eMedicine Specialties > Orthopedic Surgery > Spine

Os Odontoideum: Treatment

Author: Eeric Truumees, MD, Consulting Surgeon, Department of Orthopedic Surgery, William Beaumont HospitalOrthopaedic Director, Gehring Biomechanics LaboratoryAdjunct Faculty, Bio-Engineering Center, Wayne State University
Contributor Information and Disclosures

Updated: Sep 12, 2008

Treatment

Medical Therapy

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

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.

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