Biomechanics of Spine

Biomechanics of Spine - Vertebral Blueprint

• Typical Vertebra Components:
  • Vertebral Body: Weight-bearing, anterior.
  • Vertebral Arch: Posterior, protects spinal cord.
    • Pedicles: Connect body to arch.
    • Laminae: Form roof of vertebral canal.
  • Processes (for muscle/ligament attachment & articulation):
    • Spinous Process (1)
    • Transverse Processes (2)
    • Articular Processes (4): Superior (2), Inferior (2) - form facet joints.
• Regional Variations: Adapt to function (e.g., C1/C2 for rotation, lumbar for weight-bearing).

⭐ The pars interarticularis is a critical region of the lamina between the superior and inferior articular processes; defects here (spondylolysis) are common, especially in the lumbar spine (L5).

Biomechanics of Spine - Flex & Twist Dynamics

• Flexion:
  • Anterior structures compress (disc, body).
  • Posterior elements stretch (ligaments, facets).
  • Nucleus pulposus shifts posteriorly.
  • Foramen size ↑.
• Extension:
  • Anterior elements stretch, posterior compress.
  • Nucleus pulposus shifts anteriorly.
  • Foramen size ↓.
• Axial Rotation (Twist):
  • Maximal in cervical, then thoracic; lumbar limited (facet orientation).
  • Annulus fibrosus resists torsion.
  • Coupled with lateral bending.

⭐ In lumbar spine rotation, only about 50% of annulus fibrosus fibers are loaded at any given time due to their alternating oblique orientation, providing significant torsional resistance.

Biomechanics of Spine - Load Bearing & Pressure

• Load Distribution:
  • Anterior column (vertebral bodies, IVDs): Bears ~80% axial compression.
  • Posterior column (facet joints, ligaments): Bears ~20% axial load; guides motion, resists shear/tension.
• Intervertebral Disc (IVD):
  • Nucleus Pulposus (NP): Hydrophilic; converts axial load to radial stress.
  • Annulus Fibrosus (AF): Lamellar collagen; resists tension, torsion, shear.
• Intradiscal Pressure (IDP) - L3 Disc: (Values relative to standing = 100)
  • Supine: 25 (lowest)
  • Standing: 100
  • Sitting unsupported: 140
  • Forward bending (standing): 150
  • Lifting 20kg (improperly): 340 (highest)
  • 📌 Nachemson's classic study.

⭐ The nucleus pulposus, due to its high water content, behaves hydrostatically, distributing applied loads evenly to the annulus fibrosus and endplates.

Biomechanics of Spine - Stability & Strain Patterns

• Panjabi's Model:
  • Passive: Bones, discs, ligaments (ALL, PLL, LF, ISL, SSL), capsules.
  • Active: Muscles & tendons (paraspinal, abdominal).
  • Neural: CNS/PNS control for dynamic stability.
• Denis' 3 Columns:
  • Anterior: ALL, ant. body/disc.
  • Middle: PLL, post. body/disc.
  • Posterior: Pedicles, facets, lamina, spinous proc., post. ligaments (PLC).
  • Instability if ≥2 columns disrupted.
• Strain & Injury Patterns:
  • Axial Load: Burst Fx, endplate #.
  • Flexion: Ant. wedge Fx, PLC injury.
  • Extension: Post. element Fx, ALL strain.
  • Flexion-Distraction: Chance Fx (seatbelt), PLC disruption.
  • Rotation/Shear: Facet injury, disc/annular tears.
• Neutral Zone (NZ): ↑NZ → instability.
• IAR (Instantaneous Axis of Rotation): Shifts with pathology.

⭐ An increased Neutral Zone (NZ) signifies reduced spinal stiffness and is a key indicator of clinical instability, implying higher injury risk with physiological loads.

High‑Yield Points - ⚡ Biggest Takeaways

• Nucleus pulposus resists axial compression; annulus fibrosus withstands tension and torsion.
• Spinal motion is often coupled: lateral bending usually occurs with axial rotation.
• The Instantaneous Axis of Rotation (IAR) shifts during motion and with degeneration.
• Fryette's Laws describe predictable coupled motions in thoracic/lumbar spine.
• Spinal curves: tensile forces maximal on convexity; compressive forces on concavity.
• Intradiscal pressure: lowest supine, highest sitting flexed with weight.
• Facet joints resist torsion and shear forces, vital in lumbar spine.