Life isn't perfect and neither is any method of filling and sealing root canals. What we are after is consistency as far as humanly possible and thus we have to take into account the human machine operating the endo instruments.
We are looking for a way to minimize the overfill but yet be sure that whenever humanly possible we get the apex sealed. As a result, if we have to err, we err on the side of a smaller or larger overfill rather than the seal vs. non-seal of the apex.
We have learned through trial and error that an overfill causes some problems in the short run (as long as it is not gross, or into the mandibular canal) but underfills can mean ultimate failure of the root canal months or even years in the future.
With cement fills, you can't cheat. Cement shows up on xrays and if a root canal looks good, it generally is. If it is bad it looks awful.
One of the good things about cement fills is that the cement stays fluid for enough time to make corrections in the fill even going so far as to completely remove and re-treat. If it is a bit short we can nudge the material apically and still maintain our seal. After all, we flowed it in in the first place, we can flow it a little farther if we are short.
So what we have to come up with is a practical way of doing the fill that gives us what we want.
With minimal practice the average dentist can consistently fill root canals with a syringe and deliver cement in quantities small enough that it is possible to deliver a 'puff' of cement without creating an overfill.
In some techniques it is necessary to apply an excess of pressure to get the cement to flow into the 'nooks and crannies' of the canal space because lateral canals and irregularities are filled with a combination of crushed dentin, pulp tissue and bacteria, sometimes actually plugging the apex.
If the filing method removes that debris, then all is necessary is to flow the cement into the canal under minimal pressure and it will adapt to the irregularities without applying pressure.
If you are interested in numbers by turning the plunger 1/8 of a turn you deliver 0.1666 cubic millimeters of cement or 0.0001666 cc.
When fluid flows through a pipe, there is friction and this friction increases with increases in viscosity. For example, syrup flows slower than water. Grease flows slower than oil. Thick root canal sealer flows slower than watery cement.
Thus when the plunger on a syringe is pushed(or turned in our case) the cement continues to flow after the turning stops. The highest flow rate is immediate and it tails off for several seconds...the number of seconds depends on the thickness.
In our case, we mix our cement VERY thick making it possible for us to turn the plunger and actually time the flow limiting the flow to the amount of time we leave the needle in place.
Since the inside of the needle is pretty much standard we can't control this factor but it does have some bearing on the inside of the canal when you consider that the flow rate through a canal might be affected by the coarseness or smoothness of the canal wall.
When fluid flows through a pipe it first coats the inside of the pipe and then the fluid actually flows through the coating. That coating is a function of the thickness and stickiness of the fluid.
Since ZOE is not technically a fluid but a suspension of particles in a fluid, the actual thickness of the 'fluid' is the particle size, the coating is particles in fluid it actually makes the already small thirty gauge needle (inside diameter of 0.15mm) even smaller.
The smaller the pipe the slower the flow.
As the distance from the exit port of the needle gets greater the friction drops the pressure. As the tip of the needle gets further from the apex, the lower the pressure and the less likely it is that the cement will flow out the apex.
We use this to good advantage by backing away from the apex as soon as we are confident that the apex is sealed and filled.
Helping us control the flow OUT of the apex is back pressure, i.e. the resitance of the tissues beyond the apex to allow flow past the apex.
Fistulas provide no back pressure whatsoever and thus any cement that flows TO the apex can flow OUT of the apex.
Almost the same as fistulas although there is a small amount of resistance.
Granulomas are filled with tissue, soft friable tissue but tissue that can only be displaced, not compressed.
A vital apex consists of periodontal ligament tightly packed into the PDL space. Cement cannot be flowed into this space, it must be forced i.e. the tissue must be displaced and the bone distorted to get an overfill in a vital case. It CAN be done but only through ignorance or deliberate bypassing safeguard built into the technique.
When I see cement that has flowed through the apex, it can only be through crushing of bone or by drilling a hole in the bone by overfiling.
Cement cannot flow through blocked canals. That would seem to be obvious. What is not so obvious is that neither can air.
When a needle does not reach the end of prep that is blocked by debris, as the cement attempts to flow to the apex, the air is compressed and as soon as the pressure is relieved, it pushes back, creating a void.
Thus if you want to fill a canal that is not prepped to the apex and there is debris in the apex, either you must put enough pressure to extrude the debris, followed by the air, followed (usually) by a gusher of cement. This is NOT a good idea.
The smaller the apex, the slower cement can flow through it.
An apex whose diameter is larger than the stream of cement extruding throught the needle, i.e. size 20 or larger offers no resistance to flow.