Temporomandibular Joint | TMJ Anatomy | Mandibular Joint
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Temporomandibular joint is a bicondylar synovial joint.
The articular surfaces of the bones are covered by fibrocartilage (not hyaline), hence it is considered as atypical synovial joint.
It has two (superior and inferior) synovial cavities divided by an articular disk.
There is an upper gliding joint between the articular tubercle and mandibular fossa above and the articular disk below where forward gliding (protrusion) and backward gliding (retraction/translation) take place.
The lower hinge joint is between the disk and the mandibular head (condyle) where elevation (closing) and depression (opening) of the jaw take place. During yawning, the disk and the head of the mandible glide across the articular tubercle.
TM joint has an articular capsule that extends from the articular tubercle and the margins of the mandibular fossa to the neck of the mandible.
It is reinforced by the lateral (temporomandibular) ligament, which extends from the tubercle on the zygoma to the neck of the mandible, and the sphenomandibular ligament, which extends from the spine of the sphenoid bone to the lingula of the mandible.
Stylomandibular ligament
Pterygomandibular raphe
TM joint is innervated by the auriculotemporal and masseteric branches of the mandibular nerve.
The arterial supply is by the superficial temporal, maxillary (middle meningeal and anterior tympanic branches), and ascending pharyngeal arteries.
Stability
• The joint is much more stable when the mouth is closed (i.e., when the teeth are in occlusion) than when the mouth is open. In occlusion, the teeth themselves stabilize the mandible on maxilla and no strain is thrown on the joints when an upward blow is received on the mandible.
• Further in the occluded position, the forward movement of condyle is discouraged by the articular eminence and by the contraction of the posterior fibres of the temporalis muscle, while the backward movement of the condyle is prevented by the lateral ligament and the contraction of the lateral pterygoid muscle.
Movements
• The upper menisco-temporal compartment of TMJ permits gliding movements, during protraction (protrusion), retraction, and chewing.
• The lower menisco-temporal compartment permits rotation around two axes
o a transverse axis, during depression and elevation and
o a vertical axis during side-to-side/chewing movements.
• With these two types of movements, gliding and rotation, and with right and left TMJs working together, most of the movements of the lower jaw can be accomplished perfectly as desired. These include opening and closing the jaws and shifting the lower jaw to one side.
• The movements occurring at the temporomandibular joints are:
Depression (lowering of jaw to open mouth)
Elevation (elevating of jaw to close the mouth)
Protraction/Protrusion
Retraction
Side to side (Chewing) movements
Muscles Producing Movements
Depression (Opening of Mouth) It is produced mainly by lateral pterygoid helped by gravity. The digastric, geniohyoid, and mylohyoid muscles help when the mouth is opened widely or against resistance.
Elevation (Closing the Mouth) It is caused by medial pterygoid, masseter, and temporalis (vertical fibres). Closing the mouth is stronger action than the opening the mouth. Therefore, when attacked by a street dog, it is advisable to keep the mouth of dog closed, if possible.
Protraction It is done by lateral and medial pterygoids and masseter. Retraction It is done by posterior fibres of temporalis. It may be assisted by middle and deep fibres of the masseter, the digastric and geniohyoid muscles.
Side-to-side (Chewing) Movements These movements are performed by alternate contraction of medial and lateral pterygoids on each side.s
Clinical Aspects
• Dislocation of the mandible:
The mandible is dislocated only anteriorly. When the mouth is open, the mandibular condyles move forward and lie underneath the articular eminences. This is the most unstable position of the temporomandibular joint
• Temporomandibular joint syndrome: This syndrome consists of group of symptoms arising from temporomandibular joints and their associated masticatory muscles. The typical presenting symptoms are:
Diffuse facial pain, due to spasm of masseter muscle.
Headache, due to spasm of temporalis muscle.
Jaw pain, due to spasm of lateral pterygoid.
These symptoms may be associated with clicking and pain in the joint. .
• During surgery of temporomandibular joints, the facial nerve should be preserved with utmost care