What is Spondylolisthesis?
This is a long winded term that simply means that one vertebra has translated on the adjacent vertebra at a given motion segment. It is most common in the lower lumbar spine at L5-S1 and L4-5. Anterolisthesis or forward translation is the most likely to cause symptoms due to nerve compression or mechanical back pain. Retrolisthesis or backward translation occurs “naturally” as the disc height reduces with age and does not commonly cause significant symptoms.
There are several different underlying causes for spondylolisthesis, but most cases are either due to a pars defect or severe wearing out of the facet joints. The first type is termed lytic spondylolisthesis and the second type is degenerative spondylolisthesis. There are other causes such as acute fractures or congenital bone deformities. It can also occur after surgery when too much bone is removed during a laminectomy.
Lytic Spondylolisthesis
In this condition there is a defect in the pars interarticularis part of the vertebra. The pars connects the vertebral body to the lower facet joints and lamina of the vertebra. Thus in this condition the vertebral body translates forward and leaves behind the posterior part of the vertebra (posterior elements). It is thought to have its origin in childhood or early adolescence when the bone is potentially more vulnerable to recurrent loads and stress on the pars. A stress fracture occurs and does not heal completely leaving a cartilage bridge that can elongate over time allowing forward slip of the superior vertebra. This does not always lead to symptoms at the time and many cases have no symptoms until later adult life and some cases are only discovered by accident when an X-ray or CT is done for another reason. The most common risk factor for developing pars defects is repetitive extension exercise such as in gymnastics and cricket fast bowling.
The disc at the affected level typically undergoes premature degeneration because it takes more load than it would have done if it were connected to the posterior elements. If the disc fails and collapses then it can lead to pain and some affected individuals describe recurrent episodes of mechanical back pain but this is not universal. Interestingly slip progression in adulthood is not common and the usual reason the condition becomes a problem is that the existing nerve root at that level gets compressed from disc herniation into the foramen and this leads to pain down the leg felt as Sciatica. This is a special form of spinal stenosis.
The natural history of this condition is variable and for most patients symptoms do settle with conservative management but once nerve compression pain begins it may not resolve. This is the most common reason for the condition coming to surgical treatment.
Surgical treatment typically involves both decompression of the nerve and stabilisation of the affected segment by fusion. (see spinal fusion)
Degenerative Spondylolisthesis
In this condition the underlying problem is facet joint arthritis. The listhesis occurs as the joint wears out and erodes the underlying bone.The disc is also degenerating and often has lost the normal hydration and height. Spinal stenosis develops and is made worse by the presence of the listhesis which further reduces the space for the nerves.
Some people have a predispostiion to this condition because of the orientation of their facet joints in a sagittal plane and the condition is far more common in women.
Treatment is along the lines of that for spinal stenosis but once surgery is considered then fusion is often required to stabilise the involved segment in addition to decompression.
