AUTHOR AND EDITOR INFORMATION

Section 1 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

INTRODUCTION

Section 2 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

In 1863, separation of the odontoid process from the body of the axis was first described in a postmortem specimen. In 1886, Giacomini coined the term os odontoideum as a condition in which the dens is separated from the axis body. This entity is clinically important because the mobile or insufficient dens renders the transverse atlantal ligament (TAL) ineffective at restraining atlantoaxial motion. Translation of the atlas on the axis may lead to impingement of the upper cervical cord or vertebral artery.

Os odontoideum is rare, but the exact prevalence and incidence are unknown. Many cases are either incidentally detected or are diagnosed when patients become symptomatic. To date, no large-scale screening studies have been performed.

The age at diagnosis varies significantly from the first to the sixth decades of life. With increased awareness, however, os odontoideum has been diagnosed in younger patients. While the etiology remains controversial, an increased frequency of os odontoideum has been reported in patients with Morquio syndrome,^{1, 2} multiple epiphyseal dysplasia, and/or Down syndrome.

Related eMedicine topics:
Atlantoaxial Instability in Individuals with Down Syndrome
Atlantoaxial Instability
Atlantoaxial Injury and Dysfunction

Related Medscape topics:
CE Surgical Care in Pediatrics: An Update
Resource Center Spinal Disorders
Specialty Site Orthopaedics
American Academy of Orthopaedic Surgeons (AAOS) 75th Annual Meeting (March 5-9, 2008)
CME Highlights from the Radiological Society of North America (RSNA) 93rd Scientific Assembly and Annual Meeting

Relevant Anatomy

Successful treatment of os odontoideum requires an understanding of the unique anatomic characteristics of the cervicocranium (occiput-C2). The bony elements here develop through enchondral ossification. The tip of the dens and its associated ligaments arise from the fourth occipital through the cervical-0 (C0) somites, which do not ossify until middle childhood.^{3, 4}

The base of the dens forms from the C0 and C1 sclerotomes as 2 paired structures that ossify just before birth. The C2 and C3 sclerotomes give rise to the body of the axis, which fuses with the dens at age 4 years (see Image 1).  In a study of human embryos at 8 weeks of gestation, O’Rahilly et al reported that no transverse segmentation formed within the odontoid process at any time. An embryologic anomaly characterized by a complete/partial segmentation of 2 rostral parts will result in bipartite dens rather than an os odontoideum.⁵

The atlantoaxial joint (C1-C2) consists of biconcave articulations with loose capsules and small contact areas. Stability is therefore conferred by associated ligaments, including the TAL, which is the primary restraint to flexion and extension. Other important restraints include the apical ligaments, the alar ligaments, the tectorial membrane, and the atlanto-occipital membranes

The vertebral arteries are intricately invested in the bony anatomy of the atlantoaxial segment.  They pass just inferior to the C1-C2 facet joint, then course laterally through the transverse foramen of C2. Just above the C1 lateral mass, they turn medially and meet to progress cephalad into the foramen magnum.⁶ Aberrancy of the vertebral artery course is not rare and may limit fixation options in some patients selected for operative management of os odontoideum. This deviant course may be unilateral or bilateral and is best shown on thin-cut 1-mm CT scan. If the vertebral artery is in the path of a C1-C2 transarticular screw, an alternative fixation strategy must be considered.

Etiology

Initially, os odontoideum was thought to represent a congenital failure of fusion of the dens to the remainder of the axis. As such, the condition is usually grouped with other craniocervical junction abnormalities, such as dental aplasia and hypoplasia.^{7, 8, 9, 10, 11, 12, 13}

Today, it seems clear that failure of the secondary ossification center of the dens to fuse with the base of the odontoid represents a separate entity known as persistent ossiculum terminale.¹⁴ Differentiation between os odontoideum and persistent ossiculum terminale is clinically critical. The ossicle of ossiculum terminale is much smaller than that of os odontoideum. More important, that ossicle lies at the level of the atlantal ring above the transverse atlantal ligament. In this cranial location, ossiculum terminale, unlike os odontoideum, is not associated with significant instability.

Some authors speculate that os odontoideum represents a previous fracture of the odontoid synchondrosis before its closure at age 5-6 years.^{15, 16, 17} These authors describe os odontoideum in patients with previously normal cervical radiographs. For example, Schuler et al elegantly described the evolution of an os odontoideum following trauma in a child.¹⁸ In this model, os odontoideum develops gradually. Following a fracture of the odontoid synchondrosis, with growth, the alar ligaments carry the dens fragment away from the axis base. The cranial portion of the dens fragment continues to receive a blood supply from the apical arcade. The avascular caudad portion resorbs, leaving the characteristic rounded ossicle.

Authors who favor a congenital basis for os odontoideum point out that the craniovertebral junction is one of the most common sites for malformation. Included are clefts or aplasia of anterior and posterior arches of atlas.^{7, 10, 11, 17} On the other hand, unlike most congenital malformations, os odontoideum tends to occur as an isolated entity without other regional anomalies.¹⁹ Garg et al reported a case of os odontoideum in a myelopathic 16-year-old patient with bipartite atlas. They concluded that coexistence of these conditions support the embryologic basis for os odontoideum. In this case, the dens had an “unusual bony projection” on its anterior surface.

Crockard and Stevens reviewed the embryologic and comparative anatomy data of clinical syndromes associated with craniocervical instability. They concluded that os odontoideum is the product of excessive movement at the time of ossification of the cartilaginous dens and is analogous to the unfused type II odontoid fracture. True hypoplasia of the odontoid peg, on the other hand, was found to be part of a wider segmentation defect associated with Klippel-Feil syndrome,²⁰ occipitalized atlas, or basilar invagination and rarely was found to be associated with instability.²¹

Sankar and colleagues reviewed 519 consecutive patients with radiographic abnormalities in the occipitocervical region. Os odontoideum was confirmed in 16. Only 3 of those patients had a history of remote trauma. The authors concluded that this supported an embryologic basis for the condition.²²

The size of the os odontoideum may vary, but it typically is smaller than the normal dens, particularly at its base. Perhaps there are 2 etiologic groups. Certainly, in patients with other congenital anomalies or odontoid malformations, an embryologic basis may be assumed.  Unfortunately, individual correlations do not “prove” the case one way or the other.  

Regardless of the underlying cause, sound treatment selection requires an understanding of the natural history of this process. We know that, in a subset of patients, the secondary ligamentous restraints become lax, resulting in instability. When the instability has been long-standing, it becomes multidirectional. Less clear are the percentages and rates of progression to instability. As no population studies are available, assumptions are made from case series data of nonoperatively managed patients.

Based on the position of the dens tip, 2 types of os odontoideum are described: orthotopic and dystopic. In the orthotopic type, the dens is in anatomic position. In dystopic os odontoideum, the dens tip is in any other position. Most commonly, the fragment is located near the foramen magnum, where it may fuse with the clivus. Alternatively, the os may be fixed to the anterior ring of the atlas.

Subluxation and instability are described in both types of os odontoideum.  Some authors feel that dystopic os odontoideum is more likely to be symptomatic.

Pathophysiology

A significant, but unknown, percentage of patients with os odontoideum are and remain asymptomatic. Os odontoideum often is incidentally detected after screening or after evaluating a patient following trauma. Given the frequency of asymptomatic os odontoideum, it is difficult to determine with certainty whether the os odontoideum is the true cause of symptoms when they do occur. Typical symptoms include the following:

• Local mechanical neck pain
• Torticollis and headache
• Neurologic symptoms
• Neurovascular symptoms

Neurologic symptoms may develop in patients with cervical instability.^{23, 24} This neurologic involvement is often limited to one transitory episode of diffuse paresis following trauma. In others, a progressive myelopathy is noted.²⁵ Weakness and ataxia usually predominate over sensory changes. Less frequently, atlantoaxial instability results in vertebral artery compression precipitating neurovascular symptoms. These vascular symptoms arise from cervical cord and brainstem ischemia and encompass a bewildering array of signs and symptoms. Early sequelae include ataxia, syncope, vertigo, and visual disturbances. Later, cerebellar and brainstem infarcts and seizures are seen.^{26, 27} Sudden death is rare but can occur.

In patients suspected of having os odontoideum, a thorough physical examination is mandatory.  This assessment begins with a complete neck and cervical spine examination.  Evaluate for tenderness, range of motion (ROM), and associated anomalies.  A careful neurologic examination should include assessment of cerebellar and brainstem function, gait evaluation, and Romberg testing.  In patients with atlantoaxial instability, upper motor neuron findings are commonly reported, including spasticity, hyperreflexia, clonus, and proprioceptive loss.

WORKUP

Section 3 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Imaging Studies

Radiologic evaluation is utilized to confirm the diagnosis and estimate the degree of spinal instability. Initial evaluation includes open-mouth, anterior-posterior, and flexion-extension lateral radiographs. Os odontoideum appears as a round or oval ossicle with a smooth, uniform cortex separated from the base of the axis by a wide gap. The ossicle border does not directly match up with the axis body. The gap separating the os and the axis proper should lie above the level of the superior articular facets. Orthotopic os odontoideum (see Image 2) may appear free and in a relatively anatomic position.^{28, 29, 30}

An orthotopic os odontoideum may be difficult to differentiate from an unfused neurocentral synchondrosis, odontoid hypoplasia, or odontoid fracture nonunion. In children younger than 5 years, the neurocentral synchondrosis often has not fused. Dynamic lateral radiographs of those with an unfused synchondrosis do not demonstrate motion, whereas radiographs of individuals with an os odontoideum may.

A dystopic ossicle may be fixed to the clivus or to the anterior ring of the atlas. The remaining axis is hypoplastic as well. With a dystopic os odontoideum (see Image 3), the radiographic diagnosis is clear.  A dens fracture nonunion (see Image 4) typically exhibits a narrow gap between the axis base and dens. The normal shape and size of the dens are preserved on the open-mouth view.

With an os odontoideum, hypertrophy of the anterior arch of the atlas may be seen.³¹ This hypertrophy is believed to represent osseous reaction to chronic atlantoaxial instability and is unlikely with an acute dens fracture. Criteria for stability or instability include the following:

• Flexion-extension lateral radiographs (see Images 5-6)
• Most symptomatic patients demonstrate radiographic instability
• In one series, the average translatory motion was 1 cm
• • Instability was mainly in the anterior-posterior plane
  • Some patients were unstable in all directions

• Important prognostic indices:
• • Anterior atlantoaxial translation
  • Posterior atlantodens interval (PADI)
  • Instability index
  • Sagittal plane angulation

• In forms of atlantoaxial subluxation, the anterior atlantodens interval (AADI) is used to measure instability 

• However, with os odontoideum, the os fragment often moves with the atlas
• The AADI does not reflect the abnormal motion of the segment
• Direct measurement of the motion of C1 on the body of C2 is more useful
• The space between a line projected superiorly from the anterior border of the body of the axis and a line projected inferiorly from the posterior border of the anterior arch of the atlas are measured
• More than 3 mm of separation is pathologic. Critical evaluation of the bony anatomy of the upper cervical spine is often difficult with plain radiographs alone. Lateral tomography or, more commonly, fine 1-mm cut, sagittally reconstructed computer tomography (CT) scans allow more detailed depictions of the atlantoaxial articulation.

• Previously, the distance between the posterior border of the dens and the anterior border of the posterior ring of the atlas on plain radiographs was termed the space available for the cord (SAC)
• • More recently, this distance is called the PADI
  • These terms occasionally are used synonymously. However, with MRI or CT myelography, the actual space for the cord can be measured readily, and the SAC can be used to refer to the actual anterior-posterior canal dimension
• Space available for the cord
• • SAC refers to the PADI minus additional compression from soft tissue; soft tissue structures such as synovial cysts may further diminish the SAC in some cases of os odontoideum.
  • A PADI of less than 13 mm is associated with progressive neurologic decline.
  • The instability index refers to the change in SAC from flexion to extension.
  • The critical measurement is from the superior posterior corner of C2 to the posterior ring of C1.
• Cord compression and cord signal anomaly
• • In one study, radiographic measurements of translation and PADI did not accurately reflect clinical status. MRI measurement of cord compression was more predictive of symptomatology. Hadley’s literature review was unable to establish a linear relationship between PADI and neurologic status.³²
  • MRI also may delineate pathologic changes within the cord. A T2-weighted MRI (see Image 7) sequence may depict myelomalacia as an increased signal in the substance of the cord.^{33, 34, 35}

• A number of dynamic imaging modalities have been recommended as means to more completely understand the degree and nature of abnormal motion in patients with os odontoideum. Cine radiographs have been recommended because of their ability to define the relationship of the os to surrounding bony elements.³⁶ Also, dynamic MRI scans may detail the degree and planes of instability in real time but are rarely needed.^{34, 35}

TREATMENT

Section 4 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

In patients with os odontoideum, there are 2 main forms of management:

• Clinical and radiologic surveillance
• Operative stabilization

Surgical stabilization is utilized in 3 settings:      

• Spinal instability
• Neurologic involvement
• Intractable pain

In this setting, spinal instability is defined as cord compression or excessive motion noted radiographically. In the absence of large-cohort, prospective data, radiographic parameters can only be marginally associated with progressive neurologic dysfunction. That said, most authors agree that the following changes on flexion-extension plain lateral cervical radiographs predict neurologic decline and serve as reasonable guidelines for surgery:    

• PADI less than 13 mm
• Sagittal plane rotational angle greater than 20°
• Instability index greater than 40%
• C1-C2 translation greater than 5 mm

Surgical therapy

Surgical stabilization is least controversial in patients with obvious neurologic or neurovascular involvement. In some small series, on the other hand, based on the resolution of symptoms following transient paresis, continued nonoperative management has been recommended.

Surgical intervention in patients with axial pain is more controversial. In those with persistent, and disabling pain despite appropriate nonoperative management, stabilization may be reasonable. To justify this approach, however, surgical results need to improve regarding the natural history of this disease state. Some authors discourage surgical intervention for neck pain alone, stating that the outcomes of surgical fusion and medical treatment are no different.

Contraindications to surgery in os odontoideum patients begin with those patients not expected to benefit from stabilization.  Even in patients with a complete neurologic deficit, axial pain or the possibility of neurovascular compromise to the brainstem may still indicate surgery. In a series of patients without spinal cord symptoms, there was no difference in outcome between patients treated with surgical fusion and those treated medically.

In patients with progressive neurologic deterioration, there are few reasonable alternatives to fusion. However, when to offer surgery should be examined as well.  In smaller children without progressive deficits, waiting until the bony elements have increased in size (ages 6-7 y) may be appropriate. In smaller patients, sublaminar wire passage and screw fixation may be technically more difficult, and the risk for iatrogenic injury may be higher.  Other contraindications address one surgical technique or another.

Several surgical options have been described for os odontoideum:

• Posterior atlantoaxial  onlay fusion
• Posterior atlantoaxial wiring and fusion
• Posterior occipitocervical wiring and fusion
• Posterior Magerl screw fixation and fusion
• Harms technique of C1-2 fusion
• Anterior resection of the os fragment

Onlay fusion

Onlay fusions are technically straightforward but confer high pseudarthrosis rates. These procedures are best restricted to younger children, for whom wire passage is considered high risk.  Postoperatively, rigid external immobilization, such as a halo brace, is recommended.

Gallie fusion

Historically, the standard technique for stabilization of os odontoideum has included posterior atlantoaxial wiring (see Image 8).  Variants of this technique are based on the wire trajectory and bone graft placement. The oldest variant, Gallie fusion, incorporates wires or cables under the posterior C1 ring and around the C2 spinous process.  An onlay autogenous corticocancellous iliac crest graft is held in place by the wire. Hensinger has reported that the C2 spinous process in young children often is insufficiently ossified to reliably contain the Gallie wire. He recommends placing a Kirschner wire (K-wire) through the spinous process and wrapping the intervertebral wire around this.^{37, 38, 39} Howard An, on the other hand, states that wiring is not needed in young children.⁴⁰

Brooks fusion

In a Brooks fusion, the wire passes under the C2 lamina, as well as under the C1 ring.⁴¹ A structural or tricortical bone graft is wedged between the C1 ring and the C2 lamina, blocking extension. While sublaminar wire passage adds surgical risk, the Brooks fusion is more rigid than the Gallie fusion.  Postoperative, posterior subluxation is occasionally seen with a Gallie fusion. The physician must discuss the relative merits of Gallie versus Brooks fusions with the patient.

While posterior wiring procedures have a long track record of successful atlantoaxial stabilization, their utilization is declining in this patent population. Shortcomings include the need for postoperative halo or Minerva cast immobilization. More significant is the risk of posterior displacement of the C1 ring and os into the cord with Gallie fusions performed in more unstable spines. During sublaminar wire passage, cord injury may occur, especially in patients with an irreducible deformity.

Standard Gallie or Brooks techniques are not possible in the absence of the posterior C1 ring. Prior to screw-based techniques, these patients required occipitocervical fusion. A modification of the Brooks technique that overcomes this shortcoming has been described,⁴¹ but more typically, screw fixation is recommended.^{42, 43, 44, 45, 46, 47} Extension of the fusion to the occiput also has been recommended in patients with marginal neurologic status. In patients with irreducible dislocations with residual compression of the posterior aspect of the spinal cord from the posterior C1 ring, an occipitocervical fusion with C1 laminectomy is required.

Magerl technique

The Magerl technique provides posterior C1-C2 screw fixation and is a reliable means of atlantoaxial joint immobilization (see Image 9). While this technique does confer the most rigid fixation, it is technically demanding. The Magerl technique requires a near-anatomic reduction and normal vertebral artery anatomy.⁴⁸

Harms technique

Harms technique of fixation to the lateral mass of C1 and the pars or pedicle of C2 has become the most prominent means of stabilization in the adult population (see Image 10).⁴⁹ This technique is not as vulnerable to anatomic variability in the course of the vertebral artery as the Magerl technique. Still, careful preoperative measurement of the bony dimensions must be undertaken before recommending this technique, especially in children.

Both Harms and Magerl techniques offer rigid fixation, and postoperative bracing can be safely minimized. Neither technique is compromised in patients with deficient posterior C1 arches.  These techniques have significantly reduced the need for occipitocervical fusions in this patient population. Percutaneous approaches toward rigid C1-2 fixation have also been described.

Preoperatively, critical steps include a complete understanding of the lesion’s pathoanatomy and an attempt at reduction of any displacement.  For example, during the approach, posterior C1 ring deficits not only limit surgical options, such as standard wiring, but also render the cord and vertebral arteries vulnerable during the exposure.

Ideally, skeletal traction is used to reduce the atlantoaxial segment while the patient is awake. Hensinger recommends reduction several days before operation to decrease cord irritation.^{37, 38, 39} Halo traction can be used to maintain this positioning. If the reduction is to be performed in the operating room, do so with the patient alert either before or after awake fiberoptic intubation is performed. Some dislocations are irreducible. Displacement of the TAL in front of the ossicle may be an impediment. In these cases, consider fusion in situ rather than a risky operative reduction.

The course of the vertebral arteries and available bone stock at C1-C2 limit other options, such as Magerl screw placement. In the Park et al series of pediatric patients with atlantoaxial instability, 58% had an anomalous course of the vertebral artery, and 42% had anomalous C1–C2 bony anatomy.⁵³

Several studies have shown that screw placement is possible in most children older than 4 years. In O’Rahilly’s series, 11 of 12 pediatric patients were able to undergo transarticular fixation for atlantoaxial instability. Interestingly, the series contained several os odontoideum patients in whom previous onlay or wired graft had failed.⁵

Upper cervical spine surgery in skeletally unstable patients is technically demanding, especially in the pediatric population. Meticulous attention to every detail in reduction, maintenance of reduction, and patient positioning are critical. Monitor somatosensory-evoked potentials (SSEPs) and motor-evoked potentials (MEPs) after intubation, after positioning, and at various stages intraoperatively. Rigidly fix the patient’s head to the operating table; avoid pressure on the eyes. Mayfield tongs or a Halo ring with a Mayfield attachment of the Jackson table can be used for this purpose.

In children, limit the posterior exposure of the cervical spine to prevent inadvertent extension of the fusion to subjacent levels. Since most os odontoideums are unstable in extension as well as flexion, avoid overtightening the segment when applying fixation. Overtightening of posterior wiring may lead to posterior translation of the C1 ring and the ossicle into the canal and against the cord. Fluoroscopy may be useful to assess intraoperative motion of the affected segments. When the use of Magerl screws is attempted, image-guided surgery tools may be useful.

Occipital fusion in children must include careful awareness of the thin occipital squama because the midline bone of the skull is thicker and stronger. Plate and screw convergence toward the midline ensures better purchase.

Postoperatively, serial radiographs are obtained to ensure progression of fusion and maintenance of stability. Wound healing may be problematic in some settings and requires a layered closure with attention to careful matching of skin edges.

COMPLICATIONS

Section 5 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Complications have been reported after both operative and nonoperative management of os odontoideum. Rare, but concerning, are occasional reports of sudden death from cord compression in patients with unsuspected atlantoaxial instability.

In patients with known but stable os odontoideum, nonoperative management may be complicated by progression of instability. These changes may be radiographic, or significant and irreversible neurologic problems may develop.  In the literature, however, such irreversible neurologic decline is rare. Once identified, these patients should be followed at regular intervals. Clear instruction as to the warning signs for myelopathy must be discussed with the patient.

The morbidity and complications associated with operative intervention vary widely in the numerous small series available. With traditional wiring techniques, failure of fusion is often reported. Pseudarthrosis rates vary in the literature, but overall, they are quite low, from 0-4%. Juhl et al reported on atlantoaxial fusion in 6 patients; the fusion rate was 100%, and no complications were noted.⁵⁴ If reduction cannot be obtained or the posterior C1 arch is not intact, an occipitocervical fusion may be performed. Dai et al reported on 33 patients with satisfactory results and a 100% fusion rate.⁵⁵ The ossicle may fuse to the base of the axis following successful posterior occipitocervical fusion.

In cases of known pseudarthrosis, revision with Magerl or Harms screws is highly effective. However, pseudarthrosis with malposition of C1 on C2 or in the face of vertebral artery anomalies may preclude this option. In those difficult situations, an occiput to C2 fusion is indicated. In a case-control study, Taggard et al found that transarticular screw fixation was 21 times more likely to lead to solid fusion than wiring techniques in patients with atlantoaxial instability.⁵⁶

Neurologic decline may occur after surgery from a number of insults, including drops in blood pressure, excessive motion during positioning, and direct neural tissue trauma. A higher risk for perioperative problems is reported in patients who are highly unstable and in those with fixed dislocations, ongoing cord compression, or inherited ligamentous laxity.

More likely, however, is neurologic injury occurring during implant placement. In Spierings and Braakman’s series of 17 operative cases, 2 patients died and 1 worsened neurologically. They reported a surgical morbidity and mortality rate of 18% (3 of 17 patients).⁵⁷ In Smith et al’s series, 1 of 11 pediatric patients declined neurologically after sublaminar wire passage.⁵⁸

Less severe, but not insignificant, complications include vascular injury, posterior cervical wound infections, anesthesia complications, and ongoing muscular neck pain. Of these, injury to the vertebral arteries during screw placement is the most dangerous. Prevention, by careful review of the patient’s vertebral artery passage, is best. If the vertebral artery is injured on one side, screw placement must NOT be attempted on the contralateral side.

OUTCOME AND PROGNOSIS

Section 6 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Outcome and prognosis data for os odontoideum patients is limited to scattered case reports and small case series. These series typically describe successful outcomes with both nonoperative and surgical management.

Spierings and Braakman described their management of 37 os odontoideum patients.  At a median 7-year follow-up, of the 16 patients with neck pain only, none developed neurologic deficits. Four patients who had mild or transient myelopathy and limited radiographic instability were also followed nonoperatively. With a maximum follow-up of 14 years, 3 of the 4 had no recurrence; one had stable monoparesis. Seventeen patients with myelopathy and more instability underwent surgery. These authors found no difference in neck pain in the surgical and nonsurgical groups.⁵⁷

In a series by Dai et al, 5 patients with os odontoideum without symptoms were treated nonoperatively and were monitored for an average of 6.5 years. None of these patients had symptom progression during the follow-up period.⁵⁵

In a study by Fielding et al, 27 of 35 os odontoideum patients were radiographically unstable. Of the 27 patients, 26 underwent successful Gallie fusion. They reported “solid” fusions after 2 months of immobilization in children and after 3 months in adults. The 27th unstable patient refused surgery and remained well at 2-year follow-up examination. The 8 stable patients were managed  nonoperatively and remained well at last follow-up. Symptomatically, two thirds of Fielding’s patients had only mechanical pain. These patients reported resolution after fusion.⁵⁹

The neurologic outcome in patients with symptomatic cord compression is less clear. These patients can be divided into those with acute, incomplete cord syndromes and those with insidious myelopathic syndromes. Typically, transient neurologic signs following trauma are associated with a good prognosis. Rapid functional return parallels improvement in neurologic signs, and recurrence rates are low.

Myelopathic patients or those with cerebellar or medullary signs exhibit a more progressive course. Because these patients gradually worsen, surgery is most clearly indicated in this setting. Neurologic outcomes after surgery have been reported in small case series only, however.  Typically, symptomatic progression ceases, and most patients report significant symptomatic improvement.

FUTURE AND CONTROVERSIES

Section 7 of 9  

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• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

There are 3 evolving and controversial aspects of os odontoideum: etiology, surgical indications, and optimal management. Hadley’s 2002 consensus report called for several studies³²:

• Population-wide studies of the prevalence of os odontoideum as an incidental finding.
• A follow-up of incidentally noted and untreated os odontoideum, even with C1–C2 subluxation.
• A cooperative, multi-institutional natural history study of patients with os odontoideum without C1–C2 instability, to provide demographic and clinical factors predictive of the development of subsequent instability.
• A multi-institutional prospective, randomized trial comparing posterior wiring and fusion techniques with rigid, C1–C2 screw fixation.

In years to come both posttraumatic and congenital forms of os odontoideum may be identified. Today, however, authors in each camp often write as if their hypothesis has been all but proven. Understanding the true etiology of this disorder may be helpful in terms of identifying high-risk patients through genetic testing. If posttraumatic, the increased use of advanced imaging modalities such as spiral CT and MRI may markedly decrease the incidence of os odontoideum in the future. Further, better explication of etiology may better delineate progression risk, leading to decreased x-ray exposure during follow-up and the limiting of operative intervention only to those patients at high risk for progression. These data may also support or eliminate the need for activity restriction during the observation phase.

In the presence of progressive neurologic compromise, surgery is clearly indicated. With asymptomatic atlantoaxial hypermobility alone, on the other hand, the decision to proceed with surgery is more debatable. Larger series comparing outcomes between operative and nonoperative management may more clearly support surgery in some patients. Certainly, a better-defined discussion of surgical risks and benefits could be expected.

Finally, if surgery has been selected, the merit of each individual technique is debated. The advent of C1 lateral mass and C2 pars screw fixation has addressed the major concerns with Magerl screw fixation. The Harms technique allows rigid fixation in a wider range of patients, including many with a degree of vertebral artery ectopy or incomplete reduction for which transarticular screw placement would be too risky.

In the future, the current open method of Harms screw placement may be replaced by a more percutaneous approach. Individual “proof of concept” case reports have been published. For less unstable, but progressive, atlantoaxial instability, injections of bone morphogenetic protein (BMP) may avoid instrumentation altogether. Until there is evidence proving that the less-invasive approaches confer less operative risk, operative indications must not be relaxed.

MULTIMEDIA

Section 8 of 9  

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

Media file 1:  The axis has 5 primary and 2 secondary ossification centers. C0, C1, and C2 sclerotomes contribute to various portions of the dens. The principal portion of the dens body arises from the original center of C1.
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Media type:  Image

Media file 2:  A coronal reconstruction of an orthotopic os odontoideum. Note the wide gap between the rounded ossicle and the base of the axis.
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Media type:  CT

Media file 3:  A sagittal reconstruction of a CT scan demonstrating a dystopic os odontoideum. Note that the ossicle appears fused to the clivus (anterior portion of the foramen magnum). Also note the smooth corticated border of the ossicle.
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Media type:  CT

Media file 4:  Anterior-posterior tomogram view of a type II dens fracture. The fracture line is narrow and lower on the waist of the dens, unlike the fracture line of an os odontoideum. No cortication is noted along the fracture line.
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Media type:  X-RAY

Media file 5:  Flexion lateral radiograph of a dystopic displaced os odontoideum. Note the limited posterior interval between the dens and the posterior C1 ring (PADI). This patient had reported electric shocks radiating down the body (ie, Lhermitte sign), as well as progressive weakness and ataxia.
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Media file 6:  Extension lateral radiograph of a dystopic displaced os odontoideum. This deformity did not reduce with extension.
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Media file 7:  T2-weighted parasagittal MRI image of a patient with os odontoideum and mild compression of the upper cervical spine. This patient presented with transient quadriparesis.
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Media type:  MRI

Media file 8:  Lateral radiograph of a dystopic displaced os odontoideum 6 months after posterior wiring. After the wiring was performed, the patient had a solid arthrodesis with no motion on flexion and extension. Her neurologic symptoms resolved despite the failure to obtain a reduction. Had she continued to have severe symptoms, anterior odontoidectomy could have been considered.
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Media file 9:  Lateral radiograph demonstrating fixation of a reduced os odontoideum with Magerl screws in a patient with an incomplete posterior arch of C1.
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Media file 10:  Lateral radiograph demonstrating fixation of a reduced os odontoideum with the Harms technique.
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Media type:  Radiograph

REFERENCES

Section 9 of 9

• Authors and Editors
• INTRODUCTION
• Workup
• Treatment
• Complications
• Outcome and Prognosis
• Future and Controversies
• Multimedia
• References

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Article Last Updated: Sep 12, 2008