What does it mean if an RGP lens has no apical clearance?

When an RGP (Rigid Gas Permeable) lens has no apical clearance, it indicates that the lens is resting directly on the apex of the cornea without any space or gap between the lens and the corneal surface. This condition suggests that the lens is likely too flat for the patient's corneal curvature.

In a fitting scenario, apical clearance is necessary to allow for tear exchange and to provide comfort for the wearer. If there is no apical clearance, it means that the lens is not providing the appropriate space for the tear film, which can lead to discomfort and could potentially cause corneal irritation or edema.

In this context, describing the fit as "flat" accurately reflects the relationship between the lens and the cornea; the lens is not conforming to the natural curvature of the eye, leading to a situation where it is sitting too low or has inadequate clearance, hence highlighting the flat fit characterization.