The Munnerlyn formula used in excimer laser refractive surgery calculates ablation depth as: Depth = (Optical zone diameter)² × Refractive correction / 3. For a myopic correction of -6D with a 6 mm optical zone, the approximate stromal ablation depth would be:
- A 72 microns ✓
- B 24 microns
- C 48 microns
- D 96 microns
Correct answer: A. 72 microns
Explanation
Applying the Munnerlyn formula: Depth = (OZ diameter in mm)² × D / 3 = (6)² × 6 / 3 = 36 × 6 / 3 = 216 / 3 = 72 microns. This formula guides surgical planning to ensure adequate residual stromal bed (minimum 250 microns in LASIK, 300 microns in PRK) to prevent post-ablation ectasia. The formula highlights that both increasing optical zone size and higher corrections substantially increase ablation depth.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.