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Impact of ocular residual astigmatism on predictability of myopic astigmatism correction after small-incision lenticule extraction

Chan, Tommy C.Y. FRCS; Wan, Kelvin H. MRCS; Zhang, Lin MD; Wang, Yan MD

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Journal of Cataract & Refractive Surgery: April 2019 - Volume 45 - Issue 4 - p 525-526
doi: 10.1016/j.jcrs.2019.01.028
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Treatment planning strategies for corneal refractive surgeries are based on corneal topography or the manifest refraction. Although the anterior cornea contributes significantly to manifest astigmatism, the latter is also influenced by the crystalline lens and the posterior corneal surface, the effects of which are collectively termed ocular residual astigmatism (ORA).1

Studies2–4 report that laser in situ keratomileusis (LASIK) is less effective in correcting myopic astigmatism in eyes with high ORA. Laser in situ keratomileusis iatrogenically creates a toric cornea to compensate for the ORA in these eyes, which might reduce visual performance. Small-incision lenticule extraction (SMILE) is a flapless femtosecond laser procedure. The impact of ORA on myopic astigmatism in SMILE has not been well evaluated, and it may differ from the effect of LASIK because SMILE lacks cyclotorsion control and the centration is surgeon-dependent. We evaluated the influence of preoperative ORA on astigmatism predictability of SMILE 3 months postoperatively.

All SMILE procedures were performed by the same surgeon (Y.W.) using the VisuMax platform (Carl Zeiss Meditec AG) following a standard procedure.5 Ocular residual astigmatism was defined as the vector difference between the preoperative refractive astigmatism (R) (corneal plane) and topographic astigmatism. The former was obtained from the manifest refraction, and the latter was measured from the anterior keratometric astigmatism difference between steep keratometry and flat keratometry on corneal topography (Pentacam HR, Oculus Optikgeräte GmbH). Alpins vector analysis was used to evaluate the outcome of astigmatism correction.6 The following indexes were calculated: target-induced astigmatism, surgically induced astigmatism, difference vector, magnitude of error, absolute angle of error, correction index, and index of success. Outcomes were compared between the low-ORA group (ORA/R < 1) and high-ORA group (ORA/R ≥ 1) using the independent t test. The R value was matched between the 2 groups to mitigate bias toward eyes with high refractive astigmatism in the low-ORA group and eyes with low refractive astigmatism in the high-ORA group.2 A P value less than 0.05 was considered statistically significant.

The study comprised 134 of 134 patients (49 high-ORA group; 85 low-ORA group) with a mean preoperative spectacle plane sphere and cylinder of −5.63 diopters (D) ± 1.45 (SD) and −0.77 ± 0.28 D, respectively (Table 1). There were no significant differences in age or preoperative sphere, cylinder, and target-induced astigmatism between the 2 groups.

Table 1
Table 1:
Comparison between the high-ORA group and low-ORA group.

The difference vector, which signifies the additional astigmatism change required to achieve the target-induced astigmatism, was smaller and the absolute angle of error, which signifies better alignment of astigmatic correction, was lower in the low-ORA group (P = .026 and P = .008, respectively). The index of success in the low-ORA group was lower than in the high-ORA group (P = .020). There was a trend toward undercorrection in both groups, and the difference was not significant (P = .491) (Table 2). The anterior and posterior corneal astigmatism in the high-ORA group was higher than in the low-ORA group (P < .001). In the low-ORA group, a significant correlation was found between ORA and posterior corneal astigmatism preoperatively (r = −0.42, P < .001) but not postoperatively (P = .172). In the high-ORA group, these correlations were significant postoperatively (r = −0.49, P < .001) but not preoperatively (P = .063).

Table 2
Table 2:
Vector analysis results in the high-ORA group and low-ORA group.

All vector analysis indices suggested that SMILE is more efficacious in correcting eyes with low ORA. Similar to our findings, Piñero et al.7 reported a significant correlation between ORA and posterior corneal astigmatism after LASIK but not before LASIK. The authors hypothesized that the change in the contribution of posterior corneal astigmatism could account for the residual cylinders and poor predictability. High posterior corneal astigmatism is not unexpected in eyes with high anterior corneal astigmatism because of the positive correlation between the 2 types of astigmatism.8 Thus, in these eyes, there would still be high anterior corneal astigmatism postoperatively but at a different axis than preoperatively, resulting in different alignment between anterior and posterior corneal astigmatism. Changes in the partial compensation between anterior corneal astigmatism and posterior corneal astigmatism could be a cause of reduced predictability in eyes with high ORA and high corneal astigmatism.2

If SMILE is performed without accounting for the source of preoperative astigmatism, compensatory astigmatism created on the cornea to neutralize internal ORA can lead to unsatisfactory visual outcomes by creating excessive corneal aberrations. Qian et al.9 reported a higher index of success (0.73) 3 months after SMILE in eyes with high preoperative ORA than in eyes with low ORA (0.39); however, their study separated the groups by the magnitude of the ORA (>1.0 D or ≤1.0 D). By separating the groups using an ORA/R ratio in the present study, we were able to identify patients with predominantly anterior corneal astigmatism and those with predominantly non-anterior corneal astigmatism. Furthermore, we matched the R values between the 2 groups to minimize bias in the low ORA group, which included more eyes with a higher target induced astigmatism and thus a lower index of success.2 The index of success difference (0.16) between the 2 groups in our study is comparable to that reported after myopic astigmatism correction by LASIK (0.05 to 0.67 D) (when matched for R),2–4 suggesting that the influence of ORA on astigmatism outcome in SMILE is similar to that in LASIK.

Vector planning integrating topography parameters into the surgical planning improves astigmatic outcomes after LASIK.10 Vector planning results in better improvement in accuracy, corneal toricity, and postoperative ORA than manifest refraction planning.10 In a previous study,11 incorporating vector planning into photoastigmatic refractive keratectomy planning for eyes with forme fruste and mild keratoconus also resulted in improvements in refractive and corneal astigmatism over a follow-up of up to 10 years. Thus, the vector planning method could improve corneal regularization and reduce corneal toricity in eyes with ORA without inducing manifest refractive astigmatism.10

Preoperative ORA calculation is crucial in counseling patient expectations regarding realistic outcomes. In eyes with significant ORA, vector planning would be a preferable alternative to a lens-based procedure.

References

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