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Master the RGP challenge of hyperopia

Issue: December 1997

ByRodger T. Kame, OD, FAAO

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Comparing fluorescein patterns of a -3 D and a +3 D lens that otherwise are identical spherical lenses with similar parameters, the minus lens would tend to position in a superior position (above) and the plus in an inferior position (below). With minus lenses, the upper lid during a full blink will "attach" to the minus edge, and the center of gravity moves away from the corneal apex. With plus lenses, the center of gravity moves toward the apex and resists being pulled up.

Most of the nearly 30 million U.S. contact lens wearers are myopes and are successfully fit with both hydrogel (soft) contact lenses and rigid gas-permeable (RGP) lenses. Hyperopes remain a challenge whether soft or rigid lenses are applied.

In the case of hydrogels, it is simple to fit a comfortable, well-centered lens that provides excellent vision. However, oxygen transmission is a limitation for many due to the excessive average lens thickness, which tends to be occlusive to the cornea. This results in corneal edema. Adverse response signs include corneal striae, bulbar injection and symptoms of contradictory "dryness."

Riding low on the cornea

With RGPs, principal difficulties arise from lens mass, which tends to render the lens heavy and resistant to ride anywhere but low on the cornea. Add to that the "watermelon seed" phenomenon that squeezes the lens down away from the blink action of the upper lid. This phenomenon describes the movement of the lens toward the thickest part of the lens due to the tapered thinning curves of the plus lens profile.

Although RGP materials are available in higher Dk, the excessive lens thickness factor does limit oxygen transmission and thus relies on the tear pump to provide the bulk of oxygen to maintain optimum corneal physiology. Adverse response signs include corneal edema - either stromal striae or epithelial microcystic - 3 to 9 o'clock staining, vascularized limbal keratitis (VLK) and altered blink pattern, usually partial and viable.

Fluorescein patterns

Comparing fluorescein patterns of a -3 D and a +3 D lens that otherwise are identical spherical lenses with similar parameters, the minus lens would tend to position in a superior position and the plus in an inferior position. Two factors contribute to the final effect. With minus lenses, the upper lid during a full blink will "attach" to the minus edge and the center of gravity moves away from the corneal apex. With plus lenses, the center of gravity moves toward the apex and resists being pulled up.

Additionally, the watermelon seed effect pushes the lens away from upper lid attachment control. Furthermore, the blink amplitude tends to be attenuated, allowing the plus lens to ride in the inferior position. This partial blink pattern eventually leads to 3 to 9 o'clock staining and more complicated VLK.

Design change

A simple change can be made to the lens design to cause the plus lens to behave or perform on the cornea as if it were a minus lens. For example, permit the plus lens to "attach" to the upper lid so that normal full blinks, as are encouraged with a minus lid attached lens, can result. This lens behavior encourages better tear pumping, optimal lid/conjunctival congruity and a happier contact lens wearer. The key design change is to request a minus lenticular edge configuration.

---To alter the performance of the plus lens so it performs on the cornea as if it were a minus lens, the key design change is to request a minus lenticular edge configuration. (Top: Single Cut, Spherical, +3.00 Lens; Middle: Single Cut, Spherical, -3.00 Lens; Bottom: Minus Lenticulated, Spherical, +3.00 Lens)

This concept was first described by Donald Korb, OD, of Boston. He advocated the use of flat base curved lenses that were "fit to the upper lid, rather than the cornea" to emphasize the importance of maintaining normal blink amplitude and thus normal tear film integrity and tear function. The tear film is an essential component in the maintenance of physical and physiological stability.

In summary, the key elements for adapting the lenticular design so a thick plus lens behaves in situ as a minus lens requires a synergy of a reduced center thickness and a minus edge profile to attach to the upper lid. The fitting and adapting of RGP contact lenses for hyperopia need not be a greater challenge than that for myopia.