The mandible, often called the lower jawbone, is the largest and strongest bone of the face, forming the entire lower portion of the jaw. Unlike most other skull bones that are fused, the mandible is the skull’s only movable bone. This distinct mobility allows it to perform various actions fundamental to daily life.
Anatomy of the Mandible
The mandible’s structure resembles a horseshoe, consisting of a horizontal body and two vertical extensions known as rami. The body forms the curved front part of the jaw, holding the lower teeth along its superior alveolar border. A faint ridge, the mandibular symphysis, marks the midline of the body, indicating where the two halves fused during development.
Extending upward from the body at the angle of the jaw are the two rami. Each ramus branches into two processes at its upper end. The anterior projection is the coronoid process, which serves as an attachment point for muscles, notably the temporalis muscle. The posterior projection is the condylar process, which culminates in the mandibular condyle, a rounded articular surface.
Small openings called mental foramina are present on either side of the body. These foramina act as passageways for nerves and blood vessels, allowing sensation to the lower lip. The internal surface of the ramus also contains the mandibular foramen, which connects to the mental foramen via the mandibular canal.
Primary Functions of the Mandible
The mandible’s mobility enables fundamental actions, primarily mastication, or chewing. The lower jaw moves in opposition to the upper jaw, or maxilla, allowing for the biting, grinding, and crushing of food. Muscles such as the masseter and temporalis, which attach to the mandible, power these movements, generating significant force for food breakdown.
The mandible’s movements are also important for articulation, which is the formation of sounds for speech. By changing the shape of the oral cavity through jaw movements, along with the tongue and lips, humans can produce a wide range of vocalizations. The coordinated action of the mandible and its associated muscles helps in shaping the airflow from the lungs into distinct sounds and words.
The Temporomandibular Joint Connection
The temporomandibular joint (TMJ) connects the mandible to the skull, articulating the condylar process of the mandible with the temporal bone. This joint functions as both a hinge and a sliding joint, allowing for a broad range of jaw movements. An articular disc, composed of fibrocartilage, is positioned between the bone surfaces, ensuring smooth movement and absorbing forces.
The TMJ facilitates distinct movements of the mandible. Depression refers to the opening of the mouth, while elevation involves closing it. Protraction is the forward movement of the jaw, and retraction is its backward movement. Lateral deviation, or side-to-side motion, is also possible, enabling the grinding action needed for chewing. These coordinated movements are accomplished through the action of various muscles.
Common Mandible-Related Conditions
The mandible, despite its strength, is susceptible to several conditions. Fractures of the mandible are common facial injuries, often resulting from trauma such as road traffic accidents, assaults, or falls. The most frequently fractured sites include the body, condyle, angle, symphysis, ramus, and coronoid process. Symptoms of a fracture may include pain, swelling, difficulty moving the jaw, and changes in how the teeth fit together.
Dislocation of the mandible occurs when the condyle of the jawbone moves out of its normal position within the temporomandibular joint. This can happen due to excessive mouth opening, such as during a wide yawn, laughing, or prolonged dental procedures. Patients with a dislocated jaw often find their mouth “locked” in an open position and are unable to close it. Anterior dislocations, where the condyle is displaced forward, are the most common type.
Temporomandibular disorders (TMD) affect the TMJ and the muscles controlling jaw movement. These disorders can cause symptoms like pain in the jaw joint or surrounding muscles, clicking or popping sounds when opening or closing the mouth, and limited jaw movement. Factors such as jaw injury, arthritis, or chronic teeth grinding may contribute to the development of TMD.