Enhancing biometry

The accuracy of intraocular lens power calculations can be enhanced if surgeons take account of both the anterior and posterior surfaces of the cornea in their preoperative biometry workup, according to Oliver Findl MD, MBA. “Research has shown that selecting IOLs based on the measurements of the central and paracentral cornea may result in refractive surprises. It is clear that the posterior surface does play a role and it does improve IOL power calculations,” he told delegates attending the XXXV Congress of the ESCRS in Lisbon. Dr Findl presented results from a single-centre prospective study comparing traditional keratometric measurements to those from True Keratometry (TK), which uses data of both the posterior and anterior corneal surface obtained by swept-source OCT (IOLMaster 700, Carl Zeiss Meditec). “It is important not to confuse this with the True-K formula introduced by Graham Barrett, which measures the anterior surface and then includes an average value for the posterior surface. Here we are actually measuring both the anterior and posterior surfaces rather than using average values,” he explained. Dr Findl’s study included 49 eyes of 49 cataract patients implanted with a plate haptic monofocal lens (CT Asphina 409M/MP). IOL power calculation was performed using three well-known and widely tested formulae: Haigis, Holladay II and SRK/T. The measurements obtained were based on keratometry (K) readings of the anterior surface and TK (anterior and posterior surface) and checking the differences between them. Mean axial length was 23.16mm, and the mean astigmatism was 0.93D for both K and TK. The difference between the mean K and mean TK was actually very small (-0.03). “It is close to zero, which is good, because that means that we can use this data with the IOL constants we are using at the moment. If we had a different result, we would have needed to adapt all our IOL constants for different IOLs. Nevertheless, we can see from the standard deviation (1.49) that there is some variability, so clearly there are a few outliers there,” he said. Looking at the performance of TK overall, Dr Findl said that it showed greater accuracy for all three formulas used. “This was most pronounced for the Haigis formula with an 8% difference using TK for those patients within 0.5D of target refraction. While the differences were not statistically significant given the small sample size, there is nevertheless a slight improvement in IOL outcomes,” he said. Dr Findl added that the real benefit of such technology will probably lie in treating unconventional eyes. Oliver Findl: oliver@findl.at

Tags: iol power calculations