Keratometric index, videokeratography, and refractive surgery

Area of Science:

• Ophthalmology
• Optics
• Biomedical Engineering

Background:

• Accurate optical modeling of the cornea is essential for various ophthalmic procedures.
• Inconsistent use of refractive indices has led to confusion in corneal calculations.
• Standardized refractive index values are needed for precise diagnostic and surgical outcomes.

Purpose of the Study:

• To resolve discrepancies in refractive index values used for corneal optical modeling.
• To provide accurate refractive index recommendations for different ophthalmic applications.
• To improve the precision of calculations in refractive surgery, corneal diagnostics, and cataract surgery.

Main Methods:

• Corneal optical system modeled using 1, 2, or 3 spherical interfaces.
• Analysis performed using paraxial geometrical optics and ray tracing.
• Incorporated models for incisional and photoablative refractive surgery, using Gullstrand eye model parameters.

Main Results:

• Refractive index 1.3375 is suitable for estimating power differences in untreated or incisional keratotomy corneas.
• Refractive index 1.3315 is more accurate for absolute dioptric power values.
• Stromal refractive index 1.376 is necessary for accurate photorefractive keratectomy (PRK) ablation and dioptric change calculations.

Conclusions:

• Videokeratographic instruments should utilize three refractive indices (1.3375, 1.376, 1.3315) for comprehensive dioptric power mapping.
• Cataract surgery calculations require adjustments for intraocular lens power in patients with prior PRK.
• Standardized refractive index usage enhances accuracy in ophthalmic diagnostics and surgical planning.