There are two different kinds of back pain: mechanical and nonmechanical. Mechanical back pain occurs when some part of the soft tissue or bone structure isn't moving normally. There may be a damaged or degenerated disc or a torn ligament or a fractured bone.
Mechanical low back pain can be caused by abnormal motion. The vertebra may move in the wrong direction or slide too far in one direction. The result is to increase the load across the disc space and into the joints. A fusion puts a stop to this kind of problem.
Nonmechanical pain means that there's no evidence of a structural problem. Psychologic and emotional stress has been proven to be a major part of this type of back pain. Surgery of any kind is not usually advised. Behavioral therapy and exercise seem to work the best for nonmechanical low back pain.