readers' forum

Verifying Near Powers of Aspheric Soft Contact Lenses

BY JOE B. GOLDBERG, OD
May 1999

Although soft contact lenses dominate the contact lens industry, they're not the dominant modality for presbyopia. Practitioners have found it disappointing that, despite some manufacturers' claims of available near powers made by some manufacturers, almost all of the existing products have limited near powers. The distinction between the power in air and the power on the eye has been overlooked until now. It is important to understand how the index difference factors between the lens material and tears affect the power of the lens on the eye.

The Visionix VC-2000/20001

A new instrument, the Visionix (Dullas, Virg.) VC-2000/2001 will soon change this situation. The Visionix is a computerized vartometer that furnishes precise power profile mapping of any aspheric rigid gas permeable or soft multifocal contact lens. Manufacturers and practitioners can use this instrument to confirm the near powers of contact lenses at elliptic places: 0.50mm, 1.0mm, 1.5mm, 2.0mm and 2.5mm from the center.

Near Powers

Table 1 shows the Visionix computerized vartometer's measurements of the peripheral near powers of a V/X Green Label aspheric multifocal soft lens (GBF Contact Lens Inc). Table 2 describes the relationship between pupil sizes and near lens powers. Because pupils are smaller in presbyopic patients to begin with, and they become smaller on convergence, the results confirm that we can fit these aspheric lenses with impunity.

Aspheric Review

An aspheric surface has two peripheral radii. The tangential radius is the meridional radius and the sagittal radius is the trans-meridional radius. The relationship between the posterior apical radius (the base curve) and the peripheral radii creates the eccentricity value, or the degree of departure from circularity. There is a progressive increase of plus powers in each peripheral meridian. Therefore, although the surface is toric in the periphery, the measurements confirm that the spherical near powers at each elliptic place on the lens surface are averaging spherical powers.

Benefiting the Industry

The Visionix tool is a meaningful contribution to the industry. It lets manufacturers who make contact lens computerized lathes know if their equipment can manufacture aspheric lens surfaces with eccentricity values up to +3.00D on the eye for near vision. Also, although a lathe can generate a hyperboloidal ocular surface geometry, manufacturers need tools such as this to polish the peripheral radii that furnish the expected near powers at each elliptic place on the surface.

Visit Visionix on the Web at: www.visionix.com.

Dr. Goldberg is in private practice in Virginia Beach, VA, and is a consultant for Aero-GBF Contact Lens, Inc. He is also a diplomate in the AAO's Cornea and Contact Lenses Section.