ORIGINAL COMMUNICATION

The Obliquus Capitis Inferior Myodural Bridge

MATTHEW E. PONTELL,1* FRANK SCALI,2 EWARLD MARSHALL,1 AND DENNIS ENIX3

1Department of Anatomical Sciences, School of Medicine, St. George’s University, Grenada, West Indies2Independent Investigator, Valley Stream, New York

3Division of Research, Logan University, Chesterfield, Missouri

This study was designed to examine the obliquus capitis inferior (OCI) musclefrom a gross anatomical perspective. The objective was to isolate and identifythe OCI myodural bridge, while examining its course and contributing elements.An earlier study of the posterior cervical spine briefly reported a connectionbetween the OCI and the cervical dura mater. To the best of our knowledge, astudy has not yet been conducted specifically on this muscle and its relation tothe dura mater. In this study, the suboccipital regions of nine embalmed cadav-ers were dissected. A total of 14 OCI muscles were isolated for examination. Allfindings were documented via photograph. Of the 14 OCI muscles isolated, allemitted fibrous tissue bands from the anterolateral portion of the muscularbelly. These fibers attached to the posterolateral cervical dura mater by route ofthe atlantoaxial interspace. The OCI myodural bridge appeared to coalesce withthe rectus capitis posterior major myodural bridge, giving the appearance of asingle atlantoaxial structure that links these two muscles to the dura mater. Inconclusion, the OCI was attached to the dura mater in all of the 14 muscle speci-mens. We hypothesize that the OCI myodural bridge may play a physiologicalrole in monitoring dural tension and preventing dural infolding. It may also con-tribute to certain clinical symptoms manifesting from alterations in dural tone.

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Key words: obliquus capitis inferior; myodural bridge; atlantoaxial interspace;cervical dura mater

INTRODUCTION

Over the past 30 years, the posterior cervicalintervertebral spaces have received an increasingamount of attention in scientific literature (Kahn etal., 1992; Hack et al., 1995; Humphreys et al.,2003; Scali et al., 2011). Examination of humancadavers revealed that these interspaces containintricate connections between suboccipital muscula-ture and the cervical dura mater (Kahn et al., 1992;Hack et al., 1995, 1996, 1997; Nash et al., 2005;Scali et al., 2011). Studies of this region suggestthat the suboccipital muscles emitting these fibroustissue bridges integrate motion of the upper cervicalspine and craniovertebral joint with that of the out-ermost layer of the meninges (Hack et al., 1995;Hallgren et al., 1997; McPartland and Brodeur,1999; Scali et al., 2011, 2012). All these reports arebeginning to reveal what may be one of the humanbody’s more complex regions of anatomy. It hasbeen suggested that pathological conditions involv-

ing dural adhesions and excessive dural tension maybe simulated by these muscle-dura connections, aswell (Hack et al., 1995; Alix and Bates, 1999; Hackand Hallgren, 2004; Tagil et al., 2005; Fernandez-de-las-Penas et al., 2007; Grgic, 2007; Scali et al.,2012). These myodural connections may, therefore,play a role in clinical symptoms that may only bepartially understood at this time.

Within the atlantooccipital interspace, the anteriorfascia of the rectus capitis posterior minor (RCPmi)contributes to the atlantooccipital myodural bridge(Hack et al., 1995; Humphreys et al., 2003; Nash et

*Correspondence to: Matthew E. Pontell, 787 Lexington Avenue,Apt. 5, New York, NY 10065, USA. E-mail: mpontell@sgu.edu

Received 4 June 12; Revised 20 June 2012; Accepted 27 June2012

Published online in Wiley Online Library (wileyonlinelibrary.com).DOI 10.1002/ca.22134

VVC 2012 Wiley Periodicals, Inc.

Clinical Anatomy 00:000–000 (2012)

Clin. Anat. 00:000–000, 2012.

al., 2005; Zumpano et al., 2006; Scali et al., 2011;Kahkeshani and Ward, 2012). The RCPmi fasciafuses with the posterior atlantooccipital membraneand continues on to merge with the posterolateralcervical dura mater (Nash et al., 2005; Zumpano etal., 2006; Kahkeshani and Ward, 2012). This myo-dural bridge has been examined with gross (Hack etal., 1995; Kahkeshani and Ward, 2012), microscopic(Nash et al., 2005; Zumpano et al., 2006), and radi-ographic (Humphreys et al., 2003; Hack and Hallg-ren, 2004) modalities. This recent anatomical findingmay play a physiological role in preventing duralinfolding and monitoring dural tension (Hack et al.,1995; Hallgren et al., 1997; Rutten et al., 1997).Additionally, tensile forces transmitted through thisanatomical connection may contribute to the patho-mechanics of cervicogenic headache (Hack et al.,1995; Alix and Bates, 1999; Hack and Hallgren,2004). In one instance, sectioning the RCPmi myo-dural bridge provided relief in a case of intractablecephalgia (Hack and Hallgren, 2004).

In 2011, a study was conducted on the rectuscapitis posterior major (RCPma) myodural bridge.This study documented the anatomical attachmentof the RCPma to the posterior aspect of the cervicaldura mater (Scali et al., 2011). In 2012, a histologi-cal study conducted on the RCPma myodural bridgesupported the continuity of this soft tissue connec-tion with both the RCPma and the dura mater (Scaliet al., submitted). This myodural connection mayalso be represented by structures visible on MRI(Scali et al., 2012). Although current anatomical textreports that the yellow ligament (ligamentum fla-vum) covers the anterior boundary of the atlantoax-ial interspace (Standring, 2008), these recentreports on the RCPma refute this (Scali et al., 2011).Aside from possibly serving to monitor dural tensionthroughout movements of the head and neck, theRCPma may play a role in cervical pathologies via asimilar mechanism to that suggested of the RCPmi(Scali et al., 2011). Nevertheless, these myoduralconnections from the RCPmi and RCPma are becom-ing increasingly noteworthy in anatomical literature.

In 1992, a study of the posterior intervertebralinterspaces briefly reported that the obliquus capitisinferior (OCI) muscle also attaches to the dura mater(Kahn et al., 1992). This attachment was once againmentioned in a study on the RCPma myoduralattachment in 2011 (Scali et al., 2011). To the bestof our knowledge, there has not been a study con-ducted specifically on the OCI myodural bridge. Theobjective of this study is to examine a conveniencesample of cadaveric specimens for the presence of asoft tissue communication between the OCI and thecervical dura mater. Additionally, we aim to examineits course and composition from a gross anatomicalperspective and hypothesize as to what the functionof this connection may be.

MATERIALS AND METHODS

A convenience sample of four cadavers wasobtained from the Department of Anatomical Scien-ces at St. George’s University, School of Medicine.

An additional sample of five cadavers was obtainedfrom the Department of Anatomy at Logan College ofChiropractic. A total of nine embalmed cadavers(four male, five female) were dissected for the pur-pose of this study. Due to prior dissection, fourcadaveric specimens were only examined unilater-ally. The specimens were preserved using a forma-lin-alcohol-phenol mixture. Specimens with signs ofcervical surgery or trauma were excluded from thisstudy. All guidelines were followed for use of cadav-eric material in research. Photographic documenta-tion was recorded with a Nikon D-40 camera, using aNikon DX Af-S Nikkor 18–55 mm 1:35-5.6 GIIdetachable lens.

Each dissection began with removal of soft-tissuestructures superficial to the vertebral column inorder to prepare each specimen for a partial laminec-tomy. The muscular structures of the suboccipital tri-angle and the RCPmi remained intact as to preservethe area of interest. Using a Stryker Autopsy 810(Stryker, Kalamazoo, MI) saw, gross anatomical cutswere performed bilaterally along the laminae, fromthe third (C3) to sixth (C6) cervical vertebrae.Following this procedure, the posterior vertebralelements were removed to reveal the contents of thevertebral canal from C3 to C6.

Using a surgical scalpel, the RCPma and OCI weredetached from the spinous process of the secondcervical vertebrae (C2). The muscular attachmentsites of the RCPma and OCI were also excised fromthe inferior nuchal line and transverse process of thefirst cervical vertebrae (C1), respectively. The exci-sion of these bony attachments was performed bilat-erally.

A midsagittal cut was made on the spinous pro-cess of C2 using a Dremel 200-1/15 two-way rotary(Robert Bosch Tool Corporation, Mt. Prospect, IL)with a 426 1-1/4 in fiberglass-reinforced cutoff wheelattachment. Following this procedure, sagittal cutswere then made along the laminae bilaterally. It wasthen possible to remove the posterior arch of C2allowing visualization of the structures traversing theatlanto-axial interspace from a posterior perspective.The OCI, soft tissue bridge, and 2 cm 3 2 cm sectionof dura mater were excised as one continuous struc-ture and placed into a buffered solution for futurehistological analysis.

RESULTS

From the nine cadavers dissected, 14 OCI muscleswere isolated (six male, eight female). In all 14 OCImuscles, fibrous tissue continuous with the anteriorfascia of the OCI projected anteriorly and inferiorlyfrom the anterolateral portion of the OCI muscularbelly. The connective tissue traversed through atlan-toaxial interspace and attached to the posterolateralaspect of the cervical dura mater between the firstand second cervical vertebrae (Fig. 1). The RCPmiand RCPma myodural bridges were also identified inall of the dissections.

On further examination, the OCI myodural bridgeappeared to coalesce with the RCPma myoduralbridge in each of the 14 OCI specimens. From a

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gross perspective, both the OCI and RCPma emittedsoft tissue fibers that contributed to what appearedto be a single atlantoaxial myodural bridge (Fig. 2).Excision of the RCPma and its soft tissue bridgerevealed that the OCI remained continuous with thedura mater via a soft tissue bridging structure (Fig.3). It was, therefore, determined that the OCI wasattached to the posterolateral cervical dura mater inall the 14 of the OCI specimens.

Although the OCI and RCPma muscles are eachconnected to the dura mater, the proximity of theirmyodural connections leads to the appearance of asingle atlantoaxial myodural bridge that links boththese muscles to the dura mater. It should also benoted that the yellow ligament (ligamentum flavum)did not bound the anterior limit of the atlantoaxialinterspace. This space was occupied by the atlan-toaxial myodural bridge.

DISCUSSION

The posterior cervical musculature integrates thefine movements of the atlantooccipital joint with therest of the upper cervical spine. Namely, the musclesof the suboccipital triangle and the RCPmi areinvolved in flexion, extension, rotational, and trans-lational movements of the cervical vertebrae and oc-ciput (Standring, 2008). The results of this studysupport the fact that three of these suboccipitalmuscles, the OCI, RCPma, and the RCPmi, are fixedto the dura mater via soft tissue bridges (Kahn etal., 1992; Hack et al., 1995; Scali et al., 2011; Fig.4). Knowing that these muscles attach to the protec-

tive covering of the central nervous system, it seemsunlikely that their functions are limited to craniover-tebral movement.

It has been proposed that the suboccipital myo-dural connections of the RCPma and RCPmi serve tomonitor dural tension (Rutten et al., 1997; Scali etal., 2011). In one case, proprioceptive fibers werevisualized arising in the cervical dura mater, coursing

Fig. 1. Artist’s interpretation of a posterior uppercervical spine dissection. Image depicts the rectus capi-tis posterior major (RCPma) and obliquus capitis inferior(OCI) muscles emitting dense connective tracts contrib-uting to the atlantoaxial myodural bridge (MDB). The

myodural bridge (MDB) proceeds to communicate withthe posterior aspect of the dura mater (Dura) betweenthe atlas (C1) and the axis (C2). [Color figure can beviewed in the online issue, which is available at wileyon-linelibrary.com.]

Fig. 2. Image depicts a medial view of a midsagittalsection of a soft tissue communication (MDB) betweenthe obliquus capitis inferior (OCI), rectus capitis poste-rior major (RCPma) and the dura mater (Dura). Themuscular component and dural component maintaintheir continuity through this soft tissue connection(MDB). [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

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through the RCPma myodural bridge and continuingwithin the RCPma muscle belly (Scali et al., submit-ted). If the RCPma and RCPmi myodural bridges doreceive input from the dura mater, the OCI myoduralbridge may function in a similar manner, and thesesoft tissue bridges may collectively function to assistwith dural tension monitoring. Because the OCI func-tions as an ipsilateral rotator (Standring, 2008), andmost rotation of the neck occurs at the atlantoaxialjoint (Bates, 1991), the OCI may contribute to duraltension monitoring throughout these movements.

From a clinical perspective, the RCPma and RCPmimyodural bridges have been suggested to play a rolein conditions resulting from excess dural tension,namely, cervicogenic cephalgia. Hypertrophy of thesemuscles may result in excess tension on the duramater through the myodural bridges manifesting clin-ically as head pain (Hack et al., 1995; Alix and Bates,1999; Hack and Hallgren, 2004; Scali et al., 2011).The OCI myodural bridge may be involved in similarpathophysiology. Additionally, studies have demon-strated convergence between the upper three cervicalnerves and the trigeminal nucleus (Bogduk, 2001).As the infratentorial dura mater is innervated by theupper cervical nerves, pain arising from the cervical

Fig. 4. Image depicts the excision of the rectuscapitis posterior minor (RCPmi) muscles, rectus capitisposterior major (RCPma) muscles, obliquus capitis infe-rior (OCI) muscles. and the posterior aspect of the cer-vical dura mater (Dura) as one complete unit. Allmuscles were removed from their origin and insertionpoints before the posterior aspect of the dural sleeve

was excised. One complete structure was removed con-sisting of the six muscles and the dura mater. All sixmuscles remained attached to the dura mater by theirrespective myodural bridges. The right OCI myoduralbridge is labeled by an asterisk (*). [Color figure can beviewed in the online issue, which is available atwileyonlinelibrary.com.]

Fig. 3. Image depicts an excised section of the obli-quus capitis inferior (OCI) that communicates with thedura mater (Dura) by way of the OCI myodural bridge(MDB). [Color figure can be viewed in the online issue,which is available at wileyonlinelibrary.com.]

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dura due to excess tension may be referred throughthe distribution of the trigeminal nerve (Bogduk,2001). Should the myodural bridges of the RCPma,RCPmi, and OCI become firmly implicated in dural pa-thology, this scenario may contribute the clinical rele-vance of these structures.

The RCPma and RCPmi myodural bridges havealso been suggested to prevent dural infolding, espe-cially during extension movements of the head andneck (Hack et al., 1995; Rutten et al., 1997). As theOCI, RCPma, and RCPmi myodural bridges are ofsimilar structure, they may act together to maintaindural posture during these movements. Atrophicchanges in any of these muscles may, therefore,contribute to misalignment of the dura mater duringmovements of the upper cervical spine (Hallgren etal., 1997; Rutten et al., 1997).

Limitations of this study include the lack of histo-logical analysis of the OCI myodural bridge and thesmall sample size used in this study. Additionally,four cadavers only had a single intact OCI muscle,and these specimens could not be examined forbilateral continuity of the OCI with the cervical duramater. As this is a newly reported structure, thor-ough examination is warranted to determine itsprevalence in the population and further examine itscomposition.

In summary, our study reports that the OCI mus-cle attaches to the cervical dura mater via a fibroustissue connection that travels through the atlantoax-ial interspace. Defining the OCI myodural bridge mayserve to augment prior anatomical knowledge of thecraniocervical region. Gross identification of thisstructure is especially important with respect to dis-section, anatomical instruction, and even surgicalprocedure. Additionally, the OCI myodural bridgemay contribute to pathological conditions arisingfrom dural tension in a similar manner to the RCPmaand RCPmi myodural bridges. The presence of theseconnections suggests that the suboccipital region isan area of complex anatomy and further explorationis encouraged.

ACKNOWLEDGMENTS

The authors thank Kathleen Bubb, MD, and gradu-ate student Patrick Battaglia for assisting withcadaveric dissections. They also thank anatomicalillustrator Danny Quirk for his contribution of theoriginal anatomical artwork that accompanies thismanuscript.

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