Armand Imbert (1850–1922) and

Adolf Fick Adolf Eugen Fick (3 September 1829 – 21 August 1901) was a German-born physician and physiologist. Early life and education Fick began his work in the formal study of mathematics and physics before realising an aptitude for medicine. He t ...

(1829–1901) both demonstrated, independently of each other, that in

ocular tonometry Tonometry is the procedure that eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometers are calib ...

the tension of the wall can be neutralized when the application of the tonometer produces a flat surface instead of a convex one, and the reading of the tonometer (P) then equals (T) the IOP," whence all forces cancel each other. This principle was used by Hans Goldmann (1899–1991) who referred to it as the Imbert-Fick "law", thus giving his newly marketed

tonometer Tonometry is the procedure that eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometers are calib ...

(with the help of the Haag-Streit Company) a quasi-scientific basis; it is mentioned in the ophthalmic and

optometric Optometry is the healthcare practice concerned with examining the eyes for visual defects, prescribing corrective lenses, and detecting eye abnormalities. In the United States and Canada, optometrists are those that hold a post-baccalaureate f ...

literature, but not in any books of

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

. According to Goldmann, "The law states that the pressure in a sphere filled with liquid and surrounded by an infinitely thin

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...

is measured by the counterpressure which just flattens the membrane." "The law presupposes that the membrane is without thickness and without rigidity...practically without any extensibility." A

sphere A sphere (from Ancient Greek, Greek , ) is a surface (mathematics), surface analogous to the circle, a curve. In solid geometry, a sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...

formed from an inelastic membrane and filled with

incompressible Incompressible may refer to: * Incompressible flow, in fluid mechanics * incompressible vector field, in mathematics * Incompressible surface, in mathematics * Incompressible string, in computing {{Disambig ...

liquid cannot be indented or applanated even when the pressure inside is zero, because a sphere contains the maximum volume with the minimum surface area. Any deformation necessarily increases

surface area The surface area (symbol ''A'') of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the d ...

, which is impossible if the membrane is inelastic. The physical basis of tonometry is

Newton's third law of motion Newton's laws of motion are three physical laws that describe the relationship between the motion of an object and the forces acting on it. These laws, which provide the basis for Newtonian mechanics, can be paraphrased as follows: # A body r ...

: "If you press an eyeball with an object, the object is also pressed by the eyeball." The law is this: :Intraocular pressure = Contact force/Area of contact The law assumes that the

cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...

is infinitely thin, perfectly

elastic Elastic is a word often used to describe or identify certain types of elastomer, Elastic (notion), elastic used in garments or stretch fabric, stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rub ...

, and perfectly flexible. None of these assumptions are accurate. The cornea is a membrane that has thickness and offers resistance when pressed. Therefore, in Goldmann tonometry, readings are normally taken when an area of 3.06mm diameter has been flattened. At this point the opposing forces of corneal rigidity and the tear film are roughly approximate in a normal cornea and cancel each other out allowing the pressure in the eye to be inferred from the force applied.''The Glaucoma Book'', Paul N. Schacknow, John R. Samples, p.79. Springer, 2010. .

Notes

{{DEFAULTSORT:Imbert-Fick Law Pressure Ophthalmology

See also

Notes