Purpose: To show current approaches for overcorrecting astigmatism and "flipping" its axis need be reconsidered in light of methods we introduce that take into account both mismatch and misalignment of the toric intraocular lens (tIOL) with respect to the astigmatism to be corrected at the time of cataract surgery.
Setting: Private Practice and Research Center. The EYE Center. Champaign, IL, USA.
Design: Formal Analytical Study
Methods: In the most common surgical situation where both mismatch and misalignment exist, we present an analysis of the point at which overcorrection and undercorrection residuals coincide, yielding a simple but powerful methodology to predict the optimal degree of overcorrection with a tIOL. The method is illustrated for tIOLs used in surgical practice.
Results : The minimum astigmatism appropriate to overcorrect with a tIOL is given by, A_min= m/cos2χ , where m is the midpoint threshold used by "split-the-difference" algorithms and χ is the estimate of tIOL misalignment due to all causes. Correspondingly, the maximum overcorrection, Ω_max, that should be attempted is Ω_max= α/2 [1-γ(2 n-1)] ) where α=σ/τ is the dioptric step at the corneal plane, with σ = H - B, where H = n σ is the cylinder of the overcorrecting tIOL and B = (n - 1)σ is the cylinder of the undercorrecting tIOL, both at the IOL plane, τ is the toricity ratio and γ relates to the angle of misalignment χ by γ = 1/cos2χ - 1 which can be approximated by γ ≈ (χ_deg/40)^2. Ω_max factors elegantly in the product of α/2, the (ideal) midpoint correction for perfect alignment, by the bracketed term, representing the percent reduction of the ideal value in a realistic surgical situation with estimated misalignment χ. To illustrate: an eye of average dimensions (τ ~ 3/2) and tIOLs from major manufacturers (σ = 3/4), with A = 2.35 D dictating n = 5. For a misalignment of 10 degrees Ω_max ≅ 0.10 D is the maximum overcorrection that should be accepted, significantly smaller than the midpoint α/2 = σ/2τ = 0.25 D, recommended by many current tIOL calculators.
Conclusion: An optimal method is presented for the selection of an overcorrecting tIOL at the time of refractive cataract surgery with improvement over current tIOL calculators' methods.
IMMEDIATELY SEQUENTIAL BILATERAL CATARACT SURGERY - «FOR» AND «CONS»
