Predictability of matrix cutting in femtosecond laser micro-incision matrix lens extraction

Abstract

AIM: To compare the difference between the actual cutting amount and the preoperative predicted amount of corneal stroma after the small incision lenticule extraction(SMILE), and evaluate the predictability and accuracy of SMILE for corneal stroma. METHODS: Prospective study. A total of 113 myopic patients(220 eyes)who had taken SMILE in the Affiliated Hospital of Yunnan University were selected, and routine examinations were carried out before and 1,3mo after operation, including visual acuity, non-contact tonometer(NCT), spherical equivalents(SE), mean corneal curvature, spherical coefficient of anterior corneal surface and Pentacam anterior segment analysis. All the 102 eyes in the research objects were randomly selected to measure the central corneal thickness(CCT)with the A-supersonic cornea thickness gauge before and 3mo after operation. The actual cutting amount after operation is the difference between the thickness of the thinnest spot of the cornea before and after operation, and the error amount is the difference between the predicted cutting amount before operation and the actual cutting amount after operation. The cutting error amount was observed and its correlation with physiological parameters before operation was analyzed. RESULTS: SMILE had a good performance and the corneal morphology and visual acuity were relatively stable 1 and 3mo after operation. The consistency was good between the data measured by the A-supersonic cornea thickness gauge and the data of the thinnest spot of the cornea in the Pentacam anterior segment analysis, where the difference had no statistical significance(t= -1.877, P=0.063). The difference between the predicted cutting amount before operation(101.36±18.91)μm, and the actual cutting amount 1mo after operation(88.89±18.69)μm and 3mo after operation(84.95±18.64)μm(F=334.65, P<0.01)had statistical significance; There was statistical difference between the cutting amount 1 and 3mo after operation, and the predicted errors before operation [(12.59±9.78)μm and(16.50±9.21)μm]. The cutting amount errors were only correlated with the preoperative equivalent diopter(r=0.299, P<0.01)and(r=0.305, P<0.01). The equivalent diopter at 1 and 3mo after operation was correlated with the cutting amount error at the same time(r=-0.275, P<0.01)(r= -0.306, P<0.01). With the increase of the cutting amount error, the postoperative spherical equivalent shifted to negative.CONCLUSION: The actual cutting amount of corneal stroma after SMILE is smaller than the predicted preoperative cutting amount, and the predicted cutting amount error increases with the increase of preoperative diopter. As the cutting amount error increases, postoperative diopter gradually shifted to negative. The error, however, does not influence the target's visual acuity in the early postoperative period.

Keywords

• refractive surgery
• small incision lenticule extraction
• ablation error
• predictability