A 24-year old male showing a normal kyphosis curve after a 13-level spinal fusion to correct Scheuermann's disease
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.
Reasons for spinal fusion 
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.
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.
Conditions where spinal fusion may be considered:
- degenerative disc disease
- spinal disc herniation
- discogenic pain
- spinal tumor
- vertebral fracture
- kyphosis (i.e., Scheuermann's disease)
- Posterior Rami Syndrome
- other degenerative spinal conditions
- any condition that causes instability of the spine
Types of spinal fusion 
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:
- Anterior lumbar interbody fusion (ALIF)- the disc is accessed from an anterior abdominal incision
- Posterior lumbar interbody fusion (PLIF) - the disc is accessed from a posterior incision
- Transforaminal lumbar interbody fusion (TLIF) - the disc is accessed from a posterior incision on one side of the spine
- Transpsoas interbody fusion (DLIF or XLIF) - the disc is accessed from an incision through the psoas muscle on one side of the spine
In most cases, the fusion is augmented by a process called fixation, meaning the placement of metallic screws (pedicle screws often made from titanium), rods or plates, or cages to stabilize the vertebra to facilitate bone fusion. The fusion process typically takes 6–12 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, but have not yet been adopted on a widespread basis in the US. 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 addition to lumbar fusions, cervical spinal fusions may also be performed on the neck. The purpose of a cervical spinal fusion is to join certain bones in the back. 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 the vertebrae to eventually heal together. Cervical spinal fusion can be performed for several reasons. Following injury, this surgery can help in stabilizing the neck and preventing fractures of the spinal column which could damage 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.
Pedicle Screw Insertion: In spinal fusion surgery, the accuracy with which screws are inserted in the pedicle 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 the surgeon. For example, Manbachi A. et. al is one of the recent review articles that presents an overview of the need and the current status of the guidance methods available for improving the surgical outcomes in spinal fusion procedures. It also describes educational aids that have the potential for reducing the training process. 
- "Low Back Disorders (revised 2007)". Occupational Medicine Practice Guidelines (2 ed.). American College of Occupational and Environmental Medicine. 2007. p. 210.
- Manbachi A, Cobbold RSC and Ginsberg HJ (April 2013). "Guided pedicle screw insertion: techniques and training". The Spine Journal 13 (4): S1529–9430(13)00324–0. doi:10.1016/j.spinee.2013.03.029.
"Cervical Spinal Fusion." WebMD - Better Information. Better Health. Ed. William H. Blahd. WebMD, 02 Aug. 2010. Web. 07 Dec. 2011. <http://www.webmd.com/back-pain/cervical-spinal-fusion>. "MD Spine Center - Anterior Cervical Fusion." University of Maryland Medical Center | Home. University of Maryland Medical Center, 28 Sept. 2007. Web. 07 Dec. 2011. <http://www.umm.edu/spinecenter/education/anterior_cervical_fusion.htm>.
Further reading 
- Boatright, K. Craig; Boden, Scott D. (17 February 2005). "Chapter 12: Biology of Spine Fusion". In Lieberman, Jay R.; Friedlaender, Gary E. Bone Regeneration and Repair. Totowa, New Jersey: Humana Press. pp. 225–239. doi:10.1385/1-59259-863-3:225. ISBN 978-0-89603-847-9.
- Holmes, CF, Hershman, EB, McCance, SE (May, 2005). "Chapter 9: Cervical Spine Injuries". In Schenck, RF; AAOS. Athletic training in sports medicine. Jones & Bartlett Publishers. pp. 197–218. ISBN 0-89203-172-7
- Camillo, Francis X. (12 December 2007). "Chapter 36: Arthrodesis of the Spine". In Canale, S. Terry; Beaty, James H. Campbell's Operative Orthopaedics 2 (11th ed.). Philadelphia: Mosby. pp. 1851–1874. ISBN 978-0-323-03329-9.
- Williams, Keith D.; Park, Ashley L. (12 December 2007). "Chapter 39: Lower Back Pain and Disorders of Intervertebral Discs". In Canale, S. Terry; Beaty, James H. Campbell's Operative Orthopaedics 2 (11th ed.). Philadelphia: Mosby. pp. 2159–2224. ISBN 978-0-323-03329-9.
- Weyreuther, Martin; Heyde; Westphal, Michael; Zierski, Jan; Weber, Ulrich, eds. (31 October 2006). "Chapter 7: The Postoperative Spine". MRI Atlas Orthopedics and Neurosurgery The Spine. trans. Bettina Herwig. Berlin: Springer-Verlag. pp. 273–288. doi:10.1007/978-3-540-33534-4_7. ISBN 978-3-540-33533-7. More than one of
- Tehranzadehlow, Jamshid; Ton, Jimmy D.; Rosen, Charles D. (April 2005). "Advances in spinal fusion". Seminars in Ultrasound, CT, and MRI 26 (2): 103–113. doi:10.1053/j.sult.2005.02.007. ISSN 0887-2171.
- Resnick, Daniel K.; Haid, Jr., Regis W.; Wang, Jeffrey C., eds. (2 September 2008). Surgical management of low back pain (2nd ed.). Rolling Meadows, Illinois: American Association of Neurosurgeons. ISBN 978-1-60406-035-5.
- Wheeless III, Clifford Roberts; Nunley II, James A.; Urbaniak, James R. (eds.). "Fusion of the Spine". Wheeless' Textbook of Orthopaedics. Retrieved 3 February 2010.
- "Spinal Fusion". American Academy of Orthopaedic Surgeons. September 2007. Retrieved 10 February 2010.
- Spinasanta, Susan (8 August 2000). "What is Spinal Instrumentation and Spinal Fusion?". Fort Myers, Florida: SpineUniverse. Retrieved 10 February 2010.
- "Spinal fusion". Encyclopedia of Surgery. Retrieved 10 February 2010.
- "Degenerative Disease of the Lumbar Spine". Neurosurgical FOCUS. American Association of Neurosurgeons. March 2006. Retrieved 10 February 2010.
- Support & discussion board for spine patients - Hosted by the Arthroplasty Patient Foundation, a non-profit organization since 2005