contact lens Q&A

Designing Minus Carrier RGP Lenses

BY CHRISTOPHER SNYDER, OD, MS
DECEMBER 1998

A "carrier" will minimize the center thickness of low minus powered and plus powered lenses while making the overall lens thickness profile more uniform. Consider prescribing a minus carrier for RGP lenses in powers from -1.00 to plano and with any plus powered

carrier for RGP lenses in powers from -1.00D to plano and for any plus powered lens when centration needs to be improved. Configuring the carrier profile in a minus carrier design provides a lens edge that's thicker than the thickness of the junction between the center and carrier portion (Fig. 1). This mimics the periphery of a higher minus powered lens "wedge," giving the upper lid a lens thickness profile to embrace (Fig. 2), rather than release (Fig. 3), after the blink. This yields a lens that is more stable, better centered and seems to be attached to the movements of the upper lid. Possible payoffs include better vision, better comfort, easier adaptation and less staining due to the higher and stable lens position.

FIG. 1: Peripheral thickness profiles of regular (no carrier), minus carrier and plus carrier RGPs.

FIG. 2: Minus carrier lens in average post-blink position. Arrow indicates carrier with specular reflection, highlighting the change in anterior lens peripheral radius.

FIG. 3: Standard tricurve RGP in post-blink position.

Minimizing the minus nature of the carrier will lead to a lens profile described as plano or plus carrier. This still minimizes lens center thickness, but denies the upper lid a "wedge advantage."

How do you design minus carrier lenses?

ANSWER: Here are suggestions:

• Make the overall diameter of the lens 9.4mm or larger.
• Make the carrier width at least 0.8mm (i.e., front optic zone at least 1.6mm smaller than overall diameter).
• Make the back optic zone diameter equal to or less than the front optic zone, and design the back surface peripheral curves as you normally would for a relatively large diameter RGP lens.
• Order a junction thickness of 0.14mm. This will ensure that your lens center thickness is reasonable and minimal.
• Order a finished edge thickness greater than the junction thickness, typically 0.06mm greater than the junction thickness.

The greater the difference between the junction and finished edge thickness, the more minus the carrier is and the higher the lens will position, and vice versa. Therefore, you can titrate the amount of lid attachment and lid vertical positioning by controlling the degree of minus wedge provided in the carrier.

Minus carriers are well worth using because they can optimize your ability to fit RGPs in low minus and plus powers.

Dr. Snyder is a professor of optometry and serves as chief of contact lens patient care at the School of Optometry at the University of Alabama at Birmingham.