Non-Invasive Measurement of Ocular Rigidity With a Novel Instrument

Abstract

Purpose: : Ocular rigidity has so far been calculated with an invasive technique during intraocular surgery. The purpose of the present study is to perform pilot measurements of ocular rigidity and intraocular pressure (IOP) with a minimally invasive device.

Methods: : A minimally invasive device consists of a glass lens that is used to contact and deform the corneal surface. A wide infrared beam is propagated through the lens and cornea. Partial index matching at the area of contact modulates the back reflected intensity of the beam according to the diameter of the applanated corneal zone. This optical sensor is mounted on a load cell that measures the force required to achieve the above-mentioned deformation. Signals both from the opto-electronic sensor and the force sensor are pre-amplified and captured by means of an analog to digital converter PC card for post processing. For any given area of contact the displaced volume can be calculated. The rigidity of the eye is calculated as the slope of the IOP vs displaced volume curve.The device has been calibrated by taking pressure measurements on enucleated rabbit eyes and compared to Goldmann Applanation tonometry (GAT). Twenty right eyes of twenty healthy young subjects were enrolled in the initial evaluation of the instrument’s measurement. In addition, intraocular pressure provided by the instrument was compared to GAT.

Results: : Twenty young ocular-disease-free persons (6 men and 14 women) with a mean age of 30 + 6,2 (mean + standard deviation(SD)) performed a measurement of ocular rigidity and IOP with the above-mentioned instrument. The average ocular rigidity was 0.562 + 0,254 mmHg/ uL (mean + SD) which in Friedenwald’s notation corresponds to 0,0157 + 0,0071. The average IOP estimated was 21 + 9.3 mmHg (mean + SD). Average IOP using GAT was 18,5 + 3,8 mmHg.

Conclusions: : Establishing a novel technique for assessment of ocular rigidity is an essential step in ocular hydrodynamics. This is a novel non-invasive method of measuring ocular rigidity. It works in a way similar to GAT and does not require an operating theatre. The calculated values of ocular rigidity are comparable to values reported in the literature. Collecting data of parameters interfering ocular rigidity may lead to better understanding of the pathogenic pathway of degenerative ocular diseases.

Keywords: clinical (human) or epidemiologic studies: systems/equipment/techniques • intraocular pressure • aging