Anatomy clinical correlates: Bones, fascia and muscles of the neck

The neck contains many vital structures, including blood vessels, nerves, and lymphatics, as well as organs like the thyroid and parathyroid glands, parts of the airway like the larynx and trachea, and parts of the digestive tract like the pharynx and esophagus. All these structures are protected by the bones, fascia and muscles of the neck.

The skeleton of the neck is formed by the cervical spine, the hyoid bone, the manubrium of the sternum, and the clavicles. All of these structures are prone to injuries, so hopefully learning about them in this video won't be too much of a pain in the neck!

Let's get started! First, let’s look at cervical spine fractures. The cervical spine is the most flexible and mobile part of the vertebral column. But that flexibility comes with a price, making the cervical spine vulnerable to injury. Now, cervical spine fractures can be stable, meaning the spinal cord is at minimal to no risk of injury due to the fracture pattern, or unstable, meaning the spinal cord is at a much greater risk of injury due to the fracture pattern.

Let’s take a look at some important types of cervical fractures. Let’s start from the C1, or atlas, vertebra. 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 either the anterior arch, the posterior arch, or both.

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. These fractures don’t necessarily cause a spinal cord injury, because the diameter of the vertebral ring actually increases. However, spinal cord injuries could happen if the transverse ligament ruptures, potentially resulting in compression of the spinal cord by the dens of the C2 vertebra, also called the odontoid process.

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. Now, 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.

One of the most common injuries to the cervical vertebrae is a fracture 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 people 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 can occur, which can cause 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. The dens of the axis 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 can 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. These fractures heal 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.

Okay, now, if the force causing a C1 or C2 fracture is really strong, for example during a car crash that causes extreme flexion or extension, it can rupture the transverse ligament. This can result in anterior dislocation of the atlas on the axis, or atlantoaxial subluxation. Additionally, if there’s an associated fracture of the dens and preservation of the transverse ligament, the dens dislocates, moving together with the atlas.

Other than trauma, some other conditions can also cause atlantoaxial subluxation. First, there can be pathological softening or loosening of the transverse ligament, leading to an incompetent transverse ligament that’s unable to maintain the position of the atlantoaxial joint. This typically results from connective tissue disorders, like rheumatoid arthritis, systemic lupus erythematosus, and ankylosing spondylitis. Second, there is laxity of the transverse ligament, which is often associated with congenital disorders like Marfan syndrome, Ehlers Danlos Syndrome, or Down syndrome. Finally, there can be transverse ligament agenesis, which is when an individual is born without the transverse ligament, and this can also be associated with Down syndrome.

Pathology to the transverse ligament is significant because the spinal cord can become impinged between the posterior arch of the atlas and the dens. Even though most individuals with atlantoaxial subluxation are asymptomatic or just experience mild neck pain, a sudden or large movement may lead to compression and injury of the spinal cord. This can result in sensory loss and paralysis in all four limbs, or even injury to the brainstem leading to death.

Initial management is to place the individual in spinal precautions and immobilize the cervical spine, and depending on the stability of the injury the patient may need a halo which is a rigid structure that is secured to the frontal and parietal skull bones with metal pins, or a cervical spine collar. Stable fractures can be managed conservatively however more severe injury may require surgery.

All right, that was a lot of info! Feel free to pause the video and see if you can recognize these three images and recall the three types of odontoid process fracture.

Okay! Now, remember - the vertebrae are not the only bones of the neck! There’s also the U shaped hyoid bone. The hyoid bone is a mobile bone located in the anterior part of the neck at the level of C3 vertebra, which serves as an attachment for anterior neck muscles and functions to help keep the airway open.