In LASIK, the flap creation with microkeratome followed by excimer laser ablation is designed to reshape the anterior corneal curvature. For myopia correction, the excimer laser (193 nm ArF) ablates tissue in which pattern?

• A Central cornea more than periphery (prolate ablation) — deepening the central curvature to increase corneal power
• B Central cornea more than periphery — flattening the central corneal curvature to reduce total refractive power of the cornea ✓
• C Peripheral cornea more than center — steepening the peripheral cornea to redirect light toward the central axis
• D Uniform ablation across the entire optical zone — converting the cornea to a cylinder shape

Explanation

For myopia correction, excimer laser removes more tissue centrally than peripherally (Munnerlyn formula: ablation depth ∝ D × r²/3, where D = diopters corrected and r = ablation zone radius). This flattens the central cornea, reducing its curvature and thereby decreasing the total refractive power of the cornea, moving the focal point posteriorly onto the retina. The ablation pattern converts the prolate corneal surface (normally steeper centrally) toward an oblate shape. For hyperopia, the opposite occurs — peripheral ablation steepens the central cornea. For astigmatism, a toric ablation profile (different curvature in principal meridians) is applied. The 193 nm UV wavelength of ArF excimer laser photoablates corneal tissue by breaking C-C and C-N bonds (photoablation) rather than thermal coagulation, yielding precise tissue removal.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.