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    Corneal biomechanics and small-incision lenticule extraction: What is the evidence?

    A conservative approach, as with LASIK and PRK, should be adopted while evidence mounts

    The introduction of small-incision lenticule extraction (SMILE) in the clinical practice of refractive surgery has led to an increase of debates regarding the preservation of corneal biomechanics following laser refractive surgery. One of controversial topics discussed has been the potential major advantage of SMILE over LASIK in terms of corneal biomechanics.

    Considering that Bowman’s layer is stiffer than anterior stroma, and in turn, the anterior stroma stiffer than the posterior,1 as well as these stiffer layers being much preserved in SMILE, it can be hypothesised that SMILE provides a better preservation of corneal biomechanics than LASIK and photorefractive keratectomy (PRK).2 In fact, mathematical biomechanical models have suggested that deeper corrections in the stroma may be possible in SMILE without additional risk for ectasia, because the residual stromal bed supports a greater stress in LASIK than in SMILE.3

    However, simple inferences have to be made with caution, since the characterisation of the biomechanical behaviour of the cornea is a complex process, and cannot be predicted with simple mathematical relationships. Furthermore, the link between corneal stress and ectasia has yet to be defined.

    Indeed, a conservative approach similar to that used for LASIK and PRK should be adopted in clinical practice considering that some ectasias after SMILE have been reported in subjects with normal4 and suspicious topographies.5-7 Despite the suggestions of simulations and some clinical studies, there is an absence of clear clinical evidence favouring the SMILE advantage over other laser refractive surgery techniques in terms of preservation of corneal mechanical properties.

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