Radiology · patient case question
Where does the radiopacity in Patient A lie?
Patient case: Reading two radiographs
Patient: Two patients
Chief Complaint: A: routine recall. B: “This front tooth has to come out.”
Background: A, 44-year-old female: asymptomatic. B, 39-year-old male: tooth #8 is non-restorable following a vertical root fracture and extraction is planned with a small midline palatal flap.
Findings: A: a well-defined radiopacity is superimposed on the crown of a mandibular molar. A second periapical is taken with the tube shifted MESIALLY, and on that film the radiopacity has also moved MESIALLY relative to the tooth.
Show answer and explanation
Correct answer: A. On the lingual aspect
- A. CORRECT. This is the SLOB rule — Same Lingual, Opposite Buccal. When two images are taken with a horizontal tube shift, an object that moves in the SAME direction as the tube lies LINGUALLY (or palatally); one that moves in the OPPOSITE direction lies BUCCALLY. It is also called the buccal object rule or Clark's tube-shift technique, and it is the standard method of recovering third-dimension information from two two-dimensional films.
- B. An occlusal restoration is superimposed centrally on the crown and, like a pulp stone, moves little relative to the tooth. Where the two need separating, the discriminators are DENSITY and MARGIN — a restoration is denser with a sharp geometric outline, a pulp stone is less dense with a rounded outline.
- C. A BUCCAL object moves in the OPPOSITE direction to the tube shift. This one moved in the same direction, so it is not buccal.
- D. A PULP STONE lies within the pulp chamber, close to the centre of the tooth and therefore close to the axis of rotation, so it does NOT shift appreciably relative to the tooth when the horizontal angulation changes. That lack of movement is exactly how a pulp stone is distinguished from an object lying on a surface. It also has a rounded outline continuous with the chamber and lower density than a restoration.
Note: Three routine uses of SLOB worth having ready: localising an impacted maxillary canine as palatal or buccal, separating the two canals of a mandibular molar root, and distinguishing a superimposed anatomical structure such as the mental foramen from a genuine periapical lesion. Where precise three-dimensional relationships genuinely matter, CBCT supersedes the technique.