The mandible is the only bone in the entire cranium that doesn’t articulate with its adjacent skull bones via sutures. When the skull is observed purely as a bony structure, there is nothing anatomically holding the rest of the skull and the mandible together.
This bone is also known as the lower jaw and it articulates dentally with the upper jaw or the maxilla in the viscerocranium via the teeth when the mouth is closed. It also articulates to the neurocranium via the temporal bone, forming the temporomandibular joint (TMJ).
The mandible is a singular bone that has a distinctive horse-shoe shape and is symmetrical on both sides. It is the moving part of the jaws when the body is engaged in the feeding process and for that reason all the muscles of mastication including the medial and lateral pterygoid muscles, the temporal muscle and the masseter muscle attach to it.
The lower jaw consists of no less than five individual parts that intramembranously ossify together, especially around Meckel’s cartilage.
The ramus is the second largest part of the mandible after the body and it extends cranially from the angle of the mandible, away from the body at an angle of 110 degrees. Various structures attach themselves to both the lateral and medial walls of the ramus, including the masseter muscle laterally and the medial pterygoid muscle, and the sphenomandibular ligament to the medial side.
At the most superior point of the ramus, it divides into two processes, which are separated by a mandibular notch. Anteriorly, sits the coronoid process and posteriorly, the condylar process, which articulates with the temporal bone.
The coronoid process, as previously mentioned, is the foremost structure at the head of the ramus, attaches to the temporalis muscle, which is utilized during mastication.
Although it is not directly part of the temporomandibular joint, it still aids the various functions of the jaws, such as opening and closing, due to its proximity to the TMJ and it involvement with its adjacent structures.
The condylar process is the bony extrusion behind the coronoid process, which forms the lower bony component of the temporomandibular joint, along with the temporal bone. It is formed differently to the coronoid process, because it has a much more slender stalk with a greater protuberance to top it off. This design creates a neck for the condyle and allows the lateral pterygoid muscle to attach to the pterygoid fovea upon it.
The body of the mandible is a large and almost rectangular element that sits parallel or perpendicular to the floor, depending on when the person is standing upright or lying down flat respectively. This part of the bone contains the most anatomical landmarks of the mandible, which are discussed below, as they are part of the mandible’s osseous development.
The last part of the mandible is the alveolar process. This is arguably the most important part of the bone in its entirety, since it holds the teeth via a joint mechanism known as gomphosis. The teeth are responsible for biting, chewing, cutting and grinding, as well as speech and pronunciation and facial tissue support.
This part of the mandible extends superiorly from the body and consists of two bony plates including a thick buccal part and a thin lingual part. Symmetrically, each side of the mandible contains five primary teeth and seven to eight permanent teeth, depending on whether the wisdom teeth or third molars form during embryonic development.
The medial side of the ramus contains the mandibular foramen, which encapsulates the inferior alveolar nerve and its branches. The lateral body of the mandible contains the mental foramen anteriorly, which houses the mental nerve and its corresponding vessels.
Also on the lateral side, the external oblique line can be seen. On the medial side of the mandible there are seven major structures that are worth noting, including the superior and inferior genial tubercles and the digastric fossa, which are found in the midline, as well as the mylohyoid line, whose posterior border allows for the attachment of the pterygomandibular raphe.
The mylohyoid line divides the submandibular and the sublingual fossae.
Clinically, there are many types of pathological conditions within the mandible. Here, the most common bony disturbances have been noted:
- Alveolar bone resorption occurs when the teeth are lost; there is a lack of structures to support the bone and there is increased pressure upon the bone due to chronic denture wearing.
- Intra- and extracapsular condylar fractures are the most frequent mandibular fractures and usually result due to car accidents or indirect force due to violence.
- Other mandibular fracture areas include the body, the angle, the symphysis, the ramus, the alveolus and the coronoid process (in decreasing order of frequency).
- Tooth Aplasia is not uncommon in the third molars, the premolars and especially the lateral incisors. This can lead to gaps in the teeth and an uneven alveolar ridge.
- Osteoradionecrosis is a disorder that occurs due to cancer treatment and the bone disintegrates because of radiation.
- Osteomyelitis is an infection that can cause chronic sequestrations and bone disintegration within the mandible. It is irreversible and the mandible often needs resecting.
- Cyst formation most often happens in the mandible where the molars sit. There are many types of cyst but the common symptom is large bone resorption and bone weakening if the cyst is left untreated.