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School of Medicine
Spondylolysis and Spondylolisthesis
One cause of low back pain is a stress fracture in one or more of the vertebra that make up the spinal column. This condition is called spondylolysis (spon-dee-low-lye-sis). It usually affects the forth and fifth lumbar vertebra in the lower back. Over time, the stress fracture weakens the vertebra until it is unable to maintain its proper position and shifts out of place. This condition is called spondylolisthesis (spon-dee-low-lis-thee-sis) and may be the result of sports or activities that put stress on the lower back, constant hyperextension of the spine, trauma or heredity. For many people, spondylolysis and spondylolisthesis cause no symptoms, or they experience pain in their lower back similar to a muscle strain. If too much slippage occurs, the vertebra may begin to press on the nerves, causing pain and/or weakness and tingling in the legs, tightness in the thigh muscles and pain with exercise. At this point, surgery may be necessary to correct the condition.
Spine fusions are used to treat injuries to the vertebra, protrusion and degeneration of the disk between the vertebra, abnormal curvatures of the spine and weakness or instability of the spine caused by infections or tumors. This procedure stops motion at the painful vertebral segments, decreasing pain in that joint. There are several different approaches to lumbar spine fusion surgery. All involve adding bone graft to the area of the spine causing the problem. This causes the area to fuse and stops motion in that segment. Metal rods and screws are often used to prevent motion and allow the bony fusion to form. Spine fusions may cause minimal reduction in flexibility of the spine. However, some patients report increased flexibility due to the absence of back pain and spasm after successful surgery.
Bone grafts are a basic component of spine fusion and are used in two main ways during orthopedic/neurosurgical procedures: 1) to stimulate the bone to heal, and 2) to provide support to the skeleton by filling in gaps between two bones.
When bone is removed from a person’s own body, living bone cells (called osteocytes) can survive transfer to the new location and continue to grow new bone. Even bone taken from someone else will stimulate bone to heal, but this usually takes a little longer.
Bone graft is also used for structure. In this instance, larger pieces of bone are used to fill a gap between two bones; if the surgeon removes a vertebra or disc, he/she may use a bone graft to fill the space. Because bone is rigid, it will hold the bones apart while the body fuses the bone graft at either end.
Over time, the entire bone graft will be “remodeled” and replace the bone and disc that was removed. It is a slow process and may take months, depending upon how big the piece of bone is. Bone taken from a person’s own body is called autograft and uses living bone tissue to speed the fusion process. It is often harvested from the patient’s iliac crest (hip) and involves making another incision over the hip area to harvest the graft. The most common problem is that, occasionally, the graft site tends to be more painful than the surgery.
Bone graft taken from someone else (cadaver bone) is called allograft. Allograft is usually removed from organ donors and placed in bone banks. The bone bank follows procedures to sterilize the bone graft and make it safe for use and also performs tests for diseases such as hepatitis and HIV (just like a blood bank). An allograft can come from many types of bones in many different forms, but because it is not taken from a live patient, it does not contain any living cells and has fewer chemicals to stimulate growth of new bone. An allograft does not always heal as well or as quickly as an autograft, but a bone-growing protein can be added to the site to make up for the lack of this substance. The advantage is that this eliminates the need to harvest the patient’s own bone, so the surgery is shorter and there may be less postoperative pain. While allograft also carries a risk of transferring infectious diseases, with modern procurement and sterilization methods, the risk is extremely small.
Allograft is very useful when the operation will require more bone graft than a patient’s own body can supply. Some major spine fusions need a lot of bone graft and the surgeon may mix allograft with autograft. Some surgeries need large pieces of structural bone graft that could cause a problem in the bone-harvest area, such as the lower leg, if it were taken from the patient’s own body.
Because allograft bone heals slowly, there has been a great deal of research to design bone graft substitutes, chemicals and devices that can stimulate the bone to fuse. Many surgeons use electrical stimulation devices to speed up bone growth and fusion.
Be sure to discuss the different options with your surgeon.