COT Biometry and A-Scans 5 — Questions and Answers
Question 1: The 'effective lens position' (ELP) in IOL formulas refers to:
- The distance from cornea to retina
- The predicted postoperative position of the IOL principal plane (Correct answer)
- The thickness of the IOL optic
- The distance from the iris to the retina
Correct 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.
Question 2: A-scan biometry in a pseudophakic eye (already has an IOL) requires:
- No modification
- A specific pseudophakic velocity setting for the IOL material (Correct answer)
- Optical biometry only; A-scan is contraindicated
- A higher frequency probe (20 MHz)
Correct 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.
Question 3: Which parameter does the Holladay 1 formula use in addition to axial length and keratometry?
- Anterior chamber depth
- Surgeon factor (SF) (Correct answer)
- Lens thickness
- Corneal diameter
Correct answer: Surgeon factor (SF)
Holladay 1 uses the surgeon factor (SF) to predict ELP based on axial length and keratometry.
Question 4: Which of the following axial length ranges classifies an eye as 'long'?
- >22.5 mm
- >24.5 mm (Correct answer)
- >25.5 mm
- >26.0 mm
Correct answer: >24.5 mm
Eyes with axial lengths greater than 24.5 mm are generally classified as long, corresponding to myopic eyes.
Question 5: What is the purpose of taking multiple A-scan measurements and averaging them?
- To reduce gain artifact
- To improve accuracy and reduce variability from fixation or probe alignment (Correct answer)
- To correct for silicone oil velocity
- To calibrate the IOL formula constant
Correct 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.
Question 6: Which lens constant is used with the SRK/T formula?
- Surgeon factor (SF)
- A-constant (Correct answer)
- a0/a1/a2 Haigis constants
- pACD (personalized ACD)
Correct answer: A-constant
The SRK/T formula uses the A-constant specific to each IOL model to calculate predicted IOL power.
Question 7: When is B-scan ultrasonography preferred over A-scan for preoperative assessment?
- When axial length is needed
- When media opacities prevent visualization of posterior segment structures (Correct answer)
- When keratometry is unobtainable
- When the patient cannot cooperate with contact measurement
Correct 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.
COT Biometry and A-Scans 5
The 'effective lens position' (ELP) in IOL formulas refers to: