Let's take a moment to define Fixed Sagittal Imbalance (FSI) and discuss how this disorder relates to the spine.
In FSI, the word ‘fixed’ means the spine (or spinal segment) is not mobile or correctable. ‘Sagittal’ is a medical term associated with the Sagittal Plane, which refers to the appearance of the spine when viewed from the side (clinical picture). The sagittal plane is illustrated below. Notice the sagittal plane divides the body vertically front and back (anteroposterior). Combining these terms with the word ‘imbalance’ means the spine is ‘fixed’ prohibiting the patient from standing upright.
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Spinal Curvature and FSI
A normal spinal column is designed to curve naturally. These natural curvatures are kyphotic (forward) and lordotic (backward). In Figure 2 (x-ray), the patient’s loss of normal lumbar lordosis causes the patient to be hyperkyphotic or bent excessively forward. Normal lumbar lordosis is illustrated by hand in Figure 2 (see dots). In Figure 3, the effects of FSI are illustrated as the patient is seen standing awkwardly. Although patients with this disorder may appear relatively normal in appearance while seated, they are unable to stand up straight.The patient’s shoulders and head are in front of their hips and feet when standing/walking.
This deformity can make it difficult for the patient to stand and walk. Many activities of daily living become difficult or impossible. Most patients appear older.
Causes - A Few Examples
Osteoporosis is a ‘silent bone thinning’ disease known to adversely affect bone density. As bone strength diminishes vertebral compression fractures may occur and lead to spinal deformity. Severe Degenerative Disc Disease is known to cause loss of disc height anteriorly (front) which results in loss of lordosis. Another disorder, Ankylosing Spondylitis, a chronic inflammatory disease is characterized by pain, progressive spinal stiffness, and kyphosis. Most commonly FSI is seen following an extensive lumbar fusion, which heals in kyphosis.
Variations of FSI
Type I, or compensated deformity means the patient is able to ‘compensate’ by means of unaffected vertebral segments above and below the fused (‘fixed’) segments. The patient may hyperextend (bend backward) to control or maintain balance, which can cause the intervertebral discs to prematurely wear.
Type II, or decompensated deformity means the patient does not have enough vertebral segments above or below the fixed segments to rebalance. The patient in Figures 1 and 2 illustrate Type II.
The objective of surgery is to restore erect posture, which further serves to enable the patient to walk upright, normalizes appearance, rebuilds self-esteem and confidence, and generally makes everyday life more normal.
A surgical option used to restore sagittal (upright) balance includes Smith-Petersen osteotomies and pedicle subtraction procedures. An osteotomy is the surgical removal of bone. The following two figures illustrate how the spine may be restored.
Figure 4 illustrates a Smith-Petersen Osteotomy (SPO). The shaded area in the ‘before’ drawing indicates bone targeted for resection (removal). The results of bone resection are seen in the ‘after’ rendering, which reflects sagittal curve restoration.
Figure 5 is a Three Column Pedicle Subtraction Osteotomy (PSO). The anatomy of the spine includes three columns as follows: (1) posterior elements, (2) middle column, and (3) anterior column. Pedicles are tube-like processes that extend posteriorly from the vertebra and connect the front and back of the spine. ‘Pedicle Subtraction Osteotomy’ means removal of all of the pedicles and part of the vertebral body. This procedure reduces somewhat the need for anterior surgery when compared to SPO.
Further, in Figure 5 a wedge-shaped area is shaded in the ‘before’ drawing. This wedge includes portions of the lamina, pedicles, and vertebral body. Removal of the ‘wedge’ results in sagittal curve restoration seen in the ‘after’ drawing.
Surgical procedures utilized to correct this disorder also include spinal instrumentation and spinal fusion. Instrumentation serves to stabilize the spine during fusion, which is facilitated by bone graft. Depending on the patient and the extent of the disorder, multiple procedures may be required both anteriorly and posteriorly.
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The results of surgery can be seen Figures 6 and 7. Notice in Figure 6(x-ray) normal lumbar (low back) lordosis (backward curvature) has been restored allowing the patient to stand up straight. In most cases, these procedures lead to patient satisfaction and success.
Figures 4 and 5 reprinted with permission from Booth KC, Bridwell, KH, Lenke LG, Baldus CR, Blanke KM: Complications and predictive factors for the successful treatment of flatback deformity (fixed sagittal imbalance). Spine 24(16);1712-20, 1999.