Shoulder

The shoulder consists of three bones and four
articulations. The bones include the clavicle,
scapula, and proximal humerus. The articulations include the glenohumeral, acromioclavicular, and sternoclavicular joints and the scapulothoracic articulation (or scapulothoracic motion
interface).
The proximal humerus can be divided into
four parts: the anatomic head, the greater tuberosity, the lesser tuberosity, and the surgical neck,
which joins the other parts to the shaft (Fig 1) (1).
The anatomic head is the articular portion of the
humeral head. It is normally angled 130°–140°
superomedial to the long axis of the humeral shaft
(2). The anatomic neck is a shallow waist extending nearly circumferentially around the base of
the anatomic head, where the glenohumeral joint
capsule inserts.

The greater tuberosity is a protuberance at the lateral humeral head, to which the supraspinatus,
infraspinatus, and teres minor tendons attach. On radiographs, the greater tuberosity is best seen
when the shoulder is externally rotated; with internal rotation, it is superimposed over the humeral head, making the head appear rounded.

The lesser tuberosity is a small tubercle where the subscapularis tendon inserts at the anterior
humeral head and is situated inferior to the greater tuberosity. Because of its anterior position, the lesser tuberosity is best visualized when the shoulder is internally rotated; with external rotation, it is superimposed over the humeral shaft. The bicipital groove (or intertubercular sulcus) runs along the anterior humeral neck and separates the greater and lesser tuberosities.

The surgical neck is less strictly defined, but
the term generally refers to the flared portion of
bone joining the proximal shaft just below the
level of the tuberosities. This is the location of the
primary physis of the proximal humerus in pediatric patients. During development, the ossification centers of the greater and lesser tuberosities
fuse with that of the anatomic head to form the
proximal humeral epiphysis, which is separated
from the metaphysis and humeral shaft by the
primary physis (4). From a surgical perspective,
however, in adult patients the greater and lesser
tuberosities might more easily be considered as
part of the metaphysis, contributing cortical components to the metaphyseal medullary bone connecting the surgical and anatomic necks.

The anterior and posterior circumflex arteries
arise from the axillary artery (Fig 2), with each
giving rise to an ascending branch that enters
the humerus and flows retrograde (distal to
proximal) into the anatomic head as the arteria
arcuata (1). Minimal additional arterial contribution derives from the arteries of the rotator
cuff, which enter the greater tuberosity, and
from the intraosseous metaphyseal artery via the
humeral shaft.
The scapula is a triangular bone with medial,
lateral, and superior borders. The scapular spine
runs along the posterior aspect of the scapular
body and continues laterally as the acromion
process. The scapular spine separates the scapular
body into supraspinous and infraspinous portions. At the upper part of the lateral scapular
margin is the glenoid process, composed of the
glenoid neck and glenoid fossa (Fig 1). The glenoid fossa (or glenoid cavity) is a pear-shaped
articular surface of the lateral scapula that is covered with articular cartilage, except in its central “bare area.” The shallow glenoid fossa accommodates a broad range of motion by the humeral
head. The osseous glenoid rim is augmented by
the fibrocartilaginous glenoid labrum, and the
glenohumeral articulation is further stabilized
statically by the synovium-lined joint capsule, including focal thickenings known as glenohumeral
ligaments, and dynamically by the myotendinous
rotator cuff (5).
The glenoid usually demonstrates mild superior inclination and posterior version relative to
the scapular body, although this morphology varies between genders and races (6). The glenoid
neck connects the articular glenoid fossa to the
scapular body. The coracoid process arises from
the anterosuperior aspect of the glenoid neck,
near the rim. The anatomic neck of the glenoid
extends from the junction of the glenoid neck
and the scapular body to the lateral margin of the
coracoid base (Fig 1). The surgical neck of the
glenoid extends to the suprascapular notch, medial to the coracoid base.

The arm is supported by an osseoligamentous
ring composed of the acromion process, acromioclavicular joint capsule, distal clavicle, coracoclavicular ligaments, coracoid process, and
glenoid process. In aggregate, these structures
are referred to as the superior shoulder suspensory complex (SSSC) (Fig 3) (7) and provide a
stable ring of supporting structures to prevent
or reduce displacement of the other elements
within the complex. Additional support is provided by the deltoid, trapezius, pectoralis, and
rotator cuff muscles. Multiple injuries within the
SSSC may potentially destabilize the arm relative to the shoulder girdle and are important to
recognize.