2010 RWISO Journal - Roth Williams International Society of ...

found that there can be extreme variation in the soft tissue
around the ear. 34 This variation can make it difficult to locate
the hinge point with an arbitrary earpiece face-bow.
The present study found larger mean values for the arbitrary
earpiece face-bow measurements. This suggests that the true
hinge face-bow may not be as sensitive to anatomical changes
as the arbitrary earpiece face-bow.
Goska and Christensen conducted a similar study to
to the present study, in which they compared the positions
of maxillary cast permanent first molars in three planes of
space, using four different face-bow techniques. A true hinge
face-bow determined axis point was chosen as a baseline
against which to compare the other three arbitrary face-bow
techniques. 28 They found that deviations between this baseline
and the other three face-bow mountings ranged from 1.5
mm to 4 mm. 28 They found that deviations between the true
hinge face-bow and the arbitrary earpiece face-bow ranged
from 1.9 mm to 3.8 mm. Like the authors of the present
study, they concluded that variations in the arbitrary earpiece
face-bows might have resulted from naturally occurring
variations in ear anatomy or the fact that the arbitrary
earpiece face-bow is an average measurement. 28
In general, the present study suggests that error introduced
from arbitrary earpiece face-bow hinge axis location
may produce occlusal discrepancies caused by malpositioning
of the maxillary cast. The present study differs from
other previous studies in that it evaluates changes at the
occlusal level of the maxillary cast, as opposed to looking
at the joint level when comparing arbitrary and true hinge
mounting techniques. This study also differs from previous
studies in that it does not measure the occlusal discrepancies
that result from contacts during the mandibular arc of
closure, since the mandibular cast was not incorporated into
the measurements.
Zuckerman, in analog tracing the arc of the incisal edge,
verified that no A-P change occurred in the arc of closure, as
long as the mandible rotated along the accurate hinge axis.
However, when an error in the arbitrary earpiece face-bow
hinge axis occurred anterior to the true hinge, the incisor arc
of closure was anterior to the actual arc of closure, and when
the arbitrary earpiece face-bow hinge axis occurred posterior
to the true hinge axis, the opposite effect occurred. Errors in
the vertical position of the arbitrary earpiece face-bow hinge
axis were found to produce the largest A-P discrepancies
upon mandibular closing41 (Figure 9).
52 Freeland et al | Comparison of Maxillary Cast Positions
Figure 9-a True hinge mandibular cast vs. estimated hinge
maxillary cast: True hinge mounting.
Figure 9-b True hinge mandibular cast vs. estimated hinge
maxillary cast: Estimated hinge mounting substituted for true
hinge maxillary mounting.
Zuckerman found that an anterior incisor displacement
of 1.5 mm could occur if the arbitrary hinge axis was off from
the true hinge axis by approximately 10 mm. 41 Although the
method for the present study does not incorporate the mandibular
cast arc of closure, wax bite thickness, or condylar
positioning, it is interesting to note that the largest discrepancy
in maxillary cast position occurred in the A-P direction
with a mean difference greater than 3 mm in all three areas
measured (maxillary right and left first permanent molar and
the upper right permanent central incisor).
Gordon et al conducted a mathematical study to calculate
the amount of cusp height and mesiodistal error at the
second molar that results from arbitrary earpiece face-bow
hinge axis location 5 mm and 8 mm anterior, superior, posterior,
and inferior to the true hinge axis. 6 They concluded
that incorrect location of the hinge axis caused a positional
change in the occlusal relationship between the maxilla and
the mandible, resulting in various premature contacts. De-