Table Of ContentONLINE CASE REPORT
Ann R Coll Surg Engl 2012; 94: e208–e209
doi 10.1308/003588412X13373405384657
Congenital absence of the anterior arch of the atlas:
a normal variant
D Thavarajah, P McKenna
Royal Berkshire Hospital, Reading, UK
ABSTRACT
Congenital absence of the anterior arch of the atlas is incredibly rare with only two cases reported previously in the litera-
ture. We present a third case of a medically fit patient who suffered neck trauma with an abnormal odontoid peg x-ray, which
subsequently demonstrated a congenital non-fusion of the anterior vertebral arch of C1 on computed tomography. This case
highlights the need to have an open diagnosis to include congenital anomaly when interpreting abnormal odontoid peg x-rays.
KEYWORDS
Congenital – Anomaly – Atlas – C1
Accepted 8 May 2011; published online 26 September 2012
CORRESPONDENCE TO
Dushan Thavarajah, Royal Berkshire Hospital, London Road, Reading, Berkshire RG1 1AN, UK
E: [email protected]
Congenital absence of the anterior arch of the atlas (C1) Discussion
is incredibly rare with only two previously reported cases
The cause of a congenital absence of the arch of the at-
in the literature.1 In a study of congenital defects of the
las is due to failure of fusion of the synchondroses. It is
C1 arch in 1,153 postmortem dissections and cervical
during the seventh week of gestation that the ossification
computed tomography(CT), there were no aplastic anterior
of the atlas begins, initially with ossification of the lateral
arch anomalies seen. There were, however, 11 posterior
masses (20% of newborns have an ossification centre in the
arch defects identified, therefore a relatively more
anterior arch) that then extends dorsally. By the age of four
common occurrence of the atlas anomaly.2
the neural arches have fused posteriorly.3,4
At birth, the anterior arch of the atlas is commonly car-
Case history tilaginous and forms ventral extensions from the lateral
masses with fusion occurring by the ages of six to eight
A 35-year-old healthy housewife presented to the emer- years.3,5 The lines of union extend across the anterior por-
gency department with a 4-day history of neck pain. She in- tions of the superior articular facets.
jured her neck following a crash into a tyre wall safety bar- Attachments to the anterior arch include the longus colli
rier while go-karting. Her head jolted forward and hit the muscle and the anterior longitudinal ligament to the ante-
steering wheel. She was wearing a safety helmet. Over the rior turbercle, which is present in the centre anteriorly. Pos-
following four days she felt no ease in the pain, which she terior to the anterior arch is the articular surface with the
described as a dull ache that was unremitting to analgesia. dens, which has articular cartilage.
On examination, she walked into the emergency depart- The upper and lower borders of the anterior arch give
ment with a torticollis-type appearance to her head and attachment to the anterior atlanto-occipital membrane and
neck. She had no neurological deficit. There was no cervi- the anterior atlanto-axial ligament respectively. The former
cal spine tenderness but there was left sternocleidomastoid connects it with the occipital bone above and the latter with
tenderness. In view of the history of rapid deceleration and the axis below.
direct impact with the steering wheel, cervical spine views
were obtained. The odontoid peg view gave cause for con-
Conclusions
cern (Fig1) and she was triple immobilised as a suspected
atlas fracture. The lateral masses of the atlas appeared The clinical significance of this anomaly despite an abnor-
asymmetrical. CT demonstrated the rare congenital anoma- mal odontoid peg view, which raised the suspicion of frac-
ly: a congenital absence and non-fusion of the anterior arch ture, is that congenital anomaly should be borne in mind
of C1 (Fig 2). The patient was mobilised and discharged when assessing the patient and x-rays. This patient had not
home with analgesia for a whiplash injury with no sequelae. previously complained of neck pain or had any neurological
e208 Ann R Coll Surg Engl 2011; 93: e208–e209
THAvARAjAH MCKENNA CONgENITAL ABSENCE Of ThE ANTERIOR ARCh Of ThE ATLAS:
A NORmAL vARIANT
compromise so it was felt that she had a stable atlas and did
not warrant any further intervention.
Comparison can be made to the rare traumatic anterior
atlas arch fracture, also referred to as a ‘plough’ fracture,
whereby in a hyperextension-type injury forward propul-
sion of the odontoid peg shears off the anterior arch of the
atlas in a plough-type fashion (Fig 3).6
A B
figure 2 Coronal (A) and axial (B) computed tomography
demonstrating non-fusion of the anterior arch of the atlas in the
midline
figure 3 Axial computed tomography demonstrating a
traumatic anterior arch atlas fracture
figure 1 Odontoid peg view of asymmetrical lateral masses of References
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