Vertebral subluxation
Vertebral subluxation is a chiropractic term that is used by some chiropractors to describe a myriad of signs and symptoms thought to occur as a result of a misaligned or dysfunctional spinal segment. Since its conception by DD Palmer, the definition has undergone many refinements in an effort to describe the conditions that chiropractors treat. In 1996, the Association of Chiropractic Colleges, representing all chiropractic colleges, unified the definition as, "a complex of functional and/or structural and/or pathological articular changes that compromise neural integrity and may influence organ system function and general health." 
The concept of chiropractic vertebral subluxation remains a uniquely chiropractic construct that does not enjoy mainstream medical support.  It should not be confused with the orthopedic subluxation. While the orthopedic and medical definition of a subluxation includes objectively verifiable misalignment and sometimes nerve damage, it is considered an uncommon occurrence.

# Background
Chiropractors who believe in the vertebral subluxation complex propose that it can negatively affect general health by altering the proper circulation of information via the nervous system. Although not always painful, some chiropractors claim that a subluxation interferes with the proper function and healing of your body due to the role of the nervous system as a controller of many bodily functions.
A vertebral subluxation is theorized to affect one spinal disc (i.e. degenerative disc disease), a section of the spine, or the entire spine. For example, a functional scoliosis or abnormal curvature of the spine, viewed in chiropractic terms, may be due to a subluxation of one vertebra that causes a distortion of the entire spine.
Chiropractic treatment of vertebral subluxation focuses on delivering a chiropractic adjustment to the affected part of the spine in an effort to reduce the subluxation. Spinal manipulation is the primary procedure used by chiropractors in the adjustment. It has been shown to help some symptoms of subluxations such as low back pain, neck pain and tension type headaches, but there are only limited clinically controlled studies to evaluate the effects on organ function.

# Chiropractic and medical definitions
The chiropractic vertebral subluxation is not defined the same way as the medical subluxation. In the following statements, the World Health Organisation explains the difference by starting with a standard chiropractic definition:
In the footnote to that definition it then explains a significant difference:
It thus implies that the degree of structural displacement of the chiropractic vertebral subluxation may not necessarily be "visible on static imaging studies."

## Historical definitions
Long before the advent of chiropractic, the possible neurological consequences of the medical subluxation were described by Harrison in 1821:
In the chiropractic system developed by Daniel D. Palmer in the late 1800's, Palmer originally believed that he had discovered the cause of all diseases suffered by mankind  the vertebral subluxation.
Chiropractors use and have used various terms to express this concept: subluxation, vertebral subluxation (VS), vertebral subluxation complex (VSC), "killer subluxations," the "silent killer," or a "bone out of place" (BOOP).

# Functional theory of vertebral subluxation
As a continuation  of the brain, the spinal cord contains nerve tracts which are the neurological pathways through which the brain communicates with most of the body.  While the brain is protected by the skull, the spinal cord is protected by the vertebral column. As nerves branch off the spinal cord, they form the spinal nerve roots which exit the vertebral column through an opening made by two adjacent vertebrae, called the intervertebral foramen.
V. Strang, D.C., describes several hypotheses on how a misaligned vertebra may cause interference to the nervous system in his book, Essential Principles of Chiropractic.
- Nerve compression hypothesis: suggests that when the vertebrae are out of alignment, the nerve roots and/or spinal cord can become pinched or irritated.  While the most commonly referenced hypothesis, and easiest for a patient to understand, it may be the least likely to occur.
- Proprioceptive insult hypothesis: focuses on articular alterations causing hyperactivity of the sensory nerve fibers.
- Somatosympathetic reflex hypothesis: all the visceral organ functions can be reflexly affected by cutaneous or muscular stimulation.
- Somatosomatic reflex hypothesis: afferent impulses from one part of the body can result in reflex activity in other parts of the body.
- Viscerosomatic reflex hypothesis: visceral afferent fibers cause reflex somatic problems.
- Somatopsychic hypothesis: the effects of a subluxation on the ascending paths of the reticular activating system.
- Neurodystrophic hypothesis: focuses on lowered tissue resistance that results from abnormal innervation.
- Dentate ligament-cord distortion hypothesis: upper cervical misalignments can cause the dentate ligaments to put a stress on the spinal cord.
- Psychogenic hypothesis: emotions, such as stress, causing contraction in skeletal muscles.
The vertebral subluxation has been described as a syndrome with signs and symptoms which include: altered alignment; aberrant motion; palpable soft tissue changes; localized/referred pain; muscle contraction or imbalance; altered physiological function; reversible with adjustment/manipulation; focal tenderness. Others believe that a subluxation itself is a sign, not a syndrome.

# Treatment of vertebral subluxation
When chiropractors believe a vertebral subluxation is present they may apply a specific adjustment to the spinal bone considered to be subluxated. While there are a number of different chiropractic techniques, each can be characterized by the description of having a short-lever, high velocity, low amplitude (HVLA) thrust and specific line of correction.
While chiropractors do attempt to direct treatment at specific movement segments of the spinal column, no other profession claims to intend to treat vertebral subluxations.
Once these bones or vertebrae are theoretically restored to their proper position and/or motion, the spinal cord and/or its nerve roots are no longer considered to be  hindered or compressed. Thus, without interference from a subluxation, the brain is considered to be enabled to transmit and receive all the messages through the spinal cord and nerve roots to all the parts of the body supplied by those nerves. The spine is considered "in line" and thus it is assumed the body functions in a coordinated manner resulting in increased health.

# Scientific investigation of vertebral subluxation
The investigation of vertebral subluxation has been ongoing since it was first postulated in 1895. The early practitioners used palpation and the anatomy of the nervous system as a guide (meric system). In their efforts to be more specific, they seized the newly discovered X-ray technology and introduced the neurocalometer (a heat sensing device). It was during those early years that the medical establishment first criticized the chiropractic profession, saying that the conditions that those early chiropractors were treating were only psychophysiologic disorders. To prove that chiropractic patients had real conditions, BJ Palmer opened a research clinic as a part of the Palmer College of Chiropractic. When a patient entered the clinic, they were first examined by medical doctors and a diagnosis was formulated. They were then sent to the chiropractic part of the clinic, treated, and sent back to the medical doctors for evaluation. Since then, chiropractors have sought a greater understanding of the mechanisms and effects of the vertebral subluxation. Today we see motion x-rays, surface EMG, and digital thermography.
As research projects are able to employ new techniques and technologies to evaluate nervous system function and effects, further support for chiropractic principles has surfaced.  Chiropractors have long suggested that spinal joint fixation that results from subluxation will result in degenerative effects that break down the spinal joints.  A 2004 research team at the National University of Health Sciences evaluated changes of the lumbar vertebral column following fixation (immobility) by surgically fusing spinal joints in experimental rats.  The fixated joints showed significant degeneration compared to the mobile joints, confirming that surgical fixation results in time-dependent degenerative changes of the zygapophysial joints.
Chiropractic also asserts that spinal health and function are directly related to general health and well being.  Research concerning the intricate functioning of the nervous system suggests that this speculation may have some support.  Seaman reviewed the work of several researchers concerning autonomic nervous system relationship to the somatic tissues of the spine.  He noted that Feinstein et al. were the first to clearly describe some symptoms associated with noxious irritation of spinal tissues. They injected hypertonic saline into interspinous tissues and paraspinal muscles of normal volunteers for the purpose of characterizing local and referred pain patterns that might develop. His observations included:
Feinstein referred to these symptoms as autonomic concomitants . It is likely that these autonomic concomitants were caused by nociceptive stimulation of autonomic centers in the brainstem, particularly the medulla. Feinstein indicated that "this is an example of the ability of deep noxious stimulation to activate generalized autonomic responses independently of the relay of pain to conscious levels." In other words, pain may not be the symptomatic outcome of nociceptive stimulation of spinal structures. Such a conclusion has profound implications for the chiropractic profession. Clearly, patients do not need to be in pain to be candidates for spinal adjustments.
Questions concerning the effects of vertebral subluxation on organ systems remain. To complicate matters, Nansel finds that:
Considering this phenomenon, Seaman suggests that the chiropractic concept of joint complex (somatic) dysfunction should be incorporated into the differential diagnosis of pain and visceral symptoms because these dysfunctions often generate symptoms similar to those produced by true visceral disease and notes that this mimicry leads to unnecessary surgical procedures and medications.
Researchers at the RMIT University-Japan, Tokyo studied reflex effects of subluxation with regards to the autonomic nervous system. They found that "recent neuroscience research supports a neurophysiologic rationale for the concept that aberrant stimulation of spinal or paraspinal structures may lead to segmentally organized reflex responses of the autonomic nervous system, which in turn may alter visceral function." 
Professor Philip S. Bolton of the School of Biomedical Sciences at University of Newcastle, Australia writes in JMPT, "The traditional chiropractic vertebral subluxation hypothesis proposes that vertebral misalignment cause illness, disease, or both. This hypothesis remains controversial." His objective was, "To briefly review and update experimental evidence concerning reflex effects of vertebral subluxations, particularly concerning peripheral nervous system responses to vertebral subluxations. Data source: Information was obtained from chiropractic or, scientific peer-reviewed literature concerning human or animal studies of neural responses to vertebral subluxation, vertebral displacement or movement, or both."  He concluded, "Animal models suggest that vertebral displacements and putative vertebral subluxations may modulate activity in group I to IV afferent nerves. However, it is not clear whether these afferent nerves are modulated during normal day-to-day activities of living and, if so, what segmental or whole-body reflex effects they may have."
Conclusions: Monitoring mixed-nerve root discharges in response to spinal manipulative thrusts in vivo in human subjects undergoing lumbar surgery is feasible. Neurophysiologic responses appeared sensitive to the contact point and applied force vector of the spinal manipulative thrust. Further study of the neurophysiologic mechanisms of spinal manipulation in humans and animals is needed to more precisely identify the mechanisms and neural pathways involved. 
Researchers at the Department of Physiology, University College London studied the effects of compression upon conduction in myelinated axons. Using pneumatic pressure of varying degrees on the sciatic nerves of frog specimens, the studied supported the idea of nerve conduction failure as a result of compression. 
A model for chronic nerve root compression studies. 

# Critiques
An area of debate among chiropractors is whether "vertebral subluxation" is a metaphysical concept (as posited in B. J. Palmer's philosophy of chiropractic) or a real phenomenon.
In an article on vertebral subluxation, the chiropractic authors wrote:
Since its inception, the concept of vertebral subluxation has been a source of definitional debate. Tedd Koren, DC offers this explanation as a possible cause of the confusion: