Anatomy clinical correlates: Bones, joints and muscles of the back

The vertebral column is a very complex boney structure with numerous articulating joints and multiple muscles that support it and the vertebral canal. As with any part of our body, all of these structures are prone to injury. If you aren’t familiar yet with what type of injuries, don’t worry, we got your back!

Let’s start with fractures involving the C1 vertebra, or atlas. These fractures are also called Jefferson or burst fractures. As you might remember, C1 is a ring shaped bone that has paired wedge shaped lateral masses connected by thin anterior and posterior arches and a transverse ligament. The C1 vertebra sustains the weight of the cranium, kind of like how the God Atlas of Greek mythology bore the weight of the world on his shoulders.

Now, because the taller side of the lateral mass is directed laterally, when there are vertical forces that compress the lateral masses between the occipital condyles above, and the C2 or axis below, this compressive force drives the two lateral masses of the C1 vertebrae apart, which can lead to fractures in one or both of the anterior or posterior arches. A classic example of this is striking the bottom of the pool with the top of your head when diving. If the force is really strong, it could even rupture the transverse ligament.

The Jefferson fracture doesn’t necessarily lead to spinal cord injury. This is because the diameter of the vertebral ring actually increases. However, spinal cord injury could happen if the transverse ligament ruptures as well, potentially resulting in the dens of the C2 vertebra, or the odontoid process, compressing on the spinal cord which we will get to shortly.

On a CT-scan, a C1 fracture looks something like this. You can see where the bone has been broken and how the lateral mass shifts laterally. Moving on, the C2 vertebra, or the axis, can also be fractured. C2 is called the axis because it has a bony protrusion called the dens of the axis that fits within the atlas ring, so this articulation allows rotation of the neck from side to side, like shaking your head no.