The complex enforcements emphasize the importance of knowing how the existence and orientation of collagen fibrils influences the overall corneal mechanical behavior. It has been proposed that the preferential fibril orientation exists in order to take up the stress of the ocular rectus muscles along the cornea. An annulus of collagen fibers that encircle the limbus was also observed. The data suggest that, on average, the two directions are populated in equal proportion at the corneal center. Approximately one-third of the fibrils throughout the stromal depth tend to lie within a 45° sector of the superior-inferior meridian, and similarly for the nasal-temporal direction. In one study, wide-angle X-ray scattering was used in order to quantify the relative number of stromal collagen fibrils directed along two preferred corneal lamellar directions. X-ray scattering studies have indicated that the preferred orientation is more prevalent in the posterior half of the stroma. The directionality of the collagen fibrin in different parts of the cornea has implications on its shape. The corneal stroma is composed of approximately 300–500 lamellae of collagen fibrils that form most of the corneal tissue and therefore has a major influence on the corneal biomechanics. The cornea thickens toward its periphery, where its value is about 0.65 mm. The central corneal thickness is 0.52 ± 0.04 mm. In healthy cases the radius of curvature of the central cornea has been reported to be 7.86 ± 0.26 mm (mean ± standard deviation). The central area that lies directly in front of the pupil is the main optical zone and is about 3-4 mm in diameter. The boundary of the cornea is referred to as the limbus region. Normal corneas have a horizontal diameter (white to white) in the range of 11-12 mm in 95% of the cases. The cornea forms the transparent outer covering of the visible colored portion of the eyeball. Conclusionsĭegradation of the circumferential fibers may very well be an initiating factor of a biomechanical process in which a bulge is gradually created from a presumably healthy cornea under normal underlying pressures and therefore, the identification of the early stages of keratoconus might be achievable by monitoring the in-vivo corneal fiber distribution. The anisotropic non-homogenous characteristics of the cornea have shown to be critical for maintaining the morphology of a healthy corneal.
For a cornea with a constant modulus of elasticity value of 0.4 MPa, 350 microns decrease in thickness resulted in a decrease of approximately 25 diopters. When the thickness was maintained at 500 microns and the stiffness was decreased from 0.4 MPa to 0.04 MPa there was an increase of more than 40 diopters. Results show that under 10mmHg intraocular pressure, decreasing the modulus of elasticity and thinning have opposite effects on the dioptric power maps of a homogenous isotropic cornea. Three additional cases that are based on a model of a thin plate were used to demonstrate the effect a transition from orthotropic to isotropic mechanical properties would have in terms of deformations and diopteric power maps.
The resulting deformations and dioptric power maps were analyzed and compared. Cases comprising of thinned regions as well as regions with degraded isotropic mechanical properties and a case of gradual stiffening towards the limbus were subjected to normal intraocular pressures. Methodsįinite element models of the cornea that are based on anatomical dimensions were developed. The goal of this study was to characterize corneal biomechanics using computer modeling techniques in order to elucidate the pathogenesis of keratoconus in biomechanical terms. The etiology of keratoconus most likely involves substantial biomechanical interactions.
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