How does chiropractic work?

Recently I was engaged in a discussion on a website with a group of people who were claiming that chiropractic is “pseudoscientific bunk” and “People who believe in chiropractic are like believers in any particular religion; you can argue with them all you want, bring up science and facts, but in the end it doesn’t matter. It comes down to belief, not understanding.”

This reminded me of a quote from E. B. White: “Prejudice is a great time saver. You can form opinions without having to get the facts.”  I realized that most of these people are just poorly informed.  So, I have decided to present the “science and facts” with regard to chiropractic manipulation.

I’m going to try to explain things as clearly as I can.  In the interest of brevity I will not include references, but feel free to contact me if you have questions or if you’d like the specific peer-reviewed references I used to build this post. 

First off, I feel the need to make this point: Everybody wants to focus on spinal manipulation when it comes to chiropractic, but that’s not all we do.  In my office I perform treatments like ultrasound when appropriate, soft-tissue techniques, joint mobilization, post-isometric stretching, McKenzie protocols, athletic taping, etc.  For a lot of patients I don’t even manipulate the spine. And, I have a complete exercise rehabilitation facility where we work on everything from core strengthening to shoulder and knee injuries.

Furthermore, chiropractors are trained and licensed to diagnose. I perform complete health histories and physical exams. For example, in my headache patients I perform opthalmoscopic exams and check cranial nerves. I also perform standard orthopedic test and do things like palpation, auscultation, and percussion. I also perform pre-employment physicals. I routinely order X-rays, MRI, MRA, CT, blood work, diagnostic ultrasounds, etc. For non-musculoskeletal conditions I refer to internists or specialists. For musculoskeletal conditions that are beyond my scope or are not responding as expected I obtain orthopedic consults.

Now on to the meat of the article:  How does chiropractic manipulation work?

The “subluxation complex”:  For many years, the chiropractic profession has referred to the concept of a “subluxation”.  The definition and understanding of the “subluxation” has changed a great deal over the years but our current concept is this:  The subluxation complex is a dysfunction of the normal motion in a spinal segment.  Aberrant motion in these joints impacts the surrounding musculature and connective tissue as well as local blood vessels and nervous tissue.  Additionally, when joints are not moving normally this can lead to the development of adhesions (sort of like scar tissue) in the zygapophysial (Z) joints.  Among the ways that this can happen is by direct injury to the joint which limits its motion or via straining the surrounding musculature which causes pain and spasm that prevents the Z joint from moving normally.  This can also happen more gradually as in the case of degenerative joint disease (aka osteoarthritis).  In support of this hypothesis, several scientific studies have identified adhesions and degenerative changes in the Z joints after hypomobility.   Further, studies show that the Z joint capsule receives significant sensory innervation and between 15% and 40% of chronic low back pain is related to the Z joints.

Mechanical actions on the joint and IVF: Spinal manipulation is also sometimes referred to as HVLA (high-velocity, low-amplitude) which means that the force is generated by performing the manipulation at a high velocity but with tightly controlled amplitude.  Numerous studies have examined the specific forces involved in HVLA manipulation.  Researchers have also shown that the Z joints gap significantly during manipulation and that this allows for improved motion in the spinal segment. As the joint is gapped during HVLA, a rapid increase in joint capsule volume is associated with a corresponding drop in the capsule pressure (ideal gas law).  As pressure drops in the capsule, some of the liquid synovial fluid quickly converts into gas resulting in a “popping” sound.  This creates a bubble of gaseous synovial fluid in the joint that helps keep the joint gapped which decreases pressure in associated intravertebral foramena (IVF) as the bubble gradually dissolves back into liquid. 

Substantial evidence has demonstrated that the dorsal roots and dorsal root ganglia are susceptible to the effects of mechanical compression.   Compressive loads as low as 10 mg applied to dorsal roots can increase the discharge of incoming (afferent) nerve fibers.  Compression on the nerve root can also alter non–impulse-based mechanisms (eg, axoplasmic transport) and cause edema and hemorrhage.  So at the most basic level, spinal manipulation mechanically opens the IVF which decreases pressure on the dorsal nerve roots. 

Neurological actions:  In muscles, there is a specific type of proprioceptor called a muscle spindle muscle spindles γ (gamma) motoneurons Spinal manipulation also helps increases joint mobility by producing a barrage of impulses in muscle spindle afferents and smaller-diameter afferents ultimately silencing facilitated as proposed by Korr http://www.jaoa.org/cgi/reprint/74/7/638.  This theory is supported by several recent studies by the Pickar lab and by findings that low back pain patients have altered proprioceptive input from muscle spindles.   Recent work has also shown that that spinal manipulation modifies the discharge of Group I and II afferents.  This has been accomplished by recording single-unit activity in muscle spindle and Golgi tendon organ afferents in an animal model during manipulation. 

Substantial evidence also shows that spinal manipulation activates paraspinal muscle reflexes and alters motoneuron excitability. These effects are still being studied and appear to differ depending on whether performed on patients in pain or pain-free subjects. Maniupulation also inhibits somatosomatic reflexes by alterating muscle spindle input. It is thought that spinal manipulation may normalize spindle biomechanics and improve muscle spindle discharge.

β-endorphin mechanisms. Studies have shown increases in β-endorphin levels after spinal manipulation but not after control interventions. This is still being debated because results have been variable and a recent study failed to show increased β-endorphins even though subjects had decreased pain.

Additional effects:  Lastly, in humans, manual therapies can decrease heart rate and blood pressure while increasing vagal afferent activity as measured by heart-rate variability. Manual therapies in rats have been shown to produce an inhibitory effect on the cardiovascular excitatory response and reduce both blood pressure and heart rate. Manual therapies such as massage have been shown to impact behavioral manifestations associated with chronic activation of the HPA axis such as anxiety and depression, while decreasing plasma, urinary, and salivary cortisol and urinary corticotropin releasing factor-like immunoreactivity (CRF-LI). Manual stimulation in rats has been shown to significantly increase glucocorticoid receptor gene expression which enhanced negative feedback inhibition of HPA activity and reduced post-stress secretion of ACTH and glucocorticoid.