Methodology of studies
In vivo studies are more representative of the true accuracy of a given apex locator (Czerw et al. 1995).
Those studies that use apex locators to find the working length, cement the file in place, extract the tooth and locate the file under magnification in the root canal emulate what will happen in clinical practice (Shabahang et al. 1996).
When extraction of the tooth has not been possible, studies have used radiographs to verify the canal length.
This method introduces the problems associated with radiographic working length assessment and working in two dimensions with anatomic variation and distortions (Cox et al. 1991).
In vitro studies use electro conductive materials to simulate the clinical situation.
Researchers have found alginate, gelatine, agar or saline to be media that give predictable results with apex locators when compared with tooth length (Czerw et al. 1994, Meares & Steiman 2002).
Some of these media can leak through the apical foramen and cause premature readings (Czerw et al. 1994).
It appears that some in vitro experimental models give greater accuracy than can be achieved clinically (Czerw et al. 1995).