Options for Presbyopes
Westcon's toric soft bifocal makes contact lenses possible for presbyopes.
BY DAVID W. HANSEN, O.D., F.A.A.O.
Do you make excuses for the perceived lack of contact lens technology when dealing with presbyopic patients? When the astigmatic presbyopic patient needs soft contact lenses, you can be a "can do" practitioner.
Westcon Contact Lens Co.
The Westcon Contact Lens Company introduced the first toric soft bifocal eight years ago. The basic concentric design has been available for many years; however, the optics were easy to distort in polishing this complex surface. Over the years, though, this patented contact lens has been refined and provides excellent optics and comfort, as well as physiological safeguards.
With the industry focusing on contact lens disposability, it's important not to forget Westcon's Horizon Bi-Con Toric Bifocal or the Horizon Progressive Toric Bifocal conventional lens products. The lenses are the same, except the Progressive has an aspheric center add.
It's all about the lens
Horizon lenses are made of methafilicon 55% material. Base curves are 8.3, 8.6 and 8.9, with 8.6 used most often. Diameters can be manufactured as 14.0 mm, 14.5 mm and 15.0 mm with spherical powers available from -10.00D to +5.00D in .25D steps. The cylindrical powers range from 1.00D to 5.00D in .25D intervals. The axes are made in one-degree increments. Add powers are designed up to +3.00D.
The lens is a concentric near center add design with a toric posterior curve. Thin zones or double slab off stabilize the lens axis and provide centration.
The earlier aspheric designs covered the entire optical zone, resulting in unpredictable distance vision. To alleviate this, Westcon designed its progressive soft lens combining aspherical and spherical curves on the anterior surface while keeping the posterior surface spherical or toric.
The aspheric curve is cut only in the center of the front surface to 2 mm on the dominant eye and 3 mm on the non-dominant eye. To provide a large distance power area, the remainder of the front optical zone is spherical. The two different center zone sizes provide all visual ranges in both eyes -- a true bifocal design!
Designing the lens
Check out "Choosing a Base Curve." This design should center and have minimal movement (approximately .5 mm). If you observe excessive movement, try the 8.3-mm base curve or, if no movement, use the 8.9-mm base curve.
Behind the design philosophy
Soft lenses shouldn't move excessively, and different zone sizes create the same accommodation needs for all distances.
The zones are generally made smaller on the dominant eye; however, if the patient has a strong need for distance viewing, the lab can manufacture the zones the same size to compensate for these visual demands. Eye dominance tests are important in the pre-diagnostic regimen.
Precise measurement of pupils to accurately determine the appropriate zone size before each eye is mandatory. "Calculating Zone Size" outlines the zone size for the corresponding mesopic pupillary measurement:
Normal-sized pupils (3.0mm to 4.5mm) require a near center zone size of 2.5 mm for the dominant eye and 3.0 mm for the non-dominant eye. On smaller pupils, particularly with hyperopes, start with 2.0mm (dominant) and 2.5 mm (non-dominant). For larger pupils, use a 3.0 mm for the dominant and 3.5 mm for the non-dominant eye.
Troubleshooting requires listening to the patient! If the patient complains of diplopia, ghosting or 3-D effect at near, make the near OZ 0.5mm larger on both eyes. If the patient complains of diplopia, ghosting or 3-D effect for distance, then make the near zone OZ 0.5 mm smaller on both eyes.
If the distance correction is good and the patient requires more plus power at near, increase the add no more than +.75D. If he needs +1.00D or more, then increase the near zone size 0.5 mm and don't increase the power..
Westcon provides orientation dots located at 3 and 9 o'clock at the edge of the lens. Stabilization is important.
Patients appreciate your effort
Astigmatic presbyopic patients appreciate doctors who can successfully design soft contact lenses for them. In addition, they usually provide future referrals, so be sure to provide all options for your patients.
DR. HANSEN, A DIPLOMATE AND FELLOW OF THE AMERICAN ACADEMY OF OPTOMETRY, IS IN PRIVATE PRACTICE IN DES MOINES, IOWA.
Choosing a Base Curve
|KERATOMETRY (FLATTEST K)||BASE CURVE|
|flatter than 41.75D||8.9 mm|
|41.75D to 45.75D||8.6 mm|
|steeper than 45.75D||8.3 mm|
Calculating Zone Size
|PUPIL SIZE||ZONE SIZE|
|up to 4mm||2.0mm to 2.5 mm|
|4mm to 5.5mm||2.5mm to 3.0mm|
|more than 5mm||3.0mm to 3.5mm|