However, true aplasia of the odontoid process would mean that there is complete absence of the dens including its base and therefore some doubt that true odontoid aplasia even exists. Rarely, the odontoid process is aplastic. The dens may be developmentally hypoplastic, appearing as a small stump at the superior portion of the C2 body. Some unossified cartilage may persist into adult life in the center portion of the dens and in the dental synchondrosis (sometimes called the C1-2 disc anlage ), located at the junction of the C2 body and odontoid base.Īlthough relatively uncommon, os odontoideum is said to be the most common anomaly of the odontoid process ( Figs. By 3 to 4 years of age, the odontoid process is completely ossified, and by age 6 it is fused to the C2 vertebral body in most people. At birth, the cephalad portion of the odontoid is usually not ossified. The inferior portion of the dens begins to ossify in the seventh fetal month and ossification progresses from caudal to cephalad. The two basal columnar-shaped ossification centers of the dens begin to fuse in the sixth fetal month. The base of the odontoid process forms the superior portion of the C2 vertebral body centrally. The normal ossiculum terminale appears at age 3 and fuses to the remainder of the dens by age 12. The ossiculum terminale develops from either mesenchyme from the fourth occipital sclerotome or proatlas (or both) and is not ossified at birth. The normal odontoid process (dens) develops from three ossification centers: a tiny terminal ossification center, the ossiculum terminale, and two larger, columnar-shaped ossification centers that form the majority of the odontoid process including its base. The C2 vertebral body is formed from the second cervical sclerotome. Symptomatic cases of os odontoideum have upper neck pain, stiffness, or neurologic dysfunction secondary to atlantoaxial instability with cord compression.Įmbryologically, the odontoid process develops from the fourth occipital sclerotome and the first cervical sclerotome. Patients with os odontoideum may or may not have symptoms referable to the upper cervical region. Os odontoideum is an uncommonly encountered condition in routine practice, but when present, is often discovered as an incidental finding on CT or MR scans of the cervical spine. Os odontoideum is a term first used by Giacomini in 1886 to describe a potentially unstable condition, whereby the odontoid process is separated from the body of C2.
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