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.