There are many causes of low back pain. Back pain may arise from the muscles, tendons, joints, disks, bones, nerves and spinal cord. Rarely back pain may arise from other organs in the abdomen or chest.
Black disks called degenerative disk disease (DDD) may produce back pain. This may be caused by injury to the disk called an annular tear. Scientists believe annular tears cause pain from ingrowth of pain nerves into the disk. As the disk disease progresses this can lead to weakness in the spine called instability. Instability produces pain from abnormal movements in the spine.
Facet joint damage can also cause back pain. The facet joints hold the back of the spine together. If the joints are injured they can be a source of pain. This is called facet arthropathy and is common when people have severe arthritis in there spine. If the facet joints wears out or break and can no longer hold the spine together then the spine may slip apart. This is called a spondylolithesis.
If the back pain arises from the torn disk then pain is relieved by removing the painful disk and replacing it with cage and bone. If the problem is instability (that is weakness and abnormal movement) the fusion and instrumentation holds the spine together preventing these abnormal movements.
Today spinal fusion can be done minimally invasively with the spinal endoscope (endoscopic lumbar interbody fusion or ELIF). This decreases skin and muscle damage, pain and suffering after surgery and allows you to get gong quicker.