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    Novel metric may determine ectasia risk

    Sao Paulo—Using a new metric seems to be an easy way to determine a patient’s risk of developing ectasia after a refractive procedure, according to Marcony R. Santhiago, MD, PhD.

    The high percentage of tissue altered (PTA) was identified as the major risk factor for development of ectasia after LASIK in eyes with normal corneal topography preoperatively.

    “With retrospective analysis, most patients who develop ectasia after LASIK have lens fiber risk factors that place them at a higher risk for this complication, especially an irregular topographic pattern,” said Dr. Santhiago, associate professor of ophthalmology, University of Sao Paulo, Brazil.” However, postoperative ectasia that develops in patients who underwent LASIK and had normal topography remains a mystery.

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    “The pathology most likely represents a reduction in the biomechanical integrity below a safe threshold that is required to maintain the corneal shape and curvature,” he continued.

    This can theoretically occur, Dr. Santhiago explained, when a relatively normal cornea already destined for ectasia undergoes surgery, when a preoperatively weak but clinically stable cornea undergoes surgery, or when a completely normal cornea is weakened below a safe threshold.

    The corneal tensile strength, he said, is not uniform throughout the central cornea, with progressive weakening of the posterior two-thirds, and the relative extent of the biomechanical alterations plays a role in the postoperative weakening after LASIK.

    The thickness of the LASIK flap contributes to the alterations because the lamellar flap does not contribute to the corneal tensile strength.

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    “Based on the structural differences, it seems reasonable that a ratio or percentage should be more representative of the change that occurs after refractive surgery than specific cut-off values related to the residual stromal bed or corneal thickness,” Dr. Santhiago said.

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