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Aim of the work: this study aimed to investigate the relationship between the Intraocular Pressure(IOP) readings measured by Goldmaan Tonometry and the amount of corneal astigmatism in healthy young subjects and to assess the reliability of the measurements obtained with the instruments. Method: intraocular pressure measurements were obtained by Goldmann Applanation Tonometer (GAT) (Haag-Streit International, Koeniz, Switzerland), while keratometry readings for corneal curvature were obtained by an Auto-Kerato-Refractometer (ARK-KR.8800, Topcon, Tokyo, Japan). An interval was kept between the keratometric measurements and the IOP measurements. The sequence of measurement was randomized with all participants. Three measurements were taken within ten minutes and the reading was set to one immediately after any measurement were taken. Result: the average age of the volunteers was 22.4 ± 1.5 years (mean age ± SD; range: 20–26 years). Mean Intraocular Pressure (IOP) of the right and left eyes in the first visit (Paired t-test: P = 0.6471) and second visit (Paired t-test: P = 0.5805). The session one means GAT ; IOP was 16.48 ± 3.33 mmHg and 16.59 ±3.47mmHg (right and left eye respectively), while in session two it was15.714±3.10 mmHg and 15.82 ± 2.76 (right and left eye respectively) . The mean corneal astigmatism was -0.24 ± 1.48D, -0.38 ± 1.53 D, right and left respectively in the first visit and -0.34 ± 1.44D, -0.40 ± 1.64D right and left eye second visit respectively. Conclusion: the GAT-IOP was less affected by the amount of corneal astigmatism and GAT-IOP was not significantly higher in eyes with greater corneal astigmatism and the repeatability results, although the differences were stastistically significant, they were not clinically significant.

Purpose:\ud To study the structural features of human post-LASIK corneas.\ud Methods:\ud A pair of post-mortem donor corneas, from a 55-year old patient who underwent uncomplicated LASIK surgery five years previously, were bisected and fixed in 4% paraformaldehyde. The right cornea and one half of the left cornea were processed for light microscopy and scanning electron microscopy. One half of the right cornea was also examined by transmission electron microscopy.\ud Results:\ud The flap-bed interface could be easily detected several years after LASIK and, although the flap appeared to be in close association with the stromal bed, there was a noticeable absence of reconnection between adjacent severed lamellae. Tissue gaps were evident at the flap margin, which once free of cellular components revealed the presence of a few bridging fibres.\ud Conclusion:\ud Examination of corneas five years after LASIK revealed evidence of primitive reparative scar development at the wound interface, but no reconnection of severed collagen lamellae. Such findings may explain the occurrence of flap dislocation following trauma in some patients months or years after surgery.

This study is a comparative study of the relationship between corneal structure, morphology, and function in a range of mammalian species. X-ray scattering patterns were gathered at regular spatial intervals over the excised cornea (and in most cases also the scleral rim) of humans, marmosets, horses, cows, pigs, rabbits, and mice. All patterns were analyzed to produce quantitative information regarding the predominant orientation of fibrillar collagen throughout the tissue. The predominant direction of corneal collagen varies between mammals. This variation is not related to the size, shape, or thickness of the cornea or the frequency with which the animal blinks. A relationship does, however, appear to exist between corneal collagen arrangement and visual acuity. An excess of collagen directed toward one or both sets of opposing rectus muscles is a feature of animals that have an intermediate to high level of visual acuity. There is a significant variation in the arrangement of corneal collagen between different mammalian species. This finding may be related to differences in the frequency of action and the forces generated by the various extraocular muscles during eye movement and image fixation.

Purpose. To study the distribution and predominant orientations of fibrillar collagen at different depths throughout the entire thickness of the human cornea. This information will form the basis of a full three-dimensional reconstruction of the preferred orientations of corneal lamellae. \ud \ud Methods. Femtosecond laser technology was used to delaminate the central zones of five human corneas into three separate layers (anterior, mid, and posterior stroma), each with predetermined thicknesses. Wide-angle x-ray diffraction was used to study the gross collagen fibril orientation and distribution within each layer. \ud \ud Results. The middle and posterior parts of the human cornea demonstrated a preferential orthogonal arrangement of collagen fibrils, directed along the superior–inferior and nasal–temporal meridians, with an increase in the number of lamellae toward the periphery. However, the anterior cornea (33% of total corneal thickness) showed no systematic preferred lamellar orientation. \ud \ud Conclusions. In the posterior two thirds of the human cornea, collagen lies predominantly in the vertical and horizontal meridians (directed toward the four major rectus muscles), whereas collagen in the anterior third of the cornea is more isotropic. The predominantly orthogonal arrangement of collagen in the mid and posterior stroma may help to distribute strain in the cornea by allowing it to withstand the pull of the extraocular muscles, whereas the more isotropic arrangement in the anterior cornea may play an important role in the biomechanics of the cornea by resisting intraocular pressure while at the same time maintaining corneal curvature.

Laser in situ keratomileusis (LASIK) is a surgical technique used to correct refractive errors by reshaping the cornea. Although LASIK is superior to other visual correction techniques, recent clinical reports show that, in some cases, it leads to serious optical problems. Therefore, the aim of this thesis was to study the corneal ultrastructure in normal and LASIK corneas and discover a reason for the deterioration of vision in some LASIK patients. Different experiments were run, from experiments to improve understanding of the collagen fibril arrangement in the human cornea to studying the corneal changes in post-LASIK ectasia. Also, different techniques were used in this study including wide-angle x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and light microscopy (LM). XRD showed that the human cornea possesses a unique orthogonal central collagen fibril orientation that was not found in the corneas of animals, such as camels or rabbits, which were found to have unidirectional or circular collagen orientation respectively. However, all human, camel, and rabbit corneas were found to have the same collagen fibril orientation around the limbus, that is, an annulus circumscribing the cornea. XRD was also used to study the effect of corneal full-thickness trephination on collagen fibril arrangement, trying to mimic penetrating keratoplasty (PKP) and LASIK procedures. The results indicate that central corneal trephination (at 4 mm diameter) will change the collagen fibril arrangement around the trephine-wound edges. This effect will be reduced if the trephination is made away from the centre, toward the limbus. These results may encourage ophthalmic surgeons to use a large graft diameter for PKP in order to avoid postoperative complications such as astigmatism. Also, these results may give a good explanation for the low post-LASIK astigmatism rate found in the literature. As LASIK surgery includes flap creation and laser ablation, it was preferable to study the collagen fibril arrangement at different corneal depths (by means of femtosecond laser and XRD) in order to understand the precise effect of the LASIK technique on collagen lamellae. It was found that the first third (33%) of the corneal thickness has an irregular collagen lamellar orientation whereas the orientation clearly becomes orthogonal towards the posterior cornea (endothelium side). Thus, it can be concluded that LASIK flap creation and laser stromal ablation usually occur in the irregular collagen fibril layers. These layers have been found to be the strongest part of the cornea and are essential to maintain corneal curvature and strength. Therefore, these findings allow us to understand the reason for high astigmatism or ectasia in some LASIK patients. In vitro LASIK was then conducted on donor human corneas to simulate the in vivo situation of LASIK so that the collagen fibril orientation and other corneal structural changes could be studied by means of XRD, SEM, and TEM. XRD results indicated that the collagen fibril orientation and distribution after LASIK are similar to those in normal corneas. This normal orientation was expected because the flap creation and the laser ablation usually occurred in the first third of the corneal thickness, which has been found to have an irregular collagen fibril orientation and hence, the full corneal thickness XRD cannot reveal the localised effects of LASIK. XRD on post-LASIK ectatic corneas showed that the collagen fibril orientation was also similar to that of the normal cornea, that is, it showed an orthogonal collagen orientation. TEM, SEM, and LM of normal and ectatic LASIK corneas showed that the flap-bed interface can be detected easily regardless of the time after surgery flap borders have been detected up to 10 years after the LASIK procedure. Moreover, the results indicate that the LASIK corneal wound healing happens superficially (epithelium healing only) and also show that the collagen lamellae do not bond with each other again after LASIK, which leaves the flap weak and, thus, explains the easy separation or dislocation of the flap from the stromal bed months or years after surgery. Also, the results give an overview of the corneal biomechanical insults caused by the LASIK flap, which seem difficult to avoid and, in some cases, may lead to ectasia. Interestingly, TEM shows that the collagen fibril diameter and interfibrillar spacing of both normal and ectatic LASIK corneas appear similar to those in normal corneas, which may explain the perfect visual acuity results obtained immediately after LASIK surgery and, also, indicates (according to the above results) that the reduced vision in ectatic patients is not a result of any disarrangement of the collagen fibrils, which is known to affect corneal transparency, but, instead, is a result of a corneal biomechanics insult due to the flap creation and tissue ablation.