In a normal Class I occlusion, the maxillary first premolar (tooth 5) occludes with which surface?

Correct answer: B. The distal aspect of tooth 28 and the mesial aspect of tooth 29

• A. This is only half the answer. Every posterior tooth except the mandibular central incisor and the maxillary last molar has TWO antagonists.
• B. CORRECT. Because the mandibular teeth sit approximately one-half tooth MESIAL to their maxillary counterparts, each posterior tooth occludes with TWO opposing teeth. The maxillary first premolar (5) therefore contacts the DISTAL half of the mandibular first premolar (28) and the MESIAL half of the mandibular second premolar (29). The universal-numbering shorthand for the mandibular arch running 32 to 17 on the patient's right through to left means 28 and 29 are the mandibular right first and second premolars.
• C. The same objection applies — the canine is not the primary opposing tooth here.
• D. Tooth 27 is the mandibular right canine, which is too far mesial to be the principal antagonist of the maxillary first premolar.

Note: The two exceptions to the one-tooth-to-two rule: the mandibular CENTRAL INCISOR and the maxillary LAST MOLAR each oppose only one tooth. This half-tooth offset is what produces cusp-to-fossa and cusp-to-embrasure relationships and gives the arch its interlocking stability.