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The mechanical integrity of the spine can be compromised by any number of conditions. Degenerative disc disease, spinal tumors, trauma, or deformities can lead to spinal instability, neurological deficits, and severe pain from nerve compression. NewYork-Presbyterian Hospital's neurological surgeons have tools to help these patients, and one is based around a simple idea — helping the body heal itself with new bone. Surgeons seek to create ideal conditions for bone to grow across the damaged area. Pieces of bone — either taken from the patient's own iliac crest or retrieved from a bone bank — are implanted across the span or packed into the open area between healthy vertebrae.
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Spinal Fusion Surgery (viewed from behind) |
Restoring Spinal Integrity
Before performing a spinal fusion, surgeons often will decompress the neural elements to alleviate pain and clear out damaged tissue. Following decompression, internal fixation may be necessary to restore structural integrity. Surgeons implant metallic devices consisting of screws, hooks, rods, or plates, to span the area between healthy bone.
The internal fixation is a mechanical device and never will be as strong as it is on the day it is inserted by the surgeon. With cyclical loading — from bending, sitting, standing — it is going to weaken and ultimately fail. The short-term mechanical solution must then give way to a long-term biologic one.
The placing of bone upon the spine acts both to induce bone growth and bone fusion. There are factors (proteins) in our bone that encourage our body's capacity to fuse bones. In ideal patients, usually young and otherwise healthy, bone grows across the span, increasing in strength as the metal implants decline in strength and providing a permanent bond between healthy bones. In properly selected patients spinal fusion is associated with good outcomes; perhaps 70 to 90 percent of patients achieve a solid fusion and derive significant benefit from the surgery.
Bone Morphogenetic Protein
Success in spinal fusion surgery can be adversely affected by a patient's age and conditions that can inhibit bone forming — such as smoking, chronic steroid use, and diabetes — and by difficulty in obtaining graft bone. When an individual is older, has undergone multiple operations, or has some metabolic problem that does not let them fuse well or heal, some help is needed.
Innovations in molecular biology have resulted in purified forms of substances — a family of bone morphogenetic proteins (BMP) that have shown exponential increases in bone growth in animal models. There are now several forms of BMP available and one that has been approved by the U.S. Food and Drug Administration for clinical use in anterior lumbar interbody fusion (ALIF). To encourage bone growth, BMP still requires a mechanical structure and a substrate to hold it in place. The approved form begins as a powder that is reconstituted and absorbed into a collagen sponge. The sponge is then inserted in a titanium cage roughly the size of an intervertebral disc, which provides immediate structural stability. Threads on the edges of the cage engage the healthy bones above and below and expose the internal surface of the bones to the contents of the cage, increasing the likelihood of a good fusion. |
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