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European Board of Ophthalmology Diploma (EBOD)

Excimer ablation, PRK and LASIK

Q: A refractive surgeon plans an excimer laser myopic ablation of -6.00 D using a 6.0 mm optical zone. Based on Munnerlyn's formula (t = \frac{S^2 \times D}{3}), what is the estimated central corneal ablation depth, and how would expanding the optical zone to 7.0 mm alter the required tissue removal?

Central ablation depth is 72 µm at 6.0 mm, increasing to 98 µm at 7.0 mm (a 36% increase in central ablation depth). According to Munnerlyn's formula, central ablation depth t = \frac{S^2 \times D}{3}. For an optical zone S = 6.0\text{ mm} and myopic correction D = 6.00\text{ D}: t = \frac{6^2 \times 6}{3} = \frac{36 \times 6}{3} = 72\,\mu\text{m}. When the optical zone is expanded to S = 7.0\text{ mm}: t = \frac{7^2 \times 6}{3} = \frac{49 \times 6}{3} = 98\,\mu\text{m}. Because tissue ablation scales quadratically with optical zone diameter (S^2), expanding the zone from 6.0 mm to 7.0 mm increases required stromal depth by 36.1% (98 / 72 = 1.361), illustrating why optical zone expansion must be strictly balanced against residual stromal bed limits.

Key Takeaways

Excimer photoablation changes corneal curvature, with depth depending strongly on optical-zone size.
PRK and LASIK differ in surface healing, flap effects and complication profiles.
Ablation planning requires refractive stability, surface assessment and screening for ectatic disease.

Last updated: October 2026

Keratorefractive surgery encompasses surgical techniques that alter the anterior corneal curvature to modify the total refractive power of the human eye ( $+43.00\text{ D}$ out of the total $+60.00\text{ D}$ ocular refractive power). By precisely reshaping the stroma, surgeons correct myopia, hyperopia, and astigmatism. A comprehensive understanding of laser-tissue interactions, stromal wound healing kinetics, biomechanical stability, and ectasia risk stratification is fundamental for the European Board of Ophthalmology Diploma (EBOD) examination.

1. Optical Physics of Excimer Laser Photoablation

The excimer (excited dimer) laser operates in the far ultraviolet spectrum at 193 nm, generated by an electrical discharge through a gaseous mixture of argon and fluorine (ArF). Each 193 nm photon carries an energy of 6.4 electron volts (eV), which exceeds the intermolecular covalent binding energy of corneal peptide and collagen bonds (typically $3.6\text{ eV}$ for carbon-carbon and carbon-nitrogen bonds).

Photochemical Ablation (Photoablation)

Rather than burning or vaporising tissue through thermal heating, the 193 nm laser induces photodecomposition (photochemical ablation):

High-energy ultraviolet photons are absorbed within a microscopic superficial layer of hydrated stromal tissue ( $< 1\,\mu\text{m}$ penetration depth).
Covalent molecular bonds are directly cleaved within picoseconds.
The fragmented molecular products undergo explosive supersonic expansion and ejection away from the stromal surface.
Surrounding adjacent tissue experiences negligible collateral thermal damage ( $< 0.3–0.5\,\mu\text{m}$ thermal zone of coagulation).
Each standard laser pulse reliably ablates approximately $0.25\,\mu\text{m}$ of corneal stroma at a fluence of $160–200\text{ mJ/cm}^2$ .

Munnerlyn's Formula for Ablation Depth

The mathematical foundation of myopic excimer photoablation was established by Charles Munnerlyn. To correct a spherical myopic error by flattening the central corneal curvature, the laser ablates a central lenticule of tissue according to the parabolic equation:

t = \frac{S^2 \times D}{3}

where:

$t$ is the central ablation depth in micrometres ( $\mu\text{m}$ )
$S$ is the diameter of the optical zone in millimetres ( $\text{mm}$ )
$D$ is the dioptric power of refractive correction in dioptres ( $\text{D}$ )

The Quadratic Impact of the Optical Zone

Because the optical zone diameter ( $S$ ) is squared in Munnerlyn's formula, even small increments in optical zone size produce dramatic increases in central tissue ablation. Conversely, ablation depth scales linearly with dioptric correction ( $D$ ):

Myopic Correction ( $D$ )	Optical Zone ( $S = 5.5\text{ mm}$ )	Optical Zone ( $S = 6.0\text{ mm}$ )	Optical Zone ( $S = 6.5\text{ mm}$ )	Optical Zone ( $S = 7.0\text{ mm}$ )
$-3.00\text{ D}$	$30.25\,\mu\text{m}$	$36.00\,\mu\text{m}$	$42.25\,\mu\text{m}$	$49.00\,\mu\text{m}$
$-6.00\text{ D}$	$60.50\,\mu\text{m}$	$72.00\,\mu\text{m}$	$84.50\,\mu\text{m}$	$98.00\,\mu\text{m}$
$-9.00\text{ D}$	$90.75\,\mu\text{m}$	$108.00\,\mu\text{m}$	$126.75\,\mu\text{m}$	$147.00\,\mu\text{m}$

Important

Expanding the optical zone from $6.0\text{ mm}$ to $7.0\text{ mm}$ for a $-6.00\text{ D}$ myopic treatment increases the central stromal tissue depth required from $72\,\mu\text{m}$ to $98\,\mu\text{m}$ —an additional $36\%$ increase in tissue removal ( $98 / 72 = 1.361$ ). Selecting an optical zone smaller than the patient's mesopic pupil diameter causes severe nocturnal glare, starbursts, and loss of contrast sensitivity due to induced spherical aberrations ( $Z_4^0$ ).

2. Photorefractive Keratectomy (PRK) & Advanced Surface Ablation

Surface ablation eliminates corneal tissue directly from the anterior stromal surface after removing or displacing the corneal epithelium, permanently removing Bowman layer. Variations include Photorefractive Keratectomy (PRK), Laser Epithelial Keratomileusis (LASEK), Epipolis-LASIK (epi-LASIK), and Transepithelial PRK (transPRK).

Epithelial Debridement Techniques

Mechanical Debridement: Epithelium is scraped centrally using a blunt hockey-knife blade or an automated rotating brush (Amoils brush).
Chemical Debridement (Alcohol-Assisted / LASEK): A well containing $20\%$ dilute ethanol in balanced salt solution is placed on the central cornea for $20–30\text{ seconds}$ , loosening hemidesmosomal attachments along the basement membrane zone. In classic PRK, the epithelial sheet is discarded; in LASEK, the sheet is folded back and repositioned over the stromal bed following ablation.
Transepithelial PTK/PRK (transPRK): A single continuous laser sequence where a preliminary phototherapeutic keratectomy (PTK) profile ablates the epithelial layer (typically calibrated to $50–55\,\mu\text{m}$ centrally) followed immediately by the refractive excimer ablation without manual manipulation.

Corneal Wound Healing & Postoperative Haze Pathogenesis

Epithelial injury in surface ablation initiates an aggressive wound healing cascade:

Damaged epithelial cells release pro-inflammatory cytokines, predominantly interleukin-1 $\alpha$ (IL-1 $\alpha$ ), IL-1 $\beta$ , and tumor necrosis factor- $\alpha$ (TNF- $\alpha$ ).
These cytokines diffuse into the underlying anterior stroma, triggering immediate keratocyte apoptosis across the anterior $50–100\,\mu\text{m}$ .
Over the subsequent 2 to 4 weeks, surviving quiescent keratocytes in the posterior stroma and periphery proliferate and migrate into the depleted anterior bed under the influence of transforming growth factor- $\beta$ (TGF- $\beta$ ) and platelet-derived growth factor (PDGF) derived from the regenerating epithelium and tear film.
Activated keratocytes differentiate into myofibroblasts, which express $\alpha$ -smooth muscle actin ( $\alpha$ -SMA) and secrete abnormal, unorganised type III collagen, biglycan, and decorin, leading to subepithelial reticular opacity known as corneal haze.

Mitomycin C and Surface Healing

Mitomycin C suppresses proliferating cells through DNA cross-linking and may reduce PRK haze in selected higher-risk treatments. Concentration and exposure are protocol-specific; it is not a guarantee against haze. Avoid uncontrolled exposure and protect the endothelium. Follow epithelial closure, infection and pressure, and distinguish haze from recurrent dystrophy or ectasia.

3. Laser In Situ Keratomileusis (LASIK)

In LASIK, a partial-thickness lamellar corneal flap is created and reflected, the exposed intrastromal bed is reshaped with the excimer laser, and the flap is repositioned without sutures. Sparing Bowman layer and the central epithelium provides rapid visual recovery (within 24 hours) and minimal postoperative pain compared to PRK.

Microkeratome vs. Femtosecond Laser Flap Creation

Parameter	Mechanical Microkeratome (Blade)	Femtosecond Laser (1053 nm Nd:glass)
Mechanism	Oscillating motorized stainless steel blade advancing across vacuum suction ring	Infrared laser-induced optical breakdown (LIOB); micro-cavitation plasma bubbles of $\text{CO}_2$ and $\text{H}_2\text{O}$
Flap Morphology	Meniscus shape: thinner centrally, thicker in the periphery	Planar shape: uniform thickness across entire diameter
Flap Thickness Precision	High variability (target $130–160\,\mu\text{m} \pm 25–30\,\mu\text{m}$ )	Exceptional reproducibility (target $90–110\,\mu\text{m} \pm 5–10\,\mu\text{m}$ )
Side-Cut Architecture	Shallow bevelled angle ( $30^\circ–45^\circ$ )	Customisable vertical or inverted bevelled angle ( $70^\circ–110^\circ$ )
Biomechanical Stability	Lower wound re-apposition strength; higher risk of displacement	Superior interlocking flap-bed adhesion; higher tensile resistance
Epithelial Ingrowth Risk	Higher ( $1–2\%$ ) due to shallow side cuts	Significantly lower ( $< 0.2\%$ ) due to tight vertical/undercut edges
Intraoperative Complications	Free caps, buttonholes, incomplete passes	Suction loss, Opaque Bubble Layer (OBL), transient light sensitivity

Tip

Inverted bevelled side cuts ( $90^\circ–110^\circ$ ) created by femtosecond lasers act as a mechanical "manhole cover", preventing sliding displacement and providing superior tectonic resistance against late traumatic dislocation compared to microkeratome flaps.

Test Your Knowledge

A refractive surgeon plans an excimer laser myopic ablation of -6.00 D using a 6.0 mm optical zone. Based on Munnerlyn's formula ( $t = \frac{S^2 \times D}{3}$ ), what is the estimated central corneal ablation depth, and how would expanding the optical zone to 7.0 mm alter the required tissue removal?

Central ablation depth is 72 µm at 6.0 mm, increasing to 98 µm at 7.0 mm (a 36% increase in central ablation depth)

Central ablation depth is 54 µm at 6.0 mm, increasing to 63 µm at 7.0 mm (a 17% increase in tissue removal)

Central ablation depth is 108 µm at 6.0 mm, increasing to 147 µm at 7.0 mm (a 36% increase in central ablation depth)

Central ablation depth is 36 µm at 6.0 mm, increasing to 49 µm at 7.0 mm (a 36% increase in central ablation depth)

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