Abstract
Abstract: :
Purpose: At present, the gold standard to evaluate corneal thickness (CT) is ultrasound pachymetry (UP). Recently, two alternative non-contact techniques has been described to measure CT: corneal elevation topography with the Orbscan, which measures corneal and tear film thickness and the Optical Coherence Tomography (OCT). This study compares corneal thickness measured with the three mentioned techniques and analyzing variations in hydrogel contact lens (HCL) wearers and after its discontinuation. Methods: Ten healthy myopic subjects (20 eyes; age range: 20-27 years of age), with no previous corneal disease and who had never worn contact lenses before, were fitted with HCL. Evaluation of corneal thickness was performed after 12 months (immediately after removing HCL) and 15 days after HCL discontinuation. Measures were taken in the following order: Orbscan, OCT (+10DP lens was placed to obtain images clearly focused on the cornea) and UP. Results: Corneal thickness evaluation by UP and Orbscan was always consistent (mean difference after removing HCL: 0.2 µm; CI95% -7.1-+6.6; p>0.05; mean difference after 15 days discontinuation: 4.1 µm; CI95% -9.8-+1.6; p>0.05) . However, OCT measurements were consistently lower; OCT compared with UP showed a mean difference of 10.6 µm (CI95% 13.7-7.5; p<0.01) after 12 months and 11.8 µm (CI95% 15.3-8.3; p<0.01) after 15 days of HCL cessation; OCT compared with Orbscan showed a mean difference of 11.8 µm (CI95% 18.8-4.8; p<0.01) after 12 months and 15.8 µm (CI95% 22.4-9.3; p<0.01) after 15 days of HCL cessation. When corneal thickness after 12 months of HCL use was compared to that obtained 15 days after HCL cessation, a significant decrease was observed with UP (3.6µm, p<0.01) and OCT (7.5µm, p<0.05), whereas no change was recorded with Orbscan (0.3µm, p>0.05).Conclusions: after fifteen days of discontinuation of HCL use, a decrease in corneal thickness is detected with both UP and OCT probably caused by the edema secondary to HCL use. If the assumption that the Orbscan also measures tear film thickness is true, an increase in its thickness after discontinuation of HCL use could be the explanation to the differences found with UP and OCT.
Keywords: cornea: clinical science • contact lens • clinical (human) or epidemiologic studies: sys