The easiest way is to put a rubber stop on the needle at the apical length and see if the needle reaches the 1mm point.
If you have made a Parabola in the prep sometimes by sliding the needle in and out you can feel the lip of the Parabola.
The dentist holds the syringe in one hand, and once the needle is in place, gives the syringe plunger a small 'nudge' actually about 1/16 to 1/8 of a turn and waits a few seconds for the cement to flow to the apex and begins the withdrawal process without turning the plunger again.
Once the needle is in the Parabola, it is safe for the dentist to use a mirror to look up into the canal and instruct the dental assistant to turn the plunger as the canal fills and the needle is withdrawn.
If the cement comes out too fast and the needle is not withdrawn fast enough there could be a pressure buildup that flows cement through the apex if the apex is large and/or the tissue at the apex is gone via resorption.
If the needle is withdrawn too fast and not enough cement is extruded, gaps can be left in the cement which will have to be repaired later.
To get a needle into a molar it is necessary to bend the needle.
It is not, however, necessary to put a permanent bend in it. It can be simply curved and thus is still free to rotate should that be necessary, much like a flexible shaft.
For consistency the tip of the needle must be 1mm from the apex.
That is often easier said than done. Various tricks can be used.
The tip of the needle might have to be curved or even buffed with a rubber wheel to make it smoother.
A rubber stop or even a magic marker pen mark can help you determine where the tip of the needle is.
If there is any doubt, a broken off needle can be inserted to what you think is proper and a radiograph taken. It is very difficult to insert the needle while it is attached to the syringe and take a radiograph.
If the canal curves sharply the blunt end of a needle can gouge into the outside wall of a canal and prevent apical movement. This is even true in cases where a similarly sized file will go but the needle won't because the tip of the file is conical and will slide around a curve.
If you curve the tip of the needle properly and the direction of the curve does not match the direction of the curve of the canal, it will not go. The solution is simple, simply rotate the needle and syringe(thus the admonition not to actually put a bend in it) until it drops into place.
In many canals the curve is in the neck or the Mc Combs part of the prep.
Make sure you hold the hemostat tightly against the needle. And no you won't collapse the needle...unless you press REALLY hard.
To enable the needle to pass around the curve.
A 'nudge' is the smallest amount you can turn the plunger and do it consistently.
"Fine Tune" the syringe by turning the plunger clockwise until the cement flows and then counter-clockwise until it stops flowing, using smaller and smaller increments until the SLIGHTEST 'nudge' clockwise starts the flow and the slightest 'nudge' counterclockwise stops it. THIS is your nudge.
Once you determine what a nudge is then you can determine the volume of cement that flows per second, keeping in mind that the diameter of the string of cement is about .20mm.
The inside of a 30 gauge needle is 0.15mm but the cement expands a bit as it flows out to produce a cylinder of cement that is 0.20mm.
A size 15 apex is 0.15 mm diameter. The ratio of the cylinder of cement to the size of the canal is 20 to 15 or 4 to 3 and the ratio of their cross sectional areas is 4 square to 3 squared or 16 to 9 or about 2 to 1.
Thus it will take .5mm of cement flowing out of a 30 gauge needle to fill one mm of size 15 canal.
Obviously this is one to one.
30 to 20 or 3 to 2 or 3 square to 2 squared or 9 to 4 or about 2 to 1 so it takes about 2mm of cement to fill 1mm of size 30 canal.
50 to 20, 5 to 2, 25 to 4 or about 6 to 1 so it takes 6mm of cement to fill 1 mm of size 50 canal.
Isn't math fun?
after giving the plunger a nudge it takes a few seconds to start the flow, and to fille the neck and to get a puff beyond. Usually I wait about 2-4 seconds, depending on whether the apex is vital or not. If I sense that the cement is going to flow fast I wait less.
Thicker materials flow slower. Allow a little extra time.
Thinner materials flow slower. Generally I recommend mixing ZOE very thick so this should rarely be a consideration. But just in case someday someone comes out with a good sealer that is thin.
Cement may stick to the needle and come out with the needle. If this happens, GOOD, you shouldn't be filling a wet canal anyway.
If you leave the needle in place and cement flows up around it and you pull the needle out, it will leave a needle shaped hollow in the filled canal. Generally simply reinsert the needle to the same depth as the hollow and refill.
By flexing the needle against the side wall of the canal and extruding slowly the friction of the cement against the canal wall will keep it in place.
As you withdraw make sure that you can see the 'white worm' of cement crawling back past the needle and you will avoid voids. If you keep pulling and pushing on the needle while backing out, you will leave horizontal bands of air.
Generally if there is a void in the neck or Mc Combs you must remove all the cement and start over.
If the void is in the pressure relief areas(parabola) you can simply insert the needle beyond the void and reinject.
Remember to extrude WHILE withdrawing the needle.
Once the canal gets nearly full you can hold a wet cotton pellet over the orifice and pull the needle out. Try to avoid putting pressure on the full canal as this can, in instances of low tissue fluid pressure or large apices, force cement out the apex.
Some people use a rubber stop for the same purpose.
Silicone is very thin and sticky. It rarely comes out with the needle. If it does it means that the silicone was already curing while you were injecting. Take it all out, make a new mix and start over.
If you absolutely MUST use a gp cone, make sure it is small and gently tease it into the cement. Pushing hard on it will likely push cement through the apex.
A small puff in a vital case is almost never a problem. There are already macrophages there and it clears up quickly. Sometimes in days.
If the periapex is healthy, very often a small puff of cement will never absorb, nor does it cause much of a problem.
Even a gross overfill in a non-vital case is often asymptomatic. The reason being is that there is room for it and the inflammatory process has already gone chronic with plenty of macrophages.
The only case you have to worry about is putting large amounts of cement past the apex in vital cases.
This can happen only in one of two ways. 1) you drilled a hole for the cement by over instrumentation or 2) you put so much pressure on it that you crushed bone in the process. In either case a lot of inflammation, pressure and pain are likely and a apicoectomy or at the very least trephination will be needed if the patient doesn't visit the oral surgeon in the meantime.