Understanding corneal reactions after small incision lenticule extraction

Nikolaus Luft MD Analyses of stromal thickness measurements obtained in eyes undergoing small incision lenticule extraction (SMILE) provide evidence of stromal remodeling during the first postoperative year. As a result of this reaction, the achieved central lenticule thickness appears to be consistently less than the target, reported Nikolaus Luft MD at the XXXIV Congress of the ESCRS in Copenhagen, Denmark. “Our findings are consistent with a previous study by Reinstein et al reporting that the actual amount of tissue removed at the lenticule centre was less than attempted, and with the hypothesis of those investigators that recoil from tension release in the collagen lamellae disrupted during lenticule creation leads to central stromal expansion and explains the difference. Our findings also show, however, that the anatomic undercorrection does not seem to affect SMILE’s refractive precision,” said Dr Luft, Cornea and Refractive Surgery Fellow, Ludwig-Maximilians-University, Munich, Germany. The study included 42 eyes of 21 patients with a mean surgical refractive correction of -4.94 dioptres and planned central lenticule thickness of 95 microns. Stromal thickness at 17 locations within the central 5mm of the cornea was calculated from serially-obtained spectral domain optical coherence tomography images using a custom algorithm published by Dr Luft and colleagues. Compared with the first postoperative day, stromal thickness at six weeks was decreased significantly at the apex (mean -10.4 microns) and at the 1.25mm and 2.5mm zones around the apex. Between six weeks and one year, stromal thickness increased significantly at the centre (+8.8 microns), but did not change significantly in the peripheral zones. “We believe the initial decrease reflects resolution of stromal oedema that occurs from intraoperative flushing of the stromal pocket, while the subsequent stromal expansion could be an indicator of the postoperative stromal remodeling hypothesised by Reinstein et al,” said Dr Luft. Also consistent with the stromal remodeling hypothesis was the finding of a significant direct correlation between the planned and achieved central stromal thickness (i.e. the difference increased with increasing target lenticule thickness). “Because more lamellae are dissected as lenticule thickness increases, it is logical that the remodeling reaction would also increase,” Dr Luft said. Analyses of measurements obtained at six weeks, three months, six months and one year showed that the lenticules were consistently too thin. At one year, the difference between the target and achieved lenticule thickness was greater than at six weeks, averaged 18.7 microns, and ranged from six to 33 microns. Analyses of refractive outcomes indicated that the “anatomic undercorrection” did not affect refractive precision nor stability of the procedure. Although there was a continuous myopic shift in MRSE between six weeks and one year, the change was not statistically significant. “Furthermore, the two eyes with the greatest difference between planned and achieved central stromal thickness reduction (lenticule thickness) achieved a nearly plano outcome,” Dr Luft said. Nikolaus Luft: nikolaus.luft@med.uni-muenchen.de