Clavicular and
Shoulder Joints Anatomy
Three joints contribute to the
considerable mobility of the arm; movement occurs
between the humerus and the scapula at the shoulder (glenohumeral)
joint, and the scapula moves on the chest wall through the joints at each end
of the clavicle. Although few muscles attach to the clavicle, the numerous
muscles attached to the scapula and upper humerus all contribute to
movement at the clavicular joints. Indeed, movement at the shoulder joint is
almost always associated with movement at the sternoclavicular and
acromioclavicular joints.
Clavicular joints
The sternoclavicular and
acromioclavicular joints are subcutaneous and easily palpable in the living
subject. Each has a tubular capsule lined by synovial membrane.
At the sternoclavicular joint (Fig.
3.80), the medial end
of the clavicle articulates with the notch on the upper border of the manubrium
and with the first costal cartilage. The joint is partitioned by an
intracapsular disc of fibrocartilage that attaches superiorly to the clavicle,
inferiorly to the first costal cartilage and around its periphery to the
capsule. There are two accessory ligaments. Above the capsule is the
interclavicular ligament, which joins the medial ends of the clavicles. Just
lateral to the joint is the costoclavicular (rhomboid) ligament, which attaches
the clavicle firmly to the first costal cartilage. Stability depends on the
disc and accessory ligaments, which limit both medial displacement and
elevation of the medial end of the clavicle. The sternoclavicular joints are
separated from the origins of the brachiocephalic veins and other structures in
the root of the neck by the sternohyoid and sternothyroid muscles.
At the acromioclavicular joint (Fig.
3.81) the lateral end
of the clavicle articulates with the medial aspect of the acromion of the
scapula. The joint capsule attaches to the edges of the articular surfaces that
lie obliquely, the clavicular facet facing laterally and inferiorly.
Stability of the joint is provided mostly by the strong coracoclavicular
ligament linking the coracoid process and the undersurface of the clavicle near
its lateral end. This ligament consists of conoid and trapezoid parts.
Both the sternoclavicular and
acromioclavicular joints are supplied by branches of the supraclavicular nerves
(C3 & C4).
The lateral end of the clavicle may
be elevated or depressed and drawn forwards or backwards. The axes of these
movements occur at the costoclavicular and coracoclavicular ligaments, rather than through the clavicular joints. Thus, the medial
end of the clavicle is elevated during depression of the scapula and moves
posteriorly when the scapula is protracted. Full abduction of the upper limb
requires rotation of the scapula so that the glenoid fossa tilts upwards.
Rotation of the clavicle through 40° at the sternoclavicular joint supplements
the 20° of movement available at the acromioclavicular joint, permitting the
scapula to rotate through about 60°. The principal muscles of scapular rotation
are trapezius and serratus anterior. Protraction is produced by pectoralis
minor and serratus anterior and retraction by trapezius and the
rhomboids.
The clavicle forms a strut that
supports the scapula against the medial pull of muscles such as pectoralis
major and latissimus dorsi. The clavicular joints are stabilized by their
accessory ligaments, which are so strong that trauma, such as falling onto the
outstretched limb, is more likely to fracture the clavicle than rupture the
ligaments. The lateral part of a fractured clavicle tends to be displaced
inferiorly by the weight of the limb and medially by spasm of pectoralis major
and latissimus dorsi muscles, whereas the medial fragment may be elevated by
the action of sternocleidomastoid muscle.
Shoulder joint
The shoulder (glenohumeral) joint
is synovial, of the ball-and-socket type, and is capable of a wide range of
movement. The hemispherical head of the humerus is directed medially and
back-wards and articulates with the much smaller glenoid fossa of the scapula (Fig.
3.82). The fossa faces
anterolaterally and is slightly deepened by the glenoid labrum, a cartilaginous
lip round its edge.
The joint capsule (Fig.
3.83) forms a loose
sleeve attaching medially to the glenoid labrum. Its humeral attachment is
around the anatomical neck except inferiorly on the medial side, where it
descends to the level of the surgical neck.
Synovial membrane lines the fibrous
capsule and covers the intracapsular part of the humeral shaft (Fig.
3.84). The cavity of
the joint usually communicates with the subscapular bursa through a deficiency
in the anterior part of the capsule. Indistinct thickenings in the capsule form
the glenohumeral ligaments. Between the greater and lesser tubercles (Fig.
3.85), the capsule
forms the transverse humeral ligament beneath which the tendon of the long head
of biceps enters the joint from the intertubercular groove. The tendon is
surrounded by a tubular sheath of synovial membrane as it passes over the
humeral head to attach to the supraglenoid tubercle (Fig. 3.86).
The joint is intimately related to
subscapularis, supraspinatus, infraspinatus and teres minor (Figs 3.86 &
3.87) whose tendons fuse with the capsule to form the rotator cuff (Fig. 3.83).
Above the joint is the coracoacromial arch formed by the coracoid process, the
acromion and the intervening coracoacromial ligament. The arch is separated
from supraspinatus by the subacromial bursa.
Articular nerves are derived from
the suprascapular and subscapular nerves and also from the axillary nerve which
passes very close to the joint. As this nerve leaves the axilla through the
quadrangular space (Fig. 3.64), it lies immediately inferior to the capsule.
The vascular supply is provided by branches of the circumflex humeral and
suprascapular arteries.
Flexion of the shoulder joint (up
to 180°) is produced mainly by the clavicular fibres of pectoralis major and
the anterior fibres of deltoid. Extension (limited to about 45°) is produced by
latissimus dorsi and the posterior fibres of deltoid. At the shoulder joint
itself, about 120° of abduction is possible, produced by supraspinatus and
deltoid; simultaneous rotation of the scapula through 60° permits full
elevation of the arm above the head.
Adduction, produced by teres major,
latissimus dorsi and pectoralis major, is limited by the area of the articular
surface of the humerus. Medial rotation is produced by pectoralis major,
subscapularis, teres major and the anterior fibres of deltoid, and lateral
rotation by infraspinatus, teres minor and the posterior fibres of deltoid.
Although the coracoacromial arch
prevents upward displacement of the humerus, stability of the shoulder joint
relies principally on the rotator cuff muscles that hold the humeral head
firmly in the glenoid fossa. Despite the labrum, the glenoid fossa is a shallow
socket. The capsular ligaments are lax in most positions and tighten only near
the extremes of movement. Dislocation of the joint, usually with anterior and
inferior displacement of the head of the humerus, associated with trauma or
weakness of the rotator cuff muscles, is relatively common and may
result in damage to the axillary nerve.
