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Combination of technologies can help achieve true bespoke management of astigmatism
Femtosecond cataract surgery, improvements in diagnostic technology and accurate astigmatism measurements optimize refractive results.
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Femtosecond refractive laser-assisted cataract surgery, or ReLACS, in combination with improved diagnostic technology and a better understanding of internal astigmatism, has the ability to unlock the potential for the true bespoke management of astigmatism. ReLACS both extends the treatment range for astigmatism and potentially improves the precision and customization of individual treatments.
Femtosecond intrastromal arcuate keratotomies are promising to be more predictable than penetrating arcuate keratotomies, although they result in less astigmatic effect per given arc length. More consistent surgically induced astigmatism from reproducible ReLACS incisions also helps to further refine treatment with toric lenses, which are currently only available in 0.5 D increments. The exact cylinder correction of a toric lens depends on the effective lens position. Therefore, a more consistent ReLACS capsulorrhexis, methods for measuring effective lens position with optical coherence tomography and perhaps future lenses optimized to this consistency may be able to further help. Once more predictable, we should be able to better estimate true toric lens powers with improved biometry and more sophisticated lens calculations.
ReLACS can allow customization to address asymmetric astigmatism with asymmetric intrastromal arcuate keratotomy incisions or skewed axis incisions if necessary. Keratoconus and other ectatic disorders should certainly be assessed in these cases. Anecdotally, we have had good experience with ReLACS in keratoconus utilizing a liquid optics interface, which is likely to be preferable with irregular and extreme corneal curvatures, to allow for superior optical coupling without striae. Other cases such as pseudoexfoliation, ectopia lentis or trauma associated with significant zonular weakness may dictate addressing ocular astigmatism with intrastromal arcuate keratotomies for safety reasons. Finally, when optimized, ReLACS, on at least one platform, has demonstrated that it can decrease both the likelihood that ultrasound energy is necessary for cataract surgery and the amount of ultrasound energy needed when necessary (Figure 1).
It is important to mention that cyclotorsional marking is different with ReLACS because epithelial indentations and ink are poorly visible after docking. Cautery marks at 3 and 9 o’clock placed at the slit lamp are a simple solution that we have validated in a small series using our previous technique (Figure 2), to which we can align a toric lens (Figure 3).
Figure 1. ReLACS with customized “snowflake” pattern, arcuate keratotomies, Malyugin ring and pseudoexfoliation syndrome with zonular laxity.
Images: Ma JJK
Figure 2. Validating intrastromal markings for cyclotorsional alignment.
Figure 3. Toric lens alignment to corneal intrastromal markings. Cautery marks visible at limbus.
Figure 4. Estimate of internal astigmatism.
Improved diagnostics and internal astigmatism
In 1890, Javal effectively first described the presence of internal astigmatism when he proposed the relationship between keratometry and astigmatism by subjective refraction. The implication was that anterior corneal curvature does not take into account a certain amount of internal astigmatism inherent in subjective refractions. Although postoperative internal astigmatism is difficult to predict, its contribution to overall refractive error can result in residual error and patient dissatisfaction. Moreover, because the magnitude of internal astigmatism is significant relative to the average amount of topographical corneal astigmatism (57.4% less than 1 D), accounting for internal astigmatism can mitigate against refractive surprises in patients who may not realize that they could have visually significant astigmatism postoperatively.
Groundbreaking work by Koch has increased awareness that posterior corneal astigmatism may affect the accuracy of toric IOL and arcuate keratotomy planning. Although our own research confirmed this, posterior corneal astigmatism measured by Scheimpflug only accounted for up to 60%, on average, of the total internal astigmatism in our own study. Therefore, either current Scheimpflug devices are not yet able to consistently and accurately measure this metric, or posterior corneal astigmatism may only be part of the story, with lens tilt, media and other sources, such as the vitreous and sclera, accounting for the rest.
Estimating internal astigmatism
We attempted to estimate the amount of postoperative internal astigmatism in our surgical population when we reviewed a series of 43 eyes in a single practice. All cases were implanted with aspheric monofocal lenses to eliminate the cylinder contribution from a toric lens. Internal and total astigmatism were calculated using manual refractions, several different Placido, slit and Scheimpflug-based topographies, as well as preoperative, intraoperative and postoperative aberrometry. Subsequently, we have also included full-gradient topography.
Based on this series, we estimated that the average postoperative internal astigmatism in our study population was +0.39 D × 169 (Figure 4). We have subsequently validated this against-the-rule estimate in a separate cohort of patients with toric IOLs, confirming that factoring internal astigmatism into calculations should indeed improve results. It is notable that both preoperative and intraoperative aberrometry measurements were poor predictors of postoperative internal astigmatism. Although promising, current diagnostic technologies still lack the precision, accuracy and resolution necessary for this prediction. Perhaps anterior segment OCT, which we are currently studying, may eventually make such a prediction possible.
Nevertheless, until then, we found that a simple, reasonable approach is to perform a vector addition of the estimated 0.39 D against-the-rule internal astigmatism to an accurately measured anterior corneal curvature.
Measuring anterior corneal astigmatism
Based on a study of the repeatability and reliability of 11 different devices, we currently use anterior corneal curvature values from two separate measurements, often including the results of a full gradient topographer. We combine this anterior corneal curvature with our estimate of internal astigmatism by vector summation to determine the angle and magnitude to correct. The repeatability of anterior corneal curvature is essential, between either at least two instruments or different exams, to confirm reliability before surgery. Furthermore, we found that the reproducibility of the qualitative maps from the full gradient topographer, especially in the analysis of smaller amounts of anterior corneal curvature (less than 1 D), which made up the largest cohort, was unrivaled. No other device was able to consistently re-create the same convincing pattern. This amount of detail and consistency was also useful in differentiating tear film, eyelid and other confounding variables.
It is noteworthy that although a higher incidence of with-the-rule anterior corneal curvature exists in younger cataract populations and against-the-rule in older cataract populations, the axis and magnitude do not appear to change significantly among individuals followed for up to 9 years. We hypothesize that with-the-rule and against-the-rule are mutually exclusive populations that may vary over time with respect to amplitude, but not axis.
Estimating total target astigmatism
To estimate total target astigmatism, one must also accurately estimate surgically induced astigmatism. I always aim my incisions either parallel or perpendicular to the steep axis, with a bias toward temporal incisions, to both increase the accuracy of our estimates and to simplify vector calculations.
For example, if one were to treat +1.30 D × 180°, total target astigmatism of +1.57 D × 180, given an internal astigmatism of +0.39 D against-the-rule and a temporal incision with +0.12 D of flattening at 180°, a 1.5 D corneal powered toric lens would be ideal.
However, a similar eye with +1.30 D × 90 would have an estimated total target astigmatism of +1.03 D × 90 with internal astigmatism and a temporal incision with coupling that induces +0.12 D of steepening at 90°. As such, a toric lens aimed at 1 D would be ideal instead.
Summary
We currently follow a customized algorithm that accounts for internal astigmatism, surgically induced astigmatism, anterior chamber depth, age, axial length and corneal diameter, among other parameters, to determine the optimal cylinder correction for each individual eye. No longer should we simply treat keratometry measurements at face value. ReLACS, improvements in diagnostic technology and estimates of total target astigmatism allow for a true bespoke management of astigmatism that optimizes refractive results and improves postoperative patient satisfaction.
Acknowledgement: John Golding for help with the photographs for this article.