Sagittal Alignment

Posted bythespinebookPosted inAdult Deformity, Topics

Importance of Sagittal Alignment 

  • Cone of Economy (Conus of Debousset) 
    • Narrow range in shape of cone formed between the head and the pelvis 
    • Outside this cone – energy consumption to maintain upright posture goes up 
  • Sagittal malalignment (PT >25 SVA >5) main driver of HRQOL and construct failure after spine surgery 
  • Glassman et al 
    • Linear relationship between magniude of sagitttal balance and adverse healh outcomes (ODI, SF 12 SRS 29) 

3 Components 

  • Global 
    • SVA (sagittal Vertical Axis) 
      • Distance between 
        • C7 plumb line 
        • Posterior Superior corner of S1 
      • Normal within 2cm, deformity is >5 cm 
      • If head is behind pelvis – negative SVA 
    • T1 Pelvic Angle 
      • More reliable; SVA changes with changes to Pelvic tilt through hip ORM. T1PA does not 
      • Angle between 
        • Femoral head to T1 centroid 
        • Femoral head to S2 centroid  
  • Regional 
    • C – 40 +/- 10 
    • T – 20-40 (PI-20) 
      • T67 Disc apex 
    • TL – 0 (T10-L2) 
    • LL – 40-60, (LL = PI within 10)  
      • 2/3 in L4-S1 
      • L34 Disk Apex 
    • PI 40-60 
    • PI + LL + TK <45 
  • Segmental 
    • Bridwelll et al 1989 
    • Match rod contour to anatomy 
    • 2/3 of LL in L4-S1 

Steps to Assess Sagittal Alignment 

  • Measure PI SS PT 
  • Check lumbopelvic balance 
    • PI 40-60 
    • SS 2/3 PT 1/3 of PI 
    • LL 
      • =PI 
      • Inflection point L23 disc or below 
      • 2/3 of LL betweeen L4-S1 
     

Pelvic Incidence 

  • PI = SS + PT 
  • PI = 40-60 
  • SS 2/3 PI 
  • PT cannot exceed 30 
    • Caps out at hip extension 
  • LL within 10 of PI ** MOST IMPORTANT PREDICTOR of PROGNOSIS (afterr surgery)  

Sagittal Balance 

  • Center of hip must always be under center of spine 
  • PT and SS dynamic 
  • Sum represents PI (which is static) 
    • SS and PT will change to maintain the fixed PI 
  • Pelvis  
    • Standing  
      • SS increases (lumbar lordosis) 
      • PT decreases (anterior pelvic tilt) 
        • PT movement comes from L5S1 primarily 
          • So fusing it locks you lordotic 
          • Hard to sit comfortably 
  • Sitting 
    • SS decreases (lumbar kyphosis) 
    • PT increases (posterior pelvic tilt) 
      • Causes acetabular anteversion 
        • Prevents anterior impingement 
        • Anterior impingement = posterior dislocation (sitting is dangerous) 
  • Hip dislocation risk with fusion 
    • Nil = 1.5% 
    • 2 levels = 2.7% 
    • 3-7 levevls = 4.6% 

Normal Sagittal Alignment 

  • Head over femoral head 
  • Landmarks of the Line of Gravity 
    • Center of Gravity COG of Skulls 
      • Sella tursica (correlates to external auditory meatus) 
    • Centroid C7 (COG skull slightly anterior to C7) 
    • Posterior superior corner S1 
    • Midpoint femoral head 
    • Midpoint knee 
    • Centroid tibial plafond 
    • *specific points at which the Line of Gravity intersects various vertebrae depends on the morphology of the spine itself (the Roussouly class) 

Deformity  Criteria 

  • Coronal  
    • cobb >20 
  • Sagittal 
    • SVA >5cm 
    • Pelvic Tilt > 25 
    • Thoracic Kyphosis >60 

Pelvic Tilt 

  • It is used to mask a deformity 
    • There is spinal malalignment somewhere 
    • The hips extend allowing for pelvic tilt (retroverted pelvis) 
    • This allows for upright posture despite the deformity 
  • Notably, the higher the PI, the more ROM available to have a bigger PT 
  • Capped out at 30 (hip extension maxed out) 
  • Anterior tilt 
    • Lumbar erector 
    • Iliopsoas 
    • Rectus femoris 
  • Posterior Tilt 
    • Glute med max 
    • Rectus abdominus 

Roussouly Classification 

  • Type 1 – Non Harmonious Flat Back 
  • Type 2 – Harmonious Flat Back 
  • Type 3 – Harmonious Regular 
  • Type 4 – Harmonious Hypercurved 
  • Remember 
    • SS goes up  
    • TK:LL percentage goes up 

Pearls 

  • PI and Lordosis Apex 
    • PI increases – lordotic apex more proximal 
  • Common sagital deformiies 
    • Adolescence 
      • Scheuremann’s 
    • Middle aged 
      • Inflammaotry, AS 
    • Elderly 
      • Degen scoliosis 
  • Compensatory mechanicms for loss of lumbar lordosis 
    • Hip hyperextension 
    • Knee flexion 
  • Required imaging 
    • 3 foot AP and lateral 
    • Lateral flex ex with bolster at apex to assess flexibility