How are discs damaged?

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Welcome back to the Forensic Chiropractor podcast.

This month’s study is Mechanical Aspects of Intervertebral disc injury and implications on biomechanics published by Geoffrey Desmoulin et al in the spine Journal in 2019.  This study was done to assess the different types of structural failures in the intervertebral discs with respect to mechanical loading.  In other words what types of mechanical loads will damage spinal discs. 

The study starts by discussing how important the spine is in daily activities such as walking, jumping, lifting and other activities.  It is fascinating what a healthy spine can do.  For those that have had spinal injuries it is amazing how a spine injury can alter so many of our activities.  They reported that the discs are quote “essential in separating and evenly spreading the load between vertebral bodies” end quote. 

As a refresher discs are made up of 3 components.  These components of a disc are important and can be seen separate on MRI to some extent.  These components can be important when trying to time date a disc injury.  We have the nucleus pulposus or inside of the disc which is largely gelatinous proteoglycans or high levels of water content.  The nucleus pulposus is the material that herniates outside of a disc when we have a herniation.  New disc herniations will often show the increased signal of the nucleus pulposus outside the disc in the herniation on a T2 view.  The second component of a disc is the annulus fibrosus or the strong collagen fibers that surround the nucleus pulposus.  This is the strong portion of a disc.  This is the only portion of a disc that has innervation or ability to feel pain, but it is only the outer few layers that have innervation. 

I covered in prior podcasts that when there is damage to the annulus fibrosus the innervation or nerves will grow inside the disc.  This makes the disc a potential pain generator with normal activities.  This is due to the fact that the body was never supposed to have nerves inside the discs. 

The third component of a disc is one we don’t hear a lot about.  It is the cartilaginous endplates.  This is a cartilage cap on the top and bottom of the discs.  These cartilage caps are also attached to the bone.  The cartilage is often damaged with disc herniations.  This also often leads to vertebral endplate damage and is often seen with modic changes.  Modic changes show damage to the bones and has been shown to often indicate microfractures.    

This months study says quote “sudden and abrupt loading can cause acute injury to one or more of these structures” end quote.  It is important to understand the 3 parts of a disc and how important the discs are in our different daily activities and then to understand that sudden and abrupt loading can damage all of this.  A MVC is the best description of a sudden and abrupt loading situation.  MVC can and do damage discs. 

This study covers what they call 8 types of structural damage to discs.  They are annulus tears, disc prolapse, endplate damage, schmorl nodes, internal disc disruption, disc narrowing, radial disc bulging and vertebral osteophytes.  Some of these types of damage is acute and some of them are degenerative changes unrelated to recent trauma.  Lets go over these 8.

First- annulus tears.  This is a rupture or tear of the annulus fibrosus.  There are 3 types of tears in the annulus.  We have circumferential tears, peripheral rim tears and radial fissures.  Circumferential tears where shown to be due to degenerative changes with stress and compressive forces.  The peripheral rim tears are tears that run parallel to the cartilaginous endplates and are typically acute and related to trauma.  The radial tears are tearing of the annulus into the center of the disc and propel the nucleus pulposus outside of the disc.  The annular tears have been shown to be caused by shear, flexion and compression forces.  These are all forces experienced in MVCs. 

The second type of disc damage they discussed is disc prolapse.  They defined disc prolapse as quote “involves a tear in the annulus fibrosus which allows the nucleus pulposus to bulge out beyond the damaged outer rings” end quote.  This sounds like the definition of a disc herniation.  This would include different types of disc herniations such as extrusions, protrusions, sequestered and fragmented disc herniations.  This type of disc damage is typically new and related to recent events or traumas.  They reported that the annular fibrosus was the primary site for damage with disc herniations and is primarily caused by a peripheral injury.  It was shown that flexion, rotation and compression is the typical mechanism of injury for disc herniations.  These are all forces seen in MVCs. 

The third type of disc damage they discussed is endplate damage.  They showed that endplate damage is seen with microdamage to the trabeculae with compressive forces.  As I’ve covered in prior podcasts this damage to the trabeculae has been shown to be microfractures.  This is seen with type 1 modic changes after an injury and type 2 modic changes months after an injury.   The endplates are highly innervated with the basivertebral nerve and can be an enormous pain generator.  Endplate damage was seen more with osteoporosis. 

The 4th type of disc damage they discussed is schmorl’s nodes.  I did an entire podcast on schmorl’s nodes.  For more information go back and listen to podcast 23.  As a quick refresher schmorl’s nodes are developmental remnants of the notochord.  They are no new fractures or damage to the disc and bone, but are often confused with endplate damage.     

The 5^th type of disc damage they discussed is internal disc disruption.  They defined internal disc disruption as biochemical loss of integrity of the annulus fibrous generally focused on the inner rings.  This is when there is fracturing of the cartilage endplate leading to loss of pressure in the nucleus pulposus and the annulus collapses. 

This study lumped 6-8 into one section.  They are disc narrowing, radial bulging and vertebral osteophytes.  This makes sense since these 3 categories are all degenerative changes.  In prior podcasts we have talked about how discs will lose height overtime following an injury.  Degenerative discs will become circumferential disc bulges.  The disc space narrowing and the disc bulging will alter the mechanics of the vertebrae above and below the disc.  This alteration of mechanics leads to the vertebral osteophytes. 

That covered the 8 types of disc failures according to this study.  They now cover 4 mechanical loadings that lead to these failures.  The 4 types of loading are compression, flexion, axial rotation and complex loading. 

First one is compression.  This has been shown to lead to annulus tears, disc prolapse aka herniations, endplate damage and disc bulging or degenerative discs.  Compressive forces are common in a lot of different types of trauma. 

The second type of loading they discussed is flexion.  Flexion has been shown to greatly increase risk of disc injury.  Flexion is one of the more common forces in the spine during a MVC.  The whiplash or moving backwards and then forwards movements of a MVC rearend collision lead to a lot of flexion and can cause damage to the discs. 

The third type of loading they discussed is axial rotation.  They showed that axial rotator or torsional loading of the discs leads to radial tears in the annulus and allowing the nucleus pulposus to herniate out.   They showed that when axial rotation is combined with flexion and extension this frequently resulted in disc damage.  Rotational forces are often seen with MVCs especially if the collision was not perfectly center or the patient was not sitting in the exact middle of a vehicle. 

The 4th type of loading they discussed is complex loading.  This was shown to be worse with flexion and extension loading combined with compression.  They showed that torsion alone is not responsible for disc injury, but when combined with flexion, extension or compression can cause disc damage.  It is important to understand that most MVCs or other traumas involve multiple types of complex loading to a disc. 

This study then discusses the change in disc homeostasis.  Discs have poor vascularization and receive nutrients through diffusion at the cartilaginous endplates and annulus fibrosus.  This diffusion is done through mechanical movement and with disc damage the homeostasis of a disc is altered.  Inflammatory factors released by damaged disc lead to degrading enzymes.  These enzymes promote growth of nerves which quote “are not normally present in healthy IVDs” end quote.   They went on to show that damaged discs are unable to resist normal motion and have altered biomechanics. This altered mechanical function can overload the spinal ligaments, muscles and joints. 

They concluded with repeating how important discs are with function in daily life.  They showed that discs can be damaged by many different types of loading.  Once a disc is damaged the biochemistry and biomechanics are altered.  This leads to permanent changes in the spine. 

Real world

It is not uncommon that someone will herniate a disc in a MVC.  This study can help with causation.  It is common that the forces of a MVC are compatible with disc damage.  We confirm the disc damage on MRI.  Multiple prior podcasts cover how to show a disc injury is new or old on MRI.  Once we have a disc herniation confirmed by MRI as a new injury and a mechanism of injury that correlates with the injury, we have a solid report.  Often times it is not enough to objectively show a causally related injury, but we need to explain what this type of injury means to a patient.  How will it change their life. 

This study showed how important discs are to daily function.   They said disc are quote “essential in separating and evenly spreading the load between vertebral bodies” end quote.  This study showed disc damage changes the disc’s homeostasis.  Discs in general do not have great healing capabilities given the lack of blood supply.  The healing of a disc will lead to degrading enzymes that not only make the disc healing poor, but promote nerve growth.  During this healing process the discs are now unable to resist normal forces and will have altered biomechanics.  This altered function will overload and alter ligaments, muscles and joints.  Not to mention the disc with increased nerves has the ability to feel pain with normal function. 

It is important to understand a couple things about this permanent injury.  First, the spine will never be the same and has the potential for pain and degeneration throughout the patient’s life.  This can be very unfair to someone who was injured and fair compensation should be given.  Second, disc injuries can be managed.  Most of my patients who come in for regular treatments will have minimal symptoms.  This is an injury that with management patients should be able to function and have a good quality of life.  This type of injury if managed is typically not a disability.  Patient once again with these types of injuries deserve fair compensation for the cost and inconvenience of future treatments.