Indications & choice of technique
All three families correct the same error; the choice turns on eye anatomy and lifestyle. The reasonable range covers myopia to ≈ −8 to −10 D, hyperopia to +3 to +4 D and astigmatism to 5–6 D; beyond, one shifts to intraocular solutions. Within this range, corneal thickness, topography and refractive stability decide, before any preference.
Surface regains the advantage whenever load-bearing tissue must be spared: thin or borderline cornea, mildly asymmetric topography, impact-prone occupations (military, combat sports) where a flap is a weak point, epithelial basement membrane dystrophy. LASIK keeps its edge on comfort and speed when thickness and topography are clearly favourable. Lenticule extraction holds an appealing middle ground: no flap, nerve sparing, near-immediate recovery.
Suspected keratoconus or forme fruste, insufficient thickness, predicted residual bed too thin, unstable refraction, severe dryness, and (to a lesser degree) autoimmune background or pregnancy. Preoperative tomographic and biomechanical screening is the main barrier against iatrogenic ectasia — not a formality.
| Situation | Preferred technique | Reason |
|---|---|---|
| Thin / borderline cornea | Surface (PRK) | No flap → load-bearing tissue preserved |
| Mildly atypical topography | Surface, with caution | Less biomechanical impact |
| Impact-risk occupation | Surface or lenticule | No displaceable flap plane |
| Favourable thickness & topo, fast recovery | LASIK | Comfort, useful from day 0 |
| Preop dryness, nerve concern | Lenticule (SMILE/SILK) | Relative nerve sparing |
| Keratoconus / FFKC / insufficient RSB | No photoablation | Ectasia risk → consider a lens implant |
Ablation limits — surface & depth
Safety rests not on a single number but on the geometry remaining after surgery. Two limits combine: the optical zone (surface) and the load-bearing stromal bed (depth).
7.1 Ablation depth — Munnerlyn's law
Ablation depth grows with the correction and with the square of the optical-zone diameter:
depth (µm) ≈ (optical zone² × D) / 3 → ≈ 12–15 µm / D at 6 mm
Hence the optical-zone dilemma: enlarging it improves night vision (fewer halos) but costs tissue; shrinking it saves stroma at the cost of scotopic optical quality.
7.2 Residual stromal bed & percent tissue altered (PTA)
For a long time the rule was one number: residual stromal bed ≥ 250 µm (Barraquer), today ≥ 300 µm as a safer margin. But 250 µm does not equally protect a thick cornea heavily ablated and a thin cornea barely touched. Hence percent tissue altered (PTA, Santhiago 2014), relating everything that weakens the cornea (flap + ablation) to initial thickness:
PTA = (flap thickness + ablation depth) / CCT · risk if ≥ 40 %
For equal correction, surface removes the flap's contribution to the PTA numerator: thicker residual bed, lower PTA. That is the biomechanical reason to prefer surface on borderline corneas. Landmark: the anterior 40% of stroma carries most of the tensile strength. Validation (Santhiago/Randleman): PTA ≥ 40% in nearly all eyes that became ectatic with normal preop topography (sensitivity ~97%, specificity 89%); mean PTA ~45% (ectatic) vs ~32% (controls).
| Parameter | Landmark | Comment |
|---|---|---|
| Depth / diopter | ≈ 12–15 µm/D at 6 mm | Munnerlyn; grows with optical zone² |
| Flap thickness | ~100–110 µm (femto) | Microkeratome more variable |
| Residual stromal bed | ≥ 300 µm | 250 µm = historical minimum |
| PTA | < 40 % | Ectasia risk threshold (Santhiago) |
| Risk score (ERSS) | ≥ 3 to watch | Topo, RSB, age, CCT, MRSE (Randleman) |
| Stromal strength | 40% ant. > 60% post. | Justifies sparing anterior lamellae |