Assessment of Corneal Sensitivity.

ACCURATE measurement of corneal sensitivity was first attempted by von Frey (1894), using as a criterion the length and bend of a horse hair through which pressure was applied and increased until the threshold of sensation was reached. The basic principle laid down by von Frey was generally accepted. Modifications were made by using different materials and adjustable plastic thread, which permitted the application of different pressures which could be measured on the same instrument in mg. of pressure (Boberg-Ans, 1955, 1956; Cochet and Bonnet, 1960; Schirmer and Mellor, 1961). The method depended on the idea that weight and surface were related in a rigid ratio. It was assumed that an increase of corneal contact had to be followed by a proportionate increase of weight to a hypothetical pressure index of 300 mg./sq. mm., which was universally regarded as mean average of corneal threshold sensitivity. However, experiments have shown that this 300 mg./sq. mm. loading is not valid when varying areas of surface contact are employed. Furthermore, it has become apparent that an additional factor, that of friction, is involved and an instrument has therefore been devised able to measure and correlate all three factors-loading, contact area, and friction.