prescribing
for presbyopia
Choosing the Proper
Seg Position for Alternating Bifocals
BY CRAIG W. NORMAN, FCLSA
Do
you find it challenging to determine the proper seg height for GP bifocals? Do
you wonder how your lens manufacturer determines a standard seg height?
Overcoming
Obstacles
In alternating vision
translating GP bifocal designs, two categories of obstacles must be overcome.
One is the physical response to the lens. Extra thickness inferiorly is
necessary to ballast (weight) these lenses, placing the optics in the proper
position for both distance and near.
The
second concern is visual. During the fitting process, it is not uncommon for the
patient to describe symptoms such as periodic doubling of images, glare from
headlights or a three-dimensional appearance of each line while reading. While
there can be various reasons for a GP bifocal wearer to have these symptoms,
most commonly it is related to the position of the segment line. In an earlier
column ("Measurement Tips to Increase Presbyopic RGP Fitting Success,"
January 2002), I discussed the importance of placing the seg line either at or
slightly below the lower edge of the pupil in normal lighting. The rationale for
this is to ensure that it is low enough not to disturb distance vision, but also
high enough to allow easy vision transfer to near objects when reading.
Two
Ways to Measure
The segment position
can be ordered by two different methods: seg height or distance from geometric
center. While each method nets the same results, it is interesting to discuss
how the measurements are determined.
First,
let's look at an example of a presbyopic eye (Figure 1) that presents for GP
lens fitting. The best lower lid position is where the lid rests at the lower
limbus in primary gaze. The ocular measurements are: 1a) vertical corneal
diameter=11.5 mm, 1b) pupil size=4.0mm, 1c) vertical cornea diameter=5.75mm, 1d)
pupil diameter=2.0mm and 1e) distance from lower lid to lower edge of the
pupil=3.75mm.
|
|
|
| Figure 1. Key
ocular measurements for choosing the seg height of a ballasted GP
bifocal lens. |
Figure 2.
Calculating the seg position from the geometric center of the lens. |
Since we can expect that the prism-ballasted
segmented bifocal GP lens designs will settle in primary position on the lower
lid, the initial seg height chosen would be 3.75mm (the distance from the lower
lid to the lower pupil edge).
Some
manufacturers refer to the seg position as the distance from the geometric
center of the lens, most commonly stated as the distance Below Geometric Center,
or BGC.
For this example, look at both
Figures 1 and 2. The corneal measurements are the same. A common overall
diameter for this lens is 9.5mm, and the standard seg position is 1.0mm BGC.
Look at the figure to see how this is calculated. Overall lens diameter = 9.5mm,
geometric center of the lens is 4.75mm and a seg position of 1mm BGC = a seg
height of 3.75mm.
Craig W. Norman is
Director of the Contact Lens Section at the South Bend Clinic in South Bend,
Indiana. He is a fellow of the Contact Lens Society of America and an advisor to
the RGP Lens Institute.
Contact Lens Spectrum, Issue: September 2002