Chapter 06

Indications & choice of technique

All three families correct the same error; the choice depends 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 a middle ground: no flap and fast recovery, with theoretical nerve sparing — but a shorter track record and an unproven superiority over LASIK. It is therefore an option, not the default reference.

When to decline any photoablation

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.

Table 2 — Guiding the choice
Situation	Preferred technique	Reason
Thin / borderline cornea	PRK	No flap → load-bearing tissue preserved
Combat sports / impact-risk occupation	PRK	No displaceable flap plane in case of trauma
Apprehension about applanation in theatre	PRK	The only technique without applanation or suction ring (LASIK and SMILE both require one)
Thin cornea + combat sports + fear of applanation	PRK — technique of choice	All three arguments converge
Astigmatism, but no time off work	LASIK	Recovery in hours; PRK requires ~1 week off
Myopia > 5 D and astigmatism > 1.5 D	LASIK	Better predictability in this profile
Mildly atypical topography	PRK, with caution	Less biomechanical impact
Preop dryness, nerve concern	Lenticule (SMILE/SILK)	Relative nerve sparing — but shorter follow-up
Keratoconus / FFKC / insufficient RSB	No photoablation	Ectasia risk → consider a lens implant

Chapter 07

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:

Figure 2 — Depth limits: PTA and residual stromal bed (RSB) by technique (original schematic).

PTA = (flap thickness + ablation depth) / CCT · risk if ≥ 40 %

Key idea — why surface protects

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).

Table 3 — The numerical limits to remember
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