Biomechanical Diagnostics of the Cornea

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Excerpt

The cornea contains the first cellular surface of the eye’s optical system and contributes approximately two thirds of the optical power of the relaxed eye. As a result, miniscule changes in the cornea’s shape induce substantial variations in the image formation process. Corneal diseases such as keratoconus as well as corneal refractive surgery can generate significant changes in the cornea’s optical properties. An ongoing challenge is to understand the relationship between the mechanical and structural changes that produce those clinically measured optical modifications in pathologic conditions and after refractive surgery and to understand their evolution over time. Better understanding of the biomechanics of the cornea is thus essential to comprehend the consequences of modifications in the geometry of the cornea after excimer laser refractive surgery or nonablative interventions such as corneal collagen crosslinking (CXL), to improve the diagnosis and management of ectatic corneal disorders such as keratoconus, and to understand the biomechanics of intraocular pressure (IOP).
The cornea is a complex biomechanical composite that responds to stress according to its structural subcomponents and their organization. The stroma and Bowman layer are the chief collagenous layers of the cornea and thus provide the majority of the cornea’s tensile strength.1–3 With regard to microstructure, around 300 collagen lamellae run across the corneal stroma and are stacked with angular offsets; this orientation becomes increasingly random in the anterior stroma, where significantly more oblique branching and interweaving are noted, which are also more extensive in the corneal periphery than in its center.4 Interweaving of collagen bundles between neighboring lamellae provides a structural mechanism for shear (sliding) resistance, sharing of tensile loads between lamellae, and distribution of stress across the cornea.5 In addition, x-ray diffraction studies provide evidence that the collagen fibrils are preferentially oriented superior to inferior and nasal to temporal in the central cornea and a predominantly circumferential in the corneal periphery that favors conservation of limbal circumferential dimensions even in ectatic disease.
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