On to anatomy:
I have always had trouble visualizing things from
pure description. I work much better by
looking at a picture or holding up a model when learning anatomy or
understanding how to get from point A to point B. For that reason, I recommend pulling out your GRAY’S ANATOMY (not
the TV show) or googling “shoulder anatomy”.
Look at a picture when going through this description. It will stick in your head much better. You
may ask "why not include a picture here?" The reason is: most sources
are protected by copyright laws. Some useful links are: http://www.webmd.com/pain-management/picture-of-the-shoulder
and for those who like video http://video.about.com/orthopedics/Anatomy-of-Shoulder.htm
Bones
and joints
Think of the shoulder as a cool-looking, weirdly
shaped, perfect structure. It consists
of the scapula (shoulder blade) aligned with two other bones, the humerus
(upper arm) and the clavicle (collar bone).
The scapula looks like a wide, flat sail
that is laid across the back of the thorax (rib cage). It is triangular in shape and has two
impressive projections that stick out forward off the top of the shoulder
blade. The higher projection is called
the acromion. The other projection is
called the coracoid process. It sticks out just below the acromion and extends
more towards the front of the body (anteriorly). Along the back of the scapula lies the scapular spine. It divides the scapula into two unequal
parts, an upper suprascapular section and a larger, lower infrascapular
region. These distinctions are
important as many of the muscles, which attach to the scapula, are generally
confined to one or the other section.
The humerus is the large arm bone that
extends from the shoulder region to the elbow.
There is nothing funny about it.
The clavicle or “collarbone” extends
from the base of the neck anteriorly (front side of the body) and travels
laterally (sideways) and posteriorly (towards the back) to the acromion, the
anteriorly-oriented projection off the scapula.
Those three bones form joints to create the
shoulder girdle.
The end of the scapula opposing the humerus is
called the glenoid. The joint between the glenoid and the humerus is the main
shoulder joint It is called the glenohumeral joint. The cavity of this joint is
covered on both sides by articular cartilage (smooth white tissue that allows
the bones to glide over each other).
This joint looks like a ball and socket: the ball being the head of the humerus and the socket being the glenoid
cavity. If you held them up against
each other you would see right away that they really aren’t a tight ball and
socket like the hip joint. That is all
in the design, allowing for a broader, flatter connection, which permits more
flexibility and less stability. The joint is stabilized by a ring of tough,
elastic tissue (fibrous cartilage), called the labrum.
The next joint is the acromioclavicular (AC)
joint. It is composed of a contact
between the lateral aspect of the clavicle and the top of the scapula (earlier
called the acromion). You can feel it
on yourself by running your fingers along your collarbone to its natural end,
which is marked by a small upward bump.
If you have dislocated your shoulder in the past or developed some
arthritis there, the bump may not be so small!
The third joint of the shoulder girdle is
actually at the other end of the clavicle.
It is between the sternum and the clavicle and thus called the
sternoclavicular joint.
A fourth articular surface is the scapulothoracic
surface. It is not an actual joint but
is an important feature of the shoulder girdle. It lies between the anterior scapular surface, which is smooth
and flat and slightly coved, and the back of the ribcage which neatly parallels
the scapula.
The
muscles
In this summary we will limit our discussion to
the group of muscles called the rotator cuff and mention also the biceps and
deltoid muscles. There are other
associated muscles that are involved in scapular stability and surrounding
structures but we will have to leave those for another day.
The rotator cuff muscles are a group of
four muscles that have a wide base of origin on the scapula and insert narrowly
on the humeral head. The visual we were
always given as med students was to think of the muscles as fingers on your
hand. The wrist is attached to the
scapula and the fingers extend to the humeral head and individually hold
on. Activation of the muscles leads to
movement of the humerus such as elevation, abduction, adduction, etc. With the simultaneous activation of muscles
to hold the scapula still (stabilizers), the humerus will move primarily
allowing for a proper arm swing like in backstroke or freestyle.
The rotator cuff muscles include the
supraspinatus, the infraspinatus, teres minor and subscapularis. The supraspinatus is responsible for
shoulder abduction (elevating the arm up and away from the body). The
infraspinatus and teres minor contribute to abduction and external rotation
(turning the arm out, like opening up a right refrigerator door to the right
with your right hand). The
subscapularis is responsible for internal rotation. (That would be the motion required to close the same refrigerator
door with the right hand.) It is the
largest of the 4 muscles.
The deltoid muscle group is not one of
the rotator cuff muscles. It lies on
top of the four rotator cuff muscles and totally covers them. The deltoids act as shoulder abductors. The deltoids arise from the scapula at the
acromion and attach to the humerus partway down the arm, not right at the top
like the rotator cuff muscles.
The biceps muscles have two main bodies
or heads. The long head crosses the
front of the head of the humerus and inserts on the scapula. The biceps contribute to shoulder movements
somewhat. Most of the effects are at
the elbow.
Bursae
Bursae are fluid filled sacs that cushion and
bathe tendons and ligaments at and around the joints. I will only mention one bursa concerning the shoulder: the subacromial bursa. It is located below the acromion process of
the scapula and enjoys the company of the rotator cuff tendons, especially the
supraspinatus tendon.
Ligaments
Ligaments are fibrous tissue connectors that stretch from one bone to another. They are not attached to a muscle though they
often are intimately involved with adjacent muscles. There are numerous tendons associated with the shoulder
girdle. Sadly we cannot explore them
today.
Nerves
and blood vessels
NCMS has
asked me not to talk about the nerves and blood vessels for fear that one of
their readers would take out a contract on me.
It was explained that most of the readers are not anatomy geeks and
would rather spend their time in front of the computer or Smartphone playing
solitaire or looking at swimming stats.
See you at
Nationals!
