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I 39 research _ working length I roots1_2010 difference between real values and the PC screen values when the EFL indicated ‘past apex’ and ‘apex’ (Fig.6a).WhentheMedicNRG-Blueindicated0.3mm to 1.4mm from the apex (Fig. 6b), the differences rangedfrom110to158µm,respectively.Thesefigures coincide with the values of the XFR model, revealing thatbothEFLshadhighprecisionandaveryhighlevel ofresolution.Differencesarefarbelowtheacceptable 0.5mm and therefore have no relevant influence on clinical work. Prof Moshonov’s tests also confirmed that the closer the measuring file tip is to the apex, the more precise the readings are and the higher the resolution of the EFL is (Fig. 6a). When the tip of the measuringfileisabitfartherfromtheapicalforamen, the readings coincide slightly less with the real distances (Fig. 6b). While testing the Raypex 5, we asked ourselves a question that every practitioner might ask: can we trust the values indicated on the EFL display and can we rely on the manufacturer’s instructions? The measuring device clearly showed values of 0.5mm and0.8mm(Fig.7),provingthattheEFLveryprecisely indicates the position of the file tip with respect to anatomical details, since the distance from the fora- men to constriction is 0.5 to 1.0mm. We recommend that practitioners follow what the display indicates aswellasthemanufacturer’sinstructions,butrecon- sider unusual or strange readings. The last test conducted in our laboratories aimed todeterminewhetherdifferentEFLsdisplaythesame values for the same distance in the same root canal. Thetipofthefingerspreader#15wasintroducedinto the canal until it reached the plastic plate barrier firmly placed at the plane of the anatomical foramen (Fig. 8). The tooth was normally mounted and each of the EFLs immediately indicated that the spreader tip wasbeyondtheforamen.Presumably,thiswasdueto the gelatine embedding medium inside the external portion of the cemental cone of the apical foramen; thusallEFLsindicatedthesame:contactwithartificial periodontal ligament. For all EFLs, the measuring device was adjusted to 0.001 mm (0.1µm) at this stage. With the micro- meter screw, the canal instrument was retreated until the display of the EFL indicated that the tip was no longer beyond but exactly at the foramen: apex reading (ApexPointer+, MedicNRG-XFR, Dentaport ZX), 0.0 reading (ProPex I) and red square segment (Raypex 5; specific marks on each EFL). At this stage, we had already received a definite answer to our question: the different EFLs do not indicate the same values for the same distance in the same root canal. After recording this value, the canal instrument was withdrawn further with the micrometer screw until the mark on the display of each EFL indicated that the tip had been moved and switched from the apical foramen to the first next mark/segment Figs. 6a & b_The MedicNRG-Blue displays extremely small differences between real values and those shown on the display: from only 1 µm (a) to 198 µm (b). Fig. 7_The Raypex 5 very accurately indicated the position of the file tip, which is approximately 0.5 to 0.8 mm between the anatomical foramen and apical constriction. Fig. 6a Fig. 6b Fig. 7