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T. Soyland, B. M. Aakre, R. C. Baraas, O. O. Pedersen; Corneal Flap Thickness After Lasik: What is Important - Keratometric Power or Corneal Thickness?. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4209.
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There seem to be a discrepancy in the literature with regards to whether preoperative keratometric power, corneal thickness, or both correlates with postoperative laser in situ keratomileusis (LASIK) corneal flap thickness. The aim of this study was to evaluate what preoperative corneal measurements that correlate with the postoperative LASIK corneal flap thickness.
Eighty Caucasian patients (32 males and 48 females) were included in this retrospective study. They all underwent LASIK surgery with a superior hinged flap using the Moria M2 microkeratome, and the last version 90µm single use head (intended to create flap thickness of 110µm) and control unit LSK evolution 2 (Moria SA, Antony, France) between April and October 2008. One surgeon performed all surgeries. Keratometric values were measured with the Allegretto Topolyzer (WaveLight Laser Technologies, Erlangen, Germany); mean preoperative keratometric power (K) was the average of K1 and K2. Preoperative and intraoperative central corneal thickness (CCT) were measured with Sonogage Corneo-Gage Plus 2 ultrasound pachymeter (Cleveland, Ohio, USA), the difference defined the corneal flap thickness. The eye that was operated on first of each patient was analyzed with linear regression and bivariate and partial correlation (α=0.05).
Eighty myopic [aged 20-59 yrs (mean age 39±10 yrs)] eyes were included. The preoperative spherical equivalent refraction ranged from -10.5 diopters (D) to -0.5 D. Mean myopic preoperative keratometric power K1 was 43.0±1.4D and K2 was 44.2±1.4D. Mean preoperative central corneal thickness (CCT) was 558±34µm. Postoperative corneal flap thickness ranged from 74 to 161 µm (mean 117±18µm). There was a significant correlation (Pearson’s) between corneal flap thickness and CCT (R=0.511, p<0.001), between mean preoperative keratometric power K and corneal flap thickness (R=-0.293, p=0.008), and between K and CCT (R=-0.338, p=0.002) for the myopes. The apparent relationship between K and flap thickness, however, was no longer significant when controlling for preoperative CCT (partial correlation, R=-0.148; p=0.192).
The main predictor of postoperative corneal flap thickness is corneal thickness. Keratometric power, however, co-varies with central corneal thickness in the group of myopes included in this study. Keratometric power may therefore not be a reliable measure for predicting postoperative corneal flap thickness.
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