Basic Information of Included Patients
According to the inclusion criteria, 191 patients were included in this study, and the specific information were listed in Table 1. As mentioned above, patients whose cervical sagittal morphology could be objectively quantified by Cobb angle method were included in Group A and the remaining patients, who suffered from “Osteophyte of C2/C7”, “Local kyphosis”, or “Curvature concentration”, were included in Group B.
Design and Description of Area Methods
Considering that in the vast majority of cases, the C5 body is the apex of cervical lordosis, we selected C2, C5, and C7 as the three points of the sagittal triangle. Specific points selection methods are further divided into the following four:
Method-1: Anterior-inferior point of C7 -- Midpoint of anterior edge of C5 -- Midpoint of anterior edge of C7 (Fig. 1A);
Method-2: Posterior-inferior point of C7 -- Midpoint of posterior edge of C5 -- Midpoint of posterior edge of C7 (Fig. 1B);
Method-3: Midpoint of inferior endplate of C2 -- Midpoint of anterior edge of C5 -- Midpoint of inferior endplate of C7 (Fig. 1C);
Method-4: Midpoint of inferior endplate of C2 -- Midpoint of anterior edge of C5 -- Midpoint of superior endplate of C7 (Fig. 1D).
In "Method-1" and "Method-2", the midpoint of the anterior margin of the vertebral body was selected mainly to avoid the influence of C5 or C7 osteophytes on the area measurement. In "Method-3" and "Method-4", the midpoint of the inferior endplate was selected to avoid the effect of C2 osteophytes on the area measurement. The Area Coefficient (AC) value was negative when the midpoint of the C5 anterior edge fell on the right side of the base of the cervical sagittal triangle. In addition, in order to offset the influence of patients' body size, the C1-APD2 (Fig. 1) was used as the reference value, and the AC obtained by comparing the area value with it was used as the final index:
There Was Strong Consistency in Results between Area Methods and Cobb Angle Method in Group A
For Group A patients, Cobb angle can objectively reflect the advantages and defects of the cervical sagittal morphology. Therefore, we quantified the cervical sagittal morphology of this part of patients by using both Cobb angle method and area method, and conducted correlation analysis of the two groups of data. As shown in the Fig. 2A and Fig. 2B, the correlations between the measurement results of Method-1 (r=0.8480, P<0.0001, SP=100.00%) and Method-2 (r=0.8321, P<0.0001, SP=100.00%) and the measurement results of Cobb angle method were strongly correlated. As shown in the Fig. 2C and Fig. 2D, the measurement results of the latter two area methods, Method-3 (r=0.7627, P<0.0001, SP=100.00%) and Method-4 (r=0.7157, P<0.0001, SP=100.00%) have relatively poor correlation with the measurement results of Cobb angle method. We consider that the latter two methods may be influenced by the antero-posterior diameters of the vertebra. In the measurement process, we found that the accuracy of Method-2 was slightly lower than that of Method-1, mainly because of that the posterior edge of the vertebral body was relatively unclear and there’re some images overlapping.
Results of Area Methods Were More Objective in Group B
According to the grouping conditions mentioned above, 68 patients were screened out from 191 patients who met the Group B criteria. Among them, 3 cases were identified as "Osteophyte of C2/C7", 47 cases were identified as "Local kyphosis", and 18 cases were identified as "Curvature concentration". It can be seen that a significant proportion (35.60%) of the patients had cervical sagittal morphology that was not suitable for evaluation by Cobb angle method. Similarly, after quantifying the cervical sagittal morphology of Group B patients using two methods, correlation analysis was conducted for the two groups of data. As shown in Fig. 3, the correlations obtained by the four methods were relatively poor, suggesting that there were differences in the cervical sagittal morphology described by Cobb angle method and area methods in this part of patients. Further, we selected a representative group of patients to demonstrate the advantages of the area method in the above conditions. As shown in Fig. 4, the size of the C2-7 Cobb angle of all four patients was basically the same. However, in the three patients of Group B (Fig. 4F, -4K, -4P), different from Group A (Fig. 4A), due to the structural abnormality of their cervical vertebrae, the size of Cobb angle could not accurately reflect the defects of the cervical sagittal morphology. From the results of the area methods, we can see that although the Cobb angle is of the same size, the AC values of patient in Group A (Fig. 4B to -4E) were significantly higher than that of the three patients in Group B (Fig. 4G to -4J, Fig. 4L to -4O, Fig. 4Q to -4T), suggesting that the cervical sagittal morphology of patient in Group A is actually better.
Stronger Correlation between Results of Area Methods and Clinical Indicators Was Identified in Group B
In order to evaluate the diagnostic value of different quantitative methods on CDD-related clinical indicators, the ROC curve between the quantitative results of Cervical sagittal morphology and clinical indicators is constructed. As shown in Fig. 5 and Table 2, both Cobb angle method and area methods have certain diagnostic value for the occurrence of "Neck or shoulder pain" and "Sympathetic or VA-related symptoms" in the whole patient group: the greater the quantified value of cervical sagittal morphology, the lower the incidence of these symptoms. As shown in Fig. 6 and Table 3, similar results were also obtained in Group A. However, in Group B, as shown in Fig. 7 and Table 4, the results of area method still had predictive value for the above two aspects of symptoms, while the predictive ability of Cobb angle method was significantly reduced.
Stability of Area Method Was Better than That of Cobb Angle
Another advantage of the area methods is the stability of results. Because the inferior endplate of C2 and C7 is not regular in most cases, different measurers may choose different reference points when selecting the standard line, thus resulting in inconsistent measurement results, which is also a situation we often encounter in our daily work. However, in the area methods, the location difference of selected points will not cause the obvious change of the size of the triangle area, that is to say, the stability of AC is better. To prove it, five measurers independently quantified the cervical sagittal morphology in 191 patients using the Cobb angle method and the area methods, so that five independent results were obtained for each measurement method in each sample. Further, the dispersion coefficient of the five independent results was calculated and compared. The results are shown in the Fig. 8. It can be seen that the dispersion coefficients of the measurement results of the four area methods are significantly smaller than those of Cobb angle method.
Area Methods Had No Advantage in Quantifying Cervical ROM
In this section, we explored the applicability of the area methods in evaluating cervical range of motion (ROM). The AC values in the extension and flexion positions were measured respectively, and the difference between the two results was used as a quantitative index of cervical ROM, and the correlation analysis was further conducted with the ROM calculated by conventional Cobb angle method. As shown in the Fig. 9, the results measured by the four area methods are strongly correlated with those by the Cobb angle method. However, in the process of subtraction, the defects and instability of the Cobb angle method are offset, and the results are objective and realistic enough. Therefore, although the results of the area methods have a strong correlation with Cobb angle, it does not have any advantages.