1.2 Corneal Hysteresis |
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Fig. 1.11 The projection of Placido disk on the anterior surface of the cornea. The shape and the distribution of the mires (rings) are affected by any distortion in the anterior part of the cornea
1.2Corneal Hysteresis
1.2.1Principles
“Corneal biomechanics” is a relatively new and increasingly important term in the field of refractive surgery. To understand this term, it is essential to understand
Fig. 1.12 The Photokeratoscope. It depends on the Placido principle except that the Placido disk is back-illuminated with a strobe flash and a camera replaces the clinician’s eye at the viewing port that takes a picture of the reflected mire pattern
Fig. 1.13 Anterior curvature map showing an irregular surface. The temporal part is steep (hot colors), the nasal part is flat (cold colors)
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1 Diagnosis of Keratoconus |
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Fig. 1.14 Photokeratoscopy of the same cornea in Fig. 1.13. The rings appear to be thin and tightly squeezed together in those regions where the curvature is steep and broadly dispersed wherever the curvature is flat
Fig. 1.15 Photokeratoscopy of a normal cornea. The rings are regular and concentric
the concepts behind, which are elasticity, viscosity, and viscoelasticity.
Elasticity is defined as the tendency of a body to return to its original shape after it has been stretched or compressed. Viscosity is defined as resistance of a liquid to shear forces (and hence to flow). A viscoelastic tissue will deform under the influence of an applied shear stress, but when the stress is removed, the tissue will slowly recover from some of the deformation.
Corneal tissue is a viscoelastic tissue. It is composed mainly of two parts: collagen fibers and the
Fig. 1.16 Photokeratoscopy in KC. The rings are distorted and grouped more closely in the region of the cone
matrix. Collagen fibers are responsible for the elastic properties and the matrix is responsible for the viscous properties of the cornea. Viscoelastic properties of the cornea are responsible for its biomechanical behavior and can explain many of the previously unexplained phenomena such as post-lasik ectasia and undercorrection after myopic correction and others.