A 40-year-old woman who underwent myopic LASIK 8 years ago now needs cataract surgery. Using her current corneal power readings in a standard vergence formula, the surgeon risks selecting an IOL that leaves the patient significantly hypermetropic. Why do conventional formulas fail in this setting?
- A Standard keratometry underestimates corneal power after myopic ablation, and the altered anterior-to-posterior curvature ratio invalidates the assumed keratometric index ✓
- B LASIK thins the cornea, which increases its true dioptric power above the measured value
- C The crystalline lens thickens after LASIK, shifting the effective lens plane forward
- D Axial length measurements become unreliable once the corneal surface is flattened
Correct answer: A. Standard keratometry underestimates corneal power after myopic ablation, and the altered anterior-to-posterior curvature ratio invalidates the assumed keratometric index
Explanation
After myopic excimer ablation, standard keratometry reads the paracentral cornea and misses the flattened central zone, underestimating corneal power. In addition, LASIK alters the normal anterior-to-posterior curvature ratio, so the fixed keratometric index of 1.3375 no longer applies. Both errors push the calculated IOL power too low, producing postoperative hypermetropia. Double-K methods and formulas such as Haigis-L correct these errors.
Reference: AAO BCSC Section 11: Lens and Cataract, latest ed.
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Written and medically reviewed by the StethoPrep medical team.