Centre of Rotation

Goal of the Study?

In this study 1, the authors investigated the flexion-extension range of motion (ROM) and centre of rotation (COR) of lumbar motion segments in a large population, as well as the relationship between lumbar movement and sex, age and intervertebral disc degeneration (IVD).

Why are they doing this study?

Research on the in vivo motion of the spine has a long history. However, many of these studies have used non-invasive technologies with inherent limitations impacting their accuracy and precision. Moreover, many studies have included a lower number of subjects, preventing the data’s ability to represent the general population.

The authors argue that the use of radiographic techniques in this study helps to overcome these limitations as the images allow for better visualization of each vertebra and movements of the lumbar segments. Additionally, the use of a large sample size for this study addresses the issue of representation and is the largest study to date looking at in vivo lumbar motion. 

What was done?

The researchers did a retrospective study looking at the lumbar spine radiographs in full flexion and extension for 602 patients, with the age and sex documented for each one.  Additionally, they used MRI scans of 354 patients. 

All spinal levels between T12-L1 and L5 – S1 were analyzed, resulting in 3612 lumbar motion segments from the radiographic images. They also examined 2124 images from the MRI scans looking at disc degeneration. ROM and COR were calculated for all lumbosacral segments using the software. They then examined the associations between motion and age, sex, spinal level and disc degeneration.

What did they find?

The median ROM in this study was 6.6 °. The researchers found an association between age and ROM, with older individuals, have lower ROMS. They argue these findings clearly demonstrate a relationship between age and lumbar spine flexibility independent of any signs of spinal degeneration. They also found that lower ROMS were associated with disc degeneration. However, they did not find any association between sex and ROM.   

In this study, they did not find an association between the COR and the spinal segment’s position. The most common COR was at the centre of the lower endplate of the IVD or slightly lower. With degeneration, particularly severe degeneration, they found the COR location spread randomly around the centre of the intervertebral space.


One of the main limitations of this study was the sole focus on the lumbar spine’s flexion-extension motion rather than including information on movements of different areas of the back. 

Why do these findings matter?

This study comprises the largest examination of the in vivo lumbar spine in flexion-extension, paying attention to age and spinal degeneration issues. Understanding the relationship between age and spinal mobility provides patients and doctors with information to better treat back pain and instability.

Facet pain

Goal of the Study?

The objective of this study 1, is to determine the prevalence of “pure” lumbar Z joint pain.


Why are they doing this study?

The prevalence of lumbar Zygapophysial joint (Z joint) is disputed, with rates ranging from 5% to 45%. The reason for this disparity lies in the use of different criteria for diagnosis. The criteria are focused on relief of pain after a diagnostic block, an injection of numbing medication into 1 or more small joints on each side of the vertebrae. However, these criteria can range from 50% to 80% to complete pain relief after a diagnostic block. Additionally, some diagnostic criteria focus on relief from pain after a single diagnostic block, with no controlled blocks. However, the researchers argue that for diagnostic blocks to be valid, pain must be abolished whenever an active agent is administered, and the use of repeat blocks provides validity.

The authors contend that no research has been able to determine the prevalence of “pure” lumbar Z joint pain, defined as complete relief of pain following a placebo-controlled diagnostic block. 

Facet pain


What was done?

All 206 patients for this study came from a private practice where they were referred for back pain. To be eligible, they had to have back pain longer than 3 months, with symptoms compatible with a potential diagnosis of lumbar Z joint pain. Patients with imaging that pointed to tumours or infections were excluded from this study, as were patients who were pregnant or had pain in the lower limbs.

The researchers used two protocols for this study. The first protocol was a placebo-controlled local anesthetic comparative blocks. For this, each patient received a local anesthetic on two occasions and normal saline on one occasion.  For the second protocol, patients could receive the same or a different local anesthetic on two or three occasions and normal saline on one or no occasion. Using saline as a placebo showed whether patients could tell the difference between an active control from a placebo.

Patients were considered to have Z joint pain if they had complete relief of pain three times when a local anesthetic was used, if they had complete relief of pain two times when the anesthetic was used, and if they had no relief, saline (the placebo) was used. 


What did they find?

The majority of patients did not report any pain relief from the initial block and therefore did not satisfy the criteria for lumbar Z joint pain. Only patients who had complete relief of pain from their initial block were eligible to continue. 

Of eligible patients, the researchers found that 45% of patients in group 1 and 30% in group 2 did not get relief from the second block. Moreover, 20% of patients in group 1 and 3% in group 3 had no relief from the second block but were completely relieved by the placebo. Results for the two groups combined showed a prevalence rate of “pure” lumbar Z joint pain of 15%. This is much lower compared to when a diagnostic criterion is less stringent. Moreover, they did not find any statistically significant differences between the two groups for gender and age. 


Why do these findings matter?

Determining the appropriate diagnostic criteria for lumbar facet pain has important implications for patients who need to have the right diagnosis and treatment for their pain.  


Goal of the Study?

The goal of this study 1 is to better understand whether the morphology of the facet joint contributes to spondylolysis.


Why are they doing this study?

Lumbar spondylolysis is one of the most common sports injuries in adolescents. This is a condition in the lower back where there is a defect in the part of the vertebra known as the pars interarticularis, which is a part of the bone that connects the facet joints at the back of the spine. 

Existing research suggests that individuals with more coronally (frontal) oriented facets in the lower lumbar vertebrae, combined with facet tropism (growth or turning) are at a greater risk of developing spondylolysis at L5. However, to date, there are only a few studies looking at how the morphology (shape or arrangement) of the facet joint is associated with unilateral and bilateral spondylolysis in adolescents. 


What was done?

The participants for the study were 68 junior athletes who went to the Funabashi Orthopedic Hospital due to lower back pain between April 2012 and June 2014. 

All participants had computer tomography (CT) and MRI scans done to determine their grouping. From this, they were classified into three groups:

  1. Group B – 22 athletes (18 males and 4 females) with L5 bilateral spondylolysis
  2. Group U – 27 athletes (21 males and 6 females) with L5 unilateral spondylolysis
    1. Reclassified into two groups: one with a spondylolysis side (UL group) and the other with a normal side (UN group)
  3. Group C – 19 athletes (13 male, 6 female) without spondylolysis whose back pain eventually disappeared. 

Patients with multivertebral spondylolysis and previous lumbar surgery were excluded. 

Using CT, a researcher measured the sagittal alignment of the L4/L5 and L5/S1 facet joint angles from the axial sections. These angles were compared for differences among the groups. All of the measurements were repeated three times by the same person to increase validity.


What did they find?

The researchers found that for participants in groups B and UL, the L4/L5 facet joint angles were significantly more coronally oriented (more in the frontal plane) than those in group C. However, there were no significant differences found in L4/L5 facet joint angle between groups:

  • B and UL
  • B and UN, UL
  • UN and C

Additionally, there were no significant differences in L5/S1 facet joint angle were found among the groups.


L4 Tropism Spondylolysis

L4 Vertebra from the Professional LxH Dynamic Disc Model. Tropism


Why do these findings matter?

Compared to a normal orientation, a more coronal (frontal) orientation of the L4/L5 facet joint angle is likely to increase the stress on the vertebral arch of L5 during trunk extension. Therefore, if an adolescent athlete has a coronally oriented L4/L5 facet joint, they are more at risk of developing unilateral spondylolysis at L5. This knowledge can assist in identifying which young athletes are more likely to develop spondylolysis and help to implement appropriate prevention efforts by limiting certain activities in young athletes. 


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.

Vacuum sign

Vacuum phenomenon or vacuum sign and mobility

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

Zygapophyseal Joint Pain in Chronic Whiplash Patients

A study 1 of sensory hypersensitivity in patients suffering from chronic whiplash associated disorder 6 months or more after being involved in a motor vehicle collision (MVC) found that the hypersensitivity was reduced, and pain thresholds were increased after receiving a medial branch block (MBB) procedure of the cervical spine.  The results of the study indicate the cervical zygapophyseal joints most likely contribute to sensory hypersensitivity caused by peripheral and centrally mediated pain.


What’s at Stake?

A common problem of people who have been involved in MVC’s is chronic whiplash associated disorder (WAD). According to research, between 32-56 percent of those involved in MVC’s may continue to suffer from related disability or pain 6 months or longer after their accident. Research has implicated the cervical zygapophyseal joint as a possible source of chronic hypersensitivity in 54-60 percent of subjects with WAD—evidence that is supported across multiple biomechanical and neurophysiological studies. It is thought that tissues that had been seemingly unaffected by the MVC experience sensory hypersensitivity when the body’s pain processing mechanisms are altered in the spinal cord. This sensory hypersensitivity and central nervous system hyperexcitability decrease the pain thresholds in the body, creating an exaggerated response for thermal, electrical, or mechanical stimuli for WAD patients. The prognosis for WAD patients suffering from sensory hypersensitivity is poor, and better understanding of the phenomenon could improve long-term treatment outcomes.

The Study


The pretest-posttest exploratory study involved 18 volunteers (15 females, 3 males) with an average age of 45 years and who had experienced WAD for 6 months or longer, with numerous neck complaints, body tenderness, and decreased range of motion. A control group of 18 healthy patients (15 females, 3 males) with an average age of 45 years also participated in the study. A group of chronic WAD patients with pain reported for 6 months or longer and who had a minimum of 80 percent decrease in neck pain intensity following an intra-articular zygapophyseal joint block procedure also took part in this study. Exclusionary criteria included pregnancy, previous history of headaches or neck pain requiring treatment, central or peripheral neurological problems, coronary artery or peripheral vascular disease.

Researchers rated the subjects’ pain intensity levels on a scale of 1-10 before and after receiving MBB procedures. Quantitative sensory testing (QST) based upon pressure pain thresholds (PPT’s) and cold pain thresholds (CPT’s) were conducted on the control and WAD groups. All measures were recorded, including patient demographic variables and their current MVC litigation status.

Cold Pain Threshold Testing

A 30mm x 30mm thermode set to 32 degrees Celsius placed over the anaesthetized articular pillars of the cervical zygapophyseal joints measured cold pain thresholds in the test subjects as the temperature was decreased at the rate of 1 degree Celsius per second. Patients used a self-controlled switch to indicate when the sensation of cold turned to pain as each bilateral site was tested. (The minimum temperature was 1 degree Celsius.) The average values were gathered for analysis.

Pressure Pain Threshold Testing

The articular pillars of the cervical zygapophyseal joints, peripheral nerve trunk of the median nerve, and the tibialis anterior were measured in the PPT tests, with the subjects using a self-controlled switch to indicate when the sensation of pressure turned to one of pain. The tests were performed three times bilaterally on each site, with a pause of 10 seconds between each test. The average values were recorded and later statistically analyzed.

Cervical spine model to demonstrate zygapophyseal joints

Dynamic Cervical Model

Diagnostic Cervical Zygapophyseal Joint Blockade

The patient group with chronic WAD underwent two diagnostic zygapophyseal joint block procedures—one, prior to the study, where a spinal needle was inserted with fluoroscopic guidance into the joint while the patient was in the prone position. An injection containing a local anesthetic and a corticosteroid was made into the affected zygapophyseal joint. If these patients experienced a relief of pain intensity of at least 80 percent but their pain later returned, they received the second MBB injection. In this study, none of the patients were excluded from the second MBB, as each of them had experienced at least an 80 percent reduction of pain from the first procedure, with the return of pain post-procedure.


The WAD patients demonstrated clinically significant changes in their sensory hyperactivity measurements after the blockade of the cervical zygapophyseal joint. These changes included a decrease in CPT’s and increase of PPT’s in the cervical spine and distal sites. This finding is unique in the study of chronic WAD patients and suggests that minimizing the source of pain—in this case, the zygapophyseal joint—may help modulate sensory hypersensitivity in chronic WAD patients, at least in the short-term. The study authors urge larger trials with long-term follow-ups of patients to gather more information and improve the treatment outcomes of patients with WAD.

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.

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.


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