Terminology and types of disc hernias
Radiological methods (MRI, CT) reliably describe the geometry of the displaced material and its connection to the parent disc:
- Protrusion (Disc protrusion): the base of the protrusion is wider than any other diameter of the displaced disc material.
- Extrusion (Extruded disc): the width of the base is smaller than the maximum diameter of the displaced material (in axial or sagittal projection). This term is preferred for patients, as 'extrusion' is rarely asymptomatic.
- Sequestrated fragment (Sequestrated fragment): a subtype of extrusion where there is a complete absence of continuity between the disc and the herniated material.
- Migration (Migration): displacement of the disc material from the site of extrusion, when the presence of a connection to the disc cannot be reliably determined from imaging.
By distribution, a hernia is considered focal if it occupies less than 25% of the disc circumference, and broad-based (Broad-based) when involving 25–50% of the circumference.
Hernias are also divided into contained (material covered by the annulus fibrosus) and non-contained (annulus fibrosus is ruptured). Accurately differentiating them on MRI or CT is difficult — discography is used for this purpose (contrast leaks in non-contained hernias).
Anatomic classification (displacement zones)
For precise communication between radiologists and surgeons, the localization of fragments is described using anatomical landmarks.
In the axial (transverse) plane:
- Central (posterior midline zone).
- Paracentral.
- Subarticular (lateral recess zone).
- Foraminal (intervertebral foramen).
- Extraforaminal (lateral to the foramen).
- Anterior (includes anterolateral zone).
In the vertical plane, reference is made to the vertebral pedicles:
- Suprapedicular level.
- Pedicular level.
- Infrapedicular level.
- Disc level.
Spontaneous hernia regression
Intervertebral disc hernias (especially lumbar) frequently undergo spontaneous regression. This occurs due to dehydration, retraction back into the cleft, or inflammatory resorption. In the cervical spine, regression is more commonly observed in median and diffuse soft-tissue hernias, whereas in the thoracic spine it is rare.
MRI predictors of regression:
- High signal intensity on T2-weighted images.
- Peripheral contrast enhancement (gadolinium) on T1-weighted images around the sequestrated fragment (indicates active absorption).
- Contrast enhancement of the posterior longitudinal ligament (inflammatory response) or epidural space (venous congestion).
Note: Material that has migrated into the epidural space resolves faster than hernias located beneath the ligament.
Bone marrow degenerative changes (Modic)
In response to disc pathology, reactive changes develop in the endplates and subchondral bone. In 1988, Modic classified these into three types, which are clearly visible on MRI:
| Modic type | Pathological process | T1-weighted signal | T2-weighted signal |
|---|---|---|---|
| Type I | Inflammation and edema | Decreased (↓) | Increased (↑) |
| Type II | Fatty degeneration | Increased (↑) | Increased or normal (↑/≈) |
| Type III | Reactive osteosclerosis | Decreased (↓) | Decreased (↓) |
MRI features in clinical examples
When analyzing MRI and CT images, specific features of the hernia structure can be identified:
- 'Nuclear trail' sign (nuclear trail sign): visualized on axial T2-weighted images in extrusions (e.g., in the thoracic spine).
- Fibrous nature of the fragment: confirmed by a hypointense (dark) signal on T2-weighted images during downward migration.
- Obliteration of fat: in extraforaminal hernias, the displaced material completely displaces the adipose tissue in the corresponding intervertebral foramen.
- Calcification: well-defined on CT (e.g., in calcified upward thoracic extrusions).
Briefly: answers to topical questions
What stages of intervertebral disc degeneration are distinguished according to the Pfirrmann classification?
According to the Pfirrmann classification, there are five stages of intervertebral disc degeneration, which are evaluated using T2-weighted MRI data.
| Grade | Intervertebral disc structure | Signal intensity | Clarity of demarcation between the annulus fibrosus and nucleus pulposus | Intervertebral disc height |
|---|---|---|---|---|
| I | homogeneous | hyperintense (white) | clear border | normal |
| II | non-homogeneous | hyperintense (white) | clear border | normal |
| III | non-homogeneous | intermediate (gray) | indistinct | normal or slightly reduced |
| IV | non-homogeneous | intermediate (gray) | absent border | normal or moderately reduced |
| V | non-homogeneous | hypointense (black) | absent border | sharply reduced |
Which neural structures are compressed in foraminal disc herniation?
In foraminal disc herniation, the spinal nerve root exiting at the corresponding level is compressed.
The herniated protrusion is directed straight into the intervertebral (neural) foramen. Compression of the nerve root corresponding by number to the affected disc occurs only in cases of very lateral bulging. Unlike posterolateral protrusions, which affect the underlying segments, foraminal localization causes compression specifically of the spinal nerve root exiting at that level.
What does the "nuclear trail sign" look like visually on MRI?
On axial MRI scans, the "nuclear trail sign" (nuclear trail sign) visually appears as a "comet tail configuration" (comet tail configuration).
This sign indicates severe destruction and degeneration of the intervertebral disc. On CT, this same sign is associated with abnormal straight or curvilinear opacities in the intervertebral space. The presence of such a sign requires differential diagnosis with natural age-related disc degeneration without rupture.
How to remember Modic changes: 1 — Water (edema, bright on T2), 2 — Fat (bright on T1), 3 — Bone (sclerosis, dark everywhere).
Class and exam questions
How to distinguish a protrusion from an extrusion on MRI?
In a protrusion, the base of the herniation is wider than the maximum size of the herniated material itself. In an extrusion, the base is narrow (less than the maximum diameter of the extruded disc portion).
Can an MRI reliably tell whether a hernia is contained or not?
Often this is impossible. Discography helps reliably differentiate a contained hernia from an uncontained one (when the annulus fibrosus is ruptured): if the annulus is ruptured, the contrast medium will leak outside the disc.
What does peripheral contrast enhancement around a sequestration mean?
It indicates an active inflammatory process and neoangiogenesis. Clinically, this is a favorable predictor of a high probability of spontaneous hernia regression (resorption).
What else to study on this topic
- Mechanisms of formation of the MRI "nuclear trail sign" in thoracic extrusions.
- Differential diagnosis of contained and uncontained hernias using discography.
- Radiological assessment of lateral recess and intervertebral foramina stenosis.
- Visualization features of calcified hernias on CT reformations.
- Differences in the resorption rate of epidural and subligamentous hernias.
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