Which muscle is most likely responsible?

Correct answer: A. Masseter

• A. CORRECT. The MASSETER is the most frequently affected muscle in masticatory myalgia and post-injection or post-appointment trismus. It is the most superficial and the most easily palpated of the group, running from the zygomatic arch to the ANGLE and lateral ramus of the mandible — exactly where the taut band is described. It is also the most powerful elevator, so its spasm limits opening directly.
• B. Temporalis spasm is a real cause of restricted opening and of temporal headache, and it is palpated over the temporal fossa — but the findings here are localised at the mandibular angle, which is masseter.
• C. The lateral pterygoid is a protruder and a translator, and its spasm produces deviation and pain on protrusion rather than an isolated palpable band at the angle. It is also largely inaccessible to direct palpation, contrary to what is often taught.
• D. The buccinator is a muscle of FACIAL EXPRESSION supplied by CN VII, not a muscle of mastication. It has no role in limiting mouth opening.

Note: Post-injection trismus most often involves the MEDIAL PTERYGOID, from needle trauma or haematoma during an inferior alveolar block, and it typically appears one to six days later. Post-appointment myalgia from prolonged opening more often involves the masseter. Both respond to the same conservative regimen: heat, soft diet, gentle stretching, an NSAID, and a short course of a muscle relaxant such as cyclobenzaprine if needed.