Figure 4-13 Simultaneous vision bifocal contact lenses. A, Aspheric, or multifocal lens. B, Diffractive lens. (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 b y Christine Gralapp.)

These lenses have various optical designs. One type is aspheric, or multifocal, as are intraocular lenses. Aspheric surfaces change in power from the center to the periphery: minus lenses decrease in power from the center to the periphery, whereas plus lenses increase (see Fig 4-13A). Another type of simultaneous vision lens is diffractive (see Fig 4-13B). Such lenses have concentric grooves on the back surfaces, such that the light rays are split into 2 focal packages: near and far. The diffractive surfaces reduce incoming light by 20% or more, thereby reducing vision in dim lighting. These lenses are less sensitive to pupil size than are aspheric multifocal designs, but they must be well centered for best vision.

No single style works for all patients, and most require highly motivated patients and fitters for success. Despite the availability of contact lenses for presbyopia, monovision is still the most common approach. A trial of monovision contact lenses may be beneficial when considering a permanent correction such as laser refractive surgery or lens implant surgery.

Jain S, Arora I, Azar DT. Success of monovision in presbyopes: review of the literature and potential applications to refractive surgery. Surv Ophthalmol. 1996;40(6):491–499.

Keratoconus and the Abnormal Cornea

Contact lenses often provide better vision than do spectacles by masking irregular astigmatism (higher orders of aberration). For mild or moderate irregularities, soft spherical, soft toric, or custom soft toric contact lenses are used. Large irregularities typically require RGP lenses to mask the abnormal surface; the anterior surface of the contact lens creates a new optic surface, and the tear lens corrects the corneal irregularities. As with nonastigmatic eyes, fitters should first find the best alignment fit and then determine the optimal power. Three-point touch can be successfully used for larger cones to ensure lens centration and stability: slight apical and paracentral touch or bearing (dark areas on the fluorescein evaluation; Fig 4-14). The ophthalmologist may use the apical clearance fitting technique to place a lens vault slightly over the cone. Fitting the abnormal cornea requires experience, an understanding patient, and willingness on the part of both the patient and the fitter to spend the time necessary to optimize the fit. When the lenses are ordered, it is best to request a warranty or exchange option; typically, several lenses are fitted before the final lens parameters are determined.

Figure 4-14 Three-point touch in keratoconus. (Courtesy of Perry Rosenthal, MD.)

Some specialized RGP lenses have been developed specifically 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 posttransplant fitting; they are known as intralimbic contact lenses. Some RGP lenses designed for keratoconus are made of new materials that have high oxygen permeability, allowing a more comfortable fit.

An alternative approach is to use a hybrid contact lens that comprises a rigid center and a soft skirt. The hybrid lens theoretically provides the good vision of an RGP lens and the comfort of a soft lens.

Hybrid contact lenses, including one designed specifically for patients with keratoconus (SynergEyes-KC, SynergEyes Inc, Carlsbad, CA), became available in the United States in January 2008. In addition to their use in patients with keratoconus or other degenerative conditions, SynergEyes lenses, which are the first hybrid contact lenses approved by the US Food and Drug Administration (FDA), can be used for patients with all types of refractive errors, in patients with corneal trauma, and in patients following refractive surgery (SynergEyes-PS) or penetrating keratoplasty. The lens has an RGP center (Dk = 145) and an outer ring whose material is similar to that 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 similar to those offered by hybrid lenses, as well as a greater choice of contact lens parameters.

Contact Lens Overrefraction

Patients with reduced vision and irregular corneas may be difficult to evaluate. Examination techniques such as potential acuity measurement and retinal imaging techniques such as optical coherence tomography and fluorescein angiography may not be helpful. Corneal irregularity may be corrected diagnostically with a rigid contact lens if the cornea is clear and there are no other significant media opacities. The examiner should choose a rigid contact lens that will center appropriately on the cornea, then perform an overrefraction. The resulting visual acuity will give the examiner the proportion of vision loss that is accounted for by the irregular cornea. This technique may be useful even in patients who are not contact lens candidates, as it helps direct other diagnostic and therapeutic modalities to the correct portion of the eye.

Gas-Permeable Scleral Contact Lenses

Scleral lenses have unique advantages over other types of contact lenses in rehabilitating the vision of eyes with damaged corneas. These lenses are entirely supported by the sclera; their centration and positional stability are independent of distorted corneal topography; and they avoid contact with a damaged corneal surface. Moreover, these lenses create an artificial tear-filled space over the cornea, thereby providing a protective function for corneas suffering from ocular surface disease.

Scleral lenses consist of a central optic that vaults the cornea and a peripheral haptic that rests on the scleral surface (Fig 4-15). The shape of the posterior optic surface is chosen so as to minimize the volume of the fluid compartment while avoiding corneal contact after the lenses have settled. The posterior haptic surface is configured to minimize localized scleral compression; the transitional zone that joins the optic and haptic surfaces is designed to vault the limbus. Historically, these lenses were composed of oxygen-impermeable PMMA. Currently, scleral lenses are composed of highly oxygenpermeable polymers.