During a left lateral excursion, a patient has contact only on the upper left first molar (#14) and lower left first molar (#19). The upper tooth contact is on the mesiopalatal cusp, and the lower is on the mesio-buccal cusp. What type of occlusal interference is present?

Correct answer: A. Working side interference (LUBL — Lower lingual, Upper Buccal = Lingual cusp to Buccal cusp on working)

• A. CORRECT. During left lateral excursion, the LEFT side is the WORKING side. Contact on the working side molars between the mesiopalatal cusp of the upper molar and mesiobuccal cusp of the lower molar represents a WORKING side interference. Per the BULL rule, working side contacts should ideally occur on buccal cusps of upper teeth to lingual cusps of lower (B-U-L-L). The identified contact is a working side event.
• B. Non-working (balancing) side interference involves contact on the OPPOSITE side from the direction of excursion (right side during left lateral movement). The described contact is ipsilateral (left side) = working side.
• C. Centric occlusion premature contacts occur when the patient closes into maximum intercuspation, not during lateral excursion.
• D. Protrusive interference occurs during forward (anterior) movement, not lateral excursion.