Defining the role of the condylar–C2 sagittal vertical alignment in Chiari malformation type I

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  • 1 Division of Pediatric Neurosurgery, Department of Neurosurgery, University of Utah, Salt Lake City, Utah; and
  • | 2 Department of Surgery, Texas Children’s Hospital, Houston, Texas
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OBJECTIVE

The authors’ objective was to better understand the anatomical load-bearing relationship between the atlantooccipital joint and the upper cervical spine and its influence on the clinical behavior of patients with Chiari malformation type I (CM-I) and craniocervical pathology.

METHODS

In a single-center prospective study of patients younger than 18 years with CM-I from 2015 through 2017 (mean age 9.91 years), the authors measured the occipital condyle–C2 sagittal vertebral alignment (C–C2SVA; defined as the position of a plumb line from the midpoint of the occiput (C0)–C1 joint relative to the posterior aspect of the C2–3 disc space), the pB–C2 (a line perpendicular to a line from the basion to the posteroinferior aspect of the C2 body on sagittal MRI), and the CXA (clivoaxial angle). Control data from 30 patients without CM-I (mean age 8.97 years) were used for comparison. The primary outcome was the need for anterior odontoid resection and/or occipitocervical fusion with or without odontoid reduction. The secondary outcome was the need for two or more Chiari-related operations.

RESULTS

Of the 60 consecutive patients with CM-I identified, 7 underwent anterior odontoid resection or occipitocervical fusion and 10 underwent ≥ 2 decompressive procedures. The mean C–C2SVA was greater in the overall CM-I group versus controls (3.68 vs 0.13 mm, p < 0.0001), as was the pB–C2 (7.7 vs 6.4 mm, p = 0.0092); the CXA was smaller (136° vs 148°, p < 0.0001). A C–C2SVA ≥ 5 mm was found in 35% of CM-I children and 3.3% of controls (p = 0.0006). The sensitivities and specificities for requiring ventral decompression/occipitocervical fusion were 100% and 74%, respectively, for C–C2SVA ≥ 5 mm; 71% and 94%, respectively, for CXA < 125°; and 71% and 75%, respectively, for pB–C2 ≥ 9 mm. The sensitivities and specificities for the need for ≥ 2 decompressive procedures were 60% and 70%, respectively, for C–C2SVA ≥ 5 mm; 50% and 94%, respectively, for CXA < 125°; and 60% and 76%, respectively, for pB–C2 ≥ 9 mm. The log-rank test demonstrated significant differences between C–C2SVA groups (p = 0.0007) for the primary outcome. A kappa value of 0.73 for C–C2SVA between raters indicated substantial agreement.

CONCLUSIONS

A novel screening measurement for craniocervical bony relationships, the C–C2SVA, is described. A significant difference in C–C2SVA between CM-I patients and controls was found. A C–C2SVA ≥ 5 mm is highly predictive of the need for occipitocervical fusion/ventral decompression in patients with CM-I. Further validation of this screening measurement is needed.

ABBREVIATIONS

C–C2SVA = occipital condyle–C2 SVA; CCJ = craniocervical junction; CM = Chiari malformation; CM-I = CM type I; CXA = clivoaxial angle; C0 = occiput; OI = occipital incidence; OS = occipital slope; OT = occipital tilt; pB–C2 = a line perpendicular to a line from the basion to the posteroinferior aspect of the C2 body on sagittal MRI; PI = pelvic incidence; SVA = sagittal vertical alignment.

OBJECTIVE

The authors’ objective was to better understand the anatomical load-bearing relationship between the atlantooccipital joint and the upper cervical spine and its influence on the clinical behavior of patients with Chiari malformation type I (CM-I) and craniocervical pathology.

METHODS

In a single-center prospective study of patients younger than 18 years with CM-I from 2015 through 2017 (mean age 9.91 years), the authors measured the occipital condyle–C2 sagittal vertebral alignment (C–C2SVA; defined as the position of a plumb line from the midpoint of the occiput (C0)–C1 joint relative to the posterior aspect of the C2–3 disc space), the pB–C2 (a line perpendicular to a line from the basion to the posteroinferior aspect of the C2 body on sagittal MRI), and the CXA (clivoaxial angle). Control data from 30 patients without CM-I (mean age 8.97 years) were used for comparison. The primary outcome was the need for anterior odontoid resection and/or occipitocervical fusion with or without odontoid reduction. The secondary outcome was the need for two or more Chiari-related operations.

RESULTS

Of the 60 consecutive patients with CM-I identified, 7 underwent anterior odontoid resection or occipitocervical fusion and 10 underwent ≥ 2 decompressive procedures. The mean C–C2SVA was greater in the overall CM-I group versus controls (3.68 vs 0.13 mm, p < 0.0001), as was the pB–C2 (7.7 vs 6.4 mm, p = 0.0092); the CXA was smaller (136° vs 148°, p < 0.0001). A C–C2SVA ≥ 5 mm was found in 35% of CM-I children and 3.3% of controls (p = 0.0006). The sensitivities and specificities for requiring ventral decompression/occipitocervical fusion were 100% and 74%, respectively, for C–C2SVA ≥ 5 mm; 71% and 94%, respectively, for CXA < 125°; and 71% and 75%, respectively, for pB–C2 ≥ 9 mm. The sensitivities and specificities for the need for ≥ 2 decompressive procedures were 60% and 70%, respectively, for C–C2SVA ≥ 5 mm; 50% and 94%, respectively, for CXA < 125°; and 60% and 76%, respectively, for pB–C2 ≥ 9 mm. The log-rank test demonstrated significant differences between C–C2SVA groups (p = 0.0007) for the primary outcome. A kappa value of 0.73 for C–C2SVA between raters indicated substantial agreement.

CONCLUSIONS

A novel screening measurement for craniocervical bony relationships, the C–C2SVA, is described. A significant difference in C–C2SVA between CM-I patients and controls was found. A C–C2SVA ≥ 5 mm is highly predictive of the need for occipitocervical fusion/ventral decompression in patients with CM-I. Further validation of this screening measurement is needed.

In Brief

In this study, the authors introduce a novel method that describes the relationship of the occiput–C1 joint to the subaxial cervical spine. This will aid in identifying high-risk children with Chiari malformation type I who may require reoperation or ventral brainstem decompression or occipitocervical fusion. This will help practitioners identify the high-risk Chiari phenotype when evaluating children; however, further validation of the measure is needed.

The concept of global spinal alignment is central to the adult deformity literature.1 Many studies have investigated cervical sagittal balance and its potential effect on spinopelvic alignment;2–5 however, few, if any, studies have investigated the same relationships in either the pediatric or Chiari malformation (CM) populations. Emerging evidence from the pediatric literature demonstrates that a regional craniocervical junction (CCJ) metric, the clivoaxial angle (CXA), is an independent predictor of the need for delayed thoracolumbar fusion for scoliosis progression in the setting of CM.6 Thus, abnormalities of the CCJ, and specifically the occipitocervical joint, may influence other aspects of spinal disease. In the adult spine literature, regional cervical alignment, characterized by the C2–7 sagittal vertical axis, correlates with health-related quality-of-life measures, such that a larger sagittal vertical axis relates to poorer outcome measures.7

Few studies, however, have investigated the potential role of occipitocervical joint position on global load bearing or spinal alignment. One attempt was described by Kim et al., who reported the occipital incidence (OI) parameter,8 which is considered to be an anatomical morphometric parameter like pelvic incidence (PI).9 The measurement is defined as the angle subtended by a line from the center of the skull to the center of the foramen magnum and perpendicular to the foramen magnum. By the same token, occipital slope (OS) was defined as the angle between a line from the center of the orbit to the center of the foramen magnum, and the occipital tilt (OT) was defined as the angle between a line from the center of the skull to the center of the foramen magnum and a vertical line that originated from the center of the skull.8

Previous work by Bollo et al. demonstrated that children with basilar invagination, CM 1.5, and a CXA < 125° were at increased risk for requiring occipitocervical fusion.10 However, despite this knowledge, there remains a great deal to learn about the influence of the complex interaction between the occiput (C0)–C1 joint position and the craniocervical load-bearing structures. We believe that the relationship of the C0–C1 joint to the most superior load-bearing disc of the cervical spine, the C2–3 disc, influences occipitocervical alignment and the clinical behavior of patients. In other words, a load at the C0–C1 joint not centered over the C2–3 disc space would ultimately lead to changes in the relationship of the dens and the overall occipitocervical biomechanics. We hypothesize that a simple measurement that determines the midposition of the C0–C1 joint relative to the inferior endplate of C2, namely, the occipital condyle–C2 sagittal vertical alignment (C–C2SVA) (Fig. 1), correlates with the need for occipitocervical fusion or ventral brainstem decompression in the CM type I (CM-I) patient population.

FIG. 1.
FIG. 1.

Illustration of the C–C2SVA measurement. The steps for determining this measure are shown in Fig. 2. Figure is available in color online only.

Methods

Data Collection

This study was approved by the University of Utah School of Medicine and Primary Children’s Hospital institutional review boards. A prospectively maintained database of patients with CM-I was kept, with consent obtained from the patient’s parent/legal guardian; no patients with syndromic conditions were included in the current study. All patients were < 18 years old and underwent Chiari decompression from 2015 through 2017. The study period was initiated before surgical intervention, at the time of neurosurgical evaluation. We retrospectively collected clinical variables, including sex and date of birth (age in months). The primary outcome was the need for anterior odontoid resection and/or occipitocervical fusion with or without odontoid reduction. The secondary outcome was the need for 2 or more Chiari-related operations.

Radiographic variables were recorded from MRI before treatment for CM. The measurements were performed by a senior attending pediatric neurosurgeon (D.L.B.) and a pediatric neurosurgery fellow (R.R.I.). The measurements included the C–C2SVA (mm), pB–C2 (mm) (a line perpendicular to a line from the basion to the posteroinferior aspect of the C2 body on sagittal MRI), and CXA (degrees) on preoperative brain or cervical spine MRI (Table 1).

TABLE 1.

Summary of measurements investigated in the current study

Measurement NameDefinition
C–C2SVAPosition of the midpoint of the atlantooccipital joint (parasagittal) to the C2–3 disc space (plumb line); continuous variable measured in mm
pB–C2A line drawn btwn the basion & posteroinferior aspect of the C2 body. A line perpendicular to this line, pB–C2, drawn through the odontoid tip to the ventral dura (≥9 mm associated w/ clinical symptoms)12
CXALine drawn along the clivus & extrapolated inferiorly. The line falls tangent to the posterior aspect of the odontoid process (<125° associated w/ basilar invagination)10

The C–C2SVA was measured using the following technique. First, a line is marked parallel to the C2 endplate on a midline cervical spine or brain MRI scan. Then, on parasagittal images, the midpoint of the occipitoatlantal joint is identified, taking into account minor variations of anatomy between the two sides. From this mid-atlantooccipital joint reference point, a line is dropped perpendicular to the C2 inferior endplate line. The distance from the posterior aspect of the C2–3 disc space to the plumb line is measured (mm) (Fig. 2). The C–C2SVA was also measured in 30 control patients who underwent cervical spine MRI for indications other than CM-I.

FIG. 2.
FIG. 2.

Steps for determining the C–C2SVA. A: Draw a line parallel to the C2 inferior endplate on the midline MR image. B: On a parasagittal MR image, identify and mark the midpoint of the C0–1 joint (star). C: Using the corresponding C0–1 reference point on the midline MR image, draw a line perpendicular to the C2 horizonal line in the midline. D: Measure the distance from the posterior aspect of C2–3 disc space to the plumb line in mm (dashed red lines). Figure is available in color online only.

Imaging exclusion criteria included studies clearly not performed in the neutral position, inability to obtain the images through the imaging archives, or unclear anatomy in which we were unable to identify the relevant structures.

Statistical Analysis

Data are summarized using means and standard deviations for continuous variables and counts and frequencies for categorical variables. Fisher’s exact test was performed to compare the proportions of C–C2SVA ≥ 5 mm, CXA < 125°, and pB–C2 ≥ 9 mm between the CM-I and control groups.

Treating the C–C2SVA ≥ 5 mm as a screening test, we were able to calculate the sensitivity and specificity for the outcome of multiple procedures and the need for ventral decompression/occipitocervical fusion. We also calculated the sensitivity and specificity of CXA < 125° and pB–C2 ≥ 9 mm and compared each of these metrics with C–C2SVA ≥ 5 mm. The Mann-Whitney U-test was performed to compare continuous values for C–C2SVA, CXA, and pB–C2 with respect to each outcome. Fisher’s exact test was used to compare binary variables of C–C2SVA ≥ 5 mm (yes/no), CXA < 125° (yes/no), and pB–C2 ≥ 9 mm (yes/no) and the primary outcome of needing multiple procedures or requiring occipitocervical fusion or ventral decompression.

The log-rank test was used to compare the time to event (occipitocervical fusion or ventral decompression) between C–C2SVA classifications; a Kaplan-Meier curve was generated from this. Statistical significance was established using a cutoff of p < 0.05. Data were analyzed using SAS version 9.3 software (SAS Institute).

Interrater Reliability

Each of the measurements were taken by two separate raters, and the measurements were compared. The mean values for CXA, pB–C2, and C–C2SVA were compared. Correlation between the measurements was recorded using a Pearson correlation coefficient. Cohen’s kappa coefficient was calculated to measure interrater reliability for CXA < 125° (yes/no), pB–C2 ≥ 9 (yes/no), and C–C2SVA ≥ 5 (yes/no).

Results

A total of 67 children with CM-I were screened, and 60 who underwent surgery during the study period were enrolled. The 7 excluded children did not have neutral imaging available for review or did not have full imaging studies to perform the needed measurements. Among the 60 patients who had surgery, 7 children underwent ventral brainstem decompression or occipitocervical fusion, while 10 children underwent 2 or more Chiari decompressive procedures.

Patients With CM-I Versus Control Patients

The mean age of the CM-I group was 9.91 years, and the mean age of the control group was 8.97 years. There were significant differences between CM-I patients and controls in the mean pB–C2 (7.7 vs 6.44 mm, p = 0.0092), mean CXA (136° vs 148°, p < 0.0001), and mean C–C2SVA (3.68 vs 0.13 mm, p < 0.0001) (Table 2). Fisher’s exact test demonstrated significant differences between CM-I and control in the proportion of children with C–C2SVA ≥ 5 mm (35% vs 3.3%, p = 0.006) but not between groups for pB–C2 ≥ 9 mm or CXA < 125° (Table 3).

TABLE 2.

Mann-Whitney test comparing measurements for CM-I and control groups

Mean ± SD
VariableCM-I GroupControl Groupp Value
pB–C2, mm7.7 ± 2.136.44 ± 1.850.0092
CXA, °136 ± 10.7148 ± 8.81<0.0001
C–C2SVA, mm3.68 ± 3.020.13 ± 3.17<0.0001

There were 60 CM-I patients and 30 control patients.

TABLE 3.

Fisher’s exact test comparing the proportion of patients in CM-I and control groups reaching cutoffs

CM-IControl
VariableNo. AvailableCount (%)No. AvailableCount (%)p Value
pB–C2 ≥9 mm5918 (31)304 (13)0.117
CXA <125°608 (13)301 (3.3)0.2624
C–C2SVA ≥5 mm6021 (35)301 (3.3)0.0006

Need for Ventral Decompression/Occipitocervical Fusion

There was a significant difference in the mean pB–C2 (9.71 ± 1.25 vs 7.43 ± 2.08 mm, p = 0.004), mean CXA (120° ± 7.13° vs 138° ± 9.13°, p < 0.0001), and mean C–C2SVA (8.86 ± 2.85 vs 3 ± 2.31 mm, p < 0.0001) between children who underwent ventral decompression/occipitocervical fusion and those who did not. Fisher’s exact test demonstrated significant differences in the proportion of children with C–C2SVA ≥ 5 mm (100% vs 26%, p = 0.0003), CXA < 125° (71% vs 5.7%, p = 0.0002), and pB–C2 ≥ 9 mm (71% vs 25%, p = 0.023) between these two populations.

The C–C2SVA ≥ 5 mm demonstrated 100% sensitivity in identifying patients who had ventral decompression/occipitocervical fusion, whereas CXA < 125° and pB–C2 ≥ 9 mm each demonstrated 71% sensitivity (Table 4). Specificity for C–C2SVA ≥ 5 mm was 74%, whereas specificity for CXA < 125° was 94% and that for pB–C2 ≥ 9 mm was 75%. The log-rank test demonstrated significant differences between C–C2SVA groups (p = 0.0007) for the primary outcome (Fig. 3).

TABLE 4.

Sensitivity and specificity for C–C2SVA ≥ 5 mm, CXA < 125°, and pB–C2 ≥ 9 mm for undergoing occipitocervical fusion

VariableSensitivity95% CISpecificity95% CI
pB–C2 ≥9 mm0.7140.379–10.750.6323–0.8677
CXA <125°0.710.37–10.940.88–1
C–C2SVA ≥5 mm10.59–10.7360.61–0.85
FIG. 3.
FIG. 3.

Kaplan-Meier analysis demonstrating differences between the time to event based on the value of the C–C2SVA ≥ 5 mm. Figure is available in color online only.

Need for Multiple Decompressive Procedures

There was a significant difference in the mean pB–C2 between children who required multiple procedures and those who did not (9.2 ± 1.4 vs 7.4 ± 2.13 mm, p = 0.0095); there were also significant differences in mean CXA (128° ± 12.7° vs 137° ± 9.58°, p = 0.039) and mean C–C2SVA (6.5 ± 3.92 vs 3.12 ± 2.5 mm, p = 0.0078). Fisher’s exact test demonstrated significant differences in the proportion of children with CXA < 125° between those who underwent multiple procedures and those who did not (50% vs 6%, respectively; p = 0.002). There was no significant difference between the proportion of children with C–C2SVA ≥ 5 mm (60% vs 30%, respectively; p = 0.143) or pB–C2 ≥ 9 mm (60% vs 24%, respectively; p = 0.0538).

A C–C2SVA ≥ 5 mm demonstrated 60% sensitivity for identifying patients who required multiple decompressive procedures, whereas the sensitivity was 50% for CXA < 125° and 60% for pB–C2 ≥ 9 mm (Table 5). The specificity for C–C2SVA ≥ 5 mm was 70% while that for CXA < 125° was 94% and that for pB–C2 ≥ 9 mm was 75%.

TABLE 5.

Sensitivity and specificity for C–C2SVA ≥ 5 mm, CXA < 125°, and pB–C2 ≥ 9 mm for having multiple procedures

VariableSensitivity95% CISpecificity95% CI
pB–C2 ≥9 mm0.60.2964–0.90360.75510.6347–0.8755
CXA <125°0.50.19–0.810.940.874–1
C–C2SVA ≥5 mm0.60.29–0.90360.70.573–0.827

Interrater Reliability

The Pearson correlation coefficient for the CXA between raters was 0.86, indicating a strong correlation. The κ coefficient was 0.27, indicating fair agreement. The Pearson correlation coefficient for pB–C2 was 0.842, indicating a strong correlation, with a κ of 0.68, indicating substantial agreement. The C–C2SVA demonstrated a Pearson correlation coefficient of 0.77 and a κ of 0.73, indicating strong correlation and substantial agreement.

Discussion

We describe a novel method of defining craniocervical load-bearing relationships, the C–C2SVA measurement. We found significant differences in C–C2SVA values between patients with CM-I and controls. Furthermore, in a consecutive series of CM-I patients, as a screening test, a C–C2SVA ≥ 5 mm was 100% sensitive in predicting the need for ventral decompression or occipitocervical fusion with a strong level of interrater reliability.

Previous work has shown that the CXA and pB–C2 can be useful in determining which patients with CM-I may require occipitocervical fusion and/or odontoid resection; however, there may be shortcomings to these measurement techniques. Specifically, since the odontoid has no craniocervical load-bearing responsibilities, the pB–C2 measurement fails to accurately reflect the true influence of abnormal craniocervical alignment. Furthermore, the pB–C2 fails to accurately reflect craniocervical bony relationships because it, by definition, includes soft tissue posterior to the odontoid that is of unclear significance. Also, neither the CXA nor the pB–C2 accounts for the position of the C2–3 disc space, which is crucial to the anatomical load-bearing status of the subaxial cervical spine.

The purpose of a screening tool is to detect potentially “at-risk” individuals in a simple manner with high sensitivity so as not to miss potential disease. The ideal screening test can identify asymptomatic individuals at an earlier stage than at the time symptoms would have developed.11 The C–C2SVA measurement can be used as a screening test, as it demonstrated excellent sensitivity for requiring occipitocervical fusion or ventral brainstem decompression and a strong correlation (κ = 0.73) between observers, with the highest level of interrater reliability in detecting a meaningful clinical cutoff among the 3 measurements.

A Review of Normal Cervical Alignment

To truly understand the impact of abnormal craniocervical bony relationships, it is important to review the concepts of normal cervical spine alignment. As defined by Beier et al., the center of the mass of the head in the sagittal plane is directly over the occipital condyle 1 cm above and anterior to the external auditory canal.12 The weight of the head is then borne through the occipital condyles, onto the lateral masses of C1, and then to the lateral masses of C2. The load is then distributed through the body of C2, through the articular pillars, and into the C2–3 disc space via the anterior column and the C2–3 facet joints via the posterior column.13 The anterior column, which includes the mid-body of C2, accounts for approximately 50% of the load distribution of the spine.13 The position of the cervical spine in the sagittal plane can be measured by either the C2 or C7 SVA, which are acquired by measuring plumb lines from the center of C2 or C7 to the posterior superior corner of the sacrum. These sagittal relationships give an idea of overall spinal alignment, with the intention of keeping the head centered over the pelvis.2 However, these and other similar metrics do not incorporate the occipitocervical joint, which is of critical interest in CM-I.

Global Spinal Alignment and Its Relationship to the CCJ

The closest attempt to investigate the role of the occipitocervical joint position on global load bearing or spinal alignment was described by Kim et al. with the OI parameter.8 Zhu et al. modified the OS as the angle between the foramen magnum and a horizontal line such that OI = OT + OS, similar to the formula for PI.14 They concluded also that there were strong correlations between the occipital parameters and cervical parameters of the C0–2 and C2–7, but the regional relationship between the C0–C1 joint and C2 was not elucidated. Chandra et al. reported on the sagittal joint inclination and craniocervical tilt correlating with basilar invagination and atlantoaxial dislocation and suggested that joint orientation plays a significant role in this setting.15 Their study included some pediatric patients but also included patients with irreducible basilar invagination and developmental causes of atlantoaxial dislocation. Although the measures correlated with severity, they did not predict behavior over time.

In light of these previous studies, we postulate that the relationship of the C0–C1 joint to the C2–3 disc influences CCJ alignment and clinical behavior. The current study, which showed that patients with a C–C2SVA ≥ 5 mm were more likely to have undergone an operation for brainstem compression (either fusion or ventral decompression), provides strong evidence in favor of this hypothesis.

Limitations

We acknowledge that there are limitations to the current investigation. First, the use of supine spinal MRI studies may be less than optimal in defining craniocervical anatomical relationships. It is difficult to say how the supine position relates to upright, physiological alignment, and how the load of the cranium is borne through the upper cervical spine. Ideally, standing neutral or scoliosis radiographs would be used to identify the proposed anatomical relationship, but precise identification of the relevant anatomical landmarks would be challenging, especially when comparing parasagittal anatomy to the midline. Second, this was a retrospective analysis of a prospectively maintained single-center cohort, so the information gathered is limited by the accuracy and availability of the medical record and imaging and a potentially homogeneous patient population. Although we believe that the C–C2SVA can be helpful as a screening technique, we emphasize that careful clinical evaluation is paramount when making all diagnostic and treatment decisions. Prospective, preferably multicenter, validation of this measure is necessary before widespread implementation.

Conclusions

We propose a screening measurement based on the relationship between the C0–C1 joint and the C2–3 disc space—the C–C2SVA—as a method of determining 1) a pattern of clinical behavior within CM-I and 2) which patients might be at greatest risk for requiring ventral decompression/occipitocervical fusion. Further multicenter validation is necessary.

Acknowledgments

We thank Evan J. Joyce, MD, MS, Christopher Wilkerson, MD, and Jonathan P. Scoville, MD, for their participation and support of the study, and Kristin Kraus, MSc, for editorial assistance in preparing this paper.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Brockmeyer. Acquisition of data: Brockmeyer, Iyer. Analysis and interpretation of data: Ravindra, Awad. Drafting the article: Brockmeyer, Ravindra. Critically revising the article: Brockmeyer, Ravindra, Iyer, Awad, Bollo. Reviewed submitted version of manuscript: Brockmeyer, Ravindra, Awad, Bollo. Approved the final version of the manuscript on behalf of all authors: Brockmeyer. Statistical analysis: Awad, Zhu.

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Illustration from Guida et al. (pp 346–352). Copyright Lelio Guida. Published with permission.

  • View in gallery

    Illustration of the C–C2SVA measurement. The steps for determining this measure are shown in Fig. 2. Figure is available in color online only.

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    Steps for determining the C–C2SVA. A: Draw a line parallel to the C2 inferior endplate on the midline MR image. B: On a parasagittal MR image, identify and mark the midpoint of the C0–1 joint (star). C: Using the corresponding C0–1 reference point on the midline MR image, draw a line perpendicular to the C2 horizonal line in the midline. D: Measure the distance from the posterior aspect of C2–3 disc space to the plumb line in mm (dashed red lines). Figure is available in color online only.

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    Kaplan-Meier analysis demonstrating differences between the time to event based on the value of the C–C2SVA ≥ 5 mm. Figure is available in color online only.

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