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

pending upon the direction in which the arbitrary earpiece
face-bow hinge axis was displaced from the true hinge axis,
the premature contacts occurred either anterior or posterior
to the actual arc of closure. Total error that could occur at
the second molar cusp ranged from 0.15 mm of open cuspal
space to 0.4 mm of excess cuspal height. The mesiodistal
error of the second molar cusps ranged from 0.51mm toward
the distal to 0.52 mm toward the mesial. 6 Brotman also
found that a 0.09-mm A-P discrepancy would occur between
occluding casts if the arbitray earpiece face-bow hinge axis
was improperly located by as much as 3 mm from the true
hinge point. Brotman concluded that if the arbitrary earpiece
face-bow hinge axis is incorrectly placed superior to the true
hinge axis, the lower cast will occlude in a more protrusive
direction, with premature contacts on the anterior teeth.
If the arbitrary earpiece face-bow hinge axis is incorrectly
placed inferior to the true hinge axis, the lower cast will occlude
in a more distal direction, with premature contacts on
the posterior teeth. 43 This conclusion resembles the findings
of Gordon et al. Weinberg and Fox drew similar conclusions;
the values they obtained for calculated horizontal error in
cusp heights closely resembled each other. 35,44 This suggests
that errors of several millimeters in axis location might produce
occlusal errors that are clinically intolerable on the part
of the patient. 43
The authors of the present study found a mean difference
in incisor position of 3.04 mm. The occlusal discrepancies
found in the present study suggest that a range greater
than 5 mm existed between hinge axis points located with
the arbitrary earpiece face- bow mounting and the true hinge
face-bow. The discrepancy in maxillary cast position found
in this study might possibly introduce a change in the closure
of the mandible into occlusion. The problems caused
by the occlusal errors resulting from inaccurate location of
the hinge axis point are illustrated in Figure 10. The photos
suggest an exaggerated discrepancy between the two casts
because two completely different face-bow techniques were
used. They serve to illustrate occlusal error that may result
from error in maxillary cast position. In some cases, however,
the autorotated mandibular casts closed with only a
small degree of occlusal error (Figure 9). Other casts showed
severe positional changes resulting in larger occlusal errors
when this was attempted. (Figure10).
Figure 10-a Mounted maxillary estimated cast vs.
true hinge mounted maxillary cast: True hinge mounting.
Figure 10-b Mounted maxillary estimated cast vs.
true hinge mounted maxillary cast: Estimated hinge mounting
substituted for true hinge maxillary mounting.
It may be difficult to detect which patients have arbitrary
earpiece face-bow hinge points naturally located within
5 mm of their true hinge point. Therefore, if any degree of
accuracy is needed or if any change in vertical dimension,
such as an occlusal equilibration or orthognathic surgery, is
planned, use of a true hinge axis face-bow should be considered.
Previous studies have suggested that location of a
kinematic true hinge axis point prior to treatment for dentulous
patients who require extensive treatment saves time and
results in a more satisfactory occlusion. 6 The present study
found a statistically significant difference in the maxillary
cast position in all three planes of space between the two
face-bow techniques compared.
Conclusions
1. Statistically significant differences (p < .001) were
found between the true hinge face-bow mounted maxillary
cast and the estimated earpiece face-bow hinge mounted max-
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