Bones of the vertebral column
Definitions & Key takeaways
The human vertebral column, also known as the spinal column or simply the spine, is a long, flexible structure made up of bones (vertebrae) separated by small cushions of intervertebral discs. It functions to protect the spinal cord, support the weight of the head, and allow movement of the trunk and limbs. The vertebral column is composed of 33 bones in total: 7 cervical (neck), 12 thoracic (chest), 5 lumbar (lower back), 5 sacral, and 4 coccygeal (fused into the coccyx). When viewed from the side, the spine has an S-shape. This is due to the inward curve of the cervical spine, and a gentle outward curve in the thoracic region. The lumbar spine slightly curves inward just like the cervical spine. All these curves help to distribute the weight of the head and body evenly and act as shock absorbers during movement.
The vertebrae come in different shapes and sizes, and they have unique features depending on their region. Typical vertebrae have a basic structure in common, consisting of a vertebral body, a vertebral arch, and 7 processes: a spinous process, 2 transverse processes, and 2 superior and 2 inferior articular processes.
The vertebral body is the thick, cylindrical, anterior portion of the vertebra, which functions in supporting weight. As you move down the spine the vertebral bodies become larger, as they bear more weight.
Posterior to the vertebral body, there’s the vertebral arch, which consists of two pedicles and two laminae. The pedicles are short, thick processes that project posteriorly from the vertebral body to meet the laminae, which are two broad, flat plates of bone, that unite in the midline and complete the vertebral arch.
The space between the walls of the vertebral arch and vertebral body is called the vertebral foramen. And when you stack all the foramina on top of each other, that forms the vertebral, or spinal, canal, which forms a protective bony case around the spinal cord.
Focusing on the 7 processes of the vertebrae, first we have the spinous process, which extends posteriorly from the midline junction of the laminae and serves as an attachment site for ligaments and muscles.
Next, the left and right transverse processes, they extend posterolaterally from the junctions of the pedicles and laminae, while also serving as important attachment sites for ligaments and muscles.
Finally we have the four articular processes. First there’s the left and right superior articular processes, which project superiorly from the junctions of the pedicles and laminae.
Next are the left and right inferior articular processes, which project inferiorly from the junctions of the pedicles and laminae.
The superior articular processes, inferior articular processes, along with the pedicles, the vertebral bodies, and intervertebral discs between vertebral bodies, go on to create U shaped indentations called the superior and inferior vertebral notch.
So, let’s look at the articulation between two typical vertebrae from a lateral angle. The superior and inferior articular processes of each vertebra allow articulation with adjacent vertebrae via their articular facets.
The inferior articular process of the top vertebra comes into contact with the superior articular process of the bottom vertebra, forming a zygapophyseal joint, also called a facet joint.
The facet joints help keep the vertebrae properly aligned, while also allowing movement of the vertebral bodies. Furthermore, their orientation differs throughout the vertebral column.
This allows for different movements of the vertebral bodies in the cervical, thoracic, and lumbar regions. While still looking from a lateral view, you can also see how the inferior vertebral notch of the vertebra on top aligns with the superior vertebral notch of the vertebra below it to form a canal called the intervertebral foramen, through which spinal nerves pass.
Okay, before moving on, let’s take a short break and see if you can recall the basic features of a typical vertebra. So now let’s look at the common features of typical vertebrae in each region.
Cervical vertebrae can be typical or atypical, and when it comes to the cervical column, the typical vertebrae are C3, C4, C5 and C6, while the atypical vertebrae are C1, C2 and C7.
The vertebral bodies of typical cervical vertebrae are smaller than the other regions, are wider from side to side, and have a concave superior surface and a convex inferior surface.
The lateral edges of the superior surface of the vertebral bodies are each called the uncus of the body, or the uncinate process.
Now, what really makes cervical vertebrae stand out is that they have a foramen transversarium, or transverse foramen, which is an opening on each of the transverse processes.
It's through these foramina that the vertebral arteries and corresponding veins pass. The lateral ends of the transverse processes have two projections, an anterior tubercle and a posterior tubercle, which are attachment sites for the levator scapulae and scalene muscles.
The vertebral foramen of the cervical vertebrae is large and triangular shaped to accommodate the cervical enlargement of the spinal cord to fit through it.
The facets on cervical vertebrae are nearly horizontal and, as such, allow for the most movement of all the vertebral regions.
This allows adjacent cervical vertebrae to sit nicely upon each other. The main movements the facet joints allow in the cervical region are: free flexion and extension, some lateral flexion, and limited rotation.
The spinous processes of the cervical vertebrae are short, where typically C2-C6 are bifid, meaning they have two protrusions at the posterior end that project posterolaterally.
But let’s not forget about the atypical cervical vertebrae! First, the C1 vertebra, also called the atlas, is a ring shaped bone that doesn’t have a body or a spinous process.
These surfaces articulate with two large cranial protuberances on either side of the foramen magnum on the occipital bone, called the occipital condyles,to form the atlanto-occipital joint.
The anterior and posterior arches extend between the lateral masses and form a complete ring. The anterior arch has an anterior tubercle and a facet for the dens, and the posterior arch has a posterior tubercle and a groove for the vertebral artery.
On the anterior arch, there’s a tubercle for the transverse ligament. The transverse ligament extends from one lateral mass to the other lateral mass on the atlas, passing between the spinal cord and the dens, which is a part of the C2 vertebra.
Speaking of which, the C2 vertebra is also called the axis, because it has a blunt tooth-like dens - or odontoid process.
The dens projects superiorly and articulates with the posterior surface of the anterior arch of the atlas, and it’s held in place by the transverse ligament.
It also has inferior articular processes, and transverse processes similar to the other cervical vertebrae. Finally, C7, has a smaller transverse foramen than the other cervical vertebrae, and it is also called the vertebra prominens due to its long spinous process.
In most people it is the most prominent spinous process, and you can actually feel it if you run your finger and press along the midline of the back of your neck!
Okay. I prominens we’re done articulating the multi-faceted details of the cervical vertebrae for now.
Take a moment to see if you can recall the key structures of the typical and atypical cervical vertebrae. All right!
Next up are the thoracic vertebrae, which are located in the upper and middle back, between the cervical and lumbar vertebrae.
They are intermediate in size, though they do get bigger further down the vertebral column. On quick glance, an easy way to identify the thoracic vertebrae is from a posterior lateral view, where they look like a giraffe!
This is compared to the lumbar vertebrae which we will talk about shortly, which look like a moose! From a superior view, the typical thoracic vertebrae, which are T2 through T11, have a heart-shaped vertebral body, a vertebral foramen that is circular and small, and a pair of transverse processes that are long, strong, and extend posterolaterally.
Let's look at a typical thoracic vertebra from a lateral view. The facets of the superior articular processes are nearly vertical and face posteriorly and slightly laterally, while the facets of the inferior articular processes are also nearly vertical but face anteriorly and slightly medially, The unique orientation of the facets allows adjacent thoracic vertebrae to articulate nicely.
This orientation also results in the main movement of rotation and some lateral flexion of the thoracic vertebrae. The spinous process of a typical thoracic vertebra is long and slopes posteroinferiorly, overlapping with the inferior vertebra.
The most characteristic features of the thoracic vertebrae are the two superior and two inferior costal facets which articulate with the heads of the ribs.
These are known as demifacets, as they are half of a facet, with two demifacets articulating with the head of a single rib.
There is also one costal facet on the anterior side of each of the transverse processes called the transverse costal facet, which articulates with the tubercle of the ribs.
And now, just a few words on the atypical thoracic vertebrae features, which are T1 and T12. T1 is similar to the C7 vertebra, so it has a long, nearly horizontal spinous process similar to that of C7.
Its lower half is more lumbar-like, with the body and pedicles being larger and thicker; as well as having a shorter, more horizontal spinous process.
Furthermore, the inferior articular processes are more laterally facing which means the movements between T12 and L1 are flexion and extension, not rotation.
Since T12 is sort of caught in between the somewhat inflexible thoracic region and more flexible and mobile lumbar region, this vertebra is subject to increased transitional stress.
This makes T12 the most commonly fractured vertebra. Okay.
Take a moment to see if you can recall the key structures of the typical and atypical thoracic giraffes…uhh, I mean vertebrae.
All right. Next up are the lumbar vertebrae, which are in the lower back, between the thoracic vertebrae and the sacrum.
They are the largest movable vertebrae, which helps with bearing the weight of the upper body. They have a massive, thick, and strong body, which is fitting as we said before from a posterior view they look a lot like a moose, an animal that also has a massive, strong body.
Typical lumbar vertebrae also have a triangle-shaped vertebral foramen, which is bigger than the thoracic vertebrae but smaller than the cervical vertebrae.
The lumbar vertebrae also have long slender transverse processes that extend laterally, and short, broad, sturdy spinous processes.
The superior articular facets are in the sagittal plane mainly facing medially and a little posteriorly, while the inferior articular facets are also in the sagittal plane mainly facing laterally and slightly anterior, allowing each lumbar vertebra to articulate well with the adjacent vertebrae.
On the posterior surface of each of the superior articular processes, there are small tubercles called the mammillary processes that are characteristic of lumbar vertebrae.
The mammillary processes project posteriorly and allow for attachment to the multifidus,and intertransversarii muscles. There is another process called the accessory process, also characteristic of lumbar vertebrae, on the posterior surface of the base of each transverse process.
The accessory process provides an attachment for the intertransversarii muscles. Now let's look at an atypical lumbar vertebra.
Among the lumbar vertebrae, L5 is atypical in that it is the largest of the movable vertebrae, with the most massive body and transverse processes.
This is because it carries the weight of the whole upper body! Its body is also taller anteriorly.
Furthermore, the facets of the L5-S1 joint are more coronal in orientation, allowing for flexion and extension, along with some lateral flexion.
Okay. Let’s stop moosin’ around and pause for a quick moment to see if you can recall the key structures of typical and atypical lumbar vertebrae.
Alright! Now let’s look at the sacral region.
The sacrum is a triangular bone made up of five fused sacral vertebrae in adults. It is located between the lumbar vertebrae and the coccyx in the vertebral column, right between the hip bones.
It stabilizes the pelvis and transfers weight to the pelvic girdle. The sacrum is thick and wide across at its base, where it plays a role in weight-bearing, but tapers down inferiorly towards its apex, where it does not bear weight.
From an anterior view, we can see how the sacrum resulted from the fusion of the five sacral vertebrae, as we notice the four transverse ridges that indicate the divisions between what originally was the five individual sacral vertebrae, S1 through S5.
The wide base of the sacrum is derived from S1, and on each side it extends laterally as a wing-like structure called the ala, which is Latin for wing.
The anterior lip of the body of S1 is called the sacral promontory, which marks the border of the pelvic inlet. Inferiorly, we can see four pairs of anterior (or ventral) sacral foramina that lie lateral to each vertebral body and allow for the passage of the anterior rami of sacral spinal nerves.
Also in the posterior view, we can see the superior articular processes of S1 and their corresponding superior articular facets on the dorsal aspect, which articulate with the inferior articular facets of L5 at the lumbosacral angle, which can vary between 130 and 160 degrees.
These auricular surfaces articulate with the ilium to form the sacroiliac joint. In the center of the sacrum, there’s the sacral canal, which is the continuation of the vertebral canal.
The sacral canal contains the spinal nerve roots that make up the cauda equina, the bundle of nerves protruding from the bottom of the spinal cord.
The intermediate sacral crests that lie just lateral to either side of the median crests are remnants of the fused articular processes of the sacral vertebrae.
Finally, the lateral sacral crests lie further laterally on each side, and they arise as a result of fusion of the transverse processes of the sacral vertebrae.
Because S5 doesn’t have a spinous process, nor laminae, that results in an upside-down U-shaped opening in the wall of the sacral canal called the sacral hiatus.
A bit further inferiorly, there are the sacral cornua, a pair of projections just lateral of the midline, which are remnants of the inferior articular processes of the S5 vertebra.
And finally, there’s the coccyx, or tailbone, which is a small triangle-shaped bone derived from the fusion of the four rudimentary coccygeal vertebrae at the very bottom of the vertebral column.
The anterior surface of the coccyx is smooth, slightly concave. The posterior surface is convex, and has rudimentary articular processes.
The coccyx serves as a site for attachment for parts of the gluteus maximus, coccygeus muscles, and the anococcygeal ligament.
Phew! That was a lot.
Generally speaking, the vertebrae across the different regions have some key components in common, including the vertebral body, pedicles, laminae, transverse processes, articular processes, and a spinous process.
Cervical vertebrae are the smallest movable vertebrae, with rectangular vertebral bodies, large triangle-shaped vertebral foramen, bifid spinous processes, and a pair of transverse foramina, which are specific to the region.
The atypical cervical vertebrae are C1 (the atlas), C2 (the axis), and C7 (the vertebra prominens). Thoracic vertebrae are a bit bigger than cervical vertebrae and increase in size down the column.
They look like giraffes and have a heart-shaped vertebral body, a small circular vertebral foramen, long transverse processes, downward-sloping spinous processes, and costal facets on the vertebral bodies and transverse processes, which are unique to the region.
The atypical thoracic vertebrae are T1 at the top of the region, which appears to be more cervical-like, and T12 at the bottom of the region, which appears to be more lumbar-like.
Lumbar vertebrae are the largest and strongest movable vertebrae, which is important since they bear the most weight, and remember: they look like a moose.
They have distinctly massive vertebral bodies, medium-sized triangular vertebral foramen, slender transverse processes, and mammillary processes characteristic of the region L5 is the lone atypical lumbar vertebra, with its massive body that is taller anteriorly, accounting for the lumbosacral angle.
The sacrum and coccyx are the triangle-shaped bones at the bottom of the column that form from the fusion of the individual vertebrae that make them up.
Bones of the Vertebral Column
Figure 1. Section of the Spine
Figure 2. Features of a Typical Lumbar Vertebrae A. Superior View of Vertebra B. Lateral View of Vertebra
Figure 3. Lateral view of typical lumbar articulation.
Figure 4. Typical Cervical Vertebra. A. Lateral view of Cervical Vertebra B. Superior view of Cervical Vertebra
Figure 5. Atypical Cervical Vertebrae A. Superior view of C1 (Atlas) B. Posterosuperior view of C2 (Axis) C. Superior View of Vertebra Prominens (C7)
Figure 6. Typical Thoracic Vertebrae A. Lateral View of Articulating Thoracic Vertebrae B. Superior View of Thoracic Vertebra
Figure 7. Features of the Sacrum A. Anterior view B. Lateral view C. Posterior view
Figure 8. Sacral Crests, Posterior View
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