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.

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.

 

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.

 

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.

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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Dynamic Disc Designs

Physical inactivity relates to many poor musculoskeletal conditions but has not been carefully looked at with regards to disc height loss.

In Arthritis and Research Therapy, a manuscript published May 7, 2015, researchers looked at seventy-two community based volunteer’s lumbar spine during 2011-2012. They used MRI to evaluate their spines after obtaining information of the level of activity by questionnaire between 2005 and 2008. At the time of MRI, they  a chronic pain scale to evaluate low back pain. Intervertebral disc height, muscle area of two spinal muscles and fat content of the multifidus muscle were measured.

The results showed that physical inactivity was related to disc height loss. There was no association seen between the size of the muscles in the spine but there was a relationship to fat within the multifidus.

It is very likely that the disc height loss seen, relates to the static load of the spinal tissues. Previous research has demonstrated static compression shows disc height loss. This was seen in a publication in Effects of static compression with different loading magnitudes and durations on the intervertebral disc: an in vivo rat-tail study.

Interestingly, too much activity, like seen with moderate-intensity running, has also shown significant disc height loss.

While research continues to expose the balance between physical inactivity and activity, too much of either looks to be deleterious to spinal discs. Spine educators have a job to teach patients about the important balance between too much or too little motion. Dynamic Disc models help doctors convey important load examples in the drive to improve spinal outcomes.

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Lumbar Disc Research - Intervertebral Disc

Lumbar Disc Assessment of Bedrest using MRI

Lumbar disc assessment of glycoaminoglycan content by MRI was investigated using gadolinium to look at the changes that took place after 21 days of bedrest. This research published in PLOS One investigated the effects of recumbancy on the lumbar discs to get a better understanding of whether the discs imbibe water or increase proteoglycan content with unloading. Spaceflight has effects of disc swelling and associated back pain.

Using bedrest as an analog to spaceflight, five volunteers, healthy, non-smoking males were subjected to 24hr bed rest with a 6 degree head down tilt for 21 days. Magnetic Resonance Imaging (MRI) scans were taken according to the delayed gadolinium-enhanced magnetic resonance imaging (dGEMRIC) protocol before and after bedrest.

Results showed significant changes in T1 which indicated that insufficient sleep can be a risk factor for low back pain. These researchers concluded that sufficient recumbency (unloading, decompression) is necessary for the intervertebral discs to recover from mechanical load and strain of daily activities. Interestingly, they not only showed how water increases to the discs with bedrest but also glycosaminoglycan content.

Clinical Tips

Clinically, patients often complain of symptoms related to sleep or lack thereof. They also complain of back pain related to the diurnal timing of load and respective disc height loss. That is, some complain of symptoms early in the morning after a period of recumbency (indicative if a degenerative disc) and others complain of symptoms later in their day, likely due to the progressive and hydraulic disc height loss of vertebral approximation irritating intradiscal tissue and/or facets with the associated articular capsule/synovial folds.

A careful dissection of the timing of patients symptoms will help point the investigator to the anatomy in question. Our models can assist in the patient education of the diurnal timing of symptoms and the importance of bedrest and unloading of interverterbal discs.

Explaining the timing of pain onset helps in the management of back pain. When patients understand that symptoms are related to hydraulic height loss, lying down or taking a nap mid-day may be a viable therapeutic strategy to increase disc height.