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?
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.