Preoperative Optical Zone Diameter Can Foretell SMILE Outcomes

Postoperative refractive outcomes of SMILE surgery are affected by preoperative optical zone diameter.

Small incision lenticule extraction (SMILE) is a safe, effective, and predictable refractive surgery for correction of myopia and myopic astigmatism, but postoperative refractive outcomes of SMILE are affected by the diameter of the preoperative optical zone, according to research published in BMC Ophthalmology.

Researchers conducted a comparative, retrospective clinical study to evaluate the influence of the preoperative optical zone diameter on myopic correction in eyes of patients who underwent SMILE.

The research team compared eyes with small optical zone diameters, ranging from 6.0 to 6.4 mm, and those with large optical zone diameters, ranging from 6.5 to 6.8 mm. They included patient data for uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), spherical, and cylinder preoperatively and 3 months postoperatively. They evaluated the influence of optical zone diameter on postoperative refractive outcomes using univariate regression analysis.

A total of 581 eyes from 316 patients underwent SMILE (52.2% men and 47.8% women). The mean patient age was 23.36±5.48 years (range, 18-49). There were 117 eyes in the small optical zone group (20%) and 464 eyes in the large optical zone group (80%). The team used propensity score matching with age, gender, eye (right/left), keratometry, and preoperative spherical equivalent to select pairs of eyes from the groups to compare refractive outcomes.

The influence of optical zones on SMILE refractive outcomes is significant in eyes with high degree of myopia, but not significant in eyes with low myopia.

Based on propensity score matching, the researchers selected 78 pairs of eyes. They observed no significant differences in the age, gender, eye (right/left), keratometry, or preoperative spherical equivalent between the small and large optical zone groups. 

The study shows a significant difference in the postoperative spherical equivalent between the groups, demonstrating that patients with larger optical zones experience less undercorrection (-0.10±0.29 vs 0.02±0.34 diopters (D); P =.018) than those with smaller preoperative optical zone measurements. 

With univariate regression analysis, the study shows  each millimeter larger optical zone results in an 8.13% or 0.39 D reduction of undercorrection (P <.001). 

“Our study demonstrated that the postoperative refractive outcomes of SMILE would be affected by preoperative optical zone, and the eyes with a larger optical zone have a tendency to less undercorrection. The influence of optical zones on SMILE refractive outcomes is significant in eyes with high degree of myopia, but not significant in eyes with low myopia,” according to the researchers.

Limitations of the study included the retrospective design, relatively small sample size, use of univariate regression analysis, rather than multivariate regression analysis, to evaluate the relationship between the optical zone and postoperative spherical equivalent, inability to evaluate the effect of optical zone on visual quality due to a lack of data, and inability to determine whether the residual stromal bed thickness confounds the effect of the optical zone on SMILE refractive outcomes.

References:

Liu P, Yu D, Zhang B, et al. Influence of optical zone on myopic correction in small incision lenticule extraction: a short-term study. BMC Ophthalmol. 2022;22(1):409. doi:10.1186/s12886-022-02631-4