Keywords
basal angle - Boogaard’s angle - clivus-canal angle - magnetic resonance imaging -
terminology
Introduction
The atlas, axis, and the occiput bone form the craniovertebral junctions. Craniometric
angles are angles formed between the posterior skull base and the cervical spine.
Basal angle, Boogaard’s angle, and clival angle are frequently used in diagnosing
the craniometric angle malformations, based on radiography/cephalogram or now more
on MRI. Chiari malformation and basilar invagination are two major clinical conditions
due to the craniovertebral junction (CVJ) malformation. Platybasia and hyperlordosis
are often associated with basilar invagination.[1]
[2]
[3] Since the anatomical bony landmarks required for the angle measurements are easily
accessible, it can be used as a standard diagnostic tool in CVJ malformations. In
the literatures, few studies have mentioned the normal range of angles based on comparative
studies. Additionally, few factors like race, sex, age, and height of an individual
can alters the CVJ craniometry.[4]
[5] To the best of our knowledge, no study has been conducted to evaluate the normal
craniovertebral angles among the south Indian population. Since the MRI is an emerging
gold standard and low-radiation exposure investigation in diagnosing the soft-tissue
pathologies, the aim of the study is to define the normal range of basal angle, Boogaard’s
angle, and clival angle for men and women among the south Indian population based
on MRI.
Materials and Methods
One hundred MRI images (50 males and 50 female) were studied retrospectively. MRI
images taken for other clinical conditions like seizure disorders and headache, reported
as normal by neuroradiologist, were taken up for the study. The images were retrieved
from the picture archiving and communication system (PACS) after obtaining approval
from the institute research monitoring and ethical committee (IEC). MRI images were
taken in 1.5 Tesla MR equipment (Magnetom Avanto Siemens, Erlangen Germany) using
a 14-channel head coil. High-resolution, heavily T2-weighted 3D sequence sampling
perfection with application optimized contrasts using different flip-angle evolution
(SPACE) was used to acquire thin 1 mm sections. The scanning parameters were TR/TE–1200/162
milliseconds; section thickness 1 mm; no. of sections–120; FOV 220 mm; flip angle
150°; matrix 192 × 256. Thin section T1-weighted magnetization prepared rapid gradient
echo (MPRAGE) sequence was also acquired with similar parameters for comparison. The
direct sagittal plane was used for better in-plane resolution. True sagittal sections
were ensured by the presence of stalk, infundibular recess, cerebral aqueduct, and
anterior-posterior commissural line in the same visualized midsagittal plane image.
The following measurements were taken in sagittal view:
-
Basal angle–angle between the line extending from nasion to dorsum sellae and from
dorsum sellae to basion ([Fig. 1A])
-
Boogaard’s angle–angle between the line extending from dorsum sellae to basion and
from basion to opisthion ([Fig. 1B])
-
Clival angle–angle between the line extending from dorsum sellae to basion and line
along the posteroinferior surface dens ([Fig. 1C])
Fig. 1 Sagittal heavily T2-weighted 3D sampling perfection with application optimized contrasts
using different flip-angle evolution (SPACE) MRI images of brain in midsagittal plane showing the basal angle (A), Boogaard’s angle (B) and the clivus-canal angle (C).
All the information was collected in a deidentified way. The data was measured thrice,
and the average was taken as final. The same was measured by the second observer to
determine the interobserver variability. Mean and the standard deviation of males
and females was calculated for basal angle, Boogaard’s angle, and clival angle, separately.
Unpaired t-test was used to analyze the significant difference (p < 0.05) between the genders. The intraclass coefficient correlation was used to analyze
the interobserver variability.
Results
[Table 1] shows the mean values of all the parameters. Fig. 2 shows the graphical representation of the parameters. The mean value of basal angle
in males and females are 113°and 114°, respectively. The mean value of Boogaard’s
angle in males and females are 120°and 121°, respectively. The mean value of clival
angle in males and females are 157°and 155°, respectively. There was no statistically
significant difference (p > 0.05) between males and females in all three angles. The
interobserver correlation coefficients for the basal, Boogard’s and clival angle were
0.70, 0.78 and 0.80, respectively.
Table 1
Showing the measurements of skull base angles in normal adult males (50) and females
(50)
|
Angle
|
Mean (100)
|
Male
(mean ± SD)
|
Range
|
Female
(mean ± SD)
|
Range
|
Significance
|
|
Basal angle
|
113
|
113°± 5°
|
111°-114°
|
114°± 4°
|
113°-116°
|
p = 0.13
|
|
Boogaard’s angle
|
120
|
120°± 10°
|
118°-123°
|
121°± 5°
|
119°-123°
|
p = 0.68
|
|
Clivus-canal angle
|
156
|
157°± 9°
|
154°-159°
|
155°± 7°
|
153°-158°
|
p = 0.41
|
Discussion
CVJ angular geometry formed by the skull base and the cervical vertebra can be determined
by craniometric angles like basal angle, Boogaard’s angle, and clivus-canal angle.[2] Anterior skull base and the clivus influences the basal angle and the Boogaard’s
angle, respectively. Basal angle is formed by the line extending from nasion to dorsum
sellae and from dorsum sellae to basion. The mean basal angle was 113°(males 113°±
5°and females 114°± 4°). There was no statistically significant difference between
males and females. The present study correlates well with the MRI study conducted
by Botelho and Ferreira, Koenigsberg et al and Hirunpat et al.[1]
[2]
[4] Boogaard’s angle is formed by the line extending from dorsum sellae to basion and
from basion to opisthion. The mean Boogaard’s angle was 120°(males 120°± 10°and females121°±
5°). There was no statistically significant difference between males and females.
Botelho et al in a previous study in Brazilian population found Boogaard’s angle to
have a slightly greater mean value of 126.[2] Clival angle is formed by the line extending from dorsum sellae to basion and from
posteroinferior surface of axis to posterosuperior surface of dens. The mean measurement
was 156°(males 157°± 9°females 155°± 7°). There was no statistically significant difference
between males and females. This measurement was slightly more than the study conducted
by Botelho and Ferreira (mean 148°) and Smoker (mean 150°).[2]
[6]
CVJ malformation can be of congenital or acquired condition. Few congenital conditions
of craniofacial anomalies are osteogenesis imperfecta, craniocleidodysostosis, Arnold–Chiari
malformation, basilar invagination (type I and type II), and platybasia. Abnormal
skull base flattening or basal angle enlargement was defined as platybasia.[1]
[2] It can be of isolated condition or associated with other conditions. The isolated
condition is mostly asymptomatic. Platybasia associated with basilar invagination
(type II) shows signs of upper cervical spinal cord and brainstem compression. Some
acquired conditions like Paget disease, osteomalacia, rickets, hyperparathyroidism,
localized bone destruction, and trauma can also be associated with CVJ deformation.[1]
[4] Basal angle and Boogaard’s angle are considered as primary angle, because these
angles are not altered by craniocervical posture and balance. The clival angle is
considered as secondary angle in evaluating the craniocervical junction malformation.
In platybasia, clival angle was more acute and the Boogaard’s angle was wide. Along
with this, the basal angle was also wide in basilar invagination.[2] Frade et al[7] measured these craniovertebral transition parameters in normal Brazilian population
and found that basal angle was 128.96°mean (SD 6.51) and the clivus canal angle was
150.5°mean (interquartile range [IQR] 143.2–157.3). Xu and Gong[8] proposed clivus-dens angle as measures in sagittal CT reformation as an alternative
to clivus-canal angle with a better diagnostic performance. Kovero et al[9] studied cephalometric evaluation of various measurements in 54 patients with osteogenesis
imperfecta and normal volunteers including the basal angle (anterior cranial base
angle) and cranio-vertebral angle (measured between nasion-sella line and longitudinal
axis of odontoid process of C2). The basal angle showed significant difference between
normal and those with platybasia due to osteogenesis imperfecta, and the angle was
larger in more severe type of OI than type I. In a study of skull base angle morphometry
of patients with Apert syndrome, Lu et al[10] did not find any significant changes in basal angle.
Skull base development is now considered a related determinant for facial growth or
jaw/maxillomandibular occlusal schemes, and basal angle changes have been studied
in those with dental/skeletal malocclusion.[11] It has been reported that the basal angle is larger or normal in class II malocclusion
and smaller or normal in class III malocclusion.[12] But Guyot et al[12] called the basal angle between the sphenoethmoidal plane and clival plane as clival
angle as described by Landzert first in 1866.
The basal angle had also been correlated with other measurements to quantify the small
posterior fossa seen in Chiari malformations.[13]
[14] The basal angle and Boogaard’s angle were wider in 22 patients with Chiari malformation
even though platybasia was noted only in three patients. Hwang in their report of
12 patients with Chiari malformation described the Boogaard’s angle and angle of the
clivus and showed that this angle and the angle between the supraoccipital and McRae’s
line were significantly larger in Chiari group, indicating the steep clivus of narrow
posterior fossa.
The basal angle is the most commonly used angle in diagnosing the platybasia and is
considered as primary angle. [Table 2] shows the comparison of angles with the previous studies. Knowledge about the normal
angle will be an important tool in planning the treatment involving the skull base.
Since there may be racial differences in the type of skull, the normal craniometric
angles also vary according to the race. The present study is an attempt to look for
the parameter of skull base angles in the normal population, so that it can be used
as a standard tool in the appropriate method of correction of the anomalies. However,
the sensitivity of the normal value is to be evaluated clinically for the south Indian
populations.
Table 2
Showing the comparison of skull base angles with previous studies
|
S. No
|
Author
|
Study
|
Basal angle
|
Clival angle
|
Boogard’s angle
|
|
1.
|
Batista et al[5]
|
CT
|
113.7
|
153.6
|
–
|
|
2.
|
Botelho et al[2]
|
MRI
|
119
|
148
|
126
|
|
3.
|
Koenigsberg et al[1]
|
MRI
|
117
|
–
|
–
|
|
4.
|
Hirunpat et al[4]
|
MRI
|
117
|
–
|
–
|
|
5.
|
Smoker[6]
|
MRI
|
132
|
150
|
–
|
|
6.
|
Xu and Gong[8]
|
CT
|
|
149.6 ± 8.7
|
|
|
7.
|
Hwang et al[14]
|
MRI
|
|
|
117.42
|
|
8.
|
Kovero[9]
|
Cephalogram
|
129.8
|
|
|
|
9.
|
Guyot et al[12]
|
Skull
|
124.14 ± 10.04
|
|
|
|
10.
|
Karagoz et al[13]
|
MRI
|
121 ± 6
|
|
137 ± 6*
|
|
11.
|
Frade et al[7]
|
MRI
|
128.96 (± 6.51)
|
150.14 (+ 15.37)
|
|
|
12.
|
Present study
|
MRI
|
113
|
156
|
120
|
There is confusion in the usage of the terminology with basal angle mentioned as anterior
cranial base angle, Welcher basal angle, and even as clival angle. Similarly, the
Boogaard’s angle is described as angle of the clivus in some studies. There is need
for uniformity and standardization of the terminology among the clinicians of radiology,
neurosurgery and orthopedics apart from the anatomists for better research data management,
which is now widely available due to routine neuroimaging using CT or MRI.
Fig. 2 Graphical representation of craniometric angles in males and females.
