Temporomandibular Articulation
The temporomandibular joint is the articulation between the fist-shaped condylar head of the mandible and the mandibular fossa of the temporal bone. It is bicondyloid - there are 2 separate joints, one either side of the skull.
It is a very specialised joint - with a number of special features:
It is a very specialised joint - with a number of special features:
- A fibrocartilage disc, separating the joint space into 2 synovial cavities, an upper and a lower cavity. The upper cavity is the larger and looser of the 2 - allowing it to stretch more (gliding movement). The disc is continuous with the capsule. The disc is thicker around its circmference than in the centre.
- Articular surfaces covered in fibrocartilage rather than hyaline cartilage
- A fibrous joint capsule - attached to the mandibular fossa and the articular tubercle superiorly and the neck of the condyle of the mandible inferiorly, along with being attached within to the fibrocartilage disc.
Ligaments of the TMJ
Internal view of left mandible demonstrating capsule, sphenomandibular and stylomandibular ligaments
There are 3 ligaments, the temporomandibular (lateral) ligament, the sphenomandibular ligament, ant the stylomandibular ligament.
- Temporomandibular ligament: from lateral surface zygomatic arch and articular tubercle, to lateral surface and posterior border of neck of mandible. This ligament is covered by the parotid gland. It is the strongest ligament, preventing lateral dislocation of the mandible.
- Sphenomandibular ligament: from spine of sphenoid bone to lingula of mandible, it is relatively weak and may afford slight prevention of ?anterior dislocation.
- Stylomandibular ligament: from styloid process to angle of mandible. This is actually a condensation of cervical fascia, and runs between the masseter and medial pterygoid and separates the parotid gland from the submandibular gland. Some of the fibres of styloglossus muscle arise from this ligament. It is weak and may help prevent ?anterior dislocation.
Movements of the TMJ
The movements of the TMJ are complex. it is best thinking of each synovial cavity as a "separate" joint, so:
The upper cavity:
The lower cavity:
In practise, slight opening of the mandible is achieved initially by depression at the hinge joint (lower). However, to open wide, you then need to protrude the mandible (anterior gliding at the upper joint) to move the mandible anteriorly away from structures posterior to it between it and the mastoid process, and from the mastoid process itself, and simultaneously depress further with the hinge joint. To close the mouth from this position, elevation occurs at the lower hinge joint whilst retraction is occurring at the gliding upper joint.
Lateral movements are a mixture of protrusion (anterior gliding) on one side with retraction (posterior gliding) on the other side, so using the upper joints.
The muscles acting to produce these movements are described in the next section.
The upper cavity:
- is the joint between the disc and the mandibular fossa
- is loose and allows a gliding movement - in both an anterior direction (protrusion), on to the articular tubercle (or eminence, whichever you want to call it) and posteriorly (retraction)
The lower cavity:
- is the joint between the disc and the condylar head of the mandible
- is tighter, and allows a hinge movement - meaning depression or elevation of the mandible
In practise, slight opening of the mandible is achieved initially by depression at the hinge joint (lower). However, to open wide, you then need to protrude the mandible (anterior gliding at the upper joint) to move the mandible anteriorly away from structures posterior to it between it and the mastoid process, and from the mastoid process itself, and simultaneously depress further with the hinge joint. To close the mouth from this position, elevation occurs at the lower hinge joint whilst retraction is occurring at the gliding upper joint.
Lateral movements are a mixture of protrusion (anterior gliding) on one side with retraction (posterior gliding) on the other side, so using the upper joints.
The muscles acting to produce these movements are described in the next section.
Muscles of Mastication
(All innervated by branches of the mandibular nerve, Vc)
Temporalis
- Arises from temporal fossa, inserts into coronoid process of mandible. Covered by temporal fascia.
- Innervation: deep temporal branches of anterior division of mandibular nerve
- Action: Elevates mandible, posterior (horizontal) fibres retract mandible
Temporalis lying in the temporal fossa, attached to coronoid process mandible
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Sphenoid bone - note small amount of greater wing that contributes to temporal fossa, and also the attachment of lateral pterygoid (pterygoideus externus in this image) to the lateral side of lateral pterygoid plate and infratemporal surface of sphenoid
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Masseter
- Arises from zygomatic arch (made up from temporal process zygomatic bone and zygomatic process temporal bone) and zygomatic process of maxilla. Inserts into lateral surface of angle and lower part ramus of mandible
- Innervation: Anterior division of Mandibular nerve (Vc)
- Action: Elevates mandible, (helps with protrusion also)
Re-visit this image to see position of masseter, lying deep to zygomaticus muscles, risorius etc
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Points of muscle attachment on mandible - note attachments of temporalis and masseter
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Medial pterygoid
- Arises from medial side of lateral pterygoid plate and pterygoid fossa (sphenoid bone), and maxillary tuberosity + pyramidal process palatine bone. Inserts into medial side angle of mandible.
- Innervation: Nerve to medial pterygoid, a branch of Mandibular nerve (Vc)
- Action: Elevates mandible, helps with protrusion
Muscle attachments - medial surface mandible - note attachments of lateral pterygoid (externus), temporalis, medial pterygoid (internus), and also mylohyoid, genoihyoid and digastric (digastricus)
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Lateral pterygoid (pterygoideus externus) and medial pterygoid (pterygoideus internus)
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Lateral pterygoid
- Arises from infratemporal surface of sphenoid bone and lateral surface of lateral pterygoid plate. Inserts into pterygoid fovea (at neck, just below head) of mandible, joint capsule and articular disc
- Innervation: Nerves to lateral pterygoid, from anterior branch of Mandibular nerve
- Action: protrudes mandible (anterior "gliding") - pulls condyle and disc anteriorly. This is required for opening mandible.
Depression of the mandible - gravity, supra/infrahyoids
Depression of the mandible relies on a number of things to occur. Opening the mouth slightly may be achieved without much protrusion, but effective opening requires protrusion - anterior gliding at the superior compartment of the TMJ. Along with protrusion, opening or rotation at the hinge joint (lower compartment) needs to occur. This is achieved by:
- Gravity: the weight of the mandible partially depresses it
- Supra and infrahyoid muscles: The infrahyoids (sternohyoid, sternothyroid, thyrohyoid, omohyoid) , in this situation, will contract to stabilise the hyoid bone, fixing it in place. With the hyoid bone fixed in place, contraction of the suprahyoids (myloyoid, geniohyoid, digastric, (stylohyoid)) will result in depression of the mandible. (Note this is opposite to what occurs in swallowing, where the mandible remains elevated and fixed, so the suprahyoids elevate the hyoid bone and so assist in elevating the larynx)
- Other minor factors, e.g. platysma, may help depress the mandible
Summary of movements of the mandible
Download the following file for a summary of the movements and the muscles responsible
| mechanism_of_movement_at_tmj.docx |
Dislocation
- Posterior dislocation not possible due to shape of the bones - the thick, strong postglenoid tubercle lies directly behind the condylar head of the mandible where it sits in the mandibular fossa
- Lateral dislocation prevented by the strong lateral temporomandibular ligament
- Anterior dislocation is prevented by the articular tubercle (/eminence), stylomandibular and sphenomandibular ligaments being weak and offering little protection from anterior dislocation
