The Anatomy Component of the Subluxation Complex      

This section is compiled by Frank M. Painter, D.C.
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Spinal Anatomy 101
A Chiro.Org info collection

This page reviews the anatomy of vertebrae, describes the function of the “Spinal Motion Unit”, and clarifys which tissues suffer degenerative changes from the subluxation complex.

An Anatomical Study of the Suboccipital Cavernous Sinus and
its Relationship with the Myodural Bridge Complex

Clinical Anatomy 2023 (Jul); 36 (5): 726–736 ~ FULL TEXT

The suboccipital cavernous sinus (SCS) and the myodural bridge complex (MDBC) are both located in the suboccipital region. The SCS is regarded as a route for venous intracranial outflow and is often encountered during surgery. The MDBC consists of the suboccipital muscles, nuchal ligament, and myodural bridge and could be a power source for cerebrospinal fluid circulation. Intracranial pressure depends on intracranial blood volume and the cerebrospinal fluid. Since the SCS and MDBC have similar anatomical locations and functions, the aim of the present study was to reveal the relationships between them and the detailed anatomical characteristics of the SCS. The study involved gross dissection, histological staining, P45 plastination, and three-dimensional visualization techniques. The SCS consists of many small venous sinuses enclosed within a thin fibrous membrane that is strengthened by a fibrous arch closing the vertebral artery groove. The venous vessels are more abundant in the lateral and medial portions of the SCS than the middle portion. The middle and medial portions of the SCS are covered by the MDBC. Type I collagen fibers arranged in parallel and originating from the MDBC terminate on the SCS either directly or indirectly via the fibrous arch. The morphological features of SCS revealed in this research could serve as an anatomical basis for upper neck surgical procedures. There are parallel arrangements of type I collagen fibers between the MDBC and the SCS. The MDBC could change the blood volume in the SCS by pulling its wall during the head movement.

The Anatomy and Morphometry of Cervical Zygapophyseal Joint Meniscoids
Surg Radiol Anat. 2015 (Sep); 37 (7): 799–807 ~ FULL TEXT

Meniscoids were identified in 86% of zygapophyseal joints examined; 50% contained both ventral and dorsal meniscoids, 7% contained a ventral meniscoid only and 29% contained a dorsal meniscoid only. Meniscoids were classified as adipose (4%), fibrous (74%), or fibroadipose (22%) based upon histological composition. There were no significant associations between meniscoid size (surface area or protrusion length) and gender, position in joint, spinal level, or articular degeneration. Increased articular degeneration was associated with fibrous meniscoid classification.

Real-Time Visualization of Joint Cavitation
PLoS One. 2015 (Apr 15); 10 (4): e0119470 ~ FULL TEXT

Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. Unfortunately, bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon. Here we present direct evidence from real-time magnetic resonance imaging that the mechanism of joint cracking is related to cavity formation rather than bubble collapse. In this study, ten metacarpophalangeal joints were studied by inserting the finger of interest into a flexible tube tightened around a length of cable used to provide long-axis traction. Before and after traction, static 3D T1-weighted magnetic resonance images were acquired. During traction, rapid cine magnetic resonance images were obtained from the joint midline at a rate of 3.2 frames per second until the cracking event occurred. As traction forces increased, real-time cine magnetic resonance imaging demonstrated rapid cavity inception at the time of joint separation and sound production after which the resulting cavity remained visible. Our results offer direct experimental evidence that joint cracking is associated with cavity inception rather than collapse of a pre-existing bubble. These observations are consistent with tribonucleation, a known process where opposing surfaces resist separation until a critical point where they then separate rapidly creating sustained gas cavities. Observed previously in vitro, this is the first in-vivo macroscopic demonstration of tribonucleation and as such, provides a new theoretical framework to investigate health outcomes associated with joint cracking.

Effects of Unilateral Facet Fixation and Facetectomy on
Muscle Spindle Responsiveness During Simulated
Spinal Manipulation in an Animal Model

J Manipulative Physiol Ther. 2013 (Nov); 36 (9): 585–594 ~ FULL TEXT

The apparent relationship between intervertebral joint mobility and the clinical success of spinal manipulation for LBP, combined with increasing evidence for proprioceptive-related changes in individuals with LBP led us to undertake a basic science investigation to determine the relationship between changes in lumbar spinal stiffness and mechanoreceptor activity from muscle spindles in the low back during a simulated High Velocity Low Amplitude spinal manipulation (HVLA-SM) in an animal preparation. The purpose of this study was to determine whether relative increases versus decreases in spinal stiffness can impact paraspinal sensory sensory responses over 5 thrust durations of HVLA-SM directed at the same level as the dysfunction. This study aims to be an important first step in concurrently examining the effects of intervertebral dysfunction and peripheral afferent signaling during a commonly used and effective therapeutic intervention for LBP.

Evaluating the Relationship Among Cavitation, Zygapophyseal
Joint Gapping, and Spinal Manipulation:
An Exploratory Case Series

J Manipulative Physiol Ther. 2011 (Jan); 34 (1): 2–14 ~ FULL TEXT

Figure 1 summarizes the theory upon which this study was designed. Gapping is the separation of the zygapophyseal (Z) joint articular facets that occurs during spinal manipulative therapy (SMT) and side-posture positioning. [1] Zygapophyseal joint gapping during SMT is considered by many to be beneficial because the separation of joint surfaces during gapping is thought to   (1)   break up connective tissue adhesions that develop in hypomobile Z joints [2–4] (steps 1–5b, Fig 1) and   (2)   stimulate afferent nerves that innervate the Z joint capsule and the small muscles of the spine, [5–7] resulting in reflex neurologic [5, 8–16] (steps 3–5a, Fig 1) and possibly immunologic [15, 17, 18] consequences.

Surgical Model of a Chronic Subluxation in Rabbits
J Manipulative Physiol Ther. 1988 (Oct); 11 (5): 366–372

Critically needed in chiropractic research is an animal model of a subluxation that will allow experimental study. Previous attempts in this, as well as other, laboratories have been only minimally successful. We report here the development of a straightforward surgical method of producing a misalignment of the thoracic spine in rabbits, one that appears to be satisfactory for further study.

Basic Science Research Related to Chiropractic Spinal Adjusting:
The State of the Art and Recommendations Revisited

FROM:   J Manipulative Physiol Ther. 2006 (Nov);   29 (9):   726–761

Anatomical Research

The topic of anatomical research will be divided into studies that relate specifically to spinal manipulation/adjusting (SM) (“Studies Related to SM”) and those that add detail to the understanding of the spine (“Studies Adding to the Knowledge Base of the Spine”).

Studies Related to SM

Gapping of the zygapophysial joints

Dysfunction of the spine that is treated by chiropractors has been described as the vertebral subluxation complex.   This complex, as described by Lantz [2] and Rosner, [3] has several components.   These components include myologic, connective, vascular, neurologic, and lymphatic tissue involvement.   Many hypothesize that a fundamental component of the vertebral subluxation complex is the development of adhesions in the zygapophysial joints (Z joints) after hypomobility of these structures. [4–6]   Spinal adjusting of the lumbar region is thought to separate the articular surfaces of the Z joints. [7–9]   This “gapping” is theoretically the action that “breaks up” adhesions.   Elimination of adhesions would allow the Z joints to become more mobile, thus helping the motion segment (2 adjacent vertebrae and the ligamentous structures connecting them) to reestablish a physiologic range of motion (ROM). [5]   Although the idea that gapping the Z joints would occur during a SM and that this action would break up adhesions seems logical to those who have performed these procedures, no anatomical or physiologic evidence existed [1] to show that gapping of the Z joints actually occurred, or that adhesions in the Z joints could potentially be broken until relatively recently. [10, 11]   Cramer et al [10, 11] found that in healthy volunteers, the lumbar Z joints did gap during both side-posture positioning and during chiropractic adjusting and the joints gapped significantly more during the latter procedure.   However, no reports have been published by any group regarding gapping of the Z joints in the low back pain population.

Degeneration of the z joints after hypomobility

Paris [12] has reportedly identified adhesions in the Z joints after hypomobility. More recently, degenerative changes have been identified in the Z joints of rats after induced hypomobility. [13]   See the section on
Animal Models for further details.

Advances in magnetic resonance imaging relevant to the spine and spinal adjusting

Functional magnetic resonance imaging (MRI) evaluation of the spinal cord maps MRI signal changes following a specific stimulus designed to change neural activity. This procedure may become fundamental in the future workup of spinal cord injury [14] and may provide intriguing possibilities to the assessment of the spinal cord following procedures such as spinal adjusting. Diffusion and perfusion MRI provides information related to the structure and function of tissue at a microscopic level and will play a more prominent role in future neurovascular imaging. [15] As MRI at the molecular level becomes possible, opportunities for advances in research (and clinical practice) increase. [16]

Research is also being done involving morphometry of the spine by means of MRI. [17] Morphometric measurements allow for an increased ability to study the structures influenced by chiropractic adjusting. [11, 17] Of related interest is that direct oblique cervical MRI provides more accurate assessment of all of the borders of the cervical intervertebral foramens (IVFs) than standard sagittal MRI. [11]

Studies Adding to the Knowledge Base of the Spine

Zygapophysial joints

Between 15% and 40% of chronic low back pain is related to the Z joints. [18] The Z joint capsule receives a significant sensory innervation, [19, 20] much of which is probably related to nociception, that is, signaling potential or real tissue damage. The medial branches of the posterior primary divisions innervating a Z joint terminate as 1 of 3 types of sensory receptors:

free nerve endings (nociceptive),
complex unencapsulated nerve endings, and
encapsulated nerve endings.

The free nerve endings are associated with nociception. The ultrastructure of these receptors has been described. [20, 21]

The Z joint capsules throughout the vertebral column are thought to do little to limit motion. [22] However, the capsules probably help to stabilize the Z joints during motions. [23] The gross and microscopic anatomy of the Z joint capsules has been described in detail. [23, 24]

See the sections on
Biomechanics and the Somatic Nervous System for further details of current research related to the Z joints and spinal adjusting.

Ligaments of the spine

Nerve tracing techniques indicate that stretching of spinal ligaments results in “a barrage of sensory feedback from several spinal cord levels on both sides of the spinal cord.” [25] The sensory information has been found to ascend to many higher (cortical) centers. Such findings provide provocative evidence that the spinal ligaments, along with the Z joint capsules, and the small muscles of the spine (interspinales, intertransversarii, and transversospinalis muscles) play an important role in mechanisms related to spinal proprioception (joint position sense) and may play a role in the neural activity related to spinal adjusting. [26]

Recent work has also been done assessing the structure of spinal ligaments. The attachment sites and dimensions of the anterior and posterior longitudinal ligaments [27, 28] and the innervation and gross and light microscopic structure of the ligamenta flava [29, 30] and iliolumbar ligaments [31] have been studied in detail.

The long posterior sacroiliac ligament may be important in transmitting loads from the lower extremity to the spine. [32] The strongest fibers course from the posterior superior iliac spine to the sacrotuberous ligament, and many important structures attach to this band, including the aponeurotic attachments of the common origin of the erector spinae muscle. The ligament is tensed during counternutation of the sacrum and slackened during nutation. [32] These findings are considered to be important by those involved with the study of the “kinetic chain concept” of load transmission from the lower extremity to the spine.

The intervertebral disk and intervertebral disk degeneration

Many relevant studies on the biology of the intervertebral disks (IVDs) have been completed in recent years. Disk degeneration is characterized by loss of fluid pressure, disruption or breakdown of collagen and proteoglycans, and sclerosis of the cartilaginous end plate and the adjacent subchondral bone. All of these hallmark signs of IVD degeneration can also occur as part of the normal aging process of the IVD. For these reasons, disk degeneration and normal aging of the disk are frequently discussed interchangeably, [33, 34] although the biochemical processes may be distinct. The IVD seems to age differently from other tissues, probably because of its lack of a blood supply, and the degenerative process may begin as early as 20 years of age (earlier in some cases). [33] In fact, certain teenagers may experience back pain because of IVD degeneration. [35] There is an extremely wide variation in aging and degeneration of the IVD. Some individuals in their 70s have disks of equivalent health to some in their 30s. The aging and degenerative stages of the IVD from prenatal development through the ninth decade of life have been worked out in considerable detail at the gross and light microscopic levels. [34–40] Calcification of the IVD during the aging process is much more common than was once thought, being found in 58.3% of subjects at autopsy. Such calcification is “significantly underestimated” by conventional radiography. [41]

Several conditions promote or even possibly initiate disk degeneration. These include traumatic Schmorl's node formation, advanced aortic atherosclerosis, [42] and possibly, nicotine consumption. [43] The biochemistry of IVD degeneration is also being elucidated. In this regard, extruded nucleus pulposus has been found to spontaneously produce increased amounts of many chemokines that not only initiate a series of events that decrease the size of the IVD bulge but also result in IVD degeneration. [26, 35, 44–47]

Intervertebral disk protrusion

The normal mechanics of the IVD continue to be investigated. [33, 48–51] In addition, the mechanisms involved in IVD protrusion and failure have been studied in detail [35, 39, 52–62], as well as the effects of changing intradiscal pressure. [63, 64, 65] In addition, a set of terms to be used when describing bulging of the IVD was established by the International Society for the Study of the Lumbar Spine. [66] This terminology included disk bulge, protrusion (tearing of some inner layers of the anulus fibrosus with the nucleus extending into the radial tear), extrusion (tearing of all layers of the anulus fibrosus allowing nuclear material to enter the vertebral canal), and sequestration (a piece of extruded nucleus breaks off of the host IVD). Much recent research related to IVD protrusion in the cervical and lumbar regions (protrusion in the thoracic region has not been studied as extensively) has found that IVD protrusion is a very dynamic process and that after approximately 1 to 3 weeks, IVD protrusion will usually begin a 2-month to 1-year process of resolution, resulting in significant resorption and, from a patient's standpoint (ie, pain), often complete remission of signs and symptoms. [33, 67–69] In fact, histologic evidence of resorption of sequestered nuclei pulposi has been found, [70, 71], and shrinkage of protruded nuclei pulposi has been seen on both computed tomography and MRI. [72] This provides hope to patients with protruded IVDs and for those using conservative methods to treat this condition. Adenovirus-mediated transfer of genes and the resultant production of therapeutic growth factors are being investigated as a means to further study the biology of the IVD and the potential for treatment of disk degeneration [73]; however, the low vascularity of the adult IVD may preclude the effective use of gene therapy in IVD disease. [34] Two published studies have shown that the inhibition of tumor necrosis factor–a (TNF-a) (extruded nucleus pulposus contains high levels of TNF-a) by a monoclonal antibody (Remicade [infliximab], Centocor, Inc., Horsham, Pa) is successful in alleviating sciatica. [74, 75] Finally, the mechanisms of radicular pain continue to be studied. [41, 76–78]

Innervation of IVDs

The significant innervation of the IVDs continues to be investigated in detail. [79–82] Degenerated disks have been found to receive increased innervation by sensory fibers conducting nociception. [83] The added innervation seems to be stimulated by Schwann cells of the nerves innervating the outer aspect of the anulus fibrosus. [84] Consequently, injured or degenerated disks are likely to be more sensitive to pain than normal disks.

Unique characteristics of the cervical IVDs

The cervical IVDs have been found to differ significantly from the lumbar disks. Rather than being made up of many lamellae, the anulus fibrosus of each cervical disk is composed of a single, crescent-shaped piece of fibrocartilage that is thick anteriorly and becomes very narrow laterally and posteriorly. [85]

Range of motion studies

Noteworthy studies measuring both the ranges of motion in various regions of the spine (eg, cervical region) and the motions between individual vertebrae continue. The latter activity has led to studies attempting to better understand the concept of coupled motions in the spine. Finally, significant findings related specifically to motions in the sacroiliac joints have also been published in recent years. These findings are summarized next.

Although ROMs (eg, cervical ROMs) can be measured reliably, [86] measurements made on different days of the same individual can vary considerably. [87] Coupled motion (eg, rotation of vertebrae during lateral flexion) of spinal segments continues to be actively studied.

Current investigators are finding that:

(1)   these motion patterns are very complex;

(2)   all spinal motions are coupled motions; and

(3)   coupling differs from 1 motion segment to the next.

Furthermore, consensus has not been reached on many of these motion patterns. [88]

The full ROM of the sacroiliac joint is not expressed until the extremes of hip motion are reached, moving an average of 7.5° (range, 3°–17°) in the sagittal plane during full flexion and extension of the hips. [89] Motions as high as 22° to 36° have been reported in preteen and early teenage children. [90] Contraction of the left and right transversus abdominis muscles increases stiffness of the sacroiliac joint, thus potentially reducing sprains of the ligaments that protect it. [91]

Morphometric studies

Morphometry means “measurement of an organism or its parts.” The past decade has seen many morphometric studies of various spinal structures. These studies allow for more accurate biomechanical and computer modeling (finite element analysis) studies to be performed and also allow for more accurate patient treatment protocols (surgical and manipulative) to be designed. Table 1 shows many of the morphometric studies performed since 1995, the region of the spine investigated, and the specific anatomical structure analyzed.

Certain anatomical findings can best be discussed with each spinal region. The following sections describe anatomical findings of particular significance in the cervical, thoracic, lumbar, and sacroiliac regions. Each of the topics discussed is related to an active area of research.

Anatomical Findings of Clinical Significance by Spinal Region: Cervical Region

Connective tissue attachments to the spinal dura mater

Connective tissue attachments to the posterior aspect of the spinal dura arising from the foramen magnum, posterior arch of C1, the spinous process of C2, [114] the rectus capitis posterior minor muscle, [115–117] the ligamentum nuchae, [118, 119] and the ligamenta flava between C1–C2 and C6–C7 [120, 121] have been described. These attachments may hold the dura mater posteriorly during cervical extension (to prevent buckling of the dura mater into the spinal cord) and flexion (to prevent the dura from moving forward and compressing the cord). Some authors have speculated that increased tension of the cervical paraspinal muscles may traction the connection between the rectus capitis posterior minor muscle and the dura, leading to headaches secondary to dural tension. [122] Others have proposed that tearing of these connective tissue attachments during the flexion component of flexion-extension (whiplash) type of injuries or other trauma to the cervical region could lead to buckling of the dura mater into the cervical segments of the spinal cord. Such dural buckling could conceivably result in the chronic neck pain, headaches, disorders of balance, and signs and symptoms of cervical myelopathy experienced by some patients who have had trauma to the cervical region. [118] The experimental work of Shinomiya et al [120, 121] lends support to these theories. In addition, homologous attachments in the lumbar region, called meningovertebral ligaments, have been shown to traction the dura mater and the related nerve roots after IVD protrusion.

Vertebral artery

The structure of the vertebral artery continues to be studied. The tortuosity of the vertebral artery can occasionally increase with severe, multilevel IVD degeneration. [123] Haynes et al [124] found that usually, there is no compression or stenosis of the vertebral artery with atlantoaxial rotation. Li et al [125] found that extreme extension and extension with rotation resulted in decreased flow in both vertebral arteries. Licht et al [126] found a decrease in flow in the vertebral artery contralateral to rotation and, for the first time, documented an increase in flow on the ipsilateral side of rotation. Mitchell et al [118] found a decrease in flow through both the left and right vertebral arteries (more in the contralateral vessel) with maximal rotation, especially in those arteries with underlying pathology (eg, atherosclerosis). Therefore, maximal rotation and extension seem to decrease flow through the vertebral arteries, but submaximal rotation seems to have less of an effect.

Anatomical Findings of Clinical Significance by Spinal Region: Thoracic Region

Idiopathic Scoliosis

The cause of idiopathic scoliosis remains unknown, but it is thought to be the result of many factors. A genetic component [127, 128] to the disorder is likely with secondary factors that include a decrease in melatonin production [129–131] and a related increase in circulating levels of the hormone calmodulin. [128] Changes in skeletal muscle, [132] connective tissue [46, 47] bone density, rib distortion, [128] decreased height of the posterior vertebral arch, [133] asymmetry of the neurocentral synchondrosis, [134] and the relatively common finding (up to 26%) of syrinx formation and other neuroanatomical abnormalities in the spinal cord [135, 136] are probably the result of the altered biomechanics of the spine and spinal cord that occur with spinal curvatures. [128] In addition, increasing evidence exists supporting the theory that the primary disorder (probably related to genetic influences) is the involvement of high (cortical) brain centers involved in processing vestibular information. [137–139] An interesting study by D'Attilio et al [140] may indicate that there could be more than 1 subpopulation of patients with idiopathic scoliosis. These investigators showed that the alignment of the spinal column may be strongly influenced by dental occlusion and temporomandibular joint disturbance. These investigators reported that all 15 rats in an experimental group with an induced malocclusion developed thoracolumbar scoliosis within 1 week after the intervention. The scoliotic curvature was then completely resolved in 83% of these rats 1 week after correction of the malocclusion. None of the 15 untreated control rats in their study developed scoliosis. They suggested that an anatomical and functional relationship between the stomatognathic apparatus and the spinal column could explain their observations. They noted the presence of convergent sensory inputs to the craniocervical cord from stomatognathic and cervical spine structures and posited that a consequential tilt of the first cervical vertebra (C1) could affect the alignment of adjacent vertebrae, destabilizing the vertical alignment of the spine.

Anatomical Findings of Clinical Significance by Spinal Region: Lumbar Region

Many of the findings described previously in the section entitled “Intervertebral Disk” were related to the lumbar IVDs and will not be repeated here.

Lumbar intervertebral foramina

Transforaminal ligaments of the IVF can be identified on MRI, with a positive predictive value of 86.7%. [141] These ligaments are present in approximately 60.0% of lumbar IVFs (66.7% of L5–S1 IVFs) [141] and have been implicated as both a cause of low back pain and nerve root entrapment. [142–146] These structures can be quite sturdy (especially at the L5–S1 region) and can calcify. [147] They have been found to decrease the dimensions of the compartment transmitting the anterior primary division of the spinal nerve by 31.5%. [148] Limiting the size of the compartment in this way may, at times, contribute to the incidence of neurologic symptoms in the region, especially after trauma or secondary to degenerative arthritic changes in the region of the IVF. [148]

Anatomical Findings of Clinical Significance by Spinal Region: Sacroiliac Joints

Acquired accessory sacroiliac joints frequently (19.1%) form within the posterior (fibrous) portion of the joint. These accessory joints are more common in obese and older individuals and are also associated with other signs of degeneration and periodic low back pain. [149]


  1. Continue to investigate the effects of spinal adjusting on the tissues of the spine and other organ systems (see section on Autonomic Nervous System) in various disease states (eg, gapping studies of the Z joints).

  2. Continue to evaluate the causes of hypomobility of vertebral segments in the general population, under what conditions such hypomobility is maintained, and continue to characterize the changes of the tissues of the spine after hypomobility (and possibly hypermobility) when normal activity is reestablished (ie, normal forces to the spine are reestablished) and also after spinal adjusting is added in an attempt to help reestablish normal forces and movement to hypomobile tissues. A combination of human and animal studies will be needed to achieve this recommendation.

  3. Evaluate the effects of spinal manipulative procedures on pain of radicular origin and on radiating pain.

  4. Further evaluation at the basic science level of the issue of vertebral and basilar artery iatrogenic pathology is warranted.

  5. Conduct descriptive studies to clarify regional differences (ie, between the cervical, thoracic, and lumbar regions) in the anatomy of the vertebral column and related spinal tissues.

  6. Conduct studies evaluating the normal development of all spinal tissues from embryogenesis through the mid 20s.

  7. Conduct studies further evaluating the aging spine (fourth through ninth decades).

  8. All of the recommendations above should be carried out in both human and animal studies at the gross (imaging and postmortem studies), light microscopic (biopsy specimens, eg, those obtained during surgery, cadaveric studies), and electron microscopic (biopsy specimens and postmortem studies) levels.


Since 3–06–1997

Updated 11-12–2023

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