Application Of Anterior Segment Optical Coherence Tomography In Refraction Surgery

Corneal thickness (CT) has been commonly used as a potential determinant of the security of corneal refractive surgery. Recently, accurate determination of CT has been proposed prior to the surgery, determination of corneal ablation, as well as the evaluation of postoperative refractive stability and security. Thus, it is imperative to measure the CT before the initiation of the surgery. The corneal epithelium, as the first protective barrier for cornea, plays a crucial role in the prevention of pathogenic microorganism invasion. Also, it serves as an important optical transduction and refracting media with a comparatively strong regeneration and repairing capacity. It has been proposed that the measurement of corneal epithelial thickness (CET) is important in the early diagnosis of keratoconus and the epithelial healing after corneal refractive surgery.Anterior segment optical coherence tomography (AS-OCT), a technique for performing high-resolution cross-sectional imaging, has been commonly used instead of Orbscan tomography as it shows higher reproducibility and accuracy. AS-OCT include time domain OCT (TD-OCT) and fourier-domain OCT (FD-OCT), to compare the low resolution in TD-OCT, FD-OCT scan depther, and it has a high axial resolution, and display clearly tissue layer and extent of pathological,,Therefore, the first part of present study is designed to measure the corneal epithelial thickness in myopic eyes, and to study the relationship between corneal epithelial thickness and biomechanical properties. And the purpose of the second part of my study is to use fourier-domain anterior segment OCT to evaluate the CT and CET in SCLs wearers subjected to refractive surgery. On this basis, we aim to summary the normal ranges and distributing characteristics of CT and CET in myopic subjects, and provide guidance for the physicians in order to improve the safety, predictability and stability of the refractive surgeries.Part I:Corneal Epithelial Thickness in Myopic Eyes and its Relationship with the Corneal Biomechanical PropertiesPURPOSErTo measure the corneal epithelial thickness in myopic eyes, and to study the relationship between corneal epithelial thickness and biomechanical properties.SETTING:Department of Ophthalmology, Shandong Provincial Hospital affiliated to ShandongUniversity. DESIGNrClinical study.METHODS:In this cross-sectional study,198right eyes of 198 myopia or myopia astigmatism patients were enrolled. The age was 18-41years old. The equivalent spherical refraction was-0.75-10.38D. Corneal epithelial thickness for different corneal zones(0-2mm,>2-5mm,>5-6mm)was measured using RTVue-OCT. Different quadrant epithelial thickness, including the superior, superior temporal, temporal, inferior temporal, inferior, inferior nasal, nasal, superior nasal of the cornea, were obtained. Correlations of corneal epithelial thickness with corneal hysteresis (CH) and corneal resistance factor(CRF).which were measured by ORA.were investigated.Three groups were divided according to the different corneal epithelial thickness. Mann-Whinney U test, Kruskal-Wallis test,multivariable linear regression method were used to analyze the data.Results The 0-2mm,>2-5mm,>5-6mmCorneal epithelial thickness median of the 198 eyes were 54.0(52.0-56.0) um,54.1 (52.6-56.4) um,53.8 (52.2-55.8) um, respectively. No relationships were found among the eight quadrants (Z>2-5mm=279.482, P<0.01;Z>5-6mm=210.442, P<0.01),no significant difference was found between nasall and temporal (Z>2-5mm=-0.481, P>0.05, Z>5-6mm=-0.381, P>0.05). Positive relationships were found between the central corneal epithelial thickness with CH and CRF. The statistically significant different were found in three corneal epithelial thickness groups(CH:Z=8.945, P<0.05; CRF:Z=9.225, P<0.05).Positive relationships were found between the central corneal epithelial thickness and the age(r=-0.185, P<0.01).Conclusion The corneal epithelial thickness is not evenly distributed across the cornea. The epithelial thickness has a Positive correlation with the corneal biomechanical properties. superior nasal of the cornea, were obtained. Correlations of corneal epithelial thickness with corneal hysteresis (CH) and corneal resistance factor(CRF).which were measured by ORA.were investigated.Three groups were divided according to the different corneal epithelial thickness. Mann-Whinney U test, Kruskal-Wallis test,multivariable linear regression method were used to analyze the data.Results The 0-2mm,>2-5mm,>5-6mmCorneal epithelial thickness median of the 198 eyes were 54.0(52.0-56.0) um,54.1 (52.6-56.4) um,53.8 (52.2-55.8) um, respectively. No relationships were found among the eight quadrants (Z>2-5mm=279.482, P<0.01;Z>5-6mm=210.442, P<0.01),no significant difference was found between nasall and temporal (Z>2-5mm=-0.481, P>0.05, Z>5-6mm=-0.381, P>0.05). Positive relationships were found between the central corneal epithelial thickness with CH and CRF. The statistically significant different were found in three corneal epithelial thickness groups(CH:Z=8.945, P<0.05; CRF:Z=9.225, P<0.05).Positive relationships were found between the central corneal epithelial thickness and the age(r=-0.185, P<0.01).Conclusion The corneal epithelial thickness is not evenly distributed across the cornea. The epithelial thickness has a Positive correlation with the corneal biomechanical properties.Part II:Effects of Long-term Soft Contact Lenses Wear on the Corneal Thickness and Corneal Epithelial Thickness in Myopic subjectsPURPOSE:To investigate the effects of long-term soft contact lenses wear on the corneal thickness (CT) and corneal epithelial thickness (CET) in myopic subjects.SETTING:Department of Ophthalmology, Shandong Provincial Hospital affiliated to ShandongUniversity.DESIGN:clinical study.METHODS:Subjects willing to receive refractive surgery in our hospital from April to July in 2013 were included. CT and CET were measured in those with immediate discontinuation of lens (group I,56 eyes), and those with discontinued wearing of lens for more than 2 weeks (group II,56 eyes). Ninety-four eyes were enrolled as normal control. The CT and CET at the positions with a radius of 0~1.0 mm,1.0～2.5 mm (divided into eight quadrants) and 2.5-3.0 mm (divided into eight quadrants) away from the corneal center was measured using RTVue-100 Fourier-domain anterior segment OCT system.RESULTS:1、Significant decrease was noted in the CT of the subjects in group II compared with that of group I and control group (P<0.05).2、Meanwhile, significant decrease was noticed in the CET in myopic subjects with long-term SCLs wear compared with the control group (P < 0.05). After discontinuation of SCLs, increase of CET was noticed, however, it can not reach the normal range as revealed in the control group.CONCLUSIONS:1. Edema of corneal stroma together with thinning of cornea stroma was noticed in myopic subjects with long-term SCLs wear.2. Additionally, thinning of corneal epithelium was noted in these subjects.