Piggyback Optics from a Clinician’s Perspective

I am writing in response to the GP Insights column titled “Piggyback Contact Lens Systems” by Karen DeLoss, OD, that appeared in the October 2021 issue. Piggyback contact lens systems by definition require that thick lens optics be considered. The back vertex power (BVP) of the soft lens and the BVP of the previous GP lens cannot be simply added, because they are components of a new thick lens system. This system also includes the tear lens, which now has a new posterior surface created by the front curve of the soft lens. This new “K” can be calculated from the soft lens specifications (index, BVP, thickness) or measured with a keratometer.

Placing a negative-power soft lens on the eye changes the refractive error and forms a new K. If the contact lens does not have a refractive index near that of tears, there will be a net decrease in minus in the system: BVP versus the sum of the two BVPs. Plus lenses have the opposite effect.

For example, suppose that a patient has a K of 7.00, the GP lens has a 7.00mm base curve and power of –3.00D, and the soft lens is a Dailies Total1 (Alcon) with –3.00D power. A sphero-cylindrical over-refraction (SCOR) over just the GP lens is plano. A SCOR over just the soft lens is plano. However, the SCOR is +0.41 when the GP lens is piggybacked over the soft lens.

The soft lens on a 7.00mm cornea with a –3.00D BVP should have a 7.40mm front surface, assuming that the center thickness is 0.10mm and the index (n) is 1.42. An on-K fit with a BVP of 7.40/plano with a –3.00D soft lens should have a plano SCOR. In this case, the full soft lens power is realized.

The original GP with base curve of 7.00mm and –3.00D power now with a –6.00D Dailies Total1 lens would yield an expected +1.00 over-refraction. This is clinically significant. The new K in this example may require a refit of the GP lens.

There is no rule-of-thumb that you can hang your hat on. If the carrier is a moderate-minus or low-plus lens, take a chance and ignore it until a GP change is needed. No power is ever lost, it is just hiding in plain sight.

A firm contact lens requires thick lens optics considerations, and piggyback lenses add an additional layer of complexity. An optically thick GP lens and a thin tear lens combination can be simplified to a BVP and a thin tear lens. In this case, the front tear lens and the BVP can be changed with predictable results. Purists will point out that it is actually the front radius of the GP lens that is changed. It remains up to the lens designer to calculate the actual surfaces required to correct the ametropia referenced to the corneal apex. It is permissible due to the high powers of the GP lens surfaces to simply add any prescription corrections to the presenting GP lens BVP. Thick lens calculations are not necessary to achieve a clinically accurate correction at the corneal apex.

The principles do not change with the addition of a soft contact lens. In this case, the refractive error should be vertexed to the apex of the soft lens, although only a small error is introduced by simply adding the soft lens BVP to the refractive error. The soft lens front radius (r front soft lens [mm]) now becomes the new K and replaces the previous tear lens back surface (–K). This change in the back tear lens is responsible for the confusion with piggyback optics.

The net result of adding the soft lens is not just the additional soft lens BVP but also the new tear lens back minus (–K). The new K can be measured with a keratometer or calculated using Morton Sarver’s formula.

If the original GP lens is retained, only a small change is created by the carrier BVP as long as plus lenses and moderate minus lenses are avoided.

Note that if an on-K (radius [r] = the carrier’s front r) fit is used that the full soft lens BVP (actually, front vertex power) will contribute to the system BVP at the apex of the carrier lens.

In summary:

1. Try a low-power soft lens carrier in addition to the original GP lens if comfort is a problem. Don’t worry about prescription changes yet.
2. Thick lens considerations can be limited to the design of the GP lens and to the calculation of the soft lens front radius (new K).
3. Corrections to the GP BVP are made in the accustomed way to correct the ametropia at the reference apex.
4. The only place to make a correction is the back vertex location of the GP (technically, the front GP curve.)

I hope that this clears up the mystery of “lost soft lens power” when piggyback systems are used.

The following formula was calculated by Morton Sarver to simplify GP lens design calculations with spreadsheets: r front soft lens (mm) = (((n soft lens – 1) x r cornea (mm) x 1000)/(BVP soft lens x r cornea (mm) + (1000 x (n soft lens – 1)))) + (((n soft lens – 1)/n soft lens) x center thickness (mm) soft lens).

John C. Heiby, OD
St. Clairsville, OH

Dr. DeLoss’ Response

I thank Dr. Heiby for his letter and for sharing his clinical experiences with piggyback lens optics in practice. It is also important to remember that piggyback lens optics are different for irregular corneas as opposed to regular corneas.

There is a great deal of evidence in the literature and information through other sources regarding the optics of piggyback lens systems and how to fit piggyback lenses, available at the following:

• Baldone JA. The fitting of hard contact lenses onto soft contact lenses in certain diseased conditions. Contact Lens Med Bull. 1973;6:15-17.
• Michaud L, Brazeau D, Corbeil M-E, Forcier P, Bernard P-J. Contribution of soft lenses of various powers to the optics of a piggy-back system on regular corneas. Cont Lens Anterior Eye. 2013 Dec;36:318-323.
• López-Alemany A, González-Méijome JM, Almeida JB, Parafita MA, Refojo MF. Oxygen transmissibility of piggyback systems with conventional soft and silicone hydrogel contact lenses. Cornea. 2006 Feb;25:214-219.
• Nichols JJ, Willcox MDP, Bron AJ, et al. The TFOS International Workshop on Contact Lens Discomfort: executive summary. Invest Ophthalmol Vis Sci. 2013 Oct 18;54:TFOS7-TFOS13.
• GP Lens Institute. Contact Lens Clinical Pocket Guide. Available at www.gpli.info/fitting-guide . Accessed Feb. 14, 2022.
• Bennett E, Henry V. Clinical Manual of Contact Lenses. 4th ed. Philadelphia: Lippincott, Williams and Wilkins; 2013.
• Romero-Jiménez M, Santodomingo-Rubido J, Flores-Rodríguez P, González-Méijome JM. Which soft contact lens power is better for piggyback fitting in keratoconus? Cont Lens Anterior Eye. 2013 Feb;36:45-48.
• Steele C. Contact lens fitting today: Fitting for the irregular cornea. Optometry Times. 2005 Oct 21:32-39. Available at www.yumpu.com/en/document/read/25026193/contact-lens-fitting-today-david-thomas-contact-lenses . Accessed Feb. 14, 2022.
• Chang CY, DeNaeyer GW. Specialty Contact Lenses: Treat Your Keratoconus Patients Right. Rev Cornea Contact Lens. 2017 Mar/Apr:14-17. Available at www.reviewofcontactlenses.com/article/specialty-contact-lenses-treat-your-keratoconus-patients-right . Accessed Feb. 14, 2022.