Disc aging: normal and pathology
The degeneration process has a linear dependence on age; however, radiography and CT distinguish two fundamentally different types of aging:
- Spondylosis deformans (Spondylosis deformans) is a variant of normal aging. It is typically characterized by the appearance of anterior and lateral marginal osteophytes, as well as a small gas collection near the apophyseal enthesis.
- Intervertebral osteochondrosis (Intervertebral osteochondrosis) is pathological aging. The process affects the nucleus pulposus (nucleus pulposus) and vertebral endplates (vertebral end plates). It is characterized by a pronounced vacuum phenomenon (a large amount of gas in the center of the disc) and reactive bone marrow changes.
Radiological signs of degenerative disease
The earliest sign on plain radiographs is decreased disc height, but its reliability is questionable since height depends on patient positioning (supine or standing) and is often reduced in asymptomatic individuals.
More reliable indicators of pathology include endplate sclerosis, osteophytes, calcification, and the vacuum phenomenon (most frequently at L2-L3 to L5-S1 levels).
On MRI, an early marker is signal loss on T2-weighted images. In normal aging, the signal decreases evenly, whereas in pathology, it decreases locally at one or two levels. This phenomenon is called the «black disc» (black disc). Degeneration typically starts in segments subjected to the highest mechanical load: the cervical (C5-C6, C6-C7) and lumbar (L4-L5, L5-S1) regions.
Intervertebral disc bulging and herniation
When evaluating axial slices, it is important to distinguish between tissue bulging and a true herniation.
Disc bulging (bulging) is the extension of disc tissue beyond the borders of the apophyseal ring. It can be symmetric (involving 50% to 100% of the circumference) or asymmetric (uneven protrusion, often seen in scoliosis).
Herniations (herniations) are classified according to the ratio of the width of the protrusion base to its apex:
- Protrusion (protrusion): the base of the herniated material is wider than its apex. Focal protrusions and broad-based protrusions are distinguished.
- Extrusion (extrusion): the base of the protrusion is wider than its apex. Imaging reveals the «toothpaste sign» (toothpaste sign), when the mass is squeezed through a narrow defect in the annulus fibrosus.
Intravertebral herniations—Schmorl's nodes (Schmorl's nodule)—are distinguished separately. They penetrate the endplate and are surrounded by a zone of bone marrow edema, which yields a hyperintense signal on T2-WI and STIR.
Topographic classification of herniations
In the axial plane, protrusions are classified by their localization:
- Central (median): directed strictly along the midline into the spinal canal.
- Paracentral: located near the midline with lateral displacement.
- Subarticular (in the lateral recess): located under the articular process, often compressing the nerve root of the inferior level.
- Foraminal: located within the intervertebral foramen, causing compression of the exiting spinal nerve root.
- Extraforaminal: displaced laterally to the intervertebral foramen.
- Anterior (ventral): directed anteriorly to the vertebral bodies, usually asymptomatic.
Annular tears
Loss of disc elasticity and overload lead to the formation of fissures and tears of the annulus fibrosus (annular tears):
- Concentric: delamination between the lamellae of the outer layers of the annulus, visually resembling «onion rings». Considered a consequence of torsional overload.
- Transverse (marginal): horizontal tears of Sharpey's fibers at the bony apophyseal ring.
- Radial: penetrate from the center of the disc outwards. A complete radial tear is necessary for progressive disc destruction.
On MRI, tears can form a high-intensity zone (HIZ). It glows on T2-WI (similar to cerebrospinal fluid) and arises due to repeated microtrauma and inflammation. The presence of HIZ is associated with discogenic pain, although the sensitivity of this sign is low.
Briefly: answers to topical questions
What types of Modic degenerative bone marrow changes are distinguished on MRI?
MRI distinguishes three types of Modic degenerative bone marrow changes.
- Type 1 changes (Type 1 changes) represent bone marrow edema associated with acute or subacute inflammation, characterized by decreased signal intensity on T1-WI and increased signal intensity on T2-WI.
- Type 2 changes (Type 2 changes) represent the replacement of normal bone tissue with fat (fatty degeneration), manifesting as increased signal intensity on T1-WI and iso- or hyperintense signal on T2-WI.
- Type 3 changes (Type 3 changes) represent reactive osteosclerosis (reactive osteosclerosis), characterized by decreased signal intensity on both T1-WI and T2-WI.
What stages of intervertebral disc degeneration are distinguished according to the Pfirrmann MRI classification?
According to the Pfirrmann MRI classification, there are five stages of intervertebral disc degeneration evaluated on T2-weighted images.
| Grade | Disc structure | Signal intensity | Border of annulus and nucleus | 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 |
To easily distinguish between types of herniations, imagine a mountain and a mushroom. A protrusion is a «mountain» (wide base, narrow apex). An extrusion is a «mushroom» (narrow stalk, wide cap).
Class and exam questions
Do MRI degenerative changes always cause pain?
No. The correlation between imaging findings and clinical presentation is debatable. Fibrosis, annular tears, and loss of height are frequently found in entirely asymptomatic patients.
What is a high-intensity zone (HIZ)?
This is a combination of coalescing radial and concentric tears of the annulus fibrosus causing inflammation. On MRI, it produces a bright signal and is considered a marker of discogenic pain.
Why does a «black disc» appear on MRI?
The decrease in signal intensity on T2-WI is associated with the loss of disc hydration (drying out) during the degeneration process.
How does disc bulging differ from protrusion?
In a bulging disc, the tissue concentrically or asynchronously extends beyond the edges of the apophyseal ring, but there is no structural defect. A protrusion is a localized outpouching of herniated material through a defect with a broad base.
What else to study on this topic
- Reactive endplate changes (Modic type).
- Extradural inflammation and its contrast-enhanced visualization.
- Features of bone marrow edema around Schmorl's nodes (STIR sequence).
- The role of collagen quality and biomechanics in the etiology of disc lesions.
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