Corneal Biomechanics

Measuring corneal biomechanics in vivo

Measuring biomechanical properties clinically is currently one of the most exciting fields in modern ophthalmology. Biomechanical properties are defined as the response of a biomechanical tissue to a force. The cornea is visco-elastic which means that it exhibits both viscous and elastic biomechanical behaviour.

 

The applications of measuring these features in clinical practice are numerous since several diseases such as keratoconus have their origin in the change of biomechanical properties. Before corneal curvature or thickness changes due to the disease, the corneal stiffness and elasticity is already altered. Therefore, measurement of biomechanical properties is crucial for the detection of subclinical keratoconus.

 

For a refractive surgeon taking biomechanical properties into account therefore leads to:

  • A higher safety, as patients at risk for developing ectasia after LASIK can be excluded
  • A higher efficiency, as surgery can be performed when patients have a stiff and stable cornea

Moreover, many corneal treatments such as laser vision correction (LVC), corneal cross-linking or corneal incisions lead to an altered corneal biomechanical response which finally influences the vision of the patient.

 

IOP-Measurements: closer to the physiological IOP

The intraocular pressure measurement by applanation tonometry is highly influenced by the biomechanical properties. Therefore, taking biomechanical properties into consideration will provide a much more accurate IOP reading, closer to the physiological IOP. In conventional Goldman tonometry IOP readings can be completely off when the biomechanical properties of the cornea are altered – as for example after LASIK. This could lead to wrong decisions in the diagnosis and management of glaucoma.

     

    Biomechanical properties as independent risk factors for glaucoma

    Despite leading to a more accurate measurement of IOP the biomechanical properties of the eye ball are supposed to be independent risk factors for glaucoma. This allows screening for normal tension glaucoma by biomechanical parameters such as stiffness.

    The biomechanical properties of the cornea can be measured by the evaluation of the response of the cornea when placed under stress. This can be achieved by an external force such as an air pulse – as done with the Corvis® ST.

    Corvis® ST: Measurement Principle

    The Corvis® ST is a combination of an air pulse tonometer with an ultra-high-speed Scheimpflug camera. Shortly before the air pulse starts the cornea is illuminated by a blue slit light.  At this moment corneal thickness is measured as well. Afterwards the high-speed camera tracks the biomechanical response of the cornea.  Within 31 ms the camera records 140 images of the horizontal sectional plane. This is a frame rate of more than 4300 images / second.

     

    The movement of the cornea is mainly influenced by three factors which can be measured by the instrument:

    1. Intraocular pressure (IOP)
    2. Biomechanical properties of the cornea
    3. Corneal thickness

    All 140 images depict a complete picture of the biomechanical response of the cornea. At the beginning the cornea is in its initial convex shape. The air pulse drives the cornea backwards until the first applanation occurs. Afterwards, the cornea is further deformed until the moment of maximal concavity. After an oscillation phase the cornea returns back to its original shape. Before it reaches the initial state it passes through a second applanation, where the cornea is flat again.

     

    During this dynamic corneal response three moments in time are of major interest:

    1. The first applanation, when the cornea is flat.
    2. The moment of highest concavity.
    3. The second applanation, when the cornea is flat again before it returns to its original state.

    The Corvis® ST is able to measure important Dynamic Corneal Response parameters during the whole process. The complete biomechanical response is described in detail by Cynthia Roberts.


    Measurement Parameters

    1. Dynamic Corneal Response Parameters

    2. Stiffness Parameter

    3. Biomechanical corrected IOP (bIOP)

    4. Corvis Biomechanical Index (CBI)

    5. Tomographic Biomechanical Index

    Biomechanics: Theory and Applications

    The understanding of corneal biomechanical properties is an area of intensive research over the last decade now. Several experts all over the world have contributed to the understanding of the biomechanical behaviour of the cornea. Others have worked on new methods to measure biomechanical properties in vivo or ex vivo. Corneal cross-linking has been established which is based on the alteration of the biomechanical properties. In the following section experts in the different field summarize parts of their work.

     

    Prof. Eberhard Spoerl, PhD

    Prof. Eberhard Spoerl, PhD is one of the inventors of corneal cross-linking. Since 1993 is head of the laboratory for biomechanics in the Technical University of Dresden and is investigating the biomechanical properties of the eye ball along with testing methods to stiffen the cornea. In the following document he describes basic concepts of biomechanical properties such as the concept of elasticity and ways to measure stress strain curves ex-vivo.

    Download: Stress-strain measurements by strip extensiometry (Prof. Eberhard Spoerl, PhD)

    Definitions

    • Visco-elasticity

    • Elasticity

    • Viscosity

    • Stress–strain curve

    • Elastic modulus

    • Stiffness

    • Hysteresis

    Pentacam

    Vital

    Biomechanics meets Tomography

    Vital for the successful measurement is interaction. In that regard OCULUS set the benchmark in the market. With the Pentacam® and the Corvis® ST, interaction performs flawlessly: By combining tomographic data from the Pentacam® examination with biomechanical data from the Corvis® ST, ophthalmologists can further improve sensitivity and specificity in the detection of patients with a significant risk for developing ectasia after refractive surgery.

    Learn how you can take care of more patients with greater safety!

    Corvis