Sulcus diameter outperforms white-to-white diameter in predicting vault

Sulcus diameter and white-to-white diameter also predicted different lens sizes.

Issue: May 10, 2012

Sulcus diameter proved more effective than white-to-white diameter in predicting vault height of a posterior chamber phakic IOL, a speaker said.

During Refractive Subspecialty Day preceding the American Academy of Ophthalmology meeting in Orlando, Fla., Dan Z. Reinstein, MD, MA, FRCSC, DABO, FRCOphth, presented results of a study on the predictability of vault height after implantation of the Visian Implantable Collamer Lens (ICL, STAAR Surgical).

“ICL sizing has commonly been done using a formula based on the white-to-white diameter, as recommended by STAAR Surgical, which assumes that there is a correlation between white-to-white and the dimensions of the posterior chamber where the lens is placed. However, numerous studies have now shown that either there is no correlation or only a very weak correlation,” Prof. Reinstein said. “For example, in our study, there was a statistically significant correlation but it was a very poor correlation. There’s a lot of scatter, meaning that if you were to predict sulcus diameter by taking white-to-white, 38% of the time you will have an error of more than half a millimeter, which equals one lens size.”

Dan Z. Reinstein

Undersizing of the ICL is known to cause cataracts, while oversizing causes other complications, Prof. Reinstein said. Shallow vault is also believed to spur cataract formation in some cases.

Measurements and calculations

“The purpose of this study was to compare the vault height predictability of the ICL based on sulcus diameter sizing or white-to-white sizing,” Prof. Reinstein said.

The study included 50 eyes with myopia ranging from –5 D to –22.5 D. Investigators used Artemis very high-frequency (VHF) digital ultrasound (ArcScan) to obtain a direct measurement of sulcus diameter

ICL size was calculated using an algorithm built around the sulcus diameter measurement and other anatomic variables such as anterior chamber depth. The algorithm was developed by Carlo F. Lovisolo, MD, who also performed the surgical procedures. Postoperatively, VHF digital ultrasound was used to measure the vault height achieved using the lens sized by this sulcus-based sizing formula.

In order to make a comparison with STAAR Surgical’s conventional white-to-white-based sizing formula, this formula was also used to calculate the lens size that would have been recommended. Investigators then used circle segment trigonometry to calculate the vault height that would have resulted if the white-to-white formula had been used to determine lens size.

“As we had direct measurements of the sulcus diameter and vault height and the size of the lens was known, in the cases where the white-to-white formula would have chosen a different lens size, we could calculate the vault height that would have been achieved if such a lens had been used, assuming that the lens haptics would have been in the same place,” Prof. Reinstein said.

Significant variations

“Had we used the STAAR formula, we would have used a different lens in 54% of cases,” Prof. Reinstein said.

Mean postoperative vault height was 367 µm as measured by VHF digital ultrasound in this population in which the ICL size had been selected based on a direct sulcus diameter measurement. Mean vault height would have been 163 µm if the lens size had been chosen based on the white-to-white measurement, Prof. Reinstein said.

“The standard deviation was 163 µm in the population,” he said. “It would have been 306 µm had the white-to-white lens sizing formula been used. If we convert that into a 95% confidence interval, we can see clearly that we have 95% confidence of not touching the crystalline lens in the sulcus diameter group, but we would have had a lot of cases of lens touch in the white-to-white STAAR formula group.”

In the U.S. Food and Drug Administration trial for the Visian ICL, investigators found that a lens vault of more than 90 µm was required to prevent cataract formation.

In Prof. Reinstein’s study, one eye (2%) had vault height less than 90 µm and no eyes developed cataracts after 6-year follow-up.

“If we had used the STAAR formula, vault height would have been less than 90 µm in 36% of eyes,” Prof. Reinstein said. “However, cataract formation does not occur in all cases of low vault or where the lens touches the cornea. In the FDA trial cohort, 0.6% of cases developed cataracts that required removal, so if this rate of cataract formation is applied to the results of our study, this translates to a 0.03% chance of cataract needing to be removed when lens sizing is calculated based on sulcus diameter measurements rather than white-to-white.” – by Matt Hasson

For more information:

Dan Z. Reinstein, MD, MA, FRCSC, DABO, FRCOphth, can be reached at the London Vision Clinic; email: dzr@londonvisionclinic.com.
Disclosure: Prof. Reinstein has a financial interest in Artemis very high-frequency digital ultrasound (ArcScan). He is also a consultant for Carl Zeiss Meditec.