical use of the excimer. The principle stayed simple: remove the central epithelium, apply photoablation directly to Bowman's layer and anterior stroma, then let the epithelium heal under a bandage lens. What evolved is epithelial management:

The historical price of surface ablation is haze — a subepithelial myofibroblast veil, mainly after high corrections and UV exposure. Intraoperative mitomycin C transformed the outlook by inhibiting this fibroblastic response, rehabilitating surface ablation for errors once reserved for LASIK.

LASIK — ablation under a flap

LASIK separates access from sculpting. First an anterior lamellar flap (epithelium, Bowman, superficial stroma) is cut and lifted on its hinge; the exposed stromal bed is photoablated; the flap is repositioned and adheres by endothelial pump, without sutures. Immediate benefit: with Bowman and epithelium intact on the flap, pain is minimal and recovery takes hours. The double price: the flap consumes load-bearing tissue and its cut severs corneal nerves (dryness).

Flap creation migrated from the mechanical microkeratome (fast but variable thickness) to the femtosecond laser, which makes a flap of planned, regular, reproducible thickness with angled edges (better adhesion, less epithelial ingrowth). This predictability is not cosmetic: it directly governs the residual stromal bed calculation, hence the safety margin.

Lenticule extraction (SMILE, SILK)

The latest generation dispenses with the excimer entirely. The femtosecond laser cuts, within intact stroma, a lenticule whose geometry carries the whole correction; it is extracted through a small peripheral incision, without lifting a flap. By breaking only a short incision rather than a full circular cut, these techniques theoretically preserve anterior lamellae and nerves better, so more favourable biomechanics and dryness are attributed to them — but to be clear, this advantage is theoretical and debated, and head-to-head studies have not shown a clear clinical superiority over LASIK (broadly comparable refractive and biomechanical outcomes).

SMILE (VisuMax) introduced a plano-convex lenticule. SILK (ELITA, CE 2023) offers an ultra-low-energy variant (~40–90 nJ, ~1 µm spot) and, crucially, a biconvex lenticule with matched faces, limiting micro-folding on removal and minimising induced spherical aberration. Series cover myopia to −12 D and astigmatism to −6 D, with fast recovery. Known limitations: shorter track record than PRK and LASIK; more difficult enhancement (no flap to re-lift); extension to astigmatism, hyperopia and presbyopia only within the last two years; operator-dependent lenticule dissection. Its place remains to be defined: this is a more recent technique, long restricted to myopia — treatment of astigmatism, hyperopia and presbyopia has only been possible for less than two years. Follow-up is therefore shorter than for PRK and LASIK, and biomechanical and nerve-regeneration data remain to be consolidated.

Differentiating the techniques

Three ways to reach the same optical profile, three relationships to tissue: surface works on the anterior face after de-epithelialisation; LASIK sculpts a bed under a flap; extraction removes an intrastromal volume through a single incision.