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Study challenges superiority of partial coherence interferometry in measuring axial length
Invest Ophthalmol Vis Sci. 2009;50(6):2547-2552.
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IOL power calculation based on axial length measurement with partial coherence interferometry showed no clinical advantage over applanation ultrasound biometry, according to a study.
“It is generally believed that [partial coherence interferometry, PCI] offers superior accuracy of the [axial length] measurement and the IOL implant power calculation compared with [applanation ultrasound biometry],” the study authors said. “However, a review of published studies related to PCI, [applanation ultrasound biometry] and cataract surgery failed to identify any randomized controlled trials that support this belief.”
The double-masked, randomized controlled trial included 205 patients. Preoperative PCI measurements were made with the IOLMaster (Carl Zeiss Meditec); applanation ultrasound biometry measurements were performed with the Microscan Model 100A+ (Sonomed). Mean absolute postoperative refractive error was the main dependent variable. The mode of biometry was the independent variable.
PCI failed to measure axial length in 36 patients. Of 169 patients who successfully underwent PCI and applanation ultrasound biometry, 84 patients were randomly assigned to receive PCI-calculated IOLs and 85 received applanation ultrasound biometry-calculated IOLs. All patients underwent phacoemulsification with IOL implantation through a 3.2-mm superior corneoscleral incision. Postop follow-up was conducted at 7 and 12 days, and 5 weeks.
The study data showed that patients with PCI-calculated IOLs had mean absolute error of 0.4 ± 0.37 D; patients with applanation ultrasound biometry-calculated IOLs had mean absolute error of 0.45 ± 0.41 D. The difference was not statistically significant, the authors said.
“It is likely that the apparent improvement of PCI over [applanation ultrasound biometry] reported in present studies will not be reinforced in future randomized clinical trials,” they said.
There are three problems with the design of the study.
1. The number of patients is too small to show a difference in a normal population. Ninety-five percent of the patients are within ± 0.2 mm of the mean axial lengths with each instrument. Only about five patients in each group would have had unusually short or long eyes, and this is where the difference would be significant between OCT and applanation ultrasound biometry.
2. SRK/T was used. Several studies have shown that the SRK/T rarely performs equal to other newer-generation IOL formulas such as the Holladay 1, Hoffer Q and Haigis formulas, and these are almost 20 years old. More recent formulas that use additional preop measurements such as horizontal corneal diameter (white-to-white), anterior chamber depth and lens thickness may have shown a difference, even with this small study.
3. The manufacturer’s lens constant was used. The constant should have been optimized for both groups to eliminate bias. Manufacturers’ lens constants were developed with ultrasound, and they are clearly different than with the IOLMaster. The A-constant, or more accurately the effective lens position, for the IOLMaster is about 0.3 D higher on the average but varies depending on the IOL. The only objective way to evaluate these is to optimize the constant for both instruments. This is a significant bias for ultrasound and would result in a significantly higher mean absolute error for the IOLMaster.
The conclusion should be that in a small group of normal axial length patients, using the ultrasonically determined manufacturer’s A-constant and the SRK/T formula, no significant difference in mean absolute prediction error was found, as would be expected.
– Jack T. Holladay, MD, MSEE
OSN Optics
Section Editor