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The SIAK value is the vector difference between the postoperative astigmatism and preoperative astigmatism. The vector should theoretically be a Jackson cross according to Gauss' law of mean curvature, requiring equal amounts of orthogonal flattening and steeping of the cornea.1 Unfortunately, the primary cataract incision is not paired (symmetrical) and is often accompanied by an unpaired secondary incision superiorly; thus, the resulting effect is rarely symmetrical (Figure  1). With multiple incisions, the SIAK vector is wherever the vector difference dictates.
Source: Journal of Cataract and Refractive Surgery - Category: Opthalmology Authors: Tags: Letter Source Type: research