Spondylolisthesis is a common diagnosis treated at Virginia Spine Institute. Spondylolisthesis means the forward slippage of one lumbar vertebra in relation to the vertebra below. Spondylolisthesis can be caused by several mechanisms. The two main causes are from either a stress fracture in the vertebra or by acquired degenerative changes in the facet joints.
To understand how both mechanisms cause slippage, it is important to understand the anatomy of the vertebral motion segment. Each level of your spine functions as a three-joint complex. There are two facet joints in the back and a large disc that acts as a joint in front. This tripod creates great stability, supports the weight above each level and provides support for movement in all directions. As long as the disc remains healthy, it can withstand these forces for many years without any symptoms.
A pars fracture is also known as a stress fracture, or as spondylolysis. Spondylolisthesis is often the result of spondylolysis. In non-medical terms, this means a stress fracture causes the forward slippage of a vertebral body. The stress fracture occurs through a fragile part of the vertebral bone called the “pars” and is often broken on both sides. The fracture may be the result of a direct trauma, by a focused strain usually from athletic activity, or from a genetic weakness in this area of the bone. This is a thin bone that can break with repeated use; imagine a paperclip that has been bent over and over and finally breaks. Most commonly, a pars stress fracture occurs as an adolescent; it is estimated up to 5-7% of adolescents have a pars fracture. It is extremely commonly in athletes in high impact sports and those experiencing rotational forces. Fortunately, these fractures often heal with a period of rest and core strengthening. If the bone does not heal, fibrous scar tissue may form to still provide support.
The pars functions as a bony hook and when fractured the posterior support for the vertebrae is broken. It can cause a forward slippage with time.
Many people with spondylolysis do not have problems for many years as their disc continues to be healthy. With a stress fracture, there is decreased posterior support in the vertebral segment causing more forces to be placed on the disc in front. This can cause degenerative disc disease, where the disc progressively breaks down and does not adequately support the bone above. Disc degeneration causes supporting collagen fibers in the disc to breakdown, allowing the disc to lose water. When it begins to dehydrate, it loses height and function. As the disc narrows, the bones above and below the disc can move or slip on each other. In addition, it is possible for the fracture gap at the pars to widen, causing the vertebra to shifts forward. This forward slippage is called spondylolisthesis. Imagine a tripod stool where two back legs have been partially cut through. With time and movement, the stool would be unstable and tip forward. There is not supposed to be any movement of these bones. This movement indicates segmental spinal instability.
When we look at the spine from the side, we can imagine a scotty dog. It is outlined above. The pars fracture is seen as a collar around the dog's neck. Instability and movement can cause the neck to widen. We will see this below in an X-ray example.
Facet Joints: The other main cause of spondylolisthesis is from acquired degeneration of the facet joints. Remember, the facet joints are the two posterior support columns in the three-joint complex. Review spinal anatomy if needed. As these joints degenerate the small ligaments supporting the joint wear out and become loose. The ligaments surrounding the facet joints are essential for support, just like any bigger joints in your body such as the ACL ligament and the knee. This laxity allows the facet joints to open more, or separate, with movement. The facet joints can be a source of pain and allow the disc to slip forward since the joint is separating more than it should.
Spondylolisthesis is most commonly found in the lumbar spine as these levels bear the most weight. To assess instability, we take x-rays of a patient bending forward (flexion) and bending backwards (extension). Normally there would be no motion. The more motion seen between flexion and extension x-rays across an injured level, the greater the patient’s symptoms usually appear.
Most symptoms can be managed non-operatively. Our deep core abdominal muscles are the major supporting and stabilizing structure for our low back. Core strengthening is an integral part of treatment. Depending on the degree of core strength a patient has, the spine may have varying degrees of instability.
Surgical intervention is often required if symptoms progress to intractable back pain, leg pain, or significant neurologic problems from a pinched nerve. The goal of a surgical intervention is to stabilize the unstable spinal segment. This is best achieved by fusing together the slipping bones. The fusion welds two slipping bones together to prevent any movement. This reduces pain and neurological symptoms. Before the bones are fused, the spine is returned to appropriate alignment and spinal height is improved. This will help to decompress, or open up any narrowed space, for nerves.
When this condition occurs in the neck, it is surgically corrected by an Anterior Cervical Discectomy and Fusion (ACDF).
Lumbar fusion surgery can have several surgical approaches. This surgery can be accomplished through:
- an anterior (front) approach called an Anterior Lumbar Interbody Fusion - ALIF
- a posterior (back) approach called a Posterior Lumbar Interbody Fusion - PLIF
- a side approach called a Lateral Interbody Fusion - DLIF, XLIF
- a combination of two approaches called a staged fusion
Patients often return to full, unrestricted activities and gain their life back once the spine is re-aligned, stabilized, decompressed and fused.