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Let's start with a quick review of a normal disc, and then we will go through the various stages of a herniated disc.
(Example is based on the L4 disc - lumbar spine segment, however, a similar mechanism occurs in both the cervical and thoracic spine segments)
Healthy disc (top view):
Fig. 1: The nucleus pulposus (pink area in the center of the disc) is a semi-fluid substance rich in proteoglycans, which is responsible for transmitting axial body load and gravitational forces from the center of one vertebra to the center of the next vertebra. It also serves as a pivot point around which rotational movement of the torso occurs. Under normal conditions, the nucleus is tightly held in place by the fibrous ring (green area #2), which is composed of highly concentric rings of type I collagen called lamellae (#9). This specially arranged collagen gives the ring the immense strength needed to keep the nucleus in place, as it is constantly under axial pressure from body weight. Due to continuous axial loading, the nucleus pulposus always attempts to escape (or herniate) from the weaker posterior portion of the disc's annular borders. It is also worth noting that the posterior one-third of the ring is innervated (contains nerves) by the sinuvertebral nerve (SN) and is capable of transmitting pain signals from the disc to the brain.
Disk herniation - the first step towards a disc herniation
In order for a disc to herniate, its structural components must first be torn or weakened. This weakening is believed to result from a condition known as Degenerative Disk Disease (DDD). Although all discs in the human spine undergo a natural aging process, some may experience an accelerated aging process due to genetic predispositions, diseases affecting connective tissue, or disc damage from strenuous physical activity/work.
Regardless of the cause, DDD weakens the fibrous ring discs and allows them to bulge most commonly in a "backward" direction, which in turn allows for symmetric disc protrusion (black arrows in figure 2), which may or may not be associated with a tear in the fibrous ring.
This condition, which can be observed in 60-80% of middle-aged individuals without back/spinal pain [6,8], is called disc herniation and typically does not cause back pain, unless it involves a tear in the fibrous ring fibers.
Torn fibers of the annulus fibrosus - the second step towards a herniated disc
As the annular discs are often incomplete/cracked, especially in the rear area of the disc, sometimes the nucleus pulposus can penetrate through them, resulting in a rupture of the disc. Only a rupture of the fibrous ring that reaches the outer one third of the disc (Fig. 3), which is innervated by nociceptive fibers from the sinuvertebral nerve, has the ability to generate back pain. These types of ruptures, known as full-thickness tears or Grade III fibrous ring ruptures, not only disrupt the normal biomechanics of the disc, but the physical contact of the nucleus pulposus with the nerve fiber causes radiating pain, sensory disturbances, and paresthesias along the irritated nerve root.
The disc herniation causing pain on the magnetic resonance imaging usually remains completely hidden within the bulging disc. It is also important to understand that disc herniation is not possible (there are rare exceptions) without a rupture of the fibrous ring.
Disc protrusion - disc herniation of the first degree
When the outer fibers of the annulus and the posterior longitudinal ligament (PLL - Posterior Longitudinal Ligament - depicted in blue) are almost completely torn, it often leads to an asymmetrical, focal bulge in the posterior disc, known as disc protrusion, which can be clearly seen in Figure 4.
Protrusions, typically around 4 mm in size, usually cause more low back pain than symptoms in the lower extremities and are considered difficult to treat.
The reason for increased low back pain is that pressure may build up in the posterior one-third of the disc (while the PLL and outer ring remain intact), mechanically irritating the nociceptors of the sinuvertebral nerve.
Closed disc herniations do not always produce symptoms and can be seen on MRI scans in about 30% of middle-aged individuals without back or leg pain. Moreover, if symptomatic, they have poor surgical outcomes (microdiscectomy), especially if they are less than 8 mm. It is important to ensure the necessity of surgery if a spine surgeon suggests operating on your 4 mm disc herniation.
Disk extrusion - disc herniation of the II degree
When the outer layers of the annulus (green color) and the posterior longitudinal ligament PLL (blue color) completely tear, a herniation of the nucleus pulposus can become quite large and cause severe limb pain (e.g. sciatica), which is usually greater than back pain.
This type of herniation is called disc extrusion and is usually not observed on MRI in asymptomatic individuals.
In Figure 5, it can be noted that the posterior longitudinal ligament PLL (blue color) has completely torn, resulting in a 10-millimeter extrusion that compresses both the traversing nerve (white star) and the exiting nerve root from the spine (green star).
Because a significant amount of pressure in the posterior part of the disc has been relieved due to the tear, patients ironically experience less back pain but more radiating limb pain.
Unlike protrusions, disc extrusions typically respond very well to microdiscectomy if the annular tear is not too large (which can be accurately determined during surgery) and if the patient did not wait too long for surgery (more than a year).
Another interesting fact about large disc extrusions is that the body has fairly good abilities to naturally absorb them (causing them to disappear). Studies have shown that 80% of large extrusions decrease in size by 50% within one year. Unfortunately, the ideal time for surgery for these herniations is ~4 months from the onset of the injury.
Disc sequestration is the final stage of disc herniation
Sometimes a piece of squeezed disc material (blue star) can detach from the main protrusion and freely move in the extradural space in the spinal canal. This type of herniation is called sequestration and is not usually observed in asymptomatic individuals on MRI.
Although some studies have shown that sequestration is not more painful than extrusion or protrusion, if the fragment is large enough, there is a concern that it may cause strong compression on the bundle of nerves known as the "cauda equina" in the lumbosacral segment. This, in turn, can affect the control of bowel or bladder function (the patient may lose control over the ability to urinate or defecate), which is considered a situation requiring urgent medical/surgical intervention, and is called cauda equina syndrome.
Patients experiencing such symptoms must immediately go to the hospital/call emergency services for evaluation and qualified assistance, as prolonged waiting can lead to permanent damage to the nerves controlling bowel/bladder function.
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