The Anatomy of the Spine
Updated November 2019
Low back pain, neck stiffness, and radiating pain down an arm or leg. What do these symptoms have in common? They can originate from the spine and its adjacent structures. In general terms, the spine is the main support of the body; it allows for movement, absorbs shock, and provides protection to the spinal cord. The spine is a complex structure, and understanding spinal anatomy can help explain symptoms caused by common injuries and conditions of the spine and how physiotherapy can help you!
The spine is made up of 33 individual bones known as vertebrae, that form the spinal column. These vertebrae are divided into five main regions:
Sacrum (5 fused)
Coccyx (4 fused)
Each region has distinct features that allow them to perform their unique functions.
Despite their differences, each vertebra except for C1, C2, and the coccyx have the same general structure.
The two main parts of a vertebra are the body (anterior portion) and the vertebral arch (posterior portion). The opening between the body and vertebral arch is known as the vertebral foramen or spinal canal, and it is through this opening where the spinal cord travels.
The vertebral body is the primary weight bearing component of a vertebra, while the vertebral arch provides protection to the spinal cord. The vertebral arch includes bony projections called processes (spinous and transverse) which provide sites for muscle and ligament attachments, and includes the facet joints, a secondary weight bearing component that supports and limits spinal movement in certain directions.
Image retrieved from: https://www.floridamedicalclinic.com/fmcspine/your-spine/
The spinous process of each vertebra is what one can typically feel along the centre of their back. Sliding fingers to either side of the bony spinous process, one can feel a cord of softer tissue typically the diameter of a pencil or finger. This tissue is the spinal extensors, a muscle group responsible for straightening the back from a bent forward position.
Image retrieved from: https://www.pinterest.ca/pin/515943701040714635/?lp=true
Cervical: The cervical spine consists of seven vertebrae (C1-C7), with its primary functions including supporting the weight of the head, connecting the skull to the body, and allowing for the greatest movement of any spinal region. Given the specialized shape and articulations of C1 on the skull, this allows for significant nodding “yes” motions of the head, and the articulation of C1 on C2 allows for significant rotation or “no” motions of the head.
Thoracic: The thoracic spine consists of twelve vertebrae (T1-T12), with its primary function being an attachment site for the ribs, which provide protection for the thorax (chest cavity) whose most notable contents are the lungs and heart. Although there is limited range of motion found in the thoracic spine compared to the cervical spine, its many joints and articulations with the ribcage should be mobile and can contribute to pain or dysfunction.
Lumbar: The lumbar spine consists of five vertebrae (L1-L5) which are relatively large in size compared to the rest of the spine. This size allows for greater distribution of body weight and improves our ability to lift and carry heavy loads. The predominant movement allowed at the lumbar spine is flexion/extension (bending straight forward and arching backward). Discs in the lumbar spine do not easily withstand loaded rotation forces, especially when in flexion.
Sacrum: The sacrum consists of five vertebrae that are fused together and connect the spine to the innominates. Each innominate is a fusion between three bones, the ilium, the ischium, and the pubis. Together, the sacrum and innominates make up the pelvis and the connection between the sacrum and each innominate is known as the left or right sacroiliac joint.
Coccyx: The coccyx consists of four fused vertebrae and functions primarily as an attachment site for various ligaments and muscles of the pelvic floor.
Image retrieved from: https://orthoinfo.aaos.org/en/diseases--conditions/pelvic-fractures/
Intervertebral discs typically exist between each unfused vertebra with the exception of C1 and C2. The height of each disc ranges from 20% to 33% of the adjacent vertebral body.1 and each disc undergoes imbibement (takes in water) when not under pressure and expresses (squeezes out water) when under pressure. This explains why one feels and likely is slightly taller in the morning after sleep and shorter at day’s end.
Intervertebral discs function as shock absorbers under load and provide for spinal flexibility. The circular outer layer of the disc is composed of crisscrossing cartilage fibres, while the inner layer of the disc is more mobile and gel-like. Injuries and conditions related to the disc will be further discussed below. These crisscrossing fibers can resist compression and distraction quite well as all fibers are involved in resisting those forces; however, they do not resist rotation well as their ‘x’ shape results in stretch of half of the fibers while the other half are slackened. This is why lumbar injuries often occur with rotation in addition to loading.
Intervertebral Foramen (IVF):
When discussing the intervertebral foramen, it is important to distinguish between it and the vertebral foramen mentioned above. While the vertebral foramen refers to the space in which the spinal cord travels, the intervertebral foramen refers the space between two adjacent vertebrae. The intervertebral foramen allows passage for nerves branching off the spinal cord (spinal nerves) to exit the vertebral foramen and travel to various body areas. These nerves are mixed nerves and carry sensory motor and automatic signals. Sensory signals arise from the periphery from receptors in the skin, muscle and bones that respond to types of pressure, temperature and vibration. These nerves transmit that sensory information from the periphery to the brain. Motor signals arise from the brain and allow for voluntary control of muscles. Autonomic signals arise from the brain as well but control involuntary function such as digestion, heart rate and subconscious breathing, to name a few.
Image retrieved from: https://en.wikipedia.org/wiki/Intervertebral_disc
Ligaments are strong fibrous bands that connect bones (in this case, vertebrae) together. Their expanse provides protection to the discs and spinal cord, and passive stability to the spine.
Three main ligaments should be considered when discussing the spine:
the anterior longitudinal ligament (ALL)
the posterior longitudinal ligament (PLL)
the ligamentum flavum
The ligamentum flavum attaches the vertebral arches (more specifically the lamina) of two adjacent vertebrae together. The ALL and PLL are continuous ligaments that run the length of the spine and connect the vertebral bodies together in the front and back respectively.
Image retrieved from: https://mayfieldclinic.com/pe-anatspine.htm
Although the many muscles contributing to the movement and stability of the spine could take up a whole blog post, in general, these muscles can be thought of as spinal flexors and extensors. Spinal extensors include, but are not limited to, the erector spinae group and a deep group of muscles called the multifidi. Spinal flexors include deep neck flexors, the abdominals, and hip flexors. While injury to any of these muscles can contribute to pain or poor strength, poor spinal mobility or control can also be contributors to pain and therefore may be a target for your physiotherapy treatment plan.
Neck and back pain and common spinal injuries:
Degenerative disc disease (DDD) - A misnomer, degenerative disc disease is not a true disease but instead refers to a loss of intervertebral disc height or thickness. While a normal part of aging, the loss of disc height can be found in younger populations. DDD is most commonly found in the cervical and lumbar regions of the spine and while not necessarily symptomatic, it can contribute to pain. In severe cases, the loss of disc height can cause a decrease in size of the IVF, and can lead to compression on the spinal nerves as they exit the spinal column. Compression of these spinal nerves can cause pain at the site of compression as well as pain along the nerve's length, which is known as referred pain. It is important to remember that the “degenerative” term used in DDD refers to the degeneration of the disc height itself, and not necessarily your symptoms!
Bulged/Herniated discs - As described above, intervertebral discs are composed of a tough outer layer and a gel-like inner layer. Various mechanisms, as well as degenerative changes to the disc, can lead portions of the disc to protrude outside of the typical disc shape. When the outer layers are predominantly involved, it is referred to as a disc bulge and when the inner disc material protrudes through the outer layers, it is considered a disc herniation. The protruding disc material typically occurs posteriorly and can compress the exiting spinal nerves or, in some cases, the spinal cord itself. Symptoms vary depending on the severity of compression from minor discomfort to debilitating pain in the back or a limb, with or without muscle weakness, loss of sensation and change in reflexes. Forward movements of the spine can cause more disc material to protrude and further compress the nerves. This means that long periods of sitting or activities like bending forward to tie your shoes are typically aggravating. Image retrieved from: http://www.sc-aktivity.at/tsnwvk.php?q=ruptured-disc-vs-herniated-disc Your therapist will help you find comfortable directions of movement to try to allow for resorption of the disc material over time. As pain can cause adjacent muscle weakness, strengthening is a key component of recovery from back pain.
Spinal stenosis - Spinal stenosis refers to the narrowing of the vertebral foramen or intervertebral foramen. This can cause pressure on the spinal cord or spinal nerves. Stenosis is typically caused by age related change including loss of disc height (i.e. DDD), thickening of ligaments, and bone spurs developing along the edges of vertebral bodies and around facet joints. Spinal stenosis can also be caused by congenital malformation of the bony structures leading to reduced space for the spinal cord and spinal nerves. Similar to disc injuries or DDD above, compression of neural structures can cause pain in the areas these structures innervate. Lumbar spinal stenosis can cause pain in the lower back, down the legs, cramping in calves, and decreased sensation in the legs. Positions like standing and walking (downhill) can produce these symptoms as the spine moves into extension, thus creating less space within the vertebral foramen and intervertebral foramen for nerves. Pain is often relieved by sitting or leaning forward such as on a shopping cart.
Image retrieved from: https://newsnetwork.mayoclinic.org/discussion/mayo-clinic-q-and-a-nonsurgical-treatments-may-decrease-pain-from-spinal-stenosis/
Spondylosis - Also known as osteoarthritis of the spine, spondylosis has many overlapping features with the above conditions. In general, spondylosis refers to degenerative changes of the intervertebral disc or the cartilage of the facet joints of the vertebrae. Not only can this cause pain for the same reasons as DDD, disc injuries, or spinal stenosis, but it can also contribute to stiffness of the spine due to the degenerative changes of the facet joints.
Spondylolisthesis.2 – Degenerative change, congenital abnormality or trauma can all cause spondylolisthesis, a condition where one vertebra slips relative to another. The most common is an anterolisthesis where the vertebra above slips forward or anterior on the vertebra below, however laterolistheses and retrolistheses are also possible. This slipping can be preceded by spondylolysis, which is a fracture or elongation of the pars that generally occurs during adolescent years and can be somewhat common in female gymnasts who experience considerable extension forces in the lumbar spine.
Sacroiliac Joint Dysfunction - Abnormal motion at the sacroiliac joint (SIJ), whether it's from too much or too little movement, can be a cause of pain. The SIJ is a joint of minimal movement with a naturally bumpy surface. As the sacrum is tapered from top to bottom, it sits like an inverted triangle against the innominates. This change in shape along with the ligaments holding the bones together is known as form closure – the weight of the body pushes the sacrum down, but the gap between the innominates inferiorly is smaller than the gap superiorly and so the sacrum does not slide. A second control for sacral stability is gained by force closure. This is where muscles surrounding the pelvis tighten to squeeze the innominates towards the sacrum, or to tilt the sacrum and tighten the
Sciatica - While often thought of as a diagnosis on its own, sciatica actually refers to the symptoms experienced due to irritation of the sciatic nerve. The sciatic nerve is composed of nerves from L4 to S3, which converge to form the largest nerve bundle in the body, and travels down the posterior thigh where it branches into smaller nerves. The most common cause of sciatica is compression of some of the nerve roots due to a herniated disc, but in some cases can occur due to spondylosis, spinal stenosis, or entrapment of the nerve in the glute or posterior thigh. Symptoms of sciatica typically include pain referring down the leg as well as weakness.
Image retrieved from: http://www.regenerativeorthoandspine.com/PatientEducation?ctl=View&mid=81481&ContentPubID=139
Stephen Baker graduated from Western University with a Masters of Physical Therapy. He has a passion for helping those with neck, hand or knee injuries return to their daily adventures. Book with Stephen today.
Humzah, M. D., and R. W. Soames. Human Intervertebral Disc: Structure and Function. 1988;220(4):337-356. https://doi.org/10.1002/ar.1092200402. Published April 1988. Accessed Oct 14, 2019.
Mohriak R, Vargas Silva PD, Trandafilov M Jr, et al. SPONDYLOLYSIS AND SPONDYLOLISTHESIS IN YOUNG GYMNASTS. Rev Bras Ortop. 2015;45(1):79–83. Published 2015 Nov 16. Accessed Oct 14, 2019. doi:10.1016/S2255-4971(15)30221-4