Zones of the Cornea
For more than 100 years, the corneal shape has been known to be aspheric. Typically, the central cornea is about 3 diopters (D) steeper than the periphery, a positive shape factor (prolate). Clinically, the cornea may be divided into zones (Fig 2-10). The central zone is 1–3 mm in diameter and closely resembles a spherical surface. It is surrounded by the paracentral zone, a 3- to 4-mm “doughnut” with an outer diameter of 7–8 mm that progressively flattens from the center. Together, the paracentral and central zones constitute the apical zone, which is used in contact lens fitting. The central and paracentral zones are primarily responsible for the refractive power of the cornea. Adjacent to the paracentral zone is the peripheral zone or transitional zone, which has an outer diameter of approximately 11 mm. This is the area of greatest flattening and asphericity in the normal cornea. Finally, there is the limbus (limbal zone), where the cornea steepens prior to joining the sclera at the limbal sulcus; its outer diameter averages 12 mm.
The optical zone is the portion of the cornea that overlies the entrance pupil of the iris. The corneal apex is the point of maximum curvature, typically temporal to the center of the pupil. The corneal vertex is the point located at the intersection of the patient’s line of fixation and the corneal surface. It is represented by the corneal light reflex when the cornea is illuminated coaxially with fixation. The corneal vertex is the center of the keratoscopic image and does not necessarily correspond to the point of maximum curvature at the corneal apex (Fig 2-11).
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.