Spinal Column Stability and Mobility

Stability and Mobility

• A reference straight column with N=0 and R=1 contrasts with a flexible column like the spine, which has curvatures leading to a resistance of R=10.
• Normal spinal curvatures show an index of 95% (normal limits: 94-96).
• Exaggerated spinal curvatures yield a Delmas index below 94%, while attenuated curvatures result in an index above 96%.

Intervertebral Disk Structure

• Composed of two parts:
  • Nucleus pulposus (central portion)
  • Annulus fibrosus (peripheral portion)
• High concentrations of water and proteoglycans in the nucleus; collagen (Type I and II) is predominant in the annulus.
• Annulus fibers organized in concentric lamellae, enclosing the nucleus and maintaining constant pressure.

Intervertebral Disk Functions - Stability

• Disk structure provides resistance against:
  • Intervertebral distraction (vertical separation)
  • Shear (sliding)
  • Torsion (twisting)
• Collagen fibers oriented vertically resist distraction but less effective against sliding and torsion movements.
• Parallel fibers to the endplate effectively resist shear and torsion but not distraction.

Intervertebral Disk Functions - Mobility

• The ratio of disk thickness to vertebral body height influences mobility; larger ratios lead to greater mobility.
• Mobility ranks (decreasing order):
  • Cervical (C) 2/5
  • Lumbar (L) 1/3
  • Thoracic (T) 1/5

Intervertebral Disc as a Shock Absorber

• Nucleus center position is optimal for function; increased hydrostatic pressure is generated from body weight and muscle contraction.
• Increased nuclear pressure elevates tension in the annulus fibrosus, counteracting radial expansion and stabilizing the structure.
• Pressure transmitted through endplates to adjacent vertebrae; key during loading activities.

Nucleus Pulposus Pressure Dynamics

• High disc pressures occur during forward bends or weight-bearing;
• Lifting while squatting minimizes lumbar disc pressure compared to lifting with straight knees.
• Slouched sitting increases disc pressure versus an erect position.

Apophyseal Joints

• Comprises 24 pairs formed by opposing facet surfaces, classified as plain joints.
• The orientation of facet surfaces influences spinal kinematics:
  • Horizontal facets favor axial rotation.
  • Vertical facets promote flexion-extension.
  • Frontal facets facilitate lateral flexion but restrict axial rotation.

Influencing Factors on Joint Motion

• Joint motion affected by:
  • Ratio of vertebral body to intervertebral discs.
  • Shape of vertebrae.
  • Local muscle actions.
  • Attachment of ribs and ligaments.