Several structural parameters can foretell risk of posterior elevation following small incision lenticule extraction (SMILE) surgery, according to a 5-year investigation, published in ACTA Ophthalmologica. Currently, few studies exist into long-term posterior corneal stability following this procedure, and only a small number of cases have been reported. 

The retrospective analysis comprised surgeries performed at Tianjin Eye Hospital, Tianjin, China, between August 2013 and March 2015. Researchers evaluated 161 eyes of 161 patients (mean age 24±5 years, 77 men and 84 women) who underwent SMILE. All participants exhibited myopia that was stable for 2 years or more at baseline — with best corrected visual acuity of 20/25 or better, spherical equivalent (SE) refraction of less than -10 D, astigmatism lower than -5 D, and residual bed thickness (RBT) thicker than 250 µm. Five-year outcomes included improved or stable corrected distance visual acuity in all eyes and astigmatism within 1 D — notably, 89% achieved uncorrected 20/20, with mean SE -0.16±0.37.

Posterior corneal elevation (PCE) changes proved to be based on time, region, and angle. Results showed slight forward posterior protrusion after SMILE, cresting near the 1-year mark, but returning to preoperative level by 5 years. Centrally, within a 3 mm diameter circle, corneal tissue tended to shift backwards, opposite peripheral forward shift (all time points P <.001). Angle-dependent changes involved highest PCE occurring along the 45° to 90° semi-meridian above best-fit sphere (BFS), and lowest PCE along the 270° contour below BFS. 


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“Specifically, as the corneal lamella is removed during the procedure, it results in relaxation of the peripheral lamellae; thus, the corneal tension is redistributed, leading to central corneal flattening and peripheral steepening,” according to the investigation. Although, prior studies have found that in myopia cases worse than -10 D central regions can shift anteriorly. Deeper ablation and thicker lenticules may create an expanded space between cap and residual bed. Generally, eyes with a thinner cornea and deeper laser ablation or thinner RBT were linked to posterior instability, in part because the front 40% of corneal tissue has more tensile strength and RBT needs to be sufficient in deeper ablations.

Risk for posterior protrusion was greatest for individuals with central corneal thickness (CCT) less than 481.0 µm to 498.5 µm, myopia worse than -8.00 D to -8.31 D, as well as maximum lenticule thickness more than 139.5 µm to 144.5 µm or RBT less than 255.5 µm to 263.5 µm. Data show the best predictive threshold to avoid forward posterior shift is to keep percentage ablation depth (PAD) between 26.9% and 28.3%, and percentage stromal bed thickness from 48.9% to 52.6%.

A retrospective design is a study limitation, as well as exclusion of individuals with ectasia. Further, calculation of risk for ectasia was assessed indirectly by using the variable of posterior corneal surface stability, yet not all real-world elevation progresses to ectasia. Investigators believe prediction of posterior stability may be helpful in evaluating candidates for SMILE.

Reference

Cao H, Zhang L, Liang S, Chen X, Jhanji V, Wang Y. Predictive factors of posterior corneal shift after small incision lenticule extraction: a 5-year follow-up study. Acta Ophthalmol. Published online on April 30, 2022. doi:10.1111/aos.15166