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Results in first eye used to hone IOL calculations in second eye
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Refractive outcomes of the first eye can be used to fine-tune IOL calculations for the second eye in refractive cataract surgery.
At Hawaiian Eye 2014, Uday Devgan, MD, Healio.com/Ophthalmology Section Editor, shared pearls on adjusting the A-constant to hone IOL calculations in the second eye.
“The simple take-home message is there are two ways of doing it,” Devgan said. “The way I like is to recalculate the first eye and alter the A-constant to produce exact results. If you don’t want to do that much math, you can approximate that about 1 D in general in the refraction is about 1.5 D on the IOL.”
Challenges of lens positioning
IOL calculations hinge on three parameters: keratometric power, axial length and effective lens position, Devgan said.
“The tough one is effective lens position,” he said. “You know from putting the lens in the sulcus vs. the bag, shifting it just a little bit actually makes a lot of difference in the resultant refraction produced by the IOL. In a post-LASIK or post-PRK calculation, our main source of error is the [keratometry] values where it is hard to measure the true central power of the cornea.”
The Holladay 2, Haigis and Olsen IOL calculation formulas yield better effective lens position than the SRK, SRK II, SRK/T, Holladay 1 and Hoffer Q because they use more data to predict the position, Devgan said. These newer formulas use measurements such as anterior chamber depth, white-to-white and refraction in addition to the standard keratometry and axial length values.
“Some of the difficulty that you’re going to have is that it’s great when you have a normal axial length and a normal anterior segment size, but you can get eyes that have a short axial length and normal [anterior chamber] depth or one very large or small,” he said.
A-constant
A-constants reflect effective lens position, optic geometry and design, and variances between biometric measuring devices, Devgan said.
“The lens position of the IOL is in the A-constant,” he said. “If you have a lens that’s vaulted posteriorly, it better have a higher A-constant. The A-constant depends on the location in the eye.”
Devgan discussed a method that relies on the A-constant for recalculating the first eye and calculating the second eye. He presented a case in which the calculation for the first eye called for a 22.0 D IOL, but instead of producing a plan outcome, the patient’s postop refraction was +1.25 D with an A-constant of 118.5.
“I know that at 118.5, my 22.0 D lens gave me a hyperopic result. So, I’ll recalculate and alter the A-constant until I get the exact results of the surgery,” he said.
He recalculated the first eye using the same formula but determined that the ideal A-constant for this IOL in this eye was 120.3.
“This new A-constant reflects what happened in the first eye. So, when I calculate the second eye, I’m going to assume that what happened in the first eye with the final position of the IOL will happen in the second eye,” Devgan said.
Using the new A-constant, the calculation called for an IOL power of +24.0 D in the second eye, and the patient achieved the desired plano result. This correlates with the short-cut estimation method as well: 1.5 times the postop refraction of the first eye (+1.25) means that the ideal IOL power should have been about 2 D higher, so it would be a 24.0 D IOL instead of a 22.0 D IOL, he said.
“I just change the lens power by virtue of changing the A-constant,” Devgan said. “With that new A-constant, I’m able to achieve the correct result, which is tailored to the patient’s specific ocular anatomy.” – by Matt Hasson