Evaluation of Fetal First and Second Cervical Vertebrae: Normal or Abnormal?

OBJECTIVES: To use 3-dimensional sonographic volumes to evaluate the variable appearance of the normal fetal cervical spine and craniocervical junction, which if unrecognized may lead to misdiagnosis of malalignment at the first and second cervical vertebrae (C1 and C2). METHODS: Three-dimensional sonographic volumes of the fetal cervical spine were obtained from 24 fetuses at gestational ages between 12 weeks 6 days and 35 weeks 1 day. The volumes were reviewed on 4-dimensional software, and the vertebral level was determined by labeling the first rib-bearing vertebra as the first thoracic vertebra. The ossification centers of the cervical spine and occipital condyles were then labeled accordingly and evaluated for alignment and structure by rotating the volumes in oblique planes. The appearance on multiplanar images was assessed for possible perceived anomalies, including malalignment, particularly at the C1 and C2 levels. Evidence of head rotation was correlated with the presence of possible malalignment at C1-C2. Head rotation was identified in the axial plane by measuring the angle of the anteroposterior axis of C1 to the anteroposterior axis of C2. RESULTS: Of the 24 fetuses, 16 had adequate quality to assess the entire cervical spine and craniocervical junction. All 16 cases showed an osseous component of C1 that did not align directly with C2 on some of the multiplanar images when the volumes were rotated, which could lead to suspected diagnosis of spinal malalignment or a segmental abnormality, as occurred in 2 clinical cases in our practice. All 16 cases showed at least some degree of head rotation, ranging from 2° to 36°, which may possibly explain the apparent malalignment. The lateral offset from C1 to C2 ranged from 0.0 to 3.3 mm. CONCLUSIONS: The normal C1 and C2 ossification centers may appear to be malaligned due to normal offsetting (lateral displacement) of C1 on C2. An understanding of the normal development of the cervical spine is important in assessing spinal anatomy.

Embryonic and early fetal period development and morphogenesis of human craniovertebral junction.

Several studies have focused on the cartilaginous, articular, and ligamentous development of the craniovertebral joint (CVJ), but there are no unifying criteria regarding the origin and morphogenetic timetable of the structures that make up the CVJ. In our study, serial sections of 53 human embryonic (n = 27) and fetal (n = 26) specimens from O'Rahilly stages 17-23 and 9-13 weeks, respectively, have been analyzed. Our results demonstrate that the chondrification of the pars basioccipitalis and exoccipitalis becomes observable at stage 19, and all future bones in the CVJ are in their cartilaginous form except for the future odontoid process. In addition, two chondrification centers appear for the body of the axis. From stage 21, the apical, alar, and transverse atlantal ligaments begin to acquire a ligamentous structure and the odontoid process initiates its chondrogenic phase. Stage 22 witnesses the first signs of the articular cavities of the atlanto-occipital joint, and by stage 23 all joints have cavities except for the transverse-odontoid joint, which will wait until week 9. In week 10, the ossification of the basilar part of the occipital bone begins, followed by the rest of the structures except for the odontoid process, which will start at week 13, thus completing the osteogenesis of all bones in the CVJ. The results of this study could help in establishing the anatomical basis of the normally functioning CVJ and for detecting its related pathologies, abnormalities, and malformations.

Hypoplastic occipital condyle and third occipital condyle: review of their dysembryology.

Disruption or embryologic derailment of the normal bony architecture of the craniovertebral junction (CVJ) may result in symptoms. As studies of the embryology and pathology of hypoplasia of the occipital condyles and third occipital condyles are lacking in the literature, the present review was performed. Standard search engines were accessed and queried for publications regarding hypoplastic occipital condyles and third occipital condyles. The literature supports the notion that occipital condyle hypoplasia and a third occipital condyle are due to malformation or persistence of the proatlas, respectively. The Pax-1 gene is most likely involved in this process. Clinically, condylar hypoplasia may narrow the foramen magnum and lead to lateral medullary compression. Additionally, this maldevelopment can result in transient vertebral artery compression secondary to posterior subluxation of the occiput. Third occipital condyles have been associated with cervical canal stenosis, hypoplasia of the dens, transverse ligament laxity, and atlanto-axial instability causing acute and chronic spinal cord compression. Treatment goals are focused on craniovertebral stability. A better understanding of the embryology and pathology related to CVJ anomalies is useful to the clinician treating patients presenting with these entities.

Fetal development of the transverse atlantis and alar ligaments at the craniovertebral junction.

Although the fetal development of the craniovertebral junction has long been of major interest to embryologists from the viewpoint of segmentation, development of the associated ligaments has received scant attention. Using semiserial horizontal sections from 18 embryos and fetuses (six embryos with a crown-rump length (CRL) of 20-26 mm or ~6-7 weeks of gestation; five fetuses with a CRL of 32-58 mm or 8-9 weeks; seven fetuses with a CRL of 90-115 mm or 14-15 weeks) without any abnormalities of cartilage configuration such as atlas assimilation, we studied the ligamentous structures along and around the odontoid process of the axis. The transverse atlantis and alar ligaments originated from a common mesenchymal condensation possibly corresponding to the proatlas segment: the former started to develop slightly earlier than the latter, and the morphologies of both were established at 7 weeks of gestation. Development of the joint cavitation around the odontoid process began in the mid-anterior area at 6 weeks, but was not fully completed even at 15 weeks (115 mm CRL). The presumptive joint cavity expressed vimentin and CD34 and contained abundant CD68-positive macrophages. We always found a mid-anterior joint cavitation facing the basi-occipital, but the embryological meaning remained unclear. The apical ligament appeared most likely to originate from the notochord sheath. The notochord was exposed from the tip of the odontoid process toward the loose epidural tissue and entered the occipital bone, but was difficult to trace to the anterior surface of the basi-occipital.

Craniovertebral junction anomalies in inherited disorders: part of the syndrome or caused by the disorder?

Patterns of skeletal abnormality at the craniovertebral junction in the normal population and in syndromes such as Down, Morquio etc, are compared and the recent embryological data and comparative anatomy reviewed. The authors' view based on their own clinical and radiological experience is that the os odontoideum is the product of excessive movement at the time of ossification of the cartilaginous dens and is exactly analogous to the unfused Type II odontoid fracture. True hypoplasia of the odontoid peg is part of a wider segmentation defect associated with Klippel Feil, occipitalised atlas and/or basilar invagination; it is hardly ever associated with instability.

Atlantoaxial anomalies with particular emphasis on os odontoideum.

Anomalies of the atlantoaxial articulation are discussed with emphasis on os odontoideum. Normal and abnormal embryological and anatomical characteristics are described, as is their relevance upon stability of the upper cervical segments. This paper further discusses and reviews theories pertaining to the congenital and acquired forms of os odontoideum and a case report is presented. Some conclusions are drawn from the presented case report.

[Structure and histo-topography of meniscoid structures of the atlanto-occipital and atlanto-axial joints]. / Stroenie i gistotopografiia meninskoidnykh struktur atlantozatylochnogo i atlantoosevykh sustavov.

Anatomical preparation, Pirogov's saw-cuts and serial histotopographic sections, made across the atlantooccipital and atlantoaxial joints in the sagittal, frontal and horizontal planes in 35 corpses have been used in reconstructing and studying the meniscoid structures in the area of the joints mentioned. The meniscoids are composed of fine fibrillar connective tissue with a large amount of fat and vessels. The meniscoid has a triangle form, its base consisting of collagenous fibers, gradually transfers into the articular capsule, and its apex is directed into the articular cavity. The meniscoid, surrounding the tooth of the axial vertebra, at the sides passes into the medial parts of the atlantoocipital and lateral atlantoaxial joints, forming a single meniscoid complex of the joints studied. A tight connection is stated between the meniscoids and the perineural formations of the I and II cervical nerves and with the vertebral vessels.

Prenatal development of the composite occipito-atlanto-axial synovial joint cavity in the dog.

In the dog, the synovial cavities of the atlanto-occipital and atlanto-axial joints communicated to form a single large composite joint cavity. The prenatal development of this composite occipito-atlanto-axial joint cavity was studied by examining 26 serially sectioned dog embryos and fetuses that ranged in size from 19 to 68 mm crown-rump length, and were between 30 and 42 days of gestational age. In the composite occipito-atlanto-axial joint, trilaminar interzones developed at 19-22 mm (30-31 days), joint cavities opened at 27-32 mm (33-34 days), and the atlanto-occipital and atlanto-axial cavities first communicated at 48 mm (37 days).

The human vertebral column at the end of the embryonic period proper. 2. The occipitocervical region.

The present investigation of the cervical region of the vertebral column at eight post-ovulatory weeks is the first such study based on precise reconstructions of staged embryos. At the end of the embryonic period proper, a typical vertebra is a U-shaped piece of cartilage characterized by spina bifida occulta. The notochord ascends through the centra and leaves the dens to enter the basal plate of the skull. The median column of the axis comprises three parts (designated X, Y, Z) which persist well into the fetal period. They are related to the first, second and third cervical nerves, respectively. Part X may project into the foramen magnum and form an occipito-axial joint. Part Z appears to be the centrum of the axis. The articular columns of the cervical vertebrae are twofold, as in the adult: an anterior (atlanto-occipital and atlanto-axial) and a posterior (from the lower aspect of the axis downwards). Alar and transverse ligaments are present. Cavitation is not found in the embryonic period in either the atlanto-occipital or zygapophysial joints, and is generally not present in the median atlanto-axial joint either. Most of the transverse processes exhibit anterior and posterior tubercles. An 'intertubercular lamella' may or may not be present, i.e. the foramina transversaria are being formed around the vertebral artery. The spinal ganglia are generally partly in the vertebral canal and partly on the neural arches, medial to the articular processes. During the fetal period, the articular processes shift to a coronal position and this alteration appears to be associated with a corresponding change in the location of the spinal ganglia.