Last updated: 11/24/2009

Spinal anatomy

The spine is a series of individual bones called vertebrae, separated by cartilaginous disks. The spine is composed of seven cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, five lumbar (lower back) vertebrae,...

and the fused vertebrae in the sacrum and coccyx that help to form the hip region.

While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow tube containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae.

The spinal disks act as shock absorbers, cushioning the spine, and preventing individual bones from contacting each other. Disks also help to hold the vertebrae together.

The weight of the upper body is transferred through the spine to the hips and the legs. The spine is held upright through the work of the back muscles, which are attached to the vertebrae.

While the normal spine has no side-to-side curve, it does have a series of front-to-back curves, giving it a gentle "S" shape. The spine curves in at the lumbar region, back out at the thoracic region, and back in at the cervical region.

Surgery for scoliosis, neuromuscular disease, and cerebral palsy

Abnormal side-to-side curvature of the spine is termed scoliosis. An excessive lumbar curve is termed lordosis, and an excessive thoracic curve is kyphosis. "Idiopathic" scoliosis is the most common form of scoliosis; it has no known cause.

Scoliosis and other curves can be caused by neuromuscular disease, including Duchenne muscular dystrophy. Progressive and perhaps uneven weakening of the spinal muscles leads to gradual inability to support the spine in an upright position. The weight of the upper body then begins to collapse the spine, inducing a curve. In addition to pain and disfigurement, severe scoliosis prevents adequate movement of air into and out of the lungs. Scoliosis also occurs in cerebral palsy, due to excess and imbalanced muscle activity pulling on the spine unevenly.

Idiopathic scoliosis, which occurs most often in adolescent girls, is usually managed with a brace that wraps the abdomen and chest, allowing the spine to develop straight. Spinal fusion is indicated in patients whose curves are more severe or are progressing rapidly. The indication for surgery in cerebral palsy is similar to that for idiopathic scoliosis.

Spinal fusion in Duchenne muscular dystrophy is usually indicated earlier than in otherwise healthy adolescents. This is because these patients lose ventilatory function rapidly through adolescence, making the surgery more dangerous as time passes. Surgery should occur before excess ventilatory function is lost.

Surgery for herniated disks, disk degeneration, and pain

As people age, their disks become less supple and more prone to damage. A herniated disk is one that has developed a bulge. The bulge can press against nerves located in the spinal cord or exiting from it, causing pain. Disks can also degenerate, losing mass and thickness, allowing vertebrae to contact each other. This can pinch nerves and cause pain. Disk-related pain is very common in the neck, which is subject to constant twisting forces, and the lower back, which experiences large compressive forces. In these cases, spinal fusion is employed to prevent the nerves from being damaged. The offending disk is removed at the same time. A fractured vertebra may also be treated with fusion to prevent it from causing future problems.

Sometimes, spinal fusion is used to treat back pain even when the anatomical source of the problem cannot be located. This is usually viewed as a last resort for intractable and disabling pain.

The spinal fusion operation

Spinal fusion is performed under general anesthesia. During the procedure, the target vertebrae are exposed. Protective tissue layers next to the bone are removed, and small chips of bone are placed next to the vertebrae. These bone chips can either be from the patient's hip or from a bone bank. The chips increase the rate of fusion. Using bone from the patient's hip (an autograft) is more successful than banked bone (an allograft), but it increases the stresses of surgery and loss of blood.

Fusion of the lumbar and thoracic vertebrae is done by approaching from the rear, with the patient lying face down. Cervical fusion is typically performed from the front, with the patient lying on his or her back.

Many spinal fusion patients also receive spinal instrumentation. During the fusion operation, a set of rods, wires, or screws will be attached to the spine. This instrumentation allows the spine to be held in place while the bones fuse. The alternative is an external brace applied after the operation.

An experimental treatment, called human recombinant bone morphogenetic protein-2, has shown promise for its ability to accelerate fusion rates without bone chips and instrumentation. This technique is only available through clinical trials at a few medical centers.

Spinal fusion surgery takes approximately four hours. The patient is intubated (tube placed in the trachea), and has an IV line and Foley (urinary) catheter in place. At the end of the operation, a drain is placed in the incision site to help withdraw fluids over the next several days. The fusion process is gradual and may not be completed for months after the operation.