2020–2021 BCSC Basic and Clinical Science Course™
3 Clinical Optics
Chapter 6: Intraocular Lenses
Chapter Exercises
Answers
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6.1. c. The formula to correct the AL is AL × VC/VI, where VC is the correct velocity and VI is the incorrect velocity. This correction eliminates the need for the patient to undergo the procedure again. There is no factor that can be added to the AL to correct such an error, and the error is not negligible and thus cannot be ignored.
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6.2. c. Multifocal IOLs, including bifocal and EDOF IOLs, necessarily trade contrast performance at distance for multifocality. Although distance acuity may be degraded in dim light in multifocal IOLs, acuity may be excellent at distance in all lenses. It is important to understand that acuity is not identical to optical performance.
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6.3. c. Multifocal IOLs present both near and distant foci to the retina at the same time. This leads to an unavoidable decrease in image quality and contrast sensitivity, particularly at low levels of illumination. Pupil size may be a factor, particularly with certain types of multifocal IOLs.
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6.4. d. Piggyback IOLs have been used to reach a total dioptric power that was unavailable in a single lens. As IOLs are becoming available in a wider range of powers, however, it is less likely that a piggyback IOL will be needed to reach an unusually high or low power. Piggyback IOLs are placed anterior to the primary lens and thus modify the light vergence before the light reaches the primary IOL.
These IOLs may be used to correct inaccurate primary IOLs in a second operation if the original IOL power was too low or too high. They are not used after removal of an incorrectly powered IOL—“piggyback” implies that a second IOL is present in the eye.
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6.5 c. Note the following comments for each option:
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The axial length (AL) is the most important factor in the formula
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The refractive error resulting from an error in AL measurement is more consequential in short eyes than in long eyes.
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Accuracy in AL measurement is relatively more important in short eyes than in long eyes. Correct answer.
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During ultrasonic measurement of AL (A-scan), sound travels faster through the crystalline lens and cornea than through the aqueous and vitreous. Therefore, there is a need to make adjustment to the AL “measurement” by correcting for the incorrect velocity of sound.
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The velocity of sound in an aphakic eye varies insignificantly between short and long eyes (ie, it is almost comparable).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.