：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.
【影像图片】CT MPR图像
【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。
矢状位的CT显示颈1椎体前弓肥大（绿箭），无明显椎旁肿胀。后方寰齿间距（又称脊髓可用空间）约12mm（小于13mm预后不良）
【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.
游离齿状突是齿状突与枢椎椎体分离的一种少见病变，有两种类型，直生性齿状突小骨位置在正常的齿突尖，而营养障碍性齿状突小骨紧贴于颈1椎体前弓或斜坡。
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.
游离齿状突的病因学仍然有争议，主要的假说认为幼童时期发生的C1和C2椎体之间的软组织损伤（经常见于婴儿从摇篮里掉出来或初学走路从楼梯上滚下来）引起，外伤后的放射学检查通常不会显示明显异常，除了咽后壁肿胀之外。
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.
外伤导致齿突尖缺血性坏死超过数月或数年。因此头侧的骨化中心不会与C2椎体融合。随着生长，翼状韧带（附着于齿状突尖端和枕部的韧带）向上牵拉小骨形成其特征性的表现。
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.
游离齿状突也会发生在先天异常的病人中，包括Down's综合症，多发性的骨骺发育不良，短颈畸形和家族性营养不良。这些病人出生伴有齿突尖的发育不良
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.
临床症状表现不一，可能颈肩部的隐痛可能是最常见的表现，其他相对少见的表现有头痛、斜颈、无力及感觉异常。最严重的并发症是C1-2椎体不稳所致的脊髓受压或椎动脉受累，这可能导致脑干症状、四肢麻痹、短暂性的意识丧失和突然死亡。
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.
评估齿状突游离的最好的方式是张口位颈椎前后位和俯曲位的颈椎侧位平片。后方环枢间距又称脊髓可用空间常常被用来作为诊断的标准。这是齿状突后缘至环椎后弓间的距离。后方环枢间距的距离小于13mm常常伴有神经学症状，俯曲位C1-C2平移超过5mm也意味着预后不良。
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.
对于有症状的游离齿状突最常见的治疗方式是C1-C2椎体后缘关节融合术，尤其是对于有明显脊髓受压的患者。
来源：影像园
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