Biometry: What You Need To Know

“I often hear surgeons say, ‘apart from the very short and long eyes, I have no biometry problems’,” said Nino Hirnschall MD, of the Vienna Institute for Research in Ocular Surgery, Austria, speaking at the ESCRS Young Ophthalmologists Programme. “But I think there’s always room to improve,” he added. The main source of error in intraocular lens (IOL) power calculation is the postoperative anterior chamber depth, which largely determines the effective IOL position. “This is responsible for about 35 per cent of errors,” Dr Hirnschall told the delegates in attendance. He added that the selection of which modern IOL formula gives the best result has been hotly debated. “Optimising the IOL constants for each IOL model for the method of axial length measurement, for example optical or acoustic, has a much greater impact on the predictability of the refractive outcome than choosing between modern IOL formulae,” he said. Dr Hirnschall reported that the percentage of eyes within +/-1D of the target refraction can improve by up to 20 per cent with optimisation. Operating the second eye offers a chance to “learn” from the first eye regarding IOL power selection. However, he cautioned against the temptation to correct for the full refractive error of the first eye. “The two eyes are not twins,” said Dr Hirnschall. “Studies have shown that we should only correct for 40 per cent of the first eye’s refractive error.” “With-the-rule astigmatism is often overstated, and against-the-rule understated, by preoperative measurements, so it’s important to use different devices to measure the cornea,” he said, regarding toric IOLs. “Post-refractive surgery eyes remain the ultimate challenge,” added Dr Hirnschall.