BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & LONG D. TRAN, OD
Rotational stability is essential for successful toric soft lens wear. Visual clarity decreases when the correcting cylinder positions or rotates away from the astigmatism axis. Snyder (1989) showed that 30 degrees of axis misalignment will result in the full amount of the correcting cylinder appearing in the over-refraction (OR), 15 degrees of misalignment results in one-half of the correcting cylinder in the OR, and 10 degrees misalignment results in one-third of the correcting cylinder in the OR.
For example, if the correcting cylinder aligns with the refraction cylinder axis, you can expect an OR of plano DS; but if the correcting cylinder of the toric lens is 3.00D and the lens axis misaligns by 15 degrees, 1.50D of cylinder at an oblique axis will appear in the OR.
Manufacturers have incorporated many methods of stabilization into toric soft lens designs since the 1970s. Prism-ballast has been the primary method. Other methods have included back-surface toricity, truncation, thin-zones, eccentric lenticulation and designs that combine more than one of these strategies.
The interaction between the eyelids and the lens edge profile is more critical in stabilizing the rotational position of a prism-ballasted contact lens than is gravity. To demonstrate this, Dr. Tom Brungardt instructed optometry students to lay down on a table top with their heads positioned upside-down over the edge of table. He would then apply a prism-ballasted rigid lens with the prism base up towards the ceiling and ask the students to blink normally. The prism base remained toward the ceiling while the students blinked, then rotated so the base positioned towards the floor when the students stopped blinking (or fainted). Dr. Tony Hanks referred to this stabilization effect of the upper eyelid as the watermelon seed effect. If you place a juicy watermelon seed between your thumb and index finger and squeeze, the rounded portion of the seed tends to precede the apex of the seed as it leaves your fingers. This demonstrates that upper eyelid pressure functions to control the rotational orientation of a toric soft contact lens by squeezing the thinnest portion of the lens periphery. Thin zones, eccentric lenticulation and prism-ballast designs utilize this theory to stabilize lens rotation and position.
A New Approach
Recently, Vistakon introduced its Accelerated Stabilization Design in the Acuvue Advance for Astigmatism silicone hydrogel lens. This design utilizes four active zones near the midperiphery of the lens to minimize lens rotation. These stabilization zones have a thicker profile than the rest of the lens. By rapidly increasing the slope of these active zones, the lens employs eyelid dynamics to quickly reorient when misaligned. Be aware that the active zones of the Acuvue Advance for Astigmatism lens are aligned within the interpalpebral fissure. Angled eyelids may require additional adjustment of the axis.
The Take Home
Eyelid dynamics and lens design play equally important roles in successful toric lens prescribing. When fitting a patient, consider eyelid tone, eyelid position and aperture size. Also, it's not necessary for the base down marking to position at 6 o'clock. What is important is consistent location of the correcting cylinder axis and the rotational stability of the lens.
To expedite the correct rotational position of a toric soft contact lens, apply the lens base down (or thin zone down). Many of today's toric soft lenses have markings to indicate the base of the prism. Or you and your patients might be able to determine the thicker prism base portion of the lens by feel or by visual inspection.
For references, please visit www.clspectrum.com/references.asp and click on document #129.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org. Dr. Tran is an assistant professor at the Southern California College of Optometry. E-mail him at email@example.com.