During endodontic access on a maxillary first premolar, which wall carries the worst prognosis?

Correct answer: B. The mesial wall

• A. The distal surface is comparatively convex and has more bulk of dentine.
• B. CORRECT. The maxillary first premolar has a pronounced MESIAL developmental concavity (the canine fossa) running down the root. Dentine there is dangerously thin, so aggressive instrumentation or an off-axis post space readily strips or perforates it. A mesial strip perforation is very hard to seal and often ends in extraction.
• C. The buccal root has adequate thickness, though it is the narrower of the two roots bucco-palatally.
• D. The palatal root is generally the widest and most forgiving of the two.

Note: Thin-dentine danger zones worth knowing: the mesial concavity of the maxillary first premolar, the distal wall of the mesial root of mandibular molars (the classic furcal 'danger zone'), and the furcal floor of any molar. Keep files centred and pre-curve to stay off the concavity.