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R. Shane Tubbs, John C. Wellons III, Jeffrey P. Blount, Paul A. Grabb and W. Jerry Oakes

Object. The quantitative analysis of odontoid process angulation has had scant attention in the Chiari I malformation population. In this study the authors sought to elucidate the correlation between posterior angulation of the odontoid process and patients with Chiari I malformation.

Methods. Magnetic resonance images of the craniocervical junction obtained in 100 children with Chiari I malformation and in 50 children with normal intracranial anatomy (controls) were analyzed. Specific attention was focused on measuring the degree of angulation of the odontoid process and assigning a score to the various degrees. Postoperative outcome following posterior cranial fossa decompression was then correlated to grades of angulation. Other measurements included midsagittal lengths of the foramen magnum and basiocciput, the authors' institutions' previously documented pB—C2 line (a line drawn perpendicular to one drawn between the basion and the posterior aspect of the C-2 body), level of the obex from a midpoint of the McRae line, and the extent of tonsillar herniation.

Higher grades of odontoid angulation (retroflexion) were found to be more frequently associated with syringomyelia and particularly holocord syringes. Higher grades of angulation were more common in female patients and were often found to have obices that were caudally displaced greater than three standard deviations below normal.

Conclusions. These results not only confirm prior reports of an increased incidence of a retroflexed odontoid process in Chiari I malformation but quantitatively define grades of inclination. Grades of angulation were not found to correlate with postoperative outcome. It is the authors' hopes that these data add to our current limited understanding of the mechanisms involved in hindbrain herniation.

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R. Shane Tubbs, Daniel B. Webb and W. Jerry Oakes

patients had a Grade III retroflexed odontoid process. We have previously reported that higher grades of retroflexion are associated with an increase incidence of syringomyelia in the CIM population. 22 It is therefore not possible to discern whether the higher grade of retroflexion in these four patients played any role in their initial failed posterior fossa decompression. Conclusions Syringomyelia may continue following a decompressive procedure. We did not find any one radiological measurement, such as odontoid retroflexion or caudal descent of the brainstem

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Patricia B. Quebada and Ann-Christine Duhaime

brainstem. Grabb, et al., 4 offer a guideline for treating these lesions, specifically for retroflexed odontoid processes, but it can be adapted for dolichoodontoid processes. They support posterior decompression alone if the extent of inclination of the odontoid is less than 9 mm from a vertical line drawn from the basion to the posterior cortex of the C-2 body. If this distance exceeds 9 mm, they support anterior decompression with posterior fusion ( Fig. 4 ). Adequate decompression prevents progressive neurological deterioration. 4 In our patient, this distance

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Matthew J. McGirt, Frank J. Attenello, Daniel M. Sciubba, Ziya L. Gokaslan and Jean-Paul Wolinsky

, Klippel-Feil, C1–2 instability, Oc–C1 assimilation ETO/OCF none 200 regular 9 3 pain resolved, myelopathy 18, F neck pain, paresthesia, dysphagia, myelopathy basilar invagination, Chiari I, platybasia, retroflexed odontoid, C1–2 instability ETO/C1–4 fusion none 90 regular 5 2 pain resolved, dysphagia improved, improved hand strength to 5/5 14, F neck pain, dysphagia, myelopathy cranial settling, osteogenesis imperfecta 1st stage: ETO; 2nd stage: OCF subluxation in Halo vest 200 PEG † 9 8 pain resolved

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Todd C. Hankinson, Anthony M. Avellino, David Harter, Andrew Jea, Sean Lew, David Pincus, Mark R. Proctor, Luis Rodriguez, David Sacco, Theodore Spinks, Douglas L. Brockmeyer and Richard C. E. Anderson

. F ig . 2. Representative images obtained in patients in Group 2 (C-1 and C-2 instrumentation). a: Preoperative sagittal reconstruction CT scan demonstrating platybasia and a retroflexed odontoid process in a patient with Chiari malformation Type 1 and ventral brainstem compression. b: Postoperative lateral radiograph of the occipitocervical junction demonstrating the position of bilateral C-2 pars screws, bilateral C-1 lateral mass screws, and an occipital plate. c: Sagittal plane reconstruction CT scan obtained 6 months postoperatively, showing the

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Todd C. Hankinson, Eli Grunstein, Paul Gardner, Theodore J. Spinks and Richard C. E. Anderson

T he CM-I is among the more common pathological entities encountered by pediatric neurosurgeons. In a minority of cases, the CM-I is complicated by irreducible ventral compressive pathological features, such as basilar invagination or a severely retroflexed odontoid process. 1 In these cases, a combined anterior-posterior approach is often necessary to achieve adequate brainstem and spinal cord decompression. The most common approach to ventral pathological entities at the craniovertebral junction is transorally. 2 , 3 , 7 , 8 In some pediatric patients

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Douglas L. Brockmeyer

relative importance and clinical significance of these findings can be debated, in our experience they have become very important for our clinical decision-making process. For example, in the last 22 patients who presented with scoliosis and syringomyelia, 7 had a CM Type 1.5 and 6 had significant ventral brainstem compression due to retroflexed odontoid (defined as a pBC 2 line > 10 mm [with pBC 2 as the line drawn from the tip of the odontoid to the ventral aspect of the dura]). These findings sometimes lead to a complex scenario in which the surgeon must decide

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Arnold H. Menezes

cord myelopathy that may be overshadowed by syringomyelia. 9 , 18 , 25 Magnetic resonance imaging has revolutionized diagnosis, led to early detection, and provided a greater understanding of the pathology, genesis, and manifestations of CM-I, and has also transformed outcome studies. In a recent comprehensive review of 364 symptomatic patients with CM-I, Milhorat et al. 25 found associated syringomyelia in 65% of cases, scoliosis in 42%, abnormal retroflexed odontoid process in 26%, and basilar invagination in 12%. One of the current concepts regarding patients

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Robert J. Bollo, Jay Riva-Cambrin, Meghan M. Brockmeyer and Douglas L. Brockmeyer

T he association of CM-I with osseous abnormalities, such as scoliosis and anomalies of the craniovertebral junction (including retroflexed odontoid process, basilar invagination, and platybasia), has long been recognized. 3–5 , 12 , 13 , 16 , 22 Such patients often present with bulbar symptoms, including vertigo, diplopia, dysphagia, and apnea, 7–9 and demonstrate ventral brainstem compression on MRI. 8 , 10–12 These symptoms are especially common in younger patients (0–2 years old). 2 Many patients with this constellation of radiographic findings

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Zachary L. Hickman, Michael M. McDowell, Sunjay M. Barton, Eric S. Sussman, Eli Grunstein and Richard C. E. Anderson

compression and upward mass effect on the midbrain due to a retroflexed odontoid process. Decompression was planned via an endoscopic transnasal approach ( Fig. 2 ). F ig . 2. Case 1. Preoperative neuroimaging. A: Axial CT scan demonstrating the location of the odontoid process at the level of the clivus. B: Sagittal CT scan showing retroflexed odontoid. C: Sagittal T2-weighted MR image demonstrating cervicomedullary edema. Initial Transnasal Approach The details of the operative technique are as described above. A complete C-1 anterior arch and