Physiology & Biochemistry · patient case question
Which muscle is the PRIMARY protruder of the mandible?
Patient case: 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.
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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