letters to the editor

Piggyback Optics

I would like to comment on “Effective Optics of Piggyback Soft Contact Lenses,” by Minhee Woo, OD, and Barry A. Weissman, OD, PhD, from the November 2011 issue.

It appears that the optical problem is not defined clearly or properly. The clinical change is actually the net power change created when a soft carrier is added to a given GP. In this case the GP back vertex power (BVP) and front tear lens surface remain unchanged. The change that occurs is the new posterior tear lens surface and the addition of the soft lens BVP. The original posterior tear lens (GP alone) in air was “ – K” (n = 1.3375). The new back tear lens is (1-nt)/rfront. Thus the change is [(1-nt)/rfront] – (– K) so that the total effect is soft BVP + [(1-nt)/rfront] + K. The “rfront” can be calculated in meters or measured (with keratometry over the soft contact lens). This calculation can be used to show that for low minus carriers (between plano and –1.25D), the net change in exit vergence is less than 0.15D and therefore can be ignored. Drs. Malooley and Faron noted this in the May 2011 issue in their article “Contact Lens Options for Irregular Corneas.”

Plus lenses and stronger minus lenses quickly require a GP prescription change, however. Table 1 is inaccurate and should be also be revisited. Two values are of clinical interest. The first is the new exit vergence. This result determines whether a change in the GP BVP is clinically necessary. The second is the change in the GP BVP required to achieve a desired system exit vergence.

It is true that the change in power of the front soft lens surface in tears is about 20 percent of that in air due primarily to index differences. However, this one internal value cannot be viewed out of context. The vergence must be recalculated from the first surface (GP front) to the end to find the net change in exit vergence. Any correction needed must be added to the GP BVP.

Furthermore, it is possible to talk of individual lenses only if every media is evaluated as if separated by a thin layer of air. Otherwise you have a centered triplet in which the four individual interfaces can provide only partial results until combined as thick lenses to form a whole. The final exit vergence will be the same no matter which calculation method is used. Only the former method allows the various components to be evaluated individually and corrected if need be.

In passing, an “on new-K” fit will produce a plano tear lens. Thus, both GP and soft lens BVPs contribute their full power corrected for effectivity. With complex lens systems, it is essential to recalculate the system exit vergence with every change.

John C. Heiby, OD, FAAO St. Clairsville, OH

Dr. Weissman's Response

I welcome Dr Heiby's comments and participation in the evolving discussion. On first glance, however, I think the exact problem is the question of why the nominal power of the underlying soft lens is not the power that this lens assumes when in a piggyback system—and what power does it assume? Hence the new power of the lens rather than the tear layer is a major issue.

SiHys Not for Everyone

I enjoyed your Point/Counterpoint “Silicone Hydrogels: Were They Really Worth It?” in the December 2011 issue. I was disappointed that Dr. Berntsen did not address corneal infiltrative events in daily wear silicone hydrogel (SiHy) lenses. Tight lenses, preservative uptake, and biofilms are potential etiologies. These complications are not always so easy to manage, as a ramped-up corneal immune system can limit refitting success.

Dr. Young reviewed the infiltrative events with daily wear and was spot on in asserting that oxygen flux is far from the be-all, end-all of contact lens success. However, SiHy lenses are far from an “experiment.” They have been our industry's best effort, but is anyone shocked that they aren't appropriate for all patients?

At least 10 percent of my patients are not candidates for this modality and will fail miserably, perhaps due to meibomian gland dysfunction or dry eye. These problems are actually worse for many SiHy patients, as individual tear chemistries seem to exacerbate silicone's natural hydrophobic properties. Patient selection and management of complications will win the day.

William B. Potter, OD, Freehold, N.J.