Third Generation Apex Locators

The third-generation apex locators are similar to the second-generation except that they use multiple frequencies to determine the distance from the end of the canal. These units have more powerful microprocessors and are able to process the mathematical quotient and algorithm calculations required to give accurate readings.

The Endex/Apit is the original third generation apex locator. The relative values of frequency response method detect the apical constriction by calculating the difference between two direct potentials picked up by filters when a 1 kHz rectilinear wave is applied to the canal. The Apit is able to measure lengths with electrolytes in the canal but needs to be calibrated in each canal.

Apex locator

The Root ZX

The main shortcoming of early apex locators (erroneous readings with electrolytes) was overcome with the introduction of the ratio method and the subsequent development of the self-calibrating Root ZX. The ratio method works on the principle that two electric currents with different sine wave frequencies will have measurable impedances that can be measured and compared as a ratio regardless of the type of electrolyte in the canal. The capacitance of a root canal increases significantly at the apical constriction, and the quotient of the impedances reduces rapidly as the apical constriction is reached.

Root ZX

The Root ZX has also been combined with a handpiece to measure canal length as a rotary file is used. This is marketed as the Tri Auto ZX with integrated handpiece, and more recently as the Dentaport ZX. The Tri Auto ZX is a cordless electric endodontic handpiece with a built-in Root ZX apex locator. The handpiece uses nickel-titanium rotary instruments that rotate at 280 ± 50 rpm. The position of the tip of the rotary instrument is continuously monitored on the LED control panel of the handpiece during the shaping and cleaning of the canal.

Root ZX combined with handpiece

The Tri Auto ZX has three automatic safety mechanisms:

a) The auto-start-stop mechanism: The handpiece automatically starts rotation when the instrument enters the canal and stops when the instrument is removed.

b) The auto-torque-reverse mechanism: The handpiece also automatically stops and reverses the rotation of the instrument when the torque threshold (30 grams/centimetre) is exceeded, a mechanism developed to prevent instrument breakage.

c) The auto-apical-reverse mechanism: The handpiece automatically stops and reverses rotation when the instrument tip reaches a distance from the apical constriction that has been preset by the clinician; a mechanism controlled by the built-in Root ZX apex locator and developed to prevent instrumentation beyond the apical constriction.

The Apex Finder AFA (All Fluids Allowed) claims to have five signal frequencies and to read four amplitude ratios. The unit is self-calibrating and can measure with electrolytes present in the canal.

Apex finder