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Updated: Jun 22 2021

Spine Biomechanics

Images sagittal balance obq11.49.jpg facets_moved.jpg
  • Introduction
    • Functional spinal unit
      • the cephalad and caudad vertebral body as well as the intervertebral disc and the corresponding facet joints
      • function is to provide physiologic motion and protect neural elements
      • intradiscal pressure depends on position
    • Spinal stability
      • defined when, under physiologic loading, there is neither abnormal strain or excessive motion in the FSU
        • maintained by
          • FSU
          • muscular tension
          • abdominal and thoracic pressure
          • rib cage support
  • Three Column Theory
    • Denis three column system
      • clinical relevance
        • only moderately reliable in determining clinical degree of stability
      • definitions
        • anterior column
          • anterior longitudinal ligament (ALL)
          • anterior 2/3 of vertebral body and annulus
        • middle column
          • posterior longitudinal ligament (PLL)
          • posterior 1/3 of vertebral body and annulus
        • posterior column
          • pedicles
          • facets
          • ligamentum flavum
          • spinous process
          • posterior ligament complex (PLC)
        • instability defined by
          • injury to middle column
            • as evidenced by widening of interpedicular distance on AP radiograph
            • loss of height of posterior cortex of vertebral body
          • disruption of posterior ligament complex combined with anterior and middle column involvement
  • Ligaments
    • FSU is surrounded by 10 ligaments with the functions:
      • protecting neural structures by restricting motion of the FSU
      • absorb energy during high speed motions
    • Contents
      • all ligaments are composed of type I collagen except ligamentum flavum (mostly elastin)
      • are viscoelastic, with nonlinear behavior
  • Posterior Ligamentous Complex
    • Integerity of PLC now considered to be one of the most critical predictor of spinal fracture stability
      • one of three primary factors in TLICS scoring system. TLICS measures as
        • intact
        • suspect/indeterminant
        • ruptured
    • Anatomy
      • consists of
        • supraspinous ligament
        • interspinous ligament
        • ligamentum flavum
        • facet capsule
    • Evaluation
      • determining the integrity of the PLC can be challenging
      • conditions where PLC is ruptured
        • bony chance fracture
        • widening of interspinous distance
        • progressive kyphosis with nonoperative treatment
        • facet diastasis
      • conditions where ambiguity
        • MRI shows signal intensity between spinous process
    • Treatment
      • nonoperative
        • according to TLICS, if PLC is intact (+0 points) in a compression (+1 point) burst fx (+1 point) in a patient without neurologic deficits (+0 points) than the patient should be treated nonoperatively
          • total score = 2 points (score < 4 points = nonoperative)
      • operative
        • according to TLICS, if PLC is ruptured (+3 points) in a compression (+1 point) burst fx (+1 point) in a patient without neurologic deficits (+0 points) than the patient should be treated with surgery
          • total score = 5 points (score > 4 points = operative)
  • Spinal Balance
    • Sagittal balance
      • is due to the normal cervical lordosis, thoracic kyphosis and lumbar lordosis.
        • cervical lordosis
          • normal range 20-40°
        • thoracic kyphosis
          • average 35°
          • normal range 20-50°
        • lumbar lordosis
          • average 60°
          • normal range 20-80°
          • as much as 75% of lumbar lordosis occurs between L4 and S1 with 47% occurring at L5/S1
      • normal alignment
        • the vertical axis runs from the center of C2 to the anterior border of T7 to the middle of the T12/L1 disc, posterior to the L3 vertebral body, and crosses the posterior superior corner of the sacrum.
        • on radiograph this is estimated by a plumb line dropped from the center of C7 to the posterior-superior corner of S1
      • negative sagittal balance
        • the axis is posterior to the sacrum and occurs in patients with lumbar hyperlordosis
      • positive sagittal balance
        • The axis is anterior to the sacrum and occurs in patients with hip flexion contracture or flat-back syndrome
  • Motion
    • The orientation of the facets (zygapophyseal) joints determines the degree and plane of motion at that level
      • varies throughout the spine to meet physiologic function
      • cervical spine (C3-7)
        • planes
          • 0° coronal
          • 45° sagittal (angled superio-medially)
        • function
          • allows flexion-extension, lateral flexion, rotation
      • thoracic spine
        • planes
          • 20° coronal
          • 55° sagittal (facets in coronal plane)
          • 6 degrees of freedom
        • function
          • allows some rotation, minimal flexion-extension (also limited by ribs)
          • prevents downward flexion on heart and lungs
      • lumbar spine
        • plane
          • 50° coronal
          • 90° sagittal (facets in sagittal plane)
        • function
          • allows flexion-extension, minimal rotation
          • helps increase abdominal pressure
    • Instantaneous axis of rotation (IAR)
      • axis about which the vertebra rotates at some instant in time
      • normal FSU
        • is confined to a small area within the FSU
      • abnormal FSU (e.g. degenerate disc)
        • shifts outside the physical space of the FSU
        • is enlarged dramatically
  • Pedicle Anatomy
    • Cervical
      • C2
        • viable for pedicle screw placement
      • C3-C6
        • pedicle small making pedicle screw instrumentation difficult
          • lateral mass scews placed at C3-C6 as alternative
      • C7
        • viable for pedicle screw placement
    • Thoracic
      • pedicle diameter
        • the pedicle wall is twice as thick medially as laterally
        • T4 has the narrowest pedicle diameter (on average)
        • T7 can be irregular and have a narrow diameter on the concave side in AIS
        • T12 usually has larger pedicle diameter than L1
      • pedicle length
        • pedicle length decreases from T1 to T4 and then increases again as you move distal in the thoracic spine
          • T1: 20mm
          • T4: 14mm (shortest pedicle)
          • T10: 20 mm
      • pedicle angle
        • transverse pedicle angle
          • varies from 10deg (mid thoracic spine) to 30deg (L5)
        • sagittal pedicle angle
          • 15-17deg cephalad for majority of thoracic spine
          • neutral (0deg) for lumbar spine except L5 (caudal)
    • Lumbar-Sacral
      • landmarks
        • midpoint of the transverse process used to identify midpoint of pedicle in superior-inferior dimension
        • lateral border of pars used to identify midpoint in medial-lateral dimension
      • pedicle angulation
        • pedicles angulate more medial as you move distal
          • L1: 12 degrees
          • L5: 30 degrees
          • S1: 39 degrees
      • pedicle diameter
        • L1 has smallest diameter in lumbar spine
        • S1 has average diameter of ~19mm
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