What does "apical clearance" refer to in lens fitting?

Apical clearance specifically refers to the amount of space or clearance between the back surface of a contact lens and the cornea at the apex of the cornea. In the context of lens fitting, achieving an appropriate apical clearance is essential for ensuring comfort, proper lens positioning, and adequate tear exchange beneath the lens. This clearance is vital because insufficient apical clearance can lead to corneal irritation, whereas excessive clearance can affect visual acuity and lens stability. Proper evaluation of apical clearance helps ensure that the lens aligns well with the corneal shape and provides an optimal fit, which is crucial for the overall success of contact lens wear.

In contrast, the other options relate to different aspects of contact lens characteristics that do not directly pertain to the specific fit against the cornea's apex. For instance, lens thickness refers to how thick the lens material is, while lens power pertains to the refractive strength of the lens. Corneal curvature describes the shape of the cornea itself, which can influence lens fit but does not specifically define the clearance at the apex.