ABERROMETRY

Intraoperative wavefront aberrometry measurements of refraction in the aphakic stage of a cataract procedure show good reproducibility, but good measurements are only possible in about half of attempts and the range of error appears to be too large to be used in a clinical setting, according to the results of a study presented by Jan O Huelle MD at the XXXII Congress of the ESCRS in London.

“More efforts are required to improve the precision and quality of measurements before intraoperative wavefront aberrometry can be used to guide the surgical refractive plan in cataract surgery,” said Dr Huelle, South West Peninsula Deanery, NHS South West Peninsula, UK, and Dr Stephan Linke, University Medical Centre Hamburg-Eppendorf (UKE), Germany.

Dr Huelle noted that studies have shown that wavefront aberrometry refraction measurements agree closely with manifest refraction. There are reports in the literature suggesting that intraoperative biometry during cataract surgery can provide a very good estimation of the appropriate intraocular lens (IOL) dioptric power needed for optimum visual outcome in a given eye. However, the aphakia formulas used in those studies are all based on a different method of measurement, namely autorefractive retinoscopy.

“Up until now there has been no data on the precision of intraoperative wavefront aberrometry measurements for IOL estimation when used with published aphakia formulas for IOL calculations. We wanted to fill that gap,” Dr Huelle said.

Their study included in 74 eyes of 74 consecutive patients with a mean age of 69 years who were undergoing straightforward cataract surgery. All eyes underwent preoperative biometry measurements with the IOL master V. All eyes also underwent seven intraoperative wavefront aberrometry measurements performed at different stages during cataract surgery. Manifest refraction at three months after surgery served as the best estimate for the refractive outcome.

In their analysis, they used automated wavefront map area calculation to obtain an objective quality grading of the aberrometry measurement. They also used Holladay refractive vergence formula to retrospectively calculate the IOL that would have provided the target refraction. They also performed regression analysis to generate formulas to predict the adjusted IOL, based on the aphakic spherical equivalent. (Figure 1)

The researchers found that, out of 814 intraoperative wavefront measurement attempts, only 478 were successful and in only 40.6 per cent of eyes were all three measurements at the aphakic stage successful. The most successful readings were in aphakia with OVD.

The highest quality wavefront measurements were achieved after clear corneal incision and the lowest when the eyes were pseudophakic with OVD. The quality of the wavefront measurements at the aphakic stage was somewhere around the middle.

Dr Huelle noted that if they had used the published autorefractive retinoscopy based aphakia formulas among those with three successful aphakic wavefront measurements, less than 20 per cent would have been within 0.5D of target refraction, and less than 30 per cent would have been within 1.0D.

However, when using the aphakia formula derived from their regression analysis, 25 per cent of eyes would have been within 0.5D of target and 53.1 per cent would have been within one dioptre. Moreover, when they modified their formula to take the axial length into account, 40.6 per cent and 70.3 per cent would have been within 0.5D and 1.0D of target refraction respectively.

“Intraoperative wavefront aberrometry shows good reproducibility in aphakia if the measurement succeeds. But clinically the ranges appear to be too large, and that is why the measurement precision needs improving,” said Dr Huelle and Dr Linke.

Jan O Huelle: jan.huelle@doctors.org.uk

Stephan Linke: s.linke@uke.de