Article Date: 12/1/2002

contact lens case reports
Try Nasal Rotation with GP Translating Multifocals
BY PATRICK J. CAROLINE, FCLSA, FAAO, & MARK P. ANDRÉ, FCLSA, FAAO

Figure 1. The "watermelon seed effect" of a prism ballast lens.

Translating GP multifocal lenses commonly feature a ballast to orient the add portion into its appropriate, inferior 6 o'clock position. In most cases, the ballast is a thickness differential in which the top portion of the lens is thinnest and the bottom portion thickest. The upper lid simply squeezes the lens into position, a phenomenon commonly referred to as the "watermelon seed effect" (Figure 1). Conventional thinking indicates that if the prism is placed at 6 o'clock, the near add should position at 6 o'clock (Figure 2). Often this is not the case.

Using Nasal Rotation

Patient EM was a 52-year-old male executive interested in full time correction of his myopia/astigmatism and presbyopia. His Rx was OD ­4.50 ­1.25 x 175 and OS ­4.75 ­1.00 x 005 with a near add of +1.75D OU. His HVIDs were 12.0mm, and the lower lids positioned at the lower limbus OU. We diagnostically fit him with translating GP bifocal lenses.

Figure 2. A prism ballast at 6 o'clock. 

Slit lamp examination revealed that both lenses moved well in primary position with excellent translation on down gaze, indicating unobstructed lens movement along the vertical meridian. Under normal room illumination, the near segment lines positioned at the bottom edge of his pupils, indicating an appropriate segment height. The only remarkable finding was that both lenses rotated nasally (Figure 3).

Nasal rotation of ballasted lenses is not an uncommon finding. As the lenses come to rest on the lower lid, the nasal movement of the lower lid during the blink generates 10 to 20 degrees of nasal rotation. While this mal-alignment wreaks havoc with front surface toric GP contact lenses, it is often desirable with multifocal contact lenses. As the patient converges to see at 14 to 18 inches, the lens's add portion is in a better position for near viewing.

Figure 3. A prism ballast at 6 o'clock with 20 degrees of nasal rotation.

Some practitioners find nasal lens position so advantageous that if the initial diagnostic lenses position at 6 o'clock, they displace the prism 15 degrees to create nasal rotation. Accomplish this by ordering the final right lens with its prism at 75 degrees and the left lens at 105 degrees, or you can order the prism displaced in the direction of the rotation.

Next time you see translating bifocal lenses rotating slightly nasal, consider leaving them in what many consider to be the ideal lens position.

 

Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.

Mark André is director of contact lens services at the Oregon Health Sciences University.

 


Contact Lens Spectrum, Issue: December 2002