A specialized myodural bridge named occipital-dural muscle in the narrow-ridged finless porpoise (Neophocaena asiaeorientalis)

A dense bridge-like tissue named the myodural bridge (MDB) connecting the suboccipital muscles to the spinal dura mater was originally discovered in humans. However, recent animal studies have revealed that the MDB appears to be an evolutionarily conserved anatomic structure which may have significant physiological functions. Our previous investigations have confirmed the existence of the MDB in finless porpoises. The present authors conducted research to expound on the specificity of the MDB in the porpoise Neophocana asiaeorientalis (N.asiaeorientalis). Five carcasses of N.asiaeorientalis, with formalin fixation, were used for the present study. Two of the carcasses were used for head and neck CT scanning, three-dimensional reconstructions, and gross dissection of the suboccipital region. Another carcass was used for a P45 plastination study. Also, a carcass was used for a histological analysis of the suboccipital region and also one was used for a Scanning Electron Microscopy study. The results revealed that the MDB of the N.asiaeorientalis is actually an independent muscle originating from the caudal border of the occiput, passing through the posterior atlanto-occipital interspace, and then attaches to the cervical spinal dura mater. Thus the so called MDB of the N.asiaeorientalis is actually an independent and uniquely specialized muscle. Based on the origin and insertion of this muscle, the present authors name it the ‘Occipital-Dural Muscle’. It appears that the direct pull of this muscle on the cervical spinal dura mater may affect the circulation of the cerebrospinal fluid by altering the volume of the subarachnoid space via a pumping action.

Scanning Electron Microscopic Observation of Myodural Bridge in the Human Suboccipital Region

STUDY DESIGN

A scanning electron microscopic study performed on three cadaveric specimens focused on the human suboccipital region, specifically, myodural bridge (MDB).

OBJECTIVE

This study showed the connection form of the MDB among the suboccipital muscles, the posterior atlanto-occipital membrane (PAOM) and the spinal dura mater (SDM), and provided an ultrastructural morphological basis for the functional studies of the MDB.

SUMMARY OF BACKGROUND DATA

Since the myodural bridge was first discovered by Hack, researches on its morphology and functions had been progressing continuously. However, at present, research results about MDB were still limited to the gross anatomical and histological level. There was no research report showing the MDB's ultrastructural morphology and its ultrastructural connection forms between PAOM and SDM.

METHODS

A scanning electron microscope (SEM) was used to observe the connection of myodural bridge fibers with PAOM and SDM in atlanto-occipital and atlanto-axial interspaces, and the connection forms were analyzed.

RESULTS

Under the SEM, it was observed that there were clear direct connections between the suboccipital muscles and the PAOM and SDM in the atlanto-occipital and atlanto-axial spaces. These connections were myodural bridge. The fibers of the myodural bridge merged into the spinal dura mater and gradually became a superficial layer of the spinal dura mater.

CONCLUSION

MDB fibers merged into the SDM and became part of the SDM in the atlanto-occipital and atlanto-axial space. MDB could transfer tension and pulling force to the SDM effectively, during the contraction or relaxation of the suboccipital muscles.

LEVEL OF EVIDENCE

N/A.

The myodural bridges' existence in the sperm whale

Recent studies have identified that the myodural bridge (MDB) is located between the suboccipital muscles and cervical dura mater in the posterior atlanto-occipital interspace within humans. The myodural bridge has been considered to have a significant role in physiological functions. However, there is little information about the myodural bridge in marine mammals; we conducted this study to investigate and examine the morphology of the myodural bridge in a sperm whale. We also aim to discuss the physiological functions of the myodural bridge. In this study, a 15.1-meter long sperm whale carcass was examined. Multiple methods were conducted to examine the bridges of the sperm whale which included dissection, P45 plastination and histological analysis. This study confirmed the existence of the myodural bridge in the sperm whale and shows there are two types of the bridge in the sperm whale: one type was the occipital-dural bridge (ODB), the other type was the MDB. A large venous plexus was found within the epidural space and this venous plexus is thought to contain a great amount of blood when in deep water and thus the movements of suboccipital muscles could be a unique power source that drives cerebrospinal fluid circulation.

The myodural bridge existing in the Nephocaena phocaenoides

Recent studies have identified that the myodural bridge (MDB) between the rectus capitis posterior minor (RCPmi) and the cervical spinal dura mater in the posterior atlanto-occipital interspace in humans. And it was supposed that the MDB may play essential physiological roles. As a result, the MDB is possibly a highly conserved structure in the evolution of mammals. However, there is little confirmative description about the existence of the MDB in marine mammals. The objective of this study was to explore the existence and the fiber property of the MDB in the Neophocaena phocaenoides. Six cadavers of the Neophocaena phocaenoides with formalin fixation were used in this study. One was used for head and neck CT scanning and three-dimensional (3D) reconstruction and suboccipital region dissection, two were for sectional observation by P45 plastinated sheets of head and neck, and three were for histological analysis of suboccipial structures. This is the first study to demonstrate the existence of the MDB in the aquatic mammals. The rectus capitis dorsal minor (RCDmi) originated from the inferior border of the occiput and inserted into the cervical spinal dura mater. At the ventral aspect of the RCDmi, the MDB directly extended through the posterior atlanto-occipital interspace and connected with the cervical spinal dura mater which was consisted of type Ⅰ collagen. In addition, the dorsal atlanto-occipital membrane was not found in the Neophocaena phocaenoides. The tendinous myodural bridge extended from the RCDmi to the spinal dura mater through the posterior atlanto-occipital interspace in the Neophocaena phocaenoides.

A Systematic Review of the Soft-Tissue Connections Between Neck Muscles and Dura Mater: The Myodural Bridge

STUDY DESIGN

Systematic review.

OBJECTIVE

To elucidate the existence of soft tissue connections between the neck muscles and cervical dura mater.

SUMMARY OF BACKGROUND DATA

Several studies discuss the existence of a cervical myodural bridge; however, conflicting data have been reported.

METHODS

Searches were conducted in the PubMed, Web of Science, Cochrane Library, and PEDro databases. Studies reporting original data regarding the continuity of non-post-surgical soft tissue between the cervical muscles and dura mater were reviewed. Two reviewers independently selected articles, and a third one resolved disagreements. Another two researchers extracted the methodology of the study, the anatomical findings, and evaluated the quality of the studies using Quality Appraisal for Cadaveric Studies Scale. A different third researcher resolved disagreements.

RESULTS

Twenty-six studies were included. A soft tissue connection between the rectus capitis posterior minor, the rectus capitis posterior major, and the obliquus capitis inferior muscles seems to be proved with a strong level of evidence for each one of them. Controversy exists about the possible communication between the dura mater and the upper trapezius, rhomboideus minor, serratus posterior superior, and splenius capitis by means of the ligamentum nuchae. Finally, there is limited evidence about the existence of a soft tissue connection between rectus capitis anterior muscle and the dura mater.

CONCLUSION

There is a continuity of soft tissue between the cervical musculature and the cervical dura mater; this might have physiological, pathophysiological, and therapeutic implications, and going some way to explaining the effect of some therapies in craniocervical disorders.

LEVEL OF EVIDENCE

N/A.

Connection of the Posterior Occipital Muscle and Dura Mater of the Siamese Crocodile

The myodural bridge was proposed initially in 1995. The myodural bridge is a connective tissue bridge that connects a pair of deep muscles at the suboccipital region to the dura mater. There have been numerous studies concerning the morphology and function of the myodural bridge. To determine whether a myodural bridge exists in reptiles, six Siamese crocodiles were investigated using gross anatomy dissection and P45 sheet plastination technologies. As a result, we demonstrated that the posterior occipital muscles of the Siamese crocodile are directly or indirectly connected to the proatlas, atlas, and intermembrane between them. Multiple trabeculae existing in the posterior epidural space extended from the ventral surface of the proatlas, atlas, and intermembrane between them to the dorsal surface of the spinal dura mater. This study showed that the posterior occipital muscle in the suboccipital region of the Siamese crocodile is connected to the spinal dura mater through the proatlas, atlas, and the trabeculae. In conclusion, a myodural bridge-like structure exists in reptiles. This connection may act as a pump to provide cerebrospinal fluid (CSF) circulation at the occipitocervical junction. We hypothesize that a physiologic role of the Siamese crocodile's myodural bridge may be analogous to the human myodural bridge. Anat Rec, 299:1402-1408, 2016. © 2016 Wiley Periodicals, Inc.

Investigation of meningomyovertebral structures within the upper cervical epidural space: a sheet plastination study with clinical implications

BACKGROUND CONTEXT

Over the past two decades, soft-tissue structures communicating with the dura mater within the epidural space have become the focus of many anatomical and histopathologic studies. The relationship between these bridging structures has yet to be evaluated in situ.

PURPOSE

This is the first study that used E12 sheet plastination to investigate the epidural space of the upper cervical spine in situ and its associated bridging structures. Given the complexity of this space, this study may prove useful to clinical anatomists and surgeons who operate within this region.

STUDY DESIGN

Anatomical and microscopic analyses of structures that communicate with the dura mater within the upper cervical region were carried out.

METHODS

Gross dissection in conjunction with microscopy was used to evaluate bridging communications of the upper cervical spine in 10 cadavers. To evaluate the in situ arrangement of these structures, E12 sheet plastination was used on 13 cadavers.

RESULTS

In all 23 specimens, suboccipital fascia coalesced with the dorsal meningovertebral ligament of the atlas, and inserted directly into the posterior surface of the dura as a single but separable laminar layer. At the level of the atlantoaxial interspace, suboccipital fasciae combined and coalesced with the dorsal meningovertebral ligament of the atlas and the axis. These structures inserted into the posterior surface of the dura mater as a single but separable layer. Microscopy validated these findings and E12 sheet plastination revealed the in situ organization of these soft-tissue structures. E12 sheet plastination also provided new information on dural arrangement at the craniocervical junction, which was observed to be composed of periosteum from the occiput but consisted mainly of deep fascia from the rectus capitis posterior minor.

CONCLUSIONS

E12 sheet plastination has provided in situ visualization of bridging structures within the cervical epidural space and offers new insight into these structures, as well as the composition and arrangement of the posterior atlantooccipital membrane and cerebrospinal dura at the craniocervical junction. This study aims to expand on the anatomical understanding of the upper cervical region while defining structures that may reduce neurosurgical complications, and aid in the understanding of the pathophysiology of certain neurogenic disorders.

Posterior atlanto-occipital and atlanto-axial area and its surgical interest

Classic anatomical studies describe two membranes – atlanto-occipital and atlanto-axial in the posterior aspect of the craniocervical region. During many surgical procedures in this area, however, we have not found such membranes. Objective To clarify the anatomical aspects and structures taking part of the posterior atlanto-occipital and atlanto-axial area. Method Analysis of histological cuts of three human fetuses and anatomical studies of 8 adult human cadavers. Results In both atlanto-occipital and atlanto-axial areas, we have observed attachment between suboccipital deep muscles and the spinal cervical dura. However, anatomical description of such attachments could not be found in textbooks of anatomy. Conclusion Our study shows the absence of the classical atlanto-occipital and atlanto-axial membranes; the occipito-C1 and C1-C2 posterior intervals are an open area, allowing aponeurotic attachment among cervical dura mater and posterior cervical muscles.

Definition of the to be named ligament and vertebrodural ligament and their possible effects on the circulation of CSF

Few studies have been conducted specifically on the dense connective tissue located in the posterior medial part of the cervical epidural space. This study was undertaken to examine the presence of this connection between the cervical dura mater and the posterior wall of spinal canal at the level of C1–C2. 30 head-neck specimens of Chinese adults were used. Gross dissection was performed on the suboccipital regions of the 20 specimens. Having been treated with the P45 plastination method, 10 specimens were sliced (9 sagittal and 1 horizontal sections). As a result, a dense fibrous band was identified in the nuchal ligament of 29 specimens (except for one horizontal section case). This fascial structure arose from the tissue of the posterior border of the nuchal ligament and then projected anteriorly and superiorly to enter the atlantoaxial interspace. It was termed as to be named ligament (TBNL). In all 30 specimens the existence of a fibrous connection was found between the posterior aspect of the cervical dura mater and the posterior wall of the spinal canal at the level of the atlas to the axis. This fibrous connection was identified as vertebrodural ligament (VDL). The VDL was mainly subdivided into three parts, and five variations of VDL were identified. These two structures, TBNL and VDL, firmly link the posterior aspect of cervical dura mater to the rear of the atlas-axis and the nuchal region. According to these findings, the authors speculated that the movements of the head and neck are likely to affect the shape of the cervical dural sleeve via the TBNL and VDL. It is hypothesized that the muscles directly associated with the cervical dural sleeve, in the suboccipital region, may work as a pump providing an important force required to move the CSF in the spinal canal.

Histological examination of the human obliquus capitis inferior myodural bridge

This study was designed to examine the anatomical relationship between the obliquus capitis inferior (OCI) muscle and the cervical dura mater at the histological level. Eight human cadavers, with an average age of 65 ± 7.9 years were selected from a convenience sample for suboccipital dissection. Twelve OCI muscle specimens were excised, 100% of which emitted grossly visible soft tissue tracts that inserted into the posterolateral aspect of the cervical dura. These 12 myodural specimens were excised as single, continuous structures and sent for H&E staining. One sample also underwent immuno-peroxidase staining. Microscopic evaluation confirmed a connective tissue bridge emanating from the OCI muscular body and attaching to the posterolateral aspect of the cervical dura mater in 75% of the specimens. Microtome slices of the remaining 25% were not able to capture muscle, connective tissue and dura within the same plane and were therefore unable to be properly analyzed. The sample sent for neuro-analysis stained positively for several neuronal fascicles traveling within, and passing through the OCI myodural bridge. This study histologically confirms the presence of a connective tissue bridge that links the OCI muscle to the dura mater and the presence of neuronal tissue within this connection warrants further examination. This structure may represent a component of normal human anatomy. In addition to its hypothetical role in human homeostasis, it may contribute to certain neuropathological conditions, as well.

Histological analysis of the rectus capitis posterior major's myodural bridge

BACKGROUND CONTEXT

In recent literature, a soft-tissue communication between the rectus capitis posterior major (RCPma) muscle and the cervical dura mater has been identified. To the best of our knowledge, this communication has yet to be validated from a histological perspective nor has it been examined for neural tissue.

PURPOSE

The purpose of this study was to examine the composition and true continuity of the communication between the RCPma and the dura mater at a microscopic level. The communication was also inspected for the presence of proprioceptive neurons.

STUDY DESIGN

An anatomical and histological analysis of a novel structure in the atlantoaxial interspace.

METHODS

Gross dissection was performed on 11 cadavers to remove the RCPma, the soft-tissue communication, and a section of posterior cervical dura mater as one continuous unit. Paraffin embedding and sectioning followed by hematoxylin and eosin staining was conducted to validate the connection. Staining with antineurofilament protein fluorescent antibodies was performed to identify proprioceptive neural tissue on one specimen, and all findings were recorded via photographic documentation.

RESULTS

Histological investigation revealed a tendinous matrix inserting into both the RCPma and the posterior aspect of the cervical dura mater in all 11 specimens. In the one specimen examined for neural tissue, antineurofilament protein fluorescence revealed proprioceptive neurons within the communication. Immunoperoxidase staining demonstrated the insertion of these neurons into both the dura mater and the belly of the RCPma.

CONCLUSIONS

The existence of a true connection between the RCPma and the cervical dura mater provides new insight in understanding the complex anatomy of the atlantoaxial interspace. The presence of a neural component within this connection suggests that it may serve another function aside from simply anchoring this muscle to the dura mater. Such a connection may be involved in monitoring dural tension and may also play a role in certain cervicogenic pathologies. This study also supports previous reports that no true membrane joins the posterior arch of the atlas to the laminae of the axis and contradicts the conventional belief that the ligamentum flavum joins these two structures.

Anatomical connection between the rectus capitis posterior major and the dura mater

STUDY DESIGN

Anatomic study performed on 13 cadaveric specimens focused on muscles of the suboccipital triangle, specifically, the rectus capitis posterior major (RCPma).

OBJECTIVE

To investigate a connection between the RCPma and the cervical dura mater.

SUMMARY OF BACKGROUND DATA

In a study of the posterior intervertebral spaces, a connection between the RCPma and the dura mater was briefly described. To the best of our knowledge, no study has been conducted specifically on this communication.

METHODS

Anatomic dissections were performed in the suboccipital regions of 13 embalmed, adult cadaveric specimens. Findings were recorded via photographic documentation.

RESULTS

In 11 of the 13 specimens, the RCPma attached to the spinous process of the axis and then continued to establish a gross anatomical connection with the dura mater in the atlantoaxial interspace. Manual traction of the RCPma resulted in gross dural movement from the spinal root level of the axis to the spinal root level of the first thoracic vertebra.

CONCLUSION

A connection was found to exist between the RCPma and the cervical dura mater. Various clinical manifestations may be linked to this anatomical relationship.