25.4 Laser In Situ Keratomileusis (LASIK), PRK, SMILE & Phakic IOLs

Laser In Situ Keratomileusis (LASIK), PRK, SMILE & Phakic IOLs

Core Clinical Mandate: Refractive surgery demands precise biophysical calculations to permanently remodel the ocular refractive apparatus. The ophthalmic medical technologist must master corneal biomechanics, residual stromal bed calculations, flap dynamics, and phakic IOL clearance parameters to prevent catastrophic complications such as post-laser corneal ectasia, interface keratitis, and secondary cataracts.

Laser Biophysics & The Munnerlyn Ablation Formula

The 193 nm Argon Fluoride (ArF) Excimer Laser

Modern corneal refractive ablation utilizes the 193 nm Argon Fluoride (ArF) excimer laser operating in the deep ultraviolet spectrum:

• Photoablation (Photochemical Vaporization): A single 193 nm photon carries an energy of 6.4 electron volts (eV), which exceeds the 3.6 eV molecular bond energy of the carbon-carbon and carbon-nitrogen bonds holding corneal collagen and glycosaminoglycans together. The laser breaks intermolecular bonds directly without generating thermal damage or acoustic shockwaves, vaporizing approximately 0.25 µm of corneal tissue per pulse with sub-micron precision.

The Munnerlyn Formula

The central depth of corneal tissue ablated during myopic spherical correction can be estimated mathematically using the Munnerlyn Formula:

$AD \approx \frac{D^2 \times S}{3}$

where:

• $AD$ = Central ablation depth in microns (µm)
• $D$ = Diameter of the optical ablation zone in millimeters (mm)
• $S$ = Dioptric sphere corrected in diopters (D)

Clinical Example: Correcting -6.00 D of myopia using a 6.0 mm optical zone:

$AD \approx \frac{(6.0)^2 \times 6.0}{3} = \frac{36 \times 6.0}{3} = 72\ \mu\text{m}$

If the optical zone is enlarged to 7.0 mm (to avoid night glare in a patient with large scotopic pupils):

$AD \approx \frac{(7.0)^2 \times 6.0}{3} = \frac{49 \times 6.0}{3} = 98\ \mu\text{m}$

Key Clinical Takeaway: Ablation depth scales with the square of the optical zone diameter ($D^2$). Enlarging the optical zone dramatically increases stromal tissue removal, directly threatening corneal biomechanical safety.

Biomechanical Safety: RSB, PTA & Ectasia Risk Scoring

Iatrogenic corneal ectasia represents the most devastating long-term complication of laser refractive surgery, characterized by progressive biomechanical thinning, steepening, irregular astigmatism, and visual loss resembling advanced keratoconus.

Residual Stromal Bed (RSB) Calculation

The Residual Stromal Bed (RSB) represents the thickness of untouched, intact posterior corneal stroma remaining beneath the ablation bed:

$\text{RSB} = \text{CCT} - (\text{Flap Thickness} + \text{Ablation Depth})$

• Universal Absolute Legal Minimum: $\text{RSB} \ge 250\ \mu\text{m}$.
• Modern Standard Clinical Safety Target: $\text{RSB} \ge 300\ \mu\text{m}$.
• Violating the 250–300 µm threshold severely destabilizes corneal tensile strength, drastically increasing ectasia risk.

Percent Tissue Altered (PTA)

Introduced by Santhiago and colleagues, Percent Tissue Altered (PTA) evaluates the proportional structural alteration of the cornea, integrating both flap thickness and excimer ablation depth relative to baseline central corneal thickness (CCT):

$\text{PTA} = \frac{\text{Flap Thickness} + \text{Ablation Depth}}{\text{Central Corneal Thickness}} \times 100\%$

• Clinical Threshold: A $\text{PTA} \ge 40%$ is the single most robust independent predictor of post-LASIK ectasia in eyes with normal preoperative topography.
• When $\text{PTA} \ge 40%$, LASIK is contraindicated, and surface ablation (PRK) or phakic IOLs should be selected instead.

Worked Clinical Example:

• Patient CCT = 510 µm
• Planned Femtosecond Flap = 110 µm
• Planned Ablation Depth = 100 µm (for -7.00 D)
• $\text{RSB} = 510 - (110 + 100) = 300\ \mu\text{m}$ (appears acceptable by traditional criteria)
• $\text{PTA} = (110 + 100) / 510 \times 100% = 210 / 510 = \mathbf{41.2%}$
• Clinical Decision: Because $\text{PTA} > 40%$, this eye is at high risk for ectasia despite an RSB of 300 µm. LASIK should be aborted in favor of PRK or an ICL.

Preoperative Screening & Risk Scoring

• Randleman Ectasia Risk Score System (ERSS): Evaluates 5 factors: topographic pattern (inferior steepening, asymmetric bowtie, forme fruste keratoconus), RSB, patient age (<30 years = higher risk), CCT (<510 µm = high risk), and spherical equivalent refraction.
• Tomographic Analysis: Scheimpflug-based imaging (e.g., Oculus Pentacam) evaluates the Belin-Ambrósio Enhanced Ectasia Display (BAD-D), anterior and posterior corneal elevation against an enhanced reference sphere, and corneal pachymetric progression curves.

LASIK: Flap Dynamics, Complications & Diffuse Lamellar Keratitis

Flap Creation: Mechanical Microkeratome vs. Femtosecond Laser

• Mechanical Microkeratome (e.g., Hansatome, Moria): Uses an oscillating steel blade. Tends to produce a meniscus-shaped flap (thick at the periphery, thin in the center). Prone to variable flap thickness, free caps (in flat corneas <41 D), and buttonholes (in steep corneas >46 D).
• Femtosecond Laser (e.g., IntraLase, VisuMax): Uses ultra-fast infrared laser pulses ($10^{-15}$ s) to produce photodisruption. Tightly focused pulses create microscopic cavitation bubbles (CO₂ and water vapor) that cleave corneal tissue in a horizontal lamellar plane, followed by an angled vertical side cut. Produces a uniform planar flap of exact pre-programmed thickness (e.g., 100–110 µm) with beveled edges that resist traumatic dislocation.

Flap Complications & Management

1. Free Cap: Occurs when the microkeratome passes entirely through the corneal tissue without leaving a hinge (frequently in very flat corneas <41 D). The cap is removed, laser ablation is applied to the bed, and the free cap is repositioned using radial reference ink marks, followed by a bandage contact lens.
2. Flap Buttonhole: A central hole in the flap occurring when the blade cuts into the epithelium/Bowman's layer (frequently in steep corneas >46 D or during loss of suction). The surgeon must ABORT the procedure immediately without applying laser ablation. The flap is smoothed down and allowed to heal for 3 to 6 months, after which PRK with MMC can be considered.
3. Flap Striae:
   • Microstriae: Tiny folds in Bowman's membrane; asymptomatic or managed conservatively with lubricants.
   • Macrostriae: Full-thickness undulating folds caused by flap slippage. Causes irregular astigmatism and monocular diplopia. Managed by urgent flap lifting, hydration with hypotonic BSS, and mechanical stroking/ironing with a moist lint-free swab.
4. Diffuse Lamellar Keratitis (DLK / "Sands of the Sahara"):
   • A sterile, non-infectious inflammatory infiltrate of polymorphonuclear leukocytes (PMNs) accumulating in the lamellar interface between the flap and the stromal bed.
   • Etiology: Endotoxins from surgical instrument bio-burden, cleaning residues, lubricating oils, or excessive femtosecond laser energy.
   • Linebarger Clinical Staging & Treatment: (See table below)
5. Epithelial Ingrowth: Migration of epithelial cells beneath the flap margin into the interface. If progressive, within 2 mm of the visual axis, or causing focal flap melting, the flap must be lifted and cells mechanically scraped from both the bed and the stromal underside of the flap.

PRK, SMILE & Phakic Intraocular Lenses

Photorefractive Keratectomy (PRK) & Surface Ablation

PRK removes the corneal epithelium and applies the excimer laser directly to Bowman's layer and the anterior stroma, completely eliminating flap-related complications:

• Epithelial Removal Methods: Mechanical debridement with a blunt spatula, chemical loosening with 20% ethanol in a well for 20 to 30 seconds, a rotary Amoebic brush, or transepithelial laser (tPRK).
• Mitomycin C (MMC 0.02%) in PRK:
  • Mechanism: Applied topically via a circular sponge over the ablated bed for 12 to 60 seconds, followed by copious irrigation with 20 to 30 mL of BSS.
  • Rationale: Ablations deeper than 50 µm to 75 µm or corrections greater than -4.00 D to -6.00 D disrupt the basement membrane and anterior keratocyte network, triggering intense keratocyte activation, myofibroblast transformation, and disorganized collagen synthesis, presenting clinically as subepithelial corneal haze.
  • Action: MMC cross-links keratocyte DNA, blocking myofibroblast proliferation and preventing corneal haze.

Small Incision Lenticule Extraction (SMILE)

SMILE (e.g., Zeiss VisuMax) is an all-femtosecond laser intrastromal refractive procedure:

1. Laser Photodisruption: The femtosecond laser delivers two sequential intrastromal lamellar cuts:
   • Posterior Refractive Cut: Defines the refractive curvature and power of the lenticule.
   • Anterior Cap Cut: Creates the corneal cap (typically 100 µm to 140 µm deep).
   • Side Cut: A small arcuate side-cut incision of 2.0 mm to 4.0 mm width.
2. Manual Dissection & Extraction: The surgeon uses a micro-dissector to separate the anterior plane, then the posterior plane, and extracts the refractive lenticule through the small incision with micro-forceps.
3. Clinical Advantages of SMILE:
   • Biomechanical Superiority: The anterior 40% of the corneal stroma contains the highest density of cross-linked collagen fibers, providing the majority of corneal tensile strength. By preserving this anterior cap intact, SMILE retains more biomechanical strength than LASIK.
   • Nerve Preservation & Dry Eye: The small 2–4 mm incision severs far fewer corneal subbasal nerve fibers than a 20 mm LASIK flap incision, resulting in significantly less postoperative dry eye and accelerated corneal sensitivity recovery.

Posterior Chamber Phakic IOLs (EVO Visian ICL)

For patients with extreme myopia (-8.00 D to -20.00 D), thin corneas, or elevated ectasia risk (PTA $\ge 40%$), the Implantable Collamer Lens (ICL) provides high-definition refractive correction while preserving the crystalline lens and natural accommodation.

• EVO ICL Design: Made of Collamer (a flexible, hydrophilic collagen copolymer). Features a central 0.36 mm aperture (KS-AquaPORT), which permits natural physiological aqueous flow from the posterior chamber to the anterior chamber, eliminating the need for preoperative laser peripheral iridotomy (LPI).
• Anterior Chamber Vault: The critical clinical metric evaluated via slit lamp or anterior segment OCT (AS-OCT), representing the clearance between the posterior surface of the ICL and the anterior capsule of the crystalline lens:
  • Target Optimal Vault: 250 µm to 750 µm (approximately 0.5 to 1.5 central corneal thicknesses).
  • Low Vault (<250 µm or Direct Contact): Discloses insufficient clearance; friction and impaired aqueous nutrition across the anterior capsule trigger anterior subcapsular cataract (ASC) formation.
  • High Vault (>750 µm to 1,000+ µm): Pushes the iris forward, narrowing the iridocorneal angle and causing pigment dispersion, pupillary block, and secondary angle-closure glaucoma.
  • Vault Sizing: Governed by accurate preoperative measurement of horizontal white-to-white (WTW) corneal diameter and anterior chamber depth (ACD, which must be $\ge 3.0\text{ mm}$ from endothelium to lens capsule).