Assessment of the Anterior Clinoid Process and Optic Strut in Chiari Malformation Type I: A Computed Tomography Study

Abstract

Objective  This study aimed to evaluate morphological features of the anterior clinoid process (ACP) and the optic strut (OS) in Chiari malformation Type I (CM-I).

Methods  The study universe consisted of computed tomography images of 41 CM-I patients and 45 normal subjects. Comparison of the parameters for CM-I and the control group was performed with the Student's t -test. A “ p  < 0.05” was accepted as the significance level.

Results  ACP length was smaller in CM-I than the control group ( p  < 0.001). In contrast to ACP length, ACP angle ( p  < 0.001), OS length ( p  = 0.022), and the distance between ACP and OS ( p  = 0.020) were found greater in CM-I in comparison to the control group ( p  < 0.05). ACP width ( p  = 0.233) and OS width ( p  = 0.376) were similar in both groups. ACP pneumatization in CM-I group was found as 12.20%, whereas in the control group as 8.90%. Two different types about the pneumatization were identified in CM-I group (Type 1: 4.9% and Type 2: 7.3%), whereas three different types in the control group (Type 1: 3.3%, Type 2: 4.4%, and Type 3: 1.1%). Relative to ACP, three different types about OS position were identified in CM-I group (Type C: 31.70%, Type D: 64.60%, and Type E: 3.70%) and the control group (Type C: 7.80%, Type D: 64.40%, and Type E: 27.80%).

Conclusion  Shorter ACP, wide-angled ACP, longer OS, and more anteriorly located OS were found in CM-I group compared with the normal group. Our findings showed that the pneumatization of ACP was not affected by CM-I.

Introduction

The optic canal, a part of the sphenoid bone, is a funnel-shaped structure formed by four walls, superior (the lesser wing's superior or anterior root), medial (the sphenoid bone body), inferior (the optic strut, OS: the lesser wing's posterior root), and lateral (the anterior clinoid process, ACP). ^{1 2} Lesions originating from these anatomical structures (e.g., optic glioma or anterior clinoidal meningioma) may result in compression of the optic nerve. ^{1 2 3 4 5} For decompression, transsphenoidal approaches are recommended if lesions arise from the medial or inferior wall, whereas transcranial approaches are recommended if lesions arise from the lateral or superior wall. ^{1 2 5 6 7 8} During implementation of these approaches for decompression, OS and/or ACP are entirely or partly removed. ^{1 3 4 6 9} Furthermore, these two bony structures are of great importance for neurosurgeons as reference points to ensure intraoperative orientation. ^{6 10 11 12} In this context, detailed knowledge about morphological features of ACP and OS is needed to surgeons for carrying out a successful operation. ^{1 7 13 14}

Some authors stated that significant alterations in patients with Chiari malformation Type I (CM-I) should be expected not only in the posterior cranial fossa but also in the entire skull base, probably on account of a mesodermal defect. ^{15 16} For example, Sgouros et al ¹⁵ determined a longer anterior cranial fossa in pediatric subjects with CM-I in comparison to the control group. Nwotchouang et al ¹⁶ found 38% greater volume of the sphenoid sinus of CM-I patients compared with normal subjects. In the light of these information, we think that location, size, and shape of anatomical structures (ACP, OS, the prechiasmatic sulcus, the pterygoid canal, the optic canal, etc.) around the sellar area are significantly affected by CM-I, especially due to the increased sphenoid sinus volume. Changes in morphologies of OS and ACP may affect intraoperative surgical orientation, surgical technique choice, and positioning patient's head to prevent complications (e.g., the ophthalmic artery or optic nerve injury). ^{12 17 18 19 20} However, available information pool containing surgical procedure examinations, morphometric analysis, classifications, and detailed anatomical definitions is mainly conducted on normal subjects. Therefore, the main aim of this computed tomography (CT) study is to determine whether morphologies of ACP or OS are changed in patients with CM-I in comparison to healthy samples.

Materials and Methods

Study Population

In line with the criteria ( Table 1 ), patient folders (including information as follows: sex, age, radiographic images, complaints, diagnosis, treatment procedures, and hospital admission/discharge dates) were examined. The study universe consisted of two different populations as follows: (1) CM-I group and (2) control group (an age–sex-matched set).

Table 1. The inclusion and exclusion criteria for study population.

Criteria	CM-I group	Control group
Inclusion criteria	Patients with CM-I	Patients without malformations (syndromic or genetic)
	Patients without a history of surgical intervention around ACP	Patients without fractures, infections, tumors
	Patients with good quality CT images	Patients without a history of surgical intervention around ACP
		Patients without a history of medical treatment related to ACP
		Patients with good quality CT images
Exclusion criteria	Patients with the other types of Chiari malformation	Patients with malformations (syndromic or genetic)
	Patients with a history of surgical intervention around ACP	Patients with fractures, infections, tumors
	Patients with low-quality CT images	Patients with a history of surgical intervention around ACP
		Patients with a history of medical treatment related to ACP
		Patients with low-quality CT images

Abbreviations: ACP, anterior clinoid process; CM-I, Chiari malformation Type I; CT, computed tomography.

Computed Tomography Protocol

CT images of patients' heads were obtained with a 64-row multidetector scanner (Aquillion 64, matrix: 512 × 512, field of view: 240 mm, pixel size: 0.46 mm, 0.5-mm thick slices, 230 mA, 120 kV, 0.3-mm interval; Toshiba Medical Systems, Tokyo, Japan). The raw data were transferred to a workstation for creating three-dimensional images and then reformatted in axial, coronal, and sagittal planes.

Measured Parameters

The parameters determined to detect morphologies of ACP and OS were as follows: the length (ACP-L), width (ACP-W), and angle (ACP-A) of ACP, the length (OS-L) and width (OS-W) of OS, and the distance between ACP and OS (Dis-ACP-OS). The axial plane images were used to measure ACP-L, ACP-W, ACP-A, and Dis-ACP-OS, whereas the coronal plane images were utilized to measure OS-L and OS-W ( Figs 1 and 2 ). The explanations of the parameters were as follows ^{18 21 22 23} :

Fig. 1.

The photographs show the location of ACP and OS. ACP, anterior clinoid process; DS, dorsum sella; HF, hypophysial fossa; OS, optic strut; SS, sphenoid sinus.

Fig. 2.

The photographs show the parameters. a = ACP-A, b = ACP-L, c = ACP-W, d = Dis-ACP-OS, e = OS-L, and f = OS-W.

• ACP-L: the length between the tip and base of ACP

• ACP-W: the basal width of ACP at the optic canal's medial border

• ACP-A: the angle between the lateral and medial borders of ACP

• OS-L: the length of the inferior wall of the optic canal between ACP and the sphenoid bone body

• OS-W: the width between the anterior and posterior borders of OS at the middle level

• Dis-ACP-OS: the length between ACP tip and the posterior border of OS

Classification of Anterior Clinoid Process Pneumatization

Considering Abuzayed et al ²³ 's study, ACP pneumatization was classified as three types as follows: Type 1, less than 50% pneumatization; Type 2, more than 50% pneumatization but not totally; and Type 3, total pneumatization ( Fig. 3 ).

Fig. 3.

The photographs show the pneumatization degree of ACP. ( A ) No pneumatization, ( B ) Type 1, ( C ) Type 2, and ( D ) Type 3.

Classification of the Position of Optic Strut Relative to Anterior Clinoid Process

Considering Lee et al ¹⁸ 's study, the location of OS according to ACP was classified as six types as follows: Type A, anterior to ACP root; Type B, anterior ⅕ of ACP; Type C, anterior ⅖ of ACP; Type D, anterior ⅗ of ACP; Type E, anterior ⅘ of ACP; and Type F, posterior ⅕ of ACP.

Statistical Analysis

All measurements were performed by two independent investigators with over 10 years of experience. The first researcher measured the parameters, and the dataset was used to perform statistical evaluations. A second researcher repeated the measurements twice in randomly selected cases (20% of all subjects) to check the reproducibility of measurements. The interobserver reproducibility was tested with the intraclass correlation coefficients (ICC), whereas the intraobserver reproducibility with the paired sample t -test. Using SPSS for the Windows version 22.0 package program (IBM, Armonk, New York, United States), the statistical analyses were performed. The normality check for the measured parameters was performed by Shapiro–Wilk test. Each ACP or OS was counted as an independent case, and comparison of the parameters for CM-I and the control group was performed with the independent sample t -test ( Table 2 ). To understand whether there is an individual difference, the averages of the right and left ACPs and OSs in each patient were taken, and the comparison of the two groups was made by the independent sample t -test ( Table 3 ). Comparisons of the measured parameters for male–female and right–left sides were performed with the independent sample t -test and the paired sample t -test, respectively ( Table 4 ). Distribution of ACP pneumatization and OS position types in CM-I and the control group was evaluated with the chi-square test ( Tables 5 and 6 ). A “ p  < 0.05” was accepted as the significance level.

Table 2. Comparison of Chiari malformation Type I and the control group.

Parameters	CM-I ( N  = 82)	Control ( N  = 90)	p
ACP-L	11.12 ± 2.09	12.79 ± 2.43	<0.001
ACP-W	5.55 ± 0.83	5.71 ± 0.93	0.233
ACP-A	27.23 ± 5.65	21.86 ± 4.71	<0.001
OS-L	4.49 ± 0.87	4.23 ± 0.59	0.022
OS-W	3.08 ± 0.78	2.97 ± 0.72	0.376
Dis-ACP-OS	6.04 ± 0.92	5.60 ± 1.46	0.020

Abbreviations: ACP, anterior clinoid process; ACP-A, angle of ACP; ACP-L, length of ACP; ACP-W width of ACP; CM-I, Chiari malformation Type I; Dis-ACP-OS, distance between ACP and OS; OS, optic strut; OS-L, length of OS; OS-W, width of OS.

Note: N  = numbers of sides.

Table 3. Comparison of Chiari malformation Type I and the control group according to the averages of the two anterior clinoid processes and optic strut in each subject.

Parameters	CM-I ( N  = 41)	Control ( N  = 45)	p
ACP-L	11.12 ± 2.05	12.79 ± 2.38	0.001
ACP-W	5.55 ± 0.79	5.71 ± 0.89	0.380
ACP-A	27.23 ± 5.51	21.86 ± 4.68	<0.001
OS-L	4.49 ± 0.85	4.23 ± 0.56	0.095
OS-W	3.08 ± 0.72	2.98 ± 0.69	0.508
Dis-ACP-OS	6.04 ± 0.85	5.60 ± 1.44	0.091

Abbreviations: ACP, anterior clinoid process; ACP-A, angle of ACP; ACP-L, length of ACP; ACP-W width of ACP; CM-I, Chiari malformation Type I; Dis-ACP-OS, distance between ACP and OS; OS, optic strut; OS-L, length of OS; OS-W, width of OS.

Note: N  = numbers of subjects.

Table 4. Sex and side comparisons for Chiari malformation Type I and the control group.

	Parameters	Females ( N  = 50)	Males ( N  = 32)	p	Right ( N  = 41)	Left ( N  = 41)	p
CM-I	ACP-L	11.32 ± 2.03	10.81 ± 2.18	0.291	11.18 ± 2.08	11.06 ± 2.13	0.806
	ACP-W	5.31 ± 0.69	5.91 ± 0.92	0.001	5.71 ± 0.85	5.38 ± 0.79	0.078
	ACP-A	27.17 ± 5.97	27.34 ± 5.20	0.894	27.06 ± 5.59	27.40 ± 5.78	0.787
	OS-L	4.31 ± 0.90	4.78 ± 0.76	0.017	4.46 ± 0.79	4.52 ± 0.96	0.740
	OS-W	3.03 ± 0.80	3.14 ± 0.77	0.538	3.10 ± 0.77	3.06 ± 0.81	0.816
	Dis-ACP-OS	5.98 ± 0.93	6.13 ± 0.91	0.478	6.03 ± 0.97	6.05 ± 0.88	0.916
	Parameters	Females ( N  = 52)	Males ( N  = 38)	p	Right ( N  = 45)	Left ( N  = 45)	p
Control	ACP-L	12.86 ± 2.53	12.70 ± 2.31	0.768	12.76 ± 2.41	12.83 ± 2.47	0.888
	ACP-W	5.66 ± 1.05	5.77 ± 0.73	0.582	5.80 ± 0.94	5.62 ± 0.91	0.348
	ACP-A	22.35 ± 5.61	21.19 ± 3.02	0.250	21.77 ± 4.74	21.95 ± 4.73	0.852
	OS-L	4.12 ± 0.63	4.38 ± 0.50	0.039	4.23 ± 0.58	4.23 ± 0.60	0.893
	OS-W	3.05 ± 0.73	2.88 ± 0.69	0.274	2.98 ± 0.75	2.97 ± 0.69	0.953
	Dis-ACP-OS	5.31 ± 1.44	5.99 ± 1.42	0.030	5.51 ± 1.41	5.69 ± 1.52	0.554

Abbreviations: ACP, anterior clinoid process; ACP-A, angle of ACP; ACP-L, length of ACP; ACP-W width of ACP; CM-I, Chiari malformation Type I; Dis-ACP-OS, distance between ACP and OS; OS, optic strut; OS-L, length of OS; OS-W, width of OS.

Note: N  = numbers of sides.

Table 5. Distribution of anterior clinoid process pneumatization in Chiari malformation Type I and the control group.

ACP pneumatization	CM-I	Control	Total	p
ACP pneumatization absence	72 (87.80%)	82 (91.10%)	154	0.619
ACP pneumatization presence	10 (12.20%)	8 (8.90%)	18
Total	82	90	172

Abbreviations: ACP, anterior clinoid process; CM-I, Chiari malformation Type I.

Table 6. Distribution of optic strut location types in Chiari malformation Type I and the control group.

Types	CM-I	Control	Total	p
Type C	26 (31.70%)	7 (7.80%)	33	<0.001
Type D	53 (64.60%)	58 (64.40%)	111
Type E	3 (3.70%)	25 (27.80%)	28
Total	82	90	172

Abbreviation: CM-I, Chiari malformation Type I.

Results

CM-I group consisted of CT images of 41 patients (average age: 20.24 ± 15.14 y; sex: 25 females and 16 males) admitted to the university hospital between 2010 and 2022. In addition, CT images of 45 normal subjects (average age: 21.16 ± 18.68 y; sex: 26 females and 19 males) admitted to the university hospital with different grievances (blow to head, falling from high, etc.) in 2021 were included to the investigation as the control group. Our findings were as follows:

• The intraobserver ( p  > 0.05) and interobserver (ICC score = 0.940 − 0.990 for the parameters, p  < 0.001) analyzes displayed that the reproducibility was excellent.

• Compared with CM-I, ACP-L was greater in the control group ( p  < 0.001), but ACP-A ( p  < 0.001), OS-L ( p  = 0.022), and Dis-ACP-OS ( p  = 0.020) were smaller. ACP-W ( p  = 0.233) and OS-W ( p  = 0.376) were similar in both groups ( Table 2 ).

• Considering the averages of the two ACPs and OS in each subject, compared with CM-I, ACP-L was greater in the control group ( p  = 0.001), but ACP-A was smaller ( p  < 0.001). The other parameters were similar in both groups ( p  > 0.05; Table 3 ).

• In CM-I group, ACP-W ( p  = 0.011) and OS-L ( p  = 0.017) were smaller in females than males, but in the control group, OS-L ( p  = 0.039) and Dis-ACP-OS ( p  = 0.030) were smaller in females than males ( Table 4 ). All parameters were similar statistically on both sides for CM-I and the control group ( p  > 0.05; Table 4 ).

• In CM-I group, ACP pneumatization was identified in 10 (12.20%) sides. The pneumatization in the control group was determined in eight (8.90%) sides. Distribution rate of ACP pneumatization relative to both groups was presented in Table 5 , which displayed that the pneumatization was not affected by CM-I ( p  = 0.619).

• Two different types about the pneumatization of ACP were identified in CM-I group (Type 1: 4 sides, 4.9% and Type 2: 6 sides, 7.3%), whereas three different types in the control group (Type 1: 3 sides, 3.3%; Type 2: 4 sides, 4.4%; and Type 3: 1 side, 1.1%).

• Three different types about OS position relative to ACP were identified in CM-I group (Type C: 26 sides, 31.70%; Type D: 53 sides, 64.60%; and Type E: three sides, 3.70%), similar to the control group (Type C: 7 sides, 7.80%; Type D: 58 sides, 64.40%; and Type E: 25 sides, 27.80%). Distribution rate of OS position types relative to both groups was presented in Table 6 , which displayed that the location of OS was affected by CM-I ( p  < 0.001).

Discussion

The prevalence of CM-I, the downward herniation of the cerebellar tonsil through the foramen magnum, is presented as 0.24 to 3.6% in the literature. ²⁴ Mutations in some genes such as COL4A1, SLC4A9, and OLFML2A, or chromosomal regions such as 11p15.4 may be associated with the pathogenesis of CM-I. ²⁵ Deviations in the development of occipital somite arising from the paraxial mesoderm are considered the main cause of CM-I. ²⁶ The most affected area in CM-I is the bony components of the posterior fossa, of which volume is approximately 23% smaller in comparison to normal subjects. ^{26 27} Overcrowding of the hindbrain caused by shrinking volume results in a wide variety of signs (gait instability, pain, hoarseness, hearing loss, vertigo, etc.) in CM-I. ^{26 27 28 29 30 31} However, Sgouros et al ¹⁵ found longer anterior fossa in children with CM-I compared with the control group and claimed that a mesodermal defect in such cases results in significant alterations not only in the posterior fossa but also in the entire skull base. For instance, Nwotchouang et al ¹⁶ observed 38% increased sphenoid sinus volume in CM-I (9.3 ± 3.0 cm ³ ) compared with the control group (6.7 ± 1.9 cm ³ , p  < 0.001). On the other hand, after finding enlarged pituitary gland in magnetic resonance imaging scans of some adult patients with Chiari malformation Type II (CM-II), Patel et al ³² studied systematically on the sella morphology in 21 patients for the first time in the literature. They observed longer tuberculum sella, taller pituitary gland without any pathology, shallow sella, and shorter dorsum sella in CM-II compared with the normal group; thus, they reported that normal pituitary gland, which appears to be slightly taller owing to the shallow sella, may be mistakenly interpreted as pituitary enlargement (e.g., pituitary adenoma) in CM-II patients. ³² Considering the observations in these studies, we think that dimensions and positions of all structures (including ACP and OS) around the sellar region in CM-I or CM-II patients show great differences relative to healthy subjects.

A thorough understanding of the sellar region's complex anatomy is considered as one of the substantial ways to refrain technical challenge during cure of pathological entities (optic glioma, meningiomas, aneurysms, etc.). ^{1 5 8 33} Owing to proximity of ACP and OS with neurovascular structures (e.g., the ophthalmic artery, oculomotor nerve, trochlear nerve, internal carotid artery, and optic nerve), the risk of iatrogenic injuries during operation (e.g., anterior clinoidectomy) should be considered by neurosurgeons. ^{6 9 11 17 23 33 34 35} Morphologies of OS and ACP may affect intraoperative surgical orientation, surgical technique choice, and positioning patient's head. ^{12 17 18 19 20 21 23 36 37} For example, Cecen et al ²¹ divided ACP into different types considering ACP-L, ACP-W, and ACP-A. They suggested that the wide, short, and wide-angled configuration (16.5%, 40 cases) may call for an easier surgery, whereas the narrow, long, and narrow-angled configuration (17.8%, 43 cases) for a more complex surgical processing. ²¹ Hereby, Cecen et al ²¹ proposed preoperative radiologic analysis to identify different configurations to prevent iatrogenic injuries. Furthermore, ACP pneumatization degree is of major significance for neurosurgeons in preventing complications (e.g., opening of sinuses). ^{23 38} To reduce mortality–morbidity rates, some authors suggested that a detailed morphological dataset (containing surgical procedure examinations, morphometric analysis, classifications, and detailed anatomical definitions) about ACP and OS may be helpful for surgeons. ^{1 7 10 21 23 39} Nevertheless, the current literature is mainly focused on subjects without any malformations. In this context, our data conducted on CM-I patients may be useful for neurosurgeons to obtain a successful and efficient outcome.

ACP-W was measured as 5.55 ± 0.83 mm for CM-I and 5.71 ± 0.93 for the control group ( p  = 0.233). OS-W was measured as 3.08 ± 0.78 mm for CM-I and 2.97 ± 0.72 for the control group ( p  = 0.376). Our ACP-W and OS-W values for both groups were compatible with previous studies ( Table 7 ), ^{10 12 18 19 21 22 39 40 41 42 43 44 45 46 47 48 49} in where the mean data range was given as 5.46 to 12.40 mm for ACP-W and as 2.40 to 3.88 mm for OS-W. Our data displayed that these two parameters were not affected by CM-I. However, ACP-L was smaller in CM-I (11.12 ± 2.09 mm) than the control group (12.79 ± 2.43 mm; p  < 0.001). Our average values for both groups were compatible with previous studies ( Table 7 ), ^{10 12 18 19 21 22 39 40 41 42 44 45 46 47 48 49 50} in where the mean data range was presented between 5.30 and 12.50 mm. In contrast to ACP-L value, ACP-A, OS-L, and Dis-ACP-OS were found greater in CM-I than the control group. Our ACP-A values for CM-I (27.23 ± 5.65 degrees) and the control group (21.86 ± 4.71 degrees) were distinctly smaller in comparison to literature data ( Table 7 ), ^{21 49} in where the mean data range was presented between 39.67 and 42.56 degrees. The main reason for this situation may be the use of different reference points during ACP-A measurement. OS-L was measured as 4.49 ± 0.87 mm for CM-I and 4.23 ± 0.59 for the control group ( p  = 0.022). Dis-ACP-OS was measured as 6.04 ± 0.92 mm for CM-I and 5.60 ± 1.46 for the control group ( p  = 0.020). Our values of these parameters for both groups were compatible with previous studies ( Table 7 ), ^{10 18 41 43 48 49 50} in where the mean data range was given as 3.94 to 6 mm for OS-L and as 5.80 to 13.84 mm for Dis-ACP-OS. On the other hand, the wide range between average values given in the previous articles may be owing to differences in study populations (e.g., age, sex, region, and side) or methods (CT, cone-beam computed tomography, digital caliper, etc.). ^{10 20 21 40 46 49 51} For instance, Adanir et al ⁴⁹ stated that ACP-L and ACP-W increased and ACP-A narrowed in the transition from fetal life to adulthood, considering Beger et al ²⁰ 's fetal study (ACP-L: 3.67 ± 0.82 mm, ACP-W: 4.65 ± 0.94 mm, and ACP-A: 59.57 ± 8.03 degrees). Cecen et al ²¹ observed statistically greater ACP-L and ACP-W values on patients (CT values) in comparison to dry skulls (measurement with digital caliper). We did not find differences between the measurements in terms of side, but Lee et al ¹⁸ observed greater ACP-L and ACP-W values on the right side compared with the left side. In this study, ACP-W in CM-I group, Dis-ACP-OS in the control group, and OS-L in both groups were smaller in females than males. Adanir et al ⁴⁹ observed smaller Dis-ACP-OS, ACP-L, ACP-W, and ACP-A values in females than males. Moreover, Sabanciogullari et al ⁴⁶ showed smaller ACP-L and ACP-W in females compared with males.

Table 7. The literature data related to the measured parameters.

Study	Region	Specimen	N	Age	Technique	ACP-L	ACP-W	ACP-A	OS-L	OS-W	Dis-ACP-OS
Andaluz et al 40	United States	Cadaver	20	Adult	Anatomic	8.67 ± 2.63	6.57 ± 1.68	–	–	–	–
Boyan et al 10	Türkiye	Dry skull	68	Unknown	Anatomic	11.50 ± 1.90	12.40 ± 2.10	–	–	–	6.90 ± 1.60
Cecen et al 21	Türkiye	Dry skull	–	Adult	Anatomic	12.50 ± 3.71	10.93 ± 2.25	42.09 ± 9.20	–	–	–
		Patient	242	18–84 y	CT scan	10.74 ± 3.35	8.05 ± 1.74	39.67 ± 12.64	–	–	–
Cheng et al 41	China	Patient (R)	102	19–70 y	CT scan	10.76 ± 1.87	–	–	5.02 ± 0.98	3.67 ± 0.55	–
		Patient (L)	102			10.43 ± 1.99	–	–	4.89 ± 0.76	3.88 ± 0.61	–
Dagtekin et al 39	Türkiye	Dry skull (R)	25	Unknown	Anatomic	9.7 (6.1–12.8)	7.3 (5.6–11.3)	–	–	–	–
		Dry skull (L)	25			9.6 (6.3–11.8)	7.2 (5.3–10.5)	–	–	–	–
Gupta and Priya 50	India	Dry skull (R)	30	Adult	Anatomic	11.1 ± 1.49	–	–	3.94 ± 1	–	–
		Dry skull (L)	30			11.61 ± 2.07	–	–	3.97 ± 1.3	–	–
Hayashi et al 43	Japan	Cadaver	66	Adult	Anatomic	–	–	–	5.4 (2.8–9.5)	2.4 (0.3–4.2)	–
Hunnargi et al 44	India	Dry skull (R)	25	Adult	Anatomic	10.68 ± 1.90	12.4 ± 2.58	–	–	–	–
		Dry skull (L)	25			9.96 ± 1.71	11.12 ± 1.81	–	–	–	–
Huynh-Le et al 22	Japan	Cadaver	55	18–91 y	Anatomic	5.3 (4–6.8)	5.6 (4–7.8)	–	6 (2.8–8.9)	3.1 (2–4.7)	5.8 (2.3–8)
Kapur and Mehic 12	Bosnia	Dry skull (MR)	109	23–91 y	Anatomic	9.9 ± 1.6	9.4 ± 1.4	–	–	–	–
		Dry skull (ML)	109			9.3 ± 1.4	9.1 ± 1.7	–	–	–	–
		Dry skull (FR)	91	19–84 y	Anatomic	9.3 ± 1.6	8.7 ± 1.5	–	–	–	–
		Dry skull (FL)	91			8.9 ± 2	8.3 ± 2.1	–	–	–	–
Lee et al 18	Korea	Dry skull	120	Adult	Anatomic	9.18 ± 1.55	9.63 ± 1.49	–	–	–	1.3–9.3
Lee et al 45	Korea	Cadaver	20	Adult	Anatomic	7.61 ± 1.50	10.82 ± 1.82	–	–	–	–
Sabanciogullari et al 46	Türkiye	Patient (MR)	439	0–90 y	CT scan	8.87 ± 2.62	11.08 ± 2.39	–	–	–	–
		Patient (ML)	439			8.93 ± 2.64	10.98 ± 2.35	–	–	–	–
		Patient (FR)	592	0–90 y	CT scan	8.32 ± 2.40	10.80 ± 2.27	–	–	–	–
		Patient (FL)	592			8.34 ± 2.35	10.53 ± 2.07	–	–	–	–
Sibuor et al 47	Kenya	Dry skull	102	Unknown	Anatomic	10.92 ± 2.79	10.43 ± 2.67	–	–	–	–
da Costa et al 42	Brazil	Patient	597	0–90 y	CT scan	10.31 ± 2.1	7.7 ± 1.73	–	–	–	–
Souza et al 48	India	Dry skull (MR)	13	Unknown	Anatomic	12.07 ± 1.77	10.30 ± 1.93	–	–	–	13.46 ± 1.21
		Dry skull (ML)	13			12.46 ± 1.39	11.23 ± 1.57	–	–	–	13.84 ± 1.29
		Dry skull (FR)	14	Unknown	Anatomic	12.21 ± 1.69	10.64 ± 1.79	–	–	–	13.28 ± 0.95
		Dry skull (FL)	14			12.28 ± 1.62	11.35 ± 1.34	–	–	–	12.85 ± 1.30
Suprasanna et al 19	India	Patient	95	18–84 y	CT scan	11.20 ± 0.95	10.65 ± 0.79	–	–	–	–
Beger et al 20	Türkiye	Fetus	48	16–28 gw	Anatomic	3.67 ± 0.82	4.65 ± 0.94	59.57 ± 8.03	1.73 ± 0.48	1.55 ± 0.33	1.22 ± 0.60
Adanir et al 49	Türkiye	Adult subject	800	36.49 y	CBCT	10.56 ± 2.42	5.46 ± 1.31	42.56 ± 14.68	–	–	6.60 ± 1.50
This study	Türkiye	CM-I	82	20.24 y	CT	11.12 ± 2.09	5.55 ± 0.83	27.23 ± 5.65	4.49 ± 0.87	3.08 ± 0.78	6.04 ± 0.92
		Control	90	21.16 y	CT	12.79 ± 2.43	5.71 ± 0.93	21.86 ± 4.71	4.23 ± 0.59	2.97 ± 0.72	5.60 ± 1.46

Abbreviations: ACP, anterior clinoid process; ACP-A, angle of ACP; ACP-L, length of ACP; ACP-W width of ACP; CBCT, cone-beam computed tomography; CM-I, Chiari malformation Type I; CT, computed tomography; Dis-ACP-OS, distance between ACP and OS; FL, female left; FR, female right; gw, gestational weeks; L, left; ML, male left; MR, male right; OS, optic strut; OS-L, length of OS; OS-W, width of OS; R, right.

Note: N  = numbers of sides.

In CM-I group, three different types related to OS position relative to ACP were identified from the largest to the smallest as follows: Type D (64.60%) > Type C (31.70%) > Type E (3.70%). Similarly, three different types were identified in the control group as follows: Type D (64.40%) > Type E (27.80%) > Type C (7.80%). These findings proved that the location of OS relative to ACP was affected by CM-I ( p  < 0.001). The attachment site of OS to ACP in CM-I is located more anteriorly in comparison to the control group. This is due to that smaller ACP-L (11.12 ± 2.09 mm for CM-I, 12.79 ± 2.43 mm for the control) and greater Dis-ACP-OS (6.04 ± 0.92 mm for CM-I and 5.60 ± 1.46 for the control) were observed in CM-I compared with the normal group. In this study, Type A, Type B, and Type F were not found in both groups. The incidence rates in both groups were incompatible with the previous adult data ( Table 8 ), ^{12 18 19 49} in where the incidence rates were given as 0 to 4.3% for Type A, as 3.16 to 14.5% for Type B, as 42 to 48.95% for Type C, as 27.5 to 37.7% for Type D, as 4.12 to 8.42% for Type E, and as 0 to 2.9% for Type F. Moreover, Beger et al ²⁰ identified three different types in fetuses as follows: Type E (68.75%) > Type D (16.67%) > Type F (14.58%). Type C is the most common attachment area of OS to ACP in adult subjects, whereas Type E in fetal samples. Therefore, OS locates more posteriorly in fetuses compared with adult subjects. ²⁰ Considering that the attachment site of OS to ACP may affect intraoperative surgical orientation, surgical technique choice, and positioning patient's head to prevent complications, ^{12 17 18 19 20} we think that neurosurgeons should not ignore the more anterior localization of OS in CM-I patients.

Table 8. The literature data related to the location of optic strut according to anterior clinoid process.

Study	Kapur and Mehic 12		Suprasanna et al 19	Lee et al 18		Beger et al 20	Adanir et al 49	This study
Region	Bosnia and Herzegovina		India	Korea		Türkiye	Türkiye	Türkiye
Specimen	Dry skulls		Patients	Dry skulls		Fetuses	Healthy subjects	CM-I	Control
Age	19–91 y		40.73 ± 15.94 y	Adult		21.54 ± 3.11 wk	36.49 ± 15.91 y	20.24 ± 15.14 y	21.16 ± 18.68 y
Sex	109 M, 91 F		49 M, 46 F	Unknown		14 M, 10 F	200 M, 200 F	16 M, 25 F	19 M, 26 F
Number of sides	(Right) 200	(Left) 200	190	(Right) 69	(Left) 69	48	800	82	90
Techniques	Anatomic		CT scans	Anatomic		Anatomic	CBCT	CT	CT
Type A	1.4%	4.3%	0	1.4%	4.3%	0	3.64%	0	0
Type B	11.6%	14.5%	3.16%	11.6%	14.5%	0	13.12%	0	0
Type C	42%	47.8%	48.95%	42%	47.8%	0	43.00%	31.70%	7.80%
Type D	37.7%	27.5%	37.37%	37.7%	27.5%	16.67%	36.12%	64.60%	64.40%
Type E	4.4%	5%	8.42%	4.3%	5.9%	68.75%	4.12%	3.70%	27.80%
Type F	2.9%	0.9%	2.11%	2.9%	0	14.58%	0	0	0

Abbreviations: CBCT, cone-beam computed tomography; CT, computed tomography; F, female; M, male.

ACP pneumatization in CM-I group was found as 12.20%, whereas in the control group as 8.90%. Distribution rate of ACP pneumatization relative to both groups displayed that the pneumatization was not affected by CM-I ( p  = 0.619). The incidence rates in both groups were compatible with the literature data, ^{23 37 46 49 52 53 54 55 56 57 58 59} in where the pneumatization rate of ACP was presented as 4 to 29.3%. According to Abuzayed et al ²³ 's classification, two different types about the pneumatization were identified in CM-I group as follows: Type 2 (7.3%) > Type 1 (4.9%), and three different types were identified in the control group as follows: Type 2 (4.4%) > Type 1 (3.3%) > Type 3 (1.1%). The incidence ranges of Abuzayed et al ²³ 's types in this study were compatible with the literature ( Table 9 ), ^{23 41 49 60} in where the incidence ranges were given as 6.6 to 14% for Type 1, 3.4 to 12% for Type 2, and 2 to 5.23% for Type 3.

Table 9. Literature data related to anterior clinoid process pneumatization types reported by Abuzayed et al ²³ .

Study	Region	Sample	N	Age	Method	Side	NP	Type 1	Type 2	Type 3
Cheng et al 41	China	Adult subjects	204	46.34 (19–70) y	CTA	–	87.7%	6.9%	3.4%	2%
Abuzayed et al 23	Türkiye	Adult subjects	1296	41 ± 19.3 y	MDCT	–	87.4%	6.6%	3.5%	2.5%
Cetinkaya et al 60	Türkiye	Patients	200	13–78 y	MDCT	Right	73%	12.5%	12%	2.5%
			200			Left	70.5%	14%	11.5%	4%
Adanir et al 49	Türkiye	Adult subjects	800	36.49 ± 15.91 y	CBCT	–	78.87%	8.87%	7%	5.26%
This study	Türkiye	CM-I	82	20.24 ± 15.14 y	CT	–	87.8%	4.9%	7.3%	0
		Control	90	21.16 ± 18.68 y	CT	–	91.2%	3.3%	4.4%	1.1%

Abbreviations: CBCT, cone-beam computed tomography; CM-I, Chiari malformation Type I; CTA, computed tomography angiography; MDCT, multidetector-row computed tomography; NP, no pneumatization.

Note: N  = numbers of sides.

Conclusion

We observed shorter ACP, wide-angled ACP, and longer OS in CM-I patients. The attachment site of OS to ACP in CM-I was located more anteriorly in comparison to the control group; thus, the location of OS was affected by CM-I. However, the distribution rate of ACP pneumatization relative to both groups showed that the pneumatization was not affected by CM-I. In our opinion, the size and shape of ACP and OS, and positional relationship between these bony structures should be evaluated during preoperative radiologic examinations to safely remove ACP and/or OS.