Corneal power correction factor for photorefractive keratectomy

Area of Science:

• Ophthalmology
• Optical Engineering
• Biomedical Optics

Background:

• Corneal power measurements for photorefractive keratectomy (PRK) typically use a refractive index of 1.3375.
• This index approximates tear fluid but not the corneal stroma's actual refractive index of 1.376.
• Previous methods may lead to inaccuracies in determining corneal power post-PRK.

Purpose of the Study:

• To determine the most appropriate refractive index for corneal power calculations following PRK.
• To evaluate the impact of using different refractive indices on optical power measurements.
• To identify potential errors in current keratometric and videokeratographic calculations.

Main Methods:

• A hypothetical optical model was employed to analyze the refractive contributions of the tear lens and corneal surface.
• Calculations were performed to assess the effective optical power of the anterior corneal surface with and without the tear layer.
• The study focused on the refractive changes induced by PRK, specifically affecting the corneal epithelium and anterior stroma.

Main Results:

• The tear layer has minimal net optical power due to opposing refractive forces.
• The refractive index of 1.376, representing the corneal epithelium and stroma, is appropriate for PRK-related power calculations.
• Utilizing a refractive index of 1.3375 introduces a consistent 11.4% error in corneal power readings.

Conclusions:

• For PRK, the corneal refractive index of 1.376 should be used for accurate radius-to-power conversions.
• Instrument algorithms for keratometry and videokeratography should be updated to 1.376, or a correction factor of 1.114 applied.
• The standard index of 1.3375 remains suitable for other keratometric applications not involving PRK-induced changes.