Fusion is a surgically created solid Bone bridge between two or more adjacent, usually freely mobile bones. In a Spinal fusion, this is used to create a stability between levels of the spine (vertebrae) that usually have some motion across a disc and the facet joints. In order to achieve a fusion, bone must grow across the desired area in a gradual and solid fashion. A number of techniques can increase the chance of this to occur. The basis principle is to place bone tissue (bone graft) into the area of desired fusion, ensure sufficient immobility across that area (brace, cast, spinal instrumentation…) and then waiting for the spinal fusion to take place (6-9 months or more).
It is important to understand that in the process of preparing an area of the spine for fusion a commitment with little room for error has been made. If a fusion fails to heal (often called a pseudarthrosis or non-union) repeat surgery (revision surgery) must often be done with more bone graft and spinal instrumentation until the fusion area heals solidly.
There are numerous spinal problems for which spinal fusion may be planned. In simple terms there are long spinal fusions (across many levels) and short segmental spinal fusions (one or a few levels).
Long spinal fusions are commonly performed for correction of deformity such as idiopathic scoliosis, adult scoliosis or Scheuermann's kyphosis. The goal is to correct a deformity and to maintain the correction with spinal instrumentation until the spinal fusion is completed and the healed bone takes over the task of the hardware to stabilize the correction.
Short spinal fusions are often performed to stabilize some form of instability ranging from the acute instability caused by a fracture or large disc extrusion to the chronic instability caused by disc, ligament and cartilage wear and tear called degeneration. In many cases spinal instrumentation is placed during the spinal fusion surgery to optimize the chances of successful bone healing. For unclear reasons, in many cases short fusions do not have a better chance of solid healing than long fusion. This fact may be due to the difficulty to achieve solid fixation (blocking all motion) between two or three mobile vertebrae in the spine. The difficulty to obtain successful spinal fusion at the lowest level of the spine (L5-S1) is illustrated by the amount of different techniques which have been proposed to achieve fusion at that level (in some textbooks there are outlines of more than 25 techniques). Despite the many different surgical techniques available to achieve short spinal fusions, they are not all alike and each has advantages and disadvantages. Each patient must be treated in an individual fashion and the optimal surgical fusion technique is dependant upon many factors.
In addition to consideration of the surgical technique applied, and the type of spinal instrumentation used (if it is necessary), there are also several options in the choice of bone graft material. It must be remembered that ultimately for a spinal fusion to be successful, bone must grow across the selected spinal levels. There are a number of possible sources for this necessary bone graft. One option is bone harvested from the patient (autologous bone graft) from the spine itself or an area near the lower spine, from the iliac crest (part of the pelvis). Another option is obtaining bone graft material from a bone bank (harvested from cadavers and sterilized), this is called allograft. A further option is using synthetic bone materials made from demineralized bone matrix (DBM) and calcium phosphates or hydroxyapatites (some are derived from sea corral).
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