mercoledì 24 ottobre 2007

Failed back surgery syndrome


Figure 1: Axial T1 post-contrast weighted image demonstrates a low signal rim-enhancing collection with posterior compression of the thecal sac.
Figure 2: An axial T2 weighted image shows a high-signal epidural collection with posterior compression of the thecal sac.
Figure 3: Sagittal T1 weighted post contrast imaging demonstrates a prior laminectomy with an elongated, peripherally enhancing epidural fluid collection with displacement of the thecal sac anteriorly.
Figure 4: Sagittal T2 weighted image reveals a septated fluid collection with posterior compression of the thecal sac consistent with an epidural abscess.


Figure 1: Axial T1 weighted image demonstrates a low signal epidural mass in the right neuroforamen with some mass effect on the thecal sac.
Figure 2: Axial T1 post contrast fat suppressed image demonstrates high signal scar or granulation tissue surrounding a low signal right neuroforaminal mass with mass effect on the thecal sac.
Figure 3: One slice below, an axial T1 post contrast fat suppressed image displays the right nerve root surrounded by high signal scar and/or granulation tissue.
Figure 5 and Figure 7: Pre- and post-contrast sagittal T1 image reveals the low signal right neuroforaminal mass to be continuous with the L5-S1 disk with rim enhancement and compression of the thecal sac consistent with a recurrent disk herniation.
Figure 4 and Figure 6: Axial and sagittal T2 weighted images demonstrate a low-signal herniated disk in the right neuroforamen with compression of the thecal sac.

Diagnosis: Failed back surgery syndrome

Failed back surgery syndrome (FBSS) presents with recurrent or persistent low back pain after lumbar spine surgery. Patients may present with weakness and numbness related to lumbar radiculopathy or generalized low back pain. The etiologies of FBSS are vast and include early or late phenomenon. “Early” phenomena include post-operative hemorrhage; infection leading to diskitis, osteomyelitis, meningitis, or epidural abscess; dural tear with pseudomeningocele; sequestered disk fragment; hardware malposition and surgery at the wrong level. These entities demonstrate typical imaging findings depending upon the cause. “Late” etiologies of FBSS include: foraminal or central stenosis (20-60%), instability (14%), recurrent disk herniation (7-12%), epidural fibrosis (5-25%) and arachnoiditis.

Causes of Failed Back Surgery Syndrome:
- Arachnoiditis
- Central or foraminal stenosis
- Epidural Fibrosis
- Infection
- Hematoma
- Surgical trauma to roots
- Insufficient decompression
- Mechanical instability
- Pseudoarthrosis
- Residual or recurrent disk
- Spondylolisthesis
- Surgery at wrong level

Diagnosis of FBSS is made with contrast enhanced MR imaging of the lumbar spine. Canal stenosis is manifest on MR imaging by a “trefoil” appearance of the spinal canal with a thickened ligamentum flavum. Enhancing nerve roots may be present on contrast enhanced imaging. Vertebral body instability can be diagnosed on flexion and extension views on plain radiographs by demonstrating a “dynamic slip” of greater than 3 millimeters. MR findings of instability include antero- or retrolisthesis or loss of disc height with loss of disc signal on T2 weighted images. Epidural fibrosis appears low in signal on T1 weighted images with slightly higher signal than disc on T2 images with diffuse contrast enhancement. This is in contrast to herniated disc material which demonstrates little or thin peripheral enhancement after contrast administration. Post-operative arachnoiditis appears on MR imaging as peripheral or central clumping of nerve roots with variable enhancement.

Because post-operative scarring does not warrant additional surgery, its differentiation from herniated disk is crucial. This is best demonstrated on contrast-enhanced T1 fat saturated weighted images as disc material will demonstrate no or peripheral enhancement while scar tissue will avidly enhance. Additional differentiating features include traction of the dural tube secondary to fibrosis and compression of nerve roots from disk material. MR is 96-100% accurate in detecting epidural fibrosis versus disc material.

Treatment of FBSS is dependent on the etiology. Therapeutic modalities include physiotherapy, epidural steroids, spinal cord stimulators and when warranted, repeat surgery for treatment of recurrent herniation, foraminal stenosis and instability.

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