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.