McLean tonometer

Historical Significance: Albrecht von Graefe's invention of the surgical iridectomy procedure for the treatment of glaucoma in 1857 was his impetus to design an instrument to measure the eye pressure so that he could record pressures before and after iridectomy. His developed the first tonometer in 1863 but was unsuccessful. Frans Donders and others attempted to develop a tonometer over the next 20 years but they all lacked accuracy due to friction in the moving parts of the instrument and the fact that the probe had to be applied through the eyelid. Carl Koller's discovery of cocaine as a local anaesthetic in 1884 allowed the next series of tonometers that made direct contact with the cornea. The emphasis was on indentation tonometers, despite the difficulty and inaccuracy associated with their use. The introduction of Hjalmar Schiotz's tonometer in 1905 eased some of the major problems. Even at that early stage Alexei Maklakov introduced his applanation tonometer, which also became popular. The McLean tonometer, an indentation tonometer was constructed to improve on the Schiotz tonometer. The instrument was made for Dr McLean by EB Meyrowitz of New York. William McLean was Professor of Ophthalmology at New York Medical College. In the Am J Ophthal, June 1919 he listed its following features: (1) avoiding the changing of weights as in the Schiotz and Gradle tonometers. (2) eliminating the chart to determine the pressure in mm Hg (3) the reading scale in a position to make it easier for the observer to both apply the instrument and take a reading. (4) To prevent capillary attraction between the plunger on the tonometer and its barrel from fluid in the conjunctival sac. McLean also read a paper on his tonometer at the Oxford Ophthalmological Congress in July 1919, (See Brit J Ophthal 1919;3:385-99). The Schiotz tonometer however remained a popular instrument in clinical practice and is still used in some parts of the world. See also Keeler et al Brit J Ophthal 2009; 93: 1131). The manufacturer, EB Meyrowitz, was founded in 1875 and is still in business (in 2016). Likewise the supplier V Mueller of Chicago, medical instrument maker and supplier was founded 1906 and is still in business