abstract Background: Knowledge of the location and morphometry of the lateral venous sinus (transverse and sigmoid) and their relationship with the asterion and other surface anatomic landmarks is imperative for posterolateral approaches to the posterior cranial fossa to avoid vascular structure injury and surgical complications. We undertook this study to determine a secure anatomic area for drilling that allows entry into the posterior cranial fossa without damaging adjacent structures and study the morphometric characteristics of the asterion, the lateral sinus and bony landmarks of the posterolateral surface of the skull. methods: Using a 1.3-mm drill, both sides of 88 dry skulls (176 hemicranias) were drilled. The anatomic landmarks studied were the asterion, apex of the mastoid process, suprameatal spine, Frankfurt horizontal plane, posterior root of the zygomatic arch, external occipital protuberance and its relationship with the transverse sinus. results: Type I asterion is prevalent in 74.4%. In 82.4% of the specimens, the lateral venous sinus can be found lower than the asterion in 12.5% and higher in 5.1%. Conclusions: Using the data from this and other studies, the initial drill must be placed 15 mm below the asterion and 15 mm posterior to it to reduce the risk of injury to the lateral venous sinus

Alejandra Nohemí Hernández-Rodríguez

Carmen Theriot-Girón

Dr Rodrigo E Elizondo Omaña

María Del

Rodolfo Morales-Ávalos

Rodrigo Enrique Elizondo-Omaña

Salvador Galindo-De León

Santos Guzmán-López

CiteSeerX

Volume 81, No. 4, July-August 2013        251
Morphometric characteristics of the asterion and the 
posterolateral surface of the skull: relationship with 
dural venous sinuses and neurosurgical importance
Salvador Galindo-de León,1 Alejandra Nohemí Hernández-Rodríguez,1 Rodolfo Morales-Ávalos,1 
María del Carmen Theriot-Girón,2 Rodrigo Enrique Elizondo-Omaña,1 Santos Guzmán-López1
1 Departamento de Anatomía Humana, Facultad de Medicina y 
Hospital Universitario Dr. José Eleuterio González, Monterrey, 
Nuevo León (NL), Mexico
2 Departamento de Anatomía Humana, Facultad de Odontología, 
Universidad Autónoma de Nuevo León, Monterrey, Nuevo León, 
México
Correspondence:
Dr. Rodrigo E. Elizondo Omaña
Departamento de Anatomía Humana
Facultad de Medicina 
Universidad Autónoma de Nuevo León
Ave. Madero s/n, Col. Mitras Centro 
Monterrey 64460, Nuevo León, México
Tel: (52) 81-83294171 
E-mail: rod_omana@yahoo.com
Received: 1-28- 2013
Accepted: 13-15-2013
abstract
Background: Knowledge of the location and morphometry of the lateral venous sinus (transverse and sigmoid) and their relationship 
with the asterion and other surface anatomic landmarks is imperative for posterolateral approaches to the posterior cranial fossa to avoid 
vascular structure injury and surgical complications. We undertook this study to determine a secure anatomic area for drilling that allows 
entry into the posterior cranial fossa without damaging adjacent structures and study the morphometric characteristics of the asterion, 
the lateral sinus and bony landmarks of the posterolateral surface of the skull.
methods: Using a 1.3-mm drill, both sides of 88 dry skulls (176 hemicranias) were drilled. The anatomic landmarks studied were the 
asterion, apex of the mastoid process, suprameatal spine, Frankfurt horizontal plane, posterior root of the zygomatic arch, external 
occipital protuberance and its relationship with the transverse sinus.
results: Type I asterion is prevalent in 74.4%. In 82.4% of the specimens, the lateral venous sinus can be found lower than the asterion 
in 12.5% and higher in 5.1%.
Conclusions: Using the data from this and other studies, the initial drill must be placed 15 mm below the asterion and 15 mm posterior 
to it to reduce the risk of injury to the lateral venous sinus.
Key words: asterion, posterior cranial fossa, venous sinus, morphometry, skull.
Cir Cir 2013;81:251-255.
introduction
During surgical approaches, consideration should be given 
to the superficial anatomic reference points of the posterior 
cranial fossa that allow the topographic location of the intra-
cranial structures to avoid causing injury or accessing them.
The posterior cranial fossa is framed by important venous 
structures such as the superior, occipital and lateral petrous 
sinuses and its confluence (Herophilus’ press). The lateral 
venous sinus is large and originates in the confluence of the 
sinuses. The right lateral venous sinus tends to be the con-
tinuation of the superior sagittal sinus and the left the con-
tinuation of the straight sinus. Each lateral venous sinus has 
an inferolateral pathway in the lateral sulcus of the occipital 
portion that has a small curve with its convexity where the 
lateral margin of the tentorium is inserted until reaching the 
base of the petrous pyramid (petromastoideo channel). This 
channel is in relationship with the mastoid portion where it 
curves inferiorly and forms an “S”, for which reason it is 
called the sigmoid sinus. It continues its trajectory towards 
the jugular foramen where it extends with the bulb of the 
internal jugular vein. This allows us to recognize three parts 
in the lateral sinus: one horizontal (transverse sinus), an el-
bow or knee and a descending (sigmoid sinus).1
The anatomic points of reference to analyze the topog-
raphy of the posterolateral surface of the skull are asterion, 
external occipital protuberance, suprameatal crest, apex of 
the mastoid process, root of the zygomatic arch, Frankfurt 
horizontal plane, and the mastoid foramen.2-4 These refer-
ence points are of great importance in surgical procedures 
252             Cirugía y Cirujanos
Galindo-de León S et al.
figure 2. a) View of bilateral venous sinuses where one can see: 
confluence of the sinuses (CS), the transverse sinus (TS) and the 
elbow of the transverse sinus (ETS). b) Right unilateral endocranial 
view of the transverse venous sinus. In this view one can better ap-
preciate the “S” trajectory of the sigmoid sinus (SS). 
to locate the site where the initial trepanning will be car-
ried out. The surgical importance of the posterior cranial 
fossa lies in its dense collection of neurovascular structures 
housed in a small, rigid space, which makes the invasive 
approach very delicate and prone to accidents or medical 
errors in surgery.
The proximity of large venous sinuses, emissary veins 
that cross the posterior portion of the occipital squama and 
the large number of noncollapsible veins explain the high 
incidence of bleeding, air embolism, thrombosis and sepsis 
in neurosurgical interventions of the posterior cranial fossa. 
In addition to the possible implications for endovascular 
procedures, these facts should take into consideration by 
the surgeon.5,6
Knowledge of the location and morphometric character-
istics of the lateral venous sinus (transverse and sigmoid) 
and their relationship with it and other superficial anatomic 
landmarks is essential during posterolateral approaches to 
the posterior cranial fossa. This facilitates locating the site 
in which the initial trepanning will be performed (strategic 
drilling), as well as to be able to plan the course of surgery. 
All this gives the neurosurgeon the knowledge necessary to 
avoid injuring these vascular structures, which is a cause of 
surgical failure and subsequent surgical complications with 
severe repercussions.
The aim of this study is to determine a safe anatomic site 
to carry out a trepanning that would allow entry to the pos-
terior cranial fossa without damaging adjacent structures 
and to study the morphometric characteristics of the aste-
rion, the lateral sinus and bony landmarks of the posterolat-
eral surface of the skull.
materials and methods
An observational, cross-sectional descriptive study based 
on the analysis of 176 structurally intact dry hemicraniums 
without any evident pathology or abnormality belonging to 
the Department of Human Anatomy of the School of Medi-
cine and Dentistry of the Universidad Autónoma de Nuevo 
León. Two independent observers analyzed and classified 
the morphology of the asterion based on the accepted con-
ventional classification.7 
a)  Asterion type I: union of the parietomastoid, lambdoid 
and occipitomastoid sutures
b)  Asterion type II: sutural bone (Wormian) located among 
the other bones that make up the asterion
The morphometric characteristics of the posterolateral 
surface of the skull were recorded, among them the distance 
from the asterion towards: a) root of the zygomatic arch 
(A-RZA), b) suprameatal spine (A-SMS), c) apex of the 
mastoid process (A-AMP), d) external occipital protuber-
ance (A-EOP) and e) horizontal Frankfurt plane (A-HFP) 
(Figure 1). 
Also recorded were the morphometry and morphology of 
the sulcus of the lateral venous sinus. The most important 
parameter of the measurement was the width of the lateral 
venous sinus canal in three sections: transverse sinus, elbow 
of the sinus and sigmoid sinus (Figure 2).  At the end the 
center of the asterion in each one of the 176 hemicraniums 
studied was perforated with a drill of 1.3 mm in diameter so 
that it would perforate the bone perpendicular to its surface. 
In cases where the asterion was classified as type II, the per-
foration was carried out in the center of the Wormian bone. 
The distance of the resulting hole on the internal surface of 
the skull (endocranium) was recorded in relation with the 
sulcus of the lateral venous sinus (Figure 3). A Vernier digi-
tal millimeter dial caliper with a precision of 0.01 mm data 
was used for each of the piece studied and all measurements 
were reported in millimeters.
The protocol was approved by the Local Committee for 
Health Research (registration AH12-001). There was no 
figure 1. View of the left side of the posterolateral base of the skull. 
Measurements from the asterion towards: 1. Root of the zygomatic 
arch (A-RZA), 2. Suprameatal spine (A-SMS), 3. Apex of the mastoid 
process (A-AMP), 4. External occipital protuberance (A-EOP) and 5. 
Frankfurt horizontal plane (A-FHP). 
Volume 81, No. 4, July-August 2013          253
Morphometric characteristics of the asterion and dural venous sinus
source of external funding; hence, no conflicts of interest 
are reported.
The measurement parameters were analyzed with the 
IBM computer program SPSS Statistics 20.0 for Windows 
XP version. For each measurement parameter, the mean and 
standard deviation were reported.
results
We analyzed the morphometric characteristics of the aste-
rion, the lateral sinus and bony landmarks of the posterolat-
eral surface of the skull for 176 hemicraniums. The preva-
lence of type I asterion was 74.4%, whereas for type II was 
25.6%. Ranges and averages for the morphometric charac-
teristics are shown in Table 1.
The mean obtained of the widths for the three portions of 
the lateral sinus were width of the transverse sinus 7.87 mm 
(±1.67), width of the elbow of the lateral sinus 9.09 mm 
(±1.66) and width of the sigmoid sinus 8.4 mm (±2.03). The 
distance of the asterion with respect to the lateral venous 
sinus is shown in Table 2. The transverse sinus is located 
in the asterion in 82.4% of the cases, above the asterion in 
12.5% and below it in 5.1% (Figure 4). 
figure 3. a) Right unilateral endocranial view in which the perforation 
(arrow) can be seen, which shows the location of the asterion at the 
level of the transverse venous sinus. b) Left unilateral endocranial 
view in which a perforation (arrow) can be appreciated that shows the 
location of the asterion below the transverse venous sinus.
discussion
In the posterolateral approaches to the posterior fossa, in 
order to avoid injury, it is essential to consider the surface 
topography of the venous sinuses. For the neurosurgeon it is 
extremely useful to know the exact location of these struc-
tures and, in particular, their relationship to bony landmarks 
used in these approaches. Some authors have looked for and 
established these topographical landmarks to locate the dif-
ferent portions of the lateral venous sinus.
table 1. Morphometries of the asterion according to 
the reference point of the cranium
Morphometric 
parameters
Total
Mean
(mm)
SD
(mm)
A-RZA 54.74 4.46
A-AMP 51.53 4.97
A-SMS 44.16 5.81
A-HFP 5.49 3.2
A-OEP 61.51 7.44
Mean and standard deviation (SD) of the distances between the 
asterion and anatomic reference points studied. Distances are 
expressed in millimeters. 
A, asterion; RZA, root of the zygomatic arch; AMP, apex of the 
mastoid process; SMS, suprameatal spine; HFP, horizontal Frankfurt 
plane; OEP, occipital external protuberance.
figure 4. Location of the lateral venous sinus in relationship with the 
asterion (independent of type I and II).
table 2. Location of the TS with respect to different 
types of asterion in three planes: level with the 
asterion, higher than the asterion, 
and below the asterion
Location of the TS
Asterion
TotalType I Type II
Level 60.2% 22.2% 82.4%
Higher 9.7% 2.8% 12.5%
Below 4.5% 0.6% 5.1%
TS, transverse sinus.
254             Cirugía y Cirujanos
Galindo-de León S et al.
In our study, distances obtained from the asterion to bony 
landmarks (root of the zygomatic arch, apex of the mastoid 
process, suprameatal spine, Frankfurt horizontal plane and 
external occipital protuberance) are consistent with those 
found by other researchers (Table 3 ). The location of bony 
landmarks in Mexican skulls coincides with those found in 
other populations.
transverse sinus is variable and somewhat different from 
what is classically described (Table 4).
Martínez et al.1 and Ucerler et al.9 reported that the as-
terion is found in a posterosuperior relationship with the 
mastoid process and, in depth, corresponds with the inferior 
border of the transverse portion of the lateral sinus. There-
fore, in posterolateral approaches to the posterior fossa, 
some neurosurgeons suggest that the opening of the trepan-
ning should be located from below and behind the asterion 
away from the sinus.
Uz et al.10 found that in 54% of the cases the transverse 
sinus was located at the level of the asterion and in 44% 
above it, in the dura of the posterior fossa. For Day et al., 
in between 55 and 61% of the cases the transverse sinus is 
located at the level of the asterion.11-13 For Martínez et al. 
the percentage of cases where the transverse sinus is found 
at the level of the asterior are somewhat higher (87.8%).1
Our findings are consistent with these studies in regard to 
the higher percentages of finding the transverse sinus at the 
level of the asterion (82.4%) and a close similarity with the 
results by Martínez et al., which suggest a high prevalence 
of finding the transverse sinus at the level of the asterion in 
Mexican skulls. Therefore, it is safe to perform initial drill-
ing placed between 15 and 25 mm posteroinferior to the 
asterion. This prevents injuring some neurovascular struc-
tures.
In conclusion, the prevalence of the type I asterion is 
three times greater than that of type II asterion. Therefore, 
there is a 60.2% probability that the transverse sinus is 
found at the level of the asterion. The most relevant bony 
landmarks for locating the asterion are the root of the zygo-
matic arch and the apex of the mastoid process. The trans-
verse sinus is located in 82.4% of cases at the level of the 
table 3. Bony relation of the asterion specific to the 
cranium (different studies carried 
out in various populations)
Authors A-RZA A-AMP A-SMS A-HFP A-OEP
Day et al. 
(2000)11
53.88
(±5.09)
49.20
(±4.68)
43.92
(±4.89)
-- --
Martínez et al. 
(2005)1
55.42
(±4.92)
49.70
(±4.80)
44.80
(±5.61)
-- 64.40
(±6.78)
Ucerler and Govsa 
(2006)9
54.60
(±5.12)
51.12
(±5.06)
45.20
(±5.56)
10.30
(±6.5)
--
Galindo-de León 
et al.
(this study)
(2012)
54.74
(±4.46)
51.33
(±4.97)
44.16
(±5.81)
6.49
(±3.2)
7.44
(±7.44)
Note: Distances are expressed in millimeters. 
A-RZA (asterion-root of the zygomatic arch), A-AMP (asterion-apex 
of the mastoid proces), A-SMS (asterion-suprameatal spine), A-HFP 
(asterion-horizontal Frankfurt plane), A-OEP (asterion-occipital 
external proturberance).
In general, we agree that the confluence of sinuses is be-
tween 5 and 10 mm above the external occipital protuber-
ance. Martínez et al.1 found that the transverse venous sinus 
is located on the outer circumference of the tentorium, on a 
line stretched from the suprameatal spine to the external oc-
cipital protuberance. The sigmoid sinus is located on a line 
drawn from a vertex of the mastoid and follows its superior 
axis, subsequently reaching the asterion. At the crossing 
point of these two lines, the elbow of the lateral sinus is 
found.
These relationships lose their value in patients with de-
velopmental abnormalities of the posterior fossa as in Chiari 
malformation and Dandy-Walker syndrome. What is typical 
is that the tentorium will suffer a caudal displacement due 
to development of the brain and is placed in its definitive 
position at the third month of intrauterine life. Alterations 
in this displacement make the final position of the tentorium 
varied and, therefore, the situation of the sinuses in relation 
to it. 8 Various anatomic studies in fresh cadavers and dry 
skulls show that the relationship of the asterion with the 
table 4. Relationship of the TS with the asterion 
(different studies were done in various populations)
Study
Level of 
TS (%)
Below TS 
(%)
Above 
TS (%)
Day et al.
(2000)11
61.0 39.0 --
Uz A et al.
(2001)10
54.0 44.0 2.0
Martínez F et al.1
(2005)
76.2 23.8 --
Galindo-de León et 
al. (present study)
(2012)
82.4 12.5 5.1
TS, transverse sinus.
Volume 81, No. 4, July-August 2013          255
Morphometric characteristics of the asterion and dural venous sinus
asterion (independent of the type of asterion) so the initial 
drilling should be done away from it. 
With the results obtained it can be observed that 
the transverse sinus has an average width of 7.87 mm 
(±1.67) and the elbow of the transverse sinus 9.09 mm 
(±1.66). Initial drilling should be located from 15 to 
25 mm in the posteroinferior direction of the asterion 
to avoid any type of injury to the venous sinus. The 
zone of maximal safety for initial drilling, which would 
avoid any type of injury to the anatomic structure, is 
from 21.42 mm (±2.38) following a posterior and infe-
rior trajectory to the asterion.
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Morphometric characteristics of the asterion and the posterolateral surface of the skull: relationship with dural venous sinuses and neurosurgical importance

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Morphometric characteristics of the asterion and the posterolateral surface of the skull: relationship with dural venous sinuses and neurosurgical importance

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