Understanding The Recovery Of The Intervertebral Disc

Picture from Wikipedia

I came across a recent paper (Feki et al, 2024) about the positions which help our intervertebral discs recover and heal when we sleep. I have written at least 5 articles on intervertebral discs. This is definitely still a topic that I get questions from most often among my friends and patients alike. Almost every single one of my friends and patients are fearful of having a 'slipped intervertebral disc' whenever they have low back pain (LBP). The intervertebral discs (IVD) have a really bad reputation for causing significant pain and disability in many people.

Here's a quick review if you did not read those 5 articles.

The IVD consists of a very tough outer layer called the annulus fibrosis (AF). It is made of of several layers of fibrocartilage consisting of Type I and II collagen fibers. The AF protects the soft, gel-like substance in the middle known as the nucleus pulposus (NP). The NP helps distribute pressure evenly across the IVD and prevent excessive forces on the spine.

See how thick the AF is from the picture above? Here's something else you need to know. There is a cartilaginous endplate between the AF and the vertebra (the spine). The endplates hold the IVD in place. It allows load to be spread evenly and to provide attachment to the IVD. This creates a super strong connection to the AF making it impossible for the IVD to 'slip' out of position.

Yes, our discs are actually very strong and hardy. How strong are our IVD's? In a published study on thoracic discs in the young (28 years old plus minus 8 years) , it took about 740 pounds of force to compress the disc height 1 mm. For the older subjects (70 years young plus minus 7 years), it took almost 460 pounds of force. Note that these are on cadavers with the muscles and bones cut away (Stemper et al, 2010).

The IVD's withstand a whole variety of complex forces in our daily activities and also when we exercise. This can lead to significant structural changes in terms of volume, area and height of the intervertebral disc. It can lead to an increase in disc stiffness and a decrease in interdiscal pressure.

In order for your discs to remain healthy and strong, it is dependent on a recovery phase which serves to prevent premature disc degeneration. This happens during a period of nocturnal rest i.e. when you sleep at night.

This phenomenon of disc recovery has been documented extensively through many studies using MRI and intradiscal pressure measurement. Fluid dynamics (water content) within the disc are considered a primary factor in recovery, while it's intricate multiscale structure and viscoelastic (behaving with both liquid-like and solid-like) properties also play key roles.

Feki et al (2024) in their review collated, analyzed and evaluated the existing in vivo (human) and in vitro (in controlled environments) on this topic to provide a comprehensive understanding of this recovery process to enable future advancements in medical treatment and biomedical enginerring solutions to enhance the natural recovery processes of intervertebral discs.

As it is a very long (and complex) review article (Feki et al, 2024), I am simply highlighting the optimal recovery positions (pictured below) for intervertebral disc rehydration. 

Picture from Feki et al, 2024

(a) prone (b) modified press up/ push up position (c) supine with under knee and back support

(d) and (e) side lying with lumbar flexion with pillows between legs

(f) and (g) side lying with and without manual distraction

(h) 50 degrees gravity assisted position (i) 110 degrees supported sitting

(j) inclined sitting with lumbar support

So, you see from the pictures that some of the supposedly 'poor' posture/ positions you were told, are actually good for your intervertebral discs.

References

Feki F, Zairi F, Tamoud A et al (2024). Understanding The Recovery Of The Intervertebral Disc: A Comprehensive Review Of In Vivo And In Vitro Studies. J Bionic Eng. DOI: 10.1007/s42235-024-00542-2

Fournier DE, Kiser PK, Shoemaker JK et al (2020). Vascularization Of The Human Intervertebral Disc: A Scoping Review. JOR Spine. 15: 3(4): e1123. DOI: 10.1002/jsp2.1123.

Stemper BD, Board D et al (2010). Biomechanical Properties Of Human Thoracic Spine Disc Segments. J Craniovert Junct Sp. 1(1): 18-22. DOI: 10.4103/09774-8237.65477

The Larger The Disc Injury The More Likely To Heal Without Surgery

Picture from article referenced below

I saw a few patients with slipped discs this past week. Most patients are still very fearful even if their 'slipped discs' happened years ago. They will complain that they have been having back problems ever since.

Having written previously about how 'slipped discs' can heal I will share with my patients the facts about how the larger the disc injury, the more likely it can heal without surgery.

The picture above shows the diffferent types of herniated disc, bulging being the mildest and sequestration the most severe.

When there is a lumbar disc herniation, there is a tear or damage to the outer layers (annulus fibrosus) leading to leakage (or herniation) of the soft, gel-like inside material (nucleus pulposus). This leakage may touch or compress (irritate) the spinal nerves which causes an inflammatory response. This results in the patient complaining of pain, sensations of numbness, tingling sensations down their leg and sometimes neurological dysfunction.

Conservative management for 6 weeks (instead of surgical management) is usually the first choice for  newly diagnosed patients. In some patients, spontaneous reabsorption of the disc herniation is a widely recognized clinical observation. The spontaneous shrinkage or disapperance of a herniated lumbar intervertebral disc without surgery is called reabsorption or resorption.

The biological mechanisms involved in herniated disc resorption includes macrophage infiltration, matrix remodelling and neovascularization.

Since our immune system recognizes the gel leakage as 'foreigners' in our vertebral epidural space, this triggers a casade of inflammatory responses including phagocytosis of inflammatory cells, enzymatic degradation, increased inflammatory mediators. All of which means that healing is taking place. As the herniation decreases after resorption, the clinical symptoms also improve.

The type and composition of the herniated disc may predict the possibility of natural resorption. Extrusion and sequestration have a higher chance for resorption since the leakage is in the epidural space, creating favorable conditions for macrophage infiltration and neovascularization.

Picture from Radiopaedia

However if the area of the spine shows Modic changes, it is not conducive to macrophage infiltration and ingrowth of blood vessels, thus preventing resorption. Modic changes in the spine occur in response to degenerative changes of the discs, pathology or infections. 

An earlier meta-analysis of 38 clinical studies done in 2015 showed that resorption of lumbar disc herniation was as high as 62-66 percent (Chiu et al, 2015).

Further research is ongoing to understand what conditions can induce or promote the reabsorbtion of 'slipped discs'. This will help clinicians to rationally formulate treatment plans for patients.

Today is exactly one year on from my 2nd bike accident. Not the kind of anniversary I like to remember but it does mean that I've come quite a bit further than where I was. Of course I'm still not working the hours I did before the accident, but definitely much more than just after the accident. So I'm testimony that you can definitely recover, even after 2 compression fractures in my spine! 

References

Chiu CC, Chuang TV, Chang KH et al (2015). The Probability Of Spontaneous Regression Of Lumbar Herniated Disc: A Systematic Review. Clic Rehabil. 29(2): 184-195. DOI:10.118/269215514540919.

Yu P, Mao F, Chen J et al (2022). Characteristics And Mechanisms Of Resorption In Lumbar Disc Herniation. Arthritis Res Ther. 24, 205. DOI: 10.1186/s13075-022-02894-8

A meta-analysis of 38 clinical studies done in 2015 showed that resorption of lumbar disc herniation was as high as 62-66 percent.

About Intervertebral Discs

Picture from M.A. Adams et al, 2010

Having learnt in anatomy class (when I was a physiotherapy student) that our intervertebral discs (IVD) are avascular (has no blood supply), I was instantly surprised when recent research showed that it may not be totally true.

picture from springer link

A little anatomy lesson before I tell you more. Our IVD's are fibrocartilaginous joints that are thought to be the largest avascular structures in the human body. They are made up of three distinct and interdependent tissues. The outer most cartilage endplates are thin layers of hyaline cartilage that anchor the IVD to the adjacent vertebral bones. The vertebral end plates have plenty of blood supply and this allows for diffusion of nutrients into the IVD through the cartilage end plate.

The annulus fibrosus (AF) is a series of super strong well organized concentric lamellae of fibrocartilage that surround and protect the nucleus pulposus (NP) of the IVD. 

The NP is the innermost jelly like substance made up mainly of water and proteoglycans. The NP helps distribute pressure evenly across the IVD and prevent excessive forces loading the spine. This is what can herniate through the AF, causing what is commonly know as a 'slipped disc' or prolapsed intervertebral disc (PID).

A group of researchers performed a comprehensive *scoping review on peer-review publications on the blood supply of human IVD's excluding disc herniations. 22 out of 3122 articles found met the inclusion criteria of fetal to > 90 years old, various health status and both sexes using gross dissection, histology or medical imaging to assess if there is blood supply.

Consistent observations from this review were that there is no blood supply in the NP of the IVD throughout life. 

Both the cartilage endplates and AF have considerable blood supply during fetal development and in infants, but decreases over our lifespan. A common feature of the cartilage endplate was the presence of channels throughout the tissue, likely from the well vascularized vertebral endplate from the adjacent vertebrae. Between birth and ten years of age, there is a drastic decrease in blood vessels within these channels; which are not seen at all in adults.

However, there are blood vessels growing into the endplates and inner layers of the AF especially when there is damaged or disrupted tissue regardless of age. This is more common in older adults. Location of blood vessels are variable. 

It is thought that annular fissures or tears associated with degenerated discs are perhaps more conducive to the ingrowth of blood vessels since there is a loss of proteoglycans (a protein compound found in connective tissue) due to the healing process. Interestingly, there are also nerves found together with the blood vessels suggesting some patients may get more pain than others with such conditions.

Through this scoping review, we now know that the IVD is not entirely avascular as often thought and cited. This is great news for patients. We always knew that you can heal from a "slipped disc", but the discs having a blood supply means a better chance that it can heal from an injury.

Reference

Fournier DE, Kiser PK, Shoemaker JK et al (2020). Vascularization Of The Human Intervertebral Disc: A Scoping Review. JOR Spine. 15: 3(4): e1123. DOI: 10.1002/jsp2.1123.

*A scoping review has a broader scope compared to traditional systematic reviews with correspondingly more expansive inclusion criteria.

* you can read more about slipped discs and how slipped discs can heal here.

What You Need To Know About Slipped Discs

We're still writing about our backs. This week I'm writing about 'slipped discs' in the spine. This is a topic that I get questions from most often among my friends and patients alike.

Patients have always been told that the intervertebral disc (IVD) causes referred pain down the back/ side of your legs. They often think that discs are very fragile and get easily injured.

Almost every single one of my patients are fearful of having a 'slipped intervertebral disc' whenever they have low back pain (LBP). The discs have a really bad reputation for causing significant pain and disability in many people.

Are these common beliefs accurate? Let's go through the anatomy of the spine and the IVD.

The IVD consists of a very tough outer layer called the annulus fibrosis (AF). It is made of of several layers of fibrocartilage consisting of Type I and II collagen fibers. The AF protects the soft, gel-like substance in the middle known as the nucleus pulposus (NP). The NP helps distribute pressure evenly across the IVD and prevent excessive forces on the spine.

See how thick the AF is from the picture above? Here's something else you need to know. There is a cartilaginous endplate between the AF and the vertebra (the spine). The endplates hold the IVD in place. It allows load to be spread evenly and to provide attachment to the IVD. This creates a super strong connection to the AF making it impossible for the IVD to 'slip' out of position.

How strong are our IVD's? In a published study on thoracic discs in the young (28 years old plus minus 8 years) , it took about 740 pounds of force to compress the disc height 1 mm. For the older subjects (70 years young plus minus 7 years), it took almost 460 pounds of force. Note that these are on cadavers with the muscles and bones cut away (Stemper et al, 2010).

The endplates also allow for hydration of the disc to take place (see picture above).

Just like your ACL is often injured by shearing forces, it is also shearing forces that is most likely to hurt your discs. Twisting, rotating your back while lifting a heavy load is definitely not recommended.

What happens after your disc is injured? Find out more next week as I write more on that topic.



Reference

Stemper BD, Board D et al (2010). Biomechanical Properties Of Human Thoracic Spine Disc Segments. J Craniovert Junct Sp. 1(1): 18-22. DOI: 10.4103/09774-8237.65477