A patient with restricted mouth opening following an inferior alveolar nerve block has trismus. Which muscle is MOST likely involved?

Correct answer: A. Medial pterygoid

• A. CORRECT. The MEDIAL PTERYGOID lies immediately medial to the ramus, and the needle for an inferior alveolar nerve block passes directly through or adjacent to it en route to the pterygomandibular space. Trauma from the needle, multiple insertions, or hemorrhage into the muscle produces reflex spasm and subsequent fibrosis. Because it is a powerful ELEVATOR of the mandible, its spasm directly restricts opening. Post-injection trismus characteristically appears 1–6 days after the injection.
• B. The digastric is a mandibular DEPRESSOR — it opens the mouth, so its involvement would not restrict opening.
• C. The temporalis is an elevator and can contribute to trismus generally, but it is not in the needle path of an IANB.
• D. The lateral pterygoid PROTRUDES the mandible and assists opening; it is not the principal muscle producing post-injection trismus.

Note: Management of post-injection trismus: heat, gentle range-of-motion exercises, analgesics, soft diet, and muscle relaxants if needed. Most resolve within days to weeks. If it worsens or is accompanied by fever and swelling, suspect infection in the pterygomandibular space.