Chiropractic is based upon three related scientific theories:

1. Pathological disease processes may be influenced by disturbances of the nervous system. Many factors have been implicated as impairing health, such as hereditary and genetic anomalies; improper rest; lack of exercise; inadequate and improper nutrition; overindulgence; foods tainted by pesticides, insecticides, herbicides, fertilizers and preservatives; improperly cooked and/or processed foods; contaminated water; air pollution; stress, bodily insults and trauma; and the relative virulence and density of pathogenic organisms (bacteria and viruses). In addition to these factors, disturbances of the nervous system mediated primarily by the musculoskeletal system may also be an important factor contributing to disease etiology. Chiropractic maintains that proper structural and biomechanical integrity are important for the maintenance of homeostatic balance and resistance to disease. Agents that irritate bodily organs and structures, upset homeostasis, precipitate a bodily response that is mediated in part by the nervous system. An impaired nervous system may diminish the body’s defensive capabilities, its ability to adapt to internal or external stress and environmental change thus contributing to its susceptibility to disease etiology. Conversely, an impaired nervous system may affect adversely homeostatic balance, lowering bodily resistance as sometimes evidenced by functional disturbances without overt pathology, and which tend to weaken the body’s ability to resist disease. In this manner, the nervous system may be the root of some pathological process.

2. Disturbances of the nervous system may be the result of derangements of the musculoskeletal structure. Two broad and otherwise general theories have been identified in chiropractic by scientific researchers. One has been called the “biomechanics hypothesis,” and the other the “nerve compression hypothesis.”(1)

According to research scientists, the former consists of “chronic vertebral deviations (classical subluxations) or postural defects which may put mechanical strain on soft tissues associated with the spine: the periosteum, ligaments, fibrous capsules of joints, discs, as well as tendons, fascia and body muscles. These tissues form an essential part of the spine as a mechanical system, enabling it to support large loads through a wide range of movement. If the mechanical disorder is uncorrected, the soft tissue may undergo change, and these would exacerbate the disorder. Thus, discs may undergo thinning or protrusion. Ligaments and joint capsules may become fibrotic and thickened. Such changes would often tend to aggravate the mechanical disorder, further disturbing the normal alignment of the vertebrae. Stretched muscles would tend to contract, owing to reflex servomechanisms, and possibly go into spasms or sustained contraction. The resultant asymmetry of forces may again exacerbate the mechanical malfunctioning.

“These effects, sustained muscle contraction and irritation of the deep tissues of the spine are potential sources of pain. Deep pain, originating in the soft tissues of the spine may be referred to peripheral structures, especially if the sensory fibers from the latter enter the cord through the same dorsal roots as those conveying pain signals from the affected tissue of the spine. The reflex reaction to the pain itself may put additional tensions on the spine. In this way vicious circles are established, which prevent restoration of normal mechanical relations within the spine and at the same time, generate abnormal reflex activity, owing to stimulation of pain receptors and reflex responses to stretch of muscles and tendons.” (2)

In contrast, the nerve compression hypothesis contends that, “aberrant neural activity results from mechanical disorders of the spine due to compression of spinal nerves at the intervertebral foramina. This hypothesis still occupies a central place in the chiropractic rationale …” (3)

The latter hypothesis above represents the “classical subluxation” theory in chiropractic. Recent research in spinal and musculoskeletal biomechanics is beginning to receive increased attention in this area. Several important research developments in the past ten years underscore the need for more research on this aspect of chiropractic theory.

Recent studies have shown that the spinal nerve roots appear to be uncharacteristically sensitive to pressure and compression in contrast to previous studies on peripheral nerves. (4-6)

According to Sharpless at the University of Colorado,

“…The dorsal [nerve] roots are exquisitely sensitive to compression block when they are compressed within a few centimeters of their entrance to the [spinal] cord, far more than had been previously supposed. Compound action potentials representing volleys of nerve impulses in myelinated fibers are reduced to about one-half their normal values by a pressure of approximately 20 mm Hg. [mercury] in the roots… ” (5) And,

“A pressure of only 10mm Hg. produced significant conduction block, the potential [of nerve impulses] falling under 60 percent of its initial value in 15 minutes. With higher levels of pressure, we have observed incomplete recovery after many hours of recording.” (6)

Further research has also shown that rapidly conducting and large diameter nerve root fibers are the most susceptible to compression and pressure blocks (7-9) and that small nerve fibers may be more sensitive to anoxia and ion imbalances created by pressure on the nerve. (8-11) It has also been found that axoplasmic flow; the movement of sustaining nutrients and chemical mediators is also susceptible to various forms of compression. (9) Dr. Luttges (PhD) another researcher at the University of Colorado has found that, “[nerve] compression produced by constrictive cuffs (approximately one-third to one-half reduction in nerve diameter) was found to produce degenerative effects different from, but as profound as, those produced by nerve section.” (10)

Intervertebral subluxations have also found a place in medical literature. In fact in the early 1950’s Hadley found evidence of nerve root compression in many postmortem examinations. These subluxations were particularly prevalent in the cervical and lumbar areas of the spine. (11-16) More recently, Epstein indicated that nerve root entrapment and compression may be responsible for such things as sciatica; intermittent claudication, an ischemic condition; and other similar types of conditions. (12) This closely parallels Sunderlands theory that “compression first leads to venous obstruction followed by capillary circulation disturbances resulting in anoxia and finally irreversible ischemic damage.”

Sunderland feels that in the absence of overt pathology, e.g., osteophytic enlargement [osteo-arthritis], malignancies, etc., that compression is the result of apophyseal joint swelling secondary to [classical] subluxation of the joint and trauma to the capsule reducing the cross-sectional area of the intervertebral foramen through which the nerve passes resulting in compressive distortions of the tissues in the foramen. (13)

Cailliet states that, “pain in and from the neck results from the mechanical factor of encroachment of space and impairment of movement. Decrease in the space in which pain-sensitive tissues lie or through which they pass compresses these tissues, resulting in possible pain and loss of functions. Pain is more apt to occur if pressure is acute and transient, whereas loss of function is more likely as pressure is prolonged and continuous. The sites at with tissues are most likely to compress are the intervertebral foramina and within the spinal canal. The tissues in these specific areas are nerves and their coverings, blood vessels, ligaments, joint capsules, and dura mater. Encroachment of space resulting in pressure upon these tissues may result in pain or loss of function.”(14)

A prominent orthopedic surgeon and researcher Kirkaldy-Willis claims that ninety percent (90%) of patients with low back pain have “dysfunction,” indicating that changes exhibited are mainly those of abnormal function with slight anatomic changes to the three joint complex, the intervertebral disc and zygapophyseal joints. Of the remaining ten percent (10%) he claims that fifty percent (50%) of those have lateral nerve root entrapment. With dynamic recurrent lateral entrapment there is a laxity of the posterior joints and of the annulus causing abnormal movement of the vertebrae resulting in a narrowing of the lateral nerve canal and tapping on the main spinal nerve as this passes along the canal. Dr. Kirkaldy-Willis indicates that manipulation is an effective method of treatment for both patients with dysfunction and lateral nerve root entrapment. (15)

Other conditions involving the motor and motion units of the spine are stretching and traction injuries to the nerve roots and chronic irritations of the neural complex from adhesions, osteoarthritis, fibrosis, etc. [nerve root entrapment], and disc lesions such as herniations, discogenic diseases etc., will affect the neural complex.

In all likelihood a multiplicity of factors are responsible for the clinical manifestations that chiropractors experience in practice rather than a single mechanism. Many of the different categories or theories of causation, however, may be classified as variant forms of subluxations in chiropractic so that the term “subluxation” assumes a broader and more diverse meaning in chiropractic than in classical literature. The above information demonstrates, albeit briefly, that musculoskeletal derangements may indeed have an affect on the nervous system. Subluxations of vertebral and pelvic segments represent common chiropractic mechanical clinical findings in man. Extended abnormal secondary involvement of the nervous system may result from disturbances, strains, and stresses arising with the musculoskeletal system due to mankind’s attempt to maintain an erect posture. These mechanical lesions or subluxations are a common result of gravitational strains, asymmetrical activities and efforts, and developmental defects or other mechanical, chemical, or psychic irritations of the nervous system.

3. Disturbances of the nervous system may aggravate pathological processes in various parts or with various functions of the body. Vertebral and pelvis subluxations may be involved in common functional disorders of a visceral and vasomotor nature, and at times may produce phenomena that relate to the special organs.

Under predisposing circumstances, almost any component of the nervous system may directly or indirectly cause reactions within any other component by means of reflex mediation. The conjunction of independent causes of bodily dysfunction may jointly have more serious debilatory effects than either cause might have had separately. Subluxations may contribute to the “triggering” or exacerbating of certain types of neurovascular and neurovisceral instabilities. Correction of the spinal [manipulative] lesion is often imperative for effective total management of a patient’s case.

References:
1. Suh CH, Sharpless SK, Macgregor RJ, Luttges MW, Researching the Fundamentals of Chiropractic (Summary of Research, 1971-1973). The University of Colorado, Boulder, Colorado, 1974, P. 11.
2. Ibid. p. 11-12.
3. Ibid. P. 26.
4. Gelfan S. Tarlov I. Physiology of the Spinal Cord, Nerve Root and Peripheral Nerve Compression. Amer J Physiol 1956; 185: 217-229.
5. Sharpless SK, Groves P, Cobb S, et. al. Neurophysiology of Nerve Compression and joint Fixation. The Fifth Annual Biomechanics Conference of the Spine, Biomechanics Laboratory, University of Colorado, Boulder, Colorado, December 1974, p. 219-277.
6. Sharpless SK, Susceptibility of spinal roots to compression block. In: Goldstein M. The Research Status of Spinal Manipulative Therapy, US Department of Health, Education and Welfare, Public Health Service, National Institute of Neurological and Communicative Disorders and Stroke monograph no. 15, Bethesda, MD 20014, DHEW Publication No. (NIH) 76-998, February 1975, p. 155.
7. Gasser H., Erlanger J. The role of fiber size in the establishment of nerve block by pressure or cocaine. Am J Physiol 1929; 88: 581-591.
8. Haldeman S, Meyer BJ. The effect of constriction on the action potential of the sciatic nerve. S Afr Med J 1970; 44: 903.
9. Luttges MW, Kelly PT, Gerren RA. Degenerative changes in mouse sciatic nerves: electrophoretic and electrophysiologic characterizations. Exp Neurol 1976; 50: 706.
10. Wilcox G. Doctoral dissertation. University of Colorado, 1975.
11. Jones D, Luttges MW. Alterations in passive electrical parameters of nervous tissue and their relation to conduction block. AAAS, Southwestem and Rocky Mountain Abstract 180, 1977.
12. Sjostrand J, et al. Impairment of intraneural micro-circulation, bold-nerve barrier and axonal transport in experimental nerve ischemia and compression, In: Korr IM, The Neurobiologic Mechanisms in Manipulative Therapy, New York, Plenum Press, 1978, p. 337-355.
13. Luttges MW, Gerren RA. Compression Physiology: Nerves and Roots. In: Haldeman S, Modern Developments in the Principles and Practice of Chiropractic, New York, Appleton, Century & Crofts, 1980, p. 65-92.
14. Hadley LA. Intervertebral joint subluxation, bony impingement and foramen encroachment with nerve root changes. Am J Roent Rad Ther 1951; 65: 377-402.
15. Hadley LA. Constriction of the Intervertebral Foramen. JAMA 1949; 140: 473-476.
16. Hadley LA. Roentgenographic studies of the cervical spine. Am J Roent 1944; 52: 173-195.
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