Before I talk about the types of spinal fusions that I perform I think it’s very important that we first discuss the concept of spinopelvic balance. Until recently this was a concept that was only considered by academic spinal deformity surgeons (those spine surgeons who treat scoliosis and other complex spinal pathology.) Over the past few years, however, data has emerged that suggests that restoration of lumbar lordosis (the normal backwards curvature of the lumbar spine) in order to maintain proper spinopelvic balance is critical even for patients who undergo one- or two-level spinal fusions. Being sure to consider spinopelvic balance before fusing a patient’s spine will maximize their chance of a good outcome.
What is spinopelvic balance? Basically, the spine should maintain an upright posture, with the head positioned directly over the pelvis, with minimal energy expenditure. This notion was elegantly described by the French orthopedic surgeon Jean Dubousset who described a “cone of economy” of an upright patient (see figure 1A). Neutral spinopelvic alignment keeps the patient at the center of the cone where he has to maintain little energy to stand upright and keep horizontal gaze. As the spine pitches forward (for a variety of reasons described below) the patient falls to the periphery of the cone and thus has to expend more energy just to stay upright (see figure 1B). If he falls too far to the periphery he’ll no longer be able to support himself and will need a cane or walker. This forward pitching of the spine is referred to positive sagittal balance and is the torment of all patients with degenerated spines. The more severe the imbalance the more disabled the patient. This was first described in a landmark study in 2005 Glassman et al. The authors examined full-length standing X-rays on 352 patients and found a direct, linear correlation between increasing positive sagittal balance and worsening patient disability (see figure 2).
Figure 1A: Dubousset’s cone of economy (source: Ames et al). A patient at the center of this cone of economy will have to expend minimal energy to keep their head upright and maintain horizontal gaze. 1B; the King of Pop WAY out of his cone of economy.
Figure 2. There is a linear correlation with increasing sagittal balance and poor clinical outcomes. SF-12 and ODI scores are clinical outcomes (HRQOL) measures used in spine surgery to assess how well a patient is doing. Lower SF-12 scores and higher ODI scores indicate worse outcomes (Source: Glassman et al.)
The balance of the spine is assessed using several spinopelvic parameters measured on AP and lateral (front and side) standing X-rays of the patient that include the femoral heads (see figure 3.) This X-ray is mandatory in my clinic for any patient who is being considered for a spinal fusion. There are dozens of various spinopelvic parameters that can be measured for a given patient and it can quickly get overwhelming trying to keep track of all of them. The Glassman study mentioned above used the sagittal vertical axis, SVA, to quantify positive sagittal balance. SVA is the best measure to describe a patient’s global spinal balance as it assesses the position of the cervical spine over the sacrum (tailbone.) The problem with SVA, in my opinion, is that it can be difficult to get full-length standing X-rays at most community imaging centers. In another study by Schwab et al in 2013 the authors prospectively studied dozens of spinopelvic parameters in nearly 500 patients with spinal deformity. These parameters were correlated with a variety of health-related quality of life (HRQOL) measures. When they analyzed the data they found that three parameters matter most:
1) SVA: which we already discussed
2) PI-LL mismatch: the amount of discrepancy between the pelvic incidence (PI, a fixed morphological characteristic of your pelvis. Basically, the way your pelvis is shaped in relation to the hip joints) and the lumbar lordosis (LL, the normal curvature of the lumbar spine as mentioned above.)
3) Pelvic tilt (PT): a measure of the extent that the pelvis is tilting backwards to compensate for lost lumbar lordosis.
Figure 3: Standing lateral X-ray including femoral heads showing measurements of pelvic tilt (PT), pelvic incidence (PI), lumbar lordosis (LL), PI-LL mismatch and segmental angles. This X-ray is mandatory for any patients in my clinic being considered for lumbar fusion.
When the authors did even more in-depth analysis they found that PI-LL mismatch was the variable that most correlated with patient disability (patients with PI-LL mismatch of 11 degrees or greater were more likely to be severely disabled.) That happens to be very convenient for spine surgeons. First, both the PI and LL can be easily calculated on standing lumbar xrays that can be done at any imaging facility (full-length films not required!) Even more important, PI-LL mismatch is the parameter that is most easily addressed with surgery. Nearly 70% of a patient’s overall LL comes from the angulation at the L4/5 and L5/S1 disc spaces. So if you’re trying to correct a patient’s PI-LL mismatch you can often do so by restoring LL with large, angled intervertebral spacers placed at one or both of these levels. I know that was a lot of complicated stuff there but if you take away nothing else, know this: PI and LL should be assessed in all patients being considered for spinal fusion surgery so that PI-LL mismatch can be corrected.
Positive sagittal balance (and remember, “positive” balance is actually a bad thing) can have several causes. First, pediatric patients can have so-called “idiopathic” scoliosis and other spinal deformities. These are entirely unique entities and I won’t discuss them here. In adults, acute changes in spinal structure such as tumor, trauma or infection can cause the spine to lose structural integrity and allow the spine to fall into positive sagittal balance. Most commonly, however, progressive degeneration of the spine allows for the slow development of sagittal imbalance. As the intervertebral discs degenerate over a patient’s lifetime, and supporting spinal muscles and ligaments weaken, the spine will lose its normal lordosis (i.e. it will flatten out, forming a so-called “flat-back” deformity). In severe cases the spine may even begin to kyphose, or bend forward (see figure 4). Compounding matters, the spine can also start to buckle under the weight of the torso leading to an S-shaped coronal deformity (see figure 5.)
Figure 4: Image on left shows a normal, healthy lumbar spine with adequate lumbar lordosis (backward curvature of the spine. Image on right shows a severely degenerated spine with loss or lordosis resulting in “flat back’. PI-LL mismatch in this patient is 23 degrees.
Figure 5. AP (front view) Xray of lumbar spine and pelvis demonstrating a severe coronal deformity with a right-sided concavity. Lumbar spine should be straight up and down on this view.
Perhaps the worst cause of sagittal imbalance is iatrogenic, when a patient is fixed into sagittal imbalance after a spinal fusion. This is when patients really suffer. First, we know that patients who are left with positive sagittal balance (as measured by PI-LL mismatch) after spinal fusion surgery have worse clinical outcomes. Even more concerning, there’s also data to suggest that patients fixed into PI-LL mismatch are more likely to develop adjacent segment degeneration (ASD) after their fusion. In a 2014 study by Rothenfluh et al, the authors reported a 10x (!) increase in the incidence of ASD when patients had PI-LL mismatch after their initial fusion. It only makes sense that when a segment of spine is locked into an alignment that is pitched forward, the level above is going to more likely to continue to fall forward! (see figure 6) (To tell you how much my understanding of this topic has evolved: one of the first articles I wrote on Spinal(con)Fusion, over 5 years ago now, was on ASD and no where in that article did I discuss positive sagittal balance. I’m now convinced that fusing someone in poor sagittal alignment is the biggest contributor to increased risk of ASD after spinal fusion.) Thus, one of the main goals of any spinal fusion surgery should be to restore lumbar lordosis to correct PI-LL mismatch. This will maximize the chances of a good clinical outcome after surgery and may decrease the risk of ASD in the future.
Figure 6. Sagittal MRI (side view) showing adjacent segment degeneration at L3/4 in a patient with previous fusion at L4/5. Notice how L4 is fused into near straight alignment in relation to L5 (there should be 10-20 degrees of angulation there.) It’s no surprise this patient fell forward above his flat fusion.
This is really important stuff people. Spinopelvic parameters should not be ignored. I will take the time to do these measurements on every patient who undergoes a spinal fusion. These days it’s easy too. I can literally snap a picture of a standing X-ray with my iPhone or iPad and an app will basically do the spinopelvic measurements for me (see figure 7). There’s just no excuse not to check. If you’re considering a spinal fusion please be certain that your surgeon is taking your spinopelvic parameters into consideration.
Figure 7. Nuvaline Pro iPhone app used to measure postoperative spinopelvic parameters in patient who underwent a fusion at L5/S1.
Thanks for reading!
J. Alex Thomas, M.D.
- Ames CP, Smith JS, Scheer JK, Bess S, Bederman SS, Deviren V, et al.: Impact of spinopelvic alignment on decision making in deformity surgery in adults: A review. J Neurosurg Spine 16:547–64, 2012
- Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F: The Impact of Positive Sagittal Balance in Adult Spinal Deformity. Spine (Phila Pa 1976) 30:2024–2029, 2005.
- Rothenfluh DA, Mueller DA: Pelvic incidence-lumbar lordosis mismatch predisposes to adjacent segment disease after lumbar spinal fusion. Eur Spine J 24:1251–8, 2014
- Schwab FJ, Blondel B, Bess S, Hostin R, Shaffrey CI, Smith JS, et al.: Radiographical Spinopelvic Parameters and Disability in the Setting of Adult Spinal Deformity. Spine (Phila Pa 1976) 38:E803–E812, 2013