Orthopedic surgeons DePalma and Rothman²³ describe the cervical spine intervertebral foramina as small ovoid canals with vertical diameters approximately 10 mm in height, with the anteroposterior diameter about one half the size of the vertical diameter. The authors state that the “…nerve roots and mixed spinal nerves completely fill the anteroposterior diameter of the intervertebral foramina. The upper one quarter of the canal is filled with areolar tissue and small veins” and “small arteries arising from the vertebra.” DePalma and Rothman continue, “any space taking lesion which pinches the anteroposterior diameter of the intervertebral foramen might be expected to cause some compression of the nervous tissue elements traversing this limited space.” In contrast, these authors describe the normal lumber IVF as five to six times the diameter of the spinal nerve, permitting relatively great freedom from constriction.

Jackson, ²⁴ an orthopedist, in describing the boundaries of the cervical IVF, states:

Parallel to the conclusions of Rothman and DePalma, Jackson concludes:

Jackson notes that ventral nerve root fibers are in intimate contact with the margins of the lateral interbody joints. The posterior fibers, or posterior nerve roots, are in intimate contact with the posterior-superior articular processes of the adjacent distal vertebrae. Jackson explains:

Jackson uses the term cervical syndrome to describe the group of symptoms and clinical findings resulting from irritation or compression of the cervical nerve roots in or about the IVF. ²⁴

Lu and Ebraheim, ³³ in dissections of the dorsal root ganglia of the second cervical nerve, found that the dorsal ganglia are all proximally placed and occupy most of the foramen. The cervical ganglia at this spinal level are confined within the foramina between the arch of the atlas and the lamina of the axis. These ganglia occupy 76% of the foramen height. Such anatomic relationships may render the second cervical dorsal ganglion vulnerable to entrapment. These authors further state that trauma with extreme rotation and extension, as occurs in whiplash injuries, at the C1-C2 joint has the potential to crush the second cervical ganglion between the arch of the atlas and the lamina of the axis and may be implicated in cervicogenic headache. Many patients with headache and or neck trauma have a history of motor vehicle accident typical of whiplash. Cervicogenic headache (see Chapter 24) may result from displacement, abnormal movements, or arthritic changes in the atlas-axis articulation, compromising the second cervical nerve root and ganglion. In addition, compressing or entrapping the second cervical ganglion involves fibers that contribute to the greater occipital nerve.

Crelin, ³⁴ an anatomist, in a 1973 article frequently quoted by opponents of chiropractic, argued that nerve roots pass through spacious intervertebral foramina and that therefore exertion of pressure on a spinal nerve does not occur. However, a review of the study’s methodology coupled with current research demonstrates that Crelin’s conclusion is in error concerning the effects of joint subluxation. ³⁵

Seeking to “prove” that the theory of spinal nerve impingement is “impossible,” Crelin³⁴ obtained vertebral columns from six individuals. Three columns were from full-term infants; the others were from adults ages 35, 73, and 76 years. The vertebral column of each was excised within 3 to 6 hours after death. The skull was disarticulated from the first cervical vertebra, and the fifth lumbar vertebra was disarticulated from the sacrum. Each spinal nerve was transected 8 cm after emerging from the IVF. Deep paraspinal musculature, ligaments, and joint capsules were left intact.

Two metal vises were clamped to a platform supported the vertebral column while it was subjected to compressive forces. Five vertebral segments of the newborn column and three of the adult columns were suspended between the vises. A Dillon force gauge was used to measure force of compression applied to the vertebra. A range of maximum compression forces, including twisting and flexion, were applied. The osseous boundary of the foramen did not come in contact with the nerve, and Crelin reported that there was never less than 1.5 mm of space completely surrounding the cervical nerves, 3 mm around the thoracic nerves, and 4 mm surrounding the lumbar nerves. Crelin explained that all spinal nerves emerging from their IVFs were exposed before testing and that gentle teasing with small forceps removed the flimsy areolar tissue surrounding the nerves to expose the border of the “spacious vertebral foramina.”

However, this very tissue that Crelin removed contains important connective tissue elements that may be compressible³⁶ or, when irritated, may release chemicals having an adverse effect on nerve function.^{37383940 and 41} Some critics of the nerve compression theory have neglected to recognize that spinal biomechanical derangements do not involve “hard bone on soft nerve.” Instead, the key issue is the potential for altered interforaminal mechanics to affect vascular and connective tissue support structures, as well as the important neural components within the IVF.

Contemporary information describes the IVF as an extended interpedicular zone, ⁴² often containing transforaminal ligaments.^{43444546 and 47} These anatomic factors, combined with current understanding of spinal nerve root sensitivity to pressure^{4849 and 50} and irritation,^{3738394041 and 51} render Crelin’s conclusions unsupportable. ⁵²

Giles⁴² maintains that the IVF should no longer be conceptualized as a two-dimensional hole, but rather as a canal or tunnel through which the spinal nerve and other related structures pass. Giles maintains that “neural and associated vascular structures within the important interpedicular zone may well be compromised due to vertebral joint subluxation. This may result in chronic compression,” adding, “The precise significance clinically… is yet to be determined.”

Giles took nine randomly chosen sections of adult lumbosacral spinal tissue and examined them histologically for measurement of the L4-L5 and L5-S1 IVF canals. The zone between the pedicles of adjacent vertebrae was found to have a horizontal length of 8.2 to 12.2 mm. At a minimum, the distance between the nerve structures and the side of the IVF canal was 0.4 to 0.8 mm for both the L4-L5 and L5-S1 segments. Giles concludes that the Crelin study was “meaningless as a basis for consideration of the possible physiological and/or pathophysiological functions of spinal nerves beyond the intervertebral canal as he did not examine the important interpedicular zone.” that contains the spinal nerve root and ganglion. Rather, he examined only “the relatively insignificant lateral border.”

As described by Golub and Silverman, ⁴³transforaminal ligaments (TFL) are ligamentous bands crossing the IVF at any spinal level (Fig. 8-1). In dissections of 15 lumbar spines representing 150 IVFs, Bachop and Hilgendorf⁴⁴ found varying numbers of TFLs. Bakkum⁴⁵ determined that TFLs, once considered an abnormality, are normal and greatly reduce the functional compartment or space available for the spinal nerve. In Bakkum’s study, four adult lumbosacral spines without visible pathology or degenerative changes were examined, yielding the following results: 35 of the 49 IVFs examined (71%) had at least one TFL. More than one quarter (27%) had two TFLs, and 8% had three or four.

According to Bachop and Janse, ⁴⁶ the higher the TFL is situated in the foramen, the less space remains for the spinal vessels. This anatomical relationship can conceivably lead to ischemia or venous congestion. On the other hand, the lower the TFL is located, the greater the possibility will be of sensory deficits, motor deficits, or both. In a study of accessory ligaments of the IVF, Amonoo-Kuoffi and colleagues⁴⁷ conclude that the spinal nerve, segmental veins and arteries, and the recurrent meningeal nerve are held in place through the openings between the accessory ligaments within the IVF. In IVFs in which multiple TFLs are present, these nerves and vessels are literally threaded through a lattice created by the TFLs.

Hadley, ⁵¹ a medical radiologist, states that the importance of the IVF lies in the fact that, except for the first and second cervical nerves, each peripheral nerve must pass through one of these openings. The cervical region contains “a close five-way interrelationship between the foramen, the nerve root which passes through it, the vertebral artery contacting the root in front, the co-vertebral joint anteriorly and the posterior cervical articulation in back.” Hadley also notes that arthrotic and degenerative changes involving these structures may become an important factor resulting in foraminal encroachment and that, “bulging disc substance, exostoses or subluxation of a posterior joint may produce pressure upon the root.”

Hadley⁵¹ goes on to state: “Subluxation (partial displacement) of the vertebral bodies …may present radiographically one or more of the following features:

Regarding cervical encroachment, Hadley⁵¹ observes that, in addition to local and referred pain, patients can suffer from bizarre symptoms called chronic cervical syndrome. These symptoms are described as paroxysmal deep or superficial pain in parts of the head, face, ear, throat, or sinuses; sensory disturbances in the pharynx; vertigo; and tinnitus, with diminished hearing. Vasomotor disturbances include sweating, flushing, lacrimation, and salivation. Hadley adds, “spontaneous subluxation at the C1-C2 level either unilateral or bilateral, is usually a sequel to an inflammatory process of the throat.”

Commenting on the thoracic region, Hadley⁵¹ states, “Since the thoracic roots are relatively small, compression of these structures does not occur in the foramina of this region.” Regarding the lumbar region, Hadley affirms that the spinal nerve occupies approximately one fifth to one fourth the diameter of the normal foramen. The remainder of the space is taken up by blood and lymphatic vessels, areolar, and fatty tissue, which together constitute a “compressible safety cushion space which allows the physiological encroachment to occur without nerve compression.” Moreover, “any abnormal constriction in the size of a normal IVF if not actually causing nerve root pressure, nevertheless decreases the reserve safety cushion space surrounding that nerve and may predispose to pressure.”

In a study of the kinematics of the lumbar IVF under normal physiologic spinal motions, Panjabi and colleagues⁵⁴ maintain that in cases of spinal degeneration, normal physiologic motion “may be enough to compromise the space around the nerve root to such a degree that very little safety margin is left.” These authors further state:

Panjabi, discussing the research of Sunderland, ⁴⁸ agrees that “in contrast to the thicker connective tissue covering of the peripheral nerve, the anatomical and mechanical weakness of the spinal nerve root more easily causes interneural fibrosis and adhesions to the surrounding IVF tissues.”

Korr, ⁵⁵ a physiologist and osteopathic researcher, observes that much of the pathway taken by nerves as they emerge from the cord is through skeletal muscle. The contractile forces of this muscle along with associated chemical changes exert profound influences on the metabolism and excitability of neurons. In such an environment, neurons are subject to considerable mechanical insult (compression and torsion), as well as chemical influences. Nerve sheaths, which are extensions of the meninges surrounding the spinal cord, extend distally along the spinal nerve roots, providing a root sleeve that allows the nerves to slide smoothly in and out without friction during a wide range of vertebral column movement. Over time, however, slight mechanical stresses may produce adhesions, constrictions, and angulations. From a chiropractic prospective, this vulnerability of nerve trunks represents effects associated with the myologic component of the VSC and also relates to its inflammatory and biochemical components.

Vascular structures pass through the IVF and provide blood supply to both the bony vertebral column and the spinal cord. ⁵⁶ Ischemia of the nerve cells within the dorsal root ganglia may lead to progressive loss of sensory function, including proprioception. Edematous pressure from even slight congestion of venous drainage may affect nerve conduction. ⁵⁶ Such relationships are associated with the vascular and neurologic components of VSC. Olmarker and colleagues, ^56a in studying the effects of experimental compression on intraneural capillary and venular blood flow through tracking the intraneural transport of tracer-labeled glucose, determined that as low as 10 mm Hg pressure applied to the dorsal roots reduced nutritional transport to the peripheral axons to 20% to 30%.

Sympathetic ganglia in the highly mobile neck area and the cervical chain of ganglia positioned against the vertebral column are subject to stress imposed by motion. This stress may exert profound influence on the physiology of sympathetic nerve cells. Furthermore, the mechanical disturbances or somatic insults previously described by Korr exert slight forces resulting in slight tissue changes within the IVF and in paraspinal structures. This condition may adversely affect nerve function. A further proposal asserts that disorders of muscle tension, tissue texture, and visceral and circulatory function are reflected at the body surface as observable diagnostic elements. ^49,50

Spinal nerve roots, as compared with peripheral nerves, have a less abundant protective epineurium, no branching fasciculi, and poor lymphatic drainage.^{5657 and 58} These facts imply that the nerve root is more susceptible to injury by mechanical forces. ⁵⁹ Panjabi and colleagues⁵⁴ state that “the nerve root is constrained in the intervertebral foramen and may be easily compressed or mechanically irritated under adverse conditions of degeneration and movement.” In consideration of the effects on nerve function by subluxation or spinal biomechanical impairment, questions arise regarding possible pathophysiologic mechanism associated with compression or mechanical irritation of nerves.

Sharpless, ⁶⁰ in a study to determine the susceptibility of spinal nerve roots to compression, concluded:

Rydevik⁶¹ determined:

Konno and colleagues⁶² reported that compression of cauda equina nerve roots decreased action potentials with as little as 10 mm Hg of pressure. Hause⁶³ proposed a mechanism of progression in which mechanical changes lead to circulatory changes, after which inflammatogenic agents produce chemical radiculitis. This action, in turn, leads to disturbed flow of cerebrospinal fluid, with defective fibrinolysis and subsequent cellular changes. In addition, the influence of the sympathetic system may result in synaptic sensitization of the central and peripheral nerves, creating a vicious circle, resulting in radicular pain. Hause also proposes that compressed nerve roots can exist without causing pain.

Mechanical tension on the pain-sensitive dura mater may contribute to headache. Hack and colleagues⁶⁴ found that the rectus capitis posterior minor muscle extends from the occiput to the posterior arch of the atlas vertebra and connects via a bridge of connective tissue to the spinal dura. This connection may resist inward folding of the dura, which may compromise cerebrospinal fluid flow when the neck is extended. The dura is extremely sensitive, and tension applied during surgery is felt as headache. The muscle-dura connection may transmit forces from the neck muscles to the pain-sensitive area of the dura. This muscle-dura connection may represent an anatomic basis for the effectiveness of manipulation/adjustment, which may decrease muscle tension and reduce pain by reducing the forces between C1 and C2 involving the rectus capitis posterior major and oblique capitis inferior muscles (see Fig. 23-5, p. 479).

In Greek, soma means “body.” In the somatosomatic reflex hypothesis, stimulus at one level of the soma or musculoskeletal system produces reflex activity in the nervous system, which is then exhibited elsewhere in the musculoskeletal system. The knee-jerk reflex is an example of a somatosomatic reflex. A light tap on the patellar ligament activates stretch receptors located within that ligament and in the tendon of the quadriceps muscle, which inserts on the patella. Impulses are conducted by sensory (afferent) neurons to the CNS, specifically at the intersegmental L3 and L4 levels, where these neurons synapse with motor neurons in the gray matter of the spinal cord. Without conscious involvement of the brain, impulses are then conducted by motor (efferent) neurons back to the quadriceps muscle. The muscle contracts in response to the impulses traveling along its motor nerve.

In this example, a stimulus was applied to a receptor in a somatic structure, eliciting a response in another somatic structure. Similarly, stimuli to receptors in spinal structures, whether from abnormal joint or muscle tension or from chiropractic adjustment⁷² or manipulative treatment^30,31,73,74 to relieve it, cause various spinal reflex responses in the musculoskeletal system.

The interneurons of the dorsal horn are proposed to be involved in pain inhibition. As an example, strong tactile stimulation, as in skin rubbing, has long been known to diminish sensations of dull aching or sharp pain originating from an injured area. This decreased sensation is the result of the effect of afferent inhibition or the gate control theory. The mechanical intervention of rubbing activates the large, fast-conducting tactile-or touch-responsive A-alpha fibers (mechanoreceptors), which, in turn, inhibit the synaptic transmission of the pain signals by blocking the synaptic gates normally used by the smaller C-fibers that convey pain signals, therefore suppressing the signals of pain.

Wyke^{3031 and 32} suggested that spinal manipulation stretches mechanoreceptors in the joint capsule and that this stimulus has an inhibitory effect, mediated through spinal cord interneurons, on nociceptive activity. Increased mechanoreceptive input from increased joint motion reduces, or closes, the gate on pain signal transmission. This proposed mechanism is an adaptation of Wall’s gate control theory. ⁷⁵

A subcomponent of the somatosomatic reflex model, referred to as the proprioceptive insult hypothesis^26,76,77 suggests that receptors in the highly innervated soft tissue in and around joints may become irritated, leading to reflex modifications in postural tone and neural integration of postural activities. The proprioceptive insult hypothesis has been described in chiropractic writings^78,79 as undue irritation and stimulation of sensory receptors (including proprioceptors) located in the articular structures and in the parasegmental spinal ligaments. This irritation may result when structures are under stress from derangement of the intervertebral motor units caused by subluxation. Janse²⁶ proposes that the afferent barrage of impulses into the nervous system may disturb equilibrium, create somatosomatic reflexes, and cause aberrant somatovisceral and somatopsychic reflexes.

Reflex muscle spasm is another example of a somatosomatic reflex. ⁸⁰ This reflex has been associated with the facilitated segment, in which muscle spasm may result from and contribute to proprioceptive irritation. Theories⁸⁰ suggest that the spinal cord segments in the vicinity of a spinal fixation have a lower threshold for firing and therefore are neurologically hyperexcitable. Korr refers to this concept as the facilitated lesion. ^81,82

The Latin meaning of viscera is internal organ. In this concept, a stimulus to nerves or receptors related to spinal structures produces reflexive responses influencing function in the visceral organs, such as those in the digestive, cardiovascular, or respiratory systems. Alternate terms for this form of spinal reflex are somatosympathetic and somatoautonomic.

Lewit, ⁵³ in a review of vertebrovisceral relations in a variety of cases, cites an example in which changes in spinal function (joint blockage or hypomobility) are linked to tachycardia. Therefore when mobility of the spinal column is normalized, heart rhythm also becomes normal and remains so as long as no relapse occurs in spinal column dysfunction. Lewit states that “although direct evidence of disturbed motor function causing organic heart disease is lacking, it would seem reasonable to grant it the role of a possible risk factor.” Lewit uses the term blockage to describe spinal movement restriction, noting that the characteristic pattern of spinal hypomobility in ischemic heart disease is “blockage affecting the thoracic spine from T3 to T5, most frequently between T4 and T5, movement restriction being most noticeable to the left, and at the cervicothoracic junction.” Of historical relevance is that as early as the 1920s, chiropractic authors such as Vedder⁸³ and Firth⁸⁴ recommended adjustment of Heart Place, identified as the second and third dorsal vertebral segments, for treating tachycardia.