Which statement correctly describes how the retentive arm interacts with the survey line and undercut?

Which statement correctly describes how the retentive arm interacts with the survey line and undercut?

• A. The terminal tip extends below the survey line into the undercut
• B. The arm remains above the survey line
• C. The arm does not interact with the undercut
• D. The arm is always on the lingual surface

Answer: (A)

Retention in a surveyed tooth comes from the retentive arm engaging the tooth’s undercut. The survey line marks the boundary above which tooth structure is not undercut; below that line lies the undercut that can grab the retentive arm. To achieve retention, the terminal tip of the retentive arm must extend into and sit within that undercut, below the survey line. If the arm stays above the survey line, it won’t engage the undercut and retention is lost. The arm’s position isn’t fixed to the lingual surface; it can be on different surfaces depending on the tooth and path of insertion.

Retention in a surveyed tooth comes from the retentive arm engaging the tooth’s undercut. The survey line marks the boundary above which tooth structure is not undercut; below that line lies the undercut that can grab the retentive arm. To achieve retention, the terminal tip of the retentive arm must extend into and sit within that undercut, below the survey line. If the arm stays above the survey line, it won’t engage the undercut and retention is lost. The arm’s position isn’t fixed to the lingual surface; it can be on different surfaces depending on the tooth and path of insertion.