Hajdu-Cheney syndrome (HJCYS) is a rare autosomal dominant skeletal disorder characterized by short stature, coarse and dysmorphic facies, bowing of the long bones, and vertebral anomalies. Facial features include hypertelorism, bushy eyebrows, micrognathia, small mouth with dental anomalies, low-set ears, and short neck. There is progressive focal bone destruction, including acroosteolysis and generalized osteoporosis. Additional and variable features include hearing loss, renal cysts, and cardiovascular anomalies (summary by Ramos et al., 1998; Simpson et al., 2011; Isidor et al., 2011).

The odontoid process, or dens, is a bony projection from the axis (C2) upward into the ring of the atlas (C1) at the top of the spine. During embryogenesis, the body of the odontoid derives from the centrum of the atlas and separates from the atlas, fusing with the superior portion of the axis. If the odontoid is hypoplastic or absent, the attachments for the apical and alar ligaments are missing, allowing for excessive rotation of the atlas, craniocervical instability, and possibly cord compression (summary by Stevens et al., 2009).