Dissertation
Dissertation > Medicine, health > Ophthalmology > Eye surgery and surgery > Retinal photocoagulation and laser eye surgery

The Study on Changes of Corneal Asphericity before and after Excimer Laser Corneal Refractive Surgery

Author HouJie
Tutor WangYan
School Tianjin Medical University
Course Ophthalmology
Keywords Cornea Myopia Asphericity Q-value Spherical aberration Keratomileusis Laser in situ
CLC R779.63
Type Master's thesis
Year 2010
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Purpose:To evaluate the asphericity and corneal spherical aberration of the anterior and posterior corneal surface with different corneal diameters in myopic eyes, and investigate the change of the asphericity (Q-value) of the posterior corneal surface with different corneal diameters after corneal refractive surgery.Methods:Sectionl included 232 eyes of 116 myopia or myopic astigmatism patients were enrolled. Only the right eyes were included for analysis. The preoperative sphere refraction was -2.25~-7.75 D, and the cyclinder refraction was-0.50~-2. OOD.The Q-value and spherical aberration of the anterior and posterior corneal surface for different corneal diameters (6,7,8 and 9mm) were measured with Pentacam. The correlations between Q-value and spherical aberration, corneal refraction and central corneal thickness (CCT) were investigated.Section 2 included 115 myopic eyes of 115 subjects (59 LASIK,56 EPI-LASIK) having corneal refractive surgery were enrolled in this prospective study. The Q-value of the posterior corneal surface for different corneal diameters (6,7,8 and 9mm) were measured with Pentacam preoperatively and postoperatively. The correlations between Q-value, Q change (△Q), and the preoperative spherical equivalent refraction (SE), CCT, central ablation depth (AD) were investigated.Results:1. The Q6mm, Q7mm, Q8mm, Q9mm of the anterior corneal surface of the 116 eyes were-0.24±0.09、-0.29±0.09、-0.33±0.09 and-0.39±0.10, respectively; The Q6mm, Q7mm, Q8mm, Q9mm of the posterior corneal surface of the 116 eyes were-0.12±0.12,-0.21±0.11,-0.30±0.10 and-0.38±0.10, respectively.2. Q-value of the anterior corneal surface were more negative than the posterior corneal surface (t6mm=-10.353, P=0.000; t7mm=-6.684, P=0.000; t8mm=-3.455, P=0.001; t9mm=-0.026, P=0.979). Positive relationships were found between the anterior corneal surface and the posterior corneal surface (r6mm=0.302, P=0.001; r7mm= 0.367, P=0.000; r8mm=0.399, P=0.000; r9mm=0.424, P=0.000)3. Positive relationships were found between anterior Q-value and the cyclinder refraction (r6mm=0.280, P=0.002; r7mm=0.268, P=0.004; r8mm=0.246, P=0.008; r9mm=0.217, P=0.019). Negative relationships were found between posterior Q-value and CCT (r7mm=-0.183, P=0.049; r8mm=-0.212, P=0.022; r9mm=-0.238, P=0.010). No statistically significant correlation between Q and age or sex was found in this study(P>0.05).4. Linear correlation between Q-value and corneal spherical aberration were found. With a 6mm corneal diameter, the regression equation of the anterior corneal surface was SA=0.663Q+0.415, and the posterior corneal surface was SA=0.159Q-0.177.5. The mean Q-value of the posterior corneal surface of the 115 preoperative eyes for the corneal diameters of 6,7,8 and 9mm were 0.08±0.16,-0.05±0.13,-0.15±0.12 and-0.26±0.11, respectively. The Q-value had a statistically significant positive shift 1 month after surgery at 6,7,8 mm corneal diameter. The mean Q-value diminished significantly 6 months compared to 1 month after surgery, and had an obvious tendency back to its preoperative level.6. No statistically significant correlation between Q and SE or CCT was found in the preoperative eyes. Also, no correlation between△Q and SE, CCT, AD and AD/CCT was found after surgery.Conclusions:1. Both the anterior and the posterior corneal surface of the myopic eyes were prolate. With increasing corneal diameter Q-value became more negative (indicating an increased rate of flattening in the peripheral cornea).2. The relationship between Q-value and corneal spherical aberration can provide more valuable instructions for refractive surgeons to design the aspheric ablation.3. The posterior corneal surface showed central flattening and peripheral steepening (oblate shift) at early stage either post-LASIK or post-EPI-LASIK, and then returned to its original state over time.

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