(movements of the scapula on the thorax)
The scapula has five degrees of freedom for movement on the thorax:
- It can move in two "straight-line" directions or "translations."
- It can move in three planes, around three different axes.
The scapula's two degrees of freedom for translatory motion permit:
- ELEVATION or DEPRESSION
- ADDUCTION (also called RETRACTION)
or ABDUCTION (also called PROTRACTION)
Although a third degree of freedom is theoretically possible, the scapula does not move anteriorly or posteriorly on the thorax.
Notice that we don't identify axes for these motions. These motions are are "translations;" they don't occur around axes!
The scapula also has three degrees of freedom for rotatory motion; it can move in three planes around three axes:
Scapular WINGING occurs around a vertical axis that passes through the AC joint.
Scapular TIPPING occurs around a lateral axis that passes through the AC joint.
Scapular UPWARD ROTATION and DOWNWARD ROTATION occur around an A-P axis that passes through
- the AC joint (as the figure illustrates)
- or through the SC joint
(The SC joint's A-P axis is a line that connects, approximately, the SC joint surface and a point at the base of the scapular spine.)
The scapula moves normally on the thorax only if the two joints by which it attaches to the axial skeleton are also free to move. These two joints are the: