The important structures of the shoulder can be divided
into several categories. These include
- bones and joints
- ligaments and tendons
- muscles
- nerves
- blood vessels
- bursae
Bones and Joints
The bones of the shoulder are the humerus (the
upper arm bone), the scapula (the shoulder blade),
and the clavicle (the collar bone). The roof of the
shoulder is formed by a part of the scapula called the
acromion.
There are actually four joints that make up the shoulder.
The main shoulder joint, called the glenohumeral joint, is formed where the ball of
the humerus fits into a shallow socket on the scapula. This
shallow socket is called the glenoid.
The acromioclavicular (AC) joint is where the clavicle
meets the acromion. The sternoclavicular (SC) joint supports the
connection of the arms and shoulders to the main skeleton on
the front of the chest.
A false joint is formed where the shoulder blade
glides against the thorax (the rib cage). This joint,
called the scapulothoracic joint, is important because it
requires that the muscles surrounding the shoulder blade
work together to keep the socket lined up during shoulder
movements.
Articular cartilage is the material that covers
the ends of the bones of any joint. Articular cartilage is
about one-quarter of an inch thick in most large,
weight-bearing joints. It is a bit thinner in joints such as
the shoulder, which don't normally support weight. Articular
cartilage is white and shiny and has a rubbery consistency.
It is slippery, which allows the joint surfaces to slide
against one another without causing any damage. The function
of articular cartilage is to absorb shock and provide an
extremely smooth surface to make motion easier. We have
articular cartilage essentially everywhere that two bony
surfaces move against one another, or articulate. In
the shoulder, articular cartilage covers the end of the
humerus and socket area of the glenoid on the scapula.
Ligaments
and Tendons
There are several important ligaments in the
shoulder. Ligaments are soft tissue structures that connect
bones to bones. A joint capsule is a watertight sac
that surrounds a joint. In the shoulder, the joint capsule
is formed by a group of ligaments that connect the humerus
to the glenoid. These ligaments are the main source of
stability for the shoulder. They help hold the shoulder in
place and keep it from dislocating.
Ligaments attach the clavicle to the acromion in the AC
joint. Two ligaments connect the clavicle to the scapula by
attaching to the coracoid process, a bony knob that
sticks out of the scapula in the front of the shoulder.
A special type of ligament forms a unique structure
inside the shoulder called the labrum. The labrum is
attached almost completely around the edge of the glenoid.
When viewed in cross section, the labrum is wedge-shaped.
The shape and the way the labrum is attached create a deeper
cup for the glenoid socket. This is important because the
glenoid socket is so flat and shallow that the ball of the
humerus does not fit tightly. The labrum creates a deeper
cup for the ball of the humerus to fit into.
The labrum is also where the biceps tendon attaches
to the glenoid. Tendons are much like ligaments,
except that tendons attach muscles to bones. Muscles move
the bones by pulling on the tendons. The biceps tendon runs
from the biceps muscle, across the front of the shoulder, to
the glenoid. At the very top of the glenoid, the biceps
tendon attaches to the bone and actually becomes part of the
labrum. This connection can be a source of problems when the
biceps tendon is damaged and pulls away from its attachment
to the glenoid.
The tendons of the rotator cuff are the next layer
in the shoulder joint. Four rotator cuff tendons connect the
deepest layer of muscles to the humerus.
Muscles
The rotator cuff tendons attach to the deep rotator cuff
muscles. This group of muscles lies just outside the
shoulder joint. These muscles help raise the arm from the
side and rotate the shoulder in the many directions. They
are involved in many day-to-day activities. The rotator cuff
muscles and tendons also help keep the shoulder joint stable
by holding the humeral head in the glenoid socket.
The large deltoid muscle is the outer layer of shoulder
muscle. The deltoid is the largest, strongest muscle of the
shoulder. The deltoid muscle takes over lifting the arm once
the arm is away from the side.
Nerves
All of the nerves that travel down the arm pass through the
axilla (the armpit) just under the shoulder joint.
Three main nerves begin together at the shoulder: the
radial nerve, the ulnar nerve, and the
median nerve. These nerves carry the signals from the
brain to the muscles that move the arm. The nerves also
carry signals back to the brain about sensations such as
touch, pain, and temperature.
Blood Vessels
Traveling along with the nerves are the large vessels
that supply the arm with blood. The large axillary artery travels through the axilla. If
you place your hand in your armpit, you may be able to feel
the pulsing of this large artery. The axillary artery has
many smaller branches that supply blood to different parts
of the shoulder. The shoulder has a very rich blood
supply.
Bursae
Sandwiched between the rotator cuff muscles and the outer
layer of large bulky shoulder muscles are structures known
as bursae. Bursae are everywhere in the body. They
are found wherever two body parts move against one another
and there is no joint to reduce the friction. A single bursa
is simply a sac between two moving surfaces that contains a
small amount of lubricating fluid.
Think of a bursa like this: If you press your hands
together and slide them against one another, you produce
some friction. In fact, when your hands are cold you may rub
them together briskly to create heat from the friction. Now
imagine that you hold in your hands a small plastic sack
that contains a few drops of salad oil. This sack would let
your hands glide freely against each other without a lot of
friction.
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