Introduction
The vertebral column is joined to the skull by the atlanto-occipital and atlanto-axial joints, which allow the head to nod, tilt and turn. This topic reviews the joints between vertebrae in general (including the sacral joints), the curvatures of the column, the craniovertebral joints and the biomechanics of spinal movement.
Joints of the column: overview
- Atlanto-occipital articulation (skull and atlas).
- Atlanto-axial articulation (atlas and axis).
- Intervertebral joints.
- Sacral joints.
Joints between the vertebrae
The two major types of joints between vertebrae are symphyses between the vertebral bodies and synovial joints between the articular processes. A typical vertebra has six joints with adjacent vertebrae: four synovial joints (two above, two below) and two symphyses (one above, one below). Each symphysis includes an intervertebral disc.
- Symphysis (cartilaginous joint) between bodies: a layer of hyaline cartilage on each vertebral body with an intervertebral disc between the layers.
- Synovial joints between the superior and inferior articular facets of adjoining vertebral arches (zygapophysial joints).
- Ligaments: the interspinous ligaments run between the spinous processes; the anterior and posterior longitudinal ligaments stabilise the column. Together the joints allow some bending.
Intervertebral disc
The intervertebral disc consists of an outer anulus fibrosus surrounding a central nucleus pulposus.
- The anulus fibrosus is an outer ring of collagen surrounding a wider zone of fibrocartilage in a lamellar arrangement; this arrangement of fibres limits rotation between vertebrae.
- The nucleus pulposus fills the centre of the disc, is gelatinous and absorbs compression forces between vertebrae.
Zygapophysial joints
The synovial joints between superior and inferior articular processes of adjacent vertebrae are the zygapophysial joints. A thin articular capsule attached to the margins of the articular facets encloses each joint.
- Cervical: the joints slope inferiorly from anterior to posterior; their shape facilitates flexion and extension.
- Thoracic: the joints are oriented vertically; their shape limits flexion and extension but facilitates rotation.
- Lumbar: the joint surfaces are curved and adjacent processes interlock, limiting the range of movement, though flexion and extension are still major movements.
Uncovertebral joints
The lateral margins of the upper surfaces of typical cervical vertebrae are raised into crests or lips called uncinate processes. These may articulate with the body of the vertebra above to form small "uncovertebral" synovial joints.
Sacral joints
- The sacrum articulates with the hip bones.
- The superior surface of the sacrum has two superior facets that articulate with the inferior articular processes of the fifth lumbar vertebra.