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.

Now, one of the most common injuries to the cervical vertebrae are fractures of the vertebral arch of the axis. The fracture usually occurs in the bony part formed by the superior articular process and inferior articular processes of the axis, also known as a traumatic spondylolysis of C2.

This happens as a result of hyperextension of the head on the neck; so, in the past, this fracture pattern was often seen in criminals who were executed by hanging. Therefore, it has also been referred to as a ‘hangman’s fracture’.

In more severe injuries, the body of C2 is displaced anteriorly. Regardless of whether there is anterior displacement of the body, injury of the spinal cord may occur, which can lead to paralysis in all four limbs and even death. You can recognize a C2 fracture on x-ray in the area of the lamina and pedicles.

Now, the dens of the axis, or odontoid process as we said, can also be fractured, usually because of forced flexion or extension in the anterior posterior plane.

There are three types of dens fractures. Type I is the most uncommon type, and it’s when the fracture occurs above the transverse ligament resulting in a relatively stable fracture.

Type II is the most common fracture pattern, and it’s usually located at the base of the dens where it attaches to C2. These fractures tend to be unstable and may be complicated by nonunion, which is when a fracture won’t heal properly because of poor blood supply, excessive movement at the fracture site, or infection.

Finally, type III fractures occur below type II fractures at the vertebral body inferior to the base of the dens. This fracture heals faster because the fragments retain their blood supply, however they are mechanically unstable as this fracture moves with the occiput as one unit.

Dens fractures are best seen on an open mouth radiograph. You can see the bone discontinuation at the base of the dens. It is easier to distinguish on a coronal CT-scan, and a sagittal CT-scan.

Before going any further, let’s see if you can remember what a Jefferson fracture is and what’s the most common mechanism that causes it. Oh, and why is the C1 vertebra called the Atlas? That last one’s just for fun!

Let’s move on to a pretty common pathology called osteoporosis, which often affects post menopausal females, and elderly individuals regardless of sex. Risk factors for osteoporosis include a decrease in estrogen levels, old age, and drug use such as steroids, alcohol and other medications.

With osteoporosis, there’s an imbalance between bone formation and resorption, in favor of the latter, resulting in gradual loss of bone mass, so bones fracture more easily. The most affected bones are the neck of the femur, the bodies of the vertebrae, the metacarpals and the radius.

Now, osteoporosis typically affects the horizontal trabeculae of the trabecular bone of the vertebral body. The remaining vertical trabeculae are less able to resist compression, and so compression fractures can occur, which lead to short and wedge-shaped vertebrae.

A vertebral compression fracture can present with low back pain, immobility, and tenderness at the level of the fracture. However, these fractures can also be chronic and painless, leading to height loss and kyphosis. On a vertebral column x-ray, you can see vertebral collapse or compression fractures, which can be seen in later stages of osteoporosis due to the decrease in bone density.

Moving on, let's talk about spondylolysis, which usually affects the lower lumbar vertebrae. This is when there’s a unilateral or bilateral defect of the vertebral arch in the vertebral pars interarticularis, which causes the posterior part of the arch to separate from the rest of the vertebrae. If the defect is bilateral, the vertebrae can slip anteriorly relative to the vertebrae below, and this is called spondylolisthesis. For example, spondylolysis of L5 can cause the L5 vertebral body to move anterior relative to the S1 vertebra and overlap the sacral promontory. Spondylolisthesis can compress spinal nerves, and typically leads to lower back pain and lower limb pain.

On this X-ray, you can observe how the L5 vertebra has slipped anteriorly relative to the S1 vertebra.

Another ailment of the vertebral column is spinal stenosis. This occurs because as the intervertebral discs degenerate, this results in corresponding loss of disc height, leaving a disproportionate load on the posterior aspect of the spinal column, resulting in osteophyte formation.

This narrowing or stenosis can lead to compression of one or more spinal nerve roots that occupy the vertebral canal, which may or may not cause symptoms such as back pain, difficulty walking, and tingling and numbness in one or both legs. More severe symptoms of nerve compression include poor control of bowel movements and urination.

This compression of the spinal nerves due to the spinal stenosis is called neurogenic claudication, which is different from another type of claudication called vascular or intermittent claudication, which is caused by decreased arterial blood flow. Symptoms of both neurogenic and vascular claudication may be similar, such as pain, cramping, and tightness in the legs, however the symptoms of neurogenic claudication are more posture dependent.

With neurogenic claudication, walking and standing in an erect position causes increased narrowing of the spinal canal further worsening symptoms, and those affected find symptom relief when leaning forward, such as when leaning over a shopping cart at the grocery store or walking uphill. With vascular claudication, on the other hand, patients find relief when standing upright.