患者，女性，31岁，发生机动车辆事故。31-year-old woman involved in a high-speed motor vehicle collision.
【影像表现】：The coronal CT neck image in Figure 1 shows a small, round os odontoideum (red arrow) separated from the body of C2 (blue arrow) by a wide radiolucent gap. It has smooth and uniform cortical margins.冠状位CT显示一个小圆形的齿突（红箭），与颈2椎体分离（蓝箭），皮质边缘均匀平滑。
The sagittal CT neck image in Figure 2 shows signs of C1 anterior arch hypertrophy (green arrow). There is no paravertebral swelling. The posterior atlanto-dens interval (PADI) also known as the space available for cord (SAC) is 12mm (less than 13mm has a poor prognosis).
【影像诊断】：Os Odontoideum 游离齿状突（齿状突小骨）
【诊断要点】：Os odontoideum is a small round or oval corticated ossicle superior to the body of C2 游离齿状突是位于C2椎体上方的小圆形或卵圆形的皮质小骨。
Etiology is most likely post-traumatic failure of fusion of the ossification centers of the C2 body and dens. 创伤后导致C2椎体和齿状突骨化中心融合失败是最可能的病因。
Radiographic features include C1 anterior arch hypertrophy and a wide radiolucent gap between the ossicle and C2 which lies above the level of the superior facets of the axis 放射学特征包括C1前弓肥厚，齿状突和C2椎体之间宽大的透光性缺口，位于枢椎上关节突上方。
Serious sequelae include spinal cord compression and death 严重的后遗症包括脊髓压迫症和死亡。
【讨论】：Os odontoideum describes a rare condition in which the dens is separated from the axis body. Two types of os odontoideum have been identified. An orthotropic os odontoideum is located in the normal position of the odontoid process, whereas a dystrophic os odontoideum is either attached to the anterior arch of C1 or the clivus.
The etiology of os odontoideum is still debated. The leading hypothesis suggests that trauma in early childhood causes injury to the soft tissues between C1 and C2. This is often seen when babies fall from their cribs or toddlers fall from stairs. Radiographic images taken after this injury usually do not show any significant pathology besides retropharyngeal swelling.
Injury results in avascular necrosis of the odontoid process over a period of months or years. Therefore, the cephalad ossification centers do not fuse with the C2 body. With growth, the alar ligaments that attach the tip of the dens to the occiput pull the ossicle upwards giving it its characteristic appearance.
Os odontoideum may also occur in patients with congenital anomalies including Down\’s syndrome, multiple epiphyseal dysplasia, Klippel-Feil malformation and Morquio syndrome. These patients are born with deficient odontoid processes.
Symptoms are variable with vague pain in the neck and shoulders being the most common. Less commonly reported symptoms include headaches, torticollis, weakness and paresthesias. The most serious complications of an os odontoideum occur when C1-C2 instability causes spinal cord compression or vertebral artery compromise. This may result in brain stem symptoms, quadraparesis, transient bouts of unconsciousness and sudden death.
The best way to evaluate an os odontoideum is by obtaining open-mouth anterior-posterior and flexion-extension lateral plain films. A criteria often used to make the diagnosis is the Posterior AtlantoDens Interval (PADI) also known as the Space Available for Cord (SAC). This is the distance between the posterior border of the dens and the anterior border of the posterior ring of the atlas. A PADI of less than 13mm is associated with neurologic decline. C1-C2 translation of more than 5mm on flexion-extension radiographs also has a poor prognosis.
The most common treatment for a symptomatic os odontoideum is C1-C2 posterior arthrodesis. This is particularly indicated in patients who exhibit signs of cord compression. Medical management is indicated for patients with only mechanical symptoms.