Posterior crossbite — occlusal interferences

Correct answer: B. Lingual cusp of the lower and buccal cusp of the upper

• A. Buccal-to-buccal contact alone is incomplete; working-side excursive contacts involve paired inclines, and in crossbite the pairing inverts.
• B. Correct per the recall key. In a NORMAL (non-crossbite) relationship the working-side contacts that are adjusted follow the BULL rule — Buccal Upper, Lingual Lower — i.e. the inner incline of the maxillary buccal cusp against the outer incline of the mandibular buccal cusp, and the maxillary lingual against the mandibular lingual. Because these teeth are in CROSSBITE, the buccolingual relationship is reversed and the mnemonic must be re-derived from the actual anatomy rather than applied by rote.
• C. Lingual-to-lingual contact alone is incomplete for the same reason.
• D. This describes the classic NON-WORKING (mediotrusive/balancing) interference in a normal occlusion — maxillary lingual cusp against mandibular buccal cusp. It is not the working-side answer given by the source.

Note: ⚠️ CONTESTED / HIGH-DIFFICULTY. The recaller explicitly said they were unsure of this set. Study the underlying anatomy rather than the mnemonic: BULL (Buccal Upper, Lingual Lower) identifies the cusps adjusted for WORKING interferences and MUDL / lingual-upper-buccal-lower identifies NON-WORKING interferences — but at a crossbitten tooth the buccolingual relationship is reversed, so the standard mnemonics invert. Work it out from the picture on exam day.

Correct answer: D. Lingual cusp of the maxillary tooth and buccal cusp of the mandibular tooth

• A. Maxillary buccal against mandibular lingual is not a physiologic contact relationship in a normal occlusion.
• B. Buccal-to-buccal contact is a working-side relationship, not a balancing interference.
• C. Lingual-to-lingual contact is a working-side relationship on the palatal aspect.
• D. Correct. The non-working (balancing) interference is the classic contact between the INNER INCLINE OF THE MAXILLARY LINGUAL CUSP and the INNER INCLINE OF THE MANDIBULAR BUCCAL CUSP. These are the most destructive interferences because the contact is far from the fulcrum and the elevator muscles are at a mechanical advantage.

Note: The recall wrote this question as 'same side non working interference LLBU', which is internally contradictory — non-working is by definition the CONTRALATERAL side. Learn the standard rule as written here.

Correct answer: D. The non-working (balancing) contact between the maxillary lingual cusp and the mandibular buccal cusp

• A. Anterior guidance on the lingual of maxillary incisors is a normal protective contact during protrusion.
• B. Working-side canine contact is protective — canine guidance disoccludes the posterior teeth and is a desirable feature.
• C. A centric stop in a central fossa is a physiologic holding contact, not an interference.
• D. Correct. Non-working (balancing) interferences are considered the most destructive occlusal contacts. They occur on the side opposite the direction of movement, place the condyle in an unstable orthopaedic position, and generate high loading because the elevator muscles act with a favourable lever arm.