Contact Lens Design & Materials
Multifocals for Astigmats
By Ronald K. Watanabe, OD, FAAO
Multifocals continue to be one of the biggest areas of opportunity for contact lens fitting. One segment of the presbyopic population that may be missing out is the astigmatic group. Most soft multifocal contact lenses are not available with toric powers, so if a patient has more than 0.75D of astigmatism, vision will be somewhat compromised. GP multifocal lenses can correct corneal astigmatism, but some patients are unwilling to try them or cannot adapt to them. Also, most GP multifocal designs do not correct internal astigmatism. In such cases, soft toric multifocal lenses may be an astigmat's best option.
Soft Toric Multifocal Designs
A soft toric multifocal is a marriage between a stable toric design and good multifocal optics. Manufacturers use established rotational stabilizers that work well over a range of cylinder powers and axes. These toric designs are coupled with multifocal designs that can work over a range of add power requirements. If the cylinder is poorly corrected or the multifocal does not provide good distance and near vision, the combination will not work.
Many soft toric multifocal contact lenses are available today. Most have center-near aspheric designs, but there are also concentric designs, combination concentric/aspheric designs, and segmented designs.
Aspheric Designs vary in their progressive power profiles and zone diameters. Some are designed to minimize lens-induced aberrations. Some allow for adjustable adds (eccentricities) and optic zones. All utilize simultaneous vision and attempt to stretch a presbyope's range of focus from infinity to the near-point.
An excellent example of an aspheric design is Unilens' C•Vue Advanced HydraVue Toric Multifocal. This monthly replacement option is made with the Definitive (Contamac) silicone hydrogel material. It features a patented center-near multifocal design with an adjustable central add zone diameter. It is available in a wide parameter range, with several base curve/diameter combinations, sphere powers up to ±20.00D, cylinder powers up to –4.00D, any cylinder axis, and add powers up to +3.00D.
Concentric Designs also utilize simultaneous vision and may be either center-distance or center-near. Some manufacturers recommend using a center-near lens on one eye and a center-distance lens on the other. Some of these designs also offer adjustable zone sizes to accommodate various pupil diameters and to improve on-eye performance. Add powers can also be specified depending on the level of presbyopia.
An example of a concentric/aspheric design is the Proclear Multifocal Toric. It incorporates CooperVision's Balanced Progressive Technology and is available in two base curves, sphere powers up to ±20.00D, cylinder powers up to –5.75D, cylinder axes in five-degree steps, and add powers up to +4.00D.
Segmented Designs must translate well on downgaze to allow patients to view through the lower reading zone. They must then re-center when patients move back into primary gaze to provide distance vision. Add powers, segment heights, and lens diameters can be adjusted to provide maximum performance. When they work well, segmented designs can deliver the best visual quality of the soft multifocals.
Many Options are Available
In addition to the two lens examples mentioned, several other custom laboratories manufacture conventional and frequent replacement soft toric multifocal contact lens options. Other laboratories that offer silicone hydrogel toric multifocal lenses include Art Optical, Metro Optics, and X-Cel Contacts. With today's advanced technology, soft toric multifocal contact lenses can be a wonderful option for astigmatic presbyopes. 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.|