Development of Tactile Eye Stiffness Sensor

This article describes the design of a novel trans-scleral tonometer based on the use of multiple force sensors forming a mechanical stiffness sensor. The approach is akin to an instrumented form of digital palpation tonometry in which manual paplation is used to infer the stiffness, and hence, the intraocular pressure of the eye. Force indentation data from multiple probes has been shown to correlate with the intraocular pressure (IOP) using encucleated porcine eyes. A noticeable amount of hysteresis has been observed during indentations at higher rate. Analysis of the experimental data indicates that stress relaxation (accommodation) in the visco-elastic corneo-scleral shell is the primary factor of the observed hysteresis. Further tests under different indentation rates show that the novel tonometer is expected to have an accuracy of ±1 mmHg when the indentation rate is kept below 0.5 mm/sec for pressure range of 10-35 mmHg. Using a calibrated finite element model of the measurement, the effect of lateral and angular misalignment is also examined. The results show that the position and orientation of the tactile sensor has to be controlled to within ±1 mm and ±3° in order to achieve a target accuracy of ±1 mmHg.