Challenge of the Week / Spine / 05.21.2021

Which of the following radiographic measurements from a lateral full-length standing radiograph correlates most closely with patient-reported outcome measures after operative correction of a thoracolumbar deformity?
    • The difference between the lumbar lordosis and pelvic tilt
    • This is not a measurement used to define sagittal parameters or outcomes. The measurement used is pelvic incidence (PI) – lumbar lordosis (LL) = <9° to 10°. PI is not pelvic tilt. PI is the angle between line drawn from the center of the femoral head to the midpoint of the S1 superior endplate and a line perpendicular to the S1 superior endplate on a lateral radiograph. Pelvic mismatch is the second most impactful sagittal parameter on clinical outcome in patients with sagittal imbalance. The first is the sagittal vertical axis (SVA).

 

    • The angle between the line along the superior endplate of S1 and the horizontal line
    • This is the sacral slope and it is not specifically correlated to outcomes.

 

    • The angle between the bisector point between the femoral heads and the midpoint of the S1 superior endplate and the vertical line
    • This is the pelvic tilt. It should be <20° to 25°. When it is elevated, that indicates a retroverted pelvis, which is a compensation that is made for positive sagittal imbalance. This is an inefficient compensation, and it leads to back/buttock muscle fatigue and pain with prolonged upright stance, which rapidly resolves with sitting.

 

    • The distance from the C7 plumb line from the posterosuperior corner of the S1 endplate
    • This measurement is called the sagittal vertical axis (SVA) (distance from the C7 plumb line to the posterosuperior corner of the S1 endplate), and it best correlates with patient-reported outcome measures in patients with thoracolumbar deformity. This distance should be <4 to 5 cm.

     

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