Evaluating the cornea in SMILE

Corneal stiffness remains intact after SMILE lenticule creation

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Significant changes occur in biomechanical properties following femtosecond laser flap creation during LASIK procedures, but not after lenticule creation during small lenticule-extraction procedures (SMILE®) procedures, said Ahmed Ghoneim MD at the 23rd ESCRS Winter Meeting in Athens Greece. In a prospective study, Dr Ghoneim and his associates at Tanta University, Tanta, Egypt, used the Corvis ST Scheimpflug tonometer (Oculus) to evaluate the changes in the cornea’s biomechanical properties before and immediately after corneal lenticule creation or LASIK flap creation. The study involved 50 eyes of 25 patients aged 20-to-30 years with -4.0D to -8D of myopia who underwent SMILE and 50 eyes of 25 patients aged 20-to-32 years of age with -4.0D to -9.0D of myopia who underwent femto-LASIK. They found that although the Corvis corneal biomechanical index (CBI) was not changed significantly following lenticule creation, changing only from 0.0 to 0.07, it was changed significantly following LASIK flap creation, from 0.0 to 0.94. BIOMECHANICAL INDEX He noted that the Corvis-ST uses an ultra-high-speed Corvis ST Scheimpflug tonometer camera to measure a range of the corneal biomechanical parameters during a constant collimated air pulse. The main parameters are deformation amplitude, applanation time, applanation length, corneal velocity, peak distance, radius and intraocular pressure values. These parameters are calculated together to determine the cornea’s biomechanical index, which research has shown to be highly sensitivities in differentiating between healthy and keratoconic or ectatic eyes. The study also showed that the deformation amplitude – which refers to the maximum amplitude when the cornea is flattened to its greatest deformation by the air-jet puffed by the Corvis ST – was also not significantly changed after SMILE lenticule creation. There were, however, significant changes in the deformation index after LASIK flap creation with the femtosecond laser. He added that the deformation amplitude also tends to increase in thinner corneas. STRIPPING TECHNIQUE Dr Ghoneim noted that the intralamellar small gas bubbles due to the vaporisation of tissue are visible in the Scheimpflug camera images, but these do not appear to influence corneal biomechanical parameters. He added that he has developed his own stripping technique for the easy and rapid removal of the lenticule during SMILE. It facilitates a crucial step of the surgery through visualisation of the edge of the lenticule, thereby increasing the ease of its removal. In this way it can give the beginner surgeon confidence in the SMILE technique and possibly reduce the duration of the learning curve.
Tags: LASIK, Small-incision lenticule extraction, SMILE