emporomandibular (jaw) joint (TMJ) is the articulation of the condyle of the mandible with the mandibular fossa of the temporal bone. You can feel its action by pressing your fingertips against the jaw immediately anterior to the ear while opening and closing your mouth. The synovial cavity of the TMJ is divided into superior and inferior chambers by an articular disc, which permits lateral and medial excursion of the mandible. Two ligaments support the joint. The lateral ligament prevents posterior displacement of the mandible. If the jaw receives a hard blow, this ligament normally prevents the condylar process from being driven upward and fracturing the base of the skull. The sphenomandibular ligament on the medial side of the joint extends from the sphenoid bone to the ramus of the mandible. A stylomandibular ligament extends from the styloid process to the angle of the mandible but is not part of the TMJ proper.
A deep yawn or other strenuous depression of the mandible can dislocate the TMJ by making the condyle pop out of the fossa and slip forward. The joint is relocated by pressing down on the molars while pushing the jaw posteriorly.
Figure 6. Temporomandibular joint (TMJ)
The Shoulder Joint
The glenohumeral (humeroscapular) joint, or shoulder joint, is where the hemispherical head of the humerus articulates with the glenoid cavity of the scapula. Together, the shoulder and elbow joints serve to position the hand for the performance of a task; without a hand, shoulder and elbow movements are almost useless. The relatively loose shoulder joint capsule and shallow glenoid cavity sacrifice joint stability for freedom of movement. The cavity, however, has a ring of fibrocartilage called the glenoid labrum around its margin, making it somewhat deeper than it looks on a dried skeleton.
The shoulder is stabilized mainly by the biceps brachii muscle on the anterior side of the arm. One of its tendons arises from the long head of the muscle, passes through the intertubercular groove of the humerus, and inserts on the superior margin of the glenoid cavity. It acts as a taut strap that presses the humeral head against the glenoid cavity. Four additional muscles help to stabilize this joint: the supraspinatus, infraspinatus, teres minor, and subscapularis. Their tendons form the rotator cuff, which is fused to the joint capsule on all sides except the inferior.
Five principal ligaments also support this joint. Three of them, called the glenohumeral ligaments, are relatively weak and sometimes absent. The other two are the coracohumeral ligament, which extends from the coracoid process of the scapula to the greater tubercle of the humerus, and the transverse humeral ligament, which extends from the greater to the lesser tubercle of the humerus and forms a tunnel housing the tendon from the long head of the biceps.
Four bursae occur at the shoulder. Their names describe their locations: the subdeltoid, subacromial, subcoracoid, and subscapular bursae. The deltoid is the large muscle that caps the shoulder, and the other bursae are named for parts of the scapula.
Figure 7. Shoulder joint
The Elbow Joint
The elbow is a hinge joint composed of two articulations: the humeroulnar joint where the trochlea of the humerus joins the trochlear notch of the ulna, and the humeroradial joint where the capitulum of the humerus meets the head of the radius. Both are enclosed in a single joint capsule. On the posterior side of the elbow, there is a prominent olecranon bursa to ease the movement of tendons over the joint. Side-to-side motions of the elbow joint are restricted by a pair of ligaments: the radial (lateral) collateral ligament and ulnar (medial) collateral ligament.
Another joint occurs in the elbow region, the proximal radioulnar joint, but it is not involved in the hinge. At this joint, the edge of the disclike head of the radius fits into the radial notch of the ulna. It is held in place by the anular ligament, which encircles the radial head and is attached at each end to the ulna. The radial head rotates like a wheel against