الفهرس 
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Abstract
The IOLMaster has simplified considerably the process of ocular biometry. It is a non contact technique, which does not require use of topical anaesthesia, thus providing comfort to the patient and preventing corneal abrasions and transmission of infections. Furthermore it has greater accuracy than ultrasound biometry because it measures the ocular axial length along the visual axis .In addition it is easier to master its use
Additionally, the use of a transducer (as in acoustical contact measurements) or of the scleral shell (as in immersion coupling techniques), which are often found unpleasant is not needed. Measurements are taken with the patient seated. Corneal radii and axial length can be measured on the same device, and the patient need not change seats or lie down. Furthermore, it permits the lens type to be chosen from a database and to be transferred to the local area PC network
However, the advent of the IOLMaster has not rendered ultrasonic biometry obsolete as a significant number of eyes still require ultrasound biometry, which is still essential in every ophthalmic practice like cataract surgery. Although this number depends on the referral patterns of the practice
Packer study showed that the immersion ultrasound method with the Prager shell presents significant advantages, especially for surgeons who use the Holladay II calculation formula because the optical method lacks the lens thickness measurement which is a required variable in this formula
Connors study showed that (8- 10%) dense ocular media, corneal scarring, mature and posterior subcapsular cataracts, prevent acquisition of optical AL measurements.
Moreover, eyes with non-optimal fixation as in cases of age related macular degeneration may result in inaccurate AL measurements as the measurements are not on the visual axis. Positioning also of patients with mobility problems on the IOLMaster machine may occasionally be a problem
Optical versus ultrasound measurement of axial length for intraocular lens power calculation.
The axial lengths measured by the optical method were significantly longer than those measured by ultrasound; however, the values obtained by the 2 methods were closely correlated
During contact ultrasound measurements, the probe can applanate the cornea, shortening the AL by an average of 0.1-0.3 mm
This effect is also seen in studies comparing contact and immersion forms of ultrasound,
Immersion ultrasound minimises the indentation of the cornea and therefore immersion AL measurements are closer to those of IOLMaster
The optical method measures along the optical axis of the eye, while the ultrasound technique more likely measures on the anatomic axis
As laser interferometry relies on optical alignment methods in which the patient fixates on a light spot, this ensures better alignment of the measurement axis with the visual axis compared with ultrasound, in which the (.operator attempts manual alignment with the estimated optical axis of the eye
This is especially important in eyes with posterior pole staphylomata because of the more precise localization of the fovea
This leads to the assumption that US examination is more dependent on the operator, especially if the operator is not experienced in the use of this device. An unskilled operator will indent the cornea with the A-scan probe more than necessary, resulting in a shallower ACD and shorter axial length even though an acceptable A-scan echogram is displayed on the screen. Also, the variability in indentation is higher with less experienced operators. Off-axis measurement usually results in a too-short measurement of intraocular distances
Also, the measuring points of the two methods are different, the optical method measures from the tear film to the retinal pigment epithelium, while the ultrasound technique measures from the cornea to the vitreoretinal interface resulting in a difference corresponding to the foveal thickness and ranging from 130μm at the fovea to 400μm within the macula
During accommodation, the lens of the eye increases in thickness and the anterior chamber decreases in thickness. There may also be a small reduction in vitreous length.
The displacement of the anterior chamber and possibly the vitreous chamber by the higher-index lens will increase the optical path length within the eye and will tend to give an overestimation of any accommodation-induced changes in the length of the eye with the IOLMaster.