contact lens practice pearls
Piggyback Lens Systems Primer
BY THOMAS G. QUINN, OD, MS, FAAO
Piggybacking a GP lens over a soft lens provides a unique system, offering benefits that can move an unsuccessful fit into the winner's column. The most common beneficiaries are patients suffering from conditions such as keratoconus that result in an irregular corneal surface.
Due to the added complexity of a dual lens system, piggyback lenses are generally reserved as a last-ditch effort to salvage an unsuccessful attempt at fitting an irregular cornea with a GP lens. Your early efforts can still bear fruit — you may be able to simply tuck a soft lens under the GP lens that you've put so much effort into designing.
Choose a soft lens power of plano to –0.75D. A lens of –0.75D will have a front surface that is nearly parallel to its back surface, so it won't dramatically alter the shape to which you're fitting the base curve of the GP lens.
If you're starting a piggyback fit from scratch, base your GP lens base curve selection on keratometry or topographic values taken over the soft lens surface.
Soft Lens Power Effects
Sometimes it's advantageous to manipulate the soft lens power to alter the surface on which you will place the GP lens. For example, a –5.00D soft lens fit over a very steep keratoconic cornea will provide a flatter, more easily fit surface upon which to place the GP lens. Alternatively, a +5.00D soft lens would steepen the fitting surface over an extremely flat post-RK cornea.
Choosing Soft Lens Materials
The potential double barrier that piggyback lens systems have on the cornea's oxygen supply dictates that we start with higher-Dk silicone hydrogel lens materials. When fitting steep corneas, as is often the case in keratoconus, begin with the steepest base curve available, such as an 8.4mm Night and Day (CIBA Vision).
In some cases, such a lens won't be steep enough, resulting in lens edge fluting. If you observe this, switch to a more flexible (lower modulus) silicone hydrogel lens, such as Acuvue Oasys (Vistakon). If edge fluting persists, switch to one of the many hydrogel lenses available in extremely steep base curves.
If struggling with poor GP lens centration over the soft lens surface, consider the Flexlens piggyback cut-out (X-Cel Contacts), a hydrogel lens that features a countersunk front surface.
Choosing GP Lens Materials
As with the soft lens material, high oxygen transmissibility in the GP lens is particularly attractive. In some cases, high-Dk GP lenses are more prone to flexure, resulting in less than optimal vision.
If the fit looks great but the vision is disappointing, consider switching to a moderate-Dk lens material, such as Boston ES (Bausch & Lomb) or Fluoroperm 60 (Paragon Vision Sciences). These materials can also solve problems with surface wetting.
Employ a GP daily cleaner to care for the GP lens, but then soak both the GP and soft lens in a multipurpose (MPS) soft lens care system. Do not use GP solutions with soft lenses.
If you employ a hydrogen peroxide-based system, you'll probably need to apply some MPS to the GP lens surface before application to achieve adequate surface wetting. If GP surface wetting is a problem, switch to a lower-Dk GP lens.
Another good strategy to avoid solution-related problems is to employ a daily disposable soft lens under the GP lens.
Only use rewetting drops approved for use with soft lenses.
A More Viable Option
In the past, the greatest challenge to success with a piggyback lens system was hypoxia. Recent advancements in both soft and GP material chemistry have largely removed this obstacle, increasing the attractiveness of this potential problem-solver. CLS
Dr. Quinn is in group practice in Athens, Ohio. He is a diplomate of the Cornea and Contact Lens Section of the American Academy of Optometry and an advisor to the GP Lens Institute.