What is Indentation Tonometry?
The popularity of corneal indentation tonometry as a means to measure intraocular pressure grew steadily during the late 19th century and early 20th century. The tonometer works by observing how much the weighted instrument sinks into the surface of the cornea. The instrument sinks less into a hard eye than a soft eye; therefore there is less of a deflection on the scale reading. The first relatively accurate indentation tonometer used in a clinical setting was developed by Hjalmar Schiötz in 1905. The Schiötz tonometer (image 5) includes a footplate that rests on the cornea and is attached to the weighted plunger. The weight can be varied by adding different size weights to the instrument, ranging from 5.5 to 15 grams. As the footplate sinks into the eye, the needle moves on a linear scale. When the Schiötz tonometer was first being used in medical practice, Schiötz specified only the weight added and the amount of deflection were recorded. The instrument was not meant to give an actual pressure reading.
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In order to make the instrument more standardized and informative, a conversion calibration table was developed to convert the weight and deflection values in millimeters of mercury. This table was not finalized until 1955 by The Committee on Standardization of the American Academy of Ophthalmology and Otolaryngology. Schiötz original design was improved during the early 20th century and become the most used tonometer worldwide. The instrument was lightweight and easy to disinfect, making it a very practical instrument for measuring intraocular pressure.
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After the development of the Schiötz tonometer, many others worked to make different models of corneal indentation tonometers. One of these attempts was by William McLean. His McLean tonometer has invented in an attempt to eliminate some of the short comings of the Schiötz tonometer but was very similar in appearance. He pointed out some of the main features of his tonometer in an article published in the British Journal of Ophthalmology in 1919. The improvements over the Schiötz included a more streamline design, in which changing of the weights was not needed and the readings were directly in millimeters of mercury so an accurate calibration chart was not needed. However, readings with the McLean tonometer were significantly higher than those of the Schiötz tonometer. The McLean tonometer was an accepted method for measuring intraocular pressure, but it never gained the same wide spread usage as the Schiötz tonometer.
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