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    How corneal biomechanics affects keratoconus and ectasia

    Diagnosing and treating these conditions requires more information

    In a landmark paper titled “Global consensus on keratoconus and ectatic diseases” published in the April 2015 issue of Cornea, Gomes et al sought to reach a consensus of the definition, concepts, clinical management, and surgical treatments of keratoconus and the family of corneal ectatic diseases. According to the researchers, “there is no primary pathophysiologic explanation for keratoconus.” However, “the panelists reached the conclusion that the pathophysiology of keratoconus is likely to include environmental, biomechanical, genetic, and biochemical disorders.”1

    More from Dr. Morgenstern: Quality of life after LASIK

    Defining corneal biomechanics

    According to Garcia-Porta et al, corneal biomechanics “is a branch of science that studies deformation and equilibrium of corneal tissue under the application of any force. The structure and hence the properties of a soft tissue, such as the cornea, are dependent on the biochemical and physical nature of the components present and their relative amounts. The mechanical properties of a tissue depend on how the fibers, cells, and ground substance are organized into a structure. Collagen and elastin are responsible for the strength and elasticity of a tissue, while the ground substance is responsible for the viscoelastic properties.”2

    In a nutshell, each individual cornea, independent of its thickness and intraocular pressure (IOP), has a collagen rigidity or flaccidity profile. For example, if you exerted the same amount of force (such as the puff from a non-contact tonometer) onto the front of two distinctly different corneas that were 535 µm each, observation with a slow-motion camera would show them depress and rebound. Each cornea would depress to different depths, rebound at different rates, and produce a unique oscillation waveform in between. Each cornea would react quite differently to an identically similar stimulus.

    The study of this variance in corneal rigidity and flaccidity is the basis for research in corneal biomechanics. To put it into a real-life clinical setting, if each keratoconic patient has a unique corneal biomechanical property, then each keratoconus patient will exhibit a different rate of progression of disease that is unpredictable without corneal biomechanical information. This is why it is typically impossible to predict the long-term outcome of the disease.

    The biomechanical properties of the cornea have a direct correlation to not only the presence or absence of keratoconus in a patient but also its severity level. In addition, we know now that biomechanical properties of the cornea have a significant effect on applanation and non-contact tonometry. Back in the early 2000s, the ophthalmic community agreed that a cornea with a central thickness above or below average needed to be compensated for when calculating IOP. Today, we know that this is not the case3—IOP and corneal thickness have a non-linear relationship. More importantly, knowing the corneal resistance and rigidity (biomechanics) plays much more of a factor in IOP measurement than pachymetry.

    As more technology develops, the future will likely bring a day where analysis of corneal biomechanics is part of a primary-care eye exam to potentially determine the keratoconic and ectatic risk profile of an otherwise healthy eye.

    Next: Corneal collagen crosslinking

    Andrew S. Morgenstern, OD, FAAO
    Dr. Morgenstern is a healthcare consultant and subject matter expert for Booz Allen Hamilton at the Walter Reed National Military ...


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