Sara Wester, M.D.
Contributing Editor
In the pre-operative evaluation of a patient for cataract surgery, A-scan ultrasonography or optic coherence biometry must be performed. These measurements, in conjunction with corneal power (determined by keratometry or corneal topography) enable accurate IOL power calculations. Some of the available biometers on the market include the Accutome A-scan, the Alcon OcuScan RxP, the DGH 5000e A-Scan, the Innovative Imaging I3 System-ABD V2, and the Quantel Medical Axis II PR, all of which use A-scan ultrasonography, and the Zeiss IOL Master, which is unique in that it uses optical coherence biometry. Each biometer has unique features to address the growing importance of accurate IOL power calculations in the era of multi-focal and accommodative IOLs.
Optical biometers use infrared light to perform analyses which produce similar lens calculations as A-scan ultrasonography and keratometry. They are different from traditional A-scan ultrasonography because they use infrared light to calculate axial length, keratometry and anterior chamber depth, all in one machine. The IOL master by Carl Zeiss Meditec is an optical biometer that was approved in March 2000. It measures the distance from the corneal vertex to the retinal pigment epithelium using a patented interference optical method called partial coherence interferometry. The measurement determines refractive axial length because the patient is fixating on a fixation light and the measurement is thus made to the center of the macula. An internal statistically confirmed algorithm which is calibrated against another ultra-high resolution biometer calculates the distance to the retina, providing the equivalent of an immersion A-scan ultrasonography axial length, albeit more precise.
In addition to the axial length measurements, corneal curvature and anterior chamber depth can also be measured with the IOL Master. Both of these measurements are determined by measuring the distance between reflected light images. Corneal curvature represents the distance between the light images projected on the cornea and anterior chamber depth represents the distance between the lens and the cornea with a lateral slit beam illumination.
Using these measurements, the computer makes suggestions for IOLs that will match the goal refractive outcome. The Holladay, SRK II, and SKR/T formulas are used most frequently, but the Haigis and HofferQ formulas may also be used.
Optical biometry has several advantages over traditional immersion and applanation A-scan ultrasonography. Optical biometers have lower technician dependence and are rapid tests. In addition, the biometers do not have contact with the cornea, which reduces the variability caused by corneal compression that occurs in applanation A-scan. In addition, the biometer measures refractive axial length not anatomic axial length (which measures from the cornea to the posterior pole nasal to the foveola and thus is not as accurate). The inclusion of the thickness of the retina in the measurement (the interferometry measures to Bruch’s membrane) also increases precision compared to ultrasound which measures to the front of the retina, necessitating a standardized value of 200 microns to be added to the axial length. As a result of these factors, it offers more precise and reproducible IOL measurements than A-scans and keratometry. According to Carl Zeiss Meditec, the IOL Master is consistently accurate to within 0.02 mm (vs. 0.10mm to 0.12mm with A-scans).
Optic biometry, however, does have limitations. Measurements are difficult and may be inaccurate through dense cataracts or corneal scars or edema. In addition, it cannot be used in patients who are unable to fixate. It is important to note, however, that patients with high refractive errors can use their glasses to fixate, as this should not change the measurement. While these are important limitations, the advantages of biometry in improving precision of most patients’ IOL calculations have made it a mainstay in cataract surgery pre-operative assessment.