SS-OCT more accurate and informative than older optical biometry technologies
Nino Hirnschall MD
New swept-source optical coherence tomography (SS-OCT) biometry devices such as the IOLMaster® 700 (Zeiss) can successfully scan axial lengths in denser cataracts than was possible with previous optical biometry devices, and can confirm fixation and screen for macular disease at the same time, according to Nino Hirnschall MD, PhD, Vienna Institute for Ocular Surgery, Hanusch Hospital, Vienna, Austria.
“I think that OCT technology is definitely the future for ocular biometry,” he said.
He noted that, among the more than one million cataract surgeries in the EUREQUO database, 12% were more than 1.0D off target. A large part of that has been due to the inability of optical biometers to successfully scan dense and subcapsular cataracts. In such cases cataract surgeons have often relied on ultrasound measurements, which are much less reliable than a successful optical biometry scan.
Newer optical biometry devices using SS-OCT are bringing the number of eyes that cannot be successfully scanned ever closer to zero. He noted that, in a study conducted by Dr Oliver Findl’s team at Hanusch Hospital involving 1,226 consecutive eyes, 6.4% could not be measured with an IOLMaster 500 partial coherence interferometry (PCI) optical biometer, whereas all but 0.5% could be successfully scanned with SS-OCT technology.
Poor fixation is another source of error in biometry measurements which SS-OCT devices can correct. Its B-scans provide a cross-sectional image of the entire eye including the retina. Therefore, if the fovea pit is not visible in the image it means the patient is not fixating properly, and consecutive scans will be less accurate and consistent.
He described the case of an eye where the foveal pit was not visible in two consecutive scans with the IOLMaster 700 and the measurements differed by 190μm in axial length and by 0.5D in keratometry. However, in a different eye where the foveal pit was visible, two consecutive scans differed by only 10 microns in axial length and 0.02D in keratometry.
The longitudinal scans provided by SS-OCT also enable the surgeon to check for macular disease in their cataract patients. He noted that he and his associates found the IOLMaster 700 to have a moderate sensitivity (between 42% and 68%), high specificity (89% to 98%) for detecting the condition among 55 healthy eyes and 65 with macular disease. The conditions were definitively diagnosed by dedicated spectral-domain retinal OCT (RTVue OCT, Optovue). Intra-observer reproducibility with the biometry device was 88.3% among the three examiners participating in the study.
The new SS-OCT devices also offers two new intriguing possibilities for the improvement of toric intraocular lens (IOL) calculation, namely the detection of lens tilt and the measurement of the posterior corneal surface, Dr Hirnschall said.
Regarding the measurement of the cornea’s posterior surface, Dr Hirnschall noted that while there have been devices available in recent years credited with that ability, their accuracy is limited by their speed. He noted that a study, which Dr Peter Hoffmann from Germany conducted together with Dr Findl and Dr Hirnschall himself, showed that SS-OCT outperformed the Scheimpflug rotating camera, automated keratometry with the Lenstar, and topography in predicting residual astigmatism after cataract surgery.
OCT technology is definitely the future for ocular biometry
However, even the best preoperative measurements can provide only an estimation of the position of the implanted IOL. That is because the combination of accurate axial length, anterior chamber depth and lens thickness values still leave uncertainty regarding the equatorial plane of the empty capsule and therefore the haptic plane of the implanted lens.
Yet once again, OCT may provide a partial solution, thanks to the relative ease with which it can be employed intraoperatively. He noted that he and his associates have been working with a prototype set-up for intraoperative measurements using a Visante anterior segment OCT combined with a surgical microscope.
So far, in a study involving 164 patients, they have found that had they used the intraoperative OCT anterior chamber depth measurements as the haptic plane in their IOL calculation they would have had a better refractive outcome than those achieved using preoperative measurements.
“We still have the problem of shift of the lens during the first weeks after surgery but this is minor in most modern lenses. So the main problem with predicting the position of the lens is definitely improved,” he added.
Nino Hirnschall: firstname.lastname@example.org