Which masticatory muscle is MOST susceptible to myositis (including myositis ossificans)?

Correct answer: D. Masseter

• A. The lateral pterygoid is deeply placed and protected; it is involved less often and usually only in association with lesions of other muscles.
• B. The mylohyoid forms the floor of the mouth and is not a target for masticatory myositis.
• C. The digastric is a suprahyoid muscle, not a muscle of mastication, and is rarely involved.
• D. CORRECT. Across published series of myositis ossificans of the masticatory muscles, the MASSETER is the most frequently affected, followed by the medial pterygoid, then the lateral pterygoid and temporalis. Its superficial lateral position on the ramus makes it the muscle most exposed to direct external trauma, and its bulk and rich vascularity favour the intramuscular haematoma that seeds heterotopic ossification. The dominant clinical sign is progressive, painless limitation of opening.

Note: ⚑ Worth knowing both figures: MASSETER is most affected overall, but in the specific subgroup of cases following a DENTAL PROCEDURE — extraction or an inferior alveolar nerve block — the MEDIAL PTERYGOID is the muscle most often involved, because that is where the needle and the surgical trauma go. Trismus that fails to resolve after a block deserves imaging.