Introduction
The temporomandibular joint (TMJ) joins each condyle of the mandible to the temporal bone. Because it links the movable mandible to the fixed cranium, it is also called the craniomandibular joint. The two joints work as one, and their position decides where the teeth meet, so every occlusal surface a dentist makes depends on them.
Parts of the joint
Bones
- Mandibular (glenoid) fossa: an oval hollow in the temporal bone just in front of the ear canal. Its roof is thin and is not a loading surface.
- Articular eminence: the bony ridge in front of the fossa. Its back slope is where the condyle works when the jaw moves forward.
- Condyle: convex on all its bearing surfaces. The long axes of the two condyles, extended inward, meet in front of the foramen magnum at about 135°.
The working surfaces are the front of the condyle and the back slope of the eminence, not the fossa. They are covered with fibrocartilage, not the hyaline cartilage of most joints.
Articular disc
A plate of dense collagen tissue between condyle and temporal bone. Its centre has no vessels and no nerves, so it can take load. From front to back it has a thick anterior band, a thin intermediate zone and a thick posterior band; behind it, the vascular retrodiscal (bilaminar) tissue attaches it to the back of the joint.
The disc splits the joint into two spaces that move in different ways:
As the mouth opens wide, the condyle slides down the eminence and the thin middle of the disc stays between the two bones, while the bilaminar tissue fills the empty fossa behind. The upper head of the lateral pterygoid steadies the disc against the eminence when the jaw closes.