Correction of astigmatism with small-incision lenticule extraction: Impact of against-the-rule and with-the-rule astigmatism

Purpose: To determine the influence of against-the-rule (ATR) and with-the-rule (WTR) astigmatism on the outcomes of small-incision lenticule extraction for myopic astigmatism.

Setting: University eye clinic, Aarhus, Denmark.

Design: Retrospective case series.

Methods: One eye of each patient had small-incision lenticule extraction for astigmatism of 1.00 diopter (D) or more. Surgery was performed with a Visumax femtosecond laser. Subjective refraction and Scheimpflug tomography (Pentacam HR) were performed preoperatively and 3 months postoperatively. Cylinder data were evaluated by vector analysis and correlation analyses. Multiple linear regression was used to predict magnitude of error from age, sex, left or right eye, target-induced astigmatism (TIA), subjective ATR/WTR astigmatism, cap diameter, cap thickness, incision width, and back-surface astigmatism.

Results: The study enrolled 829 patients (505 women; median age 37 years). The mean preoperative subjective cylinder was 1.76 D ± 0.86 (SD). Overall, 66% and 95% of patients achieved a refractive cylinder within ±0.50 D or ±1.00 D, respectively. Astigmatism was WTR in 73% of eyes and ATR astigmatism in 16% of eyes. The linear regression model significantly predicted the magnitude of error (R² = 0.23, P < .001), with TIA contributing -0.15 D per attempted diopter and subjective ATR astigmatism contributing a constant 0.32 D (both P < .001). No other parameters had a significant impact on the outcome.

Conclusions: Almost 25% of the variation after small-incision lenticule extraction for myopic astigmatism might be explained by the size of the attempted cylinder correction and ATR/WTR astigmatism. Incorporating these parameters in preoperative planning might produce more consistent results in high cylinder corrections.