Continuing from yesterday’s email about the effect of spinopelvic alignment on THA, today we’re going to discuss spinopelvic parameters (pelvic incidence, sacral slope, etc etc).
To be perfectly honest, this is a topic I’ve read or heard about several times. But it’s always so hard to remember. Maybe writing about it will help.
What I learned today
First, a question to test your understanding: If a patient has a hip flexion contracture, what is the likely compensatory change in pelvic tilt when standing?
Answer: Anterior pelvic tilt will help align the limb perpendicular to the ground in the stance phase. This can then lead to lumbar hyperlordosis.
Anyway, now we’re going to look at spinopelvic alignment from the perspective of spine surgeons, rather than hip people.1 Having normal sagittal balance is associated with higher quality-of-life and QOL outcomes in patients with spinal deformity.
Pelvic incidence (PI)
Fixed; describes pelvic morphology
Normal: 51°
The angle between (1) line drawn perpendicular to the midpoint of the sacral endplate and (2) axis of rotation of the center of the femoral head
Sacral slope (SS)
Position-dependent; describes pelvic orientation
Normal: 40°
The angle between (1) line drawn along the sacral endplate and (2) horizontal reference line
Pelvic tilt (PT)
Position-dependent; describes pelvic orientation
Normal: 11°
The angle between (1) intersection of a line drawn from the midpoint of the sacral endplate to the center of the femoral head and (2) vertical reference line
Lumbar lordosis (LL)
Normal: variable
The angle between the superior endplates of T12 and S1 (Cobb constrained method) or the greatest magnitude of lumbar lordosis anywhere in the lumbar spine (e.g. Cobb angle from L2-L5) (non-constrained method)
Now take a deep breath and put it all together, and you get this:2
Basically, this is mathematical proof that PT + SS = PI. You don’t have to memorize all the little angles in-between.
A more recent topic of study has been PI-LL mismatch. Various articles use cutoffs for PI-LL as follows:
-10° or less: hyperlordosis
-10° to 10°: normal
10° to 20°: hypolordotic (significant flatback)
20° or more: severe flatback
One last note: global spinopelvic balance is assessed using the C7 plumbline. On a standing lateral radiograph, draw a vertical line from C7 down to the level of the sacrum. It should pass through the posterosuperior corner of S1.
Sadly, this is just the tip of the iceberg. There are more measurements for L5 incidence angle, many versions of lumbosacral angles, slip angle, and so on.
Personally, I’d be content with just being able to remember these ones for once. Maybe try re-drawing them from memory?
Sources
Li, Y. & Hresko, T. M. Radiographic Analysis of Spondylolisthesis and Sagittal Spinopelvic Deformity. J Am Acad Orthop Sur20, 194–205 (2012).
Labelle, H., Roussouly, P., Berthonnaud, É., Dimnet, J. & O’Brien, M. The Importance of Spino-Pelvic Balance in L5–S1 Developmental Spondylolisthesis. Spine30, S27–S34 (2005).
What do you call a cool arthroplasty surgeon? A hip person. Yeah, made that one up myself. You can use it.
The illustrations in both referenced articles are redrawn/adapted from work by the Spinal Deformity Study Group. Specifically, the JAAOS article cites Spinal Deformity Study Group Radiographic Measurement Manual. Memphis, TN, Medtronic Sofamor Danek, 2005.