Hypoplastic occipital condyle and third occipital condyle: Review of their dysembryology

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  • REVIEW

    Hypoplastic Occipital Condyle and Third OccipitalCondyle: Review of their Dysembryology

    R. SHANE TUBBS,1 PATRICK RYAN LINGO,1

    MARTIN M. MORTAZAVI,1 AND AARON A. COHEN-GADOL2*1Pediatric Neurosurgery, Childrens Hospital, Birmingham, Alabama

    2Goodman Campbell Brain and Spine, Department of Neurological Surgery,Indiana University School of Medicine, Indianapolis, Indiana

    Disruption or embryologic derailment of the normal bony architecture of thecraniovertebral junction (CVJ) may result in symptoms. As studies of theembryology and pathology of hypoplasia of the occipital condyles and thirdoccipital condyles are lacking in the literature, the present review was per-formed. Standard search engines were accessed and queried for publicationsregarding hypoplastic occipital condyles and third occipital condyles. The liter-ature supports the notion that occipital condyle hypoplasia and a third occipitalcondyle are due to malformation or persistence of the proatlas, respectively.The Pax-1 gene is most likely involved in this process. Clinically, condylar hy-poplasia may narrow the foramen magnum and lead to lateral medullary com-pression. Additionally, this maldevelopment can result in transient vertebralartery compression secondary to posterior subluxation of the occiput. Thirdoccipital condyles have been associated with cervical canal stenosis, hypopla-sia of the dens, transverse ligament laxity, and atlanto-axial instability causingacute and chronic spinal cord compression. Treatment goals are focused oncraniovertebral stability. A better understanding of the embryology and pathol-ogy related to CVJ anomalies is useful to the clinician treating patients pre-senting with these entities. Clin. Anat. 00:000000, 2013. VVC 2013 Wiley Periodicals, Inc.

    Key words: anatomy; craniovertebral junction; occiput; pathology; skull base

    INTRODUCTION

    The craniovertebral junction (CVJ) refers to thebony structures, ligaments, and articulations thatsurround the cervicomedullary junction. This funnel-shaped enclosure, which includes the caudal portionsof the occipital bone, atlas, axis, and associated liga-ments, protects the medulla oblongata and uppercervical spinal cord (Menezes et al., 2001). As partof the occipito-atlantal joint, the occipital condylescontribute largely to flexion and extension of thehead and neck. The median (third) occipital condyle,also known as condylus tertius, is a remnant of theoccipital vertebrae anterior to the foramen magnum(Rao, 2002). This structure can articulate with thedens or the atlas and limit CVJ motion (Menezes,1998; Rao, 2002) and should be distinguished frombasilar processes (Kale et al., 2009). Hypoplasticoccipital condyles and a third occipital condyle are

    rare developmental abnormalities of the CVJ that canlead to instability and compression of important sur-rounding neurovascular structures (Fig. 1) (Menezes,1998; Piper and Traynelis, 1998). Understandingtheir embryology, anatomy, biomechanics, and path-ologic influence on the CVJ can help neurosurgeonsdetermine the best treatment of these complexanomalies. Herein we review the literature regardinghypoplasia of the occipital condyle and the presence

    *Correspondence to: Aaron A. Cohen-Gadol, MD, MSc, GoodmanCampbell Brain and Spine, Indiana University Department of Neu-rological Surgery, 355 W. 16th Street, Suite 5100, Indianapolis,IN 46202, USA. E-mail: acohenmd@gmail.com

    Received 20 July 2012; Revised 17 October 2012; Accepted 21October 2012

    Published online in Wiley Online Library (wileyonlinelibrary.com).DOI 10.1002/ca.22205

    VVC 2013 Wiley Periodicals, Inc.

    Clinical Anatomy 00:000000 (2013)

  • of a third occipital condyle. The proposed embryol-ogy, clinical presentation, and treatment are pre-sented.

    Following a search in human species and in theEnglish language using PubMed, 23 articles werefound detailing the occipital condyle and hypopla-sia or hypoplastic. A search for third occipital con-dyle resulted in six articles, and a search for con-dylus tertius resulted in five publications.

    Embryology

    During the fourth week of gestation, paraxial mes-oderm adjacent to the neural tube and notochordsegments form 42 somites. There are 4 occipital, 8cervical, 12 thoracic, 5 lumbar, 5 sacral, and 810coccygeal pairs of somites. Each somite forms anouter dermatome, inner myotome, and medial scle-rotome. The cervical through coccygeal sclerotomeseventually fuse in the midline to form the vertebralbodies of the spinal column. The first and secondoccipital sclerotomes form the clivus, and the thirdsclerotome forms the jugular tubercles. Developmentof the fourth sclerotome, also known as the proatlas(Fig. 2), is key to understanding the anatomy andmalformations of CVJ. Key to the present review, theoccipital condyles are derived from the proatlas(Menezes and VanGilder, 1989; Menezes, 1998,2008; Menezes and Fenoy, 2009). Lastly, it shouldbe mentioned that the resegmentation of thesomites is still a matter of some debate, especiallyresegmentation of their lateral portions (Aoyama andAsamoto, 2000; Huang et al., 2000).

    Unlike the first three occipital somites, the fourthoccipital somite undergoes resegmentation, a pro-cess in which the original rostral-caudal boundariesof the somites are reorganized to include cellsderived from adjacent somites (Fig. 2). The caudalhalf of the fourth occipital somite fuses with the ros-tral half of the first cervical somite to form the tran-sitional proatlas sclerotome. At the resegmentationboundary, the severance line forms, and this repre-

    sents the final cellular separation of the skull andcervical spine. After primary segmentation, Hoxgenes specify the rostral-caudal identity of cellswithin the somites. Pax-1 expression influences cel-lular partitioning between tissues and thus plays acentral role in resegmentation (Pang and Thompson,2011).

    The hypocentrum of the proatlas forms the ante-rior tubercle of the clivus, whereas the centrumforms the apex of the dens and the apical ligament.The neural arch divides into ventral-rostral and dor-sal-caudal segments. The ventral-rostral segmentgives rise to the occipital condyles, the anteriorU-shape of the foramen magnum, and the alar andcruciate ligaments. The dorsal-caudal segment formsthe posterior arch and lateral masses of the atlas(Menezes and VanGilder, 1989; Menezes, 1998,2008; Menezes and Fenoy, 2009). At an early stagein development, a dense band of connective tissueforms ventral to each vertebral segment called thehypochordal bow. The hypochordal bow associatedwith the proatlas normally regresses, but the oneassociated with the first cervical somite contributesto the formation of the anterior arch of the atlas(Menezes and VanGilder, 1989; Menezes, 1998,2008; Rao, 2002).

    Failure of any of the above developmental proc-esses can lead to anomalies of the CVJ. Thus,descriptions of CVJ developmental malformations arebased on the underlying embryologic disturbance.Developmental errors can lead to hyperplasia, apla-sia/hypoplasia, midline fusion failure, and resegmen-tation anomalies. Hypoplasia of the lateral elementsof the proatlas, including the neural arch, can lead tooccipital condylar hypoplasia (Fig. 1) (Menezes,1998; Menezes et al., 2001; Pang and Thompson,2011). Persistence or hyperplasia of the hypochordalbow of the proatlas may contribute to the formationof an abnormal articulation between the clivus, theapical dens, and the anterior arch of the atlas knownas the third occipital condyle (Menezes and Van-Gilder, 1989; Menezes, 1998, 2008; Rao, 2002). Ofnote, the notochord has a course through the loca-tion of the third occipital condyle (David et al.,1998).

    Anatomy

    The occipital condyles are convex projectionslocated at the anterior-lateral margins of the fora-men magnum (Moore and Dalley, 2006). They artic-ulate with the superior facets of the atlas to form theatlanto-occipital joints (Menezes and VanGilder,1989; Moore and Dalley, 2006). They are condyloidsynovial joints with weak capsules that provide littlestabilization. These joints are reinforced by anteriorand posterior atlanto-occipital membranes, whichextend from the anterior and posterior arches of theatlas to the basion and opisthion, respectively(Moore and Dalley, 2006).

    Blood supply to the atlanto-occipital joints is pri-marily through a vascular arcade derived from thevertebral arteries. Occipital branches of the externalcarotid artery also contribute to this blood supply.

    Fig. 1. Inferior view of the skull base of an adultskull identified with a left hypoplastic occipital condyleand third occipital condyle (TOC). The TOC and hypo-plastic left condyle and normal right-sided condyle havebeen highlighted for clarity.

    2 Tubbs et al.

  • Lymphatic drainage is via the retropharyngeal nodesthat communicate with the deep upper cervicalchain. A watershed area exists for drainage of theparanasal sinuses and nasopharynx and thus thepotential exists for retrograde inflammation of theatlanto-occipital joints from coincident sinusitis withresultant ligamentous laxity (Grisels syndrome)(Menezes and VanGilder, 1989; Menezes et al.,2001).

    Within the occipitoatlantoaxial complex, theovoid occipital condyles fit into the obliquely-oriented,elliptically cupped superior facets of the atlas. Thisunique geometry primarily allows flexion-extensionand some lateral bending, but precludes meaningfulrotation. The average range of motion at the atlanto-

    occipital joints is 1315 (Menezes and VanGilder,1989; Menezes et al., 2001; Moore and Dalley, 2006).In adults, the angle of the axis of the atlanto-occipitaljoints, the SchmidtFisher angle (Fig. 3), is normally12481278. In children, the occipital condyles aresmaller, and the SchmidtFisher angle is more obtuse,making the atlanto-occiptal joints of youth inherentlyless stable t