Os Odontoideum

Etiology

The etiology of this condition remains debated, with 2 prevailing theories—traumatic and congenital—regarding the origin of os odontoideum.

Most studies favor the traumatic theory, considering it an acquired pathology resulting from avascular necrosis caused by an odontoid fracture. This theory purports that the os odontoideum forms after a traumatic event, with the preservation of the blood supply to the fractured odontoid tip; this can occur in the prenatal or postnatal periods and may not be recalled by the patient.[11][12][13][14] There have been case reports documenting the formation of an os odontoideum after trauma in patients with previously documented normal odontoid processes.[15][16][17]

On the other hand, the congenital hypothesis considers it a segmental defect, representing a failed fusion of the odontoid and axis vertebral body,[18][19][20] an incomplete migration of the axis centrum, failure of segmentation, or nontraumatic avascular necrosis.[21] The congenital theory may explain os odontoideum in identical twins without a history of trauma or reports of familial expression.[22][23][11][24] This line of thought is reinforced by Straus et al, who demonstrated trends in gene expression profiles between os odontoideum patients and normal subjects.[25] Certain associations with upper cervical anomalies such as the hypoplastic posterior arch of C1, Klippel-Feil syndrome, trisomy 21 syndrome, Morquio syndrome, or neurofibromatosis also help support this theory.[11]

In pediatrics, awareness of normal embryology and ossification of the odontoid process is critical to reducing the rate of false-positive diagnoses. The apex of the odontoid arises from the fourth occipital sclerotome, the odontoid process and anterior arch of C1 arise from the first cervical sclerotome, and the body of the axis arises from the second sclerotome. Although the apex (ossiculum terminale, secondary ossification center) typically ossifies between the ages of 3 and 6, it does not fuse with the body of the odontoid until a child attains approximately ages 10 to 12. Similarly, the normal sub-dental synchondrosis, situated between the odontoid and the body of C2, typically fuses between the ages of 3 and 8.[22][21][7]

Epidemiology

Os odontoideum is a rare condition with an uncertain estimated prevalence due to the usual asymptomatic course and the absence of large-scale screening studies. This condition is usually found incidentally in the pediatric population or diagnosed in symptomatic patients during adulthood.[4][6] In a retrospective review conducted by Perdikakis et al, examining the magnetic resonance imaging (MRI) of odontoid process configuration in 133 patients aged between 19 and 81, os odontoideum was identified in only one case, representing a prevalence of 0.7%.[26] 

Pathophysiology

The ossiculum terminale, or the apical odontoid epiphysis, originates from the fourth occipital sclerotome, while the odontoid and axis bodies develop from the first and second cervical sclerotomes, respectively. The neurocentral synchondrosis is an epiphyseal growth plate, delineating the first and second cervical sclerotomes. Additionally, the subdental synchondrosis, a cartilaginous band, typically exists between the odontoid process and the body of the axis in most children aged 4 and younger, disappearing by around age 8.

The odontoid process depends primarily on the terminal apical arcade for its blood supply, which anastomoses caudally with the deep penetrating branches originating from the posterior ascending arteries, derived from the vertebral artery. Due to the precarious nature of this vascular supply, any insufficiency in the terminal arcade can result in ischemia and necrosis during embryologic development.[27]

History and Physical

Clinical presentation varies, ranging from asymptomatic incidental findings on imaging to neck discomfort or neurological deficits with permanent paralysis in severe cases.[10]

The initial clinical presentation is often nonspecific, with neck pain, shoulder pain, torticollis or headache, and upper extremity paresthesia-like intermittent tingling and numbness in the neck and upper limbs. Other common complaints are lower limb weakness and gait difficulties.[11] These symptoms are thought to result from static or dynamic compression or repeated minor trauma to the spinal cord.[28]

Increased motion at the C1-C2 level can lead to vertebral artery occlusion and ischemia of the brainstem and posterior fossa structures, resulting in seizures, syncope, vertigo, visual disturbances, and even sudden death after minor trauma.[29][30][31][32][33]

During the physical examination, it is expected to find neurological deficits or loss of motor milestones. Hypoesthesia, hyperreflexia (patellar tendon reflex), scapulohumeral (Shimizu) reflex, Tromner reflex, Hoffmann reflex, Babinski reflex, and clonus may be present in chronic spinal cord compression.[6] Spastic or broad-based gait and decreased hand dexterity are signs of myelopathy. All those previous signs and symptoms are nonspecific, and other diagnoses that cause myelopathy should be ruled out.

Evaluation

Early and accurate diagnosis is critical in mitigating morbidity and mortality associated with os odontoideum. Minor trauma in undiagnosed cervical instability can lead to catastrophic neurological outcomes. Therefore, routine anteroposterior and lateral cervical spine radiographs, along with an open-mouth odontoid view, serve as the initial diagnostic approach, facilitating timely identification of the condition.

The lateral radiograph of the cervical spine reveals a posterior widening of the anterior atlantoaxial distance and disruption of the spinolaminar junction line. Additionally, a well-corticated rounded ossicle is observed just dorsal and slightly cranial to the anterior arch of C1 in this projection, with no prevertebral soft tissue widening. Cervical spine flexion-extension radiographs help to determine the degree of atlantoaxial instability.[29] Some authors also perform bedside lateral radiographs during preoperative skull traction, considering them crucial for assessing atlantoaxial joint instability and subluxation, aiding in determining the appropriate surgical approach.[34]

CT images confirm the corticated margin of the ossicle, depicting a shortened odontoid process and a smooth bone ossicle. Additionally, reconstruction of CT images may reveal the congenital incomplete union of the C1 posterior arch.[11] A hypertrophied anterior arch of C1 often suggests underlying chronic instability (see Image. Hypoplastic Odontoid Process).[35] CT scans are also used to assess anatomical landmarks, including the length and direction of the pedicles, as well as vascular anatomy (VA), with particular emphasis on detecting anomalous VA.[34]

MRI reveals fluid within the widened anterior atlantoaxial space, along with a potential focal narrowing of the spinal canal due to the posteriorly tilted odontoid process pressing against the ventral cervical spinal cord. Additionally, areas of increased T2-weighted signal within the central gray matter may be present, indicative of myelomalacia.[10]

Certain measurements obtained from lateral cervical radiographs or sagittal CT reconstruction can assist in defining cervical instability or determining the appropriate surgical approach, as mentioned below.

• The atlanto-dental interval (ADI): ADI refers to the distance between the odontoid process and the posterior border of the anterior arch of the atlas. In adults, an ADI measurement greater than 3.5 mm suggests instability, with values exceeding 10 mm indicating a need for surgery. Conversely, an ADI of up to 5 mm in pediatric patients is considered normal.[6]

• Space-available-cord (SAC) or posterior ADI (PADI): Another important measurement is the SAC or PADI, which refers to the distance between the posterior surface of dens and the anterior surface of the posterior arch of the atlas. In adults, a PADI measurement of less than 14 mm is associated with an elevated risk of neurological injury and warrants consideration for surgical intervention.[36]

Treatment / Management

Treatment options vary from conservative management to surgical intervention, depending on the severity of symptoms and evidence of atlantoaxial instability as assessed through radiological imaging. For asymptomatic patients, an ongoing debate exists regarding the merits of conservative therapies with imaging surveillance versus prophylactic spinal fusion.[10] Regular clinical and radiographic follow-up is advised for those without symptoms and lacking evidence of C1-C2 instability, coupled with avoiding all contact sports to mitigate potential risks.[34](B3)

Asymptomatic patients with radiographic evidence of atlantoaxial instability should consider surgery, although there is a controversy regarding whether C1-C2 fusion can significantly influence patients' neck rotation and daily activities. Although the procedure could induce the loss of normal neck rotation by as much as 50%, the remaining joints could compensate for most of the function of C1-C2. The benefits of avoiding neurological compromise secondary to cord compression outweigh the loss of neck rotation associated with this procedure.[34]

All symptomatic patients are considered candidates for surgery.[10][11][37] Surgery is typically recommended for patients presenting with neurological involvement, regardless of its transient nature, as well as for those exhibiting radiological signs of instability, such as more than 5 mm of translation in flexion and extension x-rays. Additionally, surgical intervention may be warranted in cases of progressive instability or persistent neck pain associated with atlantoaxial instability.[9](B3)

The principles of treatment aim to prevent sudden death from neurological compromise, enhance neurological status, stabilize the cervical spine, and improve the overall quality of life. Treatment strategies for this condition can be outlined in the following 4 steps:

• Identifying the primary pathology contributing to the symptoms of myelopathy, such as the odontoid peg or posterior arch of C1.
• Determining whether the atlantoaxial subluxation is reducible or irreducible.
• Deciding whether decompression of the lesion is necessary.
• Selecting the appropriate surgical option for stabilization and fusion.

Surgical options include atlantoaxial fusion, occiput-C2 fusion, and occiput-C3 fusion.[28][7] The decision depends on spinal cord compression location, area for arthrodesis, and bone quality.[9] All of them can be complemented with an additional transoral decompression in cases of irreducible subluxation of C1–C2.[38][39] However, this procedure often has accompanying complications such as wound infection, cerebrospinal fluid leakage, neurological injury, and hardware loosening.(B2)

In a recent publication, Goel et al. reported a series of 190 patients who were effectively treated with segmental atlantoaxial fixation without bony decompression, including neither trans-oral nor foramen magnum approaches, and without incorporating the occipital bone into the construct.[40] Fusion is indicated in cases of occipito-cervical instability or following unsuccessful attempts at atlantoaxial fusion. Additionally, fusion is suggested for cases involving poor bone quality, as there is an elevated risk of screw pull-out in such instances.[41][42](B2)

Summary of Treatment Options for Os Odontoideum

Treatment options for os odontoideum vary and are tailored depending on the clinical presentation and severity of instability.

• Clinically asymptomatic os odontoideum: Clinically asymptomatic cases may be managed through clinical and radiographic surveillance or, alternatively, posterior C1-C2 internal fixation and fusion.

• Symptomatic os odontoideum or C1-C2 instability: Symptomatic cases or those with C1-C2 instability typically require posterior C1-C2 internal fixation and fusion.

• Infeasibility of achieving rigid C1-C2 fixation: In situations where achieving rigid C1-C2 fixation is not feasible, postoperative halo immobilization may be implemented.

• Os odontoideum with irreducible dorsal cervico-medullary compression: For os odontoideum cases presenting with irreducible dorsal cervico-medullary compression, with or without occipito-atlantal instability, treatment may involve occipital-cervical fixation and fusion, with or without C1 laminectomy.

• Irreducible ventral cervico-medullary compression: Irreducible ventral cervico-medullary compression often necessitates ventral decompression as part of the treatment strategy.

Differential Diagnosis

Os odontoideum is among the causes of atlantoaxial instability and dislocation.[11] The differential diagnoses that can produce similar instability include acute dens fracture (characterized by a thinner and more irregular space, in contrast to the wide and smooth space observed in os odontoideum), abnormalities in dens morphology (such as aplasia of the dens, bifid dens, dens duplicated, and persistent ossiculum terminale),[43] Down syndrome, juvenile rheumatoid arthritis, Klippel–Feil syndrome, Morquio syndrome, neurofibromatosis, normal development in the pediatric population (refer to the Etiology section), rheumatoid arthritis, and transverse ligament injury.

Prognosis

The prognosis of os odontoideum varies depending on symptoms and radiological parameters. C1-C2 instability is defined as greater than 3 mm of anterior displacement of the posterior cortex of the atlas relative to the anterior aspect of the dens in adults and more than 4 mm in children.[6] Greater displacement predicts worse outcomes. The degree of atlantoaxial instability is less sensitive than the absolute diameter of the spinal canal in predicting poor outcomes.[30][2] In addition, a PADI measurement indicating a spinal canal diameter of less than 12 mm predicts paralysis development in all cases of C1-C2 instability.[36]

Complications

Complications of os odontoideum hinge on the severity of cord compression, instability, and unforeseen events, as even minor trauma can result in catastrophic neurological damage, including sudden death. This risk arises from potential C1-C2 instability due to abnormal dens development and dysfunction in the associated atlantoaxial ligamentous complex, stemming from the anomalous position of the free ossicle. Accurate detection of radiological signs of instability is crucial to prevent complications related to spinal cord compression.