Contact Lens Design & Materials
Modern GP Multifocal Lens Designs
By Ronald K. Watanabe, OD, FAAO
Presbyopia is a challenge for any contact lens fitter, especially if the patient has very critical visual demands. For such patients, GP multifocal lens designs are excellent options. They are also appropriate for patients who have high refractive errors, high add powers, or significant astigmatism. Knowing what is currently available in GP multifocal designs can help you expand your contact lens patient base.
Aspheric multifocal designs have front- and/or back-surface curvatures that vary from center to edge. Today's aspheric surfaces have precisely designed curvature profiles that maintain good distance vision, optimize add power, and minimize aberrations. They are not all simple elliptical curves anymore. Most are center-distance designs with optic zones that can vary in diameter depending on the patient's add requirement, pupil size, lens fitting characteristics, and visual needs. High add powers are possible, as are toric surfaces for patients who have moderate-to-high astigmatism. There is even a new corneoscleral multifocal.
The main advantages of aspheric designs are good intermediate vision, near vision correction in any position of gaze (though it tends to be best in downgaze with some lens translation), and fitting characteristics that are the same as or similar to single vision GP lenses. They work well for patients who have to change focus often and rapidly and for those who have a lot of intermediate vision tasks, such as computer use.
Segmented GP multifocals are prism-ballasted designs that have a distinct distance zone in the superior portion of the lens and a separate near zone below. When fitted properly, the patient will see primarily through one power zone at a time, resulting in excellent optical clarity.
The segment line may be straight, curved, or angled. The near power may be localized to a specific zone or go across the entire lens. Some designs have an intermediate zone to provide trifocal optics, and in some cases the intermediate zone is aspheric to provide a "progressive" power change. The bottom of the lens is sometimes truncated to help lens positioning and translation. The inferior edge may also be designed to rest under as well as on the lower lid. Some even combine an aspheric surface with the segmented design to gain the benefits from both design types.
The main advantages of segmented designs are the superior optical quality of viewing through one segment at a time and the ability to provide true high add powers. Some are also available in back- and front-surface toric designs. These lenses work best for patients who have nearpoint tasks in downgaze, though a trifocal can work well for computers users as well.
Many options are available today in each of these multifocal categories. Talking to your GP laboratory consultants will help you best understand their products and the types of patients they may be appropriate for. Perusing the various laboratory Web sites or the Gas Permeable Lens Institute's Web site (www.gpli.info) will allow you to see what is available across the industry.
Regardless of what lab you use, it is important to have a discussion with each patient to determine their visual needs and then select the lens design that is best suited to delivering the type of vision they need. CLS
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at email@example.com.