For more than 100 years, the corneal shape has been known to be aspheric. Typically, the central cornea is about 3 D steeper than the periphery, a positive shape factor. Clinically, the cornea is divided into zones that surround fixation and blend into one another. The central zone of 1–2 mm closely fits a spherical surface. Adjacent to the central zone is a 3–4-mm doughnut with an outer diameter of 7–8 mm. Called the paracentral zone, this doughnut represents an area of progressive flattening from the center. Together, the paracentral and central zones constitute the apical zone, as used in contact lens fitting. The central and paracentral zones are primarily responsible for the refractive power of the cornea (Fig 2-22). Adjacent to the paracentral zone is the peripheral zone, with an outer diameter of approximately 11 mm, and adjoining this is the limbus, with an outer diameter that averages 12 mm.
The peripheral zone is also known as the transitional zone, as it is the area of greatest flattening and asphericity of the normal cornea. The limbus is adjacent to the sclera and is the area where the cornea steepens prior to joining the sclera at the limbal sulcus.
The optical zone is the portion of the cornea that overlies the entrance pupil of the iris; it is physiologically limited to approximately 5.4 mm because of the Stiles-Crawford effect. 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 2-23).