The spurs of bone on the back of each vertebra slot together to form joints that glide back and forth as the spine moves, called facet joints. The shapes of the bones limit how much each joint can move. Discs of cartilage between the bones absorb shock by squashing slightly. They also stop the bones from grinding painfully against each other as they move.
Facet joints appear to have little influence on the range of side bending (lateral flexion). These functions can be disrupted by degeneration, dislocation, fracture, injury, instability from trauma, osteoarthritis, and surgery. In the thoracic spine the facet joints function to restrain the amount of flexion and anterior translation of the corresponding vertebral segment and function to facilitate rotation. Cavitation of the synovial fluid within the facet joints is responsible for the popping sound (crepitus) associated with manual spinal manipulation, commonly referred to as “cracking the back.”
The facet joints, both superior and inferior, are aligned in a way to allow flexion and extension, and to limit rotation. This is especially true in the lumbar spine.
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