A patient is scheduled for cataract surgery. The axial length on IOL Master biometry is 28.5 mm. Using the SRK/T formula, which optical constant most critically affects the predicted postoperative refraction in this eye?
- A The corneal keratometry reading only
- B The anterior chamber depth only
- C The lens thickness measured by ultrasound
- D The A-constant of the specific IOL chosen ✓
Explanation
The SRK/T formula (and all modern IOL power calculation formulas) uses the IOL-specific A-constant, which represents the predicted effective lens position (ELP) for that particular lens design. A higher A-constant corresponds to a more anterior lens position, requiring lower IOL power; a lower A-constant means deeper position requiring higher power. The A-constant is the most critical surgeon/lens-specific variable and must be used from the manufacturer's optimised data or personalised from the surgeon's own outcomes. For a long eye (28.5 mm), modern formulas (Barrett Universal II, Olsen, Kane) are preferred over SRK/T as it tends to produce a myopic surprise in long eyes. Keratometry and ACD are also inputs but the A-constant calibrates the formula's prediction.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.