Bones and Joints
The four bones of the shoulder:
Together these four bones form four junctions, or joints:
- The humerus is the upper arm bone. This is the
"ball" of the shoulder's "ball and socket" joint.
- The scapula is the flat, triangular bone commonly
called the shoulder blade. Prominent areas of the scapula serve
as attachment points for many muscles and ligaments.
- The glenoid is the shallow "socket" on the
side of the scapula that receives the 'ball' of the humerus.
Together they form the "ball and socket" arrangement of the
- The acromion is the end of the scapular spine.
It projects forward to form the top of the shoulder.
- The coracoid process is a projection towards
the front of the scapula and is an attachment site for several
muscles and ligaments.
- The clavicle or collarbone. Although it appears
to be straight, it is actually S-shaped when seen from above.
- The thorax, or rib cage, is an anchor for several
muscles and ligaments. Although the ribs do not physically attach
to the scapula, the thorax stabilizes and maintains proper positioning
of the scapula so that the arm can function to its fullest capacity.
- The glenohumeral joint is the main joint of the
shoulder. Here, the glenoid on the scapula and the head of the
humerus come together to form a ball and socket joint. The fairly
flat socket of the glenoid surrounds only 20% - 30% of the humeral
head. Because of its poor fit, this joint relies heavily on the
surrounding ligaments and muscles for support. The labrum,
a ring of fibrocartilage tissue, attaches to the glenoid and deepens
the socket to encircle more of the humerus.
- The acromioclavicular joint, or AC joint, is the
bony point on the top of the shoulder. It anchors the scapula
to the chest, by connecting the acromion on the scapula to the
clavicle, or "collarbone". A thick disk of fibrocartilage acts
as a shock absorber between the two bones. The surrounding capsule
and ligaments give this joint great stability.
- The sternoclavicular joint, or SC joint, connects
the other end of the clavicle to the sternum, or
"breastbone". Like the AC joint, this joint contains a fibrocartilage
disk that helps the bones achieve a better fit. It also gets strong
support from its joint capsule and surrounding ligaments.
- The scapulothoracic articulation is the area where
the scapula, embedded in muscle, glides over the rib cage. The
surrounding muscles and ligaments keep the scapula properly positioned
so that the arm can move correctly.
There are two types of cartilage in the shoulder:
- Articular cartilage is the shiny white coating
that covers the end of the humeral head and lines the inside surface
of the glenoid. It has two purposes:
- To provide a smooth, slick surface for easy movement
- To be a shock absorber and protect the underlying bone
- Fibrocartilage is the thick tissue that forms
the disks of the AC and SC joints and the labrum, the ring that
deepens the glenoid. Fibrocartilage has three roles:
- To act as a cushion in shock absorption
- To help stabilize the joint by improving the fit of the bones
- To act as a spacer and improve contact between the articular cartilage surfaces
The shoulder relies heavily on ligaments for support. Ligaments attach
bone to bone and provide the "static" stability in a joint.
Ligaments keep the joint within the normal limits of movement.
Muscles and Tendons
- The glenohumeral ligaments attach in layers from
the glenoid labrum to the humerus, forming the joint capsule around
the head of the humerus.
- The coracoacromial ligament is a ligament that
spans the bony projections of the coracoid process and the acromion.
- The coracoclavicular ligaments (CC ligaments)
and the acromioclavicular ligament (AC ligament)
provide most of the support for the AC joint.
Muscles and tendons work together in the shoulder to provide the "dynamic"
stability of the shoulder.
There are four muscle groups in the shoulder:
- The rotator cuff muscles are the subscapularis,
the supraspinatus, the infraspinatus,
and the teres minor. They are the primary stabilizers
that hold the "ball" of the humerus to the glenoid "socket". The
socket is too shallow to offer much security for the humerus.
These four muscles form a "cuff" around the humeral head, securing
it firmly in the socket. As its name implies, this group of muscles
also rotates the arm. The rotator cuff protects the glenohumeral
joint from dislocation, allowing the large muscles that control
the shoulder to power the arm with great mobility.
- The biceps tendon complex also helps to keep the humeral
head in the glenoid and flex and rotate the arm.
- The scapulothoracic muscles attach the scapula
to the thorax. Their main function is to stabilize the scapula
to allow for proper shoulder motion.
- The superficial muscles of the shoulder are the
large, powerful outer layer of muscles that are important to the
overall function of the shoulder. This group includes the deltoid
muscle, which covers the rotator cuff muscles.
A bursa is a pillow-like sac filled with a small amount
of fluid. Bursae (plural) reduce friction and allow smooth gliding
between two firm structures, like bone and tendon or bone and muscle.
There are over 50 bursae in the human body; the largest is the subacromial
bursa (under the acromion) in the shoulder. The subacromial
bursa and the subdeltoid bursa (under the deltoid muscle) are often
considered as one structure. This bursa separates the rotator cuff
from the acromion and acts as a spacer between them.
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