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16. Appendices
PCT-90LH User Manual 189
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Superior-Inferior (Up-Down) Positioning Error
Positioning the dental arches within the superior- inferior (up-down) dimension of the
focal trough can be difficult to achieve, especially with children whose smaller size
reduces the distance between the shoulders and the inferior border of the chin. When
the arches are positioned incorrectly in the superior-inferior direction, the image
exhibits multiple distortions, including increased overlapping in the premolar regions.
When the arches are positioned too far up or down, the teeth will simultaneously move
into a position that is too far back or too far forward, respectively, out of the focal
trough.
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Positioning the arches too far superiorly produces a characteristic "frown" or flat,
downward- turned appearance to the occlusal plane (Figure 5) . The condyles flare out
and off the edges of the image and the palate appears as a widened, thick, dense
radiopacity. This positioning error results in a widened appearance of the palate and
obliterates the apical regions of the maxillary teeth, compromising the images of the
unerupted developing dentition. As the maxillary arch tips upward, the anterior teeth tilt
backward producing the same widened appearance that results from an incorrect
anterior- posterior position. Positioning the arches too far inferior produces a
characteristic "smile" appearance or upward curve of the occlusal plane, with the
condyles tilting inward toward the center of the image
(Figure 6) . Depending on the
severity of the downward position, the vertebrae may also curve inward and appear
superimposed over the condyles, and the hyoid bone may be superimposed over the
mandible blocking a clear view of the erupted and unerupted mandibular teeth.