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Up Front | Mar 2008

Equivalence, Differences Identified in Biometric Analyses

Anterior segment measurements between instruments are comparablein some cases, but not in all.

Biometric analyses of the anterior segment have become increasingly important in the decision-making process for any anterior segment surgical protocol, especially in the pre- and postoperative evaluation of phakic IOL implantation. The size of the phakic IOL should be selected based on the anatomic dimensions of the anterior chamber in order to avoid complications, such as pupil ovalization, endothelial loss, or flare reaction.1

Currently, the sizing of angle-supported and posterior chamber phakic IOLs is still based on calculations using the corneal or white-to-white (WTW) diameter.2,3 However, this external measurement is not necessarily related to intraocular dimensions. Theoretically, the size of angle-supported phakic IOLs should be selected according to the angle-to-angle (ATA) distance and the size of posterior chamber phakic IOLs according to the sulcus-to-sulcus distance. A device for imaging of the anterior segment is needed to obtain data on the anatomical configuration of the eye; however, not all physicians have access to this technology in their clinics.

Several devices for anterior segment imaging with different physical bases have been developed. These can be classified into three categories: Scheimpflug-based systems, very high frequency (VHF) ultrasound scanning devices, and optical coherence tomography (OCT) systems. With some instruments, corneal contact is necessary for taking measurements, or immersion of the eye in balanced saline solution is required to provide a medium for transmission of the waves generated by the transducer. Other systems are noninvasive. In our institution, we have performed two studies4,5 with these new imaging systems to evaluate their applicability for intraocular refractive surgery and interchangeability.

COMPARING VHF ULTRASOUND, OCT
The aim of the first study433'18 was to analyze the similarities and evaluate the interchangeability of two anterior segment imaging devices: a VHF ultrasound scanning system (Artemis 2; Ultralink, St. Petersburg, Florida; Figure 1) and an OCT system (Visante OCT; Carl Zeiss Meditec AG, Jena, Germany; Figure 2).

This study included 20 eyes (20 patients) selected randomly and with no active ocular pathology or previous ocular surgery. In all eyes, anterior segment analysis was performed using a horizontal scan with the two instruments, nasally and temporally measuring five anatomic parameters: anterior chamber depth (ACD), central corneal thickness (CCT), ATA distance, and iridocorneal angle size (IAS). To ensure the accuracy of the procedure, each measurement was performed three consecutive times, and the intrasession repeatability was analyzed. Bland-Altman analysis was used to assess the interchangeability of the two imaging techniques for measuring the five anatomical parameters.

No statistically significant differences were found between imaging techniques in ACD, CCT, or ATA (P>.4; bilateral unpaired Student t test). The Bland-Altman ranges of agreement were 0.2 mm, 16.11 µm, and 0.8 mm for ACD, CCT, and ATA, respectively. These ranges are not clinically relevant, meaning that both imaging techniques can be used interchangeably for measuring these three parameters. Regarding IAS, no statistically significant differences between the devices were found in the nasal (P=.78; bilateral unpaired Student t test) or temporal (P=.63; bilateral unpaired Student t test) measurements. However, the range of agreement for nasal (14.3?) and temporal (14.90?) values was relevant, indicating that the two techniques cannot be used interchangeably for IAS measurement. Excellent intrasession repeatability scores were obtained in all cases (coefficient of variation, intraclass correlation coefficient).

We concluded that the Artemis 2 VHF ultrasound system and Visante OCT system can be used interchangeably for measurement of the ACD, CCT, and ATA. For the angle measurements with both devices, it is necessary to define new objective parameters to avoid the bias introduced by subjective delimitation of the angle structure.

For measuring parameters behind the iris, such as sulcus-to-sulcus distance or ciliary body size, only ultrasonic methods such as the Artemis 2 VHF ultrasound system can be used; anterior OCT systems cannot detect these structures due to the masking effect of the pigmented layers of the anterior uvea.

COMPARING CORNEAL TOPOGRAPHY, OCT
The objective of the second study5 was to evaluate the equivalence of the traditional corneal or WTW diameter, as measured with digital calipers on the CSO corneal topographer (Costruzione Strumenti Oftalmici, Florence, Italy) and the ATA diameter as measured by Visante OCT.

As noted above, sizing of angle-supported IOLs is still based on wrong approximations, based on the WTW diameter and not on an objective measurement of the place where the lens is to be implanted. Therefore, our study assessed whether this method of lens sizing is appropriate.

Thirty eyes (19 patients) were randomly selected and included in this study. In all eyes, the horizontal WTW corneal diameter and the horizontal ATA distance were measured. WTW diameter was measured using the digital calipers on the CSO corneal topography system, and the ATA distance was obtained with the Visante OCT following the procedure described by Kohnen and colleagues.6

Each measurement was taken three times consecutively to evaluate intrasession repeatability (coefficient of variation, intraclass correlation coefficient). Interchangeability of the two measurement devices was analyzed using the Bland-Altman method.

Mean horizontal WTW distance measured with the CSO topographer was 12.25 ±0.49 mm (range, 11.34–13.16 mm). Mean horizontal ATA distance measured with the Visante OCT system was 11.76 ±0.52 mm (range, 10.03–12.92 mm). The difference between these two parameters was statistically significant (P<.01, nested mixed-effects analysis of variance). No statistically significant interaction effects were detected between eyes, persons, or groups. Correlation between WTW and ATA distances was weak but statistically significant (Pearson r=0.39; P=.03). Linear regression analysis revealed a poor model with weak predictability (R2=0.03) due to the presence of influential points and significant data dispersion (Figure 3).

Bland-Altman range of agreement was 1.10 mm below and above the mean difference, which implies a clinically relevant error. Intrasession repeatability scores were excellent, which confirms the consistency of the data.

We concluded that the WTW and ATA distance measurements were not equivalent. There was no linear model to help predict the ATA from the WTW. The great variability in the data did not allow us to extract a strong and consistent linear relation between the two parameters. We also concluded that the two parameters are not clinically interchangeable because they are not comparable. The ATA cannot be predicted accurately from the WTW using linear equations, as some phakic IOL manufacturers state. However, angle-supported phakic IOL manufacturers still recommend using linear relations rather than ATA distance measurements to obtain the proper IOL size.

Equations for phakic IOL size calculation (eg, horizontal WTW + 0.5 mm or [horizontal WTW + vertical WTW]/2) are still being used, although they provide incorrect approximations of ATA. Therefore, adequate knowledge of anterior segment configuration is mandatory for the successful implantation of phakic IOLs. Specifically, determining the ATA distance is mandatory for correct selection of the IOL size when implanting an angle-supported model.

Jorge L. Alió, MD, PhD, is a Professor and the Chairman of Ophthalmology at the Miguel Hernández University, Alicante, Spain, and the Medical Director of Vissum Corp., Spain. Professor Alió states that he has no financial interest in the products or companies mentioned. He may be reached at +34 96 515 00 25; jlalio@vissum.com.

David P. Piñero, PhD, practices at the Vissum/Instituto Oftalmológico de Alicante, Spain. Dr. Piñero states that he has no financial interest in the products or companies mentioned. He may be reached at dpinero@vissum.com.

Ana Belén Plaza Puche, OD, practices at the Vissum/Instituto Oftalmológico de Alicante, Spain. Dr. Puche states that she has no financial interest in the products or companies mentioned.

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