The mandibular ramus is a portion of the mandible, or jawbone. As the bone that stretches from the underside of either ear and gives rise to the bottom teeth, the mandible consists of the body, which is the horizontal portion crossing the chin, and the ramus, the vertical section beneath each ear. The mandibular ramus serves as the bridge attaching the jaw to the temporal bone of the skull via the temporomandibular (TMJ) joint, the joint that opens and closes the mouth. It also is where the masseter muscle attaches, the large muscle used for mastication, or chewing.
As the vertical section of the jawbone, the mandibular ramus is found on either side of the jaw. Originating just in front of either ear canal, it extends downward to the level of the underside of the jawbone. The mandibular ramus is four-sided, longer from top to bottom than it is wide, and flat in shape. It also features two bony projections called processes that jut upward from the front and back corners of the bone’s top edge, forming a U-shaped space between them known as the mandibular notch.
The processes emanating from the top of the mandibular ramus are referred to as the coronoid and condyloid processes. In front of the mandibular notch, or towards the front of the ramus, is the coronoid process. This projection is where the masseter and temporalis muscle, both muscles of chewing, attach at their lower ends. Behind the mandibular notch and just in front of the ear canal is the condyloid process, the larger of the two projections. The condyloid process forms the lower surface of the TMJ joint and is so named for its oval shape.
As one of the only movable joints in the body that features an articular disk between the articulating bones, the TMJ has an unusual joint classification. It is referred to as a ginglymoarthrodial joint, a nod to the fact that the lower portion of the joint, that between the condyloid process of the mandibular ramus and the articular disk, functions as a ginglymoid or hinge joint. In a hinge joint, the bone rotates against its articulating surface like a door hinge to produce movement in two directions, allowing the jawbone to lower and lift. The portion of the joint between the disk and the temporal bone above it, on the other hand, functions as an arthrodial or planar joint. This means that the two surfaces slide past each other in a motion known as translation, which causes the entire jawbone to shift forward and downward as the mouth opens.