COT Biometry and A-Scans 4 — Questions and Answers
Question 1: Which of the following BEST describes the Haigis formula's approach to IOL calculation?
- Uses only axial length and keratometry
- Uses three constants (a0, a1, a2) and measures ACD (Correct answer)
- Uses surgeon factor and axial length only
- Uses corneal diameter and lens thickness
Correct answer: Uses three constants (a0, a1, a2) and measures ACD
The Haigis formula uses three lens constants and incorporates measured ACD to predict effective lens position.
Question 2: On a properly aligned A-scan, the retinal spike should be:
- Broad and low
- Sharp and perpendicular (90°) with high amplitude (Correct answer)
- Slanted at 45°
- Double-peaked
Correct answer: Sharp and perpendicular (90°) with high amplitude
A perpendicular, high-amplitude retinal spike indicates the probe is aligned along the visual axis and measurement is accurate.
Question 3: What is the typical axial length of an emmetropic adult eye?
- 20.0–21.0 mm
- 21.5–22.5 mm
- 23.0–24.0 mm (Correct answer)
- 24.5–25.5 mm
Correct answer: 23.0–24.0 mm
The average emmetropic adult eye has an axial length of approximately 23.0–24.0 mm.
Question 4: Which instrument combines keratometry, corneal topography, and optical biometry in one device?
- Lenstar LS 900 (Correct answer)
- Humphrey Field Analyzer
- Orbscan II
- Specular microscope
Correct answer: Lenstar LS 900
The Lenstar LS 900 uses optical low coherence reflectometry (OLCR) to measure biometry and keratometry simultaneously.
Question 5: In a patient with a dense posterior subcapsular cataract, the BEST biometry method is:
- Contact A-scan with higher gain
- Immersion A-scan (Correct answer)
- Optical biometry (IOLMaster)
- B-scan only
Correct answer: Immersion A-scan
Immersion A-scan is preferred when optical biometry cannot penetrate a dense cataract, as it avoids corneal compression.
Question 6: The keratometry value (K) used in IOL formulas represents the:
- Central corneal thickness
- Average corneal curvature in diopters (Correct answer)
- Corneal white-to-white diameter
- Corneal refractive index
Correct answer: Average corneal curvature in diopters
K values reflect the curvature of the anterior corneal surface, expressed in diopters, and are essential for IOL power calculation.
Question 7: Post-LASIK eyes present a challenge for IOL calculation primarily because:
- The axial length changes after LASIK
- Keratometry readings underestimate the true corneal power (Correct answer)
- The lens position shifts anteriorly
- The vitreous velocity changes
Correct answer: Keratometry readings underestimate the true corneal power
Standard keratometry overestimates corneal power after LASIK due to index of refraction assumptions, leading to hyperopic surprise.
COT Biometry and A-Scans 4
Which of the following BEST describes the Haigis formula's approach to IOL calculation?