What is Spinal Fusion?
Spinal fusion is a surgical procedure used to join two or more vertebrae together. In a normal spine, there is motion at each level of the spine. Most of the motion in the spine is in the neck (cervical spine) and low back (lumbar spine). There is very little motion in the thoracic spine, which is the area between the neck and low back. The purpose of a spinal fusion is to take away this motion and make the levels that are being fused immobile.
Why is Spinal Fusion Done?
Spinal fusion is done for many reasons. Most common reason is for severe arthritis in the neck or low back that is causing pressure on nerves and abnormal motion. Patients with fractures in the spine (as a result of a car accident or falls from heights) also may need a spinal fusion. Patients with severe scoliosis or kyphosis also may need a fusion to correct the spinal deformity. Other reasons for fusion include tumors and infections.
How is Spinal Fusion Performed?
There are several techniques available for fusing the spine, and the choice of which technique to use depends on location (cervical, thoracic, lumbar), patient’s pathology, and surgeon preference.
There are 3 main approaches used for spinal fusion:
- Anterior fusion – this means the fusion is done by removing the disc and placing bone graft in its place. Anterior fusion is most commonly done in the cervical spine and lumbar spine.
- Posterior fusion – this means the fusion is done in the back of the spine (posterior means back) – bone graft is placed over the bones in the back of the spine.
- Anterior AND posterior fusion – this is sometimes called “360 degree fusion”, meaning the spine is fused in the front and back. This is necessary in some cases when the spine is very unstable or sometimes it is done to improve the chances that a fusion will occur.
Another important concept in spinal fusion is whether or not spinal instrumentation is used:
- Non-instrumented fusion – this means bone graft is used alone, without the use of any screws, rods or plates.
- Instrumented fusion – this means screws and rods (or plate) is used in addition to bone graft. Instrumentation generally increases the likelihood of the fusion healing and is recommended in most cases.
Last but not least, bone graft is used to obtain a fusion. There are several bone graft options available to surgeons and the choice of which to use is dependent on many factors. The area of the spine being fused, the approach used, patient factors (smoking, history of steroid use, etc.), and surgeon preference all influence the decision. Briefly, the main categories are the following:
- Autogeneous bone – this is the patient’s own bone – obtained most often from the pelvic area. This is referred to as “iliac crest bone graft”. This is probably the “best bone graft” to use, but the disadvantage is that its supply is limited and obtaining the graft from the pelvis can cause pain in this area.
- Allograft bone – this is cadaver bone obtained from a bone bank. It serves as a scaffolding (has structural integrity) but lacks living cells that stimulate bone formation. Therefore, it is frequently supplemented with some source of growth factors to stimulate bone formation.
- Synthetic bone such as hydroxyapatite and tricalcium phosphate.
- Growth factors – an example is bone morphogenic protein (BMP).
What is the purpose of Spinal Instrumentation?
Spinal instrumentation is used to improve the chances that a fusion will occur. Screws, rods, and plates provide immediate stability to the spine after surgery so that no movement occurs at the levels that are instrumented. A key point to remember is that these implants are only a supplement to aid the fusion process. If good surgical technique is not used (preparation of bones, placing adequate bone graft, etc.), then the fusion is unlikely to occur. If a fusion does not occur, the screws can loosen and/or the rods can break.
How long does it take for a Fusion to occur?
It takes several months for a fusion to heal after surgery. X-rays are obtained periodically during this time to make certain the screws/rods/plates have not loosened or broken. The fusion may not be apparent on X-rays until at least 1 year after surgery.
What factors hinders the Fusion process?
The following factors inhibit fusions: Smoking, osteoporosis, infection, steroid use, excessive activity/motion in early postoperative period, radiation, rheumatoid arthritis.
It is critical to quit smoking before any fusion procedure. Smoking decreases the blood supply to the vertebrae, and fusion does not occur in the absence of blood flow.