|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 distance from the C7 plumb line from the posterosuperior corner of the S1 endplate The 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.
- 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 degrees. 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 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 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 degrees. 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).