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Research has demonstrated that 100% of all root canals
result in residual infection due to the imperfect seal that
allows bacteria to penetrate. The toxins given off by
these bacteria are more toxic than mercury. These
toxins can cause systemic diseases of the heart, kidney,
uterus, and nervous and endocrine systems.  Edward
Arana, D.D.S. http://www.curezone.com/dental/ 

.... Cavitations or NICO's occur when bone is deprived of its blood
supply and dies. When the bone dies a hole in the bone develops,
literally a cavity and into this hole migrate anaerobic bacteria.
These bacteria live without oxygen, indeed oxygen is poisonous to them
[this is why many people never feel pain where pulled teeth were or in
their root canals, not the case with aerobic bacteria in an abcessed
tooth, which utilize oxygen to live and create pain and/or pus].
Bacteria organise themselves into colonies which can be visualised as
cities. Cities require food to come in (you the patient supply the
food!) and generate waste material. The waste material made by these
bacteria is toxic in the extreme and in cavitations this toxic
material is constantly being released into the body. If Mustard Gas,
used in WW1, is taken as a yard stick, then most of the bacterial
waste products are 10 or more times as toxic than Mustard Gas.
Cavitations can occur in any bone in the body usually after infection
or trauma of some sort. Naturally dental extraction can easily be the
cause of a cavitation, this is especially true of wisdom teeth
extractions. Most extractions are due to infection, they involve
stretching and sometimes fracture of the bone all predisposing factors
for cavitation formation. Added to this is the fact that if the
membrane that holds the tooth in place, the periodontal membrane, is
not removed at the same time as the tooth comes out, this to leads to
cavitation formation. It is not common for the dentist removing the
tooth to also remove the membrane at the same time, an unfortunate
fact. ....

If the nerve inside a tooth has died then the tooth may need a
Root...[canal]. A root...[canal] is simply material put into a tooth
where the nerve used to be. Traditionally a rubber like substance
called Gutta Percha is used. This can contain mercury. Other materials
include Formaldehyde, Cadmium, Steroids or even concentrated Sulphuric
Acid. The problem with root canals is both the toxic nature of the
materials used and the fact that they do not fill fully the open
spaces inside the tooth