Pathogenesis and classification of protrusions
The development of a hernia is associated with damage to the fibrous ring (annulus fibrosus), through the defects of which the pulp nucleus (nucleus pulposus) is displaced outward. The prolapsed tissues begin to mechanically compress the structures of the central canal or adjacent nerve trunks. From a biomechanical perspective, the key provoking factor is spinal flexion: in this position, the anterior part of the disc is compressed, and its internal contents are squeezed backward, stretching the posterior structures.
About 90% of all protrusions have a posterolateral (paracentral) localization. This is explained by the anatomical weakness of the posterior longitudinal ligament (PLL) in its lateral parts. Much less frequently, in about 10% of cases, foraminal hernias occur when the tissue prolapses directly into the intervertebral foramen. If a disc fragment completely detaches from the main mass, a sequester is formed.
In relation to the PLL, hernias are divided into two types:
- "Contained" (subligamentous): the ligament stretches but does not tear, holding the nucleus pulposus.
- "Uncontained" (free): the PLL fibers rupture along with the annulus fibrosus.
Topographic anatomy of nerve roots
In the lumbar region, nerves are named after the vertebral pedicle (pedicle) under which they exit the spinal canal. It is important to understand that the trunks separate from the cauda equina (cauda equina) one segment above their actual exit and descend vertically. For example, the L5 root originates at the level of the L4-L5 disc, travels downward behind the body of the fifth lumbar vertebra, and exits into the L5-S1 foramen.
Due to this complex pathway, a single nerve can be compressed in different ways. The L5 root is compressed either centrally at the L4-L5 level (transiting trunk in the recess) or laterally at the L5-S1 level (exiting trunk in the foramen itself, which is more common).
The cervical spine has a different architecture: nerves run horizontally and exit under the pedicle of the vertebra located above («mismatch» syndrome). For example, the C5 trunk passes under the C4 pedicle. Therefore, any herniation at the C4-C5 level will affect precisely the fifth cervical root, regardless of the direction of the protrusion.
Clinical presentation and neurological examination
Patients often present with axial pain against the background of disc degeneration. When nerves are involved, radiculopathy develops—pain and paresthesias radiating from the back through the buttock down the leg. Lesions at the L4-L5 level cause symptoms extending to the dorsum of the foot, and L5-S1 to the sole. Symptoms are usually unilateral and worsen with sitting or forward bending.
The examination includes assessing gait and muscle strength (from 0 to 5 points). Testing spinal nerve roots is highly specific:
- L4 root: evaluated by knee extension and ankle dorsiflexion. In case of damage, the knee jerk reflex may be absent.
- L5 root: easiest to test by extension of the big toe; also responsible for hip abduction.
- S1 root: checked by the patient's ability to walk on toes.
The physician necessarily performs the Lasegue test: passive straight leg raising to 30–70 degrees elicits radicular pain. The crossed Lasegue sign (pain in the affected leg when lifting the healthy one) has a higher specificity. It is important not to forget to check the range of motion of the hip joint, the pathology of which is easily confused with spinal pain.
Instrumental diagnostics and red flags
MRI serves as the gold standard for confirming the diagnosis; however, tomography often reveals clinically insignificant (asymptomatic) changes. In one-third of people over 60 without pain complaints, MRI will show the presence of a herniation. Therefore, the study should answer a specific question rather than serve as a screening tool. Contrast-enhanced MRI (gadolinium) is used when planning revision surgeries: scar tissue enhances with contrast, while a recurrent hernia does not.
Special attention is paid to "red flags"—signs of dangerous conditions:
- Cauda equina syndrome: an emergency condition in which a massive herniation compresses nerve bundles below the conus medullaris. It manifests as bilateral lower limb deficit, saddle anesthesia, loss of reflexes, and pelvic organ dysfunction (urinary retention or incontinence).
- Suspected metastases: unexplained weight loss or a history of oncology.
- Infections (discitis, abscess): fever, leukocytosis, history of intravenous drug use.
Briefly: answers to topical questions
What are the main anatomical differences between posterolateral and foraminal lumbar disc herniations?
The main anatomical differences lie in the direction of the protrusion and the level of spinal nerve root compression.
| Characteristic | Posterolateral herniation | Foraminal herniation |
|---|---|---|
| Localization | Caused by weakness of the posterior longitudinal ligament (posterior longitudinal ligament, PLL) in the lateral parts | Directed directly into the intervertebral (neural) foramen |
| Root compression | Compresses the transiting root descending behind the disc (e.g., L5 in an L4-L5 herniation) | Compresses the exiting root within the foramen (e.g., L5 in an L5-S1 herniation) |
Which tendon reflexes are lost during L4 and S1 root compression?
During the compression of these nerve roots, sources describe the loss of only the knee reflex, as information regarding S1 tendon reflexes is absent.
- L4 root — damage may manifest as a decrease or absence of the knee jerk reflex.
- S1 root — the absent tendon reflex is not specified (the function is tested by the patient walking on toes via plantar flexion).
How is the differential diagnosis between postoperative scar tissue and recurrent disc herniation performed on MRI?
Differential diagnosis is performed using gadolinium contrast-enhanced MRI based on the enhancement pattern on T1-weighted images.
| Abnormal | Contrast enhancement | MR signal characteristics |
|---|---|---|
| Recurrent disc herniation | Peripheral contrast enhancement | The center of the lesion does not enhance and maintains a low MR signal |
| Postoperative scar (epidural fibrosis) | Intense solid contrast enhancement | Scar tissue actively enhances with contrast and may surround the spinal nerve root |
Quick root test: L4 — heels (dorsiflexion), S1 — toes (plantar flexion), L5 — big toe (extension).
Class and exam questions
Why do disc herniations most commonly have a posterolateral location?
This is because the posterior longitudinal ligament (PLL) is weakest precisely in its lateral parts, which creates a zone of least resistance when pressure inside the disc increases.
What is the difference in the course of nerve roots in the cervical and lumbar spine?
In the cervical spine, roots run horizontally and exit above the vertebra of the same name (e.g., C5 under C4). In the lumbar spine, they descend downward and exit under the pedicle of the same name (L5 under L5).
When is gadolinium used in MRI of intervertebral discs?
Contrast is used when tumor or infection is suspected, as well as to differentiate postoperative scar tissue (which enhances) from recurrent disc herniation.
What else to study on this topic
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