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Allison Strickland, Cordell M. Baker, R. Michael Siatkowski and Timothy B. Mapstone

C hiari malformation is a hindbrain abnormality of the posterior fossa in which the cerebellar tonsils herniate through the foramen magnum. The diagnostic criterion that is often cited is tonsillar herniation greater than 5 mm below the level of the foramen magnum. 1 The abnormality is thought to result from compression of neural structures in the posterior fossa and is often associated with a spinal cord syrinx or skeletal abnormalities such as basilar impression, a retroflexed odontoid process, atlantooccipital fusion, atlantoaxial assimilation, and Klippel

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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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Jacob Archer, Meena Thatikunta and Andrew Jea

The transoral transpharyngeal approach is the standard approach to resect the odontoid process and decompress the cervicomedullary spinal cord. There are some significant risks associated with this approach, however, including infection, CSF leak, prolonged intubation or tracheostomy, need for nasogastric tube feeding, extended hospitalization, and possible effects of phonation. Other ventral approaches, such as transmandibular and circumglossal, endoscopic transcervical, and endoscopic transnasal, are also viable alternatives but are technically challenging or may still traverse the nasopharyngeal cavity. Far-lateral and posterior extradural approaches to the craniocervical junction require extensive soft-tissue dissection. Recently, a posterior transdural approach was used to resect retro-odontoid cysts in 3 adult patients. The authors present the case of a 12-year-old girl with Down syndrome and significant spinal cord compression due to basilar invagination and a retro-flexed odontoid process. A posterior transdural odontoidectomy prior to occiptocervical fusion was performed. At 12 months after surgery, the authors report satisfactory clinical and radiographic outcomes with this approach.

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James K. Liu, Jimmy Patel, Ira M. Goldstein and Jean Anderson Eloy

V arious abnormalities of the craniocervical junction can result in ventral compression of the cervicomedullary junction and require anterior decompression, including resection of the C-1 arch and odontoidectomy. 14 Several indications for anterior decompression and odontoidectomy include irreducible basilar invagina-tion, 13 , 17 , 22 , 26 , 27 , 34 , 50 , 52 , 53 severe compressive rheumatoid pannus not resolved by posterior stabilization, 14 , 40 , 51 os odontoideum, 29 , 32 and severely retroflexed odontoid processes associated with Chiari

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David A. Besachio, Ziyad Khaleel and Lubdha M. Shah


Posterior odontoid process inclination has been demonstrated as a factor associated with Chiari malformation Type I (CM-I) in the pediatric population; however, no studies to date have examined this measurement in the adult CM-I population. The purpose of this study was to evaluate craniocervical junction (CCJ) measurements in adult CM-I versus a control group.


The odontoid retroflexion, odontoid retroversion, odontoid height, posterior basion to C-2 line measured to the dural margin (pB-C2 line), posterior basion to C-2 line measured to the dorsal odontoid cortical margin (pB-C2* line), and clivus-canal angle measurements were retrospectively analyzed in adult patients with CM-I using MRI. These measurements were compared with normative values established from CT scans of the cervical spine in adults without CM-I.


A statistically significant difference was found between 55 adults with CM-I and 150 sex-matched controls (125 used for analysis) in the mean clivus-canal angle and the mean pB-C2 line.


These data suggest that there are sex-specific differences with respect to measurements at the CCJ between men and women, with women showing a more posteriorly inclined odontoid process. There were also differences between the CM-I and control groups: a more acute clivus-canal angle was associated with CM-I in the adult population. These CCJ findings could have an influence on presurgical planning.

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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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Ziyad L. Khaleel, David A. Besachio, Erica F. Bisson and Lubdha M. Shah


Posterior odontoid process inclination has been associated with Chiari malformation Type I in the pediatric population. There are varying reports to support a reliable range of odontoid inclination angles in control adults. The purpose of this study is to estimate the normal measurements in adults for odontoid retroflexion, retroversion, height, and the pB–C2 line (a line drawn through the odontoid tip from the ventral dura perpendicular to a second line from drawn the basion to the inferoposterior aspect of C-2 vertebral body) to establish a normative reference in this population.


After obtaining institutional review board approval, the authors performed a retrospective analysis of non–contrast enhanced cervical spine CT scans obtained in 150 consecutive control adults. Three neuroradiologists measured odontoid retroflexion, odontoid retroversion, odontoid height, and the pB–C2 line. The cohort was divided into sex and two age groups. Comparisons of the means with unpaired 2-tailed t-test were performed.


A total of 125 subjects met the inclusion criteria; 80 were men and 45 were women (mean age 52 years, range 18–89 years). The odontoid retroflexion angle ranged from 70° to 89° (mean 79.3° ± 4.9°), and the odontoid retroversion angle ranged from 57° to 87° (mean 71.9° ± 5.3°). The range and mean of odontoid height were 17–27 mm and 22 ± 1.8 mm, respectively. The mean pB–C2 line was 6.5 ± 2.1 mm with a range of 0–11.2 mm. The results were also compared with previously published pediatric data.


The current study demonstrates that the odontoid process in adults is anatomically different from that in children: it is longer, more posteriorly inclined, and has a greater pB–C2 line. Therefore, utilization of these parameters with previously published cutoffs in the pediatric population is not appropriate for surgical planning in adults.

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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

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Harminder Singh, Sarang Rote, Ajit Jada, Evan D. Bander, Gustavo J. Almodovar-Mercado, Walid I. Essayed, Roger Härtl, Vijay K. Anand, Theodore H. Schwartz and Jeffrey P. Greenfield

(Brainlab) for craniospinal navigation. Communication between Airo and Curve made connection to Brainlab image-guided surgery systems swift and image transfers automatic. All patients had basilar invagination with a retroflexed odontoid, causing compression of the ventral medulla. Posterior occiput-to-cervical–instrumented fusion was performed in all patients prior to the anterior endonasal procedure. The indications for odontoidectomy are varied and controversial and are not the crux of our current paper. In general, if patients have radiographic evidence of brainstem

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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