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The clavicle is a long bone that serves as the connection between the scapula and the sternum. The acromial end of the clavicle articulates with the acromion of the scapula at the acromioclavicular joint, and the sternal end articulates with the sternum at the sternoclavicular joint. The clavicle functions as a movable support for the scapula and upper limb, and it allows a large range of motion of the arm. The clavicle also provides protection for the brachial plexus, as well as the subclavian and axillary arteries. Two aspects which separate it from the other long bones of the body are that it runs horizontally and that it has no medullary cavity.

Fractures of the clavicle are common. The mechanism of injury is usually from an athlete or child extending their arm while falling to the ground. In this case, the impact of the force travels up the bones of the arm and fractures the small long bone (clavicle). In other cases, subjects simply fall directly on their shoulder. Diagnosis of clavicular fracture can be aided by palpating the bone (a fracture should be apparent). An additional sign is the slumping or low-hanging of the upper limb attached to the clavicle. This results from the fracture disabling the trapezius muscle from being able to hold up the entire weight of the limb.



Just lateral to the clavicle lie the muscles of the rotator cuff (scapulohumeral muscles). It  consists of four muscles: the teres minor, subscapularis, infraspinatus, and supraspinatus; which form a muscular cuff around the glenohumeral joint. The rotator cuff acts to stabilize the shoulder, and aids in the rotation and abduction (along with the deltoid) of the humerus. During movements of the shoulder and arm, the contraction of the muscles of the rotator cuff helps hold the head of the humerus in the glenoid cavity of the scapula.

Tears in the tendons of the muscles of the rotator cuff can result from repetitive use of the upper limb above the horizontal (seen in serving a tennis ball or weight-lifting), a sudden strain on the muscles while lifting something heavy, or from blunt trauma to the shoulder. This tearing results in poor functionality of the glenohumeral joint. Overuse of these muscles can also result in degenerative tendonitis. The tendon of the supraspinatus is the most commonly torn, which limits the person's ability to abduct the humerus.






Traveling further distally from the rotator cuff, in the anterior compartment of the arm, lies the biceps brachii. This muscle is made up of two heads which attach to the scapula by their two different tendons. The biceps brachii is capable of action with three joints: the glenohumeral joint, the humeroulnar joint, and the radioulnar joint. With these three joints, the biceps brachii helps supinate or flex the forearm. The short head also helps the shoulder resist dislocation. A membranous sheath, the bicipital aponeurosis, extends from the distal end of the biceps brachii and helps protect the brachial artery and median nerve as they pass through the cubital fossa.

Dislocation or rupture of the tendon of the long head of the biceps brachii can occur from forceful lifting (as in weight-lifting flexion) or can result from prolonged biceps tendinitis. Biceps tendinitis is the result of repetitive use or trauma to the biceps during physical activity (throwing a baseball, swinging a tennis racquet). Anatomically, the rupture or tendonitis of the tendon can result from the wear and tear it can receive if it repeatedly moves through a narrowed intertubecular sulcus of the humerus during physical activity. The rupturing of the tendon of the long head of the biceps brachii is often associated with an audible popping noise.

References:
Moore et al. "Clinically Oriented Anatomy" (2010) 6th ed. 
www.aafp.org
www.myerssportsmedicine.com