Dynamic palpation device to evaluate IOP; simulation on a mechanical eyeball system

摘要

The only sure diagnostic indicator of Glaucoma is the intra ocular pressure (IOP). When the IOP increases to about 2 times the normal average pressure of 16 mmHg (2.1kPa) the pressure in the eyeball affects the optic nerve and causes visual distortion and, ultimately, can cause blindness.The Goldmann tonometer is recognised as the "gold standard" for measuring IOP. However, this test does not take into account the effect of changes in central cornea thickness (CCT), creep of the cornea and drainage of the aqueous humour (AH) from the anterior chamber. These factors, two of which are time-dependent, play a very important role for measuring the real IOP in vivo. Whereas, static devices are not able to record these parameters, a dynamic palpation instrument is able to separate the effects of IOP, CCT, creep and fluid drainage. To assess this behaviour, a mechanical eyeball was designed capable of simulating fluid drainage through a range of nozzle sizes, CCT through a change in membrane thickness and IOP using a variable hydrostatic head. The device which was driven by a piezo actuator was used to palpate the surface of the membrane with a range of frequency up to 60Hz with an amplitude of max 0.15 mm, whilst measuring the force, displacement and pressure as a functions of time. Significant differences in amplitude ratio and phase lag were achieved each characteristic of the IOP, membrane thickness and/or nozzle size.