IOL POWER CALCULATION INNOVATIONS
Improving accuracy by using intraoperative OCT to measure ACD
Nino Hirnschall MD, PhD
Intraoperative optical coherence tomography (OCT) measurement of anterior chamber depth (ACD) in the aphakic eye improves prediction of postoperative IOL position and therefore holds promise for improving the predictability of refractive outcomes after cataract surgery. It will not, however, completely eliminate the chance of a refractive surprise, said Nino Hirnschall MD, PhD, at the 37th Congress of the ESCRS in Paris, France.“Modern IOL power formulae that use preoperative ACD for predicting IOL position are already very good, but they can be improved in the future by introducing intraoperative ACD measurements of the aphakic eye,” said Dr Hirnschall, Vienna Institute for Research in Ocular Surgery, Hanusch Hospital, Vienna, Austria.“Yet, our research shows that there are still some refractive surprises that cannot be explained by intraoperative OCT. I believe that no matter what kind of measurements and techniques that we use, there will always be the small chance of a >1D refractive surprise in eyes with a very special anatomy.”
Targeting the main
source of error
With the goal of further optimising postoperative refractive outcomes, Dr Hirnschall and colleagues have been focusing on improving prediction of postoperative IOL position, recognising that it is the main source of error in IOL power calculations and particularly in eyes that have a very short or very long axial length (AL).
“If you look at series of very short or very long eyes, not even 50% have a refractive outcome within 0.5D of target,” said Dr Hirnschall.
Use of continuous anterior segment OCT to measure the ACD in the aphakic eye is based on the concept that it will provide a more accurate determination of where the IOL will sit after surgery than the effective lens position that is determined with conventional IOL formulae using preoperatively measured parameters.
The OCT-based measurement of “intraoperative ACD” represents the distance between the corneal endothelium and the anterior lens capsule. In an initial study, Dr Hirnschall and colleagues reported that its use resulted in significantly better prediction of the postoperative IOL position compared with conventional IOL power calculation formulae.
Refining the measurement
In the initial study, a capsular tension ring (CTR) was implanted in all eyes to maintain a taut capsule during the measurement. Although there is weak evidence that having a CTR implanted has no relevant influence on the refractive outcome, Dr Hirnschall suggested that it would be best to eliminate its use in the second part of the study considering it adds cost to the surgery.
As another limitation, the initial study was done with a time domain OCT system that has only moderate resolution. As a next step, Dr Hirnschall and colleagues implemented a swept-source OCT device into the operating microscope that provides higher resolution. Its use also enabled the identification and elimination of adhesions between the capsule and iris that can affect ACD, and it incorporates an additional fixation check to assure measurement of the patient’s visual axis.
In a study including 70 eyes comprised mostly of very long or very short eyes that did not receive a CTR, the researchers found that use of intraoperative ACD outperformed preoperatively measured variables – i.e., ACD + lens thickness (LT), ACD, AL and LT – for predicting postoperative IOL position. Analyses of postoperative refraction predictions using different variables combined with AL showed that intraoperative ACD + preoperative ACD + LT + AL performed the best, with 96% of eyes predicted to be within 0.5D of their target refraction.
“This is a very good result, but it is not 100%, and it seems that although you think you know where the IOL will sit after surgery, there are outliers that cannot be explained by the intraoperative measurements,” Dr Hirnschall said.
Going forward, Dr Hirnschall and colleagues are aiming to address the role that vitreous hydration has on predicting postoperative IOL position.
“Significant vitreous hydration, which is seen in 2% of patients, remains as an unpredictable factor that could influence ACD,” he said.