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Hassan Muhammad, Roa Al-Abdalla, Reginald Farrow, Albert S Khouri, Robert D Fechtner, Gordon Thomas; A self-Tonometer yields low variation and indicates linearity in the elasticity of palpebral tissue. Invest. Ophthalmol. Vis. Sci. 2014;55(13):136.
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© ARVO (1962-2015); The Authors (2016-present)
To measure the compressibility of the eye through the palpebrum by modeling the eyelid as a linearly elastic system in order for patients to measure their own IOP at home and transfer information to a physician.
A non-invasive self-tonometer which measures IOP through the palpebrum has been designed using a stepper motor and a cantilever piezo-resistive sensor. The eyelid and cornea are compressed using a rod with a tip diameter of 3.15 mm, pushed by the cantilever for a distance <1 mm. Changes in distance and force applied to the sensor are measured at a sampling rate of 500 Hz for 200 ms. The measurements are recorded using a 12-bit N.I. analog to digital converter. The system calculates a total spring constant from the ration of the measured force and displacement, which includes the spring constants of the sensor, the cornea and the eyelid. After the total spring constant is recorded, the spring constants of the sensor and cornea (calibrated to a Goldmann applanation IOP measurement) are subtracted, leaving the spring constant of the eyelid. This constant is incorporated into subsequent measurements, solving for the corneal spring constant, allowing for a pressure measurement when multiplied by displacement and divided by cross-sectional area of the probe. We measure the IOP of three healthy volunteers and one glaucoma patient. Thirty trials are conducted consecutively on each patient and 100 measurements are recorded in each trial.
Every slope is derived through a linear fit with an R2 > .990, showing linearity of the system. The average standard deviation from the mean for each subject was 1.42 in standard units. Individual percent error (std./mean) for the healthy patients is as follows: [4.1; 6.6; 5.8]%. The percent error for the glaucoma patient is 6.5%. Error induced by noise in the recorded data is fit to a Gaussian curve with the mean centered at 0.
The regression of recorded force vs. displacement measurements support a linearly elastic model of the eyelid, allowing it to be included in measurements and easily accounted for in analysis. In a controlled laboratory setting, smaller variations in measurements performed by subjects are observed as opposed to those performed by experienced technicians in a clinical setting, as conventional error is known to be about 10%.The palpebral method appears viable for measurements by a patient of his/her own IOP.
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