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Increasing Gradients of Compressive Stress Can Lead to Annular Delamination, Collapse, and IVD Degeneration

delamination, annulus

A ISSLS Prize-winning study 1 examined how increasing gradients of compressive stress within the intervertebral disc (IVD) contributed to the progress of dis degeneration. The research findings suggest that an increased grade of disc degeneration created decreased nucleus pressure and compressive annulus stress, but anterior annular stress gradients increased by approximately 75 percent, and by 108 percent in the posterior annulus—findings that are clinically significant.

The neural arch may provide a stress-shield for the degenerating disc during mechanical loading, but delamination and collapse of the annulus are most likely caused not by loading, but by increasing gradients of compressive stress, leading to advanced disc degeneration, despite the stress-shield.

The Study

Using 191 motion segments from 42 cadavers of varied ages, researchers measured the intradiscal stresses under 1 kN of compression. A pressure transducer was pulled along the midsagittal diameter of the disc to measure the intradiscal stresses. Stress gradients in the annulus were quantified using a formula that averaged the rate of increase in compressive stress between the area of maximum stress in the anterior or posterior annuls, and the nucleus. Measurements were compared before and after applied creep-loading, as well as in flexed or erect postures. A scale of 1to 4 was used to describe the amount of macroscopic disc degeneration observed.

 

Results

An increase of disc degeneration from 2 to 4 decreased by 68 percent the amount of pressure in the nucleus, and compressive stress in the annulus was decreased by 48-64 percent, depending on the simulated posture of the segment and the location of the disc. However, anterior annular stress gradients showed an average 75 percent increase in the flexion position, and posterior annular stress gradients increased 108 percent in upright posture.

 

Conclusion

The neural-arch provides stress-shielding, but compressive stress gradients are significantly increased with an increasing grade of disc degeneration. Adjacent lamellae are sheared by the stress gradients, which may contribute to the delamination and collapse of the annulus.

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Disc hydration – Unloading / Loading during the act of sitting.

posture, disc hydration

Disc hydration fluctuates naturally and diurnally. That is, over the course of the day/night cycle we (as humans) lose up to 20% of the water out of our spinal discs. 1 The intervertebral discs are sensitive to load and because of their visceo-elastic make-up they will deform under load. Most notable changes seem to occur under sustained or static loads. 2 3 4 5 6 Therefore, it is important to offload the spine, especially when one sits for an extended period of time.

Recently, a study published in the Lancet 7 looked at the 188 countries and followed them between 1990-2013 and revealed that the number one reason for disability was back pain. Yes, back pain! Not heart disease. Could we extract from this that it is perhaps the introduction of computers and more time sitting? There could be other factors but there little doubt that the human population is moving less and fixated in front of a computer….just like myself at the moment.

Lumbar Disc Changes Associated with Prolonged Sitting

Take a Break and Off-load

This 8 off-loading strategy is thought to relieve the compressive forces of the spine to allow it to refill slightly….interupting sustained compressive loads, which we know is harmful.

Interestingly, a paper published in the Journal of Human Evolution in 2000 9 looked at knuckle walkers and ‘compared to humans, all ape samples show dramatically less spinal disease, especially when considerng vertebral body involvement’ . The authors concluded that this significant difference was likely due to the gait mechanism. And obviously, they use their upper extremities to off-load their spines during the course of their gait cycle.

Therefore, it looks like if you behave more like an ape and use your upper extremities, your spine will benefit. Teach your patients to minimize compressive loads by integrating off-loading strategies in their day to decrease the creep and compressive responses in the spine…..keeping the discs hydrated to prevent disc height loss.

 

 

  1.  Urban,J.P., McMullin,J.F., 1988. Swelling pressure of the lumbar intervertebral discs: influence of age,spinal level, composition,and degeneration. Spine 13, 179–187. ↩
  2.  Adams, M.A., Hutton,W.C., 1983. The effect of posture ont he fluid content of lumbar intervertebral discs. Spine (Philadelphia1976) 8, 665–671. ↩
  3.  Kazarian, L.E., 1975. Creep characteristics of the human spinal column. Orthop. Clin. N. Am. 6, 3–18. ↩
  4.  Keller,T.S., Spengler,D.M., Hansson,T.H. ,1987. Mechanical behavior of the human lumbar spine. Creep analysis during static compressive loading. J.Orthop.Res. 5, 467–478. ↩
  5.  Koeller,W., Funke,F., Hartmann,F., 1984a. Biomechanical behavior of human intervertebral discs subjected to long lasting axial loading. Biorheology 21, 675–686. ↩
  6.  Markolf, K.L.,1972. Deformation of the thoracolumbar intervertebral joints in response to external loads: a biomechanical study using autopsy material.J.Bone Jt. Surg.Am. 54,511–533. ↩
  7. Lancet. 2015 Aug 22; 386(9995): 743–800.  ↩
  8.  Fryer JC1, Quon JA, Smith FW. Magnetic resonance imaging and stadiometric assessment of the lumbar discs after sitting and chair-care decompression exercise: a pilot study. Spine J. 2010 Apr;10(4):297-305. ↩
  9. Jurmain, R Degenerative joint disease in African great apes: an evolutionary perspective. Journal of Human Evolution (2000) 39, 185–203 ↩
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Disc height loss under constant compression – time-dependent response.

The intervertebral disc is home to visceoelastic material than deforms under constant load. Its role is to keep vertebrae separated to avoid the bones to approximate one another. Many researchers have seen how the disc gets squashed more under a constant and sustained load. 1 2 3 4 5

compression of spine with prolonged sitting

disc height loss and sitting – spine education with dynamic model

Sustained load, as seen in sitting patients with low back pain, is a common clinical complaint that is difficult to describe for the attending doctor without an appropriate model. Why a patient experiences low back pain as a result of not doing anything can be difficult for the patient to understand but using a visceoelastic model that deforms under compression can be helpful in the compliance of improving posture strategies or in the simple drive to get up out of the chair.

Dynamic Disc Designs has just made life a lot easier for doctors to explain this compression phenomenon. Using only the finest materials, these patient education models for spine can be easily accessed to demonstrate how compression of sitting will reduce disc height loss–sending a clear message that the patient should do something to move and help recover the disc hydration.

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Diurnal Disc and Symptoms

Diurnal Disc - Dynamic Disc Designs

Back pain can be tricky to figure out but the diurnal disc can elude to the root of the issue if doctors are paying close attention to the history and associated relieving body position.

Spinal symptoms are often diurnal. In other words, patients can often explain the onset of their symptoms based on the time of day or night. A common complaint is stiffness in the morning or after a period of rest while lying down. Early morning stiffness has been thought to be a telltale sign of a degenerative disc. The behaviour of annular fissuring and wedging with the combination of early morning accelerated disc height loss, results in the symptoms of stiffness. To see this you can view the movement of the nucleus in any of our clear bones models.

Patients also complain that back and/or leg symptoms come on later on in the day and into the evening. The diurnal disc can lose up to 20% of its hydraulic height. We do know also, that with height loss, the facets slide and shingle into one another.

height loss, disc, core

Height changes of the intervertebral discs over 24 hrs

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You can see well within the contents of the video above, how the disc height can cause a shingling effect of the facets. Facets are pain generators, as can be seen in our medial branch model.

Generally, and I do mean generally, symptoms that come on later in the day are more likely to be from the facet joints. This can be further supported if the patient explains that sitting relieves their symptoms. A further possibility would be stenosis if the patient explains that sitting relieves leg symptoms. We know that sitting both opens the facet joints and the spinal canal of the lumbar spine.  This can be seen in many of of our spine models including our Stenosis Degenerated Model.

 

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Dynamic Disc Research

Dynamic Disc Model

This week, in Spine Journal, an ISSLS prize winning study was published on the dynamics of the disc.

What these researchers looked at was how nutrient uptake differed when the disc was exposed to:

  1. stationary position
  2. a high-rate, low frequency force and
  3. a low-rate, low frequency force

Their objective was to quantify the effects of mechanical loading rate on disc fluid movement into healthy and degenerative discs in vivo.

This research looked to measure what kind of forces could possibly draw more fluid into the discs. Conventional thought believed that ‘pumping’ of the disc did not have an influence on nutrient transport into discs as some previous work on diffusion rates into discs did not show that it mattered whether discs were moving or not. (Jill Urban) But now, new researchers showed that it is possible to influence nutrient flow into these avascular discs and best accomplished in a slow moving and constant way.

What they found was quite profound but makes some common sense. They observed that discs responded best to low-rate, low frequency forces. They used New Zealand white rabbits to test using an oscillative force equal to 0-200N (0-44lbs) over a 2 second period–similar to intermittent traction. Interestingly, they also found that the degenerated discs took up the fluid quicker than normal discs.

This research should have a significant influence on how we understand spine. It will influence exercise programs and also guide strategies in manual therapies like mobilization and decompression techniques.

Dynamic Disc Designs develops dynamic disc models to help with patient education and showcase the dynamics of the intervertebral discs in the management and treatment of spinal related disorders.

 

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Disc Loading – The Poroelastic Behaviour

Disc Loading - Dynamic Disc Designs

The intervertebral disc is a unique structure and has a role in disc loading through its characteristics of poroelasticity.

It is the water-binding capacity of the nucleus pulposus that allows the disc to release and absorb water when disc loading and unloading occurs, respectively.

The intervertebral disc allows flexibility of the spinal column while having a fundamental role of vertebral spacing. The discs primarily resist axial loads during daily activities to be recovered in height during sleep and recumbency.

When discs degenerate, it is thought they lose their elasticity and the interstitial flow.

In a recent (Full Text) research article in Cells and Materials, Emanuel et al. looked to answer how a degenerated disc behaves differently in poroelasticity during disc loading.

These authors used 36 lumbar discs over ten days of disc loading. They categorized the discs into degeneration categories using Pfirrmann Score (PS) by way of MRI imaging.

What they found was degenerated disc have less binding capacity and respective poroelasticity when compared to normal discs. Degenerated discs lose more disc height when loaded and cause the vertebrae to approximate one another.

Disc-Loading

Dynamic Disc Designs highlights important research in the better understanding of dynamic disc concepts for effective patient education.

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Porosity of vertebral endplate

Lumbar Model - Endplate Porosity

Porosity and thickness of the vertebral endplate depend on local mechanical loading.

In a recent publication in Spine, a group of researchers looked at the porosity and thickness of the vertebral endplate comparing mechanical stress from adjacent vertebrae to disc degeneration.

Much research has been uncovered regarding what contributes to back pain.  Some of the factors were reminded to us in the wonderful introduction to this research paper. Nutrient flow into the discs continue to be one of the hot topics in the cause and prevention of disc degeneration. Vertebral endplates are also important to understand in the realm of back pain because they are innervated by the basivertebral nerve. And modic changes are also known to cause back pain.

The endplate is a .8mm thick layer of hyaline cartilage backed by weakly bonded cortical bone which sandwiches the annulus and nucleus of the intervertebral disc. It is thinnest at the central region at the interface of the nucleus pulposus of the intervertebral disc and thickens to the periphery where the annulus resides. The perforations that contributes to the porosity are mainly at the interface where the nucleus exists. Regarding porosity, the hyaline cartilage is less permeable than cortical bone and therefore plays an important role in generating and maintaining intradiscal pressure to resist compression.  But at the same time, the permeability is important for nutrient supply to the disc.

These authors wanted to figure out the role of the porosity of the endplate and whether it is related to thickness or degeneration of the disc.

What the authors concluded was that porosity of the endplates were inversely proportional to the thickness being greatest in the central region. They also found that porosity increased with degeneration but not with age.

What they did interpret from the results was that certain areas of the endplate become thin because of reduced load and as well become more porous primarily because they become thin.

Clinically, this study is important because they showed that the nucleus pressure reduces as intervertebral disc degeneration progresses. This also has implications into vertebral fracture due to bone loss and increased porosity.

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