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Anatomy & Histology · patient case question

Which muscle is on the RESTRICTED side and therefore accounts for the direction of deviation?

Patient case: Painless mandibular deviation on opening with limited range

Patient: Female, 36 years old

Chief Complaint: “When I open, my jaw pulls over to the left side.”

Background: Reports daytime clenching under work stress. No trauma, no prior TMJ treatment. No systemic disease.

Vital Signs: BP 118/74 mmHg · HR 72 bpm

Findings: On opening, the mandible DEVIATES TO THE LEFT and remains deviated at maximum opening. Maximum interincisal opening 32 mm. NO PAIN reported on opening or on joint loading. No clicking or crepitus. Tenderness on palpation of the masticatory muscles on the left.

  1. The RIGHT medial pterygoid
  2. The RIGHT lateral pterygoid
  3. The LEFT lateral pterygoid
  4. The LEFT masseter, acting as the primary protruder
Show answer and explanation

Correct answer: C. The LEFT lateral pterygoid

  • A. The medial pterygoid elevates the mandible and assists in medial excursion; it is not the primary protruder and does not drive opening.
  • B. The right lateral pterygoid is the one working NORMALLY here. It is the unopposed translation on the right that produces the visible swing, but the muscle at fault is on the side the jaw moves toward.
  • C. CORRECT. The mandible DEVIATES TOWARD THE AFFECTED SIDE. Opening is a combination of rotation and TRANSLATION, and the inferior head of the LATERAL PTERYGOID is what pulls the condyle and disc forward and medially down the articular eminence. If the LEFT side cannot translate fully — through muscle guarding, spasm, or a restriction within the joint — the right condyle translates normally, swings the midline across, and the chin ends up pointing LEFT. Deviation to the left therefore indicates the LEFT side is the restricted one.
  • D. The masseter is an ELEVATOR, not a protruder. It closes the mandible and plays no part in the opening translation.

Note: This case is deliberately the OTHER pattern — no pain, no click, no crepitus, with muscle tenderness on palpation, which points to a MUSCULAR restriction rather than an intracapsular one. The rules to hold apart: persistent DEVIATION toward one side means restriction on that side; a DEFLECTION that corrects mid-opening with a click means disc displacement WITH reduction. Normal opening is 40–55 mm; under 40 mm is restricted.

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