If a periapical view shows the tooth apex off the top of the film, what is this fault called?

The situation described, where the tooth apex is off the top of the film in a periapical view, indicates that the angle of the X-ray beam was not positioned correctly. This fault is referred to as "elongation."

When X-ray films are taken, the goal is to project the image accurately onto the film so that all pertinent anatomical structures are visible. If the angle is too flat or the vertical angulation is insufficient, the roots of the teeth, particularly the apex, may extend beyond the upper limits of the film. This leads to a distorted image where the actual height of the tooth is misrepresented, resulting in "elongation."

In practical terms, elongation can make it difficult for dental professionals to accurately diagnose conditions related to the tooth roots and surrounding bone because not all necessary anatomical information is captured. Correct positioning and angulation are crucial in radiography to ensure the entire tooth and its surrounding structures are properly recorded on the film.