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S.E. Brodie, N. Nissirios, J. Danias; Kinematic Calibration of the Induction–Impact (Rebound) Tonometer . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1244.
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Calibration of the induction–impact tonometer has thus far been accomplished only empirically. Our purpose is to improve this calibration through a better understanding of the physics of the impact event.
The induction–impact tonometer records the velocity trajectory of a pin accelerated toward the cornea by an induction coil, and then allowed to impact on the cornea. Impact trajectories were recorded and compared with manometric intraocular pressure (IOP) measurements in the eyes of 129P3–J mice. Deceleration curves were fit to a quadratic function of time with Mathematica symbolic algebra software. A calibration curve was based on over thirty measurements in three different animals.
At an IOP of zero cm H20, post–impact deceleration curves were sigmoidal in shape, suggesting an inelastic collapse of the corneal vault. At pressures ranging from 10 cm H20 to 50 cm H20, the deceleration curves were parabolic in shape, suggesting that the eye functions as a linear spring, pre–loaded to a variable extent by the intraocular pressure. Effective spring–constants estimated by a linear regression showed a linear dependence on intraocular pressure, similar between the animals studied. (r = 0.911; p<0.0001 )
The deceleration of the induction–impact tonometer pin may be understood as the effect of the eye acting as a linear spring, pre–loaded to an extent proportional to the intraocular pressure. The system is well–suited to intraocular pressure measurements in mice and other small animals.
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