The mandible is the bone that forms the lower jaw, and forms an essential part of the facial skeleton. The maxilla forms the upper jaw. Both of these bones hold the teeth we require to chew our food. The mandible is the only moveable bone of the skull. On the medial surface of mandible is the mandibular foramen which allows the entrance of a nerve and vessels that supply the lower jaw, as well as the teeth, some oral sensation and sensation over the chin. In this article we will discuss the anatomy and clinical relevance of the mandibular foramen.
Anatomy & Relations
The mandibular foramen is an opening at the internal aspect of the ramus of the mandible. The inferior alveolar branch of the mandibular division of the trigeminal nerve, as well as the inferior alveolar vessels enter the foramen, and supply the lower jaw.
The opening has a prominent ridge in its front known as lingula of the mandible for the attachment of sphenomandibular ligament. The mandibular foramen leads to the mandibular canal which traverses the trabecular bone and ends at the mental foramen. The mylohyoid groove for the mylohyoid nerve is seen to be present near the foramen, which runs in the anteroinferior direction.
A brief description of the contents of the foramen is given below.
Inferior Alveolar Artery
This artery is one of the five branches of the first section of the maxillary artery. The artery descends with the inferior alveolar nerve, and just before it enters the mandibular foramen, it gives off a small branch that runs in the mylohyoid groove and supplies the mylohyoid muscle. Mylohyoid is the muscle that forms the floor of the oral cavity.
The inferior alveolar artery then continues and runs within the substance of the mandibular bone. The artery gives off an incisor branch, which continues to run below the teeth as far as the midline, where the branch anastomoses with the artery of the opposite side. The mental branch of the artery leaves the mandible through the mental foramen and provides branches to the chin. It also formes anastomoses with the inferior labial arteries (branches of the facial artery), and the submental arteries (also branch from the facial artery).
Inferior Alveolar Nerve
The second structure that enters the mandibular foramen is the inferior alveolar nerve, which is a sensory branch of the mandibular division of the trigeminal nerve (V3). The trigeminal nerve is a large nerve that arises from the pons, and divides into three major branches.
V1 is the ophthalmic division, and leaves the skull through the superior orbital fissure. It supplies sensation to the superior surface of the face. It also supplies sensation to the surface of the eye, and therefore provides the afferent limb of the corneal reflex.
V2 is the maxillary division, and leaves the skull through the foramen rotundum. V2 supplies sensation below the V1 division, to the middle section of the face. The inferior alveolar nerve is a sensory branch of the mandibular division of the trigeminal nerve. The nerve to mylohyoid arises just before the nerve enters into the foramen.
It runs within the substance of the mandible, and as it approaches the apex of the second molar, the nerve divides into a mental and incisive branch. The incisive branch supplies sensation to the lower teeth, and the mental branch continues in the mandibular foramen, eventually leaving the mental foramen, on either side of the midline of the chin. It then supplies sensation to a small region of skin on the lower lip, and lower face.
Inferior Alveolar Vein
The vein forms by the merging of the dental, alveolar and mental branches in the mandible. Posteriorly it drains through the mandibular foramen into the pterygoid plexus in the region of the infratemporal fossa, Anteriorly it drains through the mental foramen to join the facial vein.
Inferior Alveolar Block
This is the commonest anaesthetic procedure performed in dental surgery. Successful inferior alveolar nerve block is related to the deposition of local anesthetic material very close to the nerve before it enters the mandibular foramen.
Conventionally, the most important clinical landmarks used to locate the inferior alveolar nerve are the coronoid notch and the pterygomandibular raphe. The preferred site of needle insertion lies between these two landmarks and a few drops of local anaesthesia are deposited here. There are many modified approaches to locate the mandibular foramen and inferior alveolar nerve which aim to achieve a high success rate. The inferior alveolar nerve can also be damaged during implant surgery or during removal of the third molar.
Accessory Mandibular Foramina
There can sometimes be additional foramina through which the branches of the inferior alveolar nerve may enter the mandible. These can be utilised for additional locoregional anaesthesia before dental surgery and assist maxillofacial surgeons in planning graft surgeries.