Updated: 10/10/2017

Spine Biomechanics

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Introduction
  •  Functional spinal unit (FSU)
    • 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) than the patient should be treated with surgery
        • 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) than the patient should be treated with surgery
        • total score = 2 point (score > 4 points = nonoperative)
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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