Despite the growing popularity of scleral lenses, corneal lens designs are still useful in many cases. Sclerals are able to overcome two issues with corneal designs—comfort and centration—but there are other ways to get around these issues.
Although it’s less commonly used, “piggybacking” a corneal GP on top of a soft lens can be a useful modality in the right circumstances. This combination is used in approximately 2% of contact lens-wearing keratoconus patients (Romero-Jiménez et al, 2015). It is particularly useful in low-positioned cones, as corneal lenses alone tend to decenter inferiorly. Using this combination often improves the centration and decreases lens movement, improving patient comfort.
So how do we fit these? For the soft lens, a low-plus power (around +0.50D) is recommended for normal corneas (Michaud et al, 2013). This is thought to shift the highest soft lens elevation over the central cornea, aiding in centration of the GP. For keratoconic eyes, a moderate minus power (around –3.00D) can be useful, as the flatter front surface and thicker mid-periphery can provide a more natural shape on which the GP can rest, enhancing centration (Romero-Jiménez et al, 2015). It also allows for a flatter GP lens base curve, which can reduce its power and thickness as well.
Silicone hydrogels are typically used for their higher Dk and higher modulus, which helps the lens to better hold its shape over corneal irregularities (Sherman and Wilson, 2017). The power of the soft lens does not contribute much to the system when the GP sits on top of it; its power is reduced to about one-fifth of its labeled value (Romero-Jiménez et al, 2015). Clinically, all you need to worry about is over-refracting the piggybacked system and making any needed adjustments to the GP lens power.
Once the lenses are on the eye, assessment of the fit does not change. The fluorescein pattern, centration, and movement should all be assessed. An alignment-fit GP is the general goal (Figure 1). If the soft lens does not center well over the cornea, the GP may become unstable and also decenter. A steeper base curve soft lens may help.
There are also specialty soft lenses available with anterior cutouts in which the GP can ride. These lenses are designed to resolve GP decentration and decrease edge awareness as the GP lens is recessed in its carrier.
In theory, the piggyback system is great. However, when you double the lenses, you double the work. Two lenses per eye may increase the overall cost with the added inconvenience of cleaning and caring for an additional pair of lenses. Daily disposable lenses can make things easier for patients.
Although piggybacking is not a common lens modality, this system can be a simple fix for your patients who have developed lens intolerance or for unstable and decentering lenses. It’s a simple addition to your contact lens toolbox. CLS
For references, please visit www.clspectrum.com/references and click on document #267.