We describe a 53-year-old Japanese woman with absence of the posterior arch of the atlas. Computed tomography revealed that the residual posterior fragment which was displaced to the right side does not correspond to a rare ossification center in the midline and that there was no clear mass of the posterior arch except for a small fragment. Moreover, the midline center was not demonstrated as a soft tissue mass. Easy tilting of the posterior fragment on flexion-extension movement suggests that the cartilaginous arch was not present between the lateral masses and the posterior fragment of the atlas. These observations support the hypothesis that hypoplastic development of the arch plays a major role in the pathogenesis of this anomaly rather than incomplete ossification. Abnormal segmentation of the vertebrae may be implicated in the etiology of this condition, since fusion of the anterior vertebral masses between the C5-6 levels, as shown in Klippel-Feil syndrome, is also observed.

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Osteosarcoma of the thoracic spine developed in a 15-year-old Japanese boy. After his first admission with paralysis, multiple skeletal metastases were demonstrated in the absence of pulmonary metastasis. This rare condition may possibly be considered as a unicentric osteosarcoma with bone metastases, since there were no precursor lesions or history of exposure to radioactive materials or chemical agents. These multiple lesions may be an example of so-called "organ-specific metastasis," although this "nonstochastic" process is rare.