Consequences of Mismatch, Misalignment and Rotation of Toric Intraocular Lenses in Refractive Cataract Surgery Part 1. It Ain’t 30. The True Angle of Doom

Abstract

AbstractPurposeTo demonstrate that the total loss of astigmatism as a consequence of misalignment or rotation of a toric intraocular lens (tIOL) can occur much earlier than the widely believed and taught 30 degrees. To give a precise surgically useful estimate of that value. To clarify the role of mismatch and misalignment of toric intraocular lenses in cataract surgery beyond what is commonly recognized in the literature and make corresponding surgical recommendations.SettingPrivate Practice and Research Center. The EYE Center. Champaign, IL, USA.DesignFormal Analytical StudyMethodsThe astigmatism addition approach is used in its simplest form along with analytical tools to derive new results concerning mismatch, misalignment and rotation of toric intraocular lenses.ResultsThe often stated results of total loss of astigmatic correction by 30-degree rotation and 3.3 % loss per degree represent a usually poor approximation to realistic surgical cases. We show how they constitute a very special case in the context of a more general framework relevant to procedures performed by refractive cataract surgeons dealing with the surgical correction of astigmatism with tIOLs. Total loss of astigmatic correction can occur with as little as 20 degrees of misalignment and less than 10 degrees of tIOL rotation. A practical approximation for that angle of doom, Δ, in the surgically relevant range can be expressed by Δ ≈ 30 − 15 ω degrees, where is the fractional overcorrection of L, the cylinder of the tIOL, and A, the astigmatism to be corrected. Similarly for undercorrection we show that Δ ≈ 30 + 15 u degrees where represents the corresponding fractional undercorrection. That is to say the angle of doom is extended beyond the 30 degrees for cases of undercorrection of the astigmatism. We also demonstrate that overcorrection of astigmatism results in a significantly faster decline in astigmatism correction per degree of misalignment/rotation. The significant clinical implications and surgical recommendations, including for optimal degree of overcorrection, are a natural consequence of these novel results.ConclusionsTotal loss of astigmatism correction can occur at a significantly smaller angle than commonly believed and overcorrected astigmatism residual rises with tIOL misalignment or rotation significantly faster than undercorrected astigmatism. We provide the methodology and explicit solution for determining this behavior.