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Prosthodontics & Occlusion · patient case with 2 questions

Mobile anterior crowns

Patient: Male, 58 years old

Chief Complaint: “My two front crowns wiggle when I bite.”

Background: Porcelain-fused-to-metal crowns on teeth #8 and #9 placed six years ago. Grinds at night, no appliance. Otherwise healthy.

Findings: Grade I mobility of #8 and #9. Wear facets on the incisal edges. Heavy contact on the anterior teeth in excursions; posterior teeth show minimal wear.

Question 1

Which tooth surface and which mandibular movement are most likely responsible for the mobility of these crowns?

  1. The incisal edges, during maximum intercuspation
  2. The lingual surfaces, during protrusive movement
  3. The lingual surfaces, during lateral working movement
  4. The facial surfaces, during protrusive movement
Show answer and explanation

Correct answer: B. The lingual surfaces, during protrusive movement

  • A. In maximum intercuspation the anterior contact should be light or absent, and any load there is closer to axial than the excursive load is.
  • B. CORRECT. The maxillary anterior teeth contact the mandibular incisors on their LINGUAL surfaces, and it is the PROTRUSIVE excursion that loads that contact. Because the maxillary incisors are inclined facially, protrusive contact directs force labially rather than down the long axis — a non-axial load that produces mobility. This is exactly why anterior guidance must be evaluated whenever anterior crowns loosen.
  • C. In lateral working movement the CANINES should be doing the disclusion. If the incisors are carrying working-side load, that is a separate problem, but protrusive is the movement that specifically loads the central incisors.
  • D. The facial surfaces of maxillary anteriors have no antagonist and receive no occlusal load at all.

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Question 2

Which muscle is the PRIMARY protruder of the mandible?

  1. Lateral pterygoid
  2. Temporalis
  3. Medial pterygoid
  4. Masseter
Show answer and explanation

Correct answer: A. Lateral pterygoid

The lateral pterygoid muscle, via its inferior head pulling the condyle/disc forward along the articular eminence, is the primary muscle responsible for mandibular protrusion. Temporalis and masseter are primarily elevators (and temporalis also retrudes via its posterior fibers), and medial pterygoid mainly assists in elevation and contributes to some protrusive/lateral movement, but is not the primary protruder.
  • A. The mylohyoid forms the floor of the mouth and elevates the tongue and hyoid during swallowing; it is a weak depressor of the mandible, not a protruder.
  • B. The digastric depresses the mandible and retrudes it, working with the other suprahyoids when the hyoid is fixed — the opposite of protrusion.
  • C. The masseter is a powerful ELEVATOR, running from the zygomatic arch to the lateral ramus and angle. The medial pterygoid assists protrusion but is chiefly an elevator.
  • D. CORRECT. The inferior head of the LATERAL PTERYGOID runs from the lateral pterygoid plate backward to the condylar neck. Contracting bilaterally, it pulls both condyles forward and downward along the articular eminence — protrusion. Acting on one side only, it produces contralateral lateral movement. It is also the only muscle of mastication that opens rather than closes.

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