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CROWN- AND BRIDGE-WORK. 163

again, where a temporary crown is desired, to serve, as is
sometimes necessary, until the patient or operator can make
suitable engagements for more permanent work.
The objections made to the use of this class of crowns
for permanent work are that the pin or post upon which
almost the entire support of the crown is thrown acts as a
lever in the root canal, and sooner or later many of the
weaker roots are fractured, thus ending their usefulness as
a support; and again, the pin or post, entering, as it does,
deeply into the body of the porcelain, weakens it at this
point, and not infrequently do patients return with the
crowns fractured through the center from the force of mas-
tication. Then, again, unless a perfect joint is secured be-
tween the root and tlie crown, which, too frequently, is not
accomplished and decay of the root follows, undermining
the operation.
Fig. 12=;.

A general description of the Logan crown will be suffi-
cient in this place, the principles underlying the adjustment
of the scA'eral forms of porcelain crowns being practically
the same.
The Logan Crown.—After all remnants of the natural
tooth ha^'e been removed, the canal is enlarged to receive
the pin of the crown, with suitable sized reamers. With a
root facer a labial slope is given to the root end, so that the