Segmental spinal instability is a term used to describe greater than normal range of motion (or “hypermobility”) between two vertebral segments. This condition often develops when a particular disc or facet joint degenerates to the point that it can no longer support the weight of the body through that segment of the spine.
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.
Segmental instability in the spine presents itself as significant back pain or spasms actually caused by nerve irritation as a result of the irregular micro-movements. In addition this pain may be accompanied by a “catch” or locking feeling of sudden sharp pain. Most often this happens when one is mid-way through standing upright from a seated position.
This slippage is called spondylolisthesis which literally 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 called spondylolysis or by acquired degenerative changes in the facet joints.
Spondylolysis: A 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. For many people this fracture does not cause problems for decades. Over time, the decreased posterior support causes more forces to be placed on the disc in front which can accelerate degenerative disc disease. When a disc narrows from degeneration, the bones above and below the disc can also move or slip on each other. The fracture gap can thus widen, causing the vertebra to shift more forward. This movement indicates segmental spinal instability.
Facet Joints: The other main cause of spondylolisthesis is from acquired degeneration of the facet joints. Remember from our anatomy section that the facet joints are the two posterior support columns in the three-joint complex. As these joints degenerate the small supporting ligaments wear out and become loose. These ligaments are essential for support. 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.
Segmental instability is traditionally evaluated by measuring the amount of bone slipping or sliding forward, backward, or sideways on standing films or flexion-extension x-ray films. This movement is most commonly found in the lumbar spine as these levels bear the most weight.
Preliminary treatment options are recommended to create stability where nature has produced instability. We often first recommended core strengthening, soft tissue release, and exercise. If these modalities do not sustain pain relief, we consider pain management techniques such as injections and/or pain medication. Once all non-operative treatment options have been explored and pain is still not eliminated, surgery may be an option. Depending upon the severity of the patient’s findings, spinal fusion is often the preferred procedure. Spinal fusion technology performed with modern techniques in a minimally invasive approach yields highly successful outcomes. Patients often return to very full and active lifestyles.