Anatomy of the Spinal Column

Structure of the Spinal Column

• The spinal column consists of 33 vertebrae, divided into 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 3 or 4 coccygeal vertebrae, along with their respective intervertebral discs.
• The spine articulates with the 12 pairs of ribs in the thoracic region, the cranium at the occipital-atlas joint, and the pelvis at the sacroiliac joint.

Functional Components of the Spine

• The spinal column is divided into anterior and posterior pillars.
• The anterior pillar is composed of vertebral bodies and intervertebral discs, providing weight-bearing, shock-absorbing, and hydraulic functions.
• The size of the intervertebral disc influences the amount of motion available between two vertebrae.
• The posterior pillar, or vertebral arch, is composed of articular processes and facet joints, providing the gliding mechanism for movement.
• The orientation of the facets influences the direction of motion.

Motions of the Spinal Column

• Motion of the spinal column is described both globally and at the functional unit or motion segment level.
• A functional unit is comprised of two vertebrae and the joints in between, including two zygapophyseal facet joints and one intervertebral disc.
• The axis of motion for each unit is in the nucleus pulposus of the intervertebral disc.
• Motion is defined by what is occurring with the anterior portion of the body of the superior vertebra.

Types of Motion

• Flexion: motion in the sagittal plane, resulting in forward bending, with the anterior portion of the bodies approximating and the spinous processes separating.
• Extension: motion in the sagittal plane, resulting in backward bending, with the anterior portion of the bodies separating and the spinous processes approximating.
• Side bending (lateral flexion): motion in the frontal plane, resulting in lateral flexion to the left or right, with the lateral edges of the vertebral bodies approximating on the side toward which the spine is bending and separating on the opposite side.
• Rotation: motion in the transverse plane, resulting in rotation, with the body of the superior vertebra moving relative to the right and its spinous process to the left, or vice versa.
• Anterior/posterior shear: forward or backward translation of the body of the superior vertebra on the vertebra below.
• Lateral shear: sideways translation of the body of the superior vertebra on the vertebra below.
• Compression/distraction: separation or approximation of the vertebral bodies with a longitudinal force, either away from or toward the vertebral bodies.

Arthrokinematics of the Zygapophyseal (Facet Joints)

• Each region of the spine has its own special considerations as pertains to arthrokinematic movement and function.
• The arthrokinematics of the craniovertebral (suboccipital) area are defined separately.
• The remainder of the cervical spine and all thoracic facets have relatively flat articular surfaces and glide on the adjacent facet joint.
• The superior facets of the lumbar spine are concave and articulate with the adjacent inferior convex facets.

Coupled Motions

• Coupled motions occur at a segmental level when a person side bends or rotates their spine.
• Coupled motion is a consistent association of one motion about an axis with another motion around a different axis.
• Coupled motions vary depending on the region, spinal posture, orientation of the facets, and factors such as extensibility of the soft tissues.