Definition: The ability of the spine to resist displacement under normal physiological loading, thus protecting neural structures from injury while preventing deformity and pain.
Motion Segment
A spinal motion segment includes two vertebrae and the soft tissues between them.
Stability arises from the interaction of three subsystems:
Vertebrae become wider and deeper as they move caudally, in response to increased loading.
Trabeculae (vertical and horizontal) form the internal structure of vertebrae under load. Loss of trabeculae (as in osteoporosis) predisposes vertebrae to compression fractures.
The S-shaped spine in the sagittal plane shifts body weight away from the central axis of rotation, aiding balance and gait.
The axis of rotation passes through the posterior portion of the vertebral body, and spinous processes provide long lever arms for posterior muscles, resisting flexion.
Facet Joints
Formed between the inferior articular facet of one vertebra and the superior articular facet of the vertebra below it.
Facet joints guide the type of movement possible between vertebrae.
They become progressively more vertical in the sagittal and coronal planes moving caudally.
Cervical spine: Resistant to translation, more susceptible to flexion/extension.
Lumbar spine: More resistant to flexion/extension.
Thoracic spine: Extra stability from the rib cage.
Spinal Ligaments
Ligamentum flavum: High elastin content, preventing buckling inward. Degeneration can cause it to buckle and contribute to spinal stenosis.
Posterior longitudinal ligament (PLL) and anterior longitudinal ligament (ALL): Resist flexion and extension forces.
Other ligaments include intertransverse and interspinous ligaments.
Intervertebral Discs
Shock absorbers between vertebrae that distribute load and allow limited movement. Their properties are discussed in other chapters.
Active Musculoskeletal System
Consists of spinal muscles, which include:
Extensor muscles: Arranged in three layers (deep, intermediate, and superficial).
Abdominal muscles: Help stabilize the spine, especially the lumbar region.
Intercostals and lower trunk muscles: Play roles in stability and respiration.
Extensor muscles:
Deep muscles: Interspinalis, intertransversalis.
Intermediate muscles: Multifidus, semispinalis (lie between the deep layer and transverse processes).
Proprioception is critical in the spine due to its constantly changing positions and loading. It helps coordinate movement and maintain balance.
Interaction of Subsystems in Spinal Stability
Spinal range of motion is divided into two zones:
Neutral zone: The initial, highly flexible range where the active and neural systems dominate, providing proprioceptive feedback to control movement.
Elastic zone: Occurs toward the end of the motion range, where the passive system limits movement and provides stability, preventing damage to neural structures.
Spinal Disease or Injury
Conditions like degenerative disc disease or burst fractures increase the size of the neutral zone beyond the normal physiological range, leading to instability.
The widened neutral zone increases the risk of neurological injury due to excess movement.
In a diseased spine, enhancing the active musculoskeletal system (i.e., muscle strengthening) can help minimize the increase in the neutral zone, restoring some stability.