Endodontics.
Initial Negotiation of Calcified and Blocked Canals
Initial negotiation means taking a small file all the way to the terminus. In some wide canals it is easy, in narrow and / or curved canals it can be more difficult.
If a canal can be negotiated to its terminus, the chances of success are greatly improved. Because canal calcification starts coronally, there is in theory no reason for failing to complete negotiation once started – the “calcified apex” does not exist in canals that are patent coronally.
Small stainless steel hand files are used for negotiation: they are stiffer than equivalent size Nickel Titanium and can push through obstacles more easily.
When Rotary NiTi is to be used, a Glide Path is created during negotiation. NiTi files are (mostly) not end-cutting – they need a path to follow that is bigger than their tip size.
The Narrow Canal: File Binding and Pre-flaring
Stainless Steel files are tapered - normally 2%. This means they get wider nearer the handle. A size 15 might have a 15 tip, but 10 mm up it has a size 35 diameter. A size 8 file has a 28 diameter 10 mm up.
Imagine a canal that has parallel sides, and is size 20 (0.2 mm) diameter. Your 15 file will enter it nicely, but before it has gone far, the file sides will be binding against the canal wall. You might think you have reached a blockage, and it is very hard to tell the difference between a blockage and a "bound file".
With a narrow canal, the only way to avoid this is to use a pre-flaring technique. We recommend a step-back method as follows:
Insert size 8 until it binds. Work it a little.
Insert size 10. It won't go as far as the 8. Work it a little.
Insert size 15. It won't go as far as the 10. Work it a little.
Continue this through sizes 20,25, and 30.
Now go back to the 8. It will easily drop into the space made by the other files, and go quite a way further before its sides bind on the wall again.
In fact, it will often go all the way to the terminus
If it doesn't, do a second wave of step-back pre-flaring up to size 30.
This time the terminus should be reached.
Video: Initial Negotiation of Narrow Canal
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The Blocked Canal
There are plenty of things that can block the progress of your file down the canal – inexperienced operators might blame "apical calcification and give up rather than try to identify the real cause.
Ledging
If your file is loose in the canal, but seems to hit a brick wall, you have probably encountered a steep curve. The more you hit the wall at the curve, the more likely you are to dig a pit that will prevent you getting round the curve. This is called ledging.
What should you do if you think you have ledged a canal?
Mud / Collagen blockages.
Mud and collagen blockages are typically seen when you negotiate fine with an 8 and a 10, but find the 15 won’t go down. You try the 8 and the 10 again, and find (depressingly) they won’t go down either!
Mud
Negotiation creates dentine chips which can be pushed up the canal and, mixed with fluid, create mud. This mud can be packed down by larger files acting as a piston against it.
How can you prevent mud from blocking the canal?
Collagen (vital cases)
If a pulp is vital, small files like size 8 and 10 will stab it, causing it to bleed and possibly create a collagenous blood clot. When larger files are later used, they can pack the collagen (again acting like pistons) and create a blockage.
How can you prevent collagen from injured pulp stumps blocking the canal?
Steel
A canal can become blocked when a file separates (fractures). The file tip, which may be very small, remains in the apical part of the canal.
How can you routinely detect a separation before you use the next file?
How can you minimise their occurrence?
Restorative materials
These love to fall into canals and block them. Temporary filling like Kalzinol should be completely removed before entering a canal. Working with a wall of Kalzinol is inviting trouble.
When an access cavity needs modification and the orifices are exposed (e.g. during dynamic access), this often means cutting back gold, porcelain, amalgam, composite etc.
How can you prevent restoration debris entering the canals during access cavity modification?
Summary
Preventing blockages is easy; remembering to do it religiously is the hard part !