Calculating Surgically Induced Astigmatism (SIA) post-cataract surgery
Planning corneal relaxing incisions (LRI/AK)
Auditing surgical techniques and surgeon-specific A-constants
Evaluating toric IOL alignment and effectiveness
Section 2
Formula & Logic
Vector Analysis (Alpins Method)
Astigmatism is a vector quantity with both magnitude (cylinder) and direction (axis). Simple subtraction of cylinder values is inaccurate because it ignores axis shifts. This calculator uses polar vector decomposition to find the net change.
Key Indices
SIA: Net vector change induced by the surgery
TIA (Target Induced Astigmatism): The intended vector change
Correction Index: SIA / TIA (Ideal = 1.0)
Section 3
Pearls/Pitfalls
Surgical Footprint
Surgeons should calculate their mean SIA over 50-100 cases to determine their personal "surgical footprint," which can then be used in IOL power calculation formulas to improve refractive outcomes.
Section 4
Evidence Appraisal
Primary Reference
A new method of analyzing vectors for changes in astigmatism
Alpins NA • J Cataract Refract Surg. 1993;19(4):524-39
