The sphenomandibular ligament is a lateral accessory ligament of the TMJ, running from the sphenoid spine to the mandibular lingula beside the mandibular foramen. The inferior alveolar nerve and vessels enter the mandibular foramen through the gap between the ligament and the ramus, making the ligament the medial guide for inferior alveolar nerve block injections. The ligament acts as a medial hinge during mouth opening.
| Origin | Spine of the sphenoid bone |
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| Insertion | Lingula of the mandible at the mandibular foramen |
| Actions | Acts as a hinge axis for mandibular depression; guides condylar rotation during mouth opening |
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The inferior alveolar nerve block is directed toward the mandibular foramen guided by the sphenomandibular ligament as the medial border — the needle is walked along the medial ramus to the foramen. Incorrect block technique placing the needle too posterior can lacerate the pterygoid venous plexus deep to the ligament.
Haematoma formation in the pterygoid space behind the sphenomandibular ligament from inadvertent pterygoid plexus penetration during dental nerve block, producing progressive jaw restriction over 24 hours.
