Re: "Worried about mercury traveling backward through 
tooth into my blood."

(It is stated that Methyl Mercury is 100 times more poisonous 
then mercury itself !! PD)

Very limited data in our published paper "Methyl Mercury in Dental Amalgams
in the Human Mouth" indicates that almost 5 times more methyl mercury is
generated from amalgam under a "cap", "crown", porcelain veneer crown (PVC),
or porcelain veneer crown bridge than any other specimen that we tested.  I
refer to MATERIALS & METHODS SECTION, second page "Methyl Mercury in Dental
Amalgams in the Human Mouth." Sellars et al published 1994.   

Previous studies in other aquatic environments would lead to reason that when (1)
mercury, (2) bacteria or other microorganism and (3) anaerobic conditions
are present, methylmercury is probably generated.  
One could also reason from the vast literature on 
methylmercury that: (1) the greater number of
anaerobic organisms present with favorable pH conditions, the higher the
constant dosage of methylmercury.

In a case of a three unit bridge, like we tested, a pumping action is
created with pumping cycles of about 2, 000 to 3, 000 strokes per 24 hours
because the human teeth close that often; sometimes with considerable
forces.  These strong constant forces can pump methylmercury into the
sublingual venous drainage system.  Buccal, sublingual administration,
periodontal pumping action, has special significance.  

Since venous drainage from the mouth is to the superior vena cava, 
the drug (for example, nitroglycerin) or mercury poison is protected 
(not going through PD.)
from rapid first-pass metabolism by the liver.
Nitroglycerin is indicated for the acute relief of an attack of angina
pectoris due to coronary artery disease. (Emergency heart attack).  It may
be a matter of life or death.  If the heart attack victim swallows the
nitroglycerin tablet, the nitroglycerin may not be effective to save the
heart attack patient's life.  Nitroglycerin is only effective when absorbed
by pathway via retained sublingually because it is nonionic and has a very
high lipid solubility and it is protected from the rapid first-pass
metabolism by the liver.  Hepatic first-pass metabolism is sufficient to
prevent the appearance of any active nitroglycerin in the systemic
circulation if the conventional nitroglycerin tablet is swallowed.
Therefore the heart attack victim may die.

It is reasonable that strong pumping actions of the teeth and
periodontal membranes could force methylmercury as well as other species of
Hg backwards into the sublingual venous drainage system.  Absorption or
constant forced pumping action from periodontal membrane from the oral
mucosa also has special significance.

This sublingual pathway or the buccal administration of methylmercury is
not swallowed and is therefore protected from the first bypass of liver
metabolism.  Therefore, methylmercury generated from amalgam is much more
powerful than the conventional DORM standards used to measure the potency of
fish methylmercury, because fish methylmercury is swallowed and absorbed by
the GI tract, which must be detoxified by the first-pass metabolism by the
liver before reaching systemic circulation.

Please see Chapter 1, Goodman & Gilman's THE PHARMACOLOGICAL BASIS OF
THERAPEUTICS, Ninth Edition; 1996 PHARMACOKINETICS, PHARMACODYNAMICS