apical clearance

clearance, apical 

The distance between the posterior surface of a contact lens and the apex of the cornea. See apical bearing.
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Apical clearance over the central cornea (optimum fit has little or no touch in rigid zone of lens--total corneal clearance--see Figure 5)
Apical clearance over the central cornea with little / no touch in the RGP portion of the lens (Figure 6)
Apical clearance over the central cornea with little/no touch in the RGP portion of the lens.