The Dura mater of the upper cervical spinal cord is directly attached to the circumference of the Foramen magnum, to the second and third cervical vertebrae and by fibrous slips to the posterior longitudinal ligament. Hinson and Zou¹ noted dense fibrous connective tissue slips binding the dura to the atlas posterior arch all the way around to the posterior margin of the lateral mass in all specimens (Fig. 15-1). They only observed variations in the density of the Epidural attachments. The Dura mater was found to be so firmly adherent to the atlas by these attachments, that the authors found it difficult to separate the dura from the posterior arch without tearing the dural tissue.² The atlas posterior arch constitutes most of the posterior and lateral aspects of the neural canal, and the axis dens represents the anterior margin, whereas the lateral masses are located in the anteriolateral aspect of the neural canal at this level. It is possible that most anatomy texts have ignored the Epidural attachments to the atlas because they do not appear to have surgical significance. However, this chapter will provide compelling evidence for how these anatomical structures play a potential role in the transfer of deleterious tension to the spinal cord as a result of upper cervical Subluxations.

Grostic³ has also described dissection evidence showing numerous strong attachments of the spinal dura to the atlas lateral masses, the atlas posterior arch, as well as the lateral and posterior walls of the axis neural canal. Epidural connective tissue below the upper cervical spine has been described as connecting the posterior spinal dura to the ligamentum flavum.⁴,⁵,⁶ The ligamentum flavum is a strong ligament that connects the laminae of the vertebrae and serves to protect the spinal cord and neural elements, as well as stabilize the spine to prevent excessive motion between the vertebral bodies. Along with the vertebral laminae, this ligament forms the posterior wall of the spinal canal.

In 1929, von Lanz⁷ described the ligamentum craniale durae matris spinalis (CDMS ligament) as fibrous strands between the Dura mater and the posterior border of the atlanto-occipital joints, the edge of the Foramen magnum, the atlas posterior arch, and the base of the spinous process and laminae of the axis (Fig. 15-2). Lang⁸ also discussed the CDMS ligament, although he failed to mention the connection between the dura and the arch of axis. Rutten et al.⁹ confirmed the attachments between the ventrocaudal side of the atlas posterior arch and the spinal dura. They also further described the CDMS complex as consisting of strands arising from the ligamentum flava between C1-C2 and C2-C3, although no connections were found below C3. These researchers were credited with first discovering median fibers of the CDMS ligament being continuous in the deep part of the nuchal ligament. Humphreys et al.¹⁰ have also reported connective tissue attachments from the base of the axis
spinous process and from the atlas posterior arch to the posterior cervical Dura mater.

The rectus capitis posterior minor (RCPm) muscle and the posterior atlanto-occipital (PAO) membrane extend from the atlas posterior arch to the occipital bone. Furthermore, the PAO membrane has been described as being intimately attached to the underlying spinal Dura mater. However, it was not until 1995 that Hack et al.¹¹ revealed a connective tissue bridge between the RCPm muscle and the PAO membrane–spinal dura complex of the upper cervical spinal cord (Fig. 15-2). The team of researchers from the University of Maryland at Baltimore credit the discovery to the unorthodox angle from which the dissections were performed. This was done to fully appreciate the three-dimensional arrangement of the anatomic structures of the head-neck region. Instead of performing the dissection in the standard manner from the back of the neck, the researchers made their approach from the side of the neck.¹² The discovery of this muscle-dural connection has been repeatedly confirmed by other independent researchers.⁹,¹⁰,¹³,¹⁴ The connective tissue bridge has even been observed on magnetic resonance imaging (MRI) scans.¹⁵ Research using the Visible Human Female cryosectioned data set from the National Library of Medicine has also demonstrated the muscle dural bridge.¹⁶ Interestingly, Hack and his research team have obtained cadaveric data suggesting suboccipital musculature traversing the atlantoaxial junction and attaching to tissue within the vertebral canal.⁹ However, more studies are necessary to identify this structure and determine if it is yet another “myodural” bridge.

Hong et al.¹³ have described a unilateral attachment between the RCPm and the dura in 11 cadavers and a bilateral attachment in two cases. One individual revealed a unilateral connection between the inferior border of the atlas posterior arch and the spinal dura. In two cadaveric specimens, a unilateral RCPm attachment was found, and a connection between the atlas posterior arch and the Dura mater was observed on the contralateral side. The mean length and width of the RCPm muscle connective tissue bridge was found to be 13.6 mm and 1.1 mm, respectively. The authors Hypothesized that patients with short and/or asymmetrical RCPm attachments to the spinal dura may have the potential of suffering from greater amounts of cord tension and cervicogenic headaches.

An attachment has also been found to the spinal dura via the ligamentum nuchae (LN).¹⁰,¹⁷,¹⁸ The connection of the LN to the Dura mater has been found between the atlas posterior arch and the axis spinous process, and it has been identified on MRI scans.¹⁰ Mitchell et al.¹⁷ have noted the significance of this finding related to an understanding of cervical biomechanics, particularly rotational movements of the head in the sagittal or transverse planes. However, Nash et al.¹⁴ have found that the nuchal ligament does not exist in the PAO interspace and question the findings of previous studies. Attachments between the LN and RCPm muscle have also been identified.¹⁰ It has been shown that tractioning (pulling) on one of these connective tissue attachments produces movement in the other attachments.¹⁰ These findings are significant because the spinal Dura mater is innervated and a possible source of pain and neurological dysfunction if chronic tension is applied to this structure.¹⁹,²⁰

Macroscopic dissections have shown that the CDMS ligamentous strands are actually stronger than the RCPm connective tissue fibers in the PAO membrane-spinal dura complex.⁹ It appears that the RCPm and CDMS connections serve an important function for stability and resisting dural infolding during movements of the upper cervical spine. This is an important function to prevent spinal dura irritation, particularly during cervical Hyperextension that can occur during “whiplash”
type injuries. It has been suggested that the RCPm muscle may “monitor” the stress on the Dura mater, whereas the infolding is guided by the collagenous and elastic fibers of the CDMS ligament.⁹ The RCPm is a viable candidate for providing proprioceptive feedback because of the great amount of muscle spindles contained in the muscle, which is discussed later in this chapter. Unfortunately, the previously discussed anatomical attachments can potentially leave the cord vulnerable to upper cervical Subluxations, which can result in spinal dura tension that is due to muscle spasticity or structural misalignment. This could also explain why the posterior upper cervical dura has been found to be much thicker than the anterior Dura mater.²¹

The dentate(or denticulate) ligaments are 21-paired lateral bands of epipial tissue located midway between the dorsal and ventral attachments of the nerve roots. Their medial border is continuous with the Pia mater of the spinal cord and is fixed laterally to the Dura mater. The apices of most denticulate ligaments are composed of two prongs that are approximately 1 mm in length. Part of their purpose is to attach the spinal cord to the dura and keep it in a central position. The cervical Dentate ligaments are thicker than those ligaments seen in the thoracic and lumbar regions.²² The first pair of Dentate ligaments ascend upward and into the Foramen magnum to attach to the dura of the posterior fossa. They are located between the vertebral artery anteriorly and the Hypoglossal nerve posteriorly, and the spinal accessory nerve ascends on the posterior aspect. The second pair of dentates are short, stout (thickest), and arranged almost horizontally at the level of the atlantoaxial articular facets (Fig. 15-3). This is in contrast to the ligaments below C2, which become significantly more gracile and project from the cord at gradually greater caudal angles.

The most cephalad ligaments are the thickest and strongest (particularly the second pair) of the dentates to help anchor the spinal cord around the Foramen magnum. The denticulate ligaments are more resistant to caudal than cephalad stresses in the spinal cord. Anterior and posterior motion is also constrained by these ligaments but to a limited degree, especially as one descends inferiorly along the cord.²² A strong attachment at the Foramen magnum is vital to prevent significant axial forces from being transmitted to the brainstem because of cervical flexion. The Dentate ligaments appear to serve a protective role for the central nervous system (CNS) during normal spinal motion; however, during abnormal movement of cervical vertebrae (i.e., uncoupled Subluxation), they are capable of transmitting pathological forces to the spinal cord and brainstem.

These ligaments are so strong that they have been found to buckle or even sever the upper cervical spinal cord in cases of severe hydrocephalus.²³ The denticulate ligaments have been implicated in the kinking of the medullary spinal junction seen in patients with a Chiari II malformation.²² It should be noted that the Dentate ligaments are affected by Davis’s law, which states that soft tissue will model according to imposed demands. Cusick et al.²⁴ have previously discussed how the “stretch Hypertrophy rule” affects living fibrous tissue structures that are exposed to a series of intermittent elongating tension loads by undergoing additional formation of collagen. The Dentate ligaments have a tendency to Hypertrophy or thicken when placed under chronic tension.²²,²⁴,²⁵,²⁶ This loss of elasticity can potentially increase the amount of tension that is transmitted to the spinal cord as a result of long-term upper cervical displacement.

The term Histodynamics has been used to define the study of the effects of dynamic forces on cellular elements. This field of study has shown that between full cervical extension and flexion, there is a change in the
length of the cervical neural canal by about 30 mm.²⁶ Cervical lateral flexion elongates the convex aspect of the cord by 6 mm and shortens the concave side by the corresponding length. The spinal cord has been found to shift to the anterior in the prone position and to the posterior in the supine position. Likewise, the cervical spinal cord shifts anterior during forward flexion and posterior on extension.²⁷ The cervical spinal cord accommodates this change by compressing somewhat in extension and stretching during flexion (prominently on the posterior aspect). However, because the Dentate ligaments are attached on each side of the cervical cord, they are capable of providing a transverse stretch of the spinal cord. Therefore, this causes the anterior to posterior (AP) dimension of the cord to be reduced, but the transverse diameter will slightly increase (Fig. 15-4).²⁸,²⁹,³⁰ This is known as Poisson’s effect: a decrease in cross-sectional area with an increase in length, or vice versa, while the total volume remains constant.²⁹

Breig²⁸ has provided photographs and discussion related to tension on the Dentate ligaments when the cervical spine is flexed. This can also occur with Cervical Kyphosis and osteophytic protrusions. This tension is then transmitted to the spinal dura, which can stretch the lateral branches of the central spinal cord arteries that supply the pyramidal tracts. Stretching the central spinal arteries can result in muscle spasticity that is typical of myelopathies. These conditions can result in other neurological signs and symptoms as well. Breig²⁸ noted one patient who could provoke or increase the spasticity in both legs by forcefully flexing his neck. The patient apparently did this to enable him to stand on his weak legs, as this stimulus resulted in immediate spastic rigidity.

Bedford et al.²⁵ conducted a postmortem examination of a patient who had suffered from severe Cervical spondylosis. The patient had reported pain and weakness of the legs, as well as numbness of the right hand and forearm, among other symptoms. Histological examination revealed degeneration of both the gray and white matter, with diffuse loss of nerve cells and severe gliosis. Demyelination was noted near the attachments of the Dentate ligaments, which were thicker and more cellular than usual. The condition of these ligaments was likely due to the chronic tension applied to these structures from the severe spondylosis and protruded disc material, which caused spinal cord displacement. Shimizu et al.³¹ surgically created kyphotic deformities in the cervical spines of birds and then performed histological evaluations to determine the effects of chronic spinal cord compression. This condition resulted in Demyelination of nerve fibers in the funiculi and neuronal loss in the anterior horn because of spinal cord compression. The histologic changes seem to be associated with both continuous mechanical compression and vascular changes in the spinal cord.

A report from the British Medical Journal revealed a court case in which the plaintiff was awarded $820,875 for posttraumatic multiple sclerosis (MS) that was apparently triggered by an automobile accident.³² However, the case was appealed, and the settlement was reduced. Chaudhuri and Behan³³ have reported on 39 cases in which MS was precipitated or exacerbated by Hyperextension-Hyperflexion cervical trauma to the spinal cord. The worsening or onset of the MS bore a striking temporal relationship to the injury. Their data suggest that CNS-specific acute cervical spine trauma may aggravate latent clinical symptoms in MS. These findings are intriguing because upper cervical chiropractic care has been found to be related to clinical improvement in patients with MS.³⁴,³⁵,³⁶,³⁷

Because of the complexity of the upper cervical spine from an anatomical and biomechanical (see Chapter 16)
standpoint, these Articulations appear to be quite susceptible to Subluxation. Dr. John D. Grostic’s Dentate ligament–cord distortion Hypothesis³,³⁸ provides a compelling theory for how these previously described anatomical connections can lead to spinal cord distortion in the presence of upper cervical misalignment (Fig. 15-5). However, there are three main challenges to this Hypothesis.

Mechanoreceptive Dysafferentation has important implications in the upper cervical region and its relationship to neurological integrity. The nervous system is separated into three functional divisions: sensory, motor and integration, and sensory receptors (i.e., Mechanoreceptors and Nociceptors) play a vital role in all of these categories. Mechanoreceptors derive their name because they are activated by mechanical deformation. These receptors are a morphologically diverse group that vary in size and function (Fig. 15-7). Mechanoreceptors are found in the skin, muscles, joints, and visceral organs; they are predominantly concerned with conveying tactile sensations and position sense to the Neuraxis.

Position sense can be divided into two categories: static position and Kinesthesia. Static position represents awareness of the orientation of different parts of the body with respect to each other. Kinesthesia is the sense that detects bodily position and movement of muscles, tendons, and joints. Mechanoreceptors are primarily responsible for the body’s position sense, which is particularly influenced by the existence in a Gravity environment. This requires the brain’s motor control centers to have detailed information regarding the momentary position and movement of each body part. This is particularly important in humans because of bipedal stance and an inherent instability of a high center of Gravity coupled with a narrow base of support. Proprioceptive organs include the ocular receptors (eye), labyrinth (inner ear), cutaneous receptors (skin), joint Mechanoreceptors, muscle spindles (MSp), and the Golgi tendon organs (GTOs).

The optic receptors provide visual information to the Neuraxis regarding the body’s relationship to the horizon. The vestibular apparatus (VA) informs the brain of the head’s position and Works to keep it Perpendicular with the ground by altering the tone of the cervical muscles. Small postural deviations of the skull are primarily detected by the membranous labyrinth, whereas the eyes play a more significant role in determining the head’s position in space with more extreme head tilts. Mechanoreceptors provide appropriate information depicting the orientation of the head with respect to the body to maintain equilibrium.