Reader and Industry Forum
Effective Optics of Piggyback Soft Contact Lenses
By Minhee Woo, OD, and Barry A. Weissman, OD, PhD
Piggyback contact lens systems, consisting of a base soft lens, usually silicone hydrogel, and a capping GP lens, have been an essential part of the contact lens practitioner's fitting techniques for almost half a century (Baldone, 1970; Westerhout, 1973).
Several questions have surrounded the clinical application of piggyback contact lens systems. For example, can the underlying corneal surface obtain enough oxygen? Weissman and Ye (2006) provided one theoretical answer: adequate oxygen supply to the corneal surface is anticipated under open-eye conditions when both rigid and soft lenses have been manufactured from materials that are considered to be of moderate to high oxygen permeabilities, greater than 60 Fatt Dk units.
Other questions involve the optics of piggyback contact lens systems. We have been trained to consider a model of the contact lens-eye system as a series of thin lenses in air, and, hence, we expect that lenses stacked one on top of the other should have powers that are additive. It has been clear from the earlier days of piggyback contact lens application, however, that this is not the case. The effective power of a soft lens under a GP lens has been known to be much less than the true lens power, but by how much and the reason behind it have been elusive.
The Optics of Piggybacks
In recent years, several reports have discussed the optics of piggyback soft lenses. Benjamin (2006) hypothesized that the lacrimal lens corrects for the power of the soft lens, and the over-refraction is essentially unchanged from that which occurs with only the GP lens on the eye.
On the other hand, in 2008, Schendowich suggested that piggybacked GP and soft lens powers in situ are not additive. He remarked: “An elegant proof … appeared via … Optcomlist several years ago in a fascinating discussion of the optics of piggyback systems between Drs. Charles McBride and John C. Heiby. The bottom line from the exchange was that the soft contact lens component will contribute about 20 percent of its power in air.”
Two recent reviews reported similar findings. During the 2010 Global Specialty Lens Symposium, Nixon noted: “Loretta Szczotka-Flynn … reminded us that the soft lens used in a piggyback system will have approximately 22 percent effective power of the labeled power on the lens.” Larson and Edrington (2010) reported: “A 2009 study by Professor Daniel Brazeau showed that a soft piggyback lens will change an over-refraction by about 23 percent of the soft lens power. For example, when combined with a GP lens, a +4.00D spherical soft lens will change the over-refraction by approximately +1.00D.”
The latter statements were presumably allusions to the yetunpublished research presented by Daniel Brazeau at the 2009 Global Specialty Lens Symposium. It appears this research was under way while the McBride-Heiby online discussion was in the realm of theoretical optics.
Evidently, several authors have reported how much of the labeled lens power contributes to the total effective power of the piggyback system—about 20 percent—yet no one has published the basis for this percentage. Therefore, we present a different explanation for the basis of this approximation while covering much the same theory as McBride-Heiby.
Determining Power Differences
To understand the optical effect of a soft lens under a rigid lens, one must evaluate the difference in power between the tear layer and the soft lens, both under the rigid lens, and then compare that value to the unflexed original power of the soft lens. In addition, one must consider the entrapped layer between the rigid lens and the anterior cornea as a thick lens not as thin lenses in air. The front surface of this thick lens is an interface between the rigid lens and either tears or a soft lens. Similarly, the back surface of the thick lens is the interface between either the tears or a soft lens and the anterior corneal surface.
We calculated the back vertex power of the thick lens using the lensmaker's equation, BVP=F1/[1-(t/n')(F1)] + F2 where F=(n'-n)/R. We calculated the first interface curvature (R1) that would be required for alignment with the front surface of flexed soft lenses for powers of −3.00D, −1.00D, −0.50D, +0.50D, +1.00D and +3.00D. We selected these powers because they are most often used as piggyback soft lenses. The flexed back surface (R2) was assumed to conform to anterior corneal surfaces with 7.80mm and 7.00mm of curvature, representing a normal cornea and a keratoconic cornea, respectively. We used a refractive index of 1.46 for the rigid lens, and an index of 1.336 and thickness of 0.004mm for the tear layer (Benjamin, 2006). For calculation purposes, we used a sample silicone hydrogel soft lens with an index of 1.42 and central thicknesses as follows:
• 0.1mm for powers of −3.00D, −1.00D, −0.50D, +0.50D
• 0.12mm for +1.00D
• 0.15mm for +3.00 D
The results of these calculations were compared to the original soft lens powers in diopters and in percentage (Table 1).
Our calculations did not account for lens flexure. Earlier work suggests that power change is minimal with flexure for minus-powered soft lenses, whereas plus-powered soft lenses tend to show decreased optical power with flexure (Weissman, 1984a; Weissman 1984b; Plainis & Charman, 1998). We can assume, therefore, that our results are accurate for minus lenses, but there may be a slightly lessened effective power for plus-powered lenses, owing to flexure. In addition, neither compression of the soft lens under the GP nor design elements, such as lenticulation, were considered in this model.
Despite our model's limitations, our results were consistent with recent reports. Our calculations, based on a reasonable first approximation, demonstrate that effective powers expected for a soft lens under a GP lens in piggyback contact lens systems should be, on average, 20 percent of their original powers in a range of 10 percent to 30 percent. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #192.
Dr Woo graduated from the UC Berkeley School of Optometry and completed a 1-year fellowship in contact lens practice at UCLA, the Jules Stein Eye Institute, supported by an educational grant from The Vision Care Institute, a Johnson & Johnson Company. Dr Weissman is also a graduate of the UC Berkeley School of Optometry. He is a professor of ophthalmology at the Jules Stein Eye Institute of the David Geffen School of Medicine at UCLA and an adjunct professor of optometry at the Southern California College of Optometry. He has consulted for Vistakon, Alcon, Bausch + Lomb and AMO.