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Spinal fusion, also known as spondylodesis or spondylosyndesis, is a surgical technique used to join two or more vertebrae. Supplementary bone tissue, either from the patient (autograft) or a donor (allograft), is used in conjunction with the body's natural bone growth (osteoblastic) processes to fuse the vertebrae.
Fusing of the spine is used primarily to eliminate the pain caused by abnormal motion of the vertebrae by immobilizing the faulty vertebrae themselves, which is usually caused by degenerative conditions. However, spinal fusion is also the preferred way to treat most spinal deformities, specifically scoliosis and kyphosis.
Spinal fusion is done most commonly in the lumbar region of the spine, but it is also used to treat cervical and thoracic problems. The indications for lumbar spinal fusion are controversial. People rarely have problems with the thoracic spine because there is little normal motion in the thoracic spine. Spinal fusion in the thoracic region is most often associated with spinal deformities, such as scoliosis and kyphosis.
Cervical spinal fusion can be performed for several reasons. Following injury, this surgery can help stabilize the neck and prevent damage to the spinal cord. It can also be used to treat misaligned vertebrae or as a follow-up for other spinal injuries. Additionally, cervical spinal fusion can be used to remove or reduce pressure on nerve roots caused by bone fragments or ruptured intervertebral disks.
Patients requiring spinal fusion have either neurological deficits or severe pain which has not responded to conservative treatment. Spinal fusion surgeries are also common in patients who suffer from moderate to severe back deformities that require reconstructive surgery.
Bone, metal plates, or screws can be used to make a bridge between adjacent vertebrae. In extreme cases, whole vertebrae can be removed before the fusion occurs. In most cases, however, only the intervertebral disk is removed, and the bone or metal graft is subsequently inserted, allowing for healing of the vertebrae.
Conditions where spinal fusion may be considered include the following:
Bone morphogenetic protein (rhBMP) should not be routinely used in any type of anterior cervical spine fusion, such as with anterior cervical discectomy and fusion. There are reports of this therapy causing swelling of soft tissue which in turn can cause life-threatening complications due to difficulty swallowing and pressure on the respiratory tract.
According to a report by the Agency for Healthcare Research and Quality (AHRQ), approximately 488,000 spinal fusions were performed during U.S. hospital stays in 2011 (a rate of 15.7 stays per 10,000 population), which accounted for 3.1% of all operating room procedures. This was a 70 percent growth in procedures from 2001. 
There are two main types of lumbar spinal fusion, which may be used in conjunction with each other:
Posterolateral fusion places the bone graft between the transverse processes in the back of the spine. These vertebrae are then fixed in place with screws and/or wire through the pedicles of each vertebra attaching to a metal rod on each side of the vertebrae.
Interbody fusion places the bone graft between the vertebra in the area usually occupied by the intervertebral disc. In preparation for the spinal fusion, the disc is removed entirely, for example in ACDF. A device may be placed between the vertebra to maintain spine alignment and disc height. The intervertebral device may be made from either plastic or titanium. The fusion then occurs between the endplates of the vertebrae. Using both types of fusion is known as 360-degree fusion. Fusion rates are higher with interbody fusion. Three types of interbody fusion are:
In most cases, the fusion is augmented by a process called fixation, involving the placement of metallic screws (pedicle screws often made from titanium), rods, plates, or cages to stabilize the vertebrae and facilitate bone fusion. The fusion process typically takes 6 to 12 months after surgery. During this time external bracing (orthotics) may be required. External factors such as smoking, osteoporosis, certain medications, and heavy activity can prolong or even prevent the fusion process. If fusion does not occur, patients may require reoperation.
Some newer technologies are being introduced which avoid fusion and preserve spinal motion. Such procedures, such as artificial disc replacement, are being offered as alternatives to fusion in the cervical spine. Their advantage over fusion has not been well established. Minimally invasive techniques have also been introduced to reduce complications and recovery time for lumbar spinal fusion.
In spinal fusion, the accuracy with which screws are inserted in the pedicles has a direct effect on the surgical outcome. Accurate placement generally involves considerable judgmental skills that have been developed through a lengthy training process. Because the impact of misaligning one or more pedicle screws can directly affect patient safety, a number of navigational and trajectory verification approaches have been described and evaluated in the literature to provide some degree of guidance to surgeons. For example, Manbachi et. al (2013) presented an overview of the need and the current status of the guidance methods available for improving the surgical outcomes in spinal fusion. They also describe educational aids that have the potential for reducing the training process. Accurate guidance systems, such as the Mazor Robotics "Renaissance", have been developed to assist with spinal fusion procedures.
According to a report by the Agency for Healthcare Research and Quality (AHRQ), approximately 488,000 spinal fusions were performed during U.S. hospital stays in 2011 (a rate of 15.7 stays per 10,000 population), which accounted for 3.1% of all operating room procedures.