One of the major issues with the results of toric intraocular lens (IOL) implantation is the misalignment due to intraoperative or postoperative factors. If the magnitude of the misalignment is larger than a critical value, realignment is required. Of course, other factors, such as the cylinder value of the toric IOL and the effective IOL position, are also playing a role in the nonplanned refraction change.
Scientific papers, presentations, and materials from the IOL companies often state, in many times without source references, that each degree of the toric IOL misalignment leads to a 3.3% loss of effectiveness, and so, 30 degrees of rotation results in no astigmatic correction.
In the brochure of the Alcon toric IOL,A one graph shows a linear correlation between misalignment and effectiveness, starting from 100% effectiveness with 0 degrees of misalignment to 0% effectiveness with 30 degrees of misalignment. This material cites 2 papers, which reference each other, and refers to a paper by Shimizu et al., which was the first paper written about toric IOLs and will be discussed later.1 The brochures for the TECNIS or the Symfony IOLB,C and the web page for the Bausch & Lomb toric IOLsD state that misalignment greater than 30 degrees may increase postoperative refractive cylinder.
Every paper on this subject cites the publication by Shimizu et al.1 This paper reports the results of 47 patients with against-the-rule astigmatism, in which 5.7 mm surgical incisions, closed by sutures, were performed. The implanted IOL was a 3-piece design type with oval optic. After the implantations, a negative effect was observed in 2 cases in which the rotations were 30 degrees. From these 2 cases, the authors draw a cautious conclusion that the acceptable limit for rotation of the lens axis is 30 degrees. Shimizu et al. demonstrate a curve called theoretical about the axis shift vs corrective effect and conclude that the maximum acceptable axis shift seems to be less than 30 degrees. Subsequently, many papers took these theoretical numbers as a fact, often without any references.
Alpins, in 1997, used the vector analysis and wrote about this problematic relationship on astigmatic corneas. His calculations showed that a 30-degree angle error leads to a 50% flattening index, and at 45 degrees, the flattening index is zero (Figure 7A).2
Later, Felipe et al. demonstrated that a toric IOL rotation of less than 10 degrees leads to a smaller than 0.5 D refraction change.3 Such a rotation does not lead to dependency on spectacles and does not require further astigmatic correction.
Tognetto et al. performed an experimental, objective testing of the image quality with a special device.4 The results showed that the reduction in image quality obtained after 30 degrees of the toric IOL rotation was less than 50%, and after 45 degrees, the image quality was the same as that of no toric correction. This working group concluded that image quality reduction was consistent with the trend of the flattening index in which a 45-degree misalignment causes a 100% loss of any toric correction. These objective examinations agree with the calculations of Alpins and support the idea that the correlation between the image quality and the amount of the toric IOL rotation is nonlinear.2 Tognetto et al. observed that a rotation of less than 10 degrees affects the image quality only minimally. However, in the case of a misalignment of more than 10 degrees, the percentage of effect-loss per degree is larger. The largest correction effect-loss is observed between 10 degrees and 20 degrees of misalignment. Tognetto et al. showed that only at a misalignment of 45 degrees the image quality was the same as that of no toric correction.
In conclusion, instead of 30 degrees, only a 45-degree toric IOL misalignment leads to the total effect-loss of cylindric correction. Besides, the relationship between the effect-loss and the magnitude of the misalignment is not linear: the first 10 degrees leads to a minimal to moderate effect-loss, ie, the tolerance concerning this amount of meridian change is much better than we thought. Obviously, it has to be emphasized that the accurate corneal marking and the precise IOL positioning during cataract surgery is extremely important. However, a dogma with false numbers was embedded into common knowledge. This is an unfortunate consequence of the improper use of the literature.
1. Shimizu K, Misawa A, Suzuki Y. Toric intraocular lenses: correcting astigmatism while controlling axis shift. J Cataract Refract Surg 1994;20:523–526
2. Alpins NA. Vector analysis of astigmatism changes by flattening, steepening, and torque. J Cataract Refract Surg 1997;23:1503–1514
3. Felipe A, Artigas JM, Díez-Ajenjo A, García-Domene C, Alcocer P. Residual astigmatism produced by toric intraocular lens rotation. J Cataract Refract Surg 2011;37:1895–1901
4. Tognetto D, Perrotta AA, Bauci F, Rinaldi S, Antonuccio M, Pellegrino FA, Fenu G, Stamatelatos G, Alpins N. Quality of images with toric intraocular lenses. J Cataract Refract Surg 2018;44:376–381
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