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, 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.