Operative dentistry aje qualtrough, jd satterthwaite la morrow, pa brunton

Canal orifice opening
The entrances to the canals can be identified with the aid of a sharp
probe such as a DG16. There is no sharp delineation between the
access preparation stage and canal cleaning and shaping, and to a
large extent, the transition stage occurs when initial widening of 
the canal orifices is carried out. This stage is a natural extension 
of access preparation, and leads to canal cleaning and shaping. Not
withstanding the above, this stage involves use of either hand or
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rotary instruments to widen the orifices of the canals. A sequential
series of instruments should be used in decreasing order (larger to
smaller), sometimes referred to as ‘stepping down’.
The aims of this procedure are to remove any features of the canal
wall anatomy which may preclude straight-line access to the canal 
terminus, to create a greater space in which irrigating solutions are
effective and to assist the ease of canal location (Figs 3.4, 3.5). Gates
Glidden drills are effective for this procedure (see section on cleaning
and shaping).
ENDODONTIC INSTRUMENTS
ISO standardisation
Before a standardised configuration was adopted, endodontic instru-
ments varied greatly from one manufacturer to another. The con-
figuration adopted is that set by the International Organization for
Standardization (ISO) and is laid down in ISO specification No. 3630.
All ISO hand instruments, along with paper points, silver points 
and standardised gutta-percha points, conform to this system. ISO
standardised files have a cutting length of 16 mm, have a specified
diameter at the tip (termed D
1
) and increase in diameter by 0.02 mm
for each millimetre along the file, so that at the end of the cutting part
(16 mm along the file) the diameter (termed D
2
) is 0.32 mm greater
than at D
1
. This is called an .02 taper. Files may vary in length, any
extra length is provided by a ‘blank’ portion. The nominal size of