This document discusses optical biometry measurements and their use in determining appropriate intraocular lens (IOL) calculations and selections. It provides an overview of biometry techniques such as ultrasound and optical methods, compares their accuracy advantages, and outlines considerations for different eye anatomies and conditions. Formulas for calculating IOL power are examined, including preferences for different eye lengths. Challenges in post-refractive surgery patients are also addressed.
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27. However, it must be noted that the human eye itself has only a small spot of sharp vision in the middle of the retina, the fovea centralis, the rest of the field of view being blurry.
28. The angle of the sharp vision being just few degrees in the middle of the view, the sharp area thus barely achieves even a single mega pixel resolution.
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30. "couching", in which a curved needle was used to push the lens into the rear of the eye and out of the field of vision. The eye would later be soaked with warm clarified butter and then bandaged.
31. Couching continued to be used throughout the Middle Ages and is still used in some parts of Africa and in Yemen.
37. Normal Measurements 23.5mm 12.5mm 245 μm 4.25 mm* 3.24 mm 0.5 – 8mm 550μm K: 43 - 44 D *LT is Age dependant andcontinuously growing
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41. Immersion Method – The Immersion method of Ultrasound Biometry is by far the most accurate way to measure the AL of the eye using Sound. This method requires that the probe is placed in a “bath” of saline solution directly over the eye. With the immersion of the probe the cornea is not touched thus eliminating the compression errors.
53. Very Long / Short Eyes / AstigmatismModern Technology / Knowledge
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55. What about the “Dust Cover”?aka Cornea To ensure proper K value along central visual axis Keratometry should be measured at the same time as Axial Length.
56. Poor Alignment * Image courtesy of Karen Bachman and Cara Fletcher from the Cincinnati Eye Institute
94. Lens Constants A-Constants are used with all IOL formulas, and are determined by the anticipated position within the eye. Surgeon Factor – is used with the first Holladay formula, and is determined by the distance from the Iris plane to the Optical plane of the implant. Effective Lens Position (ELP) is used for the Holladay II formula, and is based on the depth of the AC following Cataract surgery with the new IOL in place.
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96. The advantage of a simple regression formula is that it is relatively simple to calculate. A factor can be added to a simple regression formula to compensate for a long or a short eye. (These formulas have been discredited by their authors.)
103. SRK II – Is a formula used in short eyes <22mm as well as in long eyes >24.5mm it uses calculations based on AL and Keratometry (Not in use by most physicians)
104. Olsen – The Olsen formula addresses four areas of concern: the calculation of corneal power, the measurement of axial length, the anterior chamber depth prediction, and the IOL optics. (European)
115. Remember!! PCIOL For every .1mm error in Calculations, there is a .19D Post-Op SURPRISE! So 1.0mm equals about 2.0D ACIOL For every .1mm error in Calculations, there is a .12D Post-Op SURPRISE! So 1.0mm equals about 1.2D
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117. Clinical History Method – Is usable when both the pre-op and post-op Keratometry values are known.
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