Caution should be taken when considering procedures that rely on these measurements
Caution should be employed when using internal eye aberration measurements as the repeatability of all instruments has been found to be lacking, according to Dr Paul Gifford (Queensland University of Technology, Brisbane, Australia) when discussing the results of his recent study.1
Further to this, Dr Gifford and Prof. Swarbrick derived corneal wavefront aberrations from corneal topography and then calculated the internal eye aberrations by subtracting corneal from ocular aberrations. These were established from simultaneously captured data in the Discovery, consecutively captured OPDScan images, and from the combined IRX3/E300 data.
Lack of repeatability for HOAs
"We were hoping to establish that the new Discovery instrument provided a high level of repeatability across the two measurement days and that its measurements for internal aberrations were comparable to the IRX3/E300 combination," revealed Dr Gifford.
The team found that there were similarilties in the intrasession repeatability of the Discovery and IRX3/E300, with the latter instrument providing slightly more repeatable measurements for secondorder root mean square (RMS) aberrations. When looking at the data from all three instruments over two days, the study revealed that the repeatability was highest for Discovery, followed by IRX3/E300 and then the OPD-Scan 3.
However, repeatability was low for higher order aberrations with all the instruments. "We were surprised at the lack of repeatability for higher order aberration measurements across all of the instruments, and in particular for the OPD-Scan 3, which we understand to be widely used in cataract and refractive surgery," revealed Dr Gifford.
Repeatability was found to be moderate with the Discovery for spherical aberration and trefoil RMS and high for all derived refractive components. The repeatability of the IRX3/E300 was high for the refraction component M and moderate for J0 and the OPD-Scan 3 was found to be highly repeatable for the M component also but not for cylindrical components.
A cautionary outcome
"The results suggest caution about the reliability of internal eye aberration measurements, which should be taken into consideration when considering modifications to ocular implants or refractive surgery procedures that rely on this data," emphasized Dr Gifford. "We established that averaging repeated measurements was a way to improve repeatability and so reliability of measurement. We only captured four maps in this research so cannot predict whether capturing and averaging more maps will be beneficial, however there is sufficient evidence to suggest that anyone who relies on aberration data in a clinical setting should take an average value from at least four captures, and average more captures if considering refractive procedures which are reliant on the measurements being taken."
1. P. Gifford and H.A. Swarbrick, Optom. Vis. Sci., 2012;89(6):929–938.