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L. Sawides, S. Marcos, J. Merayo; Lasik-Induced Posterior Corneal Changes Measured by Scheimpflug Imaging: Validation on Model Corneas and Measurements on Patients. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3531.
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© ARVO (1962-2015); The Authors (2016-present)
To measure possible changes on the posterior corneal surface with LASIK corneal refractive surgery. We performed validations using model corneas to assess the reliability of a Scheimpflug imaging system (Pentacam, Oculus) to measure the posterior corneal surface after LASIK, and performed longitudinal measurements on patients.
Scheimpflug cameras allow capture of cross-sectional images of the eye anterior chamber, although these are subject to geometrical and optical distortions from the preceding ocular components. We used a Pentacam system, which nominally corrects for those distortions. We checked that measurements of the posterior corneal surface were unaffected by anterior corneal refractive changes using spherical corneal models (4 contact lenses of blue diffusing material and known anterior and posterior radii of curvature). The lenses were ablated with a standard refractive surgery laser applying different corrections (-12,-9,-5, +5 D). Measurements were made pre- and post-ablation. Custom algorithms in Matlab were used to check the reliability of the data processing. Additionally, we studied 22 eyes of 11 patients (35±9 yrs) who underwent myopic LASIK (-1.25 to -7.25 D). Measurements were done pre-op and 1-day, 1-week and 1-month postoperatively. Reconstructions of the posterior corneal surface were obtained from images at 25 meridians. Analysis was done in terms of posterior radius of curvature (R) and asphericity (Q).
1) Measurements on the model cornea posterior surface are unaffected by ablation of the anterior surface (Rpre=6.34±0.02 and Rpost=6.33±0.04 mm, p= 0.30). 2) In patients, we found a statistically significant decrease in posterior R and Q 1-day postoperatively (Rpre=6.47±0.23 and Rpost=6.44±0.19 mm; Qpre=0.92 and Qpost=0.86, p<0.04). However, there was no significant change pre- and 1-week and 1-month postoperatively. 3) We found a correlation between the change in R and the amount of correction 1-day (r=0.71, p=0.014), 1-week (r=0.65, p=0.08) and 1-month (r=0.53, p=0.033) postoperatively.
Validations using artificial model corneas show that the optical distortions by the anterior cornea do not affect the Pentacam measurements of the posterior cornea. We found that LASIK induces changes on the posterior corneal surface, but on average these are only significant immediately following surgery. The most relevant changes occur for the highest corrections. These findings are relevant to understand the optical and biomechanical changes induced by refractive surgery.
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