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J Korean Neurosurg Soc 36 : 353-357, 2004
KISEP
Clinical Article
Microanatomical Study of the Extradural
Middle Fossa Approach for Preventing
Cochlear Damage
Sang Myung Jung, M.D., Suk Jung Jang, M.D., Ph.D.,
Tae Hyoung Ahn, M.D.
Department of Neurosurgery, College of Medicine, Chosun University, Gwangju, Korea
Objective : The objective is to describe the relationship of anatomical landmark required for the middle fossa
approach to preservation of hearing.
Methods : Dissection of 16 fixed human cadaveric heads was performed. we identified a rhomboid-shaped
middle fossa landmarks that serve as a guide to minimize cochlea injury. The points of this construct are as follows ;
1) the junction of the greater superficial petrosal nerve and the trigeminal nerve ; 2) the lateral edge of the porus
trigemius ; 3) the intersection of the petrous ridge and arcuate eminence ; and 4) the intersection of the lines
extended along the axes of the greater superficial petrosal nerve and arcuate eminence. Mean, minimum, and
maximum measurements of all distances were determined.
Results : The average cochlea-geniculate ganglion distance measured in the dissected specimens was 3.0mm±
0.8 with a range of 1.2 to 4.1mm. The average cochlea-petrous carotid genu distance was 2.9mm±0.9 with a
range of 1.2 to 4.0mm. The average cochlea-internal acoustic meatus distance measured in the dissected
specimens was 9.0mm±0.5 with a range of 7.8-10.9mm. The average cochlea-mandibular nerve distance
measured was 9.4mm±0.4 with a range of 7.6-11.3mm.
Conclusion : The middle fossa approach requires special knowledge of the anantomy to reduce the risk of
damage to cochlea. It is important that the surgeon understand the surgical anantomy. The present study describes
the simple geometric construct that proposes to assist in locating the cochlea.
KEY WORDS : Middle fossa approach·Cochlea.
Introduction
M
iddle fossa approach is indispensable in operating
lesion on petrous apex and internal auditory meatus
with preservation of hearing and recently it has been used for
decompression of labyrinthine facial nerve for Bell's palsy
patients1). But, this surgical approach has many difficulties
because important structures are concentrated on operative
region and exact anatomical index which can avoid cochlea
damage for preservation of hearing was not reported.
Therefore the authors performed microanatomical study of
middle fossa structure using human cadaver for the
preservation of cochlea during operation, set position of
Received：April 22, 2004 Accepted：July 13, 2004
Address for reprints：Suk Jung Jang, M.D., Ph.D., Department
of Neurosurgery, College of Medicine, Chosun University, 588
Seosuk-dong, Dong-gu, Gwangju 501-717, Korea
Tel : 062) 220-3120, 3126, Fax : 062) 227-4575
E-mail : [email protected]
petrous bone and internal structure of important middle fossa
as anatomical area of rhomboid shape, measured distance
between each structures and examined anatomical index
which can avoid damage of cochlea in operating middle
fossa.
Materials and Methods
T
he author performed morphometric analysis with both
sides of 16 fixed human cadaver using operating
microscope, electric drill with cutting and diamond burrs and
microsurgical instruments for dissection. The authors set
rhomboid structure to remove bone in approaching middle
fossa without damage of structure for preservation of hearing,
measured each distance using a small caliper(Fig. 1) and
indicated all measured values with mean±standard deviation.
Surgical approach
The authors fixed cadaver head on 60 degree tilting position
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Extradural Middle Fossa Approach
Fig. 1. A small caliper.
on operating table using Mayfield three pin headrest and
performed inversed question mark shaped scalp incision.
Temporalis muscle was dissected from zygomatic bone and
additional removal of zygomatic bone was not followed. The
temporalis muscle and fascia are then elevated subperiosteally
and reflected anteriorly and held in place by large fish hooks.
An approximately 4×4cm rectangular bone flap was then
cut, lying one-third behind and two-third in front of the
external auditory canal. The inferior margin of the
craniectomy was flush with the temporal fossa floor. The
bony margin along the middle fossa floor was drilled flat to
alleviate any obstruction to a flat angle of view along the
petrous ridge.
Dural elevation was performed along the petrous ridge,
identifying the arcuate eminence(AE) as the primary
landmark. Elevation was then carried out anteromedially to
uncover Greater superficial petrosal nerve (GSPN) and
Fig. 2. Illustration of the left side extradural middle fossa floor with the
four landmarks(1 : AE at petrous ridge, 2 : porus trigeminus, 3 : GSPN
intersection with V3, 4 : intersection line along the axes of the GSPN and
AE) AE : arcuate eminence, GSPN : greater superficial petrosal nerve,
MMA : middle meningeal artery, R : retractor, V3 : mandibular division
of trigeminal nerve.
354 J Korean Neurosurg Soc 36
tegmen tympani
and the middle
meningeal artery
was dissected as
it joins the dura
to allow a greater extent of dural elevation form the middle fossa floor. The anterior and medial border of dural elevation was done to mandibular division
which is the third branch of trigeminal nerve
the and the petrous ridge, and
then middle fossa "rhomboid"
complex was confirmed. Anatomical structures of "rhomboid" complex included 1) intersection of arcuate eminence and petrous ridge,
2) porus trigeminus, 3) intersection of trigeminal nerve and greater superficial
petrosal nerve
and4) intersection of extended
line along axis
of greater superficial petrosal
nerve and arcuate eminence
(Fig. 2).
First internal
auditory canal was
unroofed with a
Fig. 3. Extradural exposure of the posterior
fossa, intracanalicular dura, inferior petrosal
sinus. Co : cochlea, GG : geniculate gangliion,
GSPN : greater superficial petrosal nerve, IAM :
internal auditory meatus, ICA : internal carotid
artery, R : retractor, V1 : ophthalmic nerve, V2 :
maxillary nerve, V3 : mandibular nerve.
Fig. 4. Intradural exposure of the posterior fossa
with pontine surface (Trigeminal root has been
removed) AICA : anterior inferior cerebellar
artery, BA : basilar artery, Co : cochlea, ICA :
internal carotid artery, V1 : ophthalmic nerve, V2
: maxillary nerve, V3 : mandibular nerve, GSPN :
greater superficial petrosal nerve, VII + VIII :
seventh and eighth cranial nerve.
SM Jung, et al.
the arcuate eminence measured
along petrous ridge was 12.6±
2.6mm and the average distance
from geniculate ganglion to the
intersection of greater superfGeniculate ganglion to arcuate eminence at the petrous ridge 12.6 ± 2.6
7.5-16.6
icial petrosal nerve to mandibGeniculate ganglion to internal acoustic meatus
13.2 ± 1.9
9.3-16.0
ular nerve was 15.4±1.4mm.
Internal auditory meatus to porus trigeminus
10.8 ± 1.5
7.5-13.2
In addition, the average distance
GSPN : greater superficial petrosal nerve, V3 : mandibular division of the trigeminal nerve
of exposed petrous ridge from
Table 2. Morphometric analysis of the cochlea and the middle fossa rhomboid complex based on 32 cadaver arcuate eminence to porus trigedissection (All results are expressed with mean±standard deviation)
minus was 25.4±1.7mm and
Length (mm)
Range (mm)
the average distance of porus
Cochlea to geniculate ganglion
3.0 ± 0.8
1.2-4.1
trigemius to intersection of
Cochlea to petrous carotid genu
2.9 ± 0.9
1.2-4.0
greater superficial petrosal
Cochlea to porus trigeminus
14.3 ± 0.6
17-12.3
nerve and mandibular nerve
Cochlea to IAM
9.0 ± 0.5
7.8-10.9
measured along mandibular
Cochlea to V3 at the GSPN
9.4 ± 0.4
7.6-11.3
GSPN : greater superficial petrosal nerve, IAM : internal auditory meatus, V3 : mandibular division of
nerve (V3) was 15.8±1.3mm.
trigeminal nerve
Finally the average distance
high-speed drill and drilling was started from center of angle from geniculate gang-lion to anterior lip of internal auditory
extending greater superficial petrosal nerve and arcuate canal was 13.2±1.9mm and the average distance from internal
eminence. The dura overlying the internal auditory canal was auditory canal to porus trigeminus was 10.8±1.5mm.
identified after removing about 3 to 4mm of bone, posterior
fossa dura was exposed by removing external parts of bone Morphometric analysis of the cochlea and the middle
between internal auditory canal and superior semicircular fossa rhomboid complex based on 32 cadaver
canal upward and internal auditory canal was exposed by dissection(Table 2)
removing them backward. After gently unroofing the greater
The average distance from cochlea to geniculate ganglion
superficial petrosal nerve to facial hiatus and identifying was 3.0±0.8mm and that from cochlea apex to petrous
geniculate ganglion as a further landmark defining the lateral carotid genu was 2.9±0.9mm. The average distance from
extent of the internal auditory canal, anterior bone of internal cochlea to porus trigeminus was 14.3±0.6mm, that from
auditory canal was removed to inferior petrosal sinus to cochlea to internal auditory meatus was 9.0±0.5mm and that
expose posterior fossa dura (Fig. 3). And then undersurface of from cochlea to mandibular branch of trigeminal nerve was
Meckel's cave and lateral margin of Dorello's canal were 9.4±0.4mm.
exposed by careful drilling anteriorly the tip of the petrous
apex. After securing operative field without cutting of greater Discussion
superficial petrosal nerve, the bone overlying Glasscock's
triangle was removed and medial and inferior bones of
iddle fossa approach was indispensable operative
internal carotid artery were removed without damaging any
technique to many neurosurgeons who are to preserve
neural or vascular structures.
hearing at the operative lesion of petrous apex and internal
After incision of the tentorium, pontomedullary junction, auditory meatus. But, this approach has surgical difficulties
the fifth cranial nerve, middle part of basilar artery, the sixth because major structures including internal auditory meatus,
cranial nerve and anterior inferior cerebellar artery could be facial nerve, cochlea, semicircular canal and internal carotid
artery are very adjacent to one another11). Therefore, in operobserved (Fig. 4).
ation with middle fossa approach, understanding of exact
Results
anatomical structures of temporal bone is indispensable.
In 1986, Fukushima3) divided middle fossa when he divided
Morphometric analysis of the middle fossa rhomboid structures around cavernous sinus and each compartment is
complex based on 32 cadaver dissection(Table 1)
composed of posterolateral, posteromedial, premeatal and
The average distance from geniculate ganglion to the end of postmeatal triangle.
Table 1. Morphometric analysis of the middle fossa rhomboid complex based on 32 cadaver dissection(All results
are expressed with mean±standard deviation)
Length (mm)
Range (mm)
Arcuate eminence to porus trigemius
25.4 ± 1.7
22.0-28.0
Porus trigeminus to GSPN-V3 junction
15.8 ± 1.3
13.6-17.5
GSPN-V3 junction to the geniculate ganglion
15.4 ± 1.4
12.9-17.5
M
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Extradural Middle Fossa Approach
First, posterolateral triangle is bony surface composed of
foramen spinosum, cochlea and mandibular nerve near its
intersection with the greater superficial petrosal nerve. This
area is an important landmark in exposing horizontal intrapetrous internal carotid artery. Second, posteromedial triangle
is the part bounded by cochlea, trigeminal groove and horizontal intrapetrous carotid artery. This is thin petrous apex
which is removed to approach the upper petroclival area,
inferior limit of internal auditory canal and internal carotid
artery. Third, premeatal triangle is bounded by the medial lip
of internal auditory meatus, carotid genu and geniculate
ganglion. This triangle identifies the exact location of cochlea
when drilling the petrous apex or exposing the intrapetrous
carotid artery though the middle fossa approach. The cochlea
is located in the base of this triangle6). Fourth, postmeatal
triangle is defined as bone between superior semicircular
canal and internal auditory canal. The geniculate ganglion,
lateral lip of the internal auditory canal and intersection of
arcuate eminence with petrous ridge are located at the
boundaries of this postmeatal triangle4). Such as a compartmentalization of the middle fossa can minimize damages of
major nerves and vascular structures during operation, but it
may cause loss of hearing due to cochlear damage because it
is very difficult for operator to know the exact position of
cochlea at live operative field.
Although superior bone of internal auditory canal contains a
lot of air cell anatomically, labyrinthine bone around cochlea
is composed of yellow cortical bone and the base of cochlea
is located vertically to medial part of the petrous internal
carotid artery. In addition, cochlea is located inferiorly to
geniculate ganglion8). However, since cochlear damage cannot be prevented at operative field only with the above
anatomical features, a lot of researches on anatomical structures around cochlea have been developed.
Dew et al.2) investigated anatomical relationship between
posterior genu of petrous internal carotid artery and the base
of cochlea with cadaver dissection. The average distance
from internal carotid artery to cochlea was 4.3±0.8mm,
(3.2~5.8mm) and Naguib et al.7) measured the distance from
cochlea to trigeminal ganglion. Rhoton8) reported that cochlea
is separated from the petous carotid artery by about 2.1mm
thickness bone. In this study, the others also confirmed that
average distance from cochlea to posterior genu of petrous
carotid artery was 2.9±0.9mm. Petrous anatomy of the
middle fossa apporach, Sennaroglu et al.9) published that
medial temporal bone could be removed about 8mm from
petrous ridge to lateral direction and removal of bone more
than 8mm might cause cochlear damages.
356 J Korean Neurosurg Soc 36
For the prevention of cochlear damages, its correlation with
important anatomical structures and direction of drilling
should be considered. Velut and Jan et al.12) emphasized that
for the prevention of cochlea, medial part of vertical surface
adjacent to medial wall of internal carotid artery should be
drilled. In particular, since cochlea is located at the lateral
middle of premeatal triangle which is made by genu of
internal carotid artery, geniculate ganglion and medial part of
internal auditory canal, it should be careful to remove
anteromedial and inferior bone of geniculate ganglion for the
prevention of cochlear damages.
Arcuate eminence has been recognized as an important
anatomical landmark which can identify the position of
anterior semicircular canal and Kartush et al.5) published that
arcuate eminence at temporal bone was proper landmark in
85%. Villavicencio et al.13) reported that the average distance
from middle of anterior semicircular canal to anterior border
of petrous ridge on the same sagittal plane was 5.2mm and
the average bone thickness of anterior part of lateral
semicircular canal was 8.9mm. This research found that the
average distance from geniculate ganglion to anterior
semicircular canal was 14.1±0.8mm and that from internal
auditory canal to anterior semicircular canal was 13.0±
0.3mm.
The middle meningeal artery which is one of the important
anatomical landmark in middle fossa approach entered cranial
cavity through foramen spinosum of the sphenoid bone and
Tedeschi et al.10) reported that the average distance from
foramen spinosum to anterolateral part of petrous internal
carotid artery was 4.5mm and to anterolateral part of
geniculate ganglion was 14mm. This research found that the
average distance from foramen spinosum to mandibular nerve
was 11.1±0.6mm.
Conclusion
T
he authors drilled bone covering greater superficial
petrosal nerve and from facial nerves of internal
auditory meatus to geniculate ganglion then exposed nerves,
performed morphometric analysis of anatomical relationship
of middle fossa rhomboid complex and measured distance
from cochlea to structures around it. Consequently, it was
found that the average distance from the base of cochlea to
geniculate ganglion was 3.0±0.8mm, that from the apex of
cochlea to genu of internal carotid artery was 2.9±0.9mm
and that from cochlea to internal auditory meatus was 9.0±
0.5mm. Then it is considered that recognition of morphometric structure, distance and understanding of cochlea
SM Jung, et al.
position surrounded by cortical bone in middle fossa
approach will be very helpful to preserve hearing during
operation.
Acknowledgement
This study was supported by research funds from Chosun University
Hospital, 2003.
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