Preoperative Ocular Residual Astigmatism (ORA) and its impact on surgical outcomes.
Updated: Jul 1, 2021
By Avi Wallerstein, MD, FRCSC 1,2*, Mathieu Gauvin, B. Eng., PhD1,2*
1 Department of Ophthalmology and Visual Sciences, McGill University, Montreal, QC, Canada
2 LASIK MD, Montreal, QC, Canada
*These authors equally contributed to this Commentary.
Dr. Lin published, "The contribution of ocular residual astigmatism to anterior corneal astigmatism in refractive astigmatism eyes", where the distribution of the magnitude and orientation of preoperative ocular residual astigmatism (ORA) is reported, and the importance and impact of ORA reviewed. In this commentary, we engage and stimulate the discussion about ORA and bring critical clarifications to Lin's study findings, with the goal of advancing the current scientific knowledge on ORA.
We read the article by Lin, "The contribution of ocular residual astigmatism to anterior corneal astigmatism in refractive astigmatism eyes" , where the distribution of the magnitude and orientation of preoperative ocular residual astigmatism (ORA) is reported, and the importance and impact of ORA is reviewed.
The author discusses the magnitude of ORA in the general adult population and cites two small sample size studies which are not optimal to be conclusive. The first, by Piñero et al., included 101 patients, with some younger than 18 years old, which is not representative of "normal adult eyes". The second by Plech et al. only included 31 "control" normal eyes. Lin's paper omits to reference large-scale studies that examined the distribution and role of ORA in more detail, such as our recent ORA study of 21,580 eyes  and the article by Frings et al. with 2,991 eyes . Those two large studies would have allowed more substantial comparisons with Lin's study.
The authors' claim, "the greater the RA are, the smaller the ORA are" . The findings from our big data study challenge this statement . We found a weak, but positive correlation (R = 0.07) between refractive astigmatism (RA) and ORA . Similarly, Mohammadpour et al. reported, a weak positive correlation between RA and ORA (R = 0.23) . Therefore, in other ORA studies, the greater the RA, the greater the ORA. These opposing findings could have been discussed, highlighting that the discrepancy might reflect differences in the populations being studied (myopes, hyperopes, gender, age).
Lin also reported a stronger correlation between RA and ORA (R = -0.27) than between ACA and ORA (R = 0.17). This finding also differs from our myopic population data. In our study, ORA was 6.3-fold more highly correlated with preoperative anterior corneal astigmatism (ACA) than RA (R = 0.44 vs. R = 0.07) . This higher correlation with ACA is consistent with physiological optics and corneal anatomy. Numerous studies have shown that, on average, the greater the ACA, the greater the posterior corneal astigmatism (PCA), although this relationship is uncertain in against-the-rule eyes . When ORA is calculated using the vectorial difference between RA and ACA, that difference mostly stems from PCA, internal ocular factors, and cortical perception. Therefore, on average, particularly in with-the-rule astigmatism eyes, the greater the RA, the greater the ACA, the greater the PCA, and the greater the ORA. This is why myopic-astigmatism eyes with high ORA have a significantly higher prevalence of moderate to high ACA, and to a lesser extent RA . The author should clarify what could explain the opposite trends that he found between RA and ORA, and between ACA and ORA in his study sample. One possibility is the difference in characteristics of the populations being studied, such as age, sphere, cylinder .
Moreover, Lin’s paper discusses refractive surgery outcomes related to the ratio of ORA divided by RA and not ORA per se. A critical distinction must be made between the ORA variable per se, versus the ORA divided by RA variable (ORA/RA). When dividing ORA by RA, we no longer study the effect of ORA, but that of the ratio between ORA and RA . This ORA/RA ratio, when low and high, cannot simply be renamed as "low ORA" or "high ORA". Such ratios are misleading and may lead to erroneous conclusions about actual ORA, as demonstrated in our study . For example, the study by Frings et al. concluded that higher ORA had a significant worsening effect on LASIK outcomes, when they did not use ORA per se but compared ORA/RA < 1 to ORA/RA > 1 , which concomitantly impacted their outcomes measure, the index of success (IOS).
We also showed that ORA/RA > 1 group had an inferior IOS . This is partly because the IOS is calculated by dividing postoperative RA (termed Difference Vector) by the intended astigmatism treatment target (termed the Target-Induced Astigmatism Vector). Of course, the higher the preoperative RA, the lower the preoperative ORA/RA ratio, and the lower the postoperative IOS. Eyes with ORA/RA > 1 may show an inferior IOS, but they also have less postoperative RA than eyes with ORA/RA < 1 . Therefore, studies that used the ORA/RA ratio cannot be used to conclude that eyes with high ORA result in poorer outcomes .
In another study using the ORA/RA < 1 versus ORA/RA > 1 grouping, the preoperative RA was found to be statistically identical between both groups, but this indicates that eyes with ORA/RA > 1 had on average greater preoperative ACA than eyes with ORA/RA < 1, which could explain the lesser outcomes of ORA/RA > 1 eyes in that study .
The most important clinically relevant outcome measure is the average residual postoperative cylinder, not the IOS. The IOS gives us an indication of laser performance relative to the preoperative cylinder but is not indicative of a practical clinical outcome. ORA/RA > 1 groups with a higher IOS also have less postoperative RA compared to ORA/RA < 1. Therefore, despite an inferior IOS, these ORA/RA > 1 eyes do better and achieve lower postoperative cylinder than eyes with ORA/RA < 1 .
In summary, while Lin’s study advances the knowledge on ORA, it has a limited list of key references on ORA and reports data that differ from findings reported in myopic-astigmatism eyes, without discussing those differences and formulating potential explanations to explain those disparities. The readers need to be informed of the current large-scale studies on ORA, which show positive correlation with RA, and no clinically-meaningful effect of higher ORA on refractive surgery outcomes in at least four previous large studies [2,7-9]. High ORA eyes only lead to inferior surgical outcomes when the surgeon targets the ACA instead of the RA .
Dr Wallerstein has indirect ownership in LASIK MD clinics and has no financial or commercial interests in the subject matter or materials presented in the current Commentary. Dr Gauvin has no conflict to disclose and no financial interest in the subject matter or materials presented in this Commentary.
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A.W. and M.G. wrote the main commentary text and did critical revision of the commentary.