Formal radiographic assessment is done using a standing full-length lateral radiograph obtained with extension at the hips and knees. The C7 vertebral body and the hip joint should be visualized. The optimal position of the arms has been debated. Having the arms at the side would obviously be the most natural position for the patient; however, this limits evaluation of the spine. Holding the arms in front of the patient (sleepwalker style) has been shown to result in posterior shift of the sagittal vertical axis (see below). A compromise seems to be flexion at the shoulder between 30º - 45 º with the arms holding on to a support.
A line is then drawn perpendicular to the ground (plumb line) from the center of the C7 vertebral body to the pelvis. This approximates the sagittal vertical axis. The distance of this line to the posterosuperior aspect of the S1 vertebral body is the sagittal vertical axis offset, sometimes incorrectly abbreviated to sagittal vertical axis. The normal range for the sagittal vertical axis offset has not been agreed on and may be anywhere between 2.5 cm and 5.0 cm.
- Negative sagittal imbalance: Plumb line falls behind the posterosuperior corner of the S1 vertebral body.
- Neutral sagittal balance: Plumb line intersects the posterosuperior corner of the S1 vertebral body.
- Positive sagittal imbalance: Plumb line falls in front of the posterosuperior corner of the S1 vertebral body.
Flat back syndrome is most commonly iatrogenic. Patients present with a forward tilt of the trunk, inability to stand erect, muscular pain in the upper back and lower cervical area, and thigh pain from chronic hip flexion and knee bending.
Formal radiographic assessment makes use of the plumb line on standing lateral radiographs, as described above. The plumb line can fall within 2 cm of the anterior aspect of the sacrum in patients without scoliosis or in those with idiopathic scoliosis who have not undergone corrective surgery.
The images below are from a patient with a history of discectomy and multilevel laminectomy at an outside facility. The patient's baseline images show a loss of lumbar lordosis (A). She subsequently had surgery with an attempt to re-establish some lumbar lordosis (B). The lateral upright radiograph (C) is a great example of bad technique. The arms are flexed at 90º and C7 is not seen.
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- Lu DC, Chou D. Flatback syndrome. Neurosurg Clin N Am. 2007 Apr;18(2):289-94.
- Vialle R, Levassor N, Rillardon L, Templier A, Skalli W, Guigui P. Radiographic analysis of the sagittal alignment and balance of the spine in asymptomatic subjects. J Bone Joint Surg Am. 2005 Feb;87(2):260-7.