Patients with keratoconus have progressive thinning of the corneas, which end up inducing a large magnitude of distortion and irregular corneal astigmatism. Therefore it is unlikely to achieve good vision with spectacle correction and other options must be pursued. Rigid gas permeable lenses and scleral lenses are one option because they are able to neutralize corneal astigmatism, but the patient must be able to tolerate wearing rigid lenses. If discomfort is an issue, there exist rigid lenses that come attached to a soft skirt which improves comfort of wear. Similarly, patients who have undergone corneal surgery, grafting, or is suffering from corneal ectasia will also experience distortion and irregular astigmatism and will benefit from wearing rigid gas permeables or scleral lenses.
This is the measure of the curvature of the cornea, typically at the central region. Values are either expressed as dioptric power of the cornea (ie. 42.00D) or radius of curvature (ie. 8.03mm). The lens base curve selection is dependent on this parameter. The keratometer (manual or automatic) is the device that measures this value.