Measuring IOL position for IOL power formulas

What is a primary difference between the various intraocular lens (IOL) power formulas?” asked Oliver Findl MD, of Vienna, Austria. “It is how they take into account the effective IOL position,” Dr Findl told a session of the XXXIII Congress of the ESCRS in Barcelona, Spain.

“According to a study on IOL power calculation, postoperative anterior chamber depth, and thus IOL position, is the number one cause of error. So how can we get a direct measurement of future IOL position?”

Dr Findl proposed intraoperative optical coherence tomography (OCT) as a means to accomplish this. Intraoperative OCT gives the opportunity to double-check preoperative findings, and allows a precise prediction of the IOL position in the aphakic eye, he said.

He discussed the results of his study using Visante from Carl Zeiss, an anterior segment OCT that is attached to the operating microscope and provides continuous intraoperative OCT videos. He also noted that, although intraoperative aberrometry might have value, intraoperative changes such as the effect of the lid speculum must be taken into account. Further, aberrometry offers no prediction of IOL position.

In a study of 203 primarily short and long eyes, standard cataract surgery was performed and a capsular tension ring (CTR) was placed in the bag. Once the CTR had been placed, providing a taut, positionally stable capsular bag, the intraoperative OCT was used to measure the anterior chamber depth. The study compared the measurements provided by optical biometry (IOLMaster 500, Lenstar + ACMaster), autorefraction, subjective refraction and the intraoperative OCT.

“The best predictor of IOL position turned out to be the anterior capsule at the edge of the rhexis, with the CTR in place. This was even better than using the axial length,” reported Dr Findl.

Two different IOLs were used in the study: an open-loop IOL (Tecnis one-piece from AMO) and a plate haptic IOL (Asphina from Zeiss).

The study results suggest that inserting the OCT results into an IOL power calculation formula would greatly improve the value of the anterior chamber depth variable within the formula. The predicted OCT-based results compared favourably to those predicted by fourth-generation IOL power calculation formulas.

A future goal of Dr Findl’s would be to fuse intraoperative OCT measurements with ray tracing to further refine refractive outcomes, and to use the intraoperative OCT during corneal endothelial surgery.

Oliver Findl: oliver@findl.at