C2 slope (C2S), a cervical parameter mathematically approximated as T1 slope minus cervical lordosis (T1S–CL), predicts functional improvement in cervical deformity patients. Nonetheless, C2S is a positional parameter based only on the horizontal axis. The current study aims to introduce novel odontoid parameters and establish their relationships with patient-reported health-related quality of life (HRQoL).
Lateral plain radiographs of 32 adults who underwent multilevel posterior cervical fusion were analyzed. The odontoid parameters included odontoid incidence (OI), C2S, odontoid tilt (OT), and gravity line-C2 distance (GL-C2), while the cervical parameters were the Cobb angle at C0–1, C1–2, C0–2, C2–7, C2–7 sagittal vertical axis (cSVA), T1 slope, and T1S–CL. The range of motion (ROM) of the occipito-atlantoaxial complex was measured in flexion and extension plain radiographs. Scores on the Neck Disability Index (NDI) and visual analogue scale (VAS) for axial neck (VASn) and arm pain were measured.
Compared to asymptomatic subjects, patients had larger C2S, cSVA, and T1S–CL, and smaller OT. Preoperatively, OI was significantly correlated with the ROM of C1–2 (r = 0.37, p < 0.05) and C0–2 (r = 0.46, p < 0.01). OT and C2S had significant correlations with the C0–1, C1–2, and C0–2 angles, GL-C2, and T1S–CL. Postoperative NDI scores were significantly correlated with OI (r = -0.40, p < 0.05) and OT (ρ = -0.37, p < 0.05). VASn was significantly correlated with GL-C2 (r = -0.35, p < 0.05).
The odontoid parameters were significantly correlated with established cervical parameters and HRQoL measures. OI is a constant parameter representing the individual's compensatory reservoir at the upper cervical spine.
Over the last few decades, the concept of sagittal spinal alignment in the thoracolumbar spine has been extensively studied. The idea of optimal alignment of the thoracolumbar spine is well-established, and pelvic parameters are the foundation of sagittal alignment of the spine [
To simplify the assessment of cervical malalignment, a novel parameter—C2 slope (C2S), which is mathematically approximated as T1S–CL—has been proposed [
The current study aims to introduce novel odontoid parameters and investigate their relationship with patient-reported HRQoL outcomes following multilevel posterior cervical fusion. We also sought to explore the relationship between the head position and cervical alignment.
After obtaining Institutional Review Board approval from Seoul National University Hospital (IRB approval No. B-2208-773-104) a retrospective analysis of clinical and radiographic outcomes was performed for patients who received a single-stage multilevel (3 or more) posterior cervical fusion. The patients were treated for cervical spondylotic myelopathy and/or radiculopathy, ossification of the posterior longitudinal ligament, degenerative disc disorders, and deformities at a single academic center by 5 attending spine surgeons. Standing lateral radiographs of the cervical spine were obtained with patients in a comfortable neutral position. The patients were instructed to look straight ahead, with the upper extremities positioned naturally at the side of the body. The inclusion criteria were patients with more than 1 year of follow-up, an upper instrumented vertebra below C2 to investigate changes in the axial cervical spine, and an acceptable range of the chin-brow vertical angle over -1.5° while maintaining a horizontal gaze in the neutral position in order to minimize the positional deviation in the cervical curvature [
Odontoid incidence (OI) was defined as the angle between the line perpendicular to the C2 endplate (C2EP) at its midpoint and the line connecting this point to the center of the odontoid process (the center of a circle with an anterior/posterior border and the apex of the dens as a tangent). Odontoid tilt (OT) was defined as the angle created by a line running from the C2EP midpoint to the center of the odontoid process and the vertical axis. Negative values indicated that the center of the odontoid process was placed anterior to the C2EP midpoint. C2S was defined as the angle between the C2EP and a horizontal line. A geometric construction using complementary angles showed that OI is the algebraic sum of OT and C2S (
The Cobb angle at C0–1, C1–2, C0–2, C2–7, T1S, C2–7 sagittal vertical axis (cSVA), and T1S minus CL (TIS–CL) were measured. For the C0–2 angle, an angle between the C2EP and the McRae line was measured. C0–1 angle was an angle between the McRae line and the line linking the inferior anterior and posterior arch of the atlas; C1–2 angle was defined as an angle between the line linking the inferior anterior and posterior arch of the atlas and the C2EP. T1S was defined as an angle between the T1 upper endplate and the horizontal plane. cSVA was defined as the distance between a plumb line from the centroid of C2 and the posterosuperior aspect of C7 (
Two commonly used self-assessment metrics for HRQoL were employed to measure disability after spine surgery: the Neck Disability Index (NDI) and visual analog pain scale (VAS) for the axial neck (VASn) and arm (VASa) pain.
A picture archiving and communication system (p view, Infinitt, Seoul, Korea) was used for measurements. The test for normality was done using the Shapiro-Wilk test. The correlations between the parameters and HRQoL scores were analyzed using Pearson correlation coefficients or Spearman rank-order correlation coefficients for nonparametric variables. Univariable linear regression analysis was performed to determine the possible threshold of radiographic parameters. The statistical analysis was conducted using SPSS software (version 25.0), and a p-value <0.05 was considered to indicate statistical significance.
In total, 32 patients (male, 22; female, 10) met the inclusion criteria for the study, with a mean age of 58.7± 14.3 years. The preoperative diagnosis for multilevel fusion included cervical spondylotic myelopathy and/or radiculopathy (n= 9), ossification of the posterior longitudinal ligament (n= 18), ossified ligamentum flavum (n = 1), and cervical deformity (n = 4). The average number of levels fused was 3.94± 1.08 (range, 3–6). The number of levels fused did not show a statistically significant correlation with either the radiographic parameters or HRQoL scores. Patient demographics and baseline radiographic parameters can be found in
Preoperatively, the odontoid parameters showed statistically significant correlations with established cervical parameters (
The postoperative NDI scores ranged from 0 to 39, with an average of 12.84 ± 9.12. The VASn scores ranged from 0 to 8, with a mean of 3.06± 2.51, and the VASa scores ranged from 0 to 10, with a mean of 3.53±3.04. The correlations between odontoid parameters and HRQoL measures were analyzed (
T1S–CL is a global assessment of sagittal alignment, detecting mismatches between the cervical and remaining thoracolumbar spine [
Recent studies have reported multiple cutoff values for the optimal T1S–CL. In one study, a cutoff value of 20° for the T1S–CL predicted moderate clinical disability according to the NDI score following multilevel cervical fusion [
In cervical malalignment, subsequent forward-shifting of the head results in chronic neck pain and leads to a downward gaze. Subsequently, the upper cervical spine extends to maintain a horizontal gaze (
The patient’s compensation to maintain a horizontal gaze may be represented by C2S [
In this study, we sought to elucidate the relationships of the odontoid parameters with clinical outcomes and radiographic cervical alignment in patients following multilevel posterior cervical fusion. This study bridges the gap between the conventional cervical parameters and explains the clinical improvement observed after cervical realignment surgery. C2S presents a simplified understanding of cervical alignment and is suggested as a unified key to understanding cervical alignment relative to the thoracic spine [
This study has certain limitations. First, it is a retrospective study with a small number of patients who had not been randomized. As a result, a detailed analysis was not possible, and we could not provide a valid cutoff value regarding NDI and OT. However, the study demonstrated significant correlations between the odontoid parameters and T1S–CL, allowing a simplified multiaxial assessment of cervical alignment harmony using the dens. In addition, the study was done with a heterogeneous cohort of patients. The majority of the patients in the present study underwent surgery not for cervical deformity, but for degenerative cervical disorders. Nonetheless, solid fusion was demonstrated to determine the true cause of disability, and we excluded patients with a misplaced screw, pseudarthrosis, facet arthrosis, or adjacent level disc herniation. After excluding other common causes of pain, we were able to assume that poor HRQoL was due to malalignment. The results from the current study revealed that odontoid parameters are valuable in assessing the relationship between cervical alignment and HRQoL. In the future, larger series of homogeneous populations undergoing cervical deformity corrective surgery can validate the results of our study.
Our study examined the novel odontoid parameters as an adjunct to the widely used C2S. Our findings demonstrate that similar to pelvic parameters, the severity of cervical malalignment differs due to the anatomical characteristics of each individual. The spatial orientation of the dens can be different between patients with identical C2S, since the angulation of the dens may vary. As OI represents the patient's compensatory reservoir, it is possible to assess the compensatory status of a patient and meticulously plan the optimal cervical alignment correction.
OI is the algebraic sum of OT and C2S. The odontoid parameters were significantly correlated with established cervical parameters and HRQoL measures following multilevel posterior cervical fusion. While C2S has shown utility in describing cervical deformities simply and effectively, it is limited in that it is a positional parameter, and its normal range may vary in each individual. In contrast, OI is a constant parameter and can represent the individual's compensatory reservoir at the upper cervical spine, like PI. Odontoid parameters can provide an effective tool for surgeons in assessing cervical malalignment. Based on the results of this study, in a given C2S, postoperative HRQoL scores showed better results in patients with larger OI. A larger OI resulted in larger cervical ROM, allowing more upper cervical spine extension, indicating a larger ROE. Therefore, it is essential to consider not only C2S, but all odontoid parameters, as an adjunct to C2S to assess the cervical alignment thoroughly.
The authors have nothing to disclose.
This study received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Conceptualization: JKL; Data curation: JKL; Formal analysis: JKL; Methodology: JKL; Project administration: SJH, KJK; Visualization: JKL, SJH; Writing - original draft: JKL, SJH, SHY, KJK; Writing - review & editing: JKL, SJH, SHY, KJK.
(A) Schematic drawing of the odontoid parameters. Odontoid incidence: the angle between the line perpendicular to the C2 endplate at its midpoint and the line connecting this point to the center of the odontoid process (the center of a circle with an anterior/posterior border and the apex of the dens as a tangent). Odontoid tilt: the angle created by a line running from the C2 endplate midpoint to the center of the odontoid process and the vertical axis (VRL) C2 slope: the angle between the C2 endplate and a horizontal line (HRL). (B) Schematic drawing of cervical parameters. The gravity line (GL) to C2 distance (GL-C2) was defined as the distance between the GL, defined as the plumb line from the center of the acoustic meatus, and the centroid of C2. The cervical sagittal vertical axis (cSVA) was defined as the distance between a plumb line from the centroid of C2 and the posterosuperior aspect of C7.
Linear regression analysis of the odontoid parameters and Neck Disability Index (NDI). Positive correlations between odontoid incidence (OI), odontoid tilt (OT), and Neck Disability Index (NDI) scores are noted.
Negative correlation between odontoid tilt (OT) and T1 slope minus cervical lordosis (T1S–CL). The linear regression model indicates that a T1S–CL value of 20° corresponded to an OT value of 0°.
An increase in C2 slope indicates the possibility of failure to achieve horizontal gaze. An increase in C2 slope leads to forward-shifting of the head or forward-shifting of the gravity line (yellow line). The patient extends the upper cervical spine to maintain a horizontal gaze. Patients with large odontoid incidence have larger neck extension reservoirs and can compensate with neck extension.
Schematic drawings illustrating the different spatial orientations of the dens with identical C2 slope and different odontoid incidence values. (A) A dens with a straight curvature is conducive to a small odontoid incidence, prone to anterior tilting of the center of the dens. (B) A dens with a greater posterior inclination is able to maintain the center of the dens more posteriorly.
Comparison of effects of odontoid tilt on the Neck Disability Index (NDI). (A) Even with an identical C2 slope, a smaller odontoid incidence is related to a smaller odontoid tilt and poorer health-related quality of life outcome score. (B) Postoperative improvement of odontoid parameters. The anterior inclination of the dens is related to cervical malalignment. OI, odontoid incidence; cSVA, cervical sagittal vertical axis; PO, postoperative.
Baseline demographic, radiographic, and surgical parameters (n=32)
Variable | Value |
---|---|
Demographic | |
Age (yr) | 58.72 ± 14.34 |
Male sex | 22 (68.8) |
Heigh (cm) | 163.10 ± 7.41 |
Weight (kg) | 64.75 ± 12.23 |
Body mass index (kg/m2) | 24.33 ± 4.23 |
Fused level | 3.94 ± 1.08 |
Baseline HRQoL metrics | |
VAS neck | 4.63 ± 2.96 |
VAS arm | 5.87 ± 3.18 |
Neck Disability Index | 21.44 ± 10.58 |
Mean radiographic parameters | |
Odontoid incidence | 18.22 ± 3.56 |
Odontoid tilt | -0.39 ± 12.41 |
C2 slope | 18.61 ± 12.29 |
C0–2 angle | -28.72 ± 9.76 |
Extension, C0–1 | -9.70 ± 6.08 |
Extension, C1–2 | -33.67 ± 4.96 |
Extension, C0–2 | -43.37 ± 8.12 |
Range of motion, C0–1 | 14.79 ± 5.42 |
Range of motion, C1–2 | 8.36 ± 3.62 |
Range of motion, C0–2 | 22.89 ± 6.58 |
C2–7 angle | -1.42 ± 20.83 |
Gravity line-C2 | -1.58 ± 9.11 |
Gravity line-C7 | 23.66 ± 15.74 |
C2–7 sagittal vertical axis | 25.25 ± 11.56 |
T1 slope | 23.51 ± 9.23 |
T1 slope minus cervical lordosis | 22.09 ± 15.61 |
Values are presented as mean±standard deviation or number (%).
HRQoL, health-related quality of life; VAS, visual analogue scale.
Comparison of cervical measurements to normative Data
Variable | Asymptomatic | Preoperative | p-value |
---|---|---|---|
Odontoid incidence | 17.7 ± 3.7 | 18.22 ± 3.56 | 0.570 |
Odontoid tilt | 6.7 ± 5.3 | -0.39 ± 12.41 | 0.005 |
C2 slope | 10.9 ± 6.2 | 18.61 ± 12.29 | 0.001 |
C0–2 angle | -25.6 ± 8.8 | -28.72 ± 9.76 | 0.305 |
C2–7 angle | -10.4 ± 7.3 | -1.41 ± 20.83 | 0.144 |
T1 slope | 23.1 ± 6.3 | 23.51 ± 9.23 | 0.686 |
C2–7 sagittal vertical axis | 17.80 ± 6.78 | 25.25 ± 11.56 | 0.000 |
T1 slope minus cervical lordosis | 12.7 ± 6.5 | 22.09 ± 15.61 | 0.003 |
Values are presented as mean±standard deviation.
Statistically significant differences (p<0.05).
Correlation of odontoid parameters with established parameters of the cervical spine at baseline
Variable | OI | OT | C2S |
---|---|---|---|
OI | NA | 0.30 |
0.02 |
OT | 0.30 |
NA | -0.93 |
C2S | 0.02 |
-0.93 |
NA |
C0–1 | -0.07 | 0.58 |
-0.61 |
C1–2 | -0.40 |
0.36 |
-0.50 |
C0–2 | -0.27 |
0.60 |
-0.68 |
GL-C2 | -0.13 | -0.71 |
0.71 |
GL-C7 | -0.33 | -0.52 |
0.51 |
ROM, C1–2 | 0.37 |
0.01 |
0.09 |
ROM, C0–2 | 0.46 |
-0.02 |
0.18 |
C2–7 | 0.09 |
-0.52 |
0.55 |
cSVA | -0.31 |
-0.3 |
0.29 |
T1S | 0.03 | 0.04 |
0.01 |
T1S–CL | 0.09 |
-0.84 |
0.92 |
OI, odontoid incidence; OT, odontoid tilt; C2S, C2 slope; GL-C2, gravity line-C2 distance; GL-C7, gravity line-C7 distance; ROM, range of motion; cSVA, C2-7 sagittal vertical axis; CL, cervical lordosis; T1S–CL, T1 slope minus cervical lordosis.
p<0.05.
p<0.01.
Spearman ρ.
Postoperative correlations of odontoid parameters and health-related quality of life
Variable | VAS neck | VAS arm | NDI |
---|---|---|---|
OI | -0.27 | -0.10 | -0.40 |
p-value | 0.14 | 0.57 | 0.02 |
OT | -0.13 |
-0.26 |
-0.37 |
p-value | 0.50 | 0.14 | 0.03 |
C2S | 0.08 | 0.25 | 0.31 |
p-value | 0.65 | 0.18 | 0.08 |
GL-C2 | 0.35 |
0.24 | 0.32 |
p-value | 0.04 | 0.18 | 0.08 |
GL-C7 | 0.17 | 0.17 | 0.27 |
p-value | 0.34 | 0.19 | 0.28 |
VAS, visual analogue scale; NDI, Neck Disability Index; OI, odontoid incidence; OT, odontoid tilt; C2S, C2 slope; GL-C2, Gravity line-C2 distance; GL-C7, gravity line-C7 distance.
p<0.05, statistically significant differences.
Spearman ρ.