contact lens case reports
Sometimes Two Lenses Are Better Than One
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Since the early 1970s, "piggyback" has described an underlying soft contact lens with a rigid lens fit on top. In the past, practitioners used the dual-lens system sparingly because of frequent complications secondary to limited oxygen permeability. Today, with hyper-Dk soft and GP lenses, the modality is enjoying an exciting rebirth in clinical practice.
Figure 1. The soft lens power determines the curve the GP lens fits
Piggyback lenses primarily improve patient comfort or help GP lens positioning when the anterior corneal surface is compromised. In many of these situations, anterior soft lens power creates a new surface to the eye. A minus power soft lens provides a flatter anterior surface while a plus power soft lens provides a steeper anterior surface (Figure 1).
Fitting Piggyback Systems
The best way to fit piggyback lenses is to place the soft lens on the eye and perform keratometry or corneal mapping over the lens surface. Then fit the GP lens on the resulting soft lens anterior surface as if it were the cornea. Figure 2 illustrates the anterior surface flattening effect associated with fitting myopic CIBA Vision Focus Night & Day lenses. With no lens in place, the apical radius of curvature of the patient's cornea is 44.25 diopters. The central radius flattens with the increase in minus lens power: 0.25D (43.87 diopters), 3.00D (41.87 diopters) and 6.00D (39.50 diopters).
Figure 3 illustrates how the anterior surface effectively steepens when we fit hyperopic Night & Day lenses. For the same patient, the central radius effectively steepens with the increased plus power: +0.25D (44.37 diopters), +3.00D (46.12 diopters) and +6.00D (48.25 diopters).
|Figure 2. A myopic soft lens has an anterior surface flattening effect in piggyback systems.||Figure 3. A hyperopic soft lens has an anterior surface steepening effect in piggyback systems.|
These examples show that the power of the soft lens (and to some extent the curvature of the underlying cornea) influences the curvature of the anterior soft contact lens and the overlying GP contact lens. This change in surface curvature may help in cases involving post-refractive surgery or in keratoconus in which flatter or steeper radii may be desired.
Patrick Caroline is an associate professor of optometry at Pacific University and is 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 and serves as an adjunct assistant professor of optometry at Pacific University.