Spinal instability is defined as an abnormal response to applied loads and is characterized by movement of spinal segments beyond the normal constrains. Lumbar spinal instability can be caused by degenerative disease, post-operation, trauma to the spine, developmental disorders (scoliosis), infections, tumors, or repetitive trauma.

Instability of the lumbar spine occurs often and mostly affects the region of L4-L5 or L5-S1. The clinical symptoms and signs are non-specific and can be described as low back pain with radiculair pain down the legs. These signs are observed especially when the position changes.

The spinal system has three basic functions to perform, namely carrying loads, protection of spinal cord and nerve roots and allowing movement between body parts. To make sure that the spinal system can fulfill these tasks, there needs to be a spinal stabilizing system.

The stabilizing system can be broken down to the the passive subsystem consisting of vertebrae, facet joints, intervertebral discs, spinal ligaments, joint capsules and passive muscle support. Then there is the neural feedback system containing force and motion transducers and the neural control centers. The third system is the active subsystem including the muscles and tendons surrounding the spinal column. Thus the spine needs the integrated function of the three subsystems, mentioned above, to control stability and movement. Instability is found when one of the systems fails to fulfill his task, and disturbs the balance.