The low back (lumbar region) is composed of bones (vertebrae) united in the front by discs of fibrocartilage with relatively soft centres, and united behind by small synovial joints, two at each level. A bunch of nerve roots (called the cauda equina or horse’s tail) runs through the center of the bones, and at each side of each level a nerve root emerges.
Surgery may be indicated for any of these structures, but the presence of an abnormality demonstrated on imaging (X-ray, CT, MRI etc.) alone is usually not an indication for surgery.
The indication for an operation depends mostly on finding the cause of the presenting problem. If there has been an accident or a sudden incident, that might not be difficult. If the X-ray after a motor vehicle accident shows a fracture, that probably is the cause of the new onset of pain. If the CAT scan shows a massive central protrusion of a disc at the lumbosacral junction in a patient who fell downstairs and has lost sensation around the anus, that is not only an explanation of the change it’s an indication for surgery within a very few hours or he may not recover.
But if the CAT scan shows a bulging disc and minor joint arthritis in a bricklayer who can no longer work, it is less probable he will be helped by a back operation. And when one operation has failed to relieve the pain, a second and a third operation are likely to be even more difficult.
Perhaps it is in spinal surgery that we find particular need for mature and cautious detail in determining whether the patient will benefit from the operation.