Optical Biometry Explained
Optical biometry is the current standard for intraocular lens (IOL) power calculations in clinical practice. Optical biometry is a highly accurate non-invasive automated method for measuring the anatomical characteristics of the eye. Accurate measurements are critical for determining the correct power of an IOL before it is implanted during cataract surgery .
Before implantation the correct lens power needs to be determined. The process of measuring the various anatomical characteristics of the eye that are needed for IOL power calculation is called ocular biometry. Optical biometry, also known as ophthalmic biometry, using partial coherence interferometry has become the gold standard in ocular biometry as it is highly accurate, easy to perform, non-invasive and comfortable for the patient. The accuracy of optical biometry, and in particular the IOLMaster, has been extensively confirmed across a wide range of scientific studies.
The use of optical biometry is a valuable tool when planning cataract surgery, resulting in optimization of patient outcomes.
What is ocular biometry?
Ocular biometry involves anatomical measurements of the eye, including the axial length (AL), keratometry and anterior chamber depth (ACD) and includes anterior segment biometry, for which only the front third of the eye is measured. These biometric measurements are crucial for the selection of the correct IOL power in order to achieve the desired refractive outcome after cataract surgery.1 Therefore, ocular biometry is an essential step before cataract surgery. There are currently two procedures available: ultrasound and optical biometry. Due to certain disadvantages of ultrasound biometry, optical biometry has become procedure of choice in ocular biometry.
It all started with ultrasound biometry
Ultrasound biometry is an invasive procedure that requires direct contact with the cornea and the use of anesthetics, both of which can be uncomfortable for the patient.2 Moreover, this measurement method requires significant training for the examiner in order to avoid errors due to excessive compression of the ultrasound probe on the cornea. Ultrasound biometry also requires adjustment of the ultrasound speed when different media and/or optical conditions are present, e.g. pseudophakic eyes and silicone oil.3
A new concept: optical biometry with the IOLMaster
In September 1999, the first automated non-invasive optical biometry device became available for clinicians – the IOLMaster from Carl Zeiss Meditec. The IOLMaster operates as a modified Michelson interferometer and uses infrared laser light (wavelength 780 nm) to provide repeatable and precise measurements of the AL,4 curvature of the anterior corneal surface,5 ACD,6 and the horizontal visible iris diameter (white-to-white diameter, WTW).7
The technique underlying IOLMaster biometry is partial coherence interferometry: A signal is produced as a result of the interference between the light reflected by the tear film and that reflected by the retinal pigmentary epithelium. Several studies comparing the IOLMaster device to contact and/or ultrasonographic techniques have demonstrated repeatability and accuracy of measurements. (Connors et al 2002, Sheng et al 2004).8 9
One reason for this accuracy is the reduced dependence of the biometric measurement on the examiner's experience, as only the alignment of the device to the patient´s eye is required while the rest of the process is automated.
The only limitation of optical biometers has been the inability to measure AL and ACD in eyes with densely opacified media. In the IOLMaster 500 this can be overcome by connecting it to an ultrasound probe that can provide AL measurements in those eyes with very dense corneal leukomas and/or cataracts.10 Additionally, other values can be entered manually. Thus, almost any type of eye can be measured with this combined technology. Moreover, with the newly available IOLMaster 700, featuring SWEPT Source OCT measurement, it has been shown that necessary ultrasound cases could be reduced by 92%, achieving a cataract penetration rate of 99% 11.
Accurate biometry measurements and IOL calculations can be performed quickly and easily.
1 Lee AC, Qazi MA, Pepose JS. Biometry and intraocular lens power calculation. Curr Opin Ophthalmol 2008; 19: 13-7.
2 Pierro L, Modorati G, Brancato R. Clinical variability in keratometry, ultrasound biometry measurements, and emmetropic intraocular lens power calculation. J Cataract Refract Surg 1991; 17: 91-4.
3 Lege BA, Haigis W. Laser interference biometry versus ultrasound biometry in certain clinical conditions. Graefes Arch Clin Exp Ophthalmol 2004; 242: 8-12.
4 Lam AK, Chan R, Pang PC. The repeatability and accuracy of axial length and anterior chamber depth measurements from the IOLMaster. Ophthalmic Physiol Opt 2001; 21: 477-83 .
5 Wang Q, Savini G, Hoffer KJ, Xu Z, Feng Y, Wen D, Hua Y, Yang F, Pan C, Huang J. A comprehensive assessment of the precision and agreement of anterior corneal power measurements obtained using 8 different devices. PLoS One 2012; 7(9): e45607.
6 Vogel A, Dick HB, Krummenauer F. Reproducibility of optical biometry using partial coherence interferometry: intraobserver and interobserver reliability. J Cataract Refract Surg 2001; 27: 1961-8.
7 Baumeister M, Terzi E, Ekici Y, Kohnen T. Comparison of manual and automated methods to determine horizontal corneal diameter. J Cataract Refract Surg 2004; 30: 374-80.
8 Sheng H, Bottjer CA, Bullimore MA. Ocular component measurement using the Zeiss IOLMaster. Optom Vis Sci 2004; 81: 27-34.
9 Connors R 3rd, Boseman P 3rd, Olson RJ. Accuracy and reproducibility of biometry using partial coherence interferometry. J Cataract Refract Surg 2002; 28: 235-8.
10 Srivannaboon S, Chirapapaisan C, Nantasri P, Chongchareon M, Chonpimai P. Agreement of IOL power and axial length obtained by IOLMaster 500 vs IOLMaster with Sonolink connection. Graefes Arch Clin Exp Ophthalmol 2013; 251: 1145-9.
11 Nino Hirnschall, Ralph Varsits, Birgit Döller, Oliver Findl; Increasing the number of successful axial eye length measurements using sweptsource optical coherence tomography technology compared to conventional optical biometry; paper in progress.
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