CRDTS Local Anesthesia Practice Test 2026 - Free Local Anesthesia Practice Questions and Study Guide

Trismus after a dental injection is most often due to irritation or trauma to the muscles of mastication located in the infratemporal fossa, especially the lateral and medial pterygoid muscles. When a needle or injected material reaches this area, it can cause inflammation, edema, or a hematoma that irritates these muscles or their motor nerve fibers, leading to painful muscle spasm and a limited ability to open the mouth. The infratemporal fossa houses these muscles and the vessels that can be affected during injections, making it the most likely source of post-injection trismus.

The orbit is distant from the injection sites and would not produce trismus. Involvement of the mandible condyle alone doesn’t typically explain the common pattern of limited mouth opening after injections, and submandibular space issues tend to present with swelling and floor-of-mouth concerns rather than primary muscle spasm.