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Intraoperative OCT improves precision of IOL power calculations
Continuous measurements of the anterior capsule during surgery were shown in studies to be the best predictor of IOL position after surgery.
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Intraoperative OCT can help predict the position of the lens capsule and IOL after surgery, leading to more accurate IOL power calculations, as shown by studies carried out at the Hanusch Hospital in Vienna with a microscope-mounted anterior segment OCT prototype.
“The idea is to find a better way to predict lens position after surgery. Nowadays we use modern formulae to combine keratometry, axial length and anterior chamber depth measurements taken before surgery, but no intraoperatively measured anatomical ACD measurement,” Nino Hirnschall, MD, PhD, said in an interview with Ocular Surgery News.
OCT measurements for IOL power calculations were used in the past, but only for preoperative assessment. The new concept is to do measurements intraoperatively after the lens is removed.
Studies have been performed under the supervision of Oliver Findl, MD, MBA, FEBO, and some are still underway, using a prototype by Carl Zeiss Meditec of a Visante OCT mounted on the OPMI 200 operating microscope. The initial studies showed that continuous intraoperative measurements of the lens capsule after removing the lens were a reliable predictor of postoperative IOL position.
“Measured intraoperatively, the lens capsule, which is in contact with the surface of the IOL haptics soon after surgery, is the main anatomical parameter and best prediction factor for postoperative IOL position. When you know the position of the aphakic anterior lens capsule, you also have a good idea where the haptics of your lens will be,” Hirnschall said.
Compared with conventional formulae, intraoperative measurements were shown to be less dependent on axial eye length.
Current limitations
This method has a few shortcomings, according to Hirnschall. One of them is that it might lead to wrong results if the capsule comes into contact with the iris. However, this problem can be solved easily.
“You always have to take care intraoperatively to detach the iris from the capsule. If that is done, there won’t be an error,” he said.
Variable hydration of the vitreous may also be a problem. The fluid that flows into the back of the eye during phaco is absorbed by the vitreous, which pushes the posterior capsule and makes the anterior chamber a little shallower.
Currently, the intraoperative OCT system is not fine-tuned to take real-time calculations intraoperatively.
“Our IOL power calculations are still retrospective. We collect a lot of data after surgery is performed, and we do retrospective analyses of the data. We have successfully measured 202 cases, mainly focusing on abnormally long and short eyes,” Hirnschall said. “These are the very difficult eyes, and especially when you have very short eyes, you have a lot of refractive surprises, as shown by the EUREQUO database. It is for these cases in particular that we have to improve on current methods.” – by Michela Cimberle
- For more information:
- Nino Hirnschall, MD, PhD, can be reached at VIROS, Hanusch Krankenhaus, Heinrich-Collin Straße 30, 1140 Vienna, Austria; email: nino.hirnschall@googlemail.com.
Disclosure: Hirnschall reports he holds the patent for the intraoperative OCT measurement system with Findl.