Ocular rigidity: biomechanical role, in vivo measurements and clinical significance.

Ocular rigidity (OR) refers to the relationship between pressure and volume changes in the eyeball. Since the description of the differential tonometry method for the calculation of an OR coefficient by Friedenwald, several other methodologies have been proposed to measure OR, including the anterior chamber manometry, axial length (AL) changes, measurement of pulse amplitude and fundus pulse, ultrasound elastography and evaluation of corneal hysteresis. However, most of these methodologies suffer from deficiencies, such as invasive nature, poor accuracy or reproducibility or technical complexity. Nevertheless, it is possible that OR affects the pathogenesis and clinical course of a variety of ocular conditions, including glaucoma, age-related macular degeneration, presbyopia, corneal changes following refractive surgery as well as the accuracy of IOP measurements by many tonometers. Thus, the future development of non-invasive and easy-to-use methodologies for the accurate measurement of OR in the every-day practice would be clinically important.