 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 the ulna as the forearm is pronated or supinated.
Figure 8. Elbow joint
The Hip Joint
The coxal (hip) joint is the point where the head of the femur inserts into the acetabulum of the hip bone. Because the coxal joints bear much of the body’s weight, they have deep sockets and are much more stable than the shoulder joint. The depth of the socket is somewhat greater than you see on dried bones because of a horseshoeshaped ring of fibrocartilage, the acetabular labrum, attached to its rim. Dislocations of the hip are rare, but some infants suffer congenital dislocations because the acetabulum is not deep enough to hold the head of the femur in place. If detected early, this condition can be treated with a harness, worn for 2 to 4 months, that holds the head of the femur in the proper position until the joint is stronger.
Figure 9. Hip joint
The Knee Joint
The tibiofemoral (knee) joint is the largest and most complex diarthrosis of the body. It is primarily a hinge joint, but when the knee is flexed it is also capable of slight rotation and lateral gliding. The patella and patellar ligament also articulate with the femur to form a gliding patellofemoral joint. The joint capsule encloses only the lateral and posterior aspects of the knee joint, not the anterior. The anterior aspect is covered by the patellar ligament and the lateral and medial patellar retinacula. These are extensions of the tendon of the quadriceps femoris muscle, the large anterior muscle of the thigh. The knee is stabilized mainly by the quadriceps tendon in front and the tendon of the semimembranosus muscle on the rear of the thigh. Developing strength in these muscles therefore reduces the risk of knee injury. The joint cavity contains two C-shaped cartilages called the lateral and medial menisci (singular, meniscus) joined by a transverse ligament. The menisci absorb the shock of the body weight jostling up and down on the knee and prevent the femur from rocking from side to side on the tibia.
The posterior popliteal region of the knee is supp