The musculoskeletal system includes bones; muscles with their related tendons and synovial sheaths; bursae; and joint structures such as cartilage, menisci, capsules, and ligaments.
The shoulder complex (Figure 8-1) is composed of four separate articulations:
The glenohumeral (GH) joint
The acromioclavicular (AC) joint
The sternoclavicular (SC) joint
The scapulothoracic pseudoarticulation
Anterior view of right shoulder joints. (Reproduced, with permission, from Luttgens K, Hamilton N: Kinesiology: Scientific Basis of Human Motion, 10th ed. New York, NY: McGraw-Hill. 2002:104.)
An important bony landmark in the shoulder is the coracoid process, medial to which run the major blood vessels and the brachial plexus complex. The coracoid serves as a muscular attachment for the pectoralis minor, the short head of the biceps, and the coracobrachialis. The acromion and the acromioclavicular joint are also important bony landmarks that can be easily palpated.
Injury to the long thoracic nerve leads to paralysis of the serratus anterior muscle, resulting in medial scapular winging.
The subacromial junction between the coracoacromial arch (a rigid structure above the humeral head and rotator cuff tendons) and the rotator cuff tendons—is considered by some as a fifth "articulation".1
Approximately eight bursae are distributed throughout the shoulder complex. The subdeltoid-subacromial bursae, collectively referred to as the subacromial bursa, are the most significant with relation to pathology. As the humerus elevates, the bursae permit the rotator cuff to slide easily beneath the deltoid muscle.
Complete movement at the shoulder girdle involves a complex interaction between the glenohumeral; AC and SC complex; scapulothoracic, upper thoracic, costal, and sternomanubrial joints; and the lower cervical spine.
- The convex humeral head articulates with the concave glenoid fossa of the scapula (Table 8-1). The humeral head is retroverted 20 to 30 degrees. The longitudinal axis of the head is 135 degrees from the axis of the neck.
- The glenoid is retroverted approximately 7 degrees. It faces anteriorly at an angle of approximately 45 degrees to the coronal plane, as it sits on the chest wall. The depth of the glenoid fossa is enhanced by the glenoid labrum, which can contribute up to 50% of the fossa's depth.
- The scapula is a flat triangular bone, situated over the second to seventh ribs. The glenoid fossa is located on the lateral angle of the scapula and faces anteriorly, laterally and superiorly. This orientation places true abduction at 30 degrees anterior to the frontal plane.