Measuring the biomechanics of the cornea

[caption id="attachment_7529" align="alignnone" width="300"] Cynthia Roberts[/caption] The correct measurement of the cornea’s biomechanical properties can have significant clinical applications, from the measurement of intraocular pressure to the evaluation of progression in glaucoma and keratoconus. Cynthia Roberts, Ohio, USA, discussed the details of these properties at the 21st ESCRS Winter Meeting in Maastricht, The Netherlands.

The goal is to distinguish the effects of corneal biomechanic variables from the actual measurements sought, such as IOP.

After explaining the differences between elasticity, viscoscity and viscoelasticity, Dr Roberts focused on another property, hysteresis, a viscoelastic parameter with a time-dependent component that describes the difference in response during loading and unloading of a force on a tissue. This is of particular importance for pneumotonometry, which relies on analysis of the corneal biomechanical response to an air puff. The variables encountered can be complex, particularly in pathologic conditions, such as keratoconus, and in eyes that have undergone refractive surgery. Both conditions result in a lower level of hysteresis. Conversely, corneal cross-linking causes a stiffening of the cornea, as well as a corresponding increase in hysteresis. Two commercial pneumotonometry devices have been designed to analyse these responses: the Ocular Respons Analyzer (ORA; Reichert) and the Corvis ST (Oculus). The ORA can produce several different measurements, including the corneal-compensated IOP (IOPcc) and the Goldmann-correlated IOP. “The IOPcc is less affected by corneal thickness and properties,” said Dr Roberts, which means that IOP measurements should more accurately reflect the actual pressure inside the eye. The goal is to distinguish the effects of corneal biomechanic variables from the actual measurements sought, such as IOP.