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Analyse

Measure the Pelvic Incidence

Draw a line along the upper endplate of S1 (dotted black line)

Draw a perpendicular line from the center of the previous line (solid black line)
-> This will be your first leg

Draw a line starting from the center of the endplate to the center of the femoral heads
-> This will be your second leg

Tip: Most of the time you’ll see both femoral heads. Use the middle of the two centers

Measure the sacral slope

Sacral Slope 

First leg: A line drawn along the upper S1 endplate

The second leg:  Horizontal line

Pelvic tilt

First leg: Connect the center of the femoral head with the center of the S1 upper endplate

Second leg: Vertical line

It doesn’t matter if you use SS or PT for assessing your pelvic ante-/retroversion. In the following steps, I’ll use SS only.

Remember the formula: PI = SS + PT

Go to spinebit.io

Get your normal values

Spinebit uses french values.

  • Roussouly type: Determines the profile type 1-4. Whenever possible use old x-rays for a more accurate profiling. 
    Spinebit gives you the needed information about the geometry of the roussouly types
  • Odontoid hip axis angle (OD-HA): Global balance
  • T4-Pelvic angle (T4PA)/ Spino-sacral angle (SSA): Balance thoracolumbar 
  • L1-Pelvic angle (L1PA): Balance lumbar
  • Sacral-slope (SS): Position of the pelvic. 
  • Total kyphosis T1 to Inflection point (use with cautious!)
  • Total lordosis: Inflection point to S1
  • Lordosis L4-S1: The amount of lordosis between L4-S1 is dependant of the roussouly type

Determine the Roussouly type 1-4

Roussouly types
Depending on the PI value, the sagittal profile varies and can be grouped into 4 classic types. Threshold values are 45° and 60°. The differentiation between type 1 and 2  isn’t possible with the PI alone. Check the curvature of the thoracic spine!

With the roussouly type determined, you have the clues as to what the spine should look like. 

Note: Your patient’s spine may not resemble the PI-determined profile. Depending on the level of segmental level of lordosis loss, the profile can degenerate into another one. Have a look at the paper from Amer Sebaaly (2020). 

Roussouly 1
PI<45°
Short lumbar lordosis and long kyphosis. 

Lumar apex L5
Inflection Point L3

Roussouly 2
PI<45°
Flat lumbar and thoracic spine.

Lumbar apex L4/5
Inflection Point L2

Roussouly 3
PI >45° and <60°
Harmonic spine.

Lumbar apex L4
Inflection point L1

Roussouly 4
PI >60°
Curved spine. Long lordosis and short kyphosis.

Lumbar apex L3/4
Inflection point Th12

Check the global and thorakolumbar balance 

Note that the whole spine x-ray is a snapshot of the posture of your patient. The posture is dynamic and highly dependent on muscle fatigue. The global sagittal balance (OD-HA) could be normal in a rested patient. So it’s uttermost important to ask the patient if there’s an increasing forward posture during walking.

Odontoid hip axis angle

First leg: Line from the dens axis to the center of the hips

Second leg: Vertical line

 

Normal value: +2° -5° (the image shows a positive angle)

 

 

T4 Pelvic angle

First leg: Center of T4 to the hips

Second leg: Center of the S1-Endplate to the hips.

Know the compensatory mechanisms

The global sagittal profile can be normal in a spine with a severely disturbed geometry. That’s because compensatory mechanisms are at work. 

 

Retroversion of the pelvis:  

  • The most effective mechanism. 
  • Roussouly types 1 and 2 have less reserve in potential retroversion than Roussouly types 3 and type 4.
  • Patients with Parkinson’s disease or severe hip arthrosis have sometimes a very limited retroversion potential

Segmental hyperlordosis: 

  • Can be found in the lumbar spine, thoracolumbar, and thoracic region
  • Hyperlordosis of the thoracolumbar junction can lead to spinal canal stenosis. It’s a good idea to do an MRI in severe cases

Retrolisthesis

  • Lumbar spine

Locate the deformity

Now you have the necessary information to locate the disturbance in your patient’s degenerated sagittal geometry. The spine can be in a compensated or decompensated state.

Left – Lumbar

Pelvis retroverted

Thoracolumbar balance disturbed

T4 Pelvic angle increased

Center-left – Thoracic

Pelvis retroverted

Thoracolumbar balance disturbed

T4 Pelvic angle increased

Center right – cervical

Pelvis normal to slightly retroverted

Thoracolumbar balance normal

T4 Pelvic angle normal

Right –  lower limbs

Pelvic normal – anteverted

Thoracolumbar balance normal

T4 Pelvic angle normal

Normal to hyperlordotic lumbar spine

When you only have a lumbar x-ray…

… it still makes sense to determine the sagittal parameters! Especially in roussouly types 1 and 2, fusions between L4-S1 have a great impact. Roussouly type 4 is more forgiving regarding the location of the correction because the apex is located at the level L3/4.

 

Check the following:

Rossouly type

What shape should the lumbar spine have?

=> lumbar apex/ Inflection Point

Sacral slope (blue)

Retroversion?

=> Compensatory mechanism

Anteversion?

=> Hip pathology?

Lumbar lordosis L4-S1 (red)

Hypolordotic?

=> Aim to correct between L4-S1!

=> Roussouly type 4 can be corrected also between L3/4

Total lordosis

Enough lordosis?

=> Measure between S1 and Inflection point

or

=> Measure the L1 Pelvic angle