Spinal disc herniation
Herniated discs can be more specifi cally classifi ed as protrusion or extrusion, on the basis of the shape of the displaced material.
The terminology “protruded disc” is used when the base of the disc is broader than any other diameter of the displaced material. Based on a two-dimensional assessment of the disc contour in the transverse plane, a protruded disc can be focal (involving <25% of the disc circumference) or broad-based (involving 25%–50% of the disc circumference).
The terminology “extruded disc” is used for a focal disc extension of which the base against the parent disc is narrower than the diameter of the extruded disc material, measured in the same plane. Extrusion is also used when there is no continuity between the herniated disc material beyond the disc space and that within the disc space.
If the displaced disc material has no connection with the parent disc, it is called a “sequestrated fragment”. This is synonymous with a “free fragment”.
On imaging studies it is often impossible to determine if continuity exists between the extruded disc material and the parent disc. Therefore, it is sometimes more practical to use the term “migration”, which signifi es displacement of disc material away from the site of the extrusion, regardless of whether it is sequestrated or not. The term migrated only refers to position and not to continuity. The term “contained” refers to displaced disc material which is covered by the outer annulus. If this covering is absent, the herniation is “uncontained”.
Imaging and Location
In the transverse (axial) plane, the following zones are recognized:
- Central (posterior midline)
- Right/left central (also known as paracentral)
- Right/left subarticular (lateral recess)
- Right/left foraminal (neural foramen)
- Right/left extraforaminal (outside the neural foramen, also known as far lateral) anterior zone (anterior and anterolateral regions).
In the vertical plane, three levels are defi ned (from top to bottom):
- Pedicle level
- Infrapedicle level
- Disc level
- Suprapedicle level
Spontaneous Regression of Disc Herniation
The regression can imply a decrease in size or even complete disappearance of the herniated disc. Proposed: retraction of disc material into the intervertebral space, dehydration or shrinkage of the disc, and resorption due to an infl ammatory reaction (phagocytosis by macrophages) (Slavin et al. 2001).