Precise biometry is essential for accurate outcomes in cataract and refractive surgeries. Ultrasound axial length measurements have been the gold standard for many years. With the introduction of
optical biometry, technology has become more advanced. Partial coherence interferometry based biometry presents an alternative for precise ocular measurements, used not only for axial length, but anterior chamber depth, pachymetry and lens and retinal thickness measurements.
Optical biometry uses a 780 um infrared light wave that has 8x the resolution of a 10 MHz sound wave, making the measurement of axial length very precise and avoiding operator variations in measurements. Also increasing the accuracy is the fact that contact with the cornea is not needed, eliminating variations due to compression on the cornea. Since optical biometry measures to the center of the macula, it gives the refractive axial length versus the anatomic axial length achieved with ultrasound biometry. Optical biometry also incorporates actual thickness of the retina, whereas ultrasound adds a standard 200um to the axial length.
There is still a role for ultrasound biometry amongst many ophthalmologists, for instance, to measure axial length in the presence of a very dense cataract or corneal edema where optical biometry is not useful. Ultrasound does not require the patient to fixate on a target. As well, ultrasound A-scan is fine for the majority of patients with normal eye anatomy. The most accurate form of ultrasound-based axial length measurement is a vector A/B scan. The B-scan runs through the optic nerve and the center of the macula while a simultaneous A-scan is performed. Perhaps the biggest advantage of ultrasound biometry units is the cost. It is much more affordable than optical biometry, but requires more operator skill to ensure consistent accuracy.
Interestingly, in June 2009, Raymond, et al. studied whether intraocular lens power calculations for cataract surgery as measured by postoperative refractive error was more accurate in improving postoperative outcomes using partial coherence interferometry (PCI) versus applanation ultrasound biometry (AUS). They found that the calculation of IOL power based on ocular axial length measurement with PCI technology provided no clinical advantage over conventional applanation ultrasound. Their results were published in IOVS.
Some of the available ultrasound biometers on the market are discussed below.
DGH - 5000e A-scan
The unique feature of this unit is the corneal compression detection software, which reduces the possibility of short readings. This unit can also be upgraded to include pachymetry.
Quantel Medical - Axis II PR
The unique feature of this unit is the Probeam, the company's A-probe with built in laser pointer. The pointer projects a light spot onto a wall or ceiling. When the patient fixates on the spot with the contralateral eye, the ultrasound beam lines up naturally with the patient’s field of view.
Accutome - A-scan
This updated version of Accutome's A-scan incorporates upgraded hardware for precision. It can be used in all patients and has an alignment detection feature to eliminate marginally aligned scans. It is loaded with 3rd and 4th generation as well as post refractive formulas.
Of the optical biometers, below is a discussion of two.
Carl Zeiss - IOL Master
The IOL Master was the first optical biometry model on the market since 1999. Since then, many advances have been made in this technology, although the IOL Master still provides accurate measurements, but uses slit imagery for some while others have progressed to optical biometry. Improvements have been made on this unit since its first development. For instance, Zeiss improved the ability to cut down on background noise when measuring through dense cataracts. The IOL Master has a very fast computer for the quick and reproducible acquisition of keratometry information. According to Carl Zeiss, the IOL Master is consistently accurate to within 0.02 mm (vs. 0.10mm to 0.12mm with A-scans).
Haag Streit - Lenstar LS 900
The Lenstar system allows measurements of not only axial length and keratometry, but it provides the white–to-white, pachymetry, lens thickness, anterior chamber depth, pupillometry, eccentricity of the visual axis, and retinal thickness. The Lenstar also defines anterior chamber depth more accurately as the distance from the posterior surface of the cornea to the anterior surface of the lens, rather than the anterior surface of the cornea to the anterior surface of the lens like many others do. The Lenstar takes keratometric readings at 1.65mm and 2.3mm, combining them via an iteration process for improved consistency. The Lenstar can also have the Holladay II formula installed.
The advances in technology when referring to ocular biometry have brought us to a new level of precision for cataract and refractive surgery. There are advantages and disadvantages to both ultrasound and optical biometry as outlined above, yet both have their roles in ophthalmology.