What are the four primary components of an intervertebral motion segment and their roles in stability?

• A. Vertebral bodies with endplates; the intervertebral disc (nucleus pulposus and annulus); the two facet joints and capsules; and the supporting ligaments and paraspinal muscles.
• B. Spinal cord, nerve roots, and meninges.
• C. Vertebral bodies with endplates only.
• D. Intervertebral discs only.

Answer: (A)

Stability of an intervertebral motion segment comes from four integrated parts: the vertebral bodies with their endplates, the intervertebral disc, the facet joints with their capsules, and the surrounding ligaments plus the deep paraspinal muscles. The vertebral bodies and endplates form the rigid anterior column and bear most of the axial load, with the endplates helping to distribute that load across the disc and vertebrae so the segment stays aligned under pressure. The disc sits between the vertebrae, and its nucleus pulposus acts as a hydraulic cushion while the annulus fibrosus resists radial expansion and contains the nucleus, maintaining disc height and spacing so the joints have a proper articulation and load sharing.

The facet joints and their capsules provide guidance and restraint for motion, helping to steer movement in the sagittal and horizontal planes and to limit excessive gliding or twisting. Their load sharing becomes particularly important during extension and rotation, where they bear part of the transmitted forces and help prevent abnormal translational movement. The ligaments and paraspinal muscles offer passive and active stabilization: ligaments limit or control motion in flexion, extension, and rotation, while the paraspinal muscles provide dynamic, postural control and proprioceptive feedback to stabilize the segment during activity.

If any one of these components is compromised, the others must bear more load, which can destabilize the segment and predispose to injury or degeneration. The other options refer to neural structures or omit key stabilizing elements, so they do not accurately describe the four contributors to motion-segment stability.