Biomechanics of Spine

Spinal Anatomy & Curves - Backbone Basics

• Vertebral Column: 33 vertebrae: 7 Cervical (C), 12 Thoracic (T), 5 Lumbar (L), 5 Sacral (S - fused), 4 Coccygeal (Co - fused).
• Segments:
  • Anterior: Vertebral bodies, intervertebral discs (IVD).
  • Posterior: Vertebral arch (pedicles, laminae), facet joints, processes (spinous, transverse, articular).
• Spinal Curves (Sagittal Plane):
  • Lordosis (concave posteriorly): Cervical, Lumbar.
  • Kyphosis (convex posteriorly): Thoracic, Sacral.
  • Develop postnatally (Cervical - head lifting; Lumbar - walking).
• Intervertebral Disc (IVD):
  • Annulus Fibrosus: Outer, fibrocartilaginous rings.
  • Nucleus Pulposus: Inner, gelatinous core.

⭐ The nucleus pulposus, primarily composed of water (70-90%) and proteoglycans, is responsible for the disc's ability to resist compressive loads.

• Ligaments: Anterior Longitudinal (ALL), Posterior Longitudinal (PLL), Ligamentum Flavum, Interspinous, Supraspinous. Provide stability. (📌 All People Like Fun In Summer for ligament order anterior to posterior, roughly).

Kinematics & Motion Segments - Spine's Smooth Moves

• Kinematics: Study of motion; each segment has 6 degrees of freedom (DOF):
  • Translations (3): Anterior-Posterior, Lateral, Cranial-Caudal.
  • Rotations (3): Flex/Ext (Sagittal), Lat Bend (Coronal), Axial Rot (Transverse).
• Functional Spinal Unit (FSU): Smallest motion unit.
  • Components: Two vertebrae, Intervertebral Disc (IVD), facet joints, ligaments.
  • Key for movement & stability.
• Coupled Motions: Movement in one plane induces motion in another.

  ⭐ In the cervical spine (C2-C7), lateral bending is typically coupled with axial rotation to the same side.

• Regional ROM Highlights:
  • Cervical: Max ROM. C1-C2: ~50% cervical rotation.
  • Thoracic: Max axial rotation; Flex/Ext limited by ribs.
  • Lumbar: Max Flex/Ext; rotation limited. oka

Loads on the Spine - Under Pressure Profile

• Spinal loads include: axial compression, bending (flexion, extension, lateral), and torsion.
• Intradiscal Pressure (IDP): A key indicator of the load experienced by intervertebral discs, especially lumbar.
  • Varies significantly with posture, body weight, muscle activity, and external loads.
  • Muscle co-contraction (e.g., abdominal bracing) can ↑ IDP but also ↑ spinal stability.
• Relative IDP Values (Nachemson & Elfstrom, approximate units):
  • Lying supine: ~25
  • Standing erect: ~100
  • Sitting unsupported: ~140
  • Sitting, leaning forward / Forward bending: ~185-200
  • Lifting 20kg (correctly: back straight, knees bent): ~210
  • Lifting 20kg (incorrectly: back bent, knees straight): ~340 ⚠️ (High risk!)
• Prolonged sitting, especially with poor posture, significantly ↑ IDP.

⭐ Intradiscal pressure is lowest when lying supine (~25 units), increases with standing (~100 units), and is significantly higher when sitting unsupported and leaning forward (~185 units) or lifting weights.

Spinal Stability & Muscles - Balancing Act Breakdown

• Panjabi's Three-Subsystem Model: Essential for spinal integrity under load.
  • Passive: Vertebrae, intervertebral discs, ligaments, facet joints. Provide inherent stiffness.
  • Active: Muscles and tendons (e.g., multifidus, erector spinae, abdominals). Generate forces for dynamic stability.
  • Neural Control: Central (CNS) & Peripheral Nervous Systems (PNS). Coordinate muscle responses.
• Key Stabilizing Muscles:
  • Local (Deep/Core): Multifidus, Transversus Abdominis (TrA), Quadratus Lumborum (deep fibers). Critical for segmental control & stiffness.
  • Global (Superficial): Erector spinae, Rectus abdominis, External/Internal obliques. Control trunk motion & general stability.
• Muscle Actions for Stability:
  • Co-contraction of antagonists (e.g., abdominals & back extensors).
  • Intra-Abdominal Pressure (IAP) mechanism.
  • 📌 Mnemonic: "PAL" for Panjabi's model (Passive, Active, Neural).

⭐ The multifidus muscle plays a crucial role in segmental stability of the lumbar spine, and its atrophy is often associated with chronic low back pain.

High‑Yield Points - ⚡ Biggest Takeaways

• Fryette's Laws: Type I (neutral, opposite coupling), Type II (non-neutral, same-side coupling) for spinal motion.
• IVD: Annulus fibrosus resists tension/torsion; Nucleus pulposus handles compression.
• Facet joints guide motion, bear significant load (especially in extension/rotation), and limit range.
• Instantaneous Axis of Rotation (IAR) shifts with disc degeneration, signaling instability.
• Axial compression is primary spinal load; intradiscal pressure lowest when supine.
• Denis three-column theory (anterior, middle, posterior) assesses thoracolumbar fracture stability.
• Cervical spine (C2-C7): side-bending and rotation couple to the same side (Type II-like motion).