Temporomandibular disorders are a group of disorders that all involve the temporomandibular joint, which is located between the temporal bone of the skull and mandible, or jawbone; as well as the muscles and associated structures that are involved in chewing and speech.
Normally, between the temporal bone and the mandible is a synovial cavity, which is wrapped in fibrocartilage and filled with synovial fluid, which is a protein rich fluid that reduces friction between the sliding bones.
The synovial cavity is divided into an upper and lower compartment by an articular disc within the synovial fluid.
The lower compartment is bound, inferiorly, by the condylar head of the mandible.
The lower compartment allows the mandible to rotate, which lets the mouth open and close.
The upper compartment is bound, superiorly, by two regions of the temporal bone: the mandibular fossa, in the middle and back, and articular tubercle, in the front.
Separating these two compartments is the articular disc.
The upper compartment allows the condylar head to move forward and rotate.
The movements of the temporomandibular joint are coordinated by numerous muscles, including: the temporalis, which is a fan-shaped muscle on both sides of the cranium; the masseter, which connects to the mandible and the zygomatic arch of the temporal bone; the medial pterygoid, which connects to the mandible and medial aspect of the lateral pterygoid plate; and the lateral pterygoid, found at the condylar process.
These muscles are innervated by branches of the trigeminal nerve.
Now, the causes of temporomandibular disorders can be categorized as intra-articular, within the joint, or extra-articular, involving the surrounding musculature.
Intra-articular causes, are called temporomandibular joint disorders, and they include things like abnormalities of the bones in the joint, inflammation in the joint from conditions like osteoarthritis or rheumatoid arthritis; disorders of the articular disk; laxity of the fibrocartilage allowing for temporomandibular hyper or hypo-mobility; or trauma that might result in structural damage or bleeding inside the joint.