In the optical principle underlying LASIK surgery, the Munnerlyn formula calculates the depth of ablation required. For myopia correction of −5.0 D with an optical zone of 6 mm, the approximate ablation depth is closest to:

• A 18 microns
• B 75 microns
• C 100 microns
• D 50 microns ✓

Explanation

The Munnerlyn formula estimates ablation depth as: t (microns) ≈ S² × D / 3, where S = optical zone diameter in mm and D = spherical equivalent correction in dioptres. For S = 6 mm and D = 5 D: t ≈ (36 × 5) / 3 = 60 microns (approximately 50–60 microns). Residual stromal bed must be ≥250 microns post-ablation to prevent ectasia. For each additional dioptre of correction or increase in optical zone, ablation depth increases substantially — this is why high myopia beyond −10 D or thin corneas (<480 microns) are relative contraindications.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.