Tonometry is used in all ophthalmology consultations to measure the intraocular pressure (IOP). The tonometry examination is carried out using an instrument called tonometer, usually found, today, in its contactless version. In fact the constant evolution of technology has made possible the introduction of forced-air tonometers, thus replacing the contact with the patient’s eye with the use of forced air, as it is soft and painless to the eye surface.
The forced-air tonometer measures the force required to deform the cornea in order to reduce its pressure. So, to improve the accuracy and make better the measure of the intraocular pressure, the instruments integrate, today, the measure of the cornea thickness (pachymetry) to propose a corrected value of the intraocular pressure.
Also, thanks to different automated systems integrated to the contactless tonometer, using it is quick and easy. Moreover, the addition of the auto-refracto-keratometer function to the TONOREF III gives a 4-in-1 machine (refraction, keratometry, tonometry and pachymetry).
The forced-air tonometer is based upon the Goldmann tonometer contact instrument. This device is used to assess the intraocular pressure by calculating the mechanical force required to flatten a 3.06 mm diameter corneal zone. The contactless tonometer uses an air jet to flatten the cornea.
The measuring system of the contactless tonometer works as follows: the instrument has a central air duct with, on one side, an infrared light emitter and a sensor on the other side. At rest, the cornea, which has a convex shape, diffuses light and the sensor of the instrument detects no signal. The air jet pressure is gradually increased in the duct to warp the cornea.
During the flattening, the corneal surface acts as a flat mirror reflecting the light to the sensor. Once the sensor has received the reflected light, the signal is sent to the instrument to deactivate the air jet. The intraocular pressure (IOP) is then measured from the actual air jet pressure required to flatten the cornea. As the corneal anaesthesia is not required to perform this examination, this device is the most frequently used one in ophthalmology.
The measure of pachymetry (cornea thickness) is integrated to TONOPACHY NT-530P and TONOREF III devices. This value makes possible applying a corrective factor to the value of the intraocular pressure, depending on the central cornea thickness. The central pachymetry is measured using the specular reflexion principle. The device calculates the distance between the paths of the epithelial reflexion and the endothelial reflexion on the CDD sensor.
The forced-air tonometer is a contactless device used to measure the intraocular pressure (IOP) simply using an air jet. So we can rapidly detect and make the following-up of some eye pathologies requiring applying eyestrain, such as glaucomas. Also, pachymetry information is interesting to detect and make the following-up of patient having a keratoconus.
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