Biometry and A-Scans Flashcards

The 'effective lens position' (ELP) in IOL formulas refers to:

Answer: The predicted postoperative position of the IOL principal plane

ELP is the predicted postoperative location of the IOL's principal plane, which significantly affects refractive outcome prediction.

A-scan biometry in a pseudophakic eye (already has an IOL) requires:

Answer: A specific pseudophakic velocity setting for the IOL material

PMMA, silicone, and acrylic IOLs each have different sound velocities, requiring appropriate settings for accurate measurement.

Which parameter does the Holladay 1 formula use in addition to axial length and keratometry?

Answer: Surgeon factor (SF)

Holladay 1 uses the surgeon factor (SF) to predict ELP based on axial length and keratometry.

Which of the following axial length ranges classifies an eye as 'long'?

Answer: >24.5 mm

Eyes with axial lengths greater than 24.5 mm are generally classified as long, corresponding to myopic eyes.

What is the purpose of taking multiple A-scan measurements and averaging them?

Answer: To improve accuracy and reduce variability from fixation or probe alignment

Averaging multiple readings minimizes random errors from eye movement or slight probe misalignment during the exam.

Which lens constant is used with the SRK/T formula?

Answer: A-constant

The SRK/T formula uses the A-constant specific to each IOL model to calculate predicted IOL power.

When is B-scan ultrasonography preferred over A-scan for preoperative assessment?

Answer: When media opacities prevent visualization of posterior segment structures

B-scan provides a two-dimensional image of posterior segment pathology (e.g., retinal detachment, tumor) obscured by dense cataracts.