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Accuracy of refraction measurement by wavefront devices should be examined


Most of the SE measurements . . . were within two standard deviations of each other, indicating the similarity among the three measurements.

Remarkably, the results showed that some systematic differences appear to exist in the refraction (in both sphere and cylinder) measured by these three devices, calling into question the accuracy of these instruments and the need to establish an instrument's validity first before its data are used clinically, according to Dr. Wang.

"The wavefront-guided treatment has been increasingly used today for keratorefractive surgery," said Dr. Wang, clinical associate professor of ophthalmology, University of Tennessee; attending surgeon at Saint Thomas Hospital; director of the Wang Vision Institute, Nashville, TN; and medical director of refractive surgery of Aier Eye Hospital system, China.

Basically, if a wavefront device is to provide meaningful higher-order aberration data, it should at least produce consistent and accurate results first for the two key lower-order aberrations, namely, sphere and cylinder, he continued.

"We can establish the validity of the wavefront device by comparing the refraction value that it generates with the gold standard of refraction, manifest refraction," he said. "We should also examine the accuracy and consistency of refraction measurements by a wavefront or autorefraction device today by comparing the refraction value it generates with that of other similar devices.

"Remarkably, despite the preponderance of literature on wavefront-guided treatment, few studies have been published in the peer-reviewed literature that have actually examined the validity of the wavefront data by comparing the refraction generated with the manifest refraction, and by comparing the refraction between similar devices," Dr. Wang continued.

Further, the results of the few published studies also show inconsistencies, he said. For example, one study showed that the WaveScan and iTrace instruments generated comparable refractions, while another study reported that in spherical equivalent (SE), iTrace consistently differed from the WaveScan by 1 D, he noted.

Dr. Wang emphasized the importance of finding a reliable refraction with these instruments. With the VISX WaveScan system, clinicians are instructed by the manufacturer about two things:

"To me, conceptually this does not make any sense," Dr. Wang said. "If one device is in principle more accurate, it should be used at all times.

"How can we say that on one hand the wavefront device is superior and generates more accurate eye measurement than the manifest refraction, but on the other hand whenever the measurements of these two devices differ, we should somehow just forget about our previous claim that wavefront is better and just revert to using manifest refraction again?" he posed. "This guideline is in itself inconsistent and contradictory."

Comparing refractions

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