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First Author: I.Bach Pedersen DENMARK
Co Author(s): A. Ivarsen A. Vestergaard J. Hjortdal
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Small Incision Lenticule Extraction (SMILE) is an intrastromal flap-free refractive technique, which may preserve corneal integrity better than flap-based techniques. However, SMILE may still induce corneal weakening due to the removal of corneal tissue. With the Corvis ST (Oculus Inc., Germany), it is possible to obtain high-speed Scheimpflug pictures of corneal deformation during a metered collimated air-pulse. By looking at defined lengths, velocities and time lapses during deformation and rebounce, it may be possible assess corneal biomechanical properties in vivo. The purpose of this study was to compare the corneal deformation patterns before and after SMILE using the parameters available with the Corvis ST.
Department of Ophthalmology, Aarhus University Hospital, Denmark
In a prospective study, 22 high myopic patients were measured with the Corvis ST before and three months after they had undergone SMILE, with attempted spherical equivalent corrections of -6 to -10 diopters. The inward deformation was described by the applanation length (A1 length), the inward velocity (A1 velocity), the time until the first applanation (A1 time), and the length of the deflected cornea at first applanation (A1 deflection length). The corneal flattening during rebound was described by applanation length (A2 length), outward velocity (A2 velocity), and time until second applanation (A2 time). At maximum deformation, the deformation amplitude, and deformation radius were measured.
At first applanation, we found a significantly shorter A1 Time (p<0.005) and smaller A1 velocity (p<0.005) after operation. No difference in A1 length was found (p=0.571), but the A1 deflection length was significantly shorter after operation (p<0.005). Parameters describing the second applanation were all statistically different, with a shorter A2 length (p<0.005), a longer A2 time (p<0.005) and larger A2 velocity (p<0.005) after operation. During highest concavity, a larger deformation amplitude (p<0.005) and a smaller deformation radius was observed after operation (p<0.005)
The slower deformation and faster rebounce of the cornea during an air-puff, as shown by the applanation velocities, suggest that corneal compliance is changed after an operation. Based on the measurements at maximum deformation, the cornea is more deformed after an operation, resulting in a larger deformation amplitude and smaller deformation radius. This may indicate a more compliant and elastic cornea after operation. The significantly smaller A1 deflection length after the operation may be a result of surgery related changes in corneal integrity. Due to the cutting of collagen fibers, the pressure from the air-pulse may be less distributed over corneal tissue at the initial phase of the deformation. This may result in deformation of a smaller area of the cornea, giving a shorter deflection length after the operation as seen in this study. Removal of corneal tissue with SMILE results in a more compliant cornea, when measured with the Corvis ST. But it would be preferable to compare with prospective studies of patients who underwent flap-based techniques, to see if it would result in a higher corneal elasticity when compared to SMILE.