Between each vertebra throughout the spine is a disc; a shock-absorbing
pillow that helps maintain proper spacing, stability, and motion within the
spine. Each disc has a fibrous, tire-like outer band (called the annulus
fibrosus) that encases a central, gel-like substance (called the nucleus
pulposus). The nucleus and annulus work together to absorb shock, help stabilize
the spine, and provide a controlled range of motion between each vertebra.
Motion in All Directions
To meet the needs of our daily activities, the spine and its vertebrae must
be free to move in multiple directions. Our daily activities require us to
move our spine forward, backwards, side-to-side, as well as rotate. These
types of movements travel in either of two directions: translational or rotational.
The translational planes have three types or “degrees” of movements
(forward/backward, side-to-side, and up/down). The rotational axes have three
degrees of movement as well (rotating, forward/backward bending, and side
to side bending). When the spine is completely free to move in all these
directions, it is said to have 6 degrees of freedom (3 degrees in translation
and 3 degrees in rotation).
The intervertebral disc and its structure (nucleus and annulus) provide
the key components that allow each vertebrae to move freely in
these 6 degrees of freedom.
