Shoulder
The shoulder
complex is made up of three bones: the humerus (arm bone),
the clavicle (collar bone), and the scapula
(shoulder blade). Where any two bones come together is a joint.
There are 4 joints in the shoulder complex: the sternoclavicular
(SC) joint (where the collar bone meets the center of the
chest), the acromioclavicular (AC) joint (where
the collar bone meets the shoulder blade), the scapulothoracic
joint (where the shoulder blade floats on the back of the
rib cage), and the glenohumeral joint (where the arm
bone meets the shoulder blade). When people refer to the shoulder
joint, they most likely are talking about the glenohumeral
joint.
The
glenohumeral joint is a ball and socket joint. That means
that a ball floats in a socket, which provides a great amount of
mobility. In the shoulder, the ball is the head of the humerus
(top of the arm bone) and the socket is the glenoid fossa
of the scapula (a cup located on the end of the shoulder
blade).
The glenoid
fossa is a very shallow socket. It is can be better described
as a dish rather than a cup. This provides for a tremendous amount
of mobility. The hip, another ball and socket joint, has a very
deep bony socket, which provides for less mobility, but more stability.
Whereas this bony socket provides stability in the hip, the shoulder
gets its stability from soft tissue such as muscles, ligaments,
and the joint capsule. This makes the shoulder a much more dynamic
and mobile joint, but it also means the shoulder can get into
compromised positions.
The shoulder
is best controlled by the rotator cuff, a group of four muscles
that wrap around the ball of the joint, holding it in the socket.
The subscapularis wraps the front, the supraspinatus
wraps the top, and the infraspinatus and teres minor
wrap the back. The different muscles engage at different times to
help hold the ball in the socket as the joint moves. If these muscles
become compromised due to poor flexibility, weakness, or injury,
the humerus can press and pinch structures, causing pain and inflammation.
Other major
muscles attaching through the shoulder are the teres major,
the pectoralis major/minor, the latissimus dorsi,
and the deltoid.
Click on the
links above to learn more about some common shoulder injuries. This
list is not exhaustive and is not designed to diagnose or treat
any pathology. If you suspect a shoulder injury, you should seek
a formal evaluation by a licensed sports medicine professional.
