Measuring astigmatism for visual outcomes

Accurate determination of the axis of astigmatism is critical for optimising visual outcomes with toric IOLs. Therefore, in seeking precision, surgeons often obtain multiple keratometric readings. However, results of a comparative study presented at the XXXI Congress of the ESCRS in Amsterdam suggest that the need for repeat astigmatism measurements and the extra time and effort involved can be avoided by using a new topography platform that uses colour LED triangulation technology and a unique ray tracing principle to analyse the corneal surface (Cassini, i-Optics).   In the study, the colour LED topographer, an optical biometer (Lenstar, Haag- Streit), a Scheimpflug device (Pentacam, Oculus), and a Placido ring topographer (Keratron, Optikon) were used to obtain anterior corneal measurements (flat K, steep K and flat axis) in 60 eyes of cataract surgery patients. Three measurements were obtained in each eye with each device and the standard deviation of the three within subject values was analysed to evaluate repeatability for each system. Although the four instruments had similar repeatability for determining the magnitude of astigmatism, the colour LED system had significantly better repeatability than each of the other devices in measuring astigmatism axis, reported Victor Arni Sicam PhD, inventor of the Cassini. Mean standard deviation for the axis measurements was 4.8o using the colour LED topographer, 6.5o with the optical biometer, 9.4o using the Placido topographer and 14.9o using the Scheimpflug device. Further analyses showed the repeatability of measuring astigmatism axis was improved for each of the instruments when only data from eyes with cylinder greater than 0.5 D was used. Again, however, the mean standard deviation values showed the colour LED topographer had significantly better repeatability than the optical biometer, Placido topographer and Scheimpflug device- 4.1o vs. 6.0o, 7.3o and 12.2o respectively. “Results from previously published studies using various keratometric instruments highlight there is only moderate repeatability of multiple axis measurements. The current study shows that with the colour LED system, only one measurement is needed to obtain accurate astigmatism information for toric IOL implantation. The first time will be right,” Dr Sicam told EuroTimes. He explained that the nature of its measurement technology accounts for the superior repeatability of the colour LED topographer. “The measurements are taken instantaneously and the acquired information is simple points rather than rings so that image and data processing are easier, this study used a Cassini clinical prototype and it is expected that the commercial version will perform much better,” Dr Sicam said. The study also investigated agreement of astigmatism magnitude measurements between the colour LED topographer and the other instruments based on Bland- Altman plots of the median data point for each eye. A systematic difference was noted between the colour LED topographer and the optical biometer and was even stronger comparing the colour LED topographer and the Placido ring topographer. In both cases, there was a trend for the inter-instrument difference to increase as the magnitude of astigmatism increased. “The Bland-Altman comparison can only establish differences between technologies but cannot identify which device is more accurate. For that reason we used the instruments to measure an artificial toric surface,” Dr Sicam said. For the latter measurements, the colour LED topographer and the optical biometer performed similarly and provided data within the confidence limits for accuracy. The Placido ring system measurements had an error of four to five per cent. Victor A Sicam: va.sicam@i-optics.com