Camilo A. Niño

Inflammation in Keratoconus.

Cleveland, OH §Department of Biomedical Engineering, Lerner Research Institute, Cleveland Clinic, OH ¶Transplant Center, Surgery Institute, Cleveland Clinic, OH kDepartment of Biomedical Engineering, Case Western Reserve University, OH **Department of Ophthalmology, University of Sao Paulo, Sao Paulo, Brazil ††Department of Ophthalmology, Federal University of Rio de Janeiro, Sao Paulo, Brazil ‡‡Cornea Genetic Eye Institute, Beverly Hills, CA §§Institute for Translational Genomics and Population Sciences, Los Angeles BioMedical Research Institute, Harbor-UCLA Medical Center, Torrance, CA ¶¶The Jules Stein Eye Institute, University of California-Los Angeles, Los Angeles, CA kkCullen Eye Institute, Baylor College of Medicine, Houston, TX ***Stulting Research Center, Woolfson Eye Institute, Atlanta, GA †††Department of Ophthalmology, Mount Sinai School of Medicine, New York, NY

Total Corneal Astigmatism Measurement Precision.

We read with interest the article by Savini and Naeser on refractive astigmatism following toric intraocular lens (IOL) implantation. In the group of 40 eyes phacoemulsification was performed through a 2.75-mm temporal incision. In Table 1 it was indicated that surgical-induced corneal astigmatism along the surgical meridian (mean 6 SD) was -0.15 6 0.40 diopters (D; range, -0.81 toþ0.79 D) in the whole group of eyes, -0.06 6 0.35 D (range, -0.56 toþ0.79 D) in the group of with-the-rule (WTR) astigmatism eyes, and -0.30 6 0.37 D (range, -0.81 to þ0.51 D) in the group of against-the-rule (ATR) astigmatism eyes. An interesting conclusion that might be drawn from these findings is that the range of the induced astigmatism was much wider than expected, indicating that this could be a significant factor influencing the results in a given eye. Table 2 showed that values of error in refractive astigmatism along the steeper corneal meridian (ERA KP /) for measurement models 1, 2, and 5, were for all eyes in the study -0.25 6 0.58 D (range, -1.22 to 0.92) applying model 1 KA. For model 2 TCA and for model 5 TCA they were -0.05 6 0.49 D (-1.12 to 0.98) and -0.07 6 0.48 D (-1.28 to 1.06), respectively. Results for WTR astigmatism eyes with model 1 KA were -0.59 6 0.34 D (-1.22 to 0.02), for model 2 TCA -0.13 6 0.42 D (-1.10 to 0.98), and for model 5 TCA -0.07 6 0.43 D (-1.20 to 1.15). For ATR eyes the values were 0.32 6 0.42 D (-0.87 to 0.92), 0.07 6 0.59 D (-1.12 to 0.71), and 0.12 6 0.61 D (-1.04 to 0.87) for models 1 KA, 2 TCA, and 5 TCA, respectively. The wide range (from overcorrections to undercorrections) in all the groups with the exception of model 1 for WTR astigmatism eyes is noteworthy. Since the ERA polar value along the meridian / was deemed negative for astigmatic overcorrections and positive for undercorrections, the use of a mean of those values, as it was done, could be misleading, because opposite signed values will tend to cancel each other. The use of other measures of central tendency like the median or eliminating the effect of the opposite sings using the absolute values of ERA might yield a more realistic vision of the results. When analyzing separately WTR and ATR eyes some interesting details are evident. Although the arithmetic mean of ERA KP / for WTR eyes applying model 2 TCA (-0.13 6 0.42 D) was smaller than ERA K / using model 1 KA (-0.59 6 0.34 D), the range of error was wider with model 2 TCA (-1.10 to 0.98 D with model 2 versus -1.22 to 0.02 D with model 1) and the standard deviation also was higher (0.42 vs. 0.34 D). The same situation presented in ATR eyes: The range of error was wider with model 2 TCA (-1.12 to 0.71 D with Model 2 versus -0.87 to 0.92 D with Model 1) and the standard deviation also was higher (0.59 vs. 0.42 D). Accordingly, the Figure in the article showed that the ellipses using models 2 through 5 had longer axes than those for model 1, which as the authors explained, were related to a larger variance and, therefore, less precision. All these findings suggested that at least some of the values were more dispersed. Knowing the percentage of ERA values 6 0.5 D would be helpful. Theoretically, measuring the total corneal astigmatism (including the posterior corneal astigmatism) should clearly result in better refractive outcomes than taking in account only anterior corneal astigmatism. The most probable explanation of these results (less precision when including total corneal astigmatism measurements) was, as the authors pointed out, limitations in measuring of the posterior corneal astigmatism using the Scheimpflug camera. Currently, it is difficult to determine whether accuracy of the posterior astigmatism made by Scheimpflug devices available are good enough or not, since, as the authors also stated, a gold standard is not available for posterior cornea imaging. However, based on the results of ERA in models 2 through 5 of the study by Savini and Naeser, which are related directly to posterior corneal astigmatism measurements done with the Pentacam (Oculus, Wetzlar, Germany), and since the measures of dispersion (range and standard deviation) were larger than in model 1, we can assert that precision of the measurements was lower than when using model 1. Moreover differences in alignment of refractive measurements and corneal astigmatism measurements (the former referenced to the center of the pupil, and the latter to the corneal apex), also will affect the accuracy of the postoperative refractive results in these patients. To advise surgeons confidently to base their calculations on total corneal astigmatism rather than keratometric astigmatism when implanting toric IOLs, undoubtedly additional technological improvements are needed to provide better measurements. We are in the right direction, but results from this and other studies suggest that there still is a long way to go before we really reach a clinically reliable device to measure total corneal astigmatism in those patients, who usually have high expectations, and in whom surgeons also expect to have very low postoperative astigmatism (smaller than 0.50 D).

Aphakic iris-claw intraocular lens pseudophakic pseudoaccommodation

We read with interest the article on pseudophakic pseudoaccommodation by Lincke et al. that included 13 eyes with fixated retropupillary intraocular lenses (IOLs). The authors failed to cite a similar study published in 2012 by Sch€opfer et al. that comprised 51 eyes with a retropupillary fixated iris-claw IOL (Artisan or Verisyse). In the supine position, the mean anterior chamber depth was 4.01 mm G 0.24 (SD) in the group by Lincke et al. (measured using ultrasound biomicroscopy) versus 4.31G 0.44 mm in the group by Sch€opfer et al. (measured using A-scan). In the prone position, the mean values were 3.57 G 0.41 mm and 4.15 G 0.57 mm, respectively. Undoubtedly the statistically significant differences (PZ .0214 and PZ .0010, respectively) could be explained by the different device used and, in the prone position, by a difference in the angle with respect to the horizontal plane. (In the study by Sch€opfer et al., the patient was seated in forward tilted-head position.) However, it would be interesting to have the data obtained in retropupillary aphakic IOLs by Lincke et al. using A-scan in this subgroup of eyes. An additional interesting issue is the refractive impact of these IOL shifts. Lincke et al. found that in eyes with a retropupillary IOL, the mean difference in spherical equivalent (SE) in the supine position versus the prone position was 0.30G 0.53 diopter (D), which was not statistically significant (P Z .5823). In addition, as the authors explained, reading position does not equal the prone position used in the experiment because in down gaze, the eyes are declined by only approximately 30 degrees and the change in SE would be approximately one half the difference between the sitting position and the prone position (ie, approximately 0.15D in this subgroup of eyes with retropupillary implanted IOL). In contrast, in the study by Sch€opfer et al., there was a statistically significant difference ( 0.37 D; P Z .003). A myopic shift between 0.15 D and 0.37 D seems too small to have a real impact on the near-vision capabilities of the patients. Sch€opfer et al. measured the amplitude of accommodation with an accommodometer (Clement Clarke Ltd.) and found amplitudes of 4.96 D in the supine position, 5.70 D in the sitting position, and 5.18 D in the prone position. However, those results were not congruent with their findings on SE changes and were much higher than those published for other IOLs (including accommodating models), as we pointed out. Thus, we suggested that the data and the technique used to obtain them by Sch€opfer et al. should be verified. In summary, we believe the study by Lincke et al. adds to the body of evidence that aphakic iris-fixated IOLs (Artisan or Verisyse) shift with changes in position but that it seems

Small-incision lenticule extraction and corneal collagen crosslinking in keratoconus.

Small-incision lenticule extraction and corneal collagen crosslinking in keratoconus We read with interest the article by GraueHernandez et al. on combined small-incision lenticule extraction and intrastromal corneal collagen crosslinking (CXL) to treat mild keratoconus. The authors indicated in the first paragraph: “Keratoconus is a bilateral, asymmetrical, noninflammatory, and progressive ecstatic disorder of the cornea.” First, undoubtedly they meant ectatic and not ecstatic. Second, the topic of inflammation in keratoconus has been intensively researched in the past decade, and the formerly accepted noninflammatory nature of the condition seems to be challenged. A significant role of proteolytic enzymes, cytokines, and free radicals has been found and although keratoconus does not meet all the classic criteria for an inflammatory disease, the lack of inflammation has been questioned. The majority of studies of the tears of patients with keratoconus have found increased levels of interleukin-6, tumor necrosis factor-a, and matrix metalloproteinase 9. With regard to the results showed by GraueHernadez et al. in a group of 15 eyes with mild keratoconus treated with the Aztec protocol (as described by the authors), they are very impressive. The postoperative uncorrected distance visual acuity 12 to 24months after surgery was 0.12 logMAR G 0.20 (SD) (Snellen 20/26). The authors stated, “Therefore, using smallincision lenticule extraction to correct the refractive myopic and astigmatic error while minimizing biomechanical weakening, and combining this technique with intrastromal CXL to increase stability while overcoming the potential side effects of epithelial removal, appeared to be an ideal therapeutic approach to treat refractive error in mild keratoconic patients.”However, as they correctly concluded in the last section of the article, their results must be interpreted cautiously because the study sample is small and, we would add, the follow-up length is not long enough. Recently, 2 cases of bilateral ectasia after femtosecond laser small-incision lenticule extraction in cases of subclinical keratoconus have been reported. El-Naggar described a case of bilateral ectasia in a 33-year-old patient 6 months after the surgery, and Wang et al. reported the same complication in a 19-year-old patient. In the series by Graue-Hernandez et al., before surgery the eyes reached a corrected distance visual acuity (CDVA) of 20/40 or better and a mean CDVA of 20/20. They had on average a moderate cylinder (mean 2.3 G 1.4 diopters). Taking those data into account, the procedure performed could be considered primarily refractive surgery. (No information on

Iris-clip versus iris-claw intraocular lenses

We read with interest the case series on anterior megalophthalmos by Messina et al. They indicated that in 4 eyes, an iris-clip anterior chamber intraocular lens (IOL) was implanted. There is a difference between the terms iris-clip IOL and iris-claw IOL, which appears to have been overlooked by Messina et al. The term iris-clip refers to a model of IOLs that is no longer in use. On the other hand, iris-claw IOLs (Artisan, Ophtec BV, and Verisyse, Abbott Medical Optics, Inc.) have a totally different design and are available today. Designed by Binkhorst in 1957 and called the iris-clip IOL since the first publication on this topic in 1959 to the last reports of its use in the 1970s, the iris-clip IOL had 2 wire loops, bent at right angles and attached to its posterior surface close to the equator. This allowed it to be fixated at the pupil margin with one wire loop located anteriorly to the iris and the other one located posteriorly, in a fashion similar to how a paper clip works. On the other hand, the iris-claw IOLs have a different principle based on fixation to the anterior peripheral iris stroma. A loose fold of iris tissue is grasped by resilient claw-shaped haptic tips. Iris-claw IOLs were designed by Worst, who began implanting them in aphakic eyes in 1979. These iris-claw IOLs have been used in some countries in Europe since at least 1980 and today are available almost globally. However, some ophthalmologists in many countries have limited knowledge of the properties and indications of this IOL for aphakia and this IOL has not been used to its full potential. A surgeon in our group (V.G.) was the first to implant these types of IOLs in Colombia in 1998. Still, some surgeons in our country perform complex procedures (eg, suturing IOLs to sclera in eyes with a normal iris) even though fixation of an iris-claw IOL on the anterior surface or posterior surface of the iris is much safer and easier and has yielded excellent results. We noted that in Case 2, the authors performed a pars plana vitrectomy with lensectomy. Ten months later, an aphakia iris-claw IOL was implanted in the anterior chamber. Undoubtedly, to perform the 2 procedures in the same surgical setting would have been preferable. Six years later, loss of enclavation of the iris-claw IOL at the 9 o’clock position was observed. As the authors suggested, enclavation of a generous fold of iris tissue is recommended to prevent this complication. We had a positive experience, as cited by Messina et al., of fixating an iris-claw Artisan aphakic IOL to the posterior surface of the iris in an eye with anterior megalophthalmos; the results and long-term stability were very good. This approach has the advantage of placing the IOL in the posterior chamber, farther from the endothelium. On the other hand, it can be technically more challenging for the surgeon.

Impacts of Implantable Collamer Lens V4c Placement on Angle Measurements Made by Optical Coherence Tomography: Two-Year Follow-up

WE READ WITH INTEREST THE ARTICLE ON IMPLANTABLE collamer lens (ICL) V4c by Fernández-Vigo and associates. It would be interesting to know if eyes with a vault larger than 750 mm had a larger reduction of trabecular-iris angle (TIA).Also, as vault seemed to be a significant determinant of TIA reduction, it would be important to know the way the authors defined the size of the lens to be implanted. In addition, 14.8% of the eyes showed trabecular-iris contact (TIC). Did those eyes have narrower values of TIA? In a previous study performed by the same authors, they found that the mean of TIA was around 35 degrees for emmetropic eyes with a standard deviation of approximately 2.5 degrees. Assuming a normal distribution, around 99.85% of the eyes had an anterior chamber angle equal to or wider than 27.5 degrees. It is noteworthy that in the group of eyes implanted with ICL, assuming a normal distribution, only around 84% of the eyes had 2 years after surgery TIA nasal, TIA temporal, and TIA inferior equal to or wider than 18.5, 18.7, and 21.9 degrees, respectively. In addition, among the approximately 16% of eyes with narrower angles at least some had 0 degrees of TIA nasal, 5.3 degrees of TIA temporal, and 12.2 degrees of TIA inferior. It would be important to determine if those eyes that finished with such narrow angles also had narrower preoperative angles, in order to establish a minimal value of acceptable preoperative TIA. One eye showed high intraocular pressure (IOP) 1month postoperatively (45 mm Hg), but apparently related to steroid response. Similarly, Rodrı́guez-Uña and associates found in a larger group of 763 eyes with the same model of ICL 1 case with a significant increase in IOP (26 mm Hg) 1 week after surgery, also related to steroid response. Both studies, however, had only 2 years as the longest follow-up time. A current limitation when selecting an ICL for a given eye is the scarcity of size options. As several experts have indicated (J.F. Alfonso and C. Lisa, personal communication, October 4, 2017), it would be convenient to have at least 6, instead of 4. In addition, assuming a normal distribution, around 16% of the eyes in the studied group

Wavefront-Guided LASIK and Preoperative Higher Order Aberrations.

We read with interest the article by Kung and Manche in the April 2016 issue.1 In the first part of the article, they explained that most studies seemed to suggest that the wavefront-guided excimer laser procedures were slightly superior than wavefront-optimized approaches, particularly in patients with preoperative root mean square (RMS) of higher order aberrations (HOAs) less than 0.3 μm. However, the two cited publications by the authors (the prospective, open-label, multicenter study conducted by Stonecipher and Kezirian2 and the meta-analysis by Feng et al.3) reported exactly the opposite: if the magnitude of preoperative RMS was greater than 0.3 μm, wavefront-guided ablation had a significantly better postoperative aberration profile than wavefront-optimized ablation and, on the other hand, wavefront-guided treatments had no clear advantage over wavefront-optimized treatments in eyes with preoperative RMS lower than 0.3 μm.2,3 Kung and Manche found that the two platforms produced similar self-reported symptoms in patients with RMS aberrations greater than 0.3 μm but, in eyes with RMS aberrations less than 0.3 μm, the wavefrontguided platform resulted in higher self-reported “excellent vision” and significantly fewer adverse effects (eg, problems with daytime and nighttime clarity and visual fluctuation).1 However, the authors did not report an analysis on the postoperative aberration profile comparing the two subgroups: those with HOAs higher than 0.3 μm and those with less than that magnitude of aberrations. The findings on visual symptoms are counterintuitive because one would expect the impact of customized aberration correction to be higher in the group with higher preoperative aberrations, as shown in the other studies. We think that the numbers deserve to be rechecked to ensure that there are not any inaccuracies in the capture or analysis of the data. In addition, in Table 3 there seems to be confusion in the presentation of the information. It shows in the first column of each group, under a heading that says logMAR, data on manifest sphere, cylinder, and spherical equivalent, which units are in diopters. However, they are converted to “Snellen visual acuities.” Undoubtedly it is a mistake. The same occurs with the aberrometry data. The text referring to the Table 3 is not clear either. It says: “Postoperative measurements of visual acuity showed that manifest sphere (wavefront-guided vs wavefront-optimized: -0.32 vs -0.56; P = .0001, significant) and manifest spherical equivalent refraction (wavefront-guided vs wavefront-optimized: -0.18 vs -0.41; P = .0001, significant) were superior in the wavefront-optimized group (Table 3).” Manifest sphere and spherical equivalent are not visual acuity measurements, but refractive error determinations. If the postoperative refractive error was smaller, as is shown, in the wavefront-guided group of eyes, the statement made in the text is not correct. It is essential that the authors clarify these critical points so that we as readers can have a better understanding of the results of this interesting study.

Re: Ueno et al.: Corneal thickness profile and posterior corneal astigmatism in normal corneas (Ophthalmology 2015;122:1072-8)

TO THE EDITOR: We read with interest the article by Ueno et al on corneal thickness and astigmatism. The authors stated, “It was found that the cornea was thicker in the vertical than in the horizontal direction, which can explain why the posterior cornea surface tends to be more [against-the-rule] astigmatic than the anterior corneal surface, as demonstrated by recent studies.” Clarification would be helpful. A steeper posterior corneal meridian aligned vertically correlates with the finding that the cornea is thicker along the vertical meridian. Considering that the posterior corneal surface has a negative power, the steeper the curvature of a given meridian, the more negative the power of that posterior corneal meridian. Therefore, when the steepest posterior corneal meridian is aligned vertically, it subtracts more power along the vertical meridian of the whole optical system than the power subtracted by the horizontal posterior corneal meridian along the horizontal meridian of the optical system. Thus, using traditional refraction terminology, it is acting like against-the-rule anterior corneal astigmatism. As the authors explained, Koch et al used vector analysis to calculate the error produced by estimating the total corneal astigmatism from anterior corneal measurements only. This approach, based on manual keratometry or simulated keratometry from a topographer, had until recently been used universally for determining toric intraocular lenses cylindrical power. They compared that estimation with the actually measured total corneal astigmatism (including both anterior and posterior corneal surfaces) using a combined Placido’s disk and dual Scheimpflug device (Galieli, Ziemer, Port, Switzerland). Tonn et al used a similar method with another Scheimpflug device (Pentacam HR; Oculus, Wetzlar, Germany). For their part, Ueno et al used an approach not used by clinicians: estimating posterior corneal astigmatism from anterior corneal measurements only. When planning for cataract surgery, the surgeon is less concerned about the posterior corneal astigmatism, but rather the total corneal astigmatism. It has been more understandable, and maybe more useful too, to determine the total corneal astigmatism as measured by the device (using ray tracing or another suitable method) and compare it with the total corneal astigmatism based on anterior measurements and the keratometric index. The authors indicated: “That is, the actual posterior corneal curvature is steeper than the simulated posterior corneal curvature in the vertical direction.This explains why keratometric astigmatism can misinterpret actual total corneal astigmatism in many cases.” Although this tendency has been identified in other studies, recently Tonn et al found that only 59% of eyes with steepest anterior corneal meridian aligned horizontally had the steepest posterior corneal meridian aligned vertically. This finding suggests that it might be inaccurate to assume that the posterior steepest corneal meridian always will be aligned vertically, as the

Causes of Explantation of Phakic Intraocular Lenses

Causes of Explantation of Phakic Intraocular Lenses We read with interest the article by Alió et al.1 on phakic intraocular lens (PIOL) explantation in the January issue. There is an ongoing debate about the long-term safety of PIOLs, especially angle-supported lenses.2 In late 2014, Alcon Laboratories, Inc. discontinued production of the AcrySof CACHET Phakic Lens (Alcon Laboratories, Inc., Fort Worth, TX),3 the ophthalmic industry’s latest attempt of many with this approach (all of them unsuccessful). The numbers are impressive in this clinical series, by far the largest published, and only comparable to those reported in 2006 by Alió et al.4 Our first question is why the authors did not perform endothelial keratoplasty instead of penetrating keratoplasty, when the case required bilensectomy followed by corneal transplantation.5 When analyzing the explantations due to cataract, knowing the age of this subgroup of patients would have helped to make a better analysis of the influence of surgical trauma and presence of PIOL versus senile cataract. It is not really correct to state that the reasons for endothelial cell loss are related to inadequate anatomy of the anterior chamber, as the authors did. Regardless of their anatomy, if those eyes had not undergone the lens implantation, none of them would have presented endothelial damage. There were cases of corneal decompensation in all three groups of PIOLs (angle-supported, iris-fixated, and posterior chamber). Although information on incidence is lacking, because three to seven times more cases of corneal decompensation presented in the angle-supported group (15 eyes versus 5 eyes in the iris-fixated group and 2 eyes in the posterior chamber group), it appears that this anatomical site has an increased risk of severely altering the corneal endothelium. The same trend was evident in the eyes with endothelial cell loss without corneal decompensation (11 to 23 times more cases in the anglesupported group). It would have been helpful to have the data available on the density of the population of endothelial cells in these eyes, because the clinical implications of having approximately 1,500 cells/mm2 are not the same as having approximately 600 cells/mm2. Although it is probable, at least for earlier models of posterior chamber PIOLs, and is in concordance with other studies cited by the authors, because they did not establish incidence it is not possible to affirm that the incidence of cataract formation was significantly higher with those lenses, as they did. The authors emphasized that the aim of the study was not the evaluation of the explantation ratio and recognized that this was a limitation of their study. We believe it would have been useful to have those data to put the findings in context. Having such a large group of patients, we strongly believe that calculating incidence of explantation (at least for one of the participating centers) or estimating an approximate incidence measure if the exact number of lenses of each model implanted in each is definitely not known will provide valuable information to the scientific community.

Total Corneal Astigmatism Measurement.

We read with interest the article by Tonn et al. on corneal astigmatism. At the beginning of the article, the authors indicated that: ‘‘Scheimpflug tomographers provide a map of the anterior chamber and therefore a more accurate model of the cornea, its thickness, and posterior surface.’’ Since the anterior chamber is really a space, and not a structure, it is probably clearer to say that Scheimpflug tomographers provide a map of the anterior segment. In the ‘‘Patients and Methods’’ section, they explained that exclusion criteria included ‘‘an astigmatism . . . less than 39 diopters (D) or greater than 49 D.’’ Those values seem to refer to corneal curvature and not to corneal astigmatism. The authors denoted that they chose the 3-mm zone of the total corneal refractive power, as measured by the Pentacam, to closely match the standard 158 zone of a keratometric analysis. However, the concept of the standard 158 zone is not clear. Standard manual keratometers acquire data from the reflection of the mires in an annular zone with a diameter ranging approximately from 2.8 to 3.5 mm, depending on corneal radius of curvature. We could not find any reference in the literature to a standard 158 zone with regard to keratometry. In the ‘‘Results’’ section, the authors referred to the orientation of both corneal anterior and posterior astigmatisms. Astigmatism can be written in positive or negative cylinders; therefore, the direction (axis) of the astigmatism will have opposite meanings: in the case of keratometric astigmatism, since the cornea is a positive lens, the axis of positive cylinders will indicate the steepest meridian, while the axis of negative cylinders will indicate the flattest meridian. In several instances, the authors referred to the alignment of the astigmatism assuming that the reader understands that a positive cylinder is used. However, in many regions (like in Latin America), ophthalmologists usually write astigmatism using a negative cylinder notation. It would be clearer to say: ‘‘if the steepest meridian on the anterior surface was vertical’’ than to say: ‘‘if anterior astigmatism was vertical’’; and to say: ‘‘the axis of steepest anterior corneal meridian’’ rather than: ‘‘the axis of anterior astigmatism.’’ In the second paragraph of the ‘‘Discussion’’ section, the authors indicated: ‘‘In most cases, posterior curvature created against-the-rule (ATR) astigmatism, due to its negative corneal power and vertical alignment.’’ Later, in the same section, they partially explained this apparent paradox. We think that, as posterior corneal astigmatism is rather a new topic of discussion among clinicians (due to the recent increase in the use of toric intraocular lenses and the availability of evolving technologies for measuring posterior corneal curvature), it deserves a more robust clarification. The situation can be easily grasped if it is envisaged that since the posterior corneal surface has a negative power, the steeper the curvature of a given meridian, the more negative the power of that posterior corneal meridian. Therefore, when the steepest posterior corneal meridian is aligned vertically, it will subtract more power along the vertical meridian of the whole optical system than the power subtracted by the horizontal posterior corneal meridian along the horizontal meridian of the optical system. Thus, using traditional refraction terminology, it is acting like an against-the-rule anterior corneal astigmatism. The finding that only 59% of eyes with steepest anterior corneal meridian aligned horizontally had the steepest posterior corneal meridian aligned vertically is very important, because it indicates that, as the authors concluded, it might be inaccurate to apply a simplified nomogram as has been suggested. Further studies are needed to clarify this discrepancy. Unfortunately, a gold standard for measuring posterior corneal astigmatism remains elusive and despite advances in corneal imaging (Scheimpflug tomographers, three-dimensional anterior segment optical coherence tomography), we still lack validated ways to accurately measure posterior corneal astigmatism on a given eye. In a clinical study with 41 eyes that underwent toric intraocular lens (IOL) implantation, Koch et al.—using a combined Placido’s disk–dual Scheimpflug system (Galilei tomographer, Ziemer, Port, Switzerland)—found mean prediction errors of 0.57 D in the with-the-rule group and 0.12 D in the ATR group even using dual Scheimpflug technology. In the ‘‘Discussion’’ section, the authors stated: ‘‘With IOL selection based on CASim-K, 5.8% had an error of magnitude of more than 0.50 D and 20.1% had an alignment error of more than 108.’’ As the 3818 eyes included in the study did not actually undergo toric IOL implantation, it would be clearer to say: ‘‘If the eyes included in this study had had a toric IOL selection based on CASim-K, then 5.8% would have had an error of magnitude of more than 0.50 D and 20.1% an alignment error of more than 108.’’ Additionally, in those predictions, other factors should be taken in account (e.g., surgically induced astigmatism, predicted versus actually induced, and effective lens position). In the future, as Koch recently indicated, accurate measurement mean posterior corneal power, as measured by accurate technologies, will also improve precision in determining spherical equivalent power of any intraocular lens, especially in eyes that have undergone corneal refractive surgery. As known, currently total corneal power used in biometric formulae is calculated using only the anterior radius of curvature and a fudge factor (the keratometric index, usually 1.3375), with the assumption that there is a fixed ratio between the anterior and posterior curvatures. Those emerging devices will enable ophthalmologists to accurately measure both posterior corneal astigmatism and posterior corneal power (spherical equivalent) to include those data in new biometric formulas that will make no assumptions about posterior corneal power (as currently used formulas do). Undoubtedly, we still have a long way to go to get truly reliable posterior corneal measurements and to determine the best way to analyze those data (there are several different complex vectorial methods available, which are not either straightforward or universally accepted); but at least we have taken the first crucial steps in the right direction, and clinical studies such as the one performed by Tonn et al. will help us reach the goal of better results when implanting toric intraocular lenses. However, as Alpins et al. recently stated, posterior corneal astigmatism does not seem to be the only significant factor in the equation of final manifest refractive cylinder and noncorneal contributors likely include other sources of intraocular astigmatism (in pseudophakic eyes, IOL tilt or decentration) and processing in the visual cortex. Moreover, differences in alignment of refractive measurements and corneal astigmatism measurements (the former referenced to the center of the pupil, and the latter to the corneal apex), will also affect the accuracy of the results.