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Title: Study of corneal ultrastructure in normal and post-LASIK human eyes
Author: Abahussin, Mohammad
ISNI:       0000 0004 2751 4722
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2008
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available