Why Spondylolisthesis Occurs Primarily in the Lower Back
Spondylolisthesis, or vertebral slippage, occurs primarily in the lumbar (lower back) region of the spine. It can be found in the cervical (neck) and thoracic (middle back) regions as well, but usually only when a traumatic injury, such as whiplash, is involved. Injury might also contribute to vertebral displacement in the lower back, but there are a number of other causes that are far more prevalent, most of which have to do with a genetic predisposition toward spondylolisthesis or lifelong participation in activities that place a great deal of stress on the back.
Anatomy of Spondylolisthesis
Before delving into the various causes of spondylolisthesis in the lower back, a brief primer on the anatomy of the lumbar spine may be helpful. There are five lumbar vertebrae (although, some people have six), and these are the largest bones in the spine. They are numbered L1-L5, and are located between the thoracic spine and the sacrum (pelvis). The lumbar vertebral bodies are separated and cushioned by spongy intervertebral discs and connected by bony hinges known as facet joints. Sandwiched between each vertebral body are pairs of nerve roots, which branch off the spinal cord and carry sensory and motor messages between the brain and the rest of the body. The sciatic nerve, which is the largest and longest nerve in the body, originates near the top of the lumbar spine.
Why the Lower Back is More Vulnerable
The lower back is extremely flexible, allowing for a wide range of motion including bending, twisting, and turning. It also supports most of the weight of the upper body. These two facts mean the anatomical components of the lumbar spine are subjected to more wear and tear than the other regions of the spine. Intervertebral discs and facet joints can begin to deteriorate over time, reducing spinal stability in the region. Spondylolisthesis in the lower back in adults usually is related to a degenerative spine condition such as osteoarthritis or degenerative disc disease.
In children, adolescents, and young adults, vertebral slippage often can be traced to a pre-existing genetic defect in the pars interarticularis, a small bone that is part of the vertebra. If the pars is thinner than usual at birth, repetitive strain produced by participation in contact sports or gymnastics can lead to a stress fracture. Over time, the stress fracture can cause the affected vertebra to slide out of place. Spondylolisthesis is, in fact, one of the most common sources of lower back pain among adolescents and young adults, although the presence of vertebral slippage should not be assumed unless confirmed by a doctor's diagnosis.