CT Evaluation of Hardware Position
Computed tomography is used for a reliable assessment of the correctness of spinal instrumentation placement. The physician must analyze the images using axial slices and sagittal oblique reconstructions.
In the normal position, the hardware correctly passes strictly through the thickness of the vertebral pedicles. However, in surgical practice, malposition occurs—the incorrect insertion of fixation elements. There are two main variants of screw displacement:
- Displacement laterally relative to the vertebral pedicle.
- Displacement medially relative to the pedicle, in which the hardware penetrates into the spinal canal and the lateral recess zone.
Cerebrospinal Fluid (CSF) Leak
Injury to the dura mater (dural breach) during surgery, if unrecognized or inadequately sutured, leads to CSF extravasation. There are two forms of this pathology:
- Pseudomeningocele (pseudomeningocele) — an accumulation of cerebrospinal fluid in the paraspinal soft tissues.
- Cutaneous CSF fistula (cutaneous CSF fistula) — the escape of CSF outward through the surgical wound or residual drain tract.
The method of choice in diagnostics is MRI. Tomograms visualize a fluid collection adjacent to the thecal sac, sometimes with a direct visible connection to it. An important diagnostic sign is the complete absence of contrast enhancement or only thin peripheral enhancement. If precise localization of the defect and visualization of the fistula tract are required, CT myelography is used.
When a fluid collection is detected, it is necessary to perform a differential diagnosis of pseudomeningocele with true meningocele (herniation of the meninges themselves through the defect), as well as with postoperative abscess and hematoma.
Intracranial Hypotension Syndrome
Chronic CSF leakage can provoke intracranial hypotension syndrome (CSF leakage syndrome). This formidable complication has a specific neuroimaging pattern. Characteristic manifestations of the syndrome include:
- Marked thickening and abnormal contrast enhancement of the dura mater.
- "Sagging" (sagging) of the midbrain structures.
- Cerebellar tonsillar herniation (tonsillar herniation).
- Marked venous dilatation.
- Compensatory enlargement of the pituitary gland.
- Formation of secondary subdural hematomas or hygromas.
Postoperative Hematomas and Infections
Hemorrhagic complications require careful differential diagnosis. Chronic hematoma on MRI is characterized by a heterogeneous hyperintense signal on T2-weighted images (T2WI). Unlike inflammatory or tumoral processes, it shows absolutely no contrast enhancement.
Postoperative infections complicate 0.2% to 3% of spinal surgeries. In the vast majority of cases, the etiological factor is direct staphylococcal infection (staphylococcal infection). The pathological process can affect various structures, manifesting as:
- Surgical site infections (wound infections).
- Discitis (discitis).
- Vertebral osteomyelitis.
- Epidural abscess.
Clinically, the presence of infection should be suspected in any patient with persistent back pain developing weeks or months after surgery. It is important to remember a paradoxical laboratory sign: the white cell count in such patients often remains within normal limits.
Briefly: answers to topical questions
What structures can be damaged during medial malposition of a transpedicular screw?
During medial malposition of a transpedicular screw, it penetrates into the lateral recess. The normal position of implants implies their correct passage through the vertebral pedicles. With improper insertion (malposition) with a medial displacement relative to the pedicle, the screw goes beyond its anatomical boundaries. Additional information on specific neural or vascular structures that can be directly damaged by the screw in this type of displacement is not provided in the sources.
Which MRI modes and sequences are optimal for detecting postoperative discitis and vertebral osteomyelitis?
To detect discitis and vertebral osteomyelitis according to the sources, MRI is used as the method of choice. The described MRI signs are evaluated on the following images:
- T1-weighted images (T1WI): low signal from the affected intervertebral disc; bone marrow edema also manifests as a low signal on T1WI.
- T2-weighted images (T2WI): high signal from the affected disc; bone marrow edema manifests as a high signal on T2WI.
- STIR: bone marrow edema manifests as a high signal in the STIR sequence.
- Gadolinium-enhanced T1WI: the sources describe the detection of paravertebral abscesses with a contrast-enhancing rim.
For differential diagnosis of fluid collections, remember: a chronic hematoma is heterogeneous on T2WI and does not enhance with contrast, a pseudomeningocele has thin rim enhancement (or none), and an abscess usually demonstrates thick ring-like enhancement.
Class and exam questions
How to reliably assess screw position on CT?
It is necessary to study images not only in the axial plane, but also mandatory to construct sagittal oblique reconstructions to confirm the passage of the screw through the pedicle.
What is the difference between a pseudomeningocele and a true meningocele?
A pseudomeningocele is an accumulation of CSF in the soft tissues without a wall, whereas a true meningocele represents a herniation of the dura mater itself through a defect in the spinal canal.
Why should one not rely solely on blood tests when discitis is suspected?
According to statistics, in postoperative spinal infections (even of staphylococcal etiology), the leukocyte count often remains normal despite persistent pain.
How does a chronic postoperative hematoma appear on MRI?
It has a heterogeneously hyperintense signal on T2-weighted images (T2WI) and does not accumulate contrast media.
What else to study on this topic
- Technique of CT myelography for precise visualization of the fistula tract in CSF fistula.
- MRI semiotics of discitis and postoperative vertebral osteomyelitis.
- Pathogenesis of intracranial hypotension syndrome: why compensatory pituitary enlargement occurs.
- Differential diagnosis of postoperative epidural abscess and organized hematoma.
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