Os Odontoideum Treatment & Management

Updated: Mar 01, 2019
  • Author: Eeric Truumees, MD; Chief Editor: Jeffrey A Goldstein, MD  more...
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Treatment

Approach Considerations

In patients with os odontoideum, there are two main forms of management, as follows:

  • Clinical and radiologic surveillance
  • Operative stabilization

Surgical stabilization may be recommended in the following three 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 serve as reasonable guidelines for surgery:

  • Posterior atlantodens interval (PADI) less than 13 mm
  • Sagittal plane rotational angle more than 20°
  • Instability index more than 40%
  • C1-C2 translation more than 5 mm

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. On the other hand, even in patients with a complete neurologic deficit, axial pain or the possibility of neurovascular compromise to the brainstem may still indicate surgery.

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. Yet to justify this approach, surgical results must improve on the natural history of this disease state. Some authors discourage surgical intervention for neck pain alone, stating that surgical outcomes are not sufficiently better than medical treatment to justify the risks.

Contraindications for surgery in os odontoideum patients begin with those patients not expected to benefit from stabilization. In a series of patients without spinal cord symptoms, no difference in outcome existed between those treated with surgical fusion and those treated medically.

In patients with neurologic deterioration, there are few reasonable alternatives to fusion. In smaller children without progressive deficits, it may be appropriate to wait until the bony elements have increased in size (eg, until age 6-7 years). Sublaminar wire passage and screw fixation are technically more difficult, and the risk for iatrogenic injury higher, in smaller patients. Other contraindications address one surgical technique or another. [22, 60, 61, 62]

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Medical Therapy

Observation is appropriate in most incidentally diagnosed os odontoideum patients, particularly those without radiographic evidence of significant instability. For patients with mechanical symptoms, medical management is indicated. This treatment includes cervical traction, physical therapy, occasional collar use, and anti-inflammatory medications. Activity limitation often is recommended but is difficult to enforce in the pediatric age group.

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Surgical Therapy

Several surgical options have been utilized for os odontoideum:

  • Posterior atlantoaxial onlay fusion
  • Posterior atlantoaxial wiring and fusion
  • Posterior occipitocervical wiring and fusion
  • Posterior Magerl screw fixation and fusion
  • Harms-Goel technique of C1-C2 fusion
  • Anterior resection of the os fragment

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 or Minerva cast, is recommended.

Historically, posterior atlantoaxial wiring was the standard technique for stabilization of os odontoideum (see the image below). There are a number of variants of this technique, based on the wiring scheme and bone graft placement. 

Lateral radiograph of a dystopic displaced os odon 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.

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 reported that the C2 spinous process in young children often is insufficiently ossified to contain the Gallie wire reliably. He recommended placing a Kirschner wire (K-wire) through the spinous process and wrapping the intervertebral wire around the K-wire and bone. [63, 64, 65]  An, on the other hand, stated that wiring is not needed in young children. [66]

In a Brooks fusion, the wire passes under the C2 lamina as well as under the C1 ring. [67] A structural or tricortical bone graft is wedged between the C1 ring and the C2 lamina, blocking extension. Whereas C2 sublaminar wire passage adds surgical risk, the Brooks fusion is more rigid than the Gallie. Postoperatively, posterior subluxation of the C1 ring and os into the cord are occasionally seen with a Gallie fusion.

Although posterior wiring procedures have a long record of successful atlantoaxial stabilization, their use is declining in this patient population. Shortcomings include the need for postoperative halo or Minerva cast immobilization. The Brooks technique is more stable in extension than the Gallie technique, but neither confers much stability in the patient with multiaxis instability. 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.

Before the development of screw-based techniques, these patients required occipitocervical fusion. A modification of the Brooks technique that overcomes this shortcoming has been described. [67] Today, most patients undergoing posterior fusion of an os odontoideum are offered a screw-based rigid stabilization. Preexisting C1 ring deficiency or C1 laminectomy concurrently performed for cord decompression in patients with irreducible deformity does not affect the more laterally placed screws.

The Magerl technique of posterior C2-C1 screw fixation immobilizes the atlantoaxial joint reliably and cost-effectively (see the image below). This technique, though mechanically the most rigid, is also the most technically demanding. The Magerl technique requires a near-anatomic reduction and normal vertebral artery anatomy.

Lateral radiograph demonstrating fixation of a red Lateral radiograph demonstrating fixation of a reduced os odontoideum with Magerl screws in a patient with an incomplete posterior arch of C1.

Introduced by Goel and popularized by Harms in 2001, the many versions of the Goel-Harms technique use C1 lateral mass screws connected to C2 pars, pedicle, or translaminar screws. [68] Today, Goel-Harms has become the common C1-2 stabilization option in adults and older children. [69] This segmental approach and a variety of C2 fixation options ensure its availability even in patients with aberrant vertebral artery anatomy.

Once screws are placed in C1, they can be used to further reduce some cases of os odontoideum. In patients with axial height loss, segmental height may be reestablished by placing allograft or PEEK blocks into the C1-2 facets. Still, careful preoperative measurement of the bony dimensions of the C1 lateral mass, C2 pars, pedicle, and lamina is required before this technique is recommended, especially in children.

In a small series, Hong et al concluded that the Harms C1-2 polyaxial screw-and-rod technique was the most appropriate treatment for patients with os odontoideum. [70] In Park and coworkers’ series of pediatric atlantoaxial instability, 58% had an anomalous course of the vertebral artery and 42% had anomalous C1-2 bony anatomy. [71]

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

In a series of pediatric (mean age, 9.6 years) atlantoaxial fusions, Haque et al were able to achieve stability in all of their patients. [72] In another series, Geck et al took computed tomography (CT) measurements in children aged 2-6 years; C1 lateral mass and C2 pedicle or laminar screws were possible in the majority of cases. [73] Transarticular screws, on the other hand, were typically not possible, given bony constraints.

Both the Harms and the Magerl technique offer rigid fixation, and postoperative bracing can be safely minimized. Both procedures are effective 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. Holly et al reported that minimally invasive placement of C1-2 instrumentation was technically feasible. [70]

Rarely, symptoms and cord compression persist following posterior stabilization of an irreducible dislocation. In these cases, anterior decompression with removal of the os fragment is recommended through an anterior transoral or retropharyngeal approach. [73, 74] More typically, anterior lesions (eg, synovial cysts) regress following successful posterior stabilization. [54, 75]

Wu et al, in a study of 25 consecutive patients who had os odontoideum with atlantoaxial dislocation, compared the clinical results of posterior fixation and fusion with or without anterior decompression. [76] Sixteen patients with reducible atlantoaxial dislocation were treated with single-level posterior fusion and stabilization; the other nine were treated with this procedure in conjunction with transoral decompression. The authors found that patients with a reducible atlantoaxial dislocation were effectively treated with single-level posterior fusion and stabilization, whereas combined transoral decompression and posterior fusion and stabilization was preferable for those with an irreducible dislocation.

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.

If surgery has been selected, the merits of each individual technique may be 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. Until there is evidence proving that the less invasive approaches confer less operative risk, operative indications must not be relaxed.

Operative details

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

If simple flexion-extension fails to reduce the os odontoideum, a period of skeletal traction may be initiated. Ideally, skeletal traction is used to reduce the atlantoaxial segment while the patient is awake. Hensinger recommended reduction several days preoperatively to decrease cord irritation. [63, 64, 65] One paper suggested 6 weeks of mobile traction for irreducible os odontoideum, [77] followed by occipitocervical fusion.

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 awake after fiberoptic intubation is performed. Some dislocations are irreducible. Displacement of the transverse atlantal ligament (TAL) in front of the ossicle may be an impediment. In these cases, consider fusion in situ rather than a risky operative reduction.

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 is 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 odontoideum is unstable in extension as well as flexion, when applying fixation, avoid overtightening the segment. 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. Weng et al reported the use of three-dimensional (3D) fluoroscopy with Harms and Magerl techniques in the treatment of 19 patients with symptomatic os odontoideum. [42] Of 60 screws placed, only three slightly penetrated the transverse foramen, without clinical consequences.

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

The midline incision must be long enough to allow medialization of screws, or stab incisions can be made for screw passage. Typically, an incision extending at least 2 cm above and below the C2 spinous process is required. The C1 ring must not be exposed too laterally. The vertebral artery groove becomes shallow more than 1.5 cm from the midline, so only the inferior aspect of the ring should be exposed more laterally. [78]

A plexus of veins surrounds the C2 nerve root. Some authors recommend routine sacrifice of this nerve to help control bleeding and to provide better access to the C2-3 facet and the C1 lateral mass. [79]

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.

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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 merely radiographic or may signal that significant and irreversible neurologic problems may develop. In the literature, however, such irreversible neurologic decline is rare.

Once os odontoideum has been identified, these patients should be followed at regular intervals. Clear instruction as to the warning signs or “red flags” for myelopathy must be discussed with the patient. Klimo et al reported three patients with neurologic deterioration who were initially diagnosed and treated conservatively for os odontoideum. The authors concluded that the risk of late neurologic deterioration should be considered during patient counseling. [2]

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 are quite low (0-4%).

Juhl et al reported on atlantoaxial fusion in six patients; the fusion rate was 100%, and no complications were noted. [80] 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. [40] 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. In the rare case in which C1 fixation cannot be achieved, 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. [81]

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.

Neurologic injury may occur during implant placement. In Spierings and Braakman’s series of 17 operative cases, two patients died and one worsened neurologically. They reported a surgical morbidity and mortality rate of 18% (3/17). [39] In a series reported by Smith et al, one of 11 pediatric patients declined neurologically after sublaminar wire passage. [82]

Injury to the vertebral artery may occur during an overly wide exposure of the C1 ring in wiring or screw stabilization cases, but it is more likely during screw placement. If the vertebral artery is injured on one side, screw placement must not be attempted on the contralateral side. Overpenetration of bicortical screws anteriorly may result in injury to the internal carotid artery and cranial nerve XII. [82, 83] 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.

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