In all tetrapods there is a scapula which is dorsal and a coracoid which is ventral. The humerus always articulates at the junction of the two elements. In the human shoulder, scapula and coracoid articulate with each other across the joint line. The presence of an epiphyseal cartilage across a joint line is beneficial in that it facilitates adjustment of the joint surfaces during growth of the bone ends (as at the shoulder, elbow and hip).
The shoulder joint is a synovial joint of the ball and socket variety. There is a 4 to 1 disproportion between the large round head of the humerus and the small shallow glenoid cavity. The glenoid labrum, a ring of fibrocartilage attached to the margins of the glenoid cavity, deepens slightly but effectively the depression of the glenoid ‘fossa’.
The capsule of the joint is attached to the scapula beyond the supraglenoid tubercle and the margins of the labrum. It is attached to the humerus around the articular margins of the head (i.e. the anatomical neck) except inferiorly, where its attachment is to the surgical neck of the humerus a finger’s breadth below the articular margin. At the upper end of the intertubercular groove the capsule bridges the gap between the greater and lesser tuberosities, being here named the transverse ligament. A gap in the anterior part of the capsule allows communication between the synovial membrane and the subscapularis bursa. A similar gap is sometimes present posteriorly, allowing communication with the infraspinatus bursa. The fibers of the capsule all run horizontally between scapula and humerus. The capsule is thick and strong but it is very lax, a necessity in a joint as mobile as this. Near the humerus the capsule is greatly thickened by fusion of the tendons of the short scapular muscles. The long tendon of biceps is intracapsular.
The synovial membrane is attached around the glenoid labrum and lines the capsule. It is attached to the articular margin of the head of the humerus and covers the bare area of the surgical neck that lies within the capsule at the upper end of the shaft. It ‘herniates’ through the hole in the front of the capsule to communicate with the subscapularis bursa and sometimes it communicates with the infraspinatus bursa. It invests the long head of biceps in a tubular sleeve that is reflected back along the tendon to the transverse ligament and adjoining floor of the intertubercular groove. The synovial sleeve glides to and fro with the long tendon of biceps during abduction-adduction of the shoulder.
The glenohumeral ligaments are scarcely worthy of mention, being slight thickenings above and below the opening into the subscapularis bursa in the anterior part of the capsule. They are visible only from within the joint cavity.
The coracohumeral ligament is quite strong. It runs from the under surface of the coracoid process laterally across the capsule, to which it becomes attached at the margin of the greater tuberosity, and along the transverse ligament.
From the medial border of the acromion, in front of the acromioclavicular articulation, a strong flat triangular band, the coracoacromial ligament, fans out to the lateral border of the coracoid process. It lies above the head of the humerus and serves to increase the surface upon which the head of the humerus is supported. It is separated from the ‘rotator cuff by the subacromial bursa.
The subacromial (subdeltoid) bursa is a large bursa which lies under the coracoacromial ligament, to which its upper layer is attached. Its lower layer is attached to the tendon of supraspinatus. It extends beyond the lateral border of the acromion with the arm at the side, but is rolled inwards under the acromion when the arm is abducted. Tenderness over the greater tuberosity of the humerus beneath the deltoid muscle which disappears when the arm is abducted indicates subacromial bursitis. Tearing the supraspinatus tendon brings the bursa into communication with the shoulder joint cavity, but in the normal shoulder the bursa never communicates with the joint.
Stability of shoulder joint:
The shoulder joint, thus far described, is seen to be a very unstable structure. The head of the humerus is much larger than the glenoid cavity, and the joint capsule, though strong, is very lax. True it is that the concavity of the glenoid fossa, deepened by the labrum, is a significant stabilizing factor. But this is only because the short scapular muscles hold the head in close apposition. Fracture of the labrum results in dislocation. Stability is increased by the coracoacromial arch, the fusion of tendons of scapular muscles with the capsule of the joint, and the muscles attaching the humerus to the pectoral girdle.
Coracoacromial arch: Upward displacement of the head of the humerus is prevented by the overhanging coracoid and acromion processes and the coracoacromial ligament that bridges them. The whole constitutes the coracoacromial arch and, lubricated by the subacromial bursa, functions mechanically as an ‘articular surface’ of the shoulder joint. The arch is very strong. Upward thrust on the humerus will never fracture the arch; the clavicle or the
Tendons of scapular muscles: The tendons of subscapularis, supraspinatus, infraspinatus and teres minor are not only attached very near the joint but actually in part fuse with the lateral part of the capsule. This is an indispensable factor in adding stability to the joint. The fused mass of tendon and lateral part of capsule is known surgically as the ‘rotator cuff. The rotator cuff prevents the lax capsule and its lining synovial membrane from being nipped. There is no cuff inferiorly, and here the capsule is attached well below the articular margin to prevent its being nipped. Note that supraspinatus does not have any rotatory action on the humerus, although the other three muscles do.
Muscles attaching the humerus to the pectoral girdle: All these muscles assist by their tonus in maintaining the stability of the joint. Especially active in this respect are the long heads of biceps and triceps muscles.
The long head of biceps, arising from the supraglenoid tubercle, sinks in through the capsule of the embryonic joint and is intracapsular in the mature joint. It leaves the capsule beneath the transverse ligament across the upper part of the intertubercular groove. The tendon acts as a strong support over the head of the humerus.
The long head of triceps is of importance during abduction of the joint, for in this position it lies immediately beneath the head of the humerus at the lowest part of the joint. This is the weakest part of the joint; the long head of triceps is thus of very great importance in giving stability to the abducted humerus.
Nerve supply of shoulder joint:
The capsule is supplied by branches from the axillary, musculocutaneous and suprascapular nerves. Each illustrates Hilton’s law.
Movements of shoulder joint:
The articular surface of the head of the humerus is four times the area of the glenoid; thus there is considerable freedom for a variety of movements. Movements at the joint are accompanied by movements of the scapula on the thoracic wall and by consequential movements of the clavicle.
These various movements of the shoulder joint itself, however, can be understood only if each is analysed into its constituent basic parts. These basic movements are only three: 1. flexion and extension, 2. adduction and abduction, and 3. rotation. Note that circumduction is merely a rhythmical combination in orderly sequence of flexion, abduction, extension and adduction (or the reverse), and for purposes of analysis it is not an elementary movement.
It is of clinical value to appreciate the fact that the medial epicondyle of the humerus faces in the same direction as the articular surface of the head. Note that this is not exactly medial, but rather medial and somewhat backwards. The glenoid cavity does not face exactly laterally, but peeps forward a little around the convexity of the thoracic wall, and the articular surface on the head of the humerus looks back towards it. The purist may claim that flexion and extension should be described as movements in the plane of the joint cavity, which is oblique to the sagittal plane, and that abduction and adduction should be defined as movements in a plane at right angles to this, which is oblique to the coronal plane. But for purposes of analysis of these movements the difference is negligible.