Types of Sagittal Imbalance
Sagittal imbalance is either fixed or flexible. “Flexible” means the patient can stand up straight if they work at it (with their hips and knees straight), while “fixed” suggests they cannot. Similarly, the imbalance can be either compensated or decompensated. “Compensated” means the body can adapt (usually by flexing the knees and hips), while “decompensated” implies they cannot (see below). The problem leading to the imbalance is either local (a few vertebra causing significant tilt), regional (many vertebra causing a slow forward bend), or a mix of the two (see below).
Compensated vs. Decompensated ImbalanceA) Compensated imbalance – A woman with traumatic lumbar kyphosis that compensates by extending her thoracic spine. Her weight-bearing (C7 plumb) line passes through S1.B) Decompensated fixed sagittal imbalance – the C7 plumb line is well anterior to S1.
Types of Fixed Sagittal Imbalance (FSI):
FSI can be caused by a local problem involving a few vertebra, a global problem involving many vertebra, or a mix of the two.
Fixed Sagittal Imbalance
Causes of Fixed Sagittal Imbalance (Flatback Syndrome)
- Congenital Malformation of the Vertebra
If one or several bones are wedged forward at birth (or while growing), the spine may not develop the proper balancing lumbar lordosis. In children, the spine is usually flexible enough to compensate. However, over time, the adult spine may lose its ability to adapt to the malformed vertebra.
- Developmental A compensated lumbar kyphosis may decompensate as the discs degenerate over time. Similarly, a progressive thoracic kyphosis may overcome the lumbar spine’s ability to compensate over time (click here for and example). [Posterior osteotomy example]
- Postsurgical “Postsurgical” means that the body was not able to compensate to changes in the body’s sagittal alignment after surgery. Common causes are multilevel fusions with loss of proper lordosis or postoperative infections.
- A) Progressive settling of the spine after sequential lumbar fusions caused a significant loss of lumbar lordosis.
- B) Infection above a hypolordotic fusion (not enough lordosis) led to decompensation of sagittal alignment.
- C) Distraction instrumentation led to “Flatback disorder”.
- Flatback Disorder In the early days of spinal instrumentation, a device called a “Harrington Rod” straightened the spine similar to how a car jack lifts a car. While it was effective in straightening the spine, it tended to remove normal lordosis and kyphosis leading to “Flatback Disorder”.
- Transition Syndrome When a segment of the spine is fused, the adjacent levels receive additional stress. At times the bones may fracture or discs may deteriorate at an accelerated rate.
- A) A 65-year old woman with thoracolumbar scoliosis and mild positive sagittal imbalance (head in front of her hips) underwent a T10-sacrum fusion.
- B) After surgery, her sagittal alignment was normal.
- C) After a fall, she developed a fracture over the fusion and began to lean forward.
- D) Over time, her fixed sagittal imbalanced progressed making it difficult to stand up straight.
Fixed Sagittal Imbalance
Nonoperative treatment for fixed sagittal imbalance is similar to all other types of spinal pathology. Anti-inflammatory medications and physical therapy are typically the first recommendations. Epidural steroid injections may be helpful in diagnosing and treating a lumbar radiculopathy (pinched nerve). Bracing is ineffective because it weakens the posture muscles, and does not treat the underlying pathology. If the imbalance progresses and there is significant pain, surgery is usually indicated.
Patients often need several preoperative imaging studies to fully understand the underlying problem.
- Radiographs Standing long-cassette radiographs with the knees and hips extended are critical to understand the extent of the imbalance.
- CT Scan Often a CT scan helps define the bony anatomy. The magnets of MRI reflect off metal instrumentation (if present) making it difficult to visualize the anatomy. CT-myelograms help define the position of the instrumentation and areas of neural impingement. This study can also help identify areas of failed fusion (pseudarthrosis).
- MRI MRI is the gold-standard for demonstrating the anatomy of the nerves and discs.
Bone Scan Triple-phase bone scans show areas of increased activity that may demonstrate pseudarthrosis or other anomalies.
Upright long-cassette radiographs:
These images show the importance of obtaining
long-cassette xrays with the knees and hips extended.
The left picture shows the patient’s compensated
posture, while the right picture shows the extent of the
sagittal imbalance (same person – hips and knees
Fixed Sagittal Imbalance
The decision process for surgery depends on a number of factors: the type of sagittal imbalance, a history of prior surgeries, the degree and location of neural compression, the age and health of the patient, among other things. Instrumentation and osteotomy techniques allow repositioning the spine. Below is a description of the types of spinal osteotomies available:
- Posterior osteotomy (also called “Smith-Peterson” or “Ponte” osteotomies): This procedure involves removing the facet joints and interspinous ligaments to tilt the bones posteriorly through a mobile disc space. The facet joints typically limit extension of the spine, so their removal allows the surgeon to accentuate lordosis. Over multiple levels, 5-15o of lordosis per level is possible.
- A) A side view of the spine showing the bone and facet resection.
- B) A side view after the osteotomy is closed.
- C) A lateral (side) radiograph of a woman with severe, rigid, Scheurmann’s kyphosis.
- D) A lateral xray after surgery. Multi-level posterior osteotomies allowed the surgeon to reduce the kyphosis to normal levels.
- Pedicle subtraction osteotomy: Surgeons use this procedure to cut through kyphotic segments. We call it a “closing wedge osteotomy” because a triangle of bone is removed so the bone can be angled backwards. The procedure is particularly powerful, especially in the lumbar spine where the bones are bigger, and small corrections can lead to large improvements in posture. It is similar to placing a wedge between bricks – creating a sudden backward bend in the spine. The surgery requires the support of instrumentation above and below the osteotomy.
Pedicle subtraction osteotomy:
- A) A side view showing the area of bone resection in pink.
- B) The lordotic segment after the osteotomy is closed. Note how the front of the vertebra is twice the height of the back causing lordosis.
- C) The preoperative xrays of a patient with fixed sagittal imbalance due to bone settling and infection.
- D) A lateral (side) xray showing the restored lordosis after the osteotomy.
- Vertebral column resection: This is the most powerful procedure of all spinal osteotomies. It is necessary when there is a severe bend in a small area. It involves essentially dislocating the spine in a controlled manner and realigning it in the proper direction.
- Anterior-posterior osteotomy: At times there is a failed fusion whose motion can be used to restore alignment. This may require an anterior and posterior surgery to take advantage of the motion through the failed fusion or mobile segment.