Goals of postoperative diagnostic imaging
Following lumbosacral surgery, diagnostic imaging serves strictly defined purposes:
- Computed tomography (CT): used to monitor implant status. It can detect peri-implant lucency (peri-implant lucency), indicating instability and screw loosening (this sign is most obvious in later stages).
- Magnetic resonance imaging (MRI): the method of choice for visualizing neural compression and soft tissues.
A typical finding upon examination is adjacent segment disease (Adjacent segment disease). Spinal fusion alters spinal biomechanics, leading to accelerated disc degeneration and facet joint hypertrophy above and below the surgical level. During differential diagnosis, it is also important to exclude discitis and vertebral osteomyelitis, which, unlike adjacent segment disease, typically involve the surgical site itself.
Failed Back Surgery Syndrome (FBSS)
FBSS is characterized by persistent or new low back pain following surgery. Neuroimaging (primarily contrast-enhanced MRI) plays a crucial role in determining the etiology.
Etiological factors are divided into three groups:
- Preoperative: initial misidentification of the pain source, psychosocial aspects (anxiety, secondary gain), and the consequences of revision surgeries (success rates drop by half with each repeated procedure).
- Intraoperative: surgical technique errors, wrong-level surgery, and excessive decompression (found in 14% of FBSS cases and leading to instability). Risk factors include morbid obesity, emergency surgery, and multiple operating surgeons.
- Postoperative: identified specifically via imaging data. These include recurrent disc herniations and scar tissue formation.
Differential diagnosis: fibrosis vs. herniation?
Epidural fibrosis is the inevitable formation of granulation tissue following procedures in the epidural space. It is considered normal if it does not tether nerve roots or deform the dural sac; however, in 20–36% of cases, scar tissue becomes the source of pain (due to nerve hypoxia and restricted mobility). Meanwhile, in 7–12% of FBSS cases, the cause is a residual or recurrent herniation.
Both entities look identical on standard T1- and T2-weighted images. The key method for differentiation is contrast-enhanced T1-weighted MRI with intravenous gadolinium administration. Scans must be performed immediately after injection, as contrast may eventually penetrate the disc tissue as well.
| Sign | Epidural fibrosis | Disc herniation |
|---|---|---|
| Contrast enhancement | Early and uniform | Absent (or peripheral only) |
| Contours | Irregular | Smooth |
| Effect on nerve roots | Tethers and encases | Compresses and displaces |
| Mass effect | Minimal | Pronounced |
Acute cauda equina syndrome
Large sequestered disc herniations can cause massive compression of spinal canal structures. On sagittal T2-weighted MRI, compression of the cauda equina roots is visualized (appearing as dark threads against hyperintense CSF), while axial T1-weighted images show subtotal filling of the spinal canal by the extruded disc material.
Clinically, this presents as severe lower paraparesis, perineal anesthesia, and detrusor-areflexia-type voiding dysfunction. Paradoxical ischuria (ischuria paradoxa)—overflow incontinence—occurs. Such a condition requires emergency neurosurgical intervention.
To differentiate the cause of pain on contrast-enhanced MRI, remember: "Scar tissue enhances and tethers, whereas a herniation displaces." Epidural fibrosis enhances uniformly and encases the dural sac, while a disc sequestration does not take up contrast and displaces neural structures.
Class and exam questions
Is contrast enhancement of a nerve root pathological?
Depends on the timing. In the early postoperative period, this is a normal finding reflecting transient inflammation. However, 6 months after surgery, it is considered pathological, indicating radiculitis.
What does sterile arachnoiditis look like on MRI?
It is characterized by nerve root clumping into a single mass and the "empty dural sac" sign (roots are tethered to the dura and distributed strictly peripherally). It occurs in 6–16% of postoperative cases.
What makes visualizing a disc herniation difficult in the first days after surgery?
In the early postoperative period, residual disc tissue is extremely difficult to differentiate from postoperative inflammatory debris and edema, so follow-up neuroimaging requires careful evaluation and, if possible, serial imaging.
What else to study on this topic
- Forms of spinal dysraphism: Spina bifida occulta and cystica
- Symptoms and etiology of detrusor areflexia
- Non-neurogenic bladder dysfunction (BPH and bladder outlet obstruction)
- Encephalocele and lethal neural tube defects
- The role of diagnostic blocks prior to spine surgery
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