vacuum sign

Vacuum sign is a common radiological finding. It is also referred to as a vacuum phenomenon and often associated with degenerative spinal discs, knee joints, hip joints, and shoulder joints.  Degenerative spondylolisthesis is a spinal condition whereby one vertebra slips on another. However, it is sometimes difficult to ascertain whether this slippage is stable or not. A more than 4mm movement defines instability, but some have indicated only 2mm as clinically significant.

degenerative spondylolisthesis model

Degenerative Spondylolisthesis Dynamic Disc Model

In a research paper published in World Surgery, 1 a group of authors looked at the vacuum sign in the facet joint as an indication of degenerative instability of the spine. They wanted to investigate the relationship between the vacuum facet phenomenon and lumbar instability. Why, you may ask? More and more research is directing spine researchers to the cause of pain and disability to the imbalance of motion of the individual vertebral segments of the spine. Some have coined this motion sharing.

Each vertebral motion segment consists of two vertebrae, and a disc should have a certain stiffness level. That is, it should move similar to its adjacent segment above and below in the spinal column. For this study, they looked at L4 on L5 (which is a prevalent spinal level to degenerate with age) and used flexion/ extension X-rays in both the fully bent forward (flexion) and the fully bending backwards (extension) with degenerative spondylolisthesis. Additionally, when available, they also looked at CAT scans of these same patients. To determine the slippage degree, they used a dynamic motion index to measure the degree of slippage.

In a total of 67 patients examined, 35 patients had vacuum signs on their CAT scan, and 32 patients did not. The degree of slippage appeared to correlate with the vacuum sign as well. That is, the more the vertebrae had slipped forward, the more likelihood of the presence of the vacuum sign. With this, the authors concluded a linear correlation between the degree of slippage and the presence of vacuum sign.

Commentary by Jerome Fryer

Vacuum sign or vacuum phenomenon is often considered an incidental finding. However, based on the modelling research I’ve done, I believe that the vacuum sign can be a clue into joint mechanics’ stiffness. In 2017 I published an article related to the cracking event we are familiar with, and in there, I believe in having revealed the vacuum phenomenon. In the presence of cavitation, a joint will have less stiffness, and in time I hope we can collectively use these radiographic findings to help us determine which joint requires more stability in the treatment of them. JF

Intervertebral disk

Intervertebral Disk Degeneration – Fibrotic Remodelling, Time and Entropy

Lower back pain is an issue on this planet. It affects 632 million people worldwide and plagues 70-80% of US individuals at some point in their lives. The good news is that it is often self-resolving, but repeated bouts’ of recurrence are very frequent. The most popular low back pain diagnosis is intervertebral disk degeneration, with specific structural disruptions often contributing to pain. Therefore it is only logical for researchers to look at this structure to learn more about how the structure contributes to back pain.

We have known for years that the intervertebral disk is composed of two main structural components. The annulus fibrosus, which is the tough criss-cross oriented outer layer and the nucleus pulposus, which is an inner, softer gelatinous structure that holds water to help with the resistance against compression forces. To dive a little deeper, the annulus fibrosus consists of type 1 collagen, and the nucleus pulposus consists of type 2 collagen. If you want to explore the microstructure, you can visit Eyre et al.

The key structural element of the intervertebral disk looks to be the extra-cellular matrix. This is the ‘stuff’ between cells. The scaffolding, so to speak. The infrastructure within that holds things in place—or at least, is supposed to hold things in place.

In highlighting this research, James Iatradis, a well-known spine researcher, assembled a team to look at the intervertebral disk’s microstructure and look closely at what could be going on. In a Special Issue Article in the Journal Spine 1, his team used a special technique, named second harmonic generation imaging (SHG), to quantify the collagen content and structural make-up of intervertebral disk degeneration.

The concept of entropy, which is a physics term to describe order, or lack thereof, helped the researchers describe the disorganization of the degenerated tissue compared to the disk’s non-degenerated tissue. They found the tendency of the degenerated disk to be less organized with an increasing intensity seen with the imaging technique used. In other words, more chaos or entropy signalled disorder of the tissue. Collagen, in its healthy form, is organized. These researchers found the degenerated disks contained less microstructural organization and fibrotic remodelling.

This is an important paper. It demonstrates that tissues change over time. And with structural changes, there must be a biomechanical change. At Dynamic Disc Designs, we understand that the spine is in constant change and with that, we must learn and adapt as we age. Explore our wide-ranging dynamic models with varying properties to help with the education of the spine.


The Dynamic Sitting Exercise (DSE)

Life can be busy. And with this busyness, people often do not have the time for lower back pain exercises. In 2010, Jerome Fryer (the owner and developer of Dynamic Disc Designs Corp.) set out to measure a simple seated decompression strategy for the lumbar spine. A pilot study used an upright MRI to investigate changes in the lumbar spine before and after this Dynamic Sitting Exercise (DSE) 1

It was originally coined “chair-care decompression exercise” to make it memorable. In a recent article written in the Indian Journal of Physiotherapy and Occupational Therapy, the researchers renamed it DSE and compared it to the popular McKenzie prone press-up. 2


Dynamic Sitting Exercise

Dynamic Sitting Exercise (DSE)


McKenzie Prone Press-up

McKenzie Prone Press-up

These researchers recruited thirty adults in the age range of 20-30 years with mechanical low back pain. To read the full inclusion and exclusion criteria, you can visit the full-text link in the references below. They randomly assigned the participants to two groups: the DSE group or the McKenzie prone press-up group. Each subject conducted 6 repetitions within the 5-minute interval with the exercises being conducted at the beginning of the 5 minutes, followed by 4 minutes of rest. Over the course of 30 minutes, each participant would have performed 6 repetitions multiplied by 5 sets for a total of 30 repetitions over the course of 30 minutes. This was equivalent to 2.5 minutes of active exercise over the course of 30 minutes.

Exercise Protocol

Exercise Protocol


The DSE instructions included:

  1. sit upright
  2. place hands-on seat pan
  3. push down on the seat pan to offload the lower spine
  4. participants should feel a slight stretching in the lower back
  5. hold for 5 seconds
  6. return to neutral for 5 seconds
  7. while returning to neutral, draw-in-abdomen.


The McKenzie Prone Press-Up instructions included:

  1. lie down on the abdomen
  2. extend back while on elbows and palms down (neutral position)
  3. perform press-up maneuver with straight arms for 5 seconds
  4. return to neutral for 5 seconds


Over 6 weeks, outcome measures included the Visual Analog Scale for pain and the Short Form-36 Health Survey Questionaire for quality of life.

What did they conclude?

Both forms of exercise demonstrated improvement of pain and quality of life. However, the DSE outperformed the McKenzie Prone Press-up in this group of mechanical low back pain adults.

Overall, this paper could have been written a little better. Their conclusions were bold and overly confident. It is still an important paper to share as the practicality of investing a few seconds of offloading to your work-day while sitting looks to be promising in those with lower back pain in this age range.


lordosis. degenrative joint disease

Flat Back (Lack of Lumbar Lordosis) and Disc Herniation

Lordosis, or the lack of it, has been thought to be a biomechanical precursor to disc herniation in the lumbar spine. To investigate this possible correlation, a group of researchers from Gothenburg University looked at sixteen young active young patients with a median age of 18yrs old who experienced a disc herniation and underwent discectomy surgery. 1

Lordosis is the curve in the lower back—which they measured before and after the surgery.


Researchers used the Roussouly 4-type classification system to measure the degree of lordosis in the lumbar spine.

The researchers found less lordosis in the subjects that had surgery for their disc herniation. In other words, disc herniation was twice as likely to be present in the group with a flatter back. All the disc herniations were found to be in the lowest two levels of the lumbar spine (L4-5 and L5-S1), consistent with other epidemiological studies. 2

Dynamic Disc Designs Models

At Dynamic Disc Designs Corp. we have worked to represent the anatomy accurately. Our Professional LxH  Dynamic Disc Model is created with 12mm of disc height anteriorly and 10mm posteriorly providing a slight lordotic curve. Further, the model has been created with a higher percentage of nucleus pulposus which is often found in younger lumbar spines. To demonstrate that disc herniation occurs more likely with less lordosis all one has to do is dynamically move the single-level model into a less lordosis position and manually create compression. With more lordosis, the nucleus has a more difficult time penetrating through the outer annulus fissure. This can be an important posture teaching point in the prevention of disc herniation.

If you want to take your patient education to a dynamic level, explore what Dynamic Disc Designs models can do for you, your practice and ultimately, your patients.

plastic models

Plastic Models and Sciatica

Pathology is common with lower back pain. However, it is difficult for professionals to share this information with a sciatica patient. A recent publication1 recorded notes with interviews between thirteen patients and their low back pain practitioners. The discussions around sciatica were interesting.

Some of the comments coming from participants included:

‘The way I understand it is my disc is inflamed so it’s bulging, so it’s bigger than normal, and
that’s why it’s pressing.’


‘The discs are abnormal and they’re bulging […] if they’re bulging, surely there’s got to be a way
of getting them back to normal and when they’re back to normal I won’t have that pain.’


‘A sports masseur I went to, to check my alignment. He said my alignments were fine. And the
physio told me my alignments were out. So, I kind of … I don’t think anybody really knows what
they’re doing, to be honest.’

There is a disconnect between a patient, their symptoms and what the caregivers are telling them.

In this study, the authors reported that the practitioners caring for these pain people were constructing sciatica concepts with the influence of a plastic model. The authors also stated that “participants valued clear information on their diagnosis and appreciated the use of plastic models of the spine to help explanations.”

At Dynamic Disc Designs, we have worked hard to help practitioners deliver truthful and anatomically accurate details about a patient’s sciatica symptoms. Our plastic spine models are more than just plastic; they are dynamic and move realistically. We hope, one day, our models are used as a comparison against the traditional static models. Our plastic models were not used in this study.






facet osteoarthritis

Facet osteoarthritis pain is common and thought to be a significant contributor to back pain in the US. Within the United States, it costs 100 Billion dollars annually to combat this endemic problem. However, back pain can originate from many anatomical structures, and the facet joint is only one of them but thought by many as significant. Other common pain structures are the intervertebral discs in the case of disc bulges, disc extrusions, disc protrusions and frank nuclear sequestration. There are also more severe causes of back pain like aneurysm and other organ pathology, so it is crucial to have a professional look carefully at the diagnostics of each case.

In the case of mechanical lower back pain (others use the term non-specific lower back pain), the facet joint garners good attention. The word ‘facet’ comes from the French facette (12c., Old French facete), diminutive of face “face, appearance” and are two anatomical structures that reside behind the intervertebral disc.

Facet osteoarthritis

Modeling facet osteoarthritis is tricky because of the complexity of motion at the spinal level. The intervertebral disc height plays a role with respective facet compression because it resides on the front of the spinal motion segment. It is this compression thought to be contributing to back pain.

Clincally, facet osteoarthritis pain is often unilateral in nature

In a study conducted recently 1, researchers worked to induce facet joint arthritis by creating compression with a spring. Over time the researchers found the increased expression of interleukin‑1β and tumour necrosis factor‑α expression. In other words, with more compression elapsing over time, the more the expression of the molecules related to many low back pain patients.

This is an important study linking the mechanics of compression and the associated physiology of molecules, which are thought to be markers of back pain patients.

At Dynamic Disc Designs, we have developed models to help explain the associated compression of facet joints as it relates to disc height loss and gains. We are committed to bringing the best in modelling. Explore our website for more.

pathology & pain in Peripheral Joint

A review 1 published in Arthritis Research & Therapy decided to cover the link between subchondral bone features, pain, and structural pathology during peripheral joint OA (osteoarthritis). The review concluded that the subchondral OA bone does seem to have relevance when it comes to therapeutic measures.

The Context

According to research, OA (or osteoarthritis) is the most common form of arthritis is human beings. It leads to disability and chronic pain. Also, during the process of this review, the market didn’t necessarily have licensed DMOADs (disease-modifying osteoarthritis drugs). Take note; multiple tissues are usually involved in clinical OA. Gaining a better understanding regarding the intricate relationships that exist between these tissues and structural progression (along with the symptoms) might help with identifying potential tissue targets. The subchondral bone is of particular interest because it’s significantly associated with the hyaline cartilage.

The current review was conducted to comprehensively go over the available literature on subchondral bone structure which has been assessed using non-conventional radiographic imaging modalities. The said literature, used in this review, had examined common sites of peripheral OA as well as had described the relationships between joint replacement, structural progression, subchondral bone features, and pain.

The Method

The required original articles were found using Medline, EMBASE, and the Cochrane library databases. The articles reported an association between non-conventional radiographic imaging-assessed subchondral bone pathologies and joint replacement, pain or structural progression in the knee, hip, hand, ankle and foot OA.

A total of 2456 abstracts were screened for this review. Take note; 139 papers were included (70 of which were cross-sectional, 71 were longitudinal analyses).

The Results

The results of this review showed that there was an independent association when it came to the link between BMLs (bone marrow lesions), osteophytes and bone shape with structural progression or joint replacement. Furthermore, there was an independent association between BMLs and bone shape with longitudinal change in pain as well as incident frequent knee pain (respectively).

What was Concluded?

The review concluded that there are independent associations between subchondral bone features and structural progression, pain as well as joint replacement when peripheral OA is concerned; in the hand and hip, however, particularly in the knee. Furthermore, more in-depth research needs to be conducted to collect data about other associations (namely with the ankle and foot). Also, a better understanding of the subchondral OA bone might open doors for better therapeutic strategies.