Aspheric Multifocals
for Presbyopia

BY JOE B. GOLDBERG, O.D., F.A.A.O.
SEPT. 1997

Successful fitting of any contact lens design depends upon your motivation, patience, refractive skill, patient management and product knowledge. When the product is aspheric multifocal contact lenses, it's essential that you know how lens performance is affected by design and manufacturing, and by lens surface geometry in particular.Understanding the properties that affect contact lens performance can help you make appropriate choices for your presbyopic patients.

CONICOIDS AND ECCENTRICITY

Conicoids are aspheric surface geometries that are defined mathematically by their eccentricity values, or the degree of departure from circularity. Depending upon the e-value, an aspheric conicoid can be ellipsoidal, paraboloidal or hyperboloidal (Fig. 1). An e-value of zero indicates a sphere.

Fig. 1: The progressive flattening of a conicoid ocular surface geometry. The ratio of the increase in length to the vertex depth is the eccentricity value. AF is the axis. X, X' and X" are unit intervals along AF. A'X, A"X' and A"'X' are chords perpendicular to AF which intersect the lens surface at these points. FA' is one-half unit larger than AF. FA" is one unit longer than AF and FA"' is one and one-half units longer than AF. An ellipsoidal surface results when units per unit step are less than 1.0; a paraboloidal surface when the units are 1.0; and hyperboloidal when the units are greater than 1.0.

Near powers are a function of the relationship between the e-value and the posterior apical radius (base curve), so the ability to define eccentricity is essential in designing an aspheric multifocal contact lens. For example, more near power can be created in a hyperboloidal contact lens surface by using steeper posterior apical radii and keeping the e-value constant. We do not prescribe add power for these aspheric multifocal contact lenses since the "add" varies with the e-value, apical radius and other variables.

NON-CONICOIDS ARE DIFFICULT TO DEFINE

Aspheric surfaces that are not conicoids may or may not be mathematically defined, depending upon the manufacturing equipment available. Without objective instrumentation capable of measuring eccentricity values of non-conicoids, such as the Volk Eccentriscope, non-conicoids have not reliably furnished levels of quantitative or qualitative near vision identical to those of conicoids. Furthermore, the near power furnished by non-conicoid aspheric multifocal soft lenses may be limited to the amount a lathe can be offset. Generally, the near powers are less than two diopters. Aspheric multifocal soft lenses that are made with offset spherical surfaces are pseudo-conicoids. An offset "aspheric" surface has its axis of revolution at an angle to the optic axis. The center of a homolateral surface is on the same side as the peripheral offset curve. It is a contralateral surface when its center is on the other side. Do you know what your laboratory is making and measuring?

LENS DESIGN AND FITTING

Aspheric, multifocal, progressive add soft contact lenses that are hyperboloidal are conicoids that have an e-value greater than 1.0. The distance power is in the center of the lens, and there is a progressive increase in plus powers from the center to the edge. The progressive increase in plus powers from +0.25D to +4.00D begins at a place between 0.50mm and 4.00mm from the center of the lens.

The position of an aspheric multifocal soft lens on the eye, as well as pupil size and normal aberrations of the eye are factors that affect visual acuity. A decentered lens, larger pupil size and angle alpha necessitate increasing minus power or reducing plus power up to 1.50D to correct distance vision.

Benjamin (1992) stated that any presbyopic prescription presents a unique combination of optical and neurological alterations. A progressive power change from the apex of the lens to the periphery causes an infinite number of images to merge along the visual axis. When a lens is centered properly, there may only be a minimal sacrifice of clarity at any viewing distance.

Aspheric multifocal soft lenses should center over the cornea with minimal displacement on blinking. The center of the lens should remain in the pupillary area although it may not be geometrically centered. Vision will improve when the lenses equilibrate and the eyelids adjust to the new sensation. Two or three pairs of lenses may be necessary to complete a fit. However, having a trial lens set reduces chair time.

Levy (1990) reported that an aspheric multifocal contact lens with an aspheric controlled area and predetermined power allows light rays from objects at all distances to be directed to a common focal area. The brain selectively interprets those images relevant to the object under observation. Therefore, perhaps a description of vision through an aspheric multifocal soft lens is simultaneous perception and selective vision.

References are available from the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #28. (Be sure to have a fax number ready.)