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\begin{document}

\title{RARE CONGENITAL ANOMALIES AT THE CRANIOVERTEBRAL JUNCTION; OS
ODENTOIDEUM}



\author[1]{Murat Guntel}%
\author[1]{Hanifi  Bayarogullari }%
\affil[1]{Mustafa Kemal Universitesi Tayfur Ata Sokmen Tip Fakultesi}%


\vspace{-1em}



  
  \date{June 17, 2024}


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\selectlanguage{english}
\begin{abstract}
Aim: We aimed to identify dislocation, myelomalacia, and other
congenital pathologies that may cause neurological complications arising
from the odontoid process in the craniocervical junction (CVJ) of
patients who were performed cervical tomography due to various
complaints. Materials and Methods: Patients who were performed cervical
CT with various complaints between 2010 and 2018 were retrospectively
screened and evaluated, from the hospital registry system. Segmentation
and fusion anomalies arising from the odontoid process in the
craniovertebral junction area, as well as other accompanying pathologies
such as subluxation, dislocation, compression to the spinal cord, and
myelomalacia were also recorded. An additional 1.5 Tesla cervical MRI
was performed for the 12 patients with os odontoideum. Radiological
evaluation was performed on the workstation by an experienced
radiologist. Results: 2525 patients with cervical CT were included in
the study. 67 of these patients had congenital anomalies at the CVJ. 13
of 67 patients had os odontoideum, 38 had Klippel-Feil syndrome (KFS).
Atlanto-occipital assimilation (AOA) and KFS often saw together. Only
AOA was observed in 16 cases. Neck pain and limitation of movement were
the most common findings in patients with congenital anomalies in the
CVJ region. Conclusion: Congenital anomalies at the CVJ region, although
rare, can cause mortality or morbidity. Cervical CT and MRI studies on
the CVJ region should be evaluated carefully by the clinicians due to
the risk of various complications.%
\end{abstract}%



\sloppy


\textbf{RARE CONGENITAL ANOMALIES AT THE CRANIOVERTEBRAL JUNCTION; OS
ODENTOIDEUM}

\textbf{Abstract}

\textbf{Aim:} We aimed to identify dislocation, myelomalacia, and other
congenital pathologies that may cause neurological complications arising
from the odontoid process in the craniocervical junction (CVJ) of
patients who were performed cervical tomography due to various
complaints.

\textbf{Materials and Methods:} Patients who were performed cervical CT
with various complaints between 2010 and 2018 were retrospectively
screened and evaluated, from the hospital registry system. Segmentation
and fusion anomalies arising from the odontoid process in the
craniovertebral junction area, as well as other accompanying pathologies
such as subluxation, dislocation, compression to the spinal cord, and
myelomalacia were also recorded. An additional 1.5 Tesla cervical MRI
was performed for the 12 patients with os odontoideum. Radiological
evaluation was performed on the workstation by an experienced
radiologist.

\textbf{Results:} 2525 patients with cervical CT were included in the
study. 67 of these patients had congenital anomalies at the CVJ. 13 of
67 patients had os odontoideum, 38 had Klippel-Feil syndrome (KFS).
Atlanto-occipital assimilation (AOA) and KFS often saw together. Only
AOA was observed in 16 cases. Neck pain and limitation of movement were
the most common findings in patients with congenital anomalies in the
CVJ region.

\textbf{Conclusion:} Congenital anomalies at the CVJ region, although
rare, can cause mortality or morbidity. Cervical CT and MRI studies on
the CVJ region should be evaluated carefully by the clinicians due to
the risk of various complications.

\textbf{Keywords:} Craniovertebral junction; Os Odentoideum; Multislice
Computed Tomography; Magnetic Resonance Imaging; Complications

\textbf{What's already known about this topic?}

Craniovertebral junction is a junction with a complex structure and
numerous congenital or acquired (traumatic, inflammatory) pathologies
can affect this anatomic region.

There are many studies in the literature that emphasize this anatomical
structure and underline that its anomalies may cause mortality or
morbidity.

\textbf{What does this article add?}

With this study, 2525 cervical CT scans obtained for various reasons
between 2010 and 2018 were scanned and we estimate that it is one of the
largest series researched in the literature.

Clinicians and radiologists should pay particular attention to the
evaluation of this anatomical region.

\textbf{Introduction}

Craniovertebral junction (CVJ) is a junction with a complex structure
consisting of ligaments providing stability for the bones and joints
between the head and spinal column. Bony structures in this junction are
occipital bone (1) including the foramen magnum, clivus, and condylar
process and the first two vertebrae of the cervical spinal column: atlas
(C1) and axis (C2). Atlanto-occipital joint (AOJ), atlanto-dental joint
(ADJ), and atlanto-axial joint (AAJ) are present in this complex
structure. There are multiple ligaments and membranes providing
stability on the joint structures, however, the most important of those
are cruciate and alar ligaments. C1 and C2 vertebrae together with the
other joints in this region help to move the head. While AOJ is
responsible for the anterior and posterior movement of the head, ADJ is
responsible primarily for the axial rotation of the head.
Cranio-cervical junction (CCJ) is the region where the brainstem, spinal
cord, some cranial nerves, and vertebral arteries are located and
preserved. Numerous congenital or acquired (traumatic, inflammatory)
pathologies can affect the CVJ. Among the congenital pathologies,
atlanto-occipital assimilation (AOA), Klippel-Feil syndrome (KFS), and
similar rare segmentation and ossification abnormalities, malformations
like Os Odentoideum (OO) can narrow the spinal canal and can lead to
myelomalacia and neurological findings by compressing the spinal cord
(1-3).

In this study, we aimed to present the patients who was performed
cervical tomography due to various complaints and had congenital
pathologies originated from odontoid proses in the CVJ region causing
dislocation, myelomalacia, and neurological complications.

\textbf{Materials and methods}

\textbf{Patients and imaging procedures}

2525 patients for whom cervical Multislice Computed Tomography (MSCT)
was performed due to head and neck pain, various neurological
complaints, trauma, etc. between 2010 and 2018 were evaluated for
segmentation and fusion anomalies of odontoid process in the CVJ region.
Patients with coexisting pathologies such as subluxation, dislocation,
compression to the spinal cord, and myelomalacia (MM) were included in
the study as well. Toshiba Aquilon 64 MDCT device was used for the
imaging studies. An additional 1.5 Tesla MRI (Achieva, Philips, Best,
The Netherlands) was performed for the 12 patients with os odontoideum.
Radiological evaluation was performed on the workstation by an
experienced radiologist.

Ethical approval was obtained from Mustafa Kemal University Ethical
council (Approval no: 04, dated 03.12.2020).

\textbf{Results}

1690 of the patients for whom MSCT was performed were male and 835 were
female. 67 of these patients had congenital anomalies at the CVJ. 13 of
67 patients had os odontoideum (OO), 38 had KFS, AOA was also frequently
observed in KFS. Only AOA was observed in 16 cases. KFS and AOA often
saw together. 10 patients with OO were male and 3 were female. The mean
age of the patients with OO was 43.07 (range: 14-88). 22 patients with
KFS were male and 16 were female. The mean age of these patients was
41.68 (range:0-71). 10 patients with AOA were male and 6 were female.
The mean age of these patients was 39.12 (range:10-70) (Table 1).

Neck pain and limitation in the range of motion around the neck region
were the most frequent finding in the patients who had congenital
anomalies at the CVJ region. Various neurological findings were also
present in these patients in accordance with the severity of coexisting
dislocation and degenerative changes. Additionally, atlanto-occipital
dislocation (AOD), Atlanta-dental dislocation (ADD), and atlanto-axial
dislocation (AAD) were also detected. Loss of muscle strength and
paralysis were observed clinically especially in the patients with MM
who had prominent nerve compression.

9 of 13 patients with OO had dystopic (Figure 1a-b) and 4 were
orthotopic type. 1 patient with OO had coexisting AC, 4 had AOD, 3 had
ADD, 2 had roto-AAD, and 5 had MM. Fusion with the anterior arch of C1
vertebra in 1 patient with OO (Figure 1c-e) and fusion with clivus in
another patient with OO was observed. 1 patient with OO had coexisting
AOA (Table 1).\selectlanguage{english}
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\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

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Figure 1a Figure 1b\selectlanguage{english}
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Figure 1c Figure 1d Figure 1e

Fusion between the C1 and C2 vertebrae in 2 patients with KFS and fusion
between only C2 and C3 vertebrae in the remaining 36 cases with KFS were
observed 21 patients with KFS had bilateral AOA, 1 had left partial AOA,
10 had odontoid hypoplasia (OH), 11 had basilar invagination (BI), 6 had
Arnold Chiari (AC), 1 had AOD, 9 had ADD, 3 had rotatory AAD, 5 had MM,
and 4 had coexisting syringomyelia (Figure 2 a-e). 15 patients with KFS
had a posterior arch defect in the C1 vertebra and 4 had an anterior
arch defect (Table 1).\selectlanguage{english}
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\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

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Figure
2a Figure 2b Figure 2c\selectlanguage{english}
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Figure 2d Figure 2e

1 patient with AOA had partial (right) and 15 patients had total
assimilation. 4 patients with AOA had OH, 3 had BI, 5 had AC, 1 had ADD,
2 had rotatory AAD, and 3 had MM. 3 patients with AOA had a posterior
arch defect in the C1 vertebra (Figure 3 a-c), (Table 1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
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Figure 3a Figure 3b Figure 3c

While the mean atlanto-dental instability of ADD was 5.48 mm (range
3.5-10.4 mm) in the patients with KFS, it was 6.56 mm (range: 3-8.8 mm)
in the patients with AOA. While the mean dislocation of dens towards the
cranial side in the patients with KFS and basilar invagination (BI) was
10.86 (range:7.7-15.29 mm), it was 10.07 (range:6.1-15.5 mm) in the
patients with AOA and BI (Table 1).

Table 1.\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllllllll@{}}
\toprule
& \textbf{Num.} & \textbf{Gender} & \textbf{Gender} & \textbf{Age} &
\textbf{Age} & \textbf{Age} & \textbf{OH} & \textbf{BI} & \textbf{AC} &
\textbf{AOD} & \textbf{ADD} & \textbf{RAAD} & \textbf{MM} &
\textbf{Syr.}\tabularnewline
\midrule
\endhead
& & \textbf{Male} & \textbf{Female} & \textbf{Min.} & \textbf{Mid.} &
\textbf{Max.} & & & & & & & &\tabularnewline
\textbf{OO} & \textbf{13} & \textbf{10} & \textbf{3} & \textbf{14} &
\textbf{43.07} & \textbf{88} & \textbf{-} & \textbf{-} & \textbf{1} &
\textbf{4} & \textbf{3} & \textbf{2} & \textbf{5} &
\textbf{-}\tabularnewline
\textbf{AOA} & \textbf{16} & \textbf{10} & \textbf{6} & \textbf{10} &
\textbf{39,12} & \textbf{70} & \textbf{4} & \textbf{3} & \textbf{5} &
\textbf{-} & \textbf{1} & \textbf{2} & \textbf{3} &
\textbf{-}\tabularnewline
\textbf{KFS} & \textbf{38} & \textbf{22} & \textbf{16} & \textbf{0} &
\textbf{41,68} & \textbf{71} & \textbf{10} & \textbf{11} & \textbf{6} &
\textbf{1} & \textbf{9} & \textbf{3} & \textbf{5} &
\textbf{4}\tabularnewline
\bottomrule
\end{longtable}

Abbreviation; OO (os odontoideum), AOA (atlanto-occipital assimilation),
KFS (Klippel-Feil syndrome), Num (number), OH (odontoid hypoplasia), BI
(basilar invagination), AC (Arnold Chiari), AOD (Atlanto-occipital
dislocation), ADD (atlanto-dental dislocation), RAAD (rotatory
atlantoaxial dislocation), MM (myelomalacia), Syr. (syringomyelia)

\textbf{Discussion}

The structures forming the CVJ are originated embryologically from the
4\textsuperscript{th} occipital and 1\textsuperscript{st} and
2\textsuperscript{nd} cervical sclerotomes. Occipital condyles, the
superior part of the posterior arch of the atlas, and apex of the dens
are originated from the 4\textsuperscript{th} occipital sclerotome
(proatlas). The inferior part of the posterior arch of the atlas,
anterior arch of the atlas, and dens are originated from the
1\textsuperscript{st} cervical sclerotome and the body of the axis is
originated from the 2\textsuperscript{nd} cervical sclerotome.
Congenital anomalies in the CVJ develop secondary to the segmentation
abnormalities in these sclerotomes. These anomalies can be in the form
of fusion as the fusion of two different structures or development or
formation abnormalities of the structures leading to the dislocation or
subluxation of the joints (4,5).

Secondary to the segmentation anomalies in the
4\textsuperscript{th}occipital sclerotome and 1\textsuperscript{st} and
2\textsuperscript{nd}cervical sclerotomes, AOA, KFS, os odontoideum, and
the other odontoid malformations and structural congenital anomalies
such as BI and AC that can develop secondary to and/or independent from
the former anomalies can be observed together. Instability in the joints
at the CVJ region, changes in the ligament laxity, joint dislocations,
early degenerative changes in the bony structures can be observed
secondary to these congenital malformations. Joint dislocations and
degenerative changes secondary to the congenital malformations can lead
to a narrowing at the CVJ region and compression to the medulla
spinalis, cranial nerves, and vascular structures and consequent various
neurological findings, severe neurological problems, morbidity, and
mortality (6,7).

Among the CVJ malformations, AOA was described in 1884 by Rokitansky. It
is observed in 0.12-0.72\% of the population with no gender
predilection. Atlas fused with occipital bone acts as a functional unit
of the occipital bone. Assimilation can form partially or completely.
Foramen magnum narrowing, basilar invagination, and atlanto-axial
instability are observed with this variation (8,9). In our study, 16
isolated AOA cases and 22 AOA cases with KFS were detected within 8
years. Among these 38 AOA cases, 2 were partial AOA and 36 were
bilateral AOA cases. The rate of isolated AOA was 0.63\% and the rate of
coexisting KFS was 1.5\%. Among the cases with isolated AOA, 5 had AC, 3
had BI, 4 had OH, 2 had RAAD, 1 had ADD, and 3 had MM.

KFS is one of the most frequent segmentation anomalies that develops
secondary to the segmentation abnormalities in the cervical sclerotomes
in the early gestational period and was described as the fusion of two
or more cervical vertebrae. It is described firstly by Maurice Klippel
and Andre Feil in 1912. It is observed in 1 of each 40000 births and the
male/female ratio was reported as 3/2. Short neck, low dorsal hairline,
and restricted neck mobility triad can be observed in 50\% of the
patients with KFS. Other system pathologies including spinal anomalies
such as scoliosis and kyphosis, renal anomalies, Sprengel deformity,
deafness, facial asymmetry, and congenital cardiac disorders can be
observed together with this syndrome. It is divided into three groups by
Feil as the prominent fusion of the cervical vertebra, type I; cervical
fusion at one or two levels, type II; the presence of concomitant
thoracic and lumbar anomalies, type III. The most observed anomaly at
the CVJ is the fusion of C2 and C3 leading to increased mobility of this
region causing more frequent dislocations and thus leading to neck pain
and severe neurological findings due to its spinal cord compression
(10,11). The rate of KFS was 1.5\% in our study and male patients were
more frequently affected. In our study, 2 patients had fusion at the
C1-2 level and 36 patients had fusion at the C2-3 level (type 2 KFS).
Among the KFS cases, 22 had AOA, 10 had OH, 11 had BI, 6 had AC. Among
the 13 KFS cases with dislocation, 9 were ADD, 3 were RAAD, and 1 was
AOD. Among the KFS cases, 5 had MM and 4 had a syrinx cavity.

Odontoid and atlas malformations are the anomalies at the CVJ causing
dislocations secondary to the joint instabilities. Therefore, it is
important to know the ossification and synchondrosis centers of the
atlas at the prenatal and postnatal periods. The odontoid process can be
distinguished from the axis body with a cartilaginous band,
dentocentralsynchondrosis. This area closes at 3 years of age. If it
does not close, os odontoideum (OO) develops. The odontoid process
develops from the caudal half of the first cervical sclerotome and from
the proatlas that develops from its cranial half (1,2). OO is a rare
anomaly at the CVJ region that is located on the anterior arch of the
atlas with a smooth cortical surface and it is branched from the main
body of the odontoid process. It is described firstly by Giacomini in
1866. Despite controversial etiology, it was linked to the congenital
reasons related to the absence of fusion at the synchondrosis level or
the acquired reasons related to the absence of fusion at the
synchondrosis level due to trauma-related fractures. OO was divided into
two groups as orthotopic and dystopic by Fielding. It is named as
orthotopic type in case of its normal localization at the posterior of
the anterior arch of the atlas and if it is located in another position
it is named as dystopic type. Its dystopic type can mimic a type II dens
fracture. Dystopic type is observed as a separate bone structure with
varying sizes. The lower limit of the OO is accepted as its location
above the superior facet of the axis and half of the size of a normal
odontoid process.OO is observed more frequently in the
2\textsuperscript{nd} and 3\textsuperscript{rd} decade with a male
predominance. It can be observed asymptomatically as well as it can
cause secondary neurological and vascular problems due to compression to
the cervico-medullary structure and vertebral artery related to AAD
which develops secondary to the mobile aberrant bone structure and the
insufficiency of the cruciate ligament. Some OO cases can be observed
together with BI. Neck pain, torticollis, myelopathy, and syncope can be
observed as clinical findings (12,13). In our study, the rate of OO was
0.51\% and it was observed more frequently in male patients. Dislocation
was observed in 11 of the 13 patients; 5 of them were ADD, 4 were AOD,
and 2 were RAAD. 1 patient had concomitant AC and 5 had MM.

Os terminale (OT), Condylus Tertius, and OH are the other malformations
of the odontoid process. OT develops secondary to the inability of the
secondary ossification center formed in the odontoid process to join the
structure of the odontoid process until 12 years of age. Despite low
clinical importance, it can be confused with type 1 odontoid fracture.
Condylus Tertius develops when the proatlas fuses wrongly with the
occipital sclerotome that will give rise to clivus during the
embryological development. Then, Condylus Tertius develops to form the
3\textsuperscript{rd} joint with the atlas and it is named as
3\textsuperscript{rd} occipital condyle. This structure can sometimes
cause neural compression.OH is maldevelopment of the odontoid causing it
to stay short. Its agenesis is very infrequent and leads to subluxation
by compromising the stability of the atlanto-axial joint (1,13,14). In
our study, 3 cases with OT were detected. Additionally, 14 OH cases with
concomitant KFS or AOA were detected.

BI, platybasia, and AC can develop embryologically together with the
other congenital anomalies or secondary to these anomalies. BI is the
displacement of the cervical spinal column towards the skull base and
two types of BI were described. Displacement of the dens into the
foramen magnum was described as the type 1 BI and secondary to this
displacement, atlanto-axial instability is observed. Displacement of the
odontoid bone towards the skull base without displacement into the
foramen magnum was described as type 2 BI. Atlanto-axial instability is
not observed in this type of BI (15). Craniometric measurements such as
the Chamberlain line and McGregor line are used for the detection of BI
and the most superior point of the odontoid process must be 5 and 7 mm
above these lines, respectively. AC is a CVJ anomaly that develops
secondary to the displacement of the posterior fossa structures such as
cerebellar tonsils and brainstem towards the foramen magnum and upper
spinal canal. 4 types of AC malformations have been described and type 1
is the simplest and the most commonly observed type in adult patients.
It is described as over 5 mm herniation of the cerebellar tonsils below
the foramen magnum. Type 2 and 3 were observed more frequently in child
patients. The clinical findings, especially neurological findings in the
BI and AC can vary according to the compression at the CVJ level,
narrowing in the spinal canal, and compression to the medullospinal
region or concomitant congenital anomalies such as syringomyelia
(16,17). 14 cases with BI were observed in our study and 3 of these
patients had AOA, 11 had KFS. 12 cases with AC were observed in our
study and 6 had concomitant KFS, 5 had AOA, and 1 had OO.

Instabilities and dislocations are observed in the joint structures
secondary to the laxity in the ligaments and malformations in the bony
structures due to congenital anomalies in the CVJ region. Dislocations
at the AOJ, ADJ, and AAJ levels narrow the spinal canal and lead to
several clinical findings secondary to the compression of the neural
structures. The classification systems used for the joint dislocations
are as follows: Traynelis classification for the AOJ dislocation, the
atlantodental interval for the ADJ, and Fielding and Hawkins
classification for the AAJ. While most of the AOJ dislocations develop
after a high energy trauma, a limited number of nontraumatic cases were
reported in the literature. Congenital dislocations and luxations at the
CVJ are observed at the ADJ and AAJ. Congenital dislocations are not
detected in the childhood period, rather they are detected after the
2\textsuperscript{nd} and 3\textsuperscript{rd} decade due to more
frequent ligament laxity and joint dislocation at these ages (18,19).
ADD is defined as above 3 mm of atlanto-dental interval at the childhood
period and above 5 mm of atlanto-dental interval at the adults. While
Greenberg and Wang classifications are used to direct the management of
the dislocations, Fielding and Hawkins classification are utilized for
determining the way of dislocation. While dislocations are divided into
two groups as correctable and uncorrectable in the Greenberg
classification, Wang classification added 2 new parameters, instability
and bone dislocation, based on the Greenberg classification. Treatment
protocols are generated according to these classifications (20). The
joint dislocations are observed more frequently at the advancing ages
consistent with the previous reports and the most frequently observed
dislocation was ADD. 9 cases with KFS, 5 cases with OO, and 1 case with
AOA had concomitant ADD. 7 cases with RAAD were observed and 3 of them
had KFS, 2 had AOA, and another 2 had OO. AOD was observed in 5 cases
and 4 of them had concomitant OO and 1 had KFS.

This study includes some limitations. Firstly, the study was conducted
retrospectively by investigation of the medical records of the patients.
Secondly, because some patients were referred to other centers,
postoperative examinations of especially the patients who had surgery
were not present. Thirdly, a complete spinal scan could not be performed
to detect other concomitant vertebral anomalies in the patients with
fusion that was detected with cervical MSCT.

\textbf{Conclusion}

Congenital anomalies at the CVJ region, although rare, can lead to
morbidity and mortality secondary to the compression to the vital
structures at the CVJ region due to degenerative changes in the bone
structure and dislocations in the joints with advancing ages. Therefore,
MSCT and MRI studies on the CVJ region should be evaluated carefully at
every age and the detected pathologies should be followed due to the
risk of complications at advancing ages.

\textbf{References}

1. Pang D, Thompson DN. Embryology and bony malformations of the
craniovertebral junction.Childs NervSyst 2011 Apr;27(4):523-564.

2.da Silva OT, Ghizoni E, Tedeschi H, Joaquim AF. Role of dynamic
computed tomography scans in patients with congenital craniovertebral
junction malformations.World J Orthop 2017 Mar 18;8(3):271-277.

3. Dickman CA, Lekovic GP. Biomechanical considerations for
stabilization of the craniovertebral junction.Clin Neurosurg
2005;52:205-213.

4. Lustrin ES, Karakas SP, Ortiz AO, Cinnamon J, et al. Pediatric
cervical spine: normal anatomy, variants, andtrauma. Radiographics 2003
May-Jun;23(3):539-560.

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