Chiropractic done by hand

Synovial Fold -Noise- and Suction. With recent data collected using MRI on the metacarpophylangeal joint, the current idea around a cracking joint appears to have been seriously challenged.

Current belief is that cavitation, a process of bubble formation and subsequent collapse, is responsible for the noise. But curiously, when you look up ‘cavitation’, it is an unwanted process–causing damage to steel like that seen with marine propellers. Then why are we seeing positive outcomes for spinal manipulation? And why have we not been able to show damage to the cartilage if the noise process is a cavitation?

In the development of an audible release model, Dr. Jerome Fryer believes that the sound generator of a classic spinal adjustment is not the process of a gas formation or the subsequent collapse, but rather a sound generates from the same physical mechanism seen from a suction cup release from a polished surface.

The synovial fold (also known as a meniscoid, meniscus, or tag) is a ring like structure that varies in size and shape and involutes into a synovial joint. It is situated in between the cartilagenous surfaces and acts to stabilize a joint’s motion. It has been shown to house nerves and has been thought to be the reason why some people respond well to manipulation: to free this entrapped tissue.

To get a better understanding of what this fold looks like, you can see the image below that Dr. Fryer commissioned Danny Quirk (medical artist) to draw.

Synovial Joint - Synovial Fold

The anatomy of the metacarpophylangeal (knuckle) joint is very similar to all other synovial joints in the body, including the facet joints in the spine. Its structures include the joint capsule (1) , the synovial fold (2), the synovial fluid (3) and the hyaline cartilage (from the latin root, glass-like) also known as articular cartilage (4). As stated earlier, the synovial fold has other synonyms: meniscoid or tag.  For the purpose of this topic, the synovial fold will be talked about because it is believed to be responsible for the sound as it lifts off the cartilage.

The synovial fold is continuous with the joint capsule and is considered to be part of the synovium. Synovium (also known as the synovial lining) is the lining of cells that peripherally borders the inside of the joint without covering the hyaline cartilage. It is a layer of cells that monitors and produces the nutrient and fluid exchange between the nutrient rich blood and nutrient weak synovial fluid. It is an important layer that governs what sort of things enter (and exit) the synovial joint.

Note: Synovial comes from the Greek root: syn, meaning same and ovum, meaning egg-like. The synovial fluid is similar to consistency of a raw egg white

It is interesting to see that many images of the synovial joint do not usually include the synovial fold. Perhaps this is why it has not been on the radar when thinking of it as part of the noise generator. The synovial fold has had more recent interest however, although it has been researched for decades with respect to its pain mechanisms in the neck.

In the manuscript titled: Synovial Folds-a pain in the neck? these researchers looked carefully at the anatomy in the cervical spine and determined that there are three basic types of folds : fibrous, fibrous-adipose and adipose. They also varied in shape and size depending on the facet joints. They are also elastic.

We know that a healthy synovial joint has a net negative sub-atmospheric pressure holding the joint together in a suction like fashion. It is slight but nonetheless, it is negative–about -3mmHg. Nobody really knows why this negative pressure exists but it is logical to think it is for general stability of the joint plus it also provides a net influx force for hydraulic flow.

To understand the sound source (or potential sound source) one has to understand how a suction cup sound works.

First, in order to elicit the snapping sound from a suction cup, one has to first push the suction cup against a glossy surface. It also has to be a tight seal because as one pulls the suction cup away, negative pressure is created between the cup and the glossy surface. If the cup is irrelgular on its contact with the polished surface, a pressure differential cannot establish.

If initially the edges of the cup ‘adhere’ as the cup is pulled away further, a pressure differential can establish and develops. Eventually, the suction cup reaches its elastic endrange and cannot stretch any further as the volume under the cup increases. At some point, a breach occurs at the edge of the suction cup and lifts off starting a cascade reaction. Different suction cups make different noises and so do the varying synovial joints in the body.

To see a slow motion suction release:

A synovial fold is believed to behave very similarly. Firstly, there is no requirement to push the fold onto the cartilage because the negative pressure has already been naturally established through synovium pump mechanisms. To read more you can Levick’s work here. And with an already established negative pressure, the joint is believed to be primed to release if enough separation is induced. With the elastic properties within the fold itself, it is ready to release and snap from the hyaline cartilage.

Once the ‘crack’ occurs, we know that there is a refractory period. This is the time required for the joint to re-establish the negative pressure to allow the joint to ‘crack’ again. Levick’s work explains how the protein gradients generate this natural pump. Everyone’s refractory period is a little different and likely due to the efficency of this natural synovium pump mechanism.

This type of event mechanism can also explain why certain joints make certain sounds. A second metacarpohylangeal joint ( index knuckle) will make a different sound when compared to the fifth (pinky knuckle). A cervical C1-2 manipulation will sound different when compared to sacroiliac audible release sound. Unsworth’s work in 1971 suggested that the size joints relate to the sound emitted during a ‘cracking’ event. This can explain why such variations in the crack sound heard between individuals.

How is it then that a suction cup even resembles a syovial fold? To understand this, one has to appreciate suction cup design.

Suction Cup Noise Proposed Mechanism

Suction Cup Release Noise, Analogy to a Synovial Joint Crack

The Event Sequence of a Suction Cup Release

  1. Elastic cup is at a distance from glass surface and air pressure is equivalent in all spaces
  2. As cup is pushed down onto polished surface, the area of air under cup is displaced
  3. Without compression or distraction of cup, pressure is equal above and below cup surface
  4. Upon initial distraction of cup, a pressure differential begins to develop as the air under the cup is now subjected to increased volume. This differential in pressure holds the edges of the cup onto the surface to prevent slippage.
  5. As further distraction is elicited, the cup material begins to stretch and begins to reach its elastic end barrier. As the pressure differential continues to increase, the cup edges are held even firmer against the glass but eventually a breach at the edge of the cup edge occurs to which starts the event of a lift off from the polished surface.
  6. As the cup lifts from the edge surfaces, a circumferential wave of air quickly rushes in to the space to equilibrate the vacuum formed by the distraction.
  7. As the air is compressed to a center core, the outward expansion leads to the generation of a sound wave through waves of condensation and rarefaction generating the sound heard as a crack.

Overall, the sound source of manipulation is important. Many people have put to rest that the sound source is due to cavitation. This has not been unequivocally defined. If we have a better understanding of the tissues involved during this event, a better understanding of the event will move us forward.

Manipulation has been shown to provide good and sometimes, not so good, outcomes. Identifying the anatomical structures responsible for the noise(s) will provide us with a better snap-shot of the event and will help guide us in improving manual and therapeutic strategies for synovial joint health. If the event can be proven to be a suction phenomenon, could this facilitate the increase of fluid flow into the joint space along with increasing joint space width? Only time will tell.

Detailed custom anatomical model

Identifying the anatomy of pain generators in the diagnosis of back pain should always be at the forefront of any workup.

Even though research has not helped us much in assisting physicians with reliable and valid orthopedic tests, that does not mean they do not exist. Dynamic Disc Designs(ddd) believes better initial physical exam workups can help with the identification of pain specific tissues. Dr. Jerome Fryer BSc DC, the founder of ddd, suggests dividing the the spine into anterior and posterior pain producers can simplify and aid the physician in the initial exam. Understanding the anatomical structures that are challenged with dynamic flexion and/or extension and how the patient responds when the pain presents can be a real clue in identifying the sensitive tissue.

He explains: If someone with back pain is guarded during the movement of active lumbar flexion and is only able to achieve 15 degrees, then quickly stops and utilizes the upper extremities by placing their hands on their thighs, Dr. Jerome Fryer would suggest that the unloading of the intervertebral disc is occurring with this avoidance behaviour. A painful intradiscal lesion should be in the differential. On the other hand, if a patient is able to move through the lumbar flexion but there is a “sore spot” in the middle of the range, one has to think about a painful facet as these facets glide on each other and translate anteriorly while approximating.”

These models were developed for patient education primarily but have also been developed to help in the differential diagnosis of painful anatomical spine structures. Spine doctors are finding them an invaluable resource for education.

  1. Anatomy
  2. diagnosis
  3. treatment
Audible articular release

Audible articular release sounds are very common with spinal manipulation.

There are still many people afraid of the sound and prevent them from trying spinal manipulation as a treatment for back or neck pain. To improve the explanation of this common procedure, Dr. Jerome Fryer, chief innovations officer at Dynamic Disc Designs Corp., has developed an audible articular release model ( spinal model ) that emits and audible “snap” when the facets are manually gapped as as in the actual procedure. This model has brand new features to help the practitioner explain precisely the procedure of spinal manipulation among other reasons as to why this type of procedure would be helpful.

Details include:

  • life-size L4 and L5 vertebrae connected by a flexible 2-part disc including a nucleus pulposus and annulus fibrosus, herniating nucleus under load
  • Synovial fold (noise generator)
  • Palpable polished hyaline cartilage on three of the four facets to allow the patient to touch what healthy cartilage should feel like
  • One facet has a roughened surface to simulate degradation of cartilage with a fractured surface and early degenerative sharp ostephyte growth to allow patients to understand clearly the early break down of cartilage in the osteoarthritis pathway
  • A signature dynamic disc to show the relationship of dynamic disc height loss and facet joint
  • an optional cauda equina

Dynamic Disc Designs flexible spine models are becoming the standard in spine education. Explore and find which model can help you the most with your education to improve clinical outcomes.

Synovial Joints, Synovial Fold

The classic “pop” or “snap” or audible noises associated with a synovial joint distraction is a curious sound.

There seems to be a generalized consensus that the structure responsible for the noise is a gas bubble. For a review you can read here and find out that we don’t really know where the sound is coming from.

Dr. Jerome Fryer (Chief Innovation Officer at Dynamic Disc Designs Corp.) believes that the sound is being emitted from the elastic recoil of the synovial fold (meniscoid, synovial tag) rather than the bubble.

As you can see above, the synovial tag can act like a suction cup. As each facet cartilage pulls away from each other, the synovial tag is drawn in either direction and recoils back into original position after it reaches its elongation end range. To understand the mechanics of action, you will need understand how a suction cup works….read here.

As a suction cup is pressed against a surface, air is pushed out. As the cup is pulled away, the now negative pressure under the the cup is similar to the negative pressure found in a healthy synovial joint (approx -3mmHg). When the suction cup is pulled from the surface, the surrounding atmospheric pressure pushes the cup to the surface until the cup reaches its elastic end point. At this point, there is an elastic recoil of the cup and it snaps back into its original shape and the atmospheric pressure surrounding the cup is equilibrated.

We know that a gas bubble presents itself after the “pop” event of distracting a joint. But there has been no definitive research that actually proves that the sound generator is the bubble. One of the references is 27 years old now and used phonoarthrography. In this 1986 study, Meal and Scott saw two sounds with the pop from a synovial joint.  In Dr. Fryer’s opinion, if the sound was coming from a bubble, we should have seen one sound, not two.  Their two sounds could be explained with this new hypothesis of  the synovial fold being the noise generator. This anatomy is not a point source but rather a ring of tissue able to generate two sounds from a single microphone, which is what these researchers used in their methods.

As we begin to unravel the precise definitions of audible joint sounds, Dr. Fryer’s  hope is we become better diagnosticians for synovial joints. And as a result of his new hypothesis, research is now moving forward with the University of Alberta using MRI.

(March 29, 2014) As the ongoing investigation of this hypothesis develops, Dr. Fryer decided to write a brief hypothesis article on this topic. This manuscript was submitted to Chiropractic and Manual Therapies on March 2, 2014. The result was a rejection. A revision was then submitted to JCCA on March 18, 2014. A rejection was the initial result and the Editor suggested a resubmission as a commentary. Jerome Fryer decided to upload the hypothesis manuscript.  If you want to read more, click Is the sound of manipulation from the synovial fold?

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