New frontiers in ocular biometry

Until the last decade, A-mode ultrasound (the 'A' being from amplitude) was considered the gold standard for measuring the axial length (AL) of the eye. However, apart from its inherent reliability issues due to corneal indentation, there are other disadvantages such as the risk of corneal infection or abrasions and potential off-axis measurements. Immersion ultrasound was a big step forward with better reproducibility, but was not uniformly adopted. Ultimately, the main problem with ultrasound is that it uses a relatively large 10 MHz sound wave to measure a relatively small distance.

Era of optical biometry

IOLMaster

The IOLMaster (Carl Zeiss Meditec, Jena, Germany) was the first commercially available PCI-based optical biometry instrument, introduced in 1999. It measures AL, anterior chamber depth (ACD), and 'white-to-white' (WTW) distance. AL measurement uses a 780 nm laser diode infrared light, but (ACD) is measured between the corneal epithelium and anterior lens surface through a lateral slit imagery technique. The anterior corneal curvature is calculated at 6 reference points in a 2.3 mm optical zone in a hexagonal pattern. The IOL Master AL measurements are consistently accurate to within 0.02 mm,⁴ compared to A-Scans which have a resolution of approximately 0.10 mm to 0.12 mm only.⁵

Since its first development, manufacturers have developed the technology considerably. The latest version (IOLMaster 500) incorporates advanced technology for digital signal processing in AL measurement mode. New features include real-time analysis of AL; a focus indicator for keratometry and more rapid ACD measurements as well as the manufacturers claiming reduced background noise when measuring through dense cataracts.

Lenstar LS 900

In 2008, a new optical biometry device based on OLCR, the Lenstar LS 900 (Haag-Streit AG, Koeniz, Switzerland) became available. It uses an 820 mm superluminescent diode (SLD) for measurement, a technique described as Optical Low Coherence Reflectometry (OLCR), although essentially the same as PCI.³ In addition to keratometry, AL, ACD and WTW parameters, the Lenstar system allows other measurements such as pachymetry, lens thickness, pupillometry, eccentricity of the visual axis and retinal thickness.

The Lenstar generates enough signal compared to noise to allow ACD (between the corneal endothelium to the anterior surface of the lens) to be measured with PCI (rather than less accurate imaging systems used by other biometers). Additionally, in an attempt to improve consistency, Lenstar system takes keratometry readings by analysing reflections from the anterior corneal curvature in two optical zones at 2.30 mm and 1.65 mm and combining them by an iteration process. The biometry measurements obtained with Lenstar are comparable to IOLMaster.³