What Is Spondylolisthesis?
Written by Kamiah A. Walker; Reviewed by Jason M. Highsmith, MD
Spondylolisthesis is an intimidating word at first glance, but all you need to do is break it into 4 sections: spondy-lo-lis-thesis. Then it’s not so hard to say. Spondylolisthesis is also not so hard to understand if you think about its Greek root words: spondylo means “vertebra” and listhesis means “to slip.”
Spondylolisthesis is when one vertebra slips forward over the vertebra below it. Most often, that happens in the low back (lumbar spine) because that part of your spine bears a lot of weight and absorbs a lot of directional pressures. In other words, your lumbar spine has to move quite a bit (rotate in various directions) while carrying your body weight. Sometimes, this combination can put so much stress on the vertebrae that one of them slips forward.
Some activities make you more susceptible to spondylolisthesis. Gymnasts, linemen in football, and weight lifters all put significant pressure and weight on their low backs. Think about gymnasts and the positions they put their body in: They practically bend in half backwards—that’s an extreme arched back. They also twist through the air quickly when doing flips and then land, absorbing the impact through their legs and low back. Those movements put substantial stress on the spine, and spondylolisthesis can develop as a result of repeated excessive strains and stress.
The x-ray below gives you a good example of spondylolisthesis. Look at the area the arrow is pointing to: You can see that the vertebra above the arrow isn’t in line with the vertebra below it. It’s slipped forward; it’s spondylolisthesis.
They are 5 different grades for spondylolisthesis, depending on how far forward the vertebra has slipped. During your appointments, then, you may hear your doctor talk about “grade I spondylolisthesis.” For a full explanation of what the grades mean and how the doctor determines your grade, please read Exams and Tests for Spondylolisthesis.
Anatomy of Spondylolisthesis
Spondylolisthesis, or the forward slip of a vertebra over the one beneath it, involves several main parts of your vertebrae. First off, the vertebrae are the bones that make up your spine. Most people have 33 vertebrae in their spinal column.
Those 33 vertebrae are divided by region: your neck (cervical spine), mid-back (thoracic spine), and low back (lumbar spine). At the lower end of your spine, you also have the sacrum and the coccyx, which is commonly called your tailbone. Spondylolisthesis usually happens in your lumbar and sacrum regions.
On the vertebra, here are the structures that you need to know in order to understand spondylolisthesis:
• Facet joints: At the top and bottom of each vertebrae, there are joints called the facets. They work like hinges, and they help stabilize your spine and control your movements. They’re composed of the superior and inferior articular processes. Two superior articular processes are on the top of the vertebra, and two inferior articular processes are on the bottom.
• Lamina: Think of this as the roof of your spine. The lamina is located on the back of your vertebra, and it helps protect your spinal cord. The pars interarticularis is part of the lamina.
• Pars interarticularis: This is a region of the lamina located between the facet joints. The pars interarticularis can fracture or seperate, leading to spondylolisthesis.
• Transverse processes: You have two of these on each vertebra—one on each side. Ligaments and tendons connect to them.
Between each vertebra, you have an intervertebral disc. It works as a cushion, absorbing shock as you move, and it allows you to move your spine in multiple directions. There are two parts to the disc: the center part is called the nucleus pulposus, and the outer part is called the annulus fibrosus. Think of your disc as a jelly donut (it’ll help you visualize the structure). The nucleus pulposus is the jelly; it’s made of a gel-like substance and is the part that acts as a shock absorber. Around the “jelly” is the tougher annulus fibrosus, which holds the nucleus in place.
The annulus fibrosus and the nucleus pulposus are both made of collagen, water, and proteoglycans. However, the nucleus has more water and proteoglycans—more fluid—than the annulus, and that’s what gives it its gel-like characteristic.
Many people with spondylolisthesis are symptom free. Sometimes, spondylolisthesis is discovered when the patient has an x-ray for an unrelated problem. However, some patients do have symptoms that range from mild to severe.
Listed below are several symptoms usually related to spondylolisthesis:
• Buttock pain
• Thigh and leg pain and/or weakness (one or both)
• Difficulty controlling bowel and bladder functions
• Tight hamstring muscles
• Walking resembles waddling movements
• Protruding abdomen
Spondylolisthesis has several main causes. Doctors have developed a classification system to help talk about the different causes of spondylolisthesis.
Type I: This is also called dysplastic or congenital spondylolisthesis. Congenital means that you’re born with it. Type I spondylolisthesis, then, is a defect in the articular processes of the vertebra (the part that’s supposed to control the movement of the vertebra; your facet joints are made of the articular processes). It usually happens in the area where the lumbar spine and the sacrum come together: the L5-S1 area. The defect allows the L5 vertebra (your last vertebra in your lumbar spine) to slip forward over the sacrum.
Type II: Also called isthmic, this is the most common kind of spondylolisthesis. With type II spondylolisthesis, there’s a problem with the pars interarticularis, a particular region of your vertebra. (You can see an illustration of this in Anatomy of Spondylolisthesis). Isthmic spondylolisthesis is divided further:
• Type II A: Gymnasts, weight lifters, and football linemen are especially prone to this kind of spondylolisthesis. It’s caused by multiple micro-fractures on the pars interarticularis—micro-fractures that occur because of hyperextension (overarching) and overuse. The pars fractures completely in type II A.
• Type II B: This type is also caused by micro-fractures. The difference, though, is that the pars don’t fracture completely. Instead, new bones grow as the pars heals, causing it to stretch. A longer pars can then cause the vertebra to slide forward.
• Type II C: Like type II A, this type has a complete fracture. However, it’s caused by trauma. The impact in a car accident could fracture your pars, for example.
A pars fracture can lead to a mobile piece of bone; the detached inferior articular process can actually move around. This bone fragment, also called a Gill fragment, can pinch the exiting nerve root, so the bone may need to be removed.
Problems with the pars interarticularis can also be called spondylolysis. The word looks like spondylolisthesis, and they are somewhat related. Micro-fractures in the pars interarticularis—the kind that gymnasts, football linemen, and weightlifters are prone to—are a form of spondylolysis. The fractures are called spondylolysis; if the vertebra slips forward because it’s not being held in place properly, it’s called spondylolisthesis.
Type III: Aging can also cause spondylolisthesis. As you grow older, the parts of your spine can degenerate; they can wear out. Usually your intervertebral discs change first. The older you get, the less water and proteoglycan content the discs have—and less fluid makes them less able to handle movement and shock. Less fluid can also cause the disc to thin, and a thinner disc brings the facet joints closer together. Without the disc acting as the cushion, the facets can’t control the spine’s movement as well, and they become hypermobile. Eventually, a vertebra can slip forward because the facets aren’t holding it in place effectively as the spine moves. Type III spondylolisthesis usually happens at the L4-L5 region (the fourth and fifth vertebrae in your low back), and it’s more common in women older than 50 years old.
Type IV: Similar to type II C, type IV involves a fracture. However, it’s a fracture of any other part of the vertebra other than the pars interarticularis. Your facet joints, for example, can fracture, separating the front part of your vertebra from the back part.
Type V: Tumors on the vertebra can also cause spondylolisthesis because they weaken the bones and can cause fractures that split your vertebra, leading to instability and a potential slip.
Type VI: You have this type of spondylolisthesis if surgery caused your vertebra to slip forward. It’s also known as iatrogenic spondylolisthesis, and it’s caused by a weakening of the pars, often as a result of a laminectomy (a typical back surgery, but type VI spondylolisthesis isn’t a typical result of the surgery).
As a quick summary, spondylolisthesis can be caused by:
• A birth defect
• spondylolysis (a defect or fracture in the pars interarticularis)
• Degeneration due to age or overuse
It’s hard to tell if you have spondylolisthesis because you may not have any symptoms or overwhelming pain—most people don’t. Spondylolisthesis is usually discovered when you’re being tested for something else and the doctor notices the slipped vertebra on an x-ray.
X-rays are the best way to diagnose spondylolisthesis. During the x-ray, you’ll probably stand facing the side—that’s so the doctor can get a lateral (side) view, which most clearly shows the slip. Looking at the lateral x-ray below, you can see that one of the vertebra has slipped off the spinal column. The arrow points to the spondylolisthesis.
Your doctor may also order an oblique x-ray. Oblique means that the x-ray is taken at an angle from the back, a view that will help the doctor see the lamina, facet joints, and pars interarticularis.
To see if your spondylolisthesis is unstable and moving, the doctor may perform flexion and extension views from the side. These are also called lateral bending views. A flexion x-ray is taken with you bending forward; an extension x-ray is taken with you bending backwards.
For further confirmation of spondylolisthesis, you may need to have a CT scan.
If the slipped vertebra is pressing on nerves, the doctor may order a myelogram. In this test, you’ll have a special dye injected into the area around your nerves—your nerves are in a sac, so the dye will go into that sac. (Before that happens, the area will be numbed.) Then you’ll have an x-ray or a CT scan. The image will provide a detailed anatomic picture of your spine, especially of the bones, that will help your doctor to identify any abnormalities.
Part of your visit to the doctor will include physical and neurological exams. In the physical exam, your doctor will observe your posture, range of motion (how well and how far you can move certain joints), and physical condition, noting any movement that causes you pain. Your doctor will feel your spine, note its curvature and alignment, and feel for muscle spasms. Spondylolisthesis can cause you to walk abnormally, so the doctor may need to watch you walk.
During the neurological exam, your spine specialist will test your reflexes, muscle strength, other nerve changes, and pain spread (that is—does your pain travel from your back and into other parts of your body?). The physical and neurological exams will give your doctor a good picture of how the slipped vertebra is affecting your body and life.
Using the lateral (side) x-ray, your doctor will grade your spondylolisthesis. He or she will use a grade I through grade V scale that describes how far forward your vertebra has slipped.
• Grade I is a less than 25% slip.
• Grade II is a 25% to 49% slip.
• Grade III is a 50% to 74% slip.
• Grade IV is a 75% to 99% slip.
• Grade V is for a vertebra that has fallen off the vertebra below it. (That’s an extreme case of spondylolisthesis that has its own name: spondyloptosis.)