The intervertebral discs of the spine is where all your movement originates from. The joints in the back, also known as the ‘facet’ joints, control the direction in which the spine’s vertebral blocks can move (eg. forward, backwards, sideways, etc.), but the extent of movement is decided by the discs.
The spinal column is not a single long bone. Instead, it is made up of 34 blocks (ie. Vertebra), stacked on top of each other. Some of these blocks are fused together. Others blocks (ie. Vertebra) have discs between them which allow all movement. Remember, that bones cant change shape, so all movement in the body comes from your ‘joints’ in between those bones.
The spine is similar in that all movement comes from the joints between the spinal vertebral bones. When a disc is removed and the bones are fused, you still have movement in your spine from the levels above and below. However, those remaining levels may have to compensate.
On the other hand, when a discectomy is performed, the inner nucleus pulposus material is removed, but the outer annulus fibrosus layer is kept intact. By removing the nucleus pulposus, it then results in its replacement with type of scarred material known as fibrocartilage. This still allows movement within the disc, although its ability to absorb forces and impacts is much lower.