where A is the IOL-specific A constant and pACD is the Hoffer pACD (ELP). So, for example, an A constant of 113.78, 116.35, or 118.92 converts to a pACD of 2.50 mm, 4.00 mm, or 5.50 mm, respectively.
It is prudent to calculate the power of an alternate IOL before surgery. If not calculated in advance, the power of an IOL intended for bag placement can be decreased for sulcus placement with subtraction of 0.75–1.50 D, depending on the AL value.
Formula choice
Several studies have indicated that the Hoffer Q formula is more accurate for eyes shorter than 24.5 mm; the Holladay 1, for eyes ranging from 24.5 to 26.0 mm; and the SRK/T, for eyes longer than 26.0 mm (very long eyes). A recent (2011) study conducted in the United Kingdom proved the statistical significance of these recommendations in more than 8000 eyes by use of optical AL values. For long eyes, the Haigis formula may achieve equivalent results.
The choice of formula is, of course, up to the surgeon, but whatever the method, every effort should be made to ensure that the biometry is as accurate as possible. The operating surgeon should review preoperative AL values and K readings. If a reading is suspect because it lies outside normal limits, biometry should be repeated during or immediately after the initial reading. Similarly, it is prudent to measure both eyes and recheck the readings if there is a large discrepancy between the 2 eyes. Great care should be taken in the measurement of eyes that have undergone previous refractive surgery (corneal or phakic IOL), as well as those that have undergone an encircling band treatment of a retinal detachment.
Piggyback and Supplemental Intraocular Lenses
When an IOL is inserted into an eye that already has an IOL, the second IOL is called a piggyback IOL. The piggyback IOL can be inserted at the time the first IOL is implanted to produce a high power that is commercially unavailable. It can also be inserted secondarily to correct a postoperative refractive error. Computer programs can be used to calculate the power of the second IOL and to make adjustments, which may be needed if the posterior IOL is displaced posteriorly. However, these adjustments are minor, and using one of the following formulas is the easiest way to calculate them:
Myopic correction: P = 1.0 × error
Hyperopic correction: P = 1.5 × error
where P is the needed power in the piggyback lens, and error refers to the residual refractive error that needs to be corrected.
Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL. Formula choice: Hoffer Q, Holladay 1, or SRK/T and refractive outcomes in 8108 eyes after cataract surgery with biometry by partial coherence interferometry. J Cataract Refract Surg. 2011;37(1):63–71.
Intraocular Lens Power Calculation After Corneal Refractive Surgery
IOL power calculation is a problem in eyes that have undergone radial keratotomy (RK) or laser corneal refractive procedures such as photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), and laser subepithelial keratomileusis (LASEK). The difficulty stems from 3 sources of errors: (1) instrument error, (2) index of refraction error, and (3) formula error.
Instrument Error
Instrument error was first described by Koch in 1989. The instruments used by ophthalmologists to measure corneal power (keratometers and corneal topographers) cannot obtain accurate measurements in eyes that have undergone corneal refractive surgery. These instruments often miss the central, flatter zone of effective corneal power. The flatter the cornea is, the larger the zone of measurement is, and the greater the error. Topography units do not correct this problem, either; rather, they usually overestimate the corneal power, leading to a postoperative hyperopic refractive error in myopic eyes.
Index of Refraction Error
The assumed index of refraction (IR) of the normal cornea is based on the relationship between the anterior and posterior corneal curvatures. This relationship changes in eyes treated with PRK, LASIK, and LASEK. Ophthalmologists long believed that IR error did not occur in eyes that have undergone RK. This situation leads to an overestimation of the corneal power by approximately 1 D for every 7 D of correction obtained. A recent study showed that in eyes treated with RK, there is greater flattening of the posterior curvature than of the anterior curvature. A manual keratometer measures only the front surface curvature and converts the radius of curvature (r) obtained to diopters (D), usually by using an IR value of 1.3375. The following formula can be used to convert diopters to radius:
To convert r to D, use
Formula Error
With the exception of the Haigis formula, all of the modern IOL power formulas (eg, Hoffer Q, Holladay 1 and 2, and SRK/T) use the AL values and K readings to predict the postoperative position of the IOL (ELP). The flatter-than-normal K value for eyes treated with RK, PRK, LASIK, or LASEK causes an error in this prediction because the anterior chamber dimensions do not actually change in these eyes commensurately with the much flatter K.