Second Generation Apex Locators

The second generation apex locators were of the single frequency impedance type which used impedance measurements instead of resistance to measure location within the canal. Impedance is comprised of resistance and capacitance. These apex locators, also known as impedance apex locators, measure opposition to the flow of alternating current or impedance. The property is utilised to measure distance in different canal conditions by using different frequencies. The change in frequency method of measuring was developed by Inoue in 1971 as the Sono- Explorer which calibrated at the periodontal pocket of each tooth and measured by the feedback of the oscillator loop. The beeping of the device indicated when the apex was reached.

A high frequency (400 kHz) wave measuring device, the Endocater was introduced in 1986. With an electrode connected to the dental chair and a sheath over the probe it was able to make measurements in canals even with conductive fluids present. The sheath caused problems because it would not enter narrow canals, could be rubbed off and was affected by autoclaving. The major disadvantage of second-generation apex locators is that the root canal has to be reasonably free of electroconductive materials to obtain accurate readings. The presence of tissue and electroconductive irrigants in the canal changes the electrical characteristics and leads to inaccurate, usually shorter measurements.