Failed Back Surgery Syndrome (FBSS)
The term FBSS is used exclusively for patients who have undergone lumbar surgery for degenerative diseases (e.g., discectomy or foraminal stenosis) but continue to experience pain and neurological deficit.
Pathology causes are divided into three groups:
- Preoperative: misidentification of the pain source, psychosocial factors, and reoperations.
- Intraoperative: technical errors, inadequate or excessive decompression, and surgery at the wrong level.
- Postoperative: residual herniations, epidural fibrosis, stenosis, and arachnoiditis.
Contrast-enhanced MRI is recognized as the best imaging modality for identifying the causes of FBSS.
Differential diagnosis: fibrosis and disc herniation recurrence
A critical task of diagnostic imaging is differentiating epidural fibrosis from recurrent disc herniation using T1-weighted imaging with gadolinium.
| Sign | Epidural fibrosis | Recurrent disc herniation |
|---|---|---|
| Contrast enhancement | Early and uniform | Does not enhance (or peripherally) |
| Contours | Irregular | Smooth |
| Mass effect | Minimal | Pronounced |
| Effect | Tethers the dural sac | Compresses and displaces the sac |
Post-contrast scans are performed immediately after injection to avoid false contrast penetration into the disc.
Hardware complications and pseudarthrosis
Spinal implants serve only for temporary support and will inevitably fail unless osseous fusion (osseous fusion) is achieved. Risk factors for nonunion include advanced age, smoking, diabetes, and osteoporosis.
The main manifestations of failure include:
- Fractures and migration: rod breakage, screw pullout, interbody cage displacement or subsidence into the spinal canal with neural compression.
- Screw loosening: CT reveals a peri-implant lucency zone around the threads.
- Pseudarthrosis: functional views demonstrate angular motion greater than 10 degrees or translation greater than 4 mm.
Infectious and other late complications
Postoperative wound infections following posterior fixation manifest as fluid collections with ring-like contrast enhancement in the subcutaneous soft tissues, while the MR signal from the discs and vertebral bodies remains normal (without spondylodiscitis).
Other late changes include:
- Adjacent segment disease: accelerated degeneration of discs and facet joints above or below the fusion level due to altered biomechanics.
- Sterile arachnoiditis: characterized by nerve root clumping and the "empty sac" sign on T2-weighted images.
- Postoperative synovial cysts: a rare finding associated with pathology of the ligamentum flavum and zygapophyseal joints.
Briefly: answers to topical questions
What imaging features are characteristic of postoperative spondylodiscitis on MRI?
Postoperative spondylodiscitis on MRI is characterized by combined involvement of the intervertebral disc and adjacent vertebral bodies.
- Disc changes — decreased height, destruction, loss of the intranuclear cleft (intranuclear cleft), hypointense signal on T1-weighted images and hyperintense on T2-weighted images.
- Bone tissue changes — vertebral body collapse (collapse), endplate destruction (endplates).
- Bone marrow edema (hypointense marrow) — low signal on T1-weighted images and high signal on T2-weighted images/STIR.
- Contrast enhancement (contrast enhancement) — enhancement in the disc, adjacent vertebrae, and paraspinal soft tissues.
How to differentiate superficial postoperative wound infection from epidural abscess on MRI?
Differential diagnosis is based on assessing the location of the pathological process and involvement of adjacent structures.
| Sign | Superficial wound infection | Epidural abscess |
|---|---|---|
| Localization | Subcutaneous and posterior paraspinal soft tissues | Epidural space |
| MR appearance | Complex mass lesion (complex mass) | Hypointense on T1-weighted and hyperintense on T2/STIR collection |
| Contrast enhancement | Intense ring enhancement (avid ring enhancement) | Data in sources ends abruptly |
| Adjacent structures | Disc and vertebral signal is normal | May be a complication of spondylodiscitis |
What are the MR signal characteristics of a postoperative synovial cyst?
On magnetic resonance imaging, a synovial (juxtafacet) cyst appears as a cystic lesion originating from the anteromedial surface of the facet joint.
- T2-weighted images — cyst contents show a moderately hyperintense signal.
- Periphery of the lesion — a low MR signal rim is visualized.
Additionally, an inflammatory reaction around the cyst may be detected, as well as signs of hemorrhage into its cavity, leading to rapid lesion enlargement.
"The scar is early and even" (epidural fibrosis enhances early and uniformly, unlike disc herniation recurrence).
Class and exam questions
What is Failed Back Surgery Syndrome (FBSS)?
This is the persistence or onset of pain and neurological symptoms following lumbar spine surgery performed for degenerative diseases.
How to differentiate epidural fibrosis from recurrent disc herniation on MRI?
Contrast-enhanced MRI on T1-weighted images is used. Epidural fibrosis shows early and uniform contrast enhancement, whereas recurrent disc herniation does not enhance (or shows faint peripheral enhancement).
What imaging signs indicate loosening of transpedicular screws?
On computed tomography, the primary reliable sign of screw instability is the appearance of a peri-implant lucency zone (halo) around the threads.
What else to study on this topic
- Biomechanics of adjacent segment disease
- Epiduroscopy in patients with negative MRI findings
- Diagnostic blocks prior to revision surgery
- The role of functional radiographs in the diagnosis of pseudarthrosis
- Pathogenesis of pain in epidural fibrosis
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