Most of the vertebrae in the human spine are joined to their immediate neighbours by facet joints. These joints are articulated: they lock the vertebrae together, whilst allowing them to move (an articulated lorry provides a useful analogy).

Abnormal joint motion or instability can result in pain, in which case fusion may be recommended, an operation in which two (or sometimes three) vertebrae are welded together. If successful, the fusion will prevent all motion at the vertebral segment and stabilise it; hence, in theory, the level of pain experienced by the patient should be diminished.

Fusion

During the operation, the spine is approached either from the front, via the abdomen (an anterior fusion), or directly from behind (a posterior fusion). A bone graft is then taken from the pelvis. Depending on the type of fusion, the bone is either:

Placed across the vertebrae (an external fusion), or In-between the vertebral bodies (an inter-body fusion). This requires the inter-vertebral disc to be removed (the diagram shows an inter-body fusion).
A solid fusion is achieved when the bone graft grows into the vertebrae, usually over a period of three months (during this period, patients are advised to avoid bending, lifting and twisting). Screws, plates and other devices are often used to assist the fusion.