Discussion
The structures forming the CVJ are originated embryologically from the
4th occipital and 1st and
2nd cervical sclerotomes. Occipital condyles, the
superior part of the posterior arch of the atlas, and apex of the dens
are originated from the 4th occipital sclerotome
(proatlas). The inferior part of the posterior arch of the atlas,
anterior arch of the atlas, and dens are originated from the
1st cervical sclerotome and the body of the axis is
originated from the 2nd 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 4thoccipital sclerotome and 1st and 2ndcervical 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
2nd and 3rd 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
3rd joint with the atlas and it is named as
3rd 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
2nd and 3rd 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.