The Munnerlyn formula relates the intended refractive correction (D) and the optical zone diameter to the depth of the ablation. If the optical zone is kept constant, doubling the attempted correction (e.g., from -3D to -6D) will approximately change the central ablation depth by a factor of:

• A 4× (quadruples)
• B 2× (doubles) ✓
• C Unchanged — central ablation depth is independent of correction
• D 1.41× (increases by √2)

Explanation

The Munnerlyn formula states: ablation depth (µm) ≈ D × OZ² / 3, where D is dioptric correction and OZ is optical zone diameter in mm. With OZ constant, ablation depth is directly proportional to D (linear relationship). Doubling D doubles ablation depth. The quadratic relationship is with the optical zone: doubling the OZ with constant D quadruples ablation depth. This formula guides the minimum residual stromal bed calculation in LASIK.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.