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The
shoulder joint, when not working properly, can cause us a lot of
pain, discomfort, and difficulty, with even the life's simplest
tasks. We all depend upon our shoulder joints for a very important
reason -- they enable us to move our arms in order to use them.
The
shoulder is a ball-and-socket joint, but it is very different from
the hip joint, which is also a ball-and-socket joint. While the
hip has more natural stability because the ball of the hip is
almost entirely surrounded by the socket of the pelvis, the
shoulder is much more like a golf ball resting on a tee.
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Bones
The two shoulder bones that form
the golf ball and tee are the upper arm, the humerus, and the
shoulder blade, or scapula.
The ball-shaped top of the humerus rests in a shallow cup, the
glenoid fossa, located on the scapula.
The ball of the humerus and the cup of
the glenoid are covered with a layer of surface cartilage.
Joint fluid naturally produced by the
body helps these two surfaces to glide with little friction.
They are held in place by ligaments called the joint capsule,
which keep the shoulder from slipping out of joint.
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The acromion is a curved section of the shoulder blade extending
from the back of the scapula over the top of the humeral head.
This part of the scapula is joined to the collarbone, or clavicle,
at the
acromio-clavicular, or A/C joint.
The collarbone is attached to the breastbone, or sternum, in the
center of the chest. The collarbone plays an important role in
helping to stabilize the entire shoulder and shoulder blade.
Muscles and
Tendons
There are many muscles attached to different parts of the
shoulder.
The deltoid muscle is attached to the shoulder blade and the
clavicle. It crosses the shoulder joint and attaches to the
humerus. The deltoid, the strongest shoulder muscle, allows you to
lift your arm up and away from the side of your body.
Under the deltoid is the rotator cuff, a group of four muscles –
the subscapularis, supraspinatus, infraspinatus and teres minor --
that form a strong cuff around the shoulder joint and help control
the rotation and position of the arm. Each of these muscles has a
tendon at the end that attaches to the humerus by growing directly
into the bone.
The subscapularis muscle is attached
to the scapula and the humeral head. It fits into a bump of bone
on the humerus called the lesser tuberosity. This muscle is used
to internally rotate the arm and bring it down to the side of the
body. It also plays a very important role in preventing the
shoulder from slipping out of the front of the joint.
The three other muscles of the
rotator cuff are attached to the back of the scapula and travel
behind the humeral head and insert
on a bump of bone called the greater tuberosity. These three
muscles together are called the posterior rotator cuff. They
externally rotate the arm and also help to bring it down to the
side of the body. The supraspinatus is the most commonly injured
part of the rotator cuff.
Ligaments
The capsule that surrounds the shoulder joint is a strong ligament
that keeps the ball and socket normally aligned. It is very
difficult to tear the ligaments of the capsule or pull the
shoulder out of joint. Such injuries usually occur only when a
great deal of force has been applied to the shoulder or the arm.
Some people have a capsule that is a little bit too loose and the
shoulder can slip out of joint. If the shoulder slips in and out
of joint twice, it is said to be unstable. Shoulder instability
can also occur in people who have a generalized looseness of their
joints, those who are very flexible and often called
"double-jointed." A bad injury can cause the shoulder to become
unstable by stretching or tearing the ligaments away from the
bone. |
