properties of the annulus, disc model

Researchers examined the effects of endplate fractures  1 on the mechanical properties of the annulus fibrosis (AF) in porcine spinal segments and found that laminate adhesion strength was significantly compromised in the fractured spines. The findings suggest that microdamage may occur beyond the vertebra, into the interlamellar matrix of the AF—information that could be helpful in the diagnosis and treatment of adolescent spinal growth-plate fractures.

The Study

The authors of this study wished to examine the effects of high-intensity pressurization on the intervertebral discs (IVD) to see how it effected the mechanical and physiological properties of the posterior AF. They used 28 fresh, recently-thawed functional porcine spinal units from 14 porcine specimens that were approximately six months old.  Control units were also used as a comparative measure against the units subjected to pressure.

A hydraulic pump and high-pressure inflation needle were used to pump hydraulic fluid into the IVD of specimens. The researchers were careful not to pierce the AF in the samples. Pressure in the needle was measured by a pressure transducer and converted from analogue to digital at 2048 Hz. The needle was subsequently removed, and the vertebral bodies were assessed for damage. Although fractured endplates created an audible ‘pop,’ the condition was only confirmed after dissection of the IVD. The control-group segments were not tested for fractures. Measurements were taken following the dissection, and the end-plate area was quantified. Bilayer AF samples were then dissected and tested for tensile endurance in the circumferential direction. A second multi-layered sample was then dissected and subjected to delamination and a peel test. Mathematical ratios were then plotted to mark the variable results for each sample.

Results

End-plate size measurements remained consistent across the control and fracture group samples. Bilayer stiffness, toe-region stretch ratio and stress, and stress at 30% stretch were consistent in the control and fracture group samples. However, there was a clinically-significant variance in peel strength—but not peel strength variability— between the two groups. In the fracture group, the peel strength was 31 percent lower than in the control group. Dissection and manual delamination were significantly easier in the fracture group of samples, as well.

Discussion

The results of this study indicate that growth-plate fracture damage may not be limited to the vertebra and may cause microdamage in the nearby AF. This was indicated by the reduction of laminate adhesion strength in the posterior AF of the fracture IVD samples subjected to pressure in the tests. This information should be taken into account when practitioners are examining and treating adolescent or childhood vertebral fractures involving the endplates.

 

KEYWORDS: damage during spinal growth-plate fractures, effects of endplate fractures on the mechanical properties of the annulus fibrosis, effects of high-intensity pressurization on the intervertebral discs, mechanical and physiological properties of the posterior AF, delamination and a peel test, Bilayer stiffness, toe-region stretch ratio and stress

 

arthritic changes, lumbar models, cervical models

Arthritic changes are very common. They are often related to a person’s pain with neck pain as one of the highest ranked common causes of disability. In this specific research article 1, the authors looked at the micro-details of neck synovial joints. With osteoarthritis known to be related to neck pain, they were looking to reveal higher anatomical detail and they were also curious about whether men or women have more of these problems.

With both neck and back pain being multifactorial (which may include both psychological and social aspects) degenerative changes within the synovial joints play a significant structural role with the development of spondylosis. This is a general term to describe a disorder of the musculoskeletal system with an emphasis on joint space narrowing, intervertebral disc height loss and frequent formation of bony spurs.

The architecture of the cervical facet joints is quite well known with most of the current knowledge around the smooth (or lack of smoothness) hyaline cartilage to allow the joint to receive and distribute loads in an efficient manner. However, there has not been much quantitative data revealing the anatomy under the hyaline cartilage designated as the subchondral bone. This bone under the cartilage (sub, meaning below and chondral, meaning cartilage) has been of recent interest as there exist nerves in this area that can cause pain. This is thought to be similar to the basivertebral nerve of the vertebral body. The innervation of the facet, however, has ascending fibres travelling through the posterior primary division which can be seen in this Medial Branch Dynamic Disc Model.

 

modeling hyaline cartilage, models

Hyaline Cartilage Modeling in our Professional and Academic LxH Dynamic Disc Models

basivertebral nerve lumbar model

Basivertebral nerve of a lumbar vertebra.

Previous research has shown that the thickness of the hyaline cartilage is .4mm in women and .5mm in men with the subchondral bone making up approximately 5% of the total cartilage thickness. It is also known that with increasing age the cartilage starts to flake off (called fibrillation) and researchers also coin the stripping of cartilage from the bone, denudation. This means being nude. A joint surface within a covering. Other terms used to describe the break down of the hyaline cartilage is erosion, fissuring and deformation. All in all, the terminology all mean that the hyaline is thinning.

arthritic changes, subchondral, joint, model

Subchondral thickening – arthritic changes

How did they do it?

These researchers looked at 72 recently deceased people and examined their joints. They used microscopes to look closely at the facet joints to help understand the pathogenesis of the arthritic changes.

When they observed the osteocartilaginous junction, the morphological changes included: flaking, splitting, eburnation, fissuring, blood vessel invasion and osteophytes. They looked at the length of the cartilage, the hyaline cartilage thickness, the calcified cartilage thickness and the subchondral bone thickness.

They found that males tended to have more severe degenerative changes described by flaking and severe fissures in the facet cartilage. Click To Tweet

Points of Key Interest

  • this was a study that looked at 1132 unique cervical spine facets from 72 humans
  • males were found to have more degenerative changes of the osteocartilaginous junction
  • the thickness of the calcified cartilage and subchondral bone increased with age whereas the hyaline cartilage decreased
  • the osteocartilaginous junction is particularly important in the pathogenesis of osteoarthritis in the cervical spine facet joints

 

At Dynamic Disc Designs, we work to bring research to the practitioner so when there is a teaching moment, Professionals are ready to explain pain triggers as they relate to a patients symptoms and movements. Empowering people about their own anatomy helps in the crafting of customized treatment plans for each unique pain patient. Explore our dynamic models and help a patient understand their arthritic changes and what that means to them.

Dr. Jerome Fryer (CEO of Dynamic Disc Designs Corp):

“Hello everyone. Dr. Jerome Fryer here of Dynamic Disc Designs. I just want to reach out to those customers that have one of my models. There’s been a lot of talk lately on social media regarding how models can be scary. I don’t know how they’re scary. Models are not scary. It really depends on the user and these models are not intended to scare anybody. It’s to teach them their own anatomy, so they can improve their posture and biomechanics to relieve their symptoms. It’s a team player. It’s like a car. You can go out there ram into people or you can drive defensively and respectfully. Anyway, so one thing that’s important when you’re using the model is to relay realistic biomechanics  and use the model in a way that simulates real-time and load.

You want to use it in a way that actually represents the actual tissue. You can talk about all sorts of things, but you can talk about disc height changes as the disc over the course of the day loses a percentage of its height. You can talk about normal loading patterns of the disc as it relates the associated nerves. But, what I would encourage is just to use real-time forces. For example if someone goes to sit down, they change their lumbar angle and they compress their disc. When they sit for a period of time, the disc actually loses further height. You want to show the subtle endplate angle changes as it relates to the facet joint for example, or in the suspected case of disc herniation, you can actually create a disc herniation.

Single-Level Disc Herniation

Model of Single-Level Disc Herniation.

One example is the changing fluid expression over the course of the day. This is an important little graph to help patients understand how first thing in the morning you’ll actually lose their height very quickly in the disc height, so the facets will actually approximate with the changing intradiscal pressure, and then over the course of the day the disc height will slowly reduce. Some people talk about around 4:00 or 5:00 in the evening as the day progresses, my symptoms become pronounced. Then also with first lie down too. You can see there’s a quick change in disc height. Anyways, I just wanted to share with you that it’s how you use the model and you want to use it in ways that are realistic with regards to movement.”

 

 

 

spine pain, models

Ed Cambridge: “Our colleague Jerome Fryer created some models for us, and this is some of the work that has come out of our lab with you and Christian Balkovec about the dynamic changes we see after herniation. Where we have disc height loss at one level, creating hypermobility at the adjacent level. So here you can see, when you move the spine around there is a stiffening effect down in the lower joint and in the upper joint hypermobility. That’s what we see when an injury propagates from one joint to the next. The patient says, “Well, the pain used to be lower but now its starting to creep up my back a little bit.” “

Stuart McGill: “Fabulous. Another little take on that … By the way, these are all cast from real human specimens. So this is the real deal. Once again, Dynamic Disc Designs has been so clever in representing the biofidelity. We start to see how this disc has been damaged, and it’s quite lax as we move it around. So those micro-movements now are triggering pain just at that level. And this joint has normal stiffness, but then look what happens. Over time, the join changes because of the change in mechanics. The lax disc now cases a bit more arthritis in those facet joints, because they are now responsible for much more motion. So then, look what happens to the cascade. As the person now extends, look what happens. The joint that was hypermobile has now bound up, has no mobility because the facets have bound up and all the motion is now left at the previously stiffened joint. The polar opposite. And then you need some kind of mobility to pop those facet joints open again after they’ve been jammed.”

inflammatory mediators

The changing spine and the anatomy. Professional LxH Dynamic Disc Model

Stuart McGill:  “So, when you understand the cascade of change that happens at a joint, it might be kicked off with a little bit of a flattened disc, which puts more load in the facet joints, which causes a little bit of arthritic growth. In two years, the joint has changed and so have the pain patterns and the mechanics. So, it really does lend insight to allow us to understand the cascade of how the patient reports those changes and their pain changes over the years. And it better allows us to show them what to do to wind down the pain sensitivity. “

 

intervertebral disc degeneration, model

Mechanobiology Research

Low back pain is a huge burden on our limited resources with limited knowledge of its pathophysiology. It is widely known that intervertebral disc degeneration (IDD) is intimately related, with the degree of degeneration associated with the severity of low back pain. The characteristics of intervertebral disc degeneration include disc height loss, proteoglycan loss, loss of water, annular fissures, and end plate calcification.

The degenerative process of the intervertebral disc has been seen as a phenotype change within the cells. This anabolic to catabolic shift seems to occur to the cells deep within the disc. One branch of research that studies the influence of mechanical forces on the biology is called Mechanobiology. In other words, can physical stressors on discs influence the process of degeneration? Can moving the disc is a certain way change the outcome of degeneration?



The Study

In this open access study, researchers were the first to investigate this kind of cyclical mechanical tension on the nucleus pulposus cell’s changing behaviour.  They extracted disc cells from caudal spines of (3-month-old) male Sprague-Dawley rats and conducted the mechanical testing using a device after the cells were cultured and prepared. They used this device to apply mechanical force on the cells of the nucleus pulposus (the centre of the disc) to see how the cells behaved under specific loading conditions.

Disc cell senescence involves telomere shortening,  free radical stress, DNA breakdown and cytokine proliferation. Mechanical loading conditions in the upright posture have been found to promote disc cell changes towards intervertebral disc degeneration in rats.  Studying the role of mechanical stress and the influence on disc health will benefit our understanding of disc pathogenesis. 

The results of this study showed a direct relationship of prolonged mechanical cyclic stress towards the catabolic shift of the cells in the nucleus pulposus. They concluded that unphysiological mechanical stress could push a disc into the degenerative cascade. They believe that eventually, too much mechanical stress can influence a cell’s behaviour and suggested that research continue searching the optimal mechanical environment for intervertebral disc cells.

At Dynamic Disc Designs, we work to bring dynamic models to the practitioner to help in the discussions related to motion and the spine.

 



Facet Joints, GAG, Annulus Fibrosus, Torsion

A recent study evaluated the role of facet joints in torsion using four different compressive preload conditions in healthy and degenerated lumbar discs—with, and without facet joints. The study also sought to develop a quantitative relationship between structure and function in tissue and torsion mechanics. The study found that annulus fibrosis GAG content substantially affects the mechanics of disc torsion.

Purpose of the Study

Because there is a large population of lower back pain (LBP) sufferers whose jobs involve excessive loading and rotating the lumbar spine, the authors of this study sought to quantify and understand how the facet joints in healthy and degenerated discs would behave under axial rotation scenarios. They did this by observing in vivo changes in spinal segments during torsional behavior. The intervertebral disc (IVD) is capable of stability and flexibility during most movement, receiving stresses and sharing them with the nearby facet joints and other surrounding structures. The facet joints should protect the disc from overload and degeneration by restricting motions that would cause damage to the spine, but some complex motions that involve axial rotation and bending during heavy loading can increase the chance of micro-damage and disc failure. How well the IVD and facet joints share loads is determined by the mode of loading and posture. Previous studies have demonstrated that up to 25 percent of axial compressive forces may be supported by the facet joints. Between 40 to 65 percent of healthy disc joint rotational and shear forces are also supported by the facet joints. Therefore, it is important to understand how the facet joints in healthy and degenerated discs react during torsion.

Study Design

Researchers obtained and imaged seven human cadaveric lumbar spine segments aged 43 to 80 years-old. The musculature and ligaments were then removed, and the intact facet joints near the discs were subdivided mid-vertebrae prior to the samples being potted in bone cement. The segments were then wrapped in gauze and stored in a phosphate solution until brought to room temperature just before testing. They were then mounted onto a testing machine and secured with screws.

The segments underwent a moderate-to-low preloaded axial compression, followed by axial rotation through the center of the disc. The cycles of compression and rotation were performed for two hours to allow the formation of creep. Ten cycles of cyclic rotation, and the samples were tested under four axial compressive preloads and allowed to recover between each test. The facet joints were then removed, and the samples were tested again, using the same loading configuration. For each round of testing, the researchers recorded the levels of force, rotation angle, displacement, and torque.

Isolating and Imaging Each Disc

Each disc was isolated and imaged after mechanical testing. Researchers measured the disc area, anterior-posterior and lateral width using a custom algorithm. Disc height was measured from the posterior, anterior, left, and right lateral sides, as well as the center. A mathematic formula determined the applied axial stress, and the images were graded and compared with radiographic-based grades.

Conclusion

The results of the tests indicated a strong correlation between creep and axial compressive preload and the loss of disc height. Removing the facet joint had no effect on this phenomenon. The presence of facet joints and an axial compressive preload did have a strong effect on torsional mechanical properties, with torsional stiffness and range decreased 50 to 60 percent for compressive loads after removing the facet joints. Energy absorption decreased about 70 percent during rotation after facetectomy, and disc-joint strain increased 74 percent, compared to only 62 percent in disc strain energy using the same axial compression.

Annulus Fibrosis GAG content in degenerated discs greatly reduced torsion mechanics, while the facet joints are integral in keeping the spine from rotating too far and helping to reduce shear stress and damage to the disc. The relationship between the biochemical-mechanical and compression-torsion levels noted in this study may help to provide for more effective and targeted biological repair methods for degenerating discs of various levels.

 

KEYWORDS: AF GAG Content Alters the Mechanics of Disc Torsion, role of facet joints in torsion, axial rotation scenarios, correlation between creep and axial compressive preload and the loss of disc height, targeted biological repair methods for degenerating discs

Instability

A study examined the relationship between lumbar disc degeneration and instability in spinal segments of three groups of volunteers and found that factors of spinal instability were closely related to disc height and the age of the study subjects and that disc height was intimately associated with age and spinal instability and was the most consistently affiliated parameter of those examined.

Patients with lower back pain (LBP) and/or sciatica often have evident disc degeneration in MRI their images, especially elderly patients. Because these patients may demonstrate no other neurological symptoms, it is commonly assumed specific evidence of LBP –aside from degeneration and the age of the patient—may not exist. Excessive motion surrounding the affected disc segment can cause LBP and spinal instability, and previous studies on the relationship between instability and LBP have been inconsistent in their findings—in part, because imaging of the subjects was performed while the patients were in the static supine position.

Study Design Utilized Flexion-Extension Standing Postured Imaging Reviews

The authors of the current study were building upon their previous research utilizing images that had been performed on patients during flexion-extension standing postures to examine the relationship between spinal instability and disc degeneration of the L4/L5 motion segment. Because disc degeneration may not be associated with LBP at all stages, the authors of the study devised a method of measurement to examine different types of segmental degeneration and any relationship it may have with spinal instability.

The subjects of the study were LBP or leg pain outpatients who had received radiologic and MRI imaging within a two-month interval during the past three years. Of the 447 patients included in the study, 268 were men, and 179 were women. Their ages ranged from 10 to 86 years, with an average age of roughly 54 years-old.

Instability was measured at the L4/L5 spinal segments during neutral, extension, and flexion postured images and was then analyzed and categorized into three variable types: Anterior slip at L4 onto L5 while in neutral position (SN), sagittal translation (ST), and segmental angulation (SA). Measurements were taken of each slip, and the results were evaluated and noted to determine the degree of apparent instability.

The disc segments were evaluated radiologically for degeneration by looking at and comparing disc height, spur formation of the anterior vertebral edges, endplate sclerosis, and evidence of vacuum phenomenon in the films taken during flexion-extension. Sixty-eight of the subjects had high disc height (HDH), 212 patients were considered to have medium disc height (MDH), and 67 patients were categorized as having low disc height (LDH). Bony spur measurements were taken, and the presence of endplate sclerosis and vacuum phenomenon were noted as either being present or not. The level of disc degeneration was evaluated by MRI and graded from 1 to 5, as “normal,” to “severe” degeneration. The patients were divided into eight groups based upon the severity of their spinal instability, and the relationship between disc height, spur size, endplate sclerosis, vacuum phenomenon, and degeneration in the MRI’s was noted in relation to the types of instability present.

The compared data indicated a link between instability, age, and a reduction in disc height. Though increased age and a loss of disc height have long been suspected to be linked to degeneration and instability of the spine, this study uses MRI to evaluate that relationship more closely, demonstrating that a lower disc height was associated at least a 3mm slippage and a higher disc height was associated with subjects who were younger in age, with larger angulation in the spinal segments. Instability was prevalent in older patients with prominent anterior spur formation and/or vacuum phenomenon.

Age and relative spinal stability were intimately related to disc height, and this instability was progressive in nature and occurred over decades.

 

KEYWORDS: Correlation Between LBP, Age-related Degeneration, and Spinal Instability, relationship between lumbar disc degeneration and instability, comparing disc height, spur formation of the anterior vertebral edges, endplate sclerosis, and evidence of vacuum phenomenon, link between instability, age, and a reduction in disc height, degeneration and instability of the spine