Temporomandibular joint
In anatomy, the temporomandibular joints are the two joints connecting the jawbone to the skull. It is a bilateral synovial articulation between the temporal bone of the skull above and the condylar process of mandible below; it is from these bones that its name is derived. The joints are unique in their bilateral function, being connected via the mandible.
Structure
The main components are the joint capsule, articular disc, mandibular condyles, articular surface of the temporal bone, temporomandibular ligament, stylomandibular ligament, sphenomandibular ligament, and lateral pterygoid muscle.Capsule
The articular capsule is a thin, loose envelope, attached above to the circumference of the mandibular fossa and the articular tubercle immediately in front; below, to the neck of the condyle of the mandible. Its loose attachment to the neck of the mandible allows for free movement.Articular disc
The unique feature of the temporomandibular joint is the articular disc. The disc is composed of dense fibrocartilagenous tissue that is positioned between the head of the mandibular condyle and the mandibular fossa of the temporal bone. The temporomandibular joints are one of the few synovial joints in the human body with an articular disc, another being the sternoclavicular joint. The disc divides each joint into two compartments, the lower and upper compartments. These two compartments are synovial cavities, which consist of an upper and a lower synovial cavity. The synovial membrane lining the joint capsule produces the synovial fluid that fills these cavities. The disc is biconcave in shape. The anterior portion of the disc serves as the insertion site for the superior head of the lateral pterygoid. The posterior portion attaches to the temporal bone. Both upper and lower compartments do not communicate with each other unless the disc is damaged.The central area of the disc is avascular and lacks innervation, thus getting its nutrients from the surrounding synovial fluid. In contrast, the posterior ligament and the surrounding capsules along have both blood vessels and nerves. Few cells are present, but fibroblasts and white blood cells are among these. The central area is also thinner but of denser consistency than the peripheral region, which is thicker but has a more cushioned consistency. With age, the entire disc thins and may undergo the addition of cartilage in the central part, changes that may lead to impaired movement of the joint. The synovial membrane covers the inner surface of the articular capsule in the TMJ, except for the surface of the articular disc and condylar cartilage.
The lower joint compartment formed by the mandible and the articular disc is involved in rotational movement—this is the initial movement of the jaw when the mouth opens. The upper joint compartment formed by the articular disc and the temporal bone is involved in translational movement—this is the secondary gliding motion of the jaw as it is opened widely.
In some cases of anterior disc displacement, the pain felt during movement of the mandible is due to the condyle compressing this area against the articular surface of the temporal bone.
Ligaments
There are three ligaments associated with the temporomandibular joints: one major and two minor ligaments. These ligaments are important in that they define the border movements, or in other words, the farthest extents of movements, of the mandible. Movements of the mandible made past the extents functionally allowed by the muscular attachments will result in painful stimuli, and thus, movements past these more limited borders are rarely achieved in normal function.- The major ligament, the temporomandibular ligament, is actually the thickened lateral portion of the capsule, and it has two parts: an outer oblique portion and an inner horizontal portion. The base of this triangular ligament is attached to the zygomatic process of the temporal bone and the articular tubercle; its apex is fixed to the lateral side of the neck of the mandible. This ligament prevents excessive retraction or moving backward of the mandible, a situation that might lead to problems with the joint.
- The two minor ligaments, the stylomandibular and sphenomandibular ligaments are accessory and are not directly attached to any part of the joint.
- * The stylomandibular ligament separates the infratemporal region from the parotid region, and runs from the styloid process to the angle of the mandible; it separates the parotid and submandibular salivary glands. It also becomes taut when the mandible is protruded.
- * The sphenomandibular ligament runs from the spine of the sphenoid bone to the lingula of mandible. The inferior alveolar nerve descends between the sphenomandibular ligament and the ramus of the mandible to gain access to the mandibular foramen. The sphenomandibular ligament, because of its attachment to the lingula, overlaps the opening of the foramen. It is a vestige of the embryonic lower jaw, Meckel cartilage. The ligament becomes accentuated and taut when the mandible is protruded.
- discomallear ligament,
- malleomandibular ligament.
Nerve supply
Blood supply
Its arterial blood supply is provided by branches of the external carotid artery, predominately the superficial temporal branch. Other branches of the external carotid artery, namely the deep auricular artery, anterior tympanic artery, ascending pharyngeal artery, and maxillary artery, may also contribute to the arterial blood supply of the joint.The fibrocartilage that overlays the is avascular in healthy subjects.
Development
Formation of the temporomandibular joints occurs at around 12 weeks in utero when the joint spaces and the articular disc develop. At approximately 10 weeks the component of the fetus' future joint becomes evident in the mesenchyme between condylar cartilage of the mandible and the developing temporal bone. Two slitlike joint cavities and intervening disk make their appearance in this region by 12 weeks. The mesenchyme around the joint begins to form the fibrous joint capsule. Very little is known about the significance of newly forming muscles in joint formation. The developing superior head of the lateral pterygoid muscle attaches to the anterior portion of the fetal disk. The disk also continues posteriorly through the petrotympanic fissure and attaches to the malleus of the middle ear.A growth center is located in the head of each mandibular condyle before an individual reaches maturity. This growth center consists of hyaline cartilage underneath the periosteum on the articulating surface of the condyle. This is the last growth center of bone in the body and is multidirectional in its growth capacity, unlike a typical long bone. This area of cartilage within the bone grows in length by appositional growth as the individual grows to maturity. Over time, the cartilage is replaced by bone, using endochondral ossification. This mandibular growth center in the condyle allows the increased length of the mandible needed for the larger permanent teeth, as well as for the larger brain capacity of the adult. This growth of the mandible also influences the overall shape of the face and thus is charted and referred to during orthodontic therapy. When an individual reaches full maturity, the growth center of bone within the condyle has disappeared.
Function
Since the TMJ is connected to the mandible, the right and left joints function together and interdependently.Each temporomandibular joint is classed as a "ginglymoarthrodial" joint since it is both a ginglymus and an arthrodial joint. The condyle of the mandible articulates with the temporal bone in the mandibular fossa. The mandibular fossa is a concave depression in the squamous portion of the temporal bone.
These two bones are actually separated by an articular disc, which divides the joint into two distinct compartments. The inferior compartment allows for rotation of the condylar head around an instantaneous axis of rotation, corresponding to the first 20mm or so of the opening of the mouth. After the mouth is open to this extent, the mouth can no longer open without the superior compartment of the temporomandibular joints becoming active.
At this point, if the mouth continues to open, not only are the condylar heads rotating within the lower compartment of the temporomandibular joints, but the entire apparatus translates. Although this had traditionally been explained as a forward and downward sliding motion, on the anterior concave surface of the mandibular fossa and the posterior convex surface of the articular eminence, this translation actually amounts to a rotation around another axis. This effectively produces an evolute which can be termed the resultant axis of mandibular rotation, which lies in the vicinity of the mandibular foramen, allowing for a low-tension environment for the vasculature and innervation of the mandible.
The necessity of translation to produce further opening past that which can be accomplished with a sole rotation of the condyle can be demonstrated by placing a resistant fist against the chin and trying to open the mouth more than 20 or so mm.
The resting position of the temporomandibular joint is not with the teeth biting together. Instead, the muscular balance and proprioceptive feedback allow a physiologic rest for the mandible, an interocclusal clearance, or freeway space, which is 2 to 4 mm between the teeth.
The temporomandibular joint is less stable when teeth are clenched together.