The human mandible, commonly known as the lower jawbone, is the largest and strongest bone within the facial skeleton. It holds a unique position as the only movable bone of the skull, excluding the tiny ossicles of the middle ear. This singular mobility allows the mandible to perform a range of actions fundamental to daily human existence.
Anatomy of the Mandible
The mandible is a horseshoe-shaped bone composed of a horizontal body and two vertical rami. The body forms the front and sides of the lower jaw, housing the lower teeth within its superior alveolar border. This curved section provides the foundation for the dental arch.
Extending upwards from the posterior ends of the body are the two rami, which connect at the mandibular angle. Each ramus features two projections: the condylar process and the coronoid process. The condylar process, located posteriorly, forms the temporomandibular joint (TMJ) by articulating with the temporal bone of the skull, enabling hinge-like jaw movement. The coronoid process, situated anteriorly, serves as an attachment point for the temporalis muscle, involved in jaw movement. The body also contains the mental foramen, small openings typically found below the second premolar teeth, which allow passage of the mental nerve and blood vessels to the lower lip and chin.
Primary Functions of the Lower Jaw
The mandible’s structure facilitates its two primary functions: mastication and speech. Mastication, or chewing, relies on the synchronized actions of the mandible and its associated muscles. Powerful muscles like the masseter, temporalis, and pterygoids attach to the mandible, enabling it to elevate, depress, protrude, retract, and move side-to-side. These movements allow the lower teeth to crush and grind food against the upper teeth, preparing it for digestion.
The precise control over mandibular movement is also fundamental for articulate speech. The jaw’s ability to move through a wide range of motions, in coordination with the tongue and lips, allows for the formation of distinct sounds and words.
Development and Changes Through Life
The human mandible undergoes significant developmental changes from infancy through old age. At birth, the mandible consists of two separate halves, initially joined by fibrocartilage at the midline. During the first year of life, these two halves fuse at the mandibular symphysis, forming a single, unified bone.
As a child grows, the mandible’s shape continues to transform, accommodating the eruption of both deciduous (baby) and permanent teeth. This growth involves a remodeling process, with bone resorption on the anterior surface of the ramus and deposition on its posterior surface, leading to increases in ramus height and anteroposterior depth. The mental foramen also shifts its position, moving more posteriorly over time.
In old age, particularly if teeth are lost, the alveolar ridge—the part of the jawbone that holds the teeth—can undergo resorption. This process leads to a reduction in the overall volume and height of the mandibular bone.
Common Mandibular Fractures and Disorders
Mandibular fractures are common injuries, often resulting from trauma such as accidents or direct impacts to the face. Fractures can occur in various locations, with the condyle being the most frequently fractured site (approximately 36% of cases). Other common fracture sites include the body of the mandible (around 21%) and the angle (about 20%).
Symptoms of a mandibular fracture can include localized pain, swelling, and bruising around the jaw or face. Individuals may experience difficulty opening their mouth fully (trismus), a misalignment of their teeth (malocclusion), or numbness in the lower lip or chin due to potential compression of the inferior alveolar nerve. Bleeding from the gums can also be observed, especially if the fracture extends into the tooth-bearing area.
Temporomandibular Joint (TMJ) disorders affect the jaw joint and the muscles controlling its movement. Common symptoms often include persistent pain in the jaw, clicking or popping sounds when opening or closing the mouth, and difficulty chewing or moving the jaw smoothly.