Do intervertebral discs have nerve innervation?

For the podcast I have gone over a few different research articles that discuss how to time date injuries with imaging and in particular MRIs.  The last podcast discussed what a disc herniation might mean to a patient.  I went over research showing the effects of a disc herniation when it takes the space of the spinal cord and nerve roots.  The last podcast showed, if a disc herniation is compressing nerve tissue this will lead to cellular death and permanent nerve damage and needs to be urgently decompressed often with surgery. 

Today I want to continue the topic of what a disc herniation may mean to the patient.  Once an injury has been causally related to a motor vehicle collision or other trauma an attorney needs to understand the nature of the injury.  Is the injury permanent, what will recovery (if any) look like, how will it affect the patient going forward. 

My first question for the listeners is Can a vertebral disc have pain?  Similar to the last podcast I want to discuss nerves, but not the nerve roots.  I want to discuss the nerve innervation of vertebral discs.  The term innervation means the nerve going to something.  Without nerve innervation a vertebral disc would not be able to sense any pain.  I want to go over 2 articles today the first is The nerve supply of the lumbar intervertebral disc published by M.A. Edgar in the Journal of Bone and Joint surgery 2007. 

Today’s first study showed that vertebral discs do have nerve innervation through the sinuvertebral nerve which used to be called the Luschka nerve.  This is a recurrent nerve meaning it re-enters the vertebral canal and has also been known as the recurrent meningeal nerve.  This recurrent meningeal nerve has fibers consistent with pain mediation. 

To understand nerve innervation of the vertebral disc it is important to understand the parts of the disc.  At the center of the disc we have the fluid filled area called the nucleus pulposus.  The nucleus pulposis is surrounded by the annulus fibrosus which is the strong tensile part of the disc.  The annulus fibrosus is surrounded by a cartilaginous endplate. 

This first study showed that the recurrent meningeal nerve only goes to the outer layers of the annulus fibrosus.  This nerve has a lot of overlap which was shown to cause poor localization of low back pain or in other words patients have a hard time determining exactly where the low back pain is coming from when a disc is involved.  This study showed that the innervation in normal vertebral discs was relatively sparse and only in the outer 3 layers of the annulus fibrosus. 

Today’s first study showed that in unhealthy damaged discs nerves extend deeper and into the inner third of the disc.  These new nerves were shown to be around new vascular areas of damaged discs showing new growth in the damaged areas.  This new nerve growth into the center of the disc was consistent with pain sensation with free nerve endings that will increase pain transmission.

This shows that healthy discs have very minimal nerve innervation and unable to sense pain beyond just the outer layer of the annulus fibrosus.  This means that healthy discs will rarely be painful or even have the capability of sensing pain throughout most the disc. 

The second study I want to go over today is titled Nerves are more abundant than blood vessels in the degenerate human intervertebral disc published by Abbie L. A. Binch et al in the Arthritis Research and therapy journal in 2015.  They reported that vertebral discs are the largest aneural structure within the human body.  This means that there are less nerves in the vertebral discs than any other structure.  The nerve fibers that go the vertebral discs are only in the outer 3 layers of the annulus fibrosus and are very fine and pain sensing nerves.  These nerves go to the outer 3 layers of the annulus and a typical annulus has around 7-15 layers. 

This second study showed that it is difficult for nerves to grow into the intervertebral disc due to the body creating factors to inhibit nerve growth there.  This study also showed that with trauma nerves do grow into the center of a vertebral disc and this is mediated by angiogenic factors in response to mechanical injury, strain and inflammation.

This study showed that tears or fissures in the disc can create a source of entry for nerves to grow.  Damage to the disc lowers the fluid pressure making nerve growth more favorable.  They showed that the increase in nerves in a damaged vertebral disc will lead to painful nerve endings in the disc that can be aggravated by normal mechanical stimulation that would not normally be painful.    This shows that after disc damage once the pain sensing nerves grow into the center of the disc that even normal motion can cause pain.  That is why the body doesn’t allow nerves to grow in the center of the disc in healthy patients. 

This second study showed that the vertebral tops and bottoms known as endplates are often damage along with discs.  This damage can be seen on MRI as modic changes (which we went over in prior podcasts).  This endplate damage has been shown to increase tumour necrosis factor which leads to nerve ingrowth.  The endplate damage has also been shown to increase enzyme production that will degrade the disc.  This damage to the discs and vertebra mediates growth factor allowing for the healing and new growth of nerves in areas where nerves should not grow.

To summarize: healthy vertebral discs have nerves or the ability to sense pain only in the outer 3 layers of the annulus fibrosis.  The annulus fibrosis has 7-15 layers which means healthy discs only have nerves or ability to feel pain on the outer 20-40% of the annulus fibrosis.  There is no nerves or ability to feel pain in the nucleus pulposis in a healthy disc.  Todays second study showed that we are not supposed to have nerves in the inside of a vertebral disc.  If we do have nerves inside a disc we will feel pain with normal mechanical stimuli that should not be painful.  Meaning normal movements would cause disc pain if we had nerves inside the discs.  This is why the human body as stated in the second article has discs that are and I will quote “the largest aneurla structure within the human body” end quote. 

Real world example

I do not want to give an example of a specific patient today, but how I use this information to help with report writing.  There are 2 main things I like to address with this research when doing a final report or an IME.

The first is healthy patients with no pre-existing disc pathology who get in a motor vehicle collision or other trauma and have new herniated discs.  As we have discussed on prior podcasts there are many ways to help determine if a disc herniation is new.  Lets assume this patient has never had prior traumas or low back pain, and has no arthritic changes surrounding a new focal disc herniation, with no loss of disc space height and has new subjective radicular pain. 

Given no prior pain or disc damage we will assume that this patient has never had disc pain or even the ability to feel disc pain given the lack of nerves inside the disc.  Now as seen in today’s articles this patient’s disc will heal or remodel into a disc with nerves where they should not be.  These nerves are pain sensing nerves that will, as seen in this study cause amplified pain sensations with new free nerve endings inside the disc with just normal mechanical stimuli. 

This means that even after the disc herniation has resorbed into a degenerative disc even normal movement can cause amplified pain sensation to free nerve endings inside the disc.  These free nerve endings will not go away or get better over time.  That is a permanent ingrowth of nerves and can lead to chronic pain over time.  I see patient’s all the time that report ever since their motor vehicle collision 20 years ago my low back has always hurt.  The radicular or nerve root problems typically resolve with conservative care or surgery, but the disc pain never went away. 

This disc pain as seen in today’s podcast is diffuse and often felt throughout the low back and buttock region.  I have seen a lot of attorneys that do not understand the significance of a disc herniation and likely do not fight for fair compensation from the insurance companies.  I have seen a lot of adjusters and insurance companies that assume a disc herniation is not a serious injury and they will make a full recovery.  This is false there will never be a full recovery.  With management often these patient’s pain will be lower and manageable, but the free nerve endings are always there in the places they should never be in a disc. 

The second topic I’d like to address is the opposite type of patient.  This next hypothetical patient has an MRI that shows an old disc injury.  There is loss of disc space height, a circumferential bulge and no focal herniation.  There is osteoarthritic surrounding the disc bulge and no new subjective radicular complaints.  This would clearly indicate an old injury. 

With this type of patient, it is often the case that their pain is much worse after the MVC than prior.  They will often report that prior to the MVC the pain was low and occasional and now it is constant and high levels of pain.  Keep in mind that this patient likely already has an ingrowth of nerves into the disc that is like a ticking time bomb for aggravation.  The new trauma often causes such intense low back pain that spreads to the buttock region that it is difficult to distinguish between disc pain and new radicular pain and an MRI is ordered. 

Once the MRI comes back with no new injuries it is easy to assume that no new injury to the disc have occurred.  It is important to remember that this patient with pre-existing problems will have a much higher ability to sense pain in the discs due to nerve ingrowth from prior injuries.  This helps explain the need for more treatment and slower progress with care.   

This is what I believe is called the egg shell plaintiff rule.  The patient’s MVC would not likely have caused as much pain in a healthy spine, but they do not have a healthy spine.  It is a clear indicator for intense pain and need for management. 

Hopefully today’s podcast helps you understand a little more about what a disc herniation means to the patients.  Please subscribe so you don’t miss out on any of the research relating to personal injury.  Thanks for listening.