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The Presbylite II™ bifocal is a translating design that has a SPHERICAL base curve so the design is fit like any spherical gas permeable lens. The fluorescein pattern should show an alignment fit. Corneas with more than 3 diopters of corneal cylinder are counter-indicated.
All the optics are on the front surface. The triangle segment contains the add power with the tip of the segment being aspheric for intermediate vision. The segment height is half of the diameter. The rest of the front optic zone is spherical and contains the distance power.
The design needs to center in the primary gaze for maximum distance acuity. The lens needs to translate up on downward gaze to get the add segment in front of the pupil. High riding lenses will not give acceptable distance acuity.
There are rotation marks on the lens. They should locate at 3 and 9 o’clock. Up to 30 degrees of nasal rotation is acceptable, but temporal rotation needs to have prism off-set to correct the problem. Use the patients spectacle add power. Base curves come on the .05 mm step only.
Fitting Guide
Here is a base curve selection method.
| Corneal Cylinder |
Base curve needed (Round to the flatter .05 mm step) |
| Spherical cornea |
Fit ½ diopter flatter than the Ks |
| .25 D to .75 D |
Fit on Flat K |
| 1.00 D to 3.00 D. |
1/3 of delta K steeper than flat K. |
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Parameters:
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| Diameters: |
9.2 mm to 10.2 in .1 mm steps |
| Distance Power: |
+10.00 D to -15.00 D |
| Add Powers: |
+1.00 to +2.75 in .25 D power steps |
| Material: |
Boston XO™ |
Fitting with the Presbylite II Software
1. The lens is fit from RX and K readings and the is guaranteed through the warranty pricing.
2. Custom fitting software is available at no charge on our web site.
3. Download the software below.
4. To fit the patients just fill in the K's, RX, add power, and the software computes the correct parameters.
ARTICLES:
Presbylite II Bifocal by Ron Beerten and Al Vaske
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