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OCT-based biometers like the ARGOS allow for accurate measurement of very dense lenses in cataractous patients.
Refractive cataract surgery has advanced at a monumental pace, bringing high patient expectations,1 more frequent procedures2 and higher disease severity.3 Even with advanced IOLs and techniques, optimal clinical outcomes for patients depend largely on precision planning, which is facilitated by ocular biometer devices.
These instruments provide surgeons with valuable biometry and keratometry parameters that can be plugged into formulae for accurately calculating IOL power. Pinpoint measurements have been shown to be essential for postoperative visual acuity, with even minor errors translating to notable postoperative refractive error.4
Current biometric paradigm
Multiple options for performing biometric measurements are available, with varying technical modalities for determining the spectrum of essential parameters. For example, the IOLMaster 500 (Carl Zeiss Meditec) utilises partial coherence interferometry via a 780-nm infrared laser diode and lateral slit illumination for measuring anterior chamber parameters. Another instrument, the Lenstar LS 900 (Haag-Streit) employs optical low-coherence reflectometry via an 820-nm superluminescent diode to measure all axial parameters.
Although the precision of these older devices is well established in the literature, they fall short in performing axial calculations in more severely cataractous lenses. Both dense nuclear and posterior subcapsular cataracts are prone to light scattering, resulting in significant signal attenuation from the retina.
The next wave of biometry
New biometers have been developed in an attempt to better serve patients with severe cases of cataractous disease. We now have several years of experience with one such biometer, the ARGOS (Movu Inc.). Unlike older technology, this device utilises a 1060-nm wavelength and 20-nm swept-source technology to perform two-dimensional, full-eye optical coherence tomography (OCT).
This allows for comprehensive measurement of standard axial parameters, central corneal and lens thickness, pupil size and aqueous humour depth. In addition, a ring LED allows for keratometry.
In our experience, OCT provides a multitude of advantages over other biometric modalities. Firstly, a swept-source OCT system allows for extended imaging axial range without a reduction in axial resolution. Even with improved axial and lateral resolution, data collection occurs quickly and efficiently, with two-dimensional image processing taking just a few hundreds of milliseconds.
Furthermore, the device utilises a safeguard system for patients unable to correctly fixate their vision during collection: a panoramic view of the eye facilitates alignment with the centre of the pupil, mitigating initial measurement errors. In addition, the system is designed to alert the user in the case of ongoing misalignment, allowing manual adjustment in particularly difficult cases.
But how does this technology benefit the patients previously mentioned, those with mature, dense, difficult-to-measure cataracts? In our experience the ARGOS far surpasses current modalities in measuring axial length in eyes with dense cataracts.
We have successfully performed measurements in many cases that would have been impossible with older devices. Our experience includes successful and accurate measurements in patients with significant posterior subcapsular changes, late-stage cortical changes and the highest severity of nuclear sclerosis.
With this biometer, axial length is truer to the absolute distance, as two-dimensional full-eye imaging allows for inclusion of characteristics that affect axial length but were previously unaccounted for, such as lens thickness. The device also uses the appropriate refractive index (based on research and consensus) for the respective ocular elements, eliminating the need for nomograms that “correct” the axial length for significantly short or long eyes.
Additionally, the OCT employs a wide-scanning beam able to travel beyond the region of the cataract, so that retinal signals are no longer attenuated. The increased wavelength facilitates deeper penetration of the cataract, removing a lot of the guesswork that went into determining accurate IOL power calculations for these patients. With this technology, we have personally measured axial length as long as 33.33 mm (Figure 1).
Dr Akeno Tamaoki, CO
Dr Tamaoki is based at Chukyo Hospital in Nagoya, Japan. Dr Tamaoki reports no financial interests in the subject matter.
Dr Carmela Palmisano, MD, FEBOphth
Dr Palmisano is based at the University of Bari Hospital in Bari, Italy and reports no financial diclosures.
Dr Noemi Misuraca, OA
Dr Miscuraca is based at the University of Bari Hospital in Bari, Italy. Dr Miscuraca has no financial disclosures to make.
1. McAlinden C, et al. The Quality of Vision questionnaire: subscale interchangeability. Optom Vis Sci. 2013;90:760-764.
2. LundstrÃ¶m M, et al. The European registry of quality outcomes for cataract and refractive surgery (EUREQUO): a database study of trends in volumes, surgical techniques and outcomes of refractive surgery. Eye Vis (Lond). 2015;2:8.
3. Gale RP,et al. Benchmark standards for refractive outcomes after NHS cataract surgery. Eye (Lond). 2009;23:149-152.
4. Lee AC, Qazi MA, Pepose JS. Biometry and intraocular lens power calculation. Curr Opin Ophthalmol. 2008;19:13-17.