Publication Exclusive: Innovations in optical biometry optimize effective lens position

Novel IOL technologies and increasing patient expectations for precise refractive results have driven a series of innovations in biometry and IOL power calculation. Notable improvements have been seen in recent years, and about 1,000 citations for optical biometry currently appear in PubMed.

Optical biometry provides detailed images of critical anatomic structures in the anterior and posterior segments of the eye. Intraoperative OCT has shown particular promise in predicting effective lens position, improving on the traditional use of axial length measurement.

ORA intraoperative aberrometry (Alcon) has been shown to improve accuracy in post-myopic LASIK patients, but it does not provide biometric measurements that are helpful in predicting the effective lens position, according to Wendell Scott, MD.

“I think that ORA was an attempt to try to improve results. It was a step in the right direction. It allowed us intraoperatively to confirm tests that we had done preoperatively. The problem, of course, is that intraoperative aberrometry is an optical wavefront measurement and not an anatomic biometric, so it is less helpful in estimating the effective lens position. So, being able to actually do intraoperative OCT, I think, will be the next methodology that really raises the predictability of our implant results,” Scott said.

Imaging and measurement of intraocular structures are particularly critical, especially during accommodation. Adrian Glasser, PhD, OSN Optics Board Member, said that accommodation is a complex and multifaceted process, underscoring the difficulty of predicting accommodative optical responses from biometric changes in the eye, the subject of three studies he co-authored.

“True accommodation is an objectively measurable dioptric change in power of the eye, such as might be measured with an autorefractor or an aberrometer. Pseudoaccommodation is a combination of factors that contribute to an increased depth of focus of the eye, but without a change in dioptric power. A pupil constriction alone, for example, increases the depth of focus of the eye. A pupil constriction alone in an eye with negative spherical aberration also acts to increase the optical power of the eye. Accommodation also normally involves both eyes, so binocular summation is also involved. All of these factors occur together,” Glasser said.

Optical biometry systems

Optical biometry has been a major step forward toward improved accuracy because it provides measurements not only of axial length, but also anterior chamber depth and thickness of the cornea, lens and retina. Currently available systems include the IOLMaster 700 (Carl Zeiss Meditec) with swept-source biometry and OCT, the AL-Scan (Nidek) optical biometer, the Lenstar LS 900 (Haag-Streit) with dual zone keratometry and optional Placido topography, the Aladdin biometer (Topcon) and Placido topographer, the Galilei G6 Lens Professional (Ziemer) dual Scheimpflug tomographer and Placido topographer with optical biometry, and the OA-2000 optical biometer and topographer (Tomey).

In a study, Oliver Findl, MD, and colleagues compared the performance of these systems in 57 eyes of 57 cataract patients, including 15 with very dense cataracts.

“We did three measurements with each device, for a total of about 1,000 measurements. The IOLMaster 700 and the OA-2000 had the highest number of successful scans, 96.7% and 98.7%, respectively. These are the two machines I would recommend if you have patients with dense cataract in your population,” Findl said.

The Galilei system had the largest number of dropouts, with only 78% successful scans in a cataract population.

“[Dropouts are] something that needs to be addressed by the manufacturers,” Findl said.

In the same study, axial length measurements of other systems were compared with the IOLMaster, which is currently the gold standard. An excellent correlation was found among all instruments.

Click here to read the full cover story published in Ocular Surgery News U.S. Edition, August 25, 2015.