Shoulder
Joint
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Articulation: This occurs between the rounded head of the humerus and the
shallow, pear-shaped glenoid cavity of the scapula. The articular surfaces are
covered by hyaline articular cartilage, and the glenoid cavity is deepened by
the presence of a fibrocartilaginous rim called the glenoid labrum.
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Type: Synovial ball-and-socket joint
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Capsule: This surrounds the joint and is attached medially to the margin of the
glenoid cavity outside the labrum; laterally, it is attached to the anatomic
neck of the humerus. The capsule is thin and lax, allowing a wide range of
movement. It is strengthened by fibrous slips from the tendons of the
subscapularis, supraspinatus, infraspinatus, and teres minor muscles (the
rotator cuff muscles).
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Ligaments: The glenohumeral ligaments are three weak bands of fibrous tissue
that strengthen the front of the capsule. The transverse humeral ligament
strengthens the capsule and bridges the gap between the two tuberosities . The
coracohumeral ligament strengthens the capsule above and stretches from the root
of the coracoid process to the greater tuberosity of the humerus.
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Accessory ligaments: The coracoacromial ligament extends between the coracoid
process and the acromion. Its function is to protect the superior aspect of the
joint
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Synovial membrane: This lines the capsule and is attached to the margins of the
cartilage covering the articular surfaces. It forms a tubular sheath around the
tendon of the long head of the biceps brachii. It extends through the anterior
wall of the capsule to form the subscapularis bursa beneath the subscapularis
muscle .
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Nerve supply: The axillary and suprascapular nerves
Movements
The shoulder joint has a wide range of movement, and the
stability of the joint has been sacrificed to permit this.
(Compare with the hip joint, which is stable but limited in
its movements.) The strength of the joint depends on the tone of the short
rotator cuff muscles that cross in front, above, and behind the joint—namely,
the subscapularis, supraspinatus, infraspinatus, and teres minor. When the
joint is abducted, the lower surface of the head of the humerus is supported by
the long head of the triceps, which bows downward because of its length and
gives little actual support to the humerus. In addition, the inferior part of
the capsule is the weakest area.
Stability
of the Shoulder Joint
The shallowness of the glenoid fossa of the scapula and the
lack of support provided by weak ligaments make this joint an unstable structure.
Its strength almost entirely depends on the tone of the short muscles that bind
the upper end of the humerus to the scapula—namely, the subscapularis in front,
the supraspinatus above, and the infraspinatus and teres minor behind. The tendons
of these muscles are fused to the underlying capsule of the shoulder joint.
Together, these tendons form the rotator cuff.
The least supported part of the joint lies in the inferior
location, where it is unprotected by muscles.
Dislocations
of the Shoulder Joint
The shoulder joint is the most commonly dislocated large
joint.
Anterior Inferior Dislocation
Sudden violence applied to the humerus with the joint fully abducted
tilts the humeral head downward onto the inferior weak part of the capsule,
which tears, and the humeral head comes to lie inferior to the glenoid fossa.
During this movement, the acromion has acted as a fulcrum. The strong flexors and
adductors of the shoulder joint now usually pull the humeral head forward and
upward into the subcoracoid position.
Posterior Dislocations
Posterior dislocations are rare and are usually caused by
direct violence to the front of the joint. On inspection of the patient with
shoulder dislocation, the rounded appearance of the shoulder is seen to be lost
because the greater tuberosity of the humerus is no longer bulging laterally
beneath the deltoid muscle. A subglenoid displacement of the head of the
humerus into the quadrangular space can cause damage to the axillary nerve, as
indicated by paralysis of the deltoid muscle and loss of skin sensation over
the lower half of the deltoid. Downward displacement of the humerus can also
stretch and damage the radial nerve.
Shoulder
Pain
The synovial membrane, capsule, and ligaments of the shoulder joint are innervated by the axillary nerve and the suprascapular nerve. The joint is sensitive to pain, pressure, excessive traction, and distention. The muscles surrounding the joint undergo reflex spasm in response to pain originating in the joint, which in turn serves to immobilize the joint and thus reduce the pain.
Injury to the shoulder joint is followed by pain, limitation
of movement, and muscle atrophy owing to disuse. It is important to appreciate
that pain in the shoulder region can be caused by disease elsewhere and that
the shoulder joint may be normal; for example, diseases of the spinal cord and
vertebral column and the pressure of a cervical rib (see page XXX) can cause
shoulder pain. Irritation of the diaphragmatic pleura or peritoneum can produce
referred pain via the phrenic and supraclavicular nerves.
