Keratometry and the Central Radius with Aspheric Corneal Surfaces

Area of Science:

• Ophthalmology
• Biomedical Optics
• Corneal Biomechanics

Background:

• Keratometry (K) and simulated keratometry (SimK) provide the radius of curvature (RS) but not the true central corneal radius (RC).
• Corneal asphericity (QC) influences the relationship between RS and RC.
• Accurate RC is crucial for refractive surgery and optical modeling.

Purpose of the Study:

• To develop a straightforward method for deriving the central corneal radius (RC) from keratometry (RS) and corneal asphericity (QC).
• To quantify the differences between RC and RS (RC-RS) using a large dataset from an anterior segment tomographer.

Main Methods:

• Derived RC and RC-RS by comparing local surface slopes to a conoid and reference sphere at the keratometry measurement zone (diameter KD).
• Evaluated RC-RS differences using a dataset from the Casia 2 tomographer.
• Analyzed the influence of corneal asphericity (QC) and keratometry zone diameter (KD) on RC-RS.

Main Results:

• RC can deviate from RS by up to 0.1 mm, depending on QC and KD.
• Prolate corneas (negative QC) lead to RS overestimating RC; oblate corneas (positive QC) lead to RS underestimating RC.
• For a typical cornea (RS=7.7 mm, QC=-0.22) with KD=3 mm, RC was 0.0322 mm steeper than RS (RC=7.6678 mm). The 95% CI for RC-RS at KD=3 mm was -0.0976 to +0.0265 mm.

Conclusions:

• The keratometric radius of curvature can be readily converted to the central corneal radius using corneal asphericity and the keratometric zone diameter.
• This conversion is valuable for corneal representations, such as conoid surfaces or paraxial calculations.
• Discrepancies between RC and RS can be clinically significant with large QC values.