Which muscle group is most commonly involved in causing trismus (limited mouth opening)?

Trismus is limited mouth opening caused by spasm or inflammation of the muscles of mastication. The muscles most commonly involved are the masseter and medial pterygoid — both are powerful jaw-closing (elevator) muscles. Inflammation from pericoronitis, odontogenic infections, or inferior alveolar nerve block injections (particularly when the needle enters the medial pterygoid muscle) can cause spasm of these muscles. The lateral pterygoid is a jaw-opener (depressor) and protrudes the mandible; it is less commonly responsible for trismus.