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:

Sub = Smaller than

Luxation = Joint dislocation

Subluxation = Smaller than a joint dislocation

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.

We now know that the “bone out of place” theory is incorrect. [1] [2] [3] [4]

Your vertebrae do not move out of place unless there is a serious injury causing a dislocation.

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:

Joint Anatomy

Joint Cavitation

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) [5]. The whole process takes milliseconds and the exact mechanics of it is still debated to this day. [6]

To summarise:

Joint Cavitation

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. [7]

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:

Joint anatomy with muscle

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 have attempted to explain these findings by developing the neuroplasticity model. [23]

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.” [24]

To summarise this further:

Areas of your spine stop being controlled correctly and this leads to further problems in the way your brain perceives the world around it, causing less than optimal functioning of the nervous system.

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 head 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:

  • Decreased reaction times. [10]
  • Increased muscle strength in the upper and lower limbs. [11]
  • Prevention of muscle fatigue from developing in certain areas. [12]
  • Reduced joint position sense error. [15]
  • Improved or altered visual acuity and visual field size. [18]

To summarise:

Neuroplasticity model

Discussion

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 detrimental 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:

Joint anatomy with adhesions

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. [25]

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 is a little eccentric.

To test his theory about joint adhesions, Cramer and his team us