The A-constant in IOL power calculation formulas is specific to each IOL model. It represents:

• A The average axial length for which the IOL was designed
• B The refractive index of the IOL material (PMMA, silicone, or acrylic)
• C The haptic angulation in degrees from the optic plane
• D An empirically derived constant that relates the predicted postoperative IOL position to the specific lens design ✓

Explanation

The A-constant is an IOL-specific empirical constant used primarily in the SRK/T and SRK-II formulas. It does not represent a direct physical measurement but rather encodes the predicted effective lens position (ELP) relative to corneal landmarks for that specific IOL model and insertion technique. Each IOL-surgeon combination may require a personalized A-constant derived from large-scale outcome audits. The A-constant influences the calculated IOL power: a higher A-constant generally yields a higher IOL power for the same biometric data. Modern third-generation formulas like Holladay 1, Hoffer Q, and SRK/T incorporate ELP prediction differently but all require IOL-specific constants.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.