How Does Chiropractic Work?
In practice I often hear how someone has “thrown their back out“, that a bone in their neck “feels out of place” or that they’ve come in to get their spine “re-aligned“.
These are all various ways of describing a situation in which a bone in the spine has moved out from where it is supposed to be.
When the founding chiropractors described what they were treating they called it a subluxation. When this word is broken down, we find that it also describes a bone being slightly out of place:
But the truth is, a lot has changed since that original definition was invented over 120 years ago. With technological developments such as MRI, electromyography and continued scientific research, our understanding of the spine and it’s related systems has advanced.
However, it is still a common misconception. Even some chiropractors and other health professionals tell their patients that they are misaligned. Chiropractic colleges continue to teach “listings”; ways of describing how a vertebra has moved out of position.
The definition of subluxation has also changed through the years and can mean a variety of things depending on the chiropractor you’re talking to.
So if chiropractors aren’t putting bones back into alignment, what are we treating?
Honestly, we are not completely sure. But we have our theories. And some of these theories have research behind them to back them up. Below I’ll talk about two of my favourites, the science behind them and why they might both be right. But first, let’s look into what chiropractors are known for: cracking joints. What’s going on there?
To help wrap our heads around the subject, try to get familiar with this simplified diagram of a spinal joint:
When a manipulative therapist (such as a chiropractor) treats the spine, the aim is to apply a short and sharp thrust in a direction that will separate a joint’s two opposing surfaces, pushing the joint apart.
As the joint rapidly separates, the synovial fluid gets caught in-between the two joint surfaces due to the sudden decrease in pressure. This is called a viscoelastic adhesion and you might have come across a similar situation when lifting a glass off of a wet tabletop. The water can get caught between the two objects and stick the glass to the surface.
With the joint surfaces continuing to separate, the negative pressure becomes too much for the synovial fluid, giving way to a process known as tribonucleation. This event occurs when the pressure gets low enough that a near-vacuum bubble forms and gases dissolved within the synovial fluid begin to evaporate into it.
The new gas bubble is easier to stretch apart that the semi-solid viscous synovial fluid, resulting in the joints being able to separate more rapidly. As it grows, the pressure in the bubble continues to decrease and gases from the surrounding synovial fluid continue to infiltrate it.
Next, the synovial fluid from the surrounding, unaffected parts of the joint begins to move in towards the cavitation bubble. The immense pressure differences cause the sides of the bubble to slam into each other with great force. It is this collision that causes the cracking sound we all know (and some love) . The whole process takes milliseconds and the exact mechanics of it is still debated to this day. 
After the cavitation, the gases that had evaporated into the bubble remain in place and take 20-30 minutes to dissolve back into the synovial fluid. Only then will you be able to crack the joint again. Not all joints can crack either. Experimental observation of joints with initial spacing larger than approximately 1.4 mm apart cannot be cracked. 
After learning about joint cavitation, you may be wondering how cracking your back can lead to the changes seen with chiropractic care. Can the process of a joint bubble collapsing lead to decreased back pain, neck pain, headaches and other neurological effects? Probably not.
The benefits of joint manipulation don’t come from the cracking sound. The sound is likely not even necessary.
Chiropractic’s real secret sauce is the sudden stretching apart of the joint and the surrounding tissues.
But why is this beneficial? Below we’ll look into some theories.
Theory 1: The Neuroplasticity Model
Time for a slight variation to our spinal joint diagram:
Over the past couple of decades, researchers have discovered that high-velocity, low-amplitude (HVLA) thrusts to the spine can have a variety of effects on a person’s nervous system. [8-22]
Researcher’s Haavik & Murphy haves attempted to explain these findings by developing the neuroplasticity model. 
They summarised their theory in plain English, calling a subluxation an area where “the central nervous system is not controlling the movement pattern of the spinal segment as it should, altering the stretching of the paraspinal muscles which changes the input to the brain which then impacts how it processes other information.” 
To summarise this further:
Neuroplasticity is the brain’s ability to reorganize itself by forming new neural connections. It is something that is happening all of the time inside everyone’s heads and it can lead to the brain adapting to both good and bad habits. For instance, when learning to ride a bike, the brain develops new connections, strengthening the ones that keep you balanced and weakening the unhelpful signals that lead to you falling. After repetitive practice, riding a bike becomes a piece of cake.
In the neuroplasticity model, the researchers believe that the brain adapts to ongoing incorrect signals caused by the dysfunction areas in the spine, reorganising to accepting the incorrect signals as normal. This reorganisation then leads the brain to interpret other sensory information incorrectly and results in less than optimal control of the body.
With this theory, chiropractic can restore normal function through the use of spinal manipulation. The HVLA thrust causes rapid stretching of the paraspinal muscles (a group of muscles that are close to the spine) and other surrounding tissues, which then improves the signals sent to the brain via the nerves and re-establish normal processing within the brain.
The brains negative adaptations would help explain why chiropractic has been shown to improve aspects of the nervous system that, at first glance, seem unrelated to the spine. These positive changes include:
This model has many research papers to back it up and it appears to make sense. The spine is literally the backbone of your body, central to the body’s movement and function while housing the vulnerable spinal cord. It is covered in muscles that are packed full of small sensory organs which, along with other tissues, tells the brain how the spine is orientated and if anything is going wrong. If this communication somehow becomes compromised, you could imagine how the brain may, therefore, interpret things incorrectly and act in a way that is different from how it normally would.
One issue with this theory is that it doesn’t provide a clear, root cause of the initial altered spinal movement. If chiropractors can correct the dysfunction, why does it come back?
It has been theorised that it could be due to something detrimental such as physical trauma, bad posture, mental health issues or other lifestyle factors. In their study, Haavik & Murphy proposed that the altered functioning of the nervous system caused by one area of spinal dysfunction, leads to further spinal dysfunction (aka: subluxation), creating a negative feedback loop.
As far as I am aware, no study has given evidence for either of these explanations.
Theory 2: The Adhesion Model
Another little update to our spinal joint diagram:
After hearing an idea from one of his mentors while studying chiropractic, researcher Gregory Cramer further developed the theory that spinal dysfunction arises from adhesions forming across either side of the spinal joints.
He theorised that spinal adjustments would gap the joint, causing the break-up of the adhesions and allow the joint to move and function correctly once more. 
You can think of adhesions as tenuous strands of fibre that build up between structures. They can range from weak spider web-like fibres to thick bands which can physically keep structures bound together.
The following video shows good examples of adhesions (he calls it fuzz) within the human body, albeit between muscles.
WARNING: the video contains cadavers (i.e. deceased human bodies) and the guy’s is a little eccentric.
To test his theory about joint adhesions, Cramer and his team used an animal model in place of humans for ethical reasons. He investigated the location of the adhesions and what happens when you physically immobilise segments of the spine through a surgically implanted device. In theory, this would be analogous to a person who is inactive and not moving their joints frequently. He then compared this immobilised group with a control group who were allowed to move around normally.
His study had a few interesting findings. Firstly, both the control group and the experimental group had minor adhesions present in their joints. So, no matter how immobile the joint was, minor adhesions were likely to develop.
They also found that joint adhesions became more numerous and thicker, the longer the joint was made immobile. The joints of the control animals were much less likely to develop medium-sized or large adhesions but they became more abundant in the immobilised subjects.
A subsequent study  involving Cramer found that 2 weeks of chiropractic adjustments of the (human) lumbar spine caused significantly more gapping of the joints than stretching alone. This additional gapping may help explain why spinal manipulation is a potentially effective tool for breaking up adhesions.
This theory gives a potential source of spinal dysfunction; adhesions that build up between the spinal joint surfaces and restrict normal motion. It also suggests a potential way to help prevent spinal dysfunction; the more you move your joints around, the fewer adhesions should develop.
There are a few caveats, however. Cramer’s study appears to be the only investigation into intra-articular adhesion of the joints within the spine. However, adhesions are known to develop throughout the body and other studies have also found them within the joints of the knee , shoulder  and jaw .
The model doesn’t offer an explanation on how manipulative therapy can positive effects on the nervous system either, as discussed in the section explaining the neuroplasticity model.
One final issue is that this study was performed on rats and not humans. What happens to them may not happen to us. Perhaps normal human sedentary behaviour doesn’t compare to the severe joint restriction caused by the surgically implanted device.
So now you know that the bones in your back do not move out of alignment. You should also understand why joints crack and that this crack is not responsible for the benefits of spinal manipulation. It does, however, confirm that the joint has been separated.
Although no one is entirely sure how chiropractic adjustments work, many scientists are investigating what is going on.
The neuroplasticity model proposes that muscles around the spine stop working normally. This then leads to incorrect sensory information going to the brain. The brain then adapts to this information, leading to incorrect processing of other incoming sensory information. This is thought to cause suboptimal nervous system control. Spinal manipulation stretches the affected muscles out, re-establishing normal sensory feedback and normal brain function.
The adhesion model attempts to explain the root cause of the issue that chiropractors and other manipulative therapists are trying to fix. It proposes that adhesions build-up across the spinal joints, connecting the two sides together. The adhesions become thicker and more abundant the less the joint is moved around. These adhesions are thought to cause limitations to the joints motion and lead to further issues. Spinal manipulation is theorised to gap the joint sufficiently enough to break these adhesions up and restore normal joint motion.
As noted in Gregory Cramer’s study, the adhesion model is not mutually exclusive to other models. Perhaps spinal joint adhesions are the initial cause of a vertebra not moving correctly in the neuroplasticity model, which then leads to all of its associated effects. The adjustment then not only breaks up the adhesions but also stretches surrounding dysfunctional tissue, bringing about normal joint motion and restoring normal nervous system function.
To wrap it all up:
Whatever the case may be, spinal joint manipulation is here to stay. It has been around for at least 2000+ years, with even Hippocrates using manipulative techniques . The more we study what is going on, the better it is for patients, manipulative therapists and all of the associated professions.
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