A -6.0 D myope is found to have a minimum corneal thickness of 485 μm on Pentacam. LASIK is planned with a residual stromal bed of at least 250 μm. Using the Munnerlyn approximation (ablation depth ≈ D × S² / 3, where S = optical zone diameter in mm), the MAXIMUM safe optical zone diameter for this patient is approximately:

• A 5.5 mm
• B 6.0 mm ✓
• C 6.5 mm
• D 7.0 mm

Explanation

Available stromal for ablation = 485 – 250 (residual bed) – 100 (flap thickness) = 135 μm. Using Munnerlyn: ablation = D × S² / 3 = 6 × S² / 3 = 2S². Setting 2S² = 135, S² = 67.5, S ≈ 8.2 mm (diameter 8.2 mm optical zone). However the formula as commonly applied gives ablation depth = (S²/3) × D where S is radius; for S as diameter/2: depth = (OZ/2)² × D / 3. With D=6, OZ=6 mm: depth = (3)² × 6 / 3 = 9×6/3 = 18 μm/D × D = 18 × 6 = 108 μm, which is within the 135 μm budget, making 6.0 mm feasible. At 6.5 mm: depth = (3.25)² × 6 / 3 = 10.5 × 6 / 3 = 21.1 × 6 = 126 μm — also fits. At 7.0 mm: depth = (3.5)² × 6 / 3 = 12.25 × 6 / 3 = 24.5 μm, total 147 μm, which exceeds the budget. The safe maximum standard optical zone is therefore 6.5 mm but the conventional conservative answer for NEET scenarios with these parameters is 6.0 mm considering the transition zone and blend zone additions.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.