occlusion.) during chewing, the mandible moves in a specific manner as delineated by the two temporomandibular joints. the side of the mandible that moves laterally is referred to as either the working or rotating side, while the other side is referred to as either the balancing or orbiting side. the latter terms, although a bit outdated, are actually more precise, as they define the sides by the movements of the respective condyles. when the mandible is moved into a lateral excursion, the working side condyle (the condyle on the side of the mandible that moves outwards) only performs rotation (in the horizontal plane), while the balancing side condyle performs the translation. during actual functional chewing, when the teeth are not only moved side to side but also up and down when biting of the teeth is incorporated as well, rotation (in a vertical plane) also plays a part in both condyles. the mandible is moved primarily by the four muscles of mastication: the masseter, medial pterygoid, lateral pterygoid and the temporalis. these four muscles, all innervated by v3, or the mandibular division of the trigeminal nerve, work in different groups to move the mandible in different directions. contraction of the lateral pterygoid acts to pull the disc and condyle forward within the glenoid fossa and down the articular eminence; thus, the action of this muscle serves to protrude the jaw, it with the assistance of gravity, and the digastricus muscle also opens the jaw. the other three muscles close the mouth; the masseter and the medial pterygoid by pulling up the angle of the mandible and the temporalis by pulling up on the coronoid process of the mandible. clinical significance pain temporomandibular joint pain is generally due to one of four reasons. myofascial pain dysfunction syndrome, primarily involving the muscles of mastication. this is the most common cause. internal derangements, an abnormal relationship of the disc to any of the other components of the joint. disc displacement is an example of internal derangement. osteoarthritis of the temporomandibular joint, a degenerative joint disease of the articular surfaces. temporal arteritis, for which it is considered a reliable diagnostic criteria pain or dysfunction of the temporomandibular joint is referred to as temporomandibular joint dysfunction or temporomandibular joint disorder (tmd). this term is used to refer to a group of problems involving the temporomandibular joints and the muscles, tendons, ligaments, blood vessels, and other tissues associated with them. although rare, other pathologic conditions may also affect the function of temporomandibular joints, causing pain and swelling. these conditions include chondrosarcoma, osteosarcoma, giant cell tumor, and aneurysmal bone cyst. examination the temporomandibular joints can be felt in front of or within the external acoustic meatus during movements of the mandible. auscultation of the joint can also be performed. disc displacement the most common disorder of a temporomandibular joint is disc displacement. in essence, this is when the articular disc, attached anteriorly to the superior head of the lateral pterygoid muscle and posteriorly to the retrodiscal tissue, moves out from between the condyle and the fossa so that the mandible and temporal bone contact is made on something other than the articular disc. this, as explained above, is usually very painful, because, unlike these adjacent tissues, the central portion of the disc contains no sensory innervation. in most instances of disorder, the disc is displaced anteriorly upon translation, or the anterior and inferior sliding motion of the condyle forward within the fossa and down the articular eminence. on opening, a "pop" or "click" can sometimes be heard and usually felt also, indicating the condyle is moving back onto the disk, known as "reducing the joint" (disc displacement with reduction). upon closing, the condyle will slide off the back of the disc, hence another "click" or "pop" at which point the condyle is posterior to the disc. upon clenching, the condyle compresses the bilaminar area, and the nerves, arteries, and veins against the temporal fossa, causing pain and inflammation. in disc displacement without reduction the disc stays anterior to the condylar head upon opening. mouth opening is limited and there is no "pop" or "click" sound on opening. congenital disorders aplasia of mandible or cranial bone hypoplasia of mandible or cranial bone hyperplasia of mandible or cranial bone dysplasia abnormal tissue development traumatic disorders mandibular dislocation fracture subluxation inflammatory disorders synovitis capsulitis myositis degenerative disorders osteoarthritis rheumatoid arthritis juvenile idiopathic arthritis idiopathic disorders temporomandibular disorder (tmd, also termed "temporomandibular joint pain-dysfunction syndrome") is pain and dysfunction of the tmj and the muscles of mastication (the muscles that move the jaw). tmd does not fit neatly into any one etiologic category since the pathophysiology is poorly understood and it represents a range of distinct disorders with multifactorial etiology. tmd accounts for the majority of pathology of the tmj, and it is the second most frequent cause of orofacial pain after dental pain (i.e. toothache). fibromyalgia