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Comparaison des Changements Biomécaniques Précoces de la Cornée après un Laser In Situ Keratomileusis de la Myopie et l’Hypermétropie Utilisant la Corneal Visualization Scheimpflug Technology

Ocular Response Analyzer has been traditionally used to study corneal biomechanics. Few non-consistent studies compared myopic to hyperopic ablation post Laser In Situ Keratomileusis (LASIK). While Shah S et al. found no significant changes in corneal hysteresis (C H) and corneal resistance factor (CRF) between myopic and hyperopic LASIK (1), Fabricio et al showed a greater decrease in C H and C RF with myopic photoablation profiles (2). Our aim is to investigate the effect of LASIK on corneal biomechanics and compare the changes between myopic and hyperopic ablation using Corneal Visualization Scheimpflug Technology (CorVis ST), a newly developed technique with real-time display of the dynamics of the deformation process.

Retrospective study of patients who underwent LASIK by one surgeon at a referral center between March and April 2018. Patients who underwent same day evaluation by Pentacam tomography and CorVis ST preoperatively and at one-week post-operative visit were included. Cases were grouped into myopic and hyperopic ablation based on spherical equivalence. Change value (Δ) for each CorVis parameter was compared between the two groups using independent t test or Mann Whitney test, based on normality studies from Shapiro-Wilk test.

Fifty-five eyes of 28 consecutive patients were included. Mean age was 36.5 ± 13.2 years. Forty-three eyes underwent myopic ablation and 12 eyes underwent hyperopic ablation. Thirty-two CorVis specific parameters were studied. At one-week post-op, multiple parameters were shown to significantly change post LASIK as compared to preoperative values. Also, Δradius (p=0.0), ΔA1 deformation amplitude (p=0.012), ΔA1 deflection area (p=0.026), ΔDA ratio max 1 mm (p=0.018) and ΔC BI (p=0.006) were significantly greater after myopic ablation when compared to hyperopic group. No significant difference in ablation depth was found between the two groups (mean 48.5 μm for myopic group vs 40.8 μm for hyperopic group, p= 0.32).

Both myopic and hyperopic LASIK ablations were shown to decrease corneal strength by increasing corneal elasticity as reflected by the significant changes of multiple parameters postoperatively. Changes were significantly more pronounced after myopic ablation which can be explained by the differential weakening profile of the central cornea as compared to the paracentral annulus in the hyperopic ablation profile, therefore possibly illustrating why ectasia post LASIK is mostly observed after myopic ablation.

Using CorVis ST, corneas post myopic LASIK showed much greater increase in elasticity as compared to corneas post hyperopic ablation, hence confirming the implication of the ablation profile as a separate parameter affecting corneal biomechanics after refractive surgery.