The head position and cervical alignment in patients with Chiari malformation: A retrospective case–control study

Abstract

Aim and Background:

This study aimed to investigate the relationship between cervical spinal alignment and the center of gravity (COG) of the head in patients with Chiari malformation (CM) compared to healthy individuals. CM is characterized by the herniation of posterior fossa structures through the foramen magnum, potentially affecting head positioning and craniovertebral junction biomechanics. Understanding these biomechanical changes is crucial for improving diagnostic and treatment strategies.

Materials and Methods:

This retrospective study included 102 CM patients and 71 healthy controls. Radiological measurements were obtained from cervical X-rays, with seven reference points used to calculate angles related to head positioning and cervical curvature. Angular parameters, including cranial incidence (CI), cranial slope (CS), cranial tilt (CT), C7 slope (C7S), and spinocranial angle (SCA), were analyzed to determine correlations with the COG of the head. Statistical analyses were performed using t-tests, ROC analysis, and Pearson/Spearman correlation tests.

Results:

CM patients had significantly higher CI, CT, STT, and SCA angles compared to controls (P < 0.05), indicating an anterior displacement of the head’s COG. The CS angle was lower in CM patients (P < 0.05), reflecting a more flexed head position. No significant differences were found in C7S and C2T angles between groups, suggesting similar cervical curvature. ROC analysis demonstrated high sensitivity and specificity of the angular measurements for diagnosing CM.

Conclusion:

CM patients exhibit distinct biomechanical alterations, including an anterior shift of the COG and a more flexed head position. These findings highlight the potential of angular measurements as noninvasive diagnostic tools for CM. Future studies should explore the implications of these biomechanical changes on CM progression and treatment outcomes.

INTRODUCTION

Chiari malformation (CM) is a pathology associated with inferior prolapse of the posterior fossa structures through the foramen magnum. It is classified into CM type 0, type I, type II, type III, type IV, type V, type 1.5, and complex CM.[1,2,3,4] Type I is the most common type, and since the increased utility of MRI in clinical practice, it is diagnosed in 1% of the general population.[5]

There are many theories on the pathophysiology of CM. There are hypotheses that CM may evolve from congenital or acquired etiologies; however, congenital etiology is more commonly accepted.[3,6,7,8] On the other hand, the number of studies concerning the acquired etiologies has increased recently. The biomechanical theory that interlinks the evolution of CM and the craniovertebral junction instability has gained popularity.[9,10,11]

Cervical curvature is a key clinical assessment reflecting the mechanical state of the cervical spine.[12] A normal lordotic curvature minimizes the energy required to maintain a horizontal gaze while upright, resulting in the smallest tensile and compressive loads on spinal structures. The cervical spine, connecting the skull and thoracic vertebrae, supports the head’s mass and enables complex neck movements. Recent studies have utilized X-ray imaging to investigate the relationship between cervical sagittal alignment parameters, including the head’s mass point, the C2 vertebral center, the T1 slope, and other related measurements.[12,13]

In this retrospective clinical and radiological study, our aim is to present the correlation between the cervical spinal alignment and the center of gravity (COG) of the head in CM patients and compare the results with healthy individuals.

MATERIALS AND METHODS

Ethical approval for this study was obtained from the Ankara Bilkent City Hospital Ethical Committee (E1-20-469). The participants were categorized into two groups: the study group included patients who were operated on for CM patients between January 1, 2015, and December 31, 2020. The control group included healthy volunteers who did not have headaches, neck pain, neck stiffness, local tenderness, or limited cervical activity. In addition, none of the volunteers had been diagnosed with a craniocervical junction anomaly or intracranial pathology. Clinical data and radiological scans of the patients in both groups were evaluated retrospectively. Measurements were taken in a double-blind manner by three experienced medical doctors. The average of these measurements was used in the study.

One hundred and two CM patients were included in the study group whose preoperative cervical anteroposterior and lateral X-rays, cervical CT, cervical MRI, and neurological assessment notes could be obtained. The control group included 71 healthy volunteers who consented to undergo cervical anterior-posterior and lateral X-rays.

Radiological measurements

In the cervical X-rays of both groups, seven reference points were determined. These points are as follows: 1) midpoint of the sella turcica, 2) hard palate, 3) lower end plate of the C2 vertebra, 4) lower end plate of the C7 vertebra, 5) midpoint of the C7 vertebra, 6) the lowest point of the occipital bone, and 7) McGregor line. Seven angles were measured using these reference points [Figure 1]. The correlation between these angles and the COG of the head and cervical lordosis was evaluated. These angles were as follows:

Figure 1.

Reference points on the lateral cervical X-ray images. (a) The midpoint of sella turcica, (b) Hard palate, (c) Lower end plate of C2 vertebra, (d) Lower end plate of C7 vertebra, (e) The midpoint of C7 vertebra, (f) The lowest point of the occipital bone, (g) McGregor line

1. C7 slope (C7S) angle: The angle between the inferior end plate of the C7 vertebrae and a horizontal line [Figure 2a]

2. Cranial incidence (CI) angle: The angle between a vertical line passing through the McGregor line and the line passing from the sella turcica to the McGregor line. This angle may show variation between individuals and does not change with head positioning[14][Figure 2b]

3. Cranial slope (CS) angle: The angle between a horizontal line and the McGregor line. It is used for determining the position of the skull base in reference to the horizontal axis. This line is positive if the McGregor line is directed superiorly and anteriorly, negative if the McGregor line is directed inferiorly and anteriorly, and zero if the McGregor line is parallel to the horizontal axis [Figure 2c]

4. Cranial tilt (CT) angle: The angle between the line that passes through the sella turcica and the midpoint of the McGregor line, and the vertical line from the midpoint of the McGregor line. It may change with positional changes of the head. It is complementary to the CS angle [Figure 2d]

5. Cervical-2 tilt (C2T) angle: The angle between a vertical line passing through the midpoint of the C7 vertebra and the line that connects the midpoint of the inferior end plate of the C2 vertebra and the midpoint of the C7 vertebra. If the C2 vertebra is anterior to the vertical line that passes through the midpoint of the C7 vertebra, the angle is considered positive and if it is posterior, the angle is considered negative [Figure 2e].

6. Sella turcica tilt (STT) angle: The angle between the vertical line passing through the midpoint of the C7 vertebra and the line between the midpoint of the sella turcica and the midpoint of the C7 vertebra. If the sella turcica is located anterior to the vertical line passing through the C7 vertebra, the angle is considered positive and if it is located posteriorly, the angle is considered negative [Figure 2f]

7. Spinocranial angle (SCA): The angle between the line that connects the midpoint of sella turcica and inferior end plate of C7 vertebra and the C7S line. It is defined as achieving an internal cervical curvature measure, similar to the spinosacral angle of the thoracolumbar spine [Figure 2g].

Figure 2.

The angular parameters are shown on the lateral cervical X-ray. (a) C7 slope, (b) cranial incidence angle, (c) cranial slope angle, (d) cranial tilt angle, (e) C2T angle, (f) Sella turcica tilt angle, (g) spinocranial angle angle

The McGregor line is taken as the landmark for defining the skull base. Sella turcica is considered the second reference landmark for the COG of the head [Figure 1a].

Statistical analysis

Statistical Package for the Social Sciences (SPSS) version 22.0 (IBM Corporation, Armonk, New York, USA) program was used for statistical analysis of the data. Frequency values were calculated for the descriptive data of the patients and other categorical variables. The results were presented with cross tables and analyzed via Pearson Chi-square analysis. The average and standard deviation and median values are determined for quantitative data, and the categorical variables are shown as the number and percentage of patients.

Kolmogorov–Smirnov and Shapiro–Wilk tests were utilized for the determination of the statistical analysis method of the hypothesis. The normality and homogeneity of the data were investigated via Levene’s test. For comparison of two independent groups, independent samples t-test was used when the parametric hypothesis was achieved, and the Mann–Whitney U test was utilized when the parametric hypothesis was not achieved. For analysis of more than two independent groups; one-way variance analysis (ANOVA) was used if the parametric hypothesis was achieved, and Kruskal–Wallis test was used if it was not achieved. If there was variance between groups, post-hoc analysis was performed.

The cutoff values for CI, CS, CT, STT, and SCA angles were determined via ROC analysis. The Youden index was used for patient selection for the study group. These patients were divided into two groups based on the cutoff values, and the sensitivity for having the pathology was shown with cross tables. The statistical significance between the quantitative variables was investigated with the Pearson correlation test for parametric values and Spearman correlation test for nonparametric values. All the analyses were performed with a 95% confidence interval and P < 0.05 was accepted as statistical significance.

RESULTS

The mean age in the control group was 37.5 (range: 18–76), and the female/male ratio was 37:34. The mean age in the study group was 36.7 (range: 13–72) and the female/male ratio was 69:33. The age distribution was similar in both groups; however, CM was observed significantly more frequent in females than males (P = 0.039) [Table 1].

Table 1.

Age and gender comparison between groups

	Control (n=71)			Study (n=102)			Test	P
	X̄	SD	Median	X̄	SD	Median
Age	37.58	13.98	32.00	36.73	13.03	34.00	−0.093	0.926
Gender, n (%)
Female		37 (34.9)			69 (65.1)		4.257	0.039*
Male		34 (50.7)			33 (49.3)

*P<0.05. SD - Standard deviation; X̄ - Average

The values of angles were compared between the groups. There was a statistically significant difference between groups for CI, CS, CT, STT, and SCA angles (t-test). The CS angle (p: 0.020) was lower in the study group; however, the CI (P < 0.001), CT (<0.001), STT (P < 0.001), and SCA (P = 0.006) angles were higher in the study group [Table 2]. The higher values of CI, CT, STT, and SCA angles for the study group are associated with the anterior displacement of the COG of the head. The low value of the CS angle is associated with the more flexed position of the head than the anatomical position in the study group [Figure 2]. The C7S and C2T angles were not significantly different in the groups, therefore, the cervical curvature was similar in the groups [Figure 2].

Table 2.

Comparison of angle measurements between control and study groups (t-test and Mann–Whitney U-test)

Angle	Control (n=71)			Study (n=102)			Test	P
	X̄	SD	Median	X̄	SD	Median
CI	20.98	3.69	21.36	26.92	5.29	26.30	−6.968+	<0.001*
CS	5.18	5.20	4.62	3.22	5.56	2.60	2.345−	0.020*
CT	15.95	5.88	16.00	23.61	5.75	24.40	−8.502−	<0.001*
C7S	22.68	5.05	22.33	22.72	8.46	22.94	−0.248+	0.804
C2T	10.15	4.44	10.03	11.10	5.33	11.50	−1.642+	0.101
STT	10.36	4.32	10.15	12.84	4.51	13.65	−3.619−	<0.001*
SCA	77.46	5.02	77.98	80.55	7.47	80.29	−2.774+	0.006*

*P<0.05; +: higher values in the study group; -: higher values in the control group. SD - Standard deviation; X̄ - Average; CI - Cranial incidence; CS - Cranial slope; CT - Cranial tilt; C7S - C7 slope; SCA - Spino-cranial angle; STT - Sella turcica tilt

There was a statistically significant difference of CI (P < 0.001), CS (P < 0.001), CT (P < 0.001), STT (P < 0.001), and SCA (P = 0.015) angles between groups; therefore, ROC analysis was performed for determination of the cutoff values [Table 3]. In the study group; the CI, CT, STT, and SCA angles were higher than the cutoff values and CS angle was lower than the cutoff value. This information could be used to diagnose CM via X-rays within a certain sensitivity and specificity [Table 4].

Table 3.

The cross-table between the control and study group for determination of the cutoff values of cranial incidence, cranial slope, cranial tilt, sella turcica tilt, and spino-cranial angles

Group	Control group (n=71), n (%)	Study group (n=102), n (%)	χ 2	P
CI angle
Control (n=84)	55 (77.5)	29 (28.4)	40.292	<0.001*
Study (n=89)	16 (22.5)	73 (71.6)
CS angle
Control (n=91)	48 (67.6)	43 (42.2)	10.874	0.001*
Study (n=82)	23 (32.4)	59 (57.8)
CT angle
Control (n=70)	50 (70.4)	20 (19.6)	44.869	<0.001*
Study (n=103)	21 (29.6)	82 (80.4)
STT angle
Control (n=87)	49 (69)	38 (37.3)	16.89	<0.001*
Study (n=86)	22 (31)	64 (62.7)
SCA angle
Control (n=93)	46 (64.8)	47 (46.1)	5.895	0.015*
Study (n=80)	25 (35.2)	55 (53.9)

*P<0.05. CI - Cranial incidence; CS - Cranial slope; CT - Cranial tilt; SCA - Spino-cranial angle; STT - Sella turcica tilt

Table 4.

The cutoff values for cranial incidence, cranial slope, cranial tilt, sella turcica tilt, and spino-cranial angle

	Cutoff value	Sensitivity (%)	Specificity (%)	P
CI	24°	71.60	77.60	<0.001*
CS	3.27°	57.80	67.60	<0.001*
CT	18.5°	80.40	70.40	<0.001*
STT	12.3°	62.70	69	<0.001*
SCA	80.1°	53.90	64.80	<0.001*

*P<0.05. ROC analysis. The sensitivity and specificity of each angle are also shown. CI - Cranial incidence; CS - Cranial slope; CT - Cranial tilt; SCA - Spino-cranial angle; STT - Sella turcica tilt

DISCUSSION

Bipedalism in the human species has provided several advantages for functionality, however, it increased balance problems including the spine, pelvis, and head. When an individual is standing on both feet, the line between two eyes should be parallel to the horizontal plane and the vertical line should be perpendicular to the floor, thus providing an optimal gaze forward. Several idiopathic or acquired spinal pathologies affect this forward gaze via altering the alignment of the spine, pelvis, or head.

The present study aimed to elucidate the relationship between cervical spinal alignment and the (COG) of the head in patients with CM compared to healthy individuals. Our findings revealed significant differences in several angular measurements between the study and control groups, highlighting distinct biomechanical alterations in CM patients.

Knowledge of the COG of the head is fundamental for analyzing the correlation between the head and spine. The COG is described as the midpoint of a line that passes between the nasion and the inion. This point lies posterior to the sella turcica and superior and slightly anterior to the external acoustic meatus radiologically.[15] The sella turcica may be seen apparently in lateral head and cervical X-rays, therefore, the midpoint of the sella turcica is defined as the cephalometric reference point for evaluation of the correlation between the chin and the skull base and determining its spatial position.[16] Sella turcica is located a few millimeters anterior to the COG of the head and it is a practical parameter for determining the COG of the head.[15]

The cervical region is the most mobile segment of the spine. The upper cervical and subaxial cervical spine have different ranges of motion and are different from the other spinal regions. Moreover, it moves in unity with the thoracolumbar spine for maintaining a normal gaze in horizontal and vertical planes. There are compensation mechanisms for straight positioning of the head in pathologies that alter the positioning of the head and change the COG of the head. Understanding this mechanism is vital for preoperative surgical planning.[13] Amabile et al. showed that the global balance is a constant value between 0 and 5 degrees, however, cervical lordosis and pelvic tilt were higher in elderly patients. They concluded that loss of lumbar lordosis is initially compensated with pelvic retroversion, followed by increasing cervical lordosis.[17]

Numerous studies have documented normal spinal alignment in healthy individuals[14,18,19,20,21] and our study’s control group P values were consistent with these established norms. However, our analysis revealed that the CI, CT, STT, and SCA were significantly higher in CM patients, while the CS angle was lower compared to healthy controls. These findings suggest an anterior displacement of the head’s COG and a more flexed head position in CM patients. Such biomechanical alterations may contribute to the symptomatology of CM, including headaches and neck pain, by changing the load distribution across the cervical spine and craniovertebral junction.

Interestingly, the C7S and C2T angles did not show significant differences between the groups, indicating that the overall cervical curvature was similar in both CM patients and healthy individuals. This suggests that while the cervical spine’s alignment might be preserved, the position and orientation of the head relative to the spine are markedly different in CM patients.

Le Huec et al. reported on the values of CI, CS, CT, SCA, and STT values in healthy individuals to determine the normal angular parameters of the cervical sagittal balance.[21] In this study, they had two reference points concerning the head; the first one was the McGregor line for the skull base and the second was sella turcica for COG of the head. They defined CI, CS, and CT angles using these reference points and performed the measurements. CI may vary between individuals and does not change with different head positions. CT may change with different head positions and be used in conjunction with CS for determining how backward the head is positioned in the sagittal plane. The significant differences in CI, CS, CT, STT, and SCA angles highlight their potential utility in the diagnostic evaluation of CM. The ROC analysis confirmed the sensitivity and specificity of these angular measurements in distinguishing CM patients from healthy individuals, offering a valuable, noninvasive diagnostic tool that can be utilized in clinical practice.

There was a statistically significant difference in angular parameters between the control and study groups, indicating that head positioning and the COG of the head differ in CM patients. The increased CI angle, which is not correlated with head position, suggests an anterior COG in CM patients. The elevated CT angle indicates an anterior head position in the sagittal plane, while the lower CS angle reflects a more horizontal head position. The CS angle shows the skull base’s position relative to the horizontal plane, with positive values when the McGregor line is directed superiorly and anteriorly, neutral when parallel, and negative when inferiorly and anteriorly directed.[21]

The higher CI, CT, STT, and SCA angles observed in CM patients suggest a compensatory mechanism to maintain a horizontal gaze despite altered craniovertebral junction anatomy. This anterior shift in the COG may increase muscular effort and strain on the cervical spine, contributing to CM symptoms. The lower CS angle further indicates a flexed head position, potentially exacerbating strain on posterior cervical structures and the foramen magnum. The SCA angle, defined as a counterpart to the spinosacral angle for evaluating cervical vertebra sagittal balance,[21] and the STT angle, suggested as complementary to the SCA angle, were found to differ significantly between control and patient groups. In contrast, the C2T and C7S angles, correlating with cervical alignment, did not differ significantly between groups. The common reference point for the SCA and STT angles is the sella turcica. The similarity in cervical alignment between groups, alongside the difference in SCA and STT angles, likely stems from the anterior displacement of the head’s COG in CM patients. These results align with our findings regarding CI, CS, and CT measurements, indicating that the altered biomechanics in CM patients are primarily due to the anterior shift of the head’s COG rather than changes in cervical curvature itself.

The variations in angles between the control and study groups result in biomechanical and pathophysiological alterations in the craniovertebral junction. Goel suggested that CM developed secondary to the instability of the craniovertebral junction and hypothesized that facet instability at the atlantoaxial joint resulted in a condition similar to the lumbosacral spondylolisthesis.[9,22] Patients with atlantoaxial joint instability often develop Chiari malformation (CM) and/or basilar invagination (BI). Musculoskeletal alterations related to BI such as short neck, torticollis, platybasia, and Klippel–Feil anomaly may be considered as signs of mild or chronic atlantoaxial instability. This shows that musculoskeletal alterations are not a result of congenital or embryonic dysgenesis, but they cause chronic instability and have a protective effect. It has also been shown that CM and the development of syringomyelia are the results of atlantoaxial instability.[9,11] The angular parameters related to the COG of the head and cervical lordosis may implicate the head positioning and the compensation mechanism of the cervical lordosis and may support the biomechanical theory.

These studies show that a failure or anomaly in craniovertebral junction embryogenesis may predispose the development of CM; thus, the head is positioned more anteriorly, therefore the patients should also be evaluated biomechanically. There are a limited number of studies that present the sagittal balance parameters in patients owing craniovertebral junction pathologies. The correlation between the head position and cervical sagittal balance in CM cases is associated with the altered morphometric angles of the skull base and the anterior displacement and posterior angulation of the head. These data may contribute to the management strategies for patients with CM and accompanying BI.

This study’s retrospective design and reliance on radiographic measurements provide a robust dataset for analyzing cervical alignment in CM. However, it is essential to acknowledge limitations such as the potential for selection bias and the need for further longitudinal studies to assess the impact of surgical intervention on these biomechanical parameters.

CONCLUSION

Our results show that the COG of the head is located more anteriorly in CM patients. Future biomechanical and morphometric studies will investigate the role of this anterior slippage in the natural history and prognosis of CM. Anterior displacement of the COG of the head may be determined from the cervical X-rays in the clinical setting as a screening tool for patients in whom a diagnosis of CM is suspected.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.