RGP Multifocals In the Blink of an Eye
BY DAVID W. HANSEN, O.D.
Lid architecture, lid tension, blink action and blink rate may all influence rigid contact lens dynamics. As people age, the structure and function of the lids change, which can adversely affect contact lens success, especially with RGP bifocals and multifocals. When prescribing bifocal contact lenses, it's extremely important to measure and follow eyelid positions and blink dynamics to improve acuity, comfort and contact lens translation.
BLINKING AND CONTACT LENS MOVEMENT
There is a delicate balance between achieving a stable contact lens position between blinks and providing sufficient lens movement during the blink to enable tear exchange under and around the contact lens. For an RGP lens to move during the blink, the upper lid force must move the lens on the cornea so that the force of the fluid holds the lens on the eye. Therefore, it's best to measure qualitative blink action (how often the patient blinks) and the size of the palpebral aperature as well as the placement of the lids over the superior and inferior cornea.
Use a biomicroscope with an eyepiece reticule with 10X magnification to measure these physical characteristics prior to fitting an RGP bifocal. Recalculate them yearly to monitor the physical changes of the eyelids and the quality of the blink.
Ptosis, or eyelid pathologies such as meibomianitis and blepharitis, can reduce the palpebral aperture, causing a bifocal lens to move drastically. Altered eyelid blink and decentered lens position are hallmark characteristics of potential problems.
If a patient who wears aspheric multifocal contact lenses develops ptosis, the lid may push against the superior portion of the contact lens driving the lens downward (Fig. 1). A blink may not center the lens to the appropriate position, so the patient may have blurry vision because he's seeing through the intermediate portion of the lens rather than the distance zone. With translating lenses, ptosis may prevent the patient from seeing with the near segment for reading.
FIG. 1: SUPERIOR LID PTOSIS PUSHING DECENTERED DECARLE DESIGN DOWN.
THE LIDS IN MOTION
Doane (1980) found that during the blink, the superior lid initially accelerates, rapidly reaching a maximum velocity at the visual axis. It then decelerates until it touches the lower lid. The lower lid produces lateral translation of a contact lens nasally, which returns centrally as the upper lid ascends. Understanding this action is critical for designing translating RGP lenses.
Traditionally, with prism ballasted lenses such as the Tangent Streak Crescent and the Decentered DeCarle design, the apex base/prism axis is ground 10 to 15 degrees off-axis to compensate for the nasal rotation (Figs. 2 & 3).
FIG. 2: SUPERIOR LID CAUSING CONTACT LENS ROTATION ON A TANGENT STREAK BIFOCAL.
FIG. 3: SUPERIOR LID CAPTURE OF A DECENTERED DECARLE BIFOCAL CONTACT LENS.
The FluoroPerm ST bifocal is thin enough not to need this compensating off-axis design to reposition the lens. You may need to reduce the diameter of an aspheric RGP multifocal to minimize lid interaction both superiorly and inferiorly. Diameter reduction may also be needed on translating lenses by thinning the superior lens edge using a CN bevel. CLS
References are available upon request to the editors at Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #25. (Have a fax number ready.)
Dr. Hansen, a cornea and contact lens diplomate and fellow of the American Academy of Optometry, is in private practice in Des Moines, Iowa.