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BLOG: IOL formulas and effective lens position

ByJosh Johnston, OD, FAAO

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Choosing the correct IOL power is key to a successful cataract surgery. This power is based on the eye’s anatomical and optical measurements.

One of the most important of these is the axial length (AL), and other components include corneal power and keratometry (K). Depending on the type of formula used, other parameters may be required, such as preoperative anterior chamber depth (ACD) and the corneal white-to-white (WTW) distance.

Where the lens ends up

Knowing where the lens will end up after surgery, the effective lens position (ELP), is the main limiting factor for refractive predictability. Manufacturers build this parameter into the IOL by using an A-constant for the specific lens, which is optimized based on the patient’s biometry and the surgeon’s technique.

IOL formulas

There are many IOL formulas; most are based on AL and K and a single IOL constant (Holladay, SRK/T and Hoffer Q3-6). The Haigis formula uses three measurements, AL, K and preoperative ACD with three IOL constants. Olsen's formula uses two additional factors based on preoperative refraction and lens thickness with one IOL constant. The Holladay 2 formula is based on seven measurements, including the patient's age and the horizontal WTW measurement. The Barrett formula uses a theoretical model eye in which ACD is related to AL and K and is also determined by the relationship between the A-constant and a “lens factor.”

Matching formulas to eyes

It has been reported that for normal ALs of 22.5 mm to 24.5 mm most formulas work well. Hoffer sought to determine which formula was the most accurate for ALs shorter or longer than normal, and he reported that the Hoffer Q formula was the most reliable results in short eyes (AL less than 22.0 mm), and the SRK/T formula was best in long eyes (AL greater than 26.0 mm).

The Hoffer Q formula provided the best refractive outcomes in eyes shorter than 21.00 mm, and the Holladay 1 and Hoffer Q formulas were equally reliable for eyes with an AL between 21.00 mm and 21.49 mm, according to a recent database study (Aristodemou et al.). The authors found that the SRK/T formula performed significantly better for eyes with an AL of 27.00 mm or longer.

The Barrett II formula may be better for more myopic patients, according to the findings. Other studies have shown high accuracy of the Haigis formula in extreme hyperopia (Zhang et al., MacLaren et al.).

Fourth- and fifth-generation formulas use more variables, which help to better predict ELP.

References:

Aristodemou P, et al. J Cataract Refract Surg. 2011;doi:10.1016/j.jcrs.2010.07.032.

Fenzl RE, et al. Ophthalmology. 1998;doi:10.1016/S0161-6420(98)99050-9.

Haigis W. J Refract Surg. 2009;25:229-234.

Haigis W, et al. Graefes Arch Clin Exp Ophthalmol. 2000;239:765-773.

Hoffer KJ. J Cataract Refract Surg. 1993;19:700-712; errata 1994;20:677.

Hoffer KJ. J Cataract Refract Surg. 2000;26:1233-1237.

Holladay JT, et al. J Cataract Refract Surg. 1988;14:17-24.

MacLaren RE, et al. Am J Ophthalmol. 2007;doi:10.1016/j.ajo.2007.02.043.

Olsen T. J Cataract Refract Surg. 2006;doi.10.1016/j.jcrs.2005.12.139.

Retzlaff JA, et al. J Cataract Refract Surg. 1990;16:333-340; correction, 528.

Zhang Y, et al. J Ophthalmol. 2016;doi:10.1155/2016/1917268.