Stable internal landmark for superimposition in the mandible

 # According to Bjork's implant studies, which of the following is considered a stable internal landmark for superimposition in the mandible?

A. The tip of the chin (pogonion).

B. The posterior border of the ramus.

C. The inner cortical border of the symphysis.

D. The lower border of the mandible.

The correct answer is C. The inner cortical border of the symphysis.

According to Arne Björk’s landmark implant studies utilizing metallic implants, the stable internal structures of the mandible that show no structural changes due to growth or remodeling—and are therefore used for longitudinal cephalometric superimposition—include:

1. The inner cortical border of the symphysis (specifically the anterior contour of the inner cortical wall).

2. The contour of the mandibular canal.

3. The trabecular structures related to the canal (the "cribriform plate" of the lower jaw).

4. The outline of the unerupted third molar germ (prior to the initiation of root formation).

Why the other options are incorrect:

• A, B, and D are all external surfaces of the mandible. Björk demonstrated that the external surfaces undergo significant, variable patterns of apposition and resorption during growth. For example, the lower border of the mandible (D) undergoes resorption posteriorly and apposition anteriorly in forward-rotating growers, while the posterior border of the ramus (B) undergoes extensive apposition to allow for the anteroposterior lengthening of the mandibular body.