The spine plays a vital role in stability, smooth movement and protection of the delicate spinal cord. It is made up of bony segments called vertebra with fibrous tissue called intervertebral discs between them. The vertebra and discs form the spinal column from the head to the pelvis, giving symmetry and support to the body.
Cervical Spine Anatomy
A single vertebra is made up of two parts, the front portion is called the body and the back portion is referred to as the vertebral or neural arch. The body is cylindrical in shape, strong and stable. Two strong pedicle bones join the vertebral arch to the body of the vertebrae.
The laminae of the vertebra can be described as a pair of flat arched bones that form a component of the vertebral arch. The transverse processes spread out from the side of the pedicles, like wings, and help to anchor the surrounding muscle to the vertebral arch. The spinous process forms a steeple at the apex of the laminae and is the part of our spine that is felt directly under the skin.
Spinal canal is formed by the placement of single vertebral foramina one on top of the other to form a canal. The purpose of the canal is to create a bony casing from the head to the lower back through which the spinal cord passes.
The spine can be divided into 4 parts: cervical, thoracic, lumber and sacral region. The cervical spine comprises the first 7 vertebrae of the spinal column, which form the neck.
The cervical spine is highly mobile compared to other regions of the spine such as thoracic or lumbar spine. In contrast to other parts of the spine, the cervical spine has transverse foramina in each vertebra through which the vertebral arteries supply blood to the brain.
Based on the structural diversity in the cervical spine, it can be divided into two parts: upper and lower cervical spine.
The upper cervical spine comprises of the atlas (C1) and axis (C2), which are different from rest of the cervical vertebrae. The atlas vertebra articulates with the occiput superiorly at the atlanto-occipital joint and with the axis inferiorly at the atlantoaxial joint. Generally, the atlantoaxial joint is accountable for half of the cervical rotational movements; while the atlanto-occipital joint is responsible for half of the flexion and extension movements of the neck.
In contrast to other vertebrae, the atlas is ring-shaped without a body. The odontoid process, or dens of the axis, represents the fused remnants of the body of the atlas. The transverse ligament firmly opposes the odontoid process to the posterior portion of the anterior arch of atlas and provides stability to the atlantoaxial joint.
The atlas comprises of a thick anterior arch, a thin posterior arch, two prominent lateral masses, and two transverse processes. The transverse process surrounds the transverse foramen, through which the vertebral artery passes. A zygapophyseal joint is present on the superior and inferior aspect of the lateral mass. The superior articular facets articulate with the occipital condyles and are kidney-shaped, concave, projecting upward and inward, while the inferior articular facets are comparatively flat projecting downward and inward and articulate with the superior facets of the axis.
The axis possesses a large vertebral body with the odontoid process or dens. The axis also has heavy pedicles, laminae, and transverse processes, which help in the attachment of muscles.
The remaining five cervical vertebrae, C3-C7, form the lower cervical spine. They are similar to each other but distinct from C1 and C2. Each of these cervical vertebrae has a body with a concave superior surface and convex inferior surface. The superior surfaces of the bodies have raised processes called uncinate processes that articulate with the inferior lateral portion of the vertebral body present above, called echancrure or anvil.
The C3-C6 vertebrae have bifid spinous processes, meaning they are split in two parts, whereas C7 has a nonbifid, rounded spinous process.
The intervertebral discs are flat and round, present between the lumbar vertebrae and act as shock absorbers when you walk or run. There is a soft, gelatinous material in the center (nucleus pulposus) which is encased in strong elastic tissue forming a ring around it called annulus fibrosus.
Facet joints are synovial joints which give the spine it’s flexibility by sliding on the articular processes of the vertebra below. The joint capsules are quite loose in the cervical region, compared to other parts of the spine, to facilitate smooth movement.
Thoracic Spine Anatomy
Thoracic spine is the central part of the spine, also called as dorsal spine, which runs from the base of the neck to the bottom of your rib cage. The thoracic spine provides flexibility that holds the body upright and protects the organs of the chest.
Spine is made up of 24 spinal bones, called as vertebra, of which, the thoracic region of the spine is made up of 12 vertebrae (T1-T12). The vertebrae are aligned on top of one another to form the spinal cord which gives the posture to our body. The different parts of the thoracic spine include bone and joints, nerves, connective tissues, muscles, and spinal segment.
Each vertebra is made up of round bone called as vertebral body. The protective bony ring attaches to each vertebral body and surrounds the spinal cord and forms the spinal canal. The bony ring is formed, when two pedicle bones joins two lamina bones which connect to the back of the vertebral body directly. These lamina bones form the outer rim of the bony ring. When vertebrae arrange on top of the other, the bony ring forms a hollow tube which surrounds the spinal cord and nerves and also provides protection to the nervous tissue.
A bony knob like structure projects out at a point, where the two lamina bones join together at the back of the spine; these projections are called as ‘spinous processes ‘and the projections at the side of the bony ring are called as ‘transverse processes’.
Between each vertebra, there are small bony knobs at the back of the spine that connect the two vertebras together, called as ‘facet joints’. Between each pair of vertebra, two facet joints are present, one on either side of the spine. The alignment of the two facet joins allows the back and forth movement of the spine. The facet joint are covered by a soft tissue called the ‘articular cartilage’, which allows the smooth movement of the bones.
On the left and right side of the vertebra, is a small tunnel called ‘neural foramen’. The two nerves that leave each vertebra pass through this neural foramen. These spinal nerves group together to form a main nerve that passes to the organs and limbs. These nerves control the muscles and organs of the chest and abdomen. An ‘intervertebral disc’ is present in front of this opening which is made up of connective tissue. The discs of thoracic region are smaller compared to cervical and lumbar spine.
Connective tissue holds the cells of the body together and ligaments attach one bone to another. Anterior longitudinal ligament runs down to the vertebral body and the posterior longitudinal ligament attaches on the back of the vertebral body. A long elastic band, called as ‘ligamentum flavum’, connects the lamina bones.
The spine muscles are arranged as layers, strap-shaped spine muscle, called as ‘erector spinae’ makes up the middle layer of the muscle. The deepest layer of muscles attaches along the back of the spine bones and connects to the vertebrae. These muscles connect one rib to the other.
Spinal segment includes two vertebrae separated by an intervertebral disc, nerves that leave the spinal column at each vertebrae, and small facet joints of the spinal column.
Lumbar Spine Anatomy
The lumbar spine is composed of the lower 5 vertebrae, which have been numbered L1–L5. The lowest vertebra of the lumbar spine (L5) is connected to the top of the sacrum, which is a triangular bone present at the base of the spine fitting into the two pelvic bones. In some cases, an extra or sixth lumbar vertebra may be present.
The lumbar vertebral bodies are seen to be taller and bulkier than the rest of the spine, as the lower back has to withstand higher pressure due to body weight and other movements such as lifting, pulling, and twisting. In addition, large and powerful muscles are found to be attached on or near the lumbar spine to provide extra strength to the lumbar vertebral bodies.
The transverse processes in the low back are broader, compared to that in the other areas of the spine, because of the attachment of large back muscles which exert heavy force on them.
There are two facet joints present between a pair of vertebrae, one on either side of the spine. A facet joint is comprised of small, bony knobs arranged along the back of the spine. Two vertebrae are connected to each other through these knobs and form a facet joint. These joints aid in the free movement of the spine.
Articular cartilage covers the surfaces of the facet joints to assist in smooth, frictionless movement between the bones in the joint.
A small tunnel called the neural foramen is present on either side of the vertebra. It is through these foramina that the two nerves leave the spine. The intervertebral disc is present at the opening of the foramen. The lumbar intervertebral discs are flat and round, present between the lumbar vertebrae and act as shock absorbers when you walk or run. There is a soft, gelatinous material in the center (nucleus pulposus) which is encased in strong elastic tissue forming a ring around it called annulus fibrosus. Ageing, injury or trauma may cause the annulus fibrosus to tear resulting in protrusion of the nucleus pulposus. This may compress the spinal nerves and/or spinal canal.
Bony spurs on the facet joint may project on the tunnel, resulting in narrowing of the foramen and compression of the nerve.
Sacral Spine Anatomy
The sacroiliac joint is one of the large joints in the body and is formed by the connection of the sacrum and the right and left iliac (pelvic) bones. The sacroiliac joints have small amount of movement and transmits all the forces of the upper body to the lower body. The sacrum is the triangular-shaped bone at the bottom of the spine, below the lumbar spine. The sacroiliac joint acts as a shock-absorbing structure. Sacroiliac joint dysfunction or Sacroiliac joint pain is one of the common causes of the low back pain.
Causes of sacroiliac joint dysfunction include:
- Traumatic injuries caused when there is a sudden impact
- Biomechanical problems such as twisted pelvis, leg length discrepancies or muscle imbalances
- Hormonal changes during pregnancy
- Inflammatory joint conditions such as arthritis and ankylosing spondylitis
The most common symptom of sacroiliac joint dysfunction is pain. Patient often experiences pain in the lower back, thigh, groin or buttocks that radiates down the leg. The pain is typically worse with standing and walking and is relieved on resting. Other symptoms include limping, fever, psoriasis, eye inflammation, and limited range of motion.
Proper diagnosis is essential because the symptoms mimic other common conditions, including sciatica, herniated disks or other low back pain problems. Sacroiliac joint pain is diagnosed by taking medical history and performing physical examination. Other imaging studies such as MRI, CT scan, bone scan, and X-rays may be needed to know the extent of joint damage.
Treatment options include adequate rest, use of pain medications, wearing a sacroiliac belt to stabilize the joint, and physical therapy. Corticosteroid injections may be given to reduce the pain and inflammation.
Surgery may be considered in patients not responding to conservative line of management.
For severe cases of pain one or a combination of the above treatments may be effective.