الفهرس 
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Abstract
Dissertation Abstract (One Page A4) : Abstract : Astigmatism associated with the posterior corneal surface is gaining interest, owing not only to its influence in overall eye astigmatism, but also to the alterations that seem to have taken place on the posterior surface at an earlier stage among keratoconic corneas. Power vectors were conceived as a way of transforming conventional refractive error, or keratometric data, into mutually independent, orthogonal components, better suited to statistical analysis. Vector analysis permits a complete description of astigmatism characteristics. The present study analyzed astigmatism of posterior corneal surface, employing vector analysis in the comparison among C, FFKc, and manifest KC. The same method was applied to investigate the correlation between anterior and posterior astigmatism within each group, assessing whether anterior and posterior corneal astigmatisms correlate in a predictable manner. Two hundred and forty four eyes of 244 attendants to the outpatient clinic of Mansoura Ophthalmic Center were enrolled into this study. For C and manifest Kc groups, only one eye was randomly selected per patient, in order to avoid any eventual correlation existing between the eyes of a single patient. For the FFKc group, only the unaffected eyes were enrolled in the study (the fellow eye of a patient with unilateral keratoconus). The majority of posterior vector parameters assessed exhibited statistically significant differences between groups (P value < 0.05) except for vector along 45 and zero degree meridian (J45, J0) which exhibited no difference among all groups. The mean magnitude of posterior M was - 6.4 ± 0.26, - 7.5 ± 0.98 and - 6.5 ± 0.35 for the control, Kc and FFKc groups respectively. APV value raged from 0.0 to 0.6 for the control group, from 0.0 to 1.75 for the Kc group and from 0.0 to 0.45 for the FFKc group. The mean magnitude of overall Blur was 6.4 ± 0.26, -7.49 ± 0.98 and 6.5 ± 0.35 for the control, Kc and FFKc groups respectively. The area under receiver operating characteristic curve (AUC) for posterior corneal APV between normal and manifest keratoconus was 0.73 (95% confidence interval): 0.66 and 0.80. By using ROC curve Sensitivity, Specificity. PPV, NPV and accuracy at cutoff 0.30 were (65.0%, 80.0%, 78.9, 66.1 and 73.1% respectively). As regard posterior corneal Blur; the AUC between normal and manifest keratoconus was 0.92 (95% confidence interval): 0.88 and 0.96. By using Roc curve Sensitivity, Specificity. PPV, NPV and accuracy at cutoff 6.65 were (85.3%, 89.0%, 90.0, 84.0 and 86.1% respectively). ompared to an APV cutoff value of 0.3 D, a Blur cutoff value of 6.65 D yielded a slightly more powerful differentiating approach. The toricity and astigmatic power vector components of the posterior corneal surface in the human eye are related to those of the anterior and therefore can be predicted consistently from the anterior toricity and astigmatic power vectors. The analysis of corneal power vectors APV, J0, and J45 revealed the presence of a statistically significant correlation between all anterior and posterior vectors’ magnitudes, especially among keratoconic corneas. Correlation coefficients found for the FFKc group most often presented slightly higher values than those found for the C group. Conclusion : Evaluation of the corneal posterior APV and Blur measurements could represent an unbiased, complementary and amiable method for the ophthalmologist in order to discriminate normal and keratoconic corneas. According to the present findings, any posterior APV value above 0.3 D or Blur above 6.65 D might raise the ophthalmologist’s suspicion toward a likely diagnosis of a keratoconic eye. Refinements in data acquisition from the posterior corneal surface could lead to further improvements in the analysis of posterior astigmatism in the future.