Keratoconus and the Abnormal Cornea
Contact lenses can provide better vision than spectacles by masking irregular corneal astigmatism. For mild or moderate irregularities, soft spherical, soft toric, or custom soft toric contact lenses are used. Large irregularities typically require an RGP lens to mask the abnormal surface; the anterior surface of the contact lens creates a smooth, spherical air–tear interface, and the tear lens fills the corneal irregularities. As with non-astigmatic eyes, fitters first find the best alignment fit and then determine the optimal power. Three-point touch can be successfully used for larger keratoconus cones to ensure lens centration and stability with slight apical and paracentral touch or bearing (dark areas on the fluorescein evaluation; Fig 5-14). Alternatively, apical clearance fitting technique places a lens vault slightly over the cone, avoiding the risk of contributing to apical scarring.
Figure 5-13 Simultaneous vision bifocal design.
(Modified with permission from Key JE II, ed. The CLAO Pocket Guide to Contact Lens Fitting. 2nd ed. Metairie, LA: Contact Lens Association of Ophthalmologists; 1998. Redrawn by Christine Gralapp.)
Figure 5-14 Three-point touch in keratoconus.
(Courtesy of Perry Rosenthal, MD.)
Specialized RGP lenses have been developed for keratoconus. Most provide a steep central posterior curve to vault over the cone and flatter peripheral curves to approximate the more normal peripheral curvature. Larger RGP contact lenses with larger optical zones (diameters >11 mm) are available for keratoconus and post-transplant fitting; they are known as intralimbic contact lenses.
A hybrid contact lens has a rigid center and a soft skirt. The aim is to provide the good vision of an RGP lens and the comfort of a soft lens.
Piggyback lens systems involve the fitting of a soft contact lens with an RGP lens fitted over it. This system may allow comfort benefits like those offered by hybrid lenses, with a greater choice of contact lens parameters, if the combination of lenses allows sufficient gas permeability to avoid hypoxia.
Scleral lenses provide an increasingly popular alternative for corneas that cannot be fit with corneal gas-permeable lenses.
Finding a good fit for an abnormal cornea is well worth the determination and patience required of both fitter and patient. The lens will need refitting after any surgical intervention.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.