الفهرس 
History of biometryOnly 14 pages are availabe for public view
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Abstract
The refractive power of the human eye depends on the power of the cornea and the lens, the position of the lens, and the length of the eye. Accurate assessment of these variables is essential in achieving optimal postoperative refractive results. If these biometric measurements and calculations are inaccurate, the patients may be left with a significant refractive error. (Olsen, 2007)
Studies conducted by Olsen showed that imprecision in measurements of anterior chamber depth (ACD), axial length and corneal power contribute to 42, 36 and 22%, respectively, of the error in predicted refraction after implantation of an intraocular lens (IOL). As a result of heightened patient expectations, there is more than ever a need to accurately predict the correct IOL power. (Olsen, 2007)
These demands have spurred the continued improvements in technology and refinements in biometry and IOL calculations.
Ultrasound biometry
Two types of A-scan ultrasound biometry are currently in use.
The first is contact applanation biometry. This technique requires placing an ultrasound probe on the central cornea. While this is a convenient way to determine the axial length for most normal eyes, errors in measurement almost invariably result from the probe indenting the cornea and shallowing the anterior chamber. Since the compression error is variable, it cannot be compensated for by a constant. IOL power calculations using these measurements will lead to an overestimation of the IOL power (Shammas HJF, 1984).
The second type is immersion A-scan biometry, which requires placing a saline filled scleral shell between the probe and the eye. Since the probe does not exert direct pressure on the cornea, compression of the anterior chamber is avoided. A mean shortening of 0.25–0.33mm has been reported between applanation and immersion axial length measurements, which can translate into an error of IOL power by approximately 1 D.
In general, immersion biometry has been shown to be more accurate than contact applanation biometry in several studies. (Watson and Armstrong ,1999)
Optical biometry
Since its introduction, optical biometry has been gaining popularity due to the fact that it offers an easy, contact-free method to quickly and accurately assess the axial length The axial length measured by optical biometry is not however directly comparable to ultrasound biometry . ( Vogel et al,2001)
Ultrasound biometry measures the distance from the anterior corneal to the inner limiting membrane, while optical biometry measures from the cornea to the retinal pigment epithelium. Thus, the measured axial length obtained from ultrasound and optical biometry cannot be expected to yield the same values. (Olsen, 2007) .
Hitzenberger and co authers found that the axial lengths measured by optical biometry were 0.18mmlonger than those measured by the immersion technique and 0.47mm longer than those measured by the applanation technique . ( Hitzenberger et al,1993).
Haigis and co authers found that the postoperative refraction was predicted correctly within 1D in 86% and within 2D in 99% of all cases using the immersion biometry data. A similar result was obtained using optical biometry. (Haigis et al,2000)
Kiss and co authers reported that the refractive outcome in cataract patients using optical biometry was comparable to that achieved with immersion biometry . (Kiss et al, 2002)
Olsen reported an average absolute IOL prediction error of 0.65D with ultrasound and 0.43D with optical biometry (P<0.00001). Sixty-two percent of predictions using optical biometry were within 0.5D compared with45% with ultrasound. (Olsen, 2007) .
Lam and co authers reported no significant difference in the axial length obtained using optical biometry between different operators. (Lam et al, 2001).
Optical biometry has several advantages over ultrasound biometry. One is that the axial length measurement is performed through the visual axis since the patient is asked to fixate into the laser spot. In highly myopic or staphylomatous eyes, this can be particularly advantageous since it can sometimes be difficult to measure the true axial length through the visual axis with an ultrasound probe. Optical biometry is also superior to ultrasound in the measurement of pseudophakic and silicone oil-filled eyes.. The preoperative axial length measurement obtained with the IOL Master was shown to be 0.07mm longer than postoperative measurement ( Prinz et al, 2006).