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Achieving the proper IOL power for patients who have had a refractive procedure such as LASIK, PRK, or RK can be challenging, and taking more measurements leads to greater accuracy, said Jack T. Holladay, MD, MSEE, FACS, Holladay LASIK Institute and Baylor College of Medicine, Houston.
Achieving the proper IOL power for patients who have had a refractive procedure such as LASIK, PRK, or RK can be challenging,and taking more measurements leads to greater accuracy, said Jack T. Holladay, MD, MSEE, FACS, Holladay LASIK Institute andBaylor College of Medicine, Houston.
"If we're going to promise our patients who get a multifocal lens that they're not going to need to wear glasses, or thatthey're not going to need to wear glasses for distance if they get a monofocal lens, there are a number of parameters thatwe're going to have to get right," he said.
"The trouble with arriving at the effective lens position is that we can't measure it preoperatively because the patientstill has a cataract, and we can't choose it like we do the postoperative refraction," Dr. Holladay said. "So we have topredict [the] value."
The first "must" in that prediction is accurate biometry, Dr. Holladay said. The standard today has become optical coherencetomography, he said, adding, however, that ultrasonic measurements still are required in about 10% to 15% of cases because ofthe presence of a dense cataract.
Axial length also is an extremely important consideration, as is anterior segment size, Dr. Holladay said. The two are notthe same, he emphasized, and an accurate eye model must include nine types of eyes: small, normal, and large anterior segmentsizes in eyes with short (<21 mm), normal, and long (>27 mm) axial lengths.
"For example, we find that short axial lengths, down to 15 mm, 80% of those have a horizontal white-to-white measurementthat's normal. In other words, the anterior segment of a very short eye is almost independent of the axial length," Dr.Holliday said. "And when you take long eyes, out to 35 mm, you find that these long eyes still have 90% normal white-to-white[measurement]."
He added that, in treating patients who have undergone LASIK or PRK, ophthalmologists trying to arrive at the proper IOLpower can "tighten the boundaries" slightly because the corneas in these patients are not as multifocal.
"RK patients, on the other hand, depending on how many incisions were made, how big the optical zone is, and so forth, have alot more variability, so it's a lot harder to get the exact target," Dr. Holladay said.