Why does a disc herniation hurt?
This blog aims to discuss the mechanism behind the pain because of disc herniations in detail which have been touched briefly in previous blogs.
Starting with, what is a disc and why is it important?
The vertebral column is made of vertebrae that encompass the spinal cord that passes through it, similar to the rib that surrounds the lung and heart or the skull that protects the brain. In between each of these vertebrae (intervertebral) there exists a fibrocartilaginous structure called the intervertebral disc.
The IV disc is divided into 3 components: the endplate, the outer annulus fibrosus, and the inner nucleus pulposus.
What is the disc made up of?
The disc which is mostly oval-shaped is made up of mostly collagen and water.
The stiff outer annulus is made of 10-20 collagen-rich concentric rings (the part that is always missing in the spine models). The inner nucleus which is a gel-like structure (always seems excessively highlighted in the spine models) and is made of proteoglycans and collagen. Proteoglycans are structures with the ability to attract water, hence the nucleus in the center tends to have relatively high water content that changes as we age.
What does the disc do?
It is generally believed that the high water content in the nucleus makes it fragile and more prone to squeezing or slipping out of its place. But there is obviously a “higher purpose” for the existence of such a structure other than just causing painful disc herniations.
Our spine is under constant load (and it is structured in a way that it can handle that load). Standing and walking transmits axial load (80-100kg, so no way the spine is fragile!) down the spine along with the force of muscles that contract to keep us upright and move around.
Therefore, the disc is designed to cope up with this load through its smart structure.
The gel-like consistency of the nucleus distributes the force, coming down through the spine, outwards or laterally, similar to pressing on a balloon at its center. This force that splays outwards in all directions is received by the stiff annulus on the outside. So the force coming down the spine not only travels down but also outwards through the nucleus from where it is dispersed into the annulus. Trust your body, it has got the basics right!
Let's not forget the cartilageous end plates, at the top and bottom of the vertebral bodies which are similar to the annulus (though not as strong as the annulus) receive the distributed load from the nucleus of the disc.
Without the annulus and the end plate accepting the force from the inner nucleus the disc would surely bulge with every movement and might even burst if under constant force, but this isn't the case in real life, a part strategically skipped during the explanation of mechanism of disc herniations using spine models.
Like almost all cells and tissues in the living body have a blood supply that provides them their nutrition as well as aids in waste removal keeping them healthy, the disc has the same setup but only till a very young age. As we start to take an upright position and walk the pressure inside the disc increases the blood vessels get limited to just the outer area of the disc.
But to sustain the disc needs nutrition which comes in through the porous endplates. As we have seen, the endplate exists between the vertebral body and the disc, therefore the disc gets about 80% of its nutrition (oxygen and blood) through diffusion from the marrow of the vertebral body.
These nutrients and oxygen are further pulled into the disc by the proteoglycans present in it. Proteoglycans are negatively charged structures that tend to draw in positively charged or neutral structures like the nutrients, oxygen, and water that keep the disc supple for force distribution.
Once the nutrients and oxygen are in the disc, they are used up the chondrocytes present in the disc which produce the proteoglycans, collagen as well as the other components that comprise the disc.
Summarizing a bit, the proteoglycans pull in the nutrients and oxygen which is then used by the chondrocytes as fuel to produce various new components of the disc. The only thing remaining is the removal of old parts, which is done by the enzymes produced by the chondrocytes themselves called the matrix metallopeptidase (MMP). Pretty simple, like any other structure in the body that regenerates and repairs itself.
Why and how do disc herniations hurt?
This can be explained primarily by the endogenous mechanism and the immune-mediated mechanism.
Like any other process of healing in the body, the process of repair and remodeling in the disc is also accompanied by inflammation where the chondrocytes produce proinflammatory mediators that aid in the inflammatory process.
Furthermore, as the disc degenerates, there is a build of waste products in it which is detected by the blood supply on the outer layer of the disc and hence triggers the blood-borne immune cells to produce an inflammatory reaction in an attempt to clear the waste out.
This ongoing inflammation in a degenerating disc can cause nerve irritation as the disc herniates out.
The immune-mediated mechanism explains that the inside of the disc is hidden from the immune system of the body except for the small outer portion with blood and nerve supply. As discussed above, the high-pressure environment and the pro-inflammatory compounds inside the disc make it hard for blood vessels to sustain.
As the outer annulus breaks and the disc material comes out, herniates, the immune system comes across something that is foreign similar to a pathogen/bacteria/virus, and hence produces an inflammatory reaction as a counter. This also explains how the inflammatory reaction brings along all the immune cells and helps in the resorption/ healing of the herniated disc. Therefore, disc herniations can heal and it is seen that the more/worse the herniation like in sequestration, the better the resorption/healing of the disc.
Why are L4-L5/ L5-S1 the commonly affected nerve roots because of herniation?
The reason why L4-L5 and L5-S1 are the most commonly affected nerve roots leading to conditions like nerve radiculopathy/radicular pain/ sciatica lies in the anatomy.
Unlike the cervical and thoracic vertebrae, the nerve that branches off from the spinal cord at the l4-21 level directly crosses over (overlaps) the intervertebral disc of that level. So any herniation at that level has a higher proximity of affecting the nerve root and the dorsal root ganglion.
Following is the diagram showing the location of the nerve root and the dorsal root ganglion (the swollen part) with respect to the disc if we section the spine horizontally (top to bottom).
Disc herniations hurt because they lead to inflammation and immune responses that irritate nearby nerves. While this process can be painful, it's also part of the body's way of healing/dealing with the herniated disc. Understanding these mechanisms can help us manage the condition effectively providing us with the probable natural history, prognosis, etc of the condition, meanwhile decreasing the fear surrounding it.
The good news is most disc herniations tend to get better with time and movemenet, so keep moving!
For more go to Physio Explored Blogs
Disclaimer: This blog is for educational purposes only.
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