If the buccal cusps of the maxillary molars occlude lingual to their normal relationship with the mandibular molars, what has occurred?

Correct answer: C. Posterior crossbite

• A. Class III is an ANTEROPOSTERIOR relationship defined by the molar and canine relationship in the sagittal plane.
• B. A scissor bite (Brodie bite) is the OPPOSITE transverse problem — the maxillary lingual cusps occlude buccal to the mandibular buccal cusps, so the maxillary teeth completely overlap the mandibular ones.
• C. CORRECT. In a normal posterior relationship the maxillary buccal cusps occlude buccal to the mandibular buccal cusps. When the maxillary buccal cusps fall lingual to that position, the transverse relationship has reversed — that is a posterior crossbite, usually from maxillary transverse deficiency.
• D. Open bite is a VERTICAL discrepancy, not a transverse one.

Note: Always determine whether a posterior crossbite is SKELETAL (true maxillary constriction, usually bilateral, treated with palatal expansion) or FUNCTIONAL (a mandibular shift on closure from a premature contact, appearing unilateral, treated by removing the interference). Check the midline in centric relation versus maximum intercuspation — a shift on closing tells you it is functional.