*Adi Abulafia, MD and Warren E. Hill, MD*

Although less frequent today than in years past, even with significant advances in measurement technology and formula accuracy, refractive surprises following all forms of lens-based surgery still occur. With patient expectations steadily increasing, surgeons should have in place a well-developed strategy for enhancing an unanticipated refractive outcome that includes the placement of a piggyback intraocular lens (IOL) or a lens exchange.

Situations in which refractive surprises occur most commonly are seen in the setting of high myopia and hyperopia, unusual anterior segments with exceptionally deep or shallow anterior chambers, small or large corneal diameters, the extremes of central corneal power, or any combination of these.

When faced with a refractive surprise, 5 basic management options are available: spectacle correction within the limits of tolerated anisometropia, the placement of a contact lens, corneal refractive surgery by LASIK or photorefractive keratectomy (PRK), a secondary piggyback IOL, or an IOL exchange.

**PIGGYBACK INTRAOCULAR LENS**

There are several requirements for a successful piggyback IOL. First, the primary IOL should be contained completely within the capsular bag and preferably have 360 degrees of anterior capsule overlap. There should also be satisfactory space for another IOL between the posterior surface of the iris and the anterior surface of the primary IOL. This can usually be determined at the slit lamp. In addition, the capsular bag IOL should not have a strongly positive shape factor in the form of a steep anterior radius, as is sometimes seen with very high plus (+) power lenses. A steep anterior radius for the capsular bag IOL may displace the ciliary sulcus IOL forward or cause it to shift to one side of the visual axis.

The first advantage of a piggyback IOL is that the power of the capsular bag IOL does not need to be known. When using a piggyback IOL, optical power is simply being added to, or subtracted from, an existing optical system. A second advantage is that visual rehabilitation is usually rapid and the procedure carries a relatively low risk when compared with that of a lens exchange. And most importantly, determining the power of a piggyback IOL is an axial length–independent exercise. If the spherical equivalent (SphEq) of the refractive error is within ± 7.00 diopters (D), and the Ks are in normal range, only the manifest refraction need be known if the intended refractive outcome is plano.^{}1

*Calculating the Power of a Piggyback Intraocular Lens*

*Calculating the Power of a Piggyback Intraocular Lens*

To calculate the power of a piggyback IOL, a simple rule of thumb is the following:

▲ For hyperopic refractive errors with a SphEq equivalent of less than +7.00 D, the SphEq is multiplied by 1.50 for a plano result. For example, for a refractive surprise of +3.75 +0.50 x 180, with Ks in a normal range, the piggyback IOL power for a plano result would be +4.00 D x 1.50 = +6.00 D.

▲ For myopic refractive errors of less than -7.00 D, the SphEq is multiplied by 1.30 for a plano result. For example, for a refractive surprise of -1.75 +0.50 x 180, with Ks in a normal range, the piggyback IOL power for a plano result would be -1.50 D x 1.30 = -1.95 D. Here, a -2.00 piggyback IOL would be used.

▲ For hyperopic and myopic refractive errors of greater than ± 7.00 D, for unusual Ks, as are seen with refractive surgery, or if the refractive target is something other than plano, more accurate results are obtained using the refractive vergence formula or the Barrett Rx formula.

Working with the refractive vergence formula requires knowing the SphEq of the manifest refraction, current Ks, and the effective lens position of the IOL to be implanted. Once again, the great advantage here is that this is an axial length–independent exercise. Repeating the calculation using the same measurements but a different theoretical formula may lead to disappointments. For most cases, it was an incorrect assumption by a theoretical formula that resulted in the original refractive surprise.

As a general rule of thumb, the effective lens position of the piggyback IOL is simply the optimized effective lens position for the capsular bag lens that has been reduced by 0.65 mm. An optimized lens constant for the piggyback IOL would not be appropriate, as this second IOL will be sitting on top of the capsular bag IOL. See Table 19-1 for a method of approximating the effective lens position based on an optimized SRK/T A-constant.

For example, a Crystalens (Bausch & Lomb) patient undergoes scleral buckling more than a year after cataract surgery. A capsulotomy has already been completed. Due to axial length elongation by the scleral buckle, the refractive error has now shifted myopic and stabilizes at -2.50 + 0.50 x 180 for a SphEq of -2.25 D. The Ks are 42.25 x 42.75, and the capsular bag IOL has an optimized effective lens position of 5.61 mm. The adjusted piggyback effective lens position would therefore be 4.96 mm. The refractive target is plano.

Abbreviation: ELP, effective lens position.

For this scenario, 3.04 D less power would be needed, added in the form of a minus (-) power secondary piggyback IOL at the plane of the ciliary sulcus in order to achieve the intended refractive outcome. The surgeon selects a -3.00 D AQ-5010V (Staar Surgical) piggyback IOL. See Figure 19-1 for the calculation carried out using the refractive vergence formula.

An explanation of how the refractive vergence formula works, along with examples, can be found on the Internet at www.doctor-hill.com/iol-main/piggyback.htm. A free, downloadable refractive vergence formula calculator in a Microsoft Excel format based on a description by Holladay^{2,3} can be downloaded at www.doctor-hill.com/physicians/download.htm.

The Holladay R formula contained within the Holladay IOL Consultant (Holladay Consulting) software is a more sophisticated, commercially available version of the refractive vergence formula.

The Rx formula, recently introduced by Graham Barrett (available at www.apacrs.org), provides a solution for 3 scenarios: piggyback IOL, IOL exchange, and the optimal rotation of an existing toric IOL to a new meridian of alignment.

The Barrett Rx formula is derived from the Barrett Universal II formula. It first determines the actual ELP for the postoperative refractive error and uses this as the basis for the calculation. For this reason, the results may be somewhat different than for the Holladay R formula, or the refractive vergence formula, which use an IOL-specific value as the basis for the ELP. A shortcoming of this method is that the predicted outcome remains unchanged when different IOL powers are implanted for a particular refractive outcome.

The Barrett Rx formula also allows surgeons to select an alternative IOL option when the calculation assumes the predicted ELP is correct. This option should be chosen for eyes with prior refractive surgery, or if the error is thought to be due to a mislabelled or unknown IOL power. In this case, the piggyback IOL prediction will be similar to that of the Holladay R and the refractive vergence formulas.

The Barrett Rx formula requires axial length, postoperative Ks, optical anterior chamber depth, power of the implanted IOL, and 2 lens constants: 1 for the actual IOL implanted and 1 for the IOL selected for an IOL exchange. The constant for the IOL can be adjusted if the IOL exchange is planned for the sulcus, and this is indicated in a popup window.

The Barrett Rx formula will predict the resultant spherical equivalent, the IOL power, and also the recommended toric cylinder for an IOL exchange or a piggyback IOL.

*Choosing a Piggyback Intraocular Lens*

*Choosing a Piggyback Intraocular Lens*

When choosing a piggyback IOL, it is generally best to select a 3-piece silicone lens with rounded edges. For powers from -4.00 D to +4.00 D, many consider the STAAR AQ-5010V (Figure 19-2) to be an ideal choice, with its relatively large 6.3-mm optic with smooth, rounded edges and a 14.0-mm haptic length. For higher plus (+) powers, the STAAR AQ-2010V is frequently used. In the United States, placing an IOL in the ciliary sulcus remains an off-label use. However, in other countries, there are available IOLs that are approved for sulcus implantation, such as the popular Sulcoflex (Rayner Intraocular Lenses Limited).

*Important Caveat*

*Important Caveat*

In general, when selecting a piggyback IOL, it is best to avoid the use of 3-piece acrylic IOLs. Unless there is a large amount of space between the anterior surface of the capsular bag IOL and the posterior iris, the combination of square, truncated edges and a semi-tacky nature of the acrylic material may result in interaction with the posterior iris. This may lead to any combination of transillumination defects, pigment dispersion, secondary glaucoma, and intermittent uveitis. A single-piece acrylic IOL would never be placed in the ciliary sulcus.^{4}

*Piggyback Intraocular Lens Surgical Technique*

*Piggyback Intraocular Lens Surgical Technique*

Placing a piggyback IOL differs from primary lens implantation in that there is generally less room in which to work and the intended location is always the ciliary sulcus. When implanting silicone optic piggyback IOLs, it is often helpful to work under a highly retentive viscoelastic, such as Healon GV (Abbott Medical Optics) and have the optic exit the insertion cartridge nozzle as slowly as possible to avoid the lens rapidly and forcefully unfolding in the anterior chamber. There is no particular alignment necessary for the haptics. Following the removal of viscoelastic, a quick check confirms that adequate space exists between the posterior iris and the anterior surface of the IOL.