With the advent of phacoemulsification, which enables cataract removal through increasingly smaller incisions, and now femtosecond laser-assisted cataract surgery, which enables the creation of these incisions with greater precision, and sophisticated intraocular lens (IOL) designs, it appears that a spectacle-free outcome is increasingly within reach. Modern biometry techniques have further enhanced our ability to minimize residual spherical refractive errors. Hence, the main challenges facing the cataract surgeon today are to achieve an increased depth of focus and the elimination of corneal astigmatism. The effect of residual corneal astigmatism is particularly disturbing to patients who did not have manifest astigmatism preoperatively, and the astigmatism only becomes unmasked after cataract removal. The constant expansion of the range of multifocal, toric monofocal, and toric multifocal IOLs reflects this desire to provide the best visual outcomes for our cataract patients as even 0.5 diopters (D) of residual astigmatism has been shown to have a significant impact on visual acuity, especially if the patient wishes to have a multifocal IOL.1,2
The introduction of the Acrysof SN6AT2 and the ReSTOR SND1T2 serves to augment the lower range of astigmatism that can be corrected, and at least 60% of the patients in the study by Levitz et al3 were able to achieve 20/20 uncorrected vision at 3 months, with at least 90% achieving a postoperative refractive cylinder within 0.5 D. Similarly good outcomes were achieved by Aujla et al.4 Nonetheless, the correction of such low corneal astigmatism needs to be approached with caution as the efficacy of the IOL is easily influenced by any inaccuracies in the biometry and surgical techniques, and patients seeking such a precise outcome are also less likely to accept any deviation from the targeted outcome than patients who have higher refractive errors preoperatively. One indication of this potential risk of a refractive surprise is the fact that 80% of the multifocal toric patients included by Levitz et al3 had preoperative corrected distance vision of 20/20.
There are a number of factors that can affect the accuracy of the biometry of which keratometry is a major component, particularly if posterior corneal astigmatism is not considered in the calculations as, for example, when keratometry is obtained with keratometers that do not measure it. As determined by a keratometer which uses Scheimpflug photography to obtain posterior corneal power values (Galilei Dual Scheimpflug Analyzer; Ziemer Ophthalmic Systems AG, Port, Switzerland), the discrepancy between total corneal astigmatism and anterior corneal astigmatism may exceed 0.5 D in 5% of eyes, and even with the inclusion of posterior corneal astigmatism, in eyes having with-the-rule astigmatism, there is still a mean prediction error of 0.57 D using current IOL formulae.5–7
The accuracy of keratometry measurements may additionally be degraded by mild ocular surface disturbances such as from dry eyes, especially if the patient has also required other investigative procedures before biometry. Hence, it may be prudent to perform biometry on a separate occasion. The construction of the IOL formulae is another factor that may affect the accuracy of the IOL calculation. In particular, the amount of surgically induced astigmatism, the eventual IOL position, and the spherical power of the IOL should be considered in addition to the total corneal astigmatic power and axial length values. For example, the Alcon toric IOL calculator uses a fixed ratio to determine the corneal plane cylinder power, whereas the Assort calculator includes the predicted IOL position in the conversion. The Barrett toric calculator incorporates all these elements as well as the posterior corneal curvature and gives the most precise prediction to date.8
Another potential source of error arises from the alignment of the IOL with the current methods of placing reference marks, which may result in a mean total error of 5 degrees of misalignment, with every 1 degree of misalignment decreasing the efficacy of the astigmatic correction by 3%.9 Thus, a number of digital markers have been developed to overcome this, such as the Verion Image guided system (Alcon, Fort Worth, TX), the Callisto Eye digital system (Carl Zeiss Meditec AG, Jena, Germany), and the Barrett toriCAM. Intraoperative wavefront aberrometry systems [ORA system with VerifEye (Alcon), Holos IntraOP (Clarity Medical Systems Inc, Pleasanton, Calif)] are other modalities that serve to improve the accuracy of the IOL placement. Nevertheless, they are also subject to errors from lid speculums, intraocular pressure, corneal clarity, and patient ability to fixate. Postoperative IOL rotation can be minimized by IOL design in terms of material and size as well as meticulous removal of viscoelastic material from behind the IOL at the end of surgery. The larger hydrophobic acrylic IOLs have been shown to have better rotational stability regardless of the haptic design.10–12
Certainly, the goal of spectacle independence can be achieved with improvements in technology. However, this independence comes at a price as these premium IOLs are not covered by standard insurance plans, and hence, the patient incurs significant additional out-of-pocket costs or an increase in insurance premiums. Moreover, the specter of potential complications of cataract surgery such as endophthalmitis and posterior capsule rupture should not be overlooked in our attempts to improve the quality of vision for our patients, and the use of cataract surgery as a form of refractive surgery for those with early cataracts should be tempered by prudence.
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— Jimmy Dean