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.

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.

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.