Validity of the keratometric index: large population-based study.

Area of Science:

• Ophthalmology
• Biomedical Optics
• Corneal Biomechanics

Background:

• The Gullstrand model eye uses a standard keratometric index (1.3315) for corneal power calculations.
• Accurate prediction of posterior corneal power is crucial for refractive surgery outcomes.
• Existing models may have limitations in predicting individual corneal power.

Purpose of the Study:

• To evaluate the accuracy of the standard keratometric index (1.3315) in predicting posterior corneal power.
• To compare the Gullstrand model's predictive accuracy with a calculated keratometric index derived from actual corneal measurements.

Main Methods:

• Corneal topography data (anterior radius of curvature, posterior radius of curvature, simulated keratometry, pachymetry) were collected from 2429 eyes using the Orbscan II.
• A calculated keratometric index (N(calc)) was derived from individual corneal measurements.
• The accuracy of posterior corneal power prediction was assessed using both the Gullstrand model and the calculated keratometric index.

Main Results:

• The mean calculated keratometric index (N(calc)) was 1.3273.
• Using N(calc), 98.3% of eyes had their posterior corneal power predicted within +/-0.50 D.
• The mean absolute error for posterior corneal power prediction was significantly lower with N(calc) (0.157 D) compared to the Gullstrand model (0.326 D).

Conclusions:

• Modifying the keratometric index based on individual corneal measurements significantly enhances the accuracy of predicting posterior corneal power.
• The calculated keratometric index (N(calc)) offers a more precise method for estimating posterior corneal power than the standard Gullstrand model.
• This improved accuracy has implications for refractive surgery planning and outcomes.