► [Small/absent odontoid]
The first cervical vertebra (atlas) and the odontoid process, or dens, of the second cervical vertebra (axis) originate from the first cervical sclerotome mesenchyme, whereas the body, lateral masses, and posterior arch of the axis arise from the second cervical sclerotome (Currarino 2002). The odontoid process forms from two independent ossification centers that appear during the 5th fetal month and
generally fuse with each other on the midline by the 7th fetal month. Delayed fusion occurring within the 2nd year is regarded as a normal variation. The body of the axis is formed from a single ossification center that appears around the 4th fetal month and fuses with the dens between 4 and 6 years of age. The neural arches each originate from an individual ossification center and fuse posteriorly at the spinous process by the age of 3-4 years and medially at the neurocentral synchondroses with the vertebral body by the age of 7 years. A secondary center of ossification for the tip of the dens, the 'ossiculum terminale,' develops from the first occipital sclerotome during the 2nd year of life and fuses with the remainder of the dens at 10-12 years of age (Fig. 3.44).
Defects of formation, segmentation, or fusion involving C-2 lead to a spectrum of odontoid anomalies, including nonfusion of the apical apophysis of the dens (ossiculum terminalis persistens), nonfusion of the dens with the body (os odontoideum), sagittal clefts, odontoid hypoplasia, and odontoid agenesis. Such anomalies as duplication of the odontoid, consisting of a partially fused midline ossicle on
the anterior arch of C-1 and fusion between the anterior lip of the foramen magnum and C-1, are exceedingly rare (Garant et al. 1997). Persistent ossiculum terminalis is diagnosed in children over 12 years of age and is a developmental variation with no clinical significance. Its importance is confined to its differentiation from more serious odontoid anomalies and fractures. The term 'os odontoideum' defines a small ossicle lying at the tip of a hypoplastic odontoid process and clearly separated from the base of the axis (Fig. 3.45). The cephalic portion of the dens normally forms from its two lateral ossification centers, but it fails to unite with the body of the axis above the level of the neurocentral synchondrosis. Whether os odontoideum represents a long-standing nonhealed fracture of a previously normally formed dens or a congenital anomaly of bony union is still not clear. It has been suggested that formation of os odon-toideum might be normal but ossification, abnormal, owing to excessive movement of the atlanto-axial joint made unstable at the time of ossification of the cartilaginous anlage (Stevens et al. 1994). Genetic factors certainly have a role in some cases, as suggested by the occurrence of os odontoideum in several members of one family (Morgan et al. 1989) and in identical twins (Kirlew et al. 1993). However, instances of acquired os odontoideum following traumatic atlanto-axial dislocation are well established (Hukuda et al. 1980). Os odontoideum can be misdiagnosed as acute dens fracture, thus prompting unnecessary invasive treatment. Hypertrophy and sclerosis of the C-1 anterior arch, representing a stress response secondary to chronic upper cervical instability, are associated in the adult population with os odontoideum but not with acute dens fractures (Holt et al. 1989). Certain rare congenital anomalies involving the anterior arch of the atlas, such as anterior arch rachischisis, can also simulate odontoid fractures on plain radiograms (Glasser and Glasser 1991). Os odontoideum must be differentiated from ossiculum terminale persistens and from ossifications of the ligamentous apparatus (Prescher 1990). Sagittal clefts of the odontoid are a normal finding in children younger than 2 years. Projection artifacts with superimposition of other structures, such as the superior incisors, can simulate odontoid clefts on frontal radiograms. Schisis of the posterior arch of C-2 is rare and often occurs in association with schi-sis of the atlas. Odontoid hypoplasia/aplasia can be an isolated anomaly (Fig. 3.46) or occur in association with other defects in the context of a syndromic disorder. In the majority of patients with odontoid hypoplasia/aplasia other anomalies involving the derivatives of the occipital and upper cervical somites are found, including atlanto-occipital assimilation, cervical vertebral fusion, basilar invagination and hindbrain deformity of Chiari type.
Developmental anomalies of the odontoid, including odontoid hypoplasia, aplasia, and os odon-toideum, can cause atlanto-axial instability and, in turn, serious neurological sequelae related to spinal cord compression. Clinical symptoms range from none to sudden death and include persistent neck
complaints, transient or persistent neurological deficits, cranial nerve irritation, and, occasionally, cerebral and brain stem ischemia resulting from compression of the vertebral arteries (Hensinger et al. 1978; Phillips et al. 1988). The effects of spinal cord compression due to atlanto-axial instability are discussed in a separate section in this book. Anomalies of the odontoid must be carefully searched for in patients in whom the prevalence of this anomaly is increased. In fact, serious neurological compromise can be prevented by proper management (Hensinger 1991).
Among the disorders with proven type II collagen defects, the following are associated with anomalies of the odontoid, ranging from mild hypoplasia to aplasia,with or without C1-2 subluxation: hypochon-drogenesis, spondyloepiphyseal dysplasia congenita, Kniest dysplasia, and spondylo-epi-metaphyseal dysplasia (Strudwick type) (Lachman 1997). At birth, spondylo-epi-metaphyseal dysplasia, Strudwick type (OMIM 184250) cannot be distinguished from spondyloepiphyseal dysplasia congenita (OMIM 183900),since major radiographic features, including odontoid hypoplasia (Fig. 3.47), lumbar lordosis, coxa vara, and delayed ossification of pubic bones and proximal femoral epiphyses, are identical in both diseases. The distinguishing feature in Strudwick dysplasia is the presence of striking metaphyseal irregularities of long bones, with sclerotic mottling and dappling, a finding that develops during infancy and early childhood (Shebib et al. 1991). Patients with spondyloepiphyseal dysplasia tarda (OMIM 184100) may show cervical vertebral abnormalities, including malformation of the odontoid process, such as hypoplasia or cone-shaped odontoid. Unlike those with spondyloepiphyseal dysplasia congenita, they do not usually show C1-2 problems, although cases presenting with cervical instability and neurological compromise have also been described (Reardon et al. 1994). In Kniest dysplasia (OMIM 156550),in which the whole spine is involved to a major extent,the odontoid is often abnormally wide and short (less frequently, it is taller than normal). Progressive fusion between the odontoid and the anterior arch of the atlas, as well as between the posterior arch of the atlas and the cranial base, may also occur (Friede et al. 1985). By contrast, Stickler dysplasia (OMIM 108300) and premature osteoarthropathy, which also belong to the group of disorders with defects in the COL2A1 gene, do not show odontoid anomalies.
Odontoid dysplasia, in the presence or absence of atlanto-axial instability, is a very common finding in patients with mucopolysaccharidoses (Hurler syndrome, Morquio disease) (Fig. 3.48). This underlines the importance of careful neurological follow-up of these patients (Nelson and Thomas 1988; Thomas et al. 1985; Tandon et al. 1996). In patients with
brachyolmia (OMIM 271530), generalized severe platyspondyly and odontoid hypoplasia are consistent features, while cervical spine instability has not been reported.
A group of conditions characterized by defective or delayed ossification of the cervical spine often show odontoid hypoplasia. These conditions include dyssegmental dysplasia [both the lethal Silverman-Handmaker type (OMIM 224410) and the less severe Rolland-Desbuquois type (OMIM 224400) (Hand-maker et al. 1977)], atelosteogenesis type I (OMIM 108720), chondrodysplasia punctata, rhizomelic type (OMIM 215100) and tibia-metacarpal type (OMIM 118651), hypophosphatasia (OMIM 241500), and metatropic dysplasia (OMIM 250600). In the last disorder, odontoid hypoplasia is found with or without C1-2 subluxation (Shohat et al. 1989). In diastrophic dysplasia (OMIM 222600) either hypoplasia or hy-perplasia of the odontoid process can be found (Fig. 3.49) in association with progressive thora-columbar kyphoscoliosis, cervical kyphosis, and mild narrowing of the interpediculate distance in the lower lumbar spine. C1-2 dislocation, with or without cord compression, is also commonly encountered. Similarly, odontoid hypoplasia or hyperplasia may be observed in Dyggve-Melchior-Clausen syndrome (OMIM 223800). Regardless of the presence of hypo- or hyperplasia,these patients can manifest significant C1-2 dislocations (Naffah and Taleb 1974). Although odontoid hypoplasia and C1-2 dislocation are not prominent features, they also occur in patients with pseudoachondroplasia (OMIM 177170) (Fig. 3.50).
Abnormalities of the odontoid have been found in as many as 22 % of patients with Down syndrome (OMIM 190685), in which the incidence of atlantoaxial instability declines with age (Elliott et al. 1988).
Was this article helpful?
Have you recently experienced hearing loss? Most probably you need hearing aids, but don't know much about them. To learn everything you need to know about hearing aids, read the eBook, Hearing Aids Inside Out. The book comprises 113 pages of excellent content utterly free of technical jargon, written in simple language, and in a flowing style that can easily be read and understood by all.