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

points. They found that the mean distance between any two
points was 1.1 mm. More than 96% of predetermined points
were within 2 mm of the true hinge axis. 23 Schallhorn also
found that approximately 98% of all true anatomical hinge
axis points were within a 5-mm radius. 26
In comparison, studies that compared maxillary cast positions
mounted with four different face-bows showed wide
variation in the mounted maxillary cast positions. All arbitrary
hinge axis points deviated from the true hinge baseline
point by anywhere from 1.5 mm to 4 mm. Therefore, the
authors of these studies concluded that it was not possible
to establish the clinical superiority of one arbitrary face-bow
over another. 28,29
Lauritzen and Bodner located 100 true hinge points on
50 subjects. They found that 67% of the axis points were
5 mm to 13 mm away from the arbitrarily marked hinge
points. This discrepancy may introduce gross errors in the
mounting of the casts on an articulator, resulting in large
occlusal errors. 30 Palik et al got similar results. They found
that only 50% of the arbitrary hinge axes located with the
arbitrary earpiece face-bow were within a 5-mm radius of
the terminal hinge axis. This indicated that the arbitrary
earpiece face-bow hinge axis location does not represent the
total population. 31 Schulte et al concluded from their study
that errors in locating the arbitrary hinge axis will produce
a three-dimensional occlusal error. 32 This study and others
have recommended that if a thick vertical dimension of wax
was used for an interocclusal record, or if the vertical dimension
will be changed with treatment, a true hinge axis should
be located on the patient. 32,33 Due to anatomical variations,
the arbitrary earpiece face-bow may introduce significant errors
in an A-P or vertical dimension, resulting in mandibular
displacement. 34,35 The only way to be relatively certain that
errors due to malpositioning of maxillary casts on an articulator
have been avoided is to locate the true hinge axis. 30,36-40
Studies indicate that coincidence between the two hinge
axis points does not usually occur. 41 This results in a discrepancy
between the arbitrary hinge axis and the true hinge axis
points. This discrepancy will cause changes in the mounted
position of the maxillary cast, which in turn can produce a
positional change of all teeth in the three planes of space. 41
Zuckerman mathematically demonstrated that discrepancies
between the true hinge axis and the arbitrary hinge axis points
can produce changes in the A-P direction of the occlusion. He
verified in his analog tracing that the arc of the incisal edge
does not change in the A-P direction in centric occlusion, as
long as the mandible is also coincident in centric relation.
However, when an error in the arbitrary hinge axis occurs
and it is anterior to the true hinge, the incisor arc of closure
is anterior to the actual arc of closure. 41 Errors in the verti-
cal position of the arbitrary hinge axis (AHA) produce the
largest A-P discrepancies upon mandibular closing. 41 Other
authors have graphically illustrated how errors in true hinge
axis location can produce occlusal aberrations. 33,35,36,42 These
authors also showed that the greatest errors occurred when
the hinge axis was incorrectly located in a vertical direction
perpendicular to the correct hinge axis closure. An arbitrary
hinge axis positioned superior to the true hinge axis also
produced premature contacts on the anterior teeth. In addition,
if the arbitrary hinge axis was placed inferior to the true
hinge axis, premature posterior contacts occured. 33,35,36,42
Brotman’s geometric representation related changes in
the hinge axis point locations between the true hinge axis
and the arbitrary axis to differences produced at the occlusal
level in mounted casts. 43 Brotman concluded that “if the
hinge axis has been improperly located by as much as 3 mm,
the error at the occluding position of the casts (anteroposteriorly)
will be about .09 mm or less than 1/250 inch.” 43
Gordon et al looked at the location of the terminal hinge
axis and its effect on the second molar cusp position on the
position of the second molar cusp. 6 Their results showed that
incorrect anterior location of the hinge axis produced the effect
of having moved the mandibular arch backward. Incorrect
posterior location of the hinge axis produced the effect
of having moved the mandibular arch forward. Incorrect inferior
location of the hinge axis caused slight retrusion of the
mandibular cast with premature posterior contacts. Incorrect
superior location of the hinge axis caused protrusion of the
mandibular cast with premature anterior contacts. 6
Since studies vary in reporting the percentage of placement
of the arbitrary hinge axis less than 5 mm from the true
hinge axis, it can be assumed that larger errors in occlusion
may occur. It has been found that an occlusal discrepancy of
0.01 inch can cause pulpitis or periodontal disease, though
the patient may not be able to detect so small a discrepancy. 44
To limit occlusal errors in mountings, it is necessary to locate
the hinge axis to within 1 mm, and the kinematic true hinge
can be done to this degree of accuracy. 44 Therefore, the importance
of the true hinge axis is substantial when changing
the vertical dimension upon mandibular closure. 38
Orthodontics deals specifically with the movement of
all teeth and their occlusal fit. Therefore, it calls for extreme
accuracy during diagnosis, treatment planning, and rendering
treatment. 9 Clinically finding the true hinge axis may be
the only way to ensure a reproducible and accurate starting
point—one from which optimum esthetic and functional results
can be obtained. 6,38,45 The purpose of this study was to
compare the maxillary cast mountings of 51 patients in three
planes of space when mounted using a true hinge axis facebow
versus an arbitrary earpiece face-bow.
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