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16. Appendices
PCT-90LH User Manual 187
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Anterior-Posterior Positioning Error
When the arches are positioned incorrectly in the anterior-posterior direction, distortion
or ghosting of the anterior anatomy occurs. Unerupted teeth in the anterior region may
not be imaged on the radiograph if positioned outside of the focal trough. It is
important to note that an error of only 3 mm to 4 mm in either direction will result in a
significantly compromised image.
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When the arches are positioned too far anterior,
the anterior teeth will appear narrow and diminished in size. The vertebrae of the
spinal column may be superimposed over the condyles at the edges of the film and,
depending on the size of the child, may be superimposed over the rami of the
mandible blocking a clear view of the posterior teeth
(Figure 3) . When the arches are
positioned too far posteriorly, the anterior teeth will appear broad or widened. If the
position is excessively posterior, anterior teeth may be completely blurred from the
image and the condyles may be cut off from the edges of the film.
To avoid these imaging errors, the anterior teeth must occlude edge-to-edge onto the
designated area of the bite block. Achieving this position is easily compromised during
exfoliation of primary teeth, making precise occlusion difficult when one tooth or
multiple teeth are missing or partially erupted. A cotton roll may be attached to the bite
block to fill in the space created by the missing tooth or teeth. Additionally, an
adjustment may be necessary when using a laser light beam guide. The
manufacturer's instructions for directing the laser light beam at a predetermined tooth
or interproximal space usually apply to adult patients. These instructions may need to
be modified for the pediatric patient with primary or mixed dentition.