Back & Spinal Cord
5. Describe the spine and the relationships of its parts. Detail the various arthroses (joints) of the spine, supportive ligaments, and muscle groups.
The spine (aka vertebral column) consists of thirty-three vertebrae that span the distance between the occipital bone of the skull ending distally with the coccyx. The spine has five regions, each comprised of a distinct type of vertebrae, some with interleaving intervertebral discs (joints). They include:
- Cervical spine - 7 cervical vertebrae,
- Thoracic spine - 12 thoracic vertebrae,
- Lumbar spine - 5 lumbar vertebrae,
- Sacrum - 5 fused (co-ossified) segments, &
- Coccyx - 4 fused (co-ossified) segments.
The most superior joints of the spine are the atlanto-occipital joints, synovial joints between the occipital condyles and the superior facets of the atlas (C1). The joints below are the atlanto-axial joints, synovial joints between the inferior facets of the atlas (C1) and the superior facets of the axis (C2), as well as a synovial pivot joint with the dens (odontoid process) of C2 forming the axis of the pivot. Moving inferiorly, all vertebrae have three types of joints:
- Anteriorly, a symphysis (a midline, cartilaginous joint) including the body of the superior vertebrae, an intervertebral disc (with an outer anulus fibrosus and inner nucleus pulposus), and the body of the inferior vertebrae
- Anulus fibrosus: consists of collagen and fibrocartilage and surrounds the nucleus pulposus
- Nucleus pulposus: a remnant of the notochord
- Posteriorly, two synovial facet joints, each between the inferior articular processes of the superior vertebra and the superior articular processes of the inferior vertebra. Additionally, joints form between spinous processes of adjacent vertebrae.
The inferior-most intervertebral disc sits between the body of L5 and the articular surface of the sacrum at the lumbosacral junction. Additionally, inferior articular processes of L5 form synovial joints with superior articular processes of the sacrum. The sacrum articulates laterally with the coxal bones (the SI joints, to be discussed in a later session) and inferiorly with the coccyx at the sacrococcygeal junction as either a symphysis, or a more mobile synovial joint.
Providing direct support to the bodies of the vertebrae and intervertebral discs are the anterior & posterior longitudinal ligaments.
- Posterior longitudinal ligament (PLL) supports the stacked vertebral bodies and intervertebral discs posteriorly. The PLL extends from the axis (C2) to the sacrum, and fuses with the anulus fibrosus of the intervertebral discs.
- Most anterior feature of the vertebral canal
The posterior ligamentous complex -- consisting of the supraspinous & interspinous ligaments, ligamenta flava, and facet joint capsules -- support the laminae, spinous processes, and facet joints.
- Supraspinous ligaments are typically only present from C7 to L4, and are frequently sparse in areas. Supraspinous ligaments tend to connect 3-4 apices of serial spinous processes, and they tend to blend with interspinous ligaments.
- Interspinous ligaments connect one spinous process to another from the root of the spinous process to the apex, and are best visualized from a lateral view. The interspinous ligaments differ with respect to region of the spine. Among cervical vertebrae, interspinous ligaments are typically absent. Among thoracic vertebrae, interspinous ligaments are gracile. Among lumbar vertebrae, interspinous ligaments are robust and often paired. These ligaments are more visible post-laminectomy.
- Ligamenta flava (sing. = ligamentum flavum) means yellow ligament and is composed predominantly of yellow elastic tissue. These span the distances between adjacent laminae within the vertebral canal and become increasingly more robust in the inferior portions of the vertebral column
The various joints, supportive ligaments, and associated muscles allow for many different types of movement. The major movements of the spine (or, more often, regions of the spine) are as follows:
- Flexion - anterior bending (e.g. touching one’s toes)
- Extension - posterior bending
- Lateral flexion - bending to a side