Setup: a 59F smoker is undergoing surgery for correction of decompensated sagittal imbalance. Because you are Genius/Greenberg savant you thought to measure the patient's sagittal vertical alignment (see below) and found it to be 8 cm (normal <4 cm per Greenberg). The conversation goes like this:
Resident: Did you take a look at this patient's imaging?
Me: Yeah, she has an SVA of 8 cm.
Resident: And what is an SVA?
Me: The SVA is measured by drawing a plumb line from the center of the vertebral body of C7 and then measuring the perpendicular distance to the posterior, superior corner of the S1 endplate.
Resident: Not bad. At the end of the case, can we check the patient's alignment using the c-arm while they are still on the table so that we could make any corrections that might be needed?
Me: That sounds like it would be give us some reassurance.
Resident: What kind of imaging do you use to measure SVA?
Me: A standing, upright fi--Oh. The patient would be lying down.
Resident: That's right. So we cannot check the new SVA at the end of the case while the patient is still on the table.
The value of SVA as a diagnostic tool is that, as shown by Glassman et al., a positive sagittal balance correlates with increasing pain and deteriorating health related quality of life.
T1 pelvic angle (discussed here) is, on paper, a very similar measurement. I include it here to highlight their differences. This measure is the angle formed between the following two lines:
a line from the center of the T1 vertebral body to the center of the femoral heads.
a line from the center of the S1 endplate to the center of the femoral heads.
N.B.: if the femoral heads are not perfectly aligned, draw a line connecting their two centers. Use the midpoint of this line for the above measure.
The authors of the linked study demonstrate that this measure is also correlated with quality of life measures and argue that it is less sensitive to postural compensations. Perhaps you can ask your attending what they think about this.
Pearls:
SVA (and most other measures of sagittal balance) must be assessed on a standing, upright lateral film with the patient's arms forward. The image must demonstrate the entire body of C7 and the femoral heads.
T1 Pelvic Angle uses the center of the S1 endplate. Sagittal vertical alignment is measured from the posterior superior corner of the S1 endplate.
Measures of sagittal balance should not be measured on CT or MRI. The patient is almost assuredly horizontal.
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