carotid rings: A microanatomical study
intracranial carotid rings were
studied in 30 specimens obtained from 15 cadaver heads fixed in
didactic purposes, this dural folds were divided into three rings:
Medial and Distal.
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first ring is found over the Internal
Carotid Artery (ICA) at its entrance in the cavernous sinus at
the level of the foramen lacerum and underneath V1 and the Trigeminal
ring is formed by the petrolingual
ligament reinforced by fibers of the endosteal
dura of the carotid canal.
removal of the anterior clinoid process allows the identification of
the “Clinoid Space”, in
this area, the clinoid segment of the internal carotid artery is
surrounded by two fibrous rings which are medial and distal.
medial ring is a formation from the dural fiber of the deep
layer of the superior wall of the cavernous sinus, and the distal ring
is formed by dural fibers from the superficial
layer of the superior wall of the cavernous sinus.
accurate understanding of the complex fibrous ring anatomy of the
parasellar region is of paramount importance when dealing with lesions
in this area and also in the strategical approaches.
Carotid Rings was studied in 30 specimens obtained from fifteen
cadaver heads fixed in formalin.
the brains were carefully removed in order to fully expose the skull
base, the heads were placed en the Sugita head holder. They were
turned 45 grade from the side of dissection and extended slightly to
simulate the surgical position.
identifying the anterior and posterior petroclinoid ligaments with the
aid of the surgical microscope the middle fossa floor dura was
dissected from anterior to posterior and lateral to medial.
bone forming the floor of the middle fossa was removed up to the
foramen rotundum, the foramen ovale and the foramen spinosum.
trigeminal nerve was dissected extradurally in order to identify the
carotid ring at the entry
of the internal carotid artery in the cavernous sinus.
optic canal was unroofed and the anterior clinoid process removed.
doing so, we exposed the
“ Clinoid Space”
within it the clinoid segment of the internal carotid artery, a part
of the ethmoidal sinus as
well as a part of the superior wall of the cavernous sinus. We also
identified the medial and
distal dural rings of the ICA in the superior and inferior limits
of this space.
particular disposition of the dural folds permits to surround
partially the internal carotid artery in the parasellar region through
of the fusion of various
dural fibers groups.
located at the exit of the level of the endocranial opening of
the foramen lacerum.
ring is formed by two fiber groups, one from the endosteal fibers of
the trigeminal impression “obturatrix membrane” and inner most
area of the superior aspect of the petrous vertex forming a tense and
hard dural fold which courses over the internal carotid artery lateral
aspect. It’s arch shaped fuses with the endosteal dura of the
lingula sphenoidalis. This dural formation is also known as the
petrolingual ligament ( Fig.
). The other fibers group arises from the endosteal dura of the
superior aspect of the petrosphenoidal suture ( Fig.
)and ends by fusing with the endosteal dura of the medial wall
of the carotid canal
two fiber groups merge at the highest part of the petrosphenoidal
suture. As a whole they are “V” shaped
and surround the internal carotid artery except in its anterior
aspect where the ring is completed by the endosteal dura of the
carotid sulcus ( Fig. ).
is possible to visualize the petrolingual ligament by dissecting
between V1 and V2. The V1 branch of the trigeminal nerve runs parallel
and above the petrolingual ligament.
ligament is the anatomical landmark which separates the
internal carotid artery cavernous segment from the petrosal one
( Fig. ).The diameter of
the proximal ring was 7.57 +-0.6 mm. The length of the petrolingual
ligament was 10.92 +- 0.9 mm, and the height was 1.22+- 0.4 mm. The
location of the sympathetic pericarotid trunk in relation to this
ligament was in the anterior third 70%, Middle third 25%, and in the
posterior third 0.5%. The abducens nerve courses parallel with the
petrolingual ligament with an average of distance 2.88 +- 0.6 mm. The
distance between the middle third of the petrolingual ligament and the
medial carotid ring was 18.12+-0.9 mm.
removed the anterior clinoid process one can identify the
clinoid space, and partially the deep layer of the superior wall of
the cavernous sinus ( Fig.
latter layer has a free border that surrounds intimately the
posterolateral aspect of the internal carotid artery immediately at
the exit of the cavernous sinus. However, in 15% this ring is less
closely related with the artery, in such a way that there is a direct
communication with the venous compartment of the cavernous sinus.
the posterior course of this ring, the interclinoid ligament joins it.
We have to emphasize that this ligament is not a direct attachment to
the anterior clinoid process.
and medially this ring fuses with the endosteal dura of the carotid
canal ( Fig. ).The
diameter of this ring was 6.1 +- 0.9 mm.
distance from the medial carotid ring to the distal carotid ring was
5.4+- 1.2 mm and the ophthalmic artery (origin) was 6.42 +- 1.4mm.
ring is formed by the convergence of dural fibers from various sites.
The dural fibers of the lateral aspect of the tubercullum sellae
together with the dural fibers of the optic canal, form a fold that
courses over the superior aspect of the internal carotid artery just
anterior to the ophthalmic artery
origin ( Fig.
It fuses afterwards with the medial fibers of the anterior
petroclinoid ligament. In the posterior segment, the ring is continuos
with the superficial layer of the superior wall of the cavernous
sinus; above the distal ring , the internal carotid artery continuous
with its supraclinoid segment. The diameter of the distal carotid ring
was 6.86+-0.9 mm.
most instances there
exist a little space between the medial aspect of the carotid artery
an the ring which corresponds to the entrance of the so called
“carotid cave”. The depth and width of this semilunar space
was 3.2 +- 0.5 mm and 1.9
+- 07 mm respectively.
distance from the proximal carotid ring was 21.0+-0.9 mm and the
ophthalmic artery (origin) was 1.86+-1.1 mm.
pericarotid connective tissue layer was found to be loosely attached
to the rings in the lateral aspect of the internal carotid artery
clinoid segment (Fig. ).
so called “Carotid rings” were described separately and at
different times, what we’re calling in the current paper proximal
ring corresponds to the “Inferior sphenopetrosal ligament”
described by Lang and Strobel (7-8), who stated that it is formed by
two parts: the pars saggitalis and the pars transversalis. The pars
saggitalis corresponds to the petrolingual ligament and the pars
to the endosteal dura of the superior aspect
of the petrosphenoidal suture. Sekhar (13)
says it’s formed by dural fibers from the carotid canal, and
Dolenc (2) calls it “the lateral ring”.
ligament is white and shiny, being an anatomical landmark to localize
the beginning of the intracavernous carotid artery. The pars
transversalis of this ligament may be missing. While performing an
inferior approach to the cavernous sinus along the
middle fossa floor, the pars
saggitalis may be identified once the Meckel’s cave has been
dissected and separated warning the surgeon of the internal carotid
we name in the current paper medial carotid ring has been called in a
number of ways, as early as 1935, Keyes (4) referred to it as what
constituted the “Clinocarotid
canal”, he wrote “this clinocarotid canal is formed by a union of
the anterior clinoid process on its medial side with the tip of the
middle clinoid process as it arises from the tuberculum sellae or the
lateral wall of the body of the sphenoid bone”. He classified the
clinocarotid canal in three types: complete, incomplete and contact,
according to the ossification degree (his study was performed of the
basis of dry skulls).
ring is formed by a condensation of fibers from the deep layer of the
superior wall of the cavernous sinus and when ossified clearly
corresponds to what Keyes stated. We can also say this ring is
partially constituted by the carotico-oculomotor membrane according to
the description made by Inoue et al (12).
medial ring corresponds to the Dolenc’s proximal ring (2). Knosp et
al (5) described a ligament that connects the anterior with the middle
clinoid process, which becomes the caroticoclinoid foramen when
ossified, suggesting for it the of “carotid or clinoid ligament”.
Umansky et al (14) published in
a microanatomical study of the superior wall, emphasizing that
the internal carotid artery extracavernous, is surrounded by two
fibrous rings: Distal and proximal rings.
the surgical point of view, Inoue et al (12) and Matsuoka et al (9)
approach the cavernous sinus through the superior wall after
removing the anterior clinoid process, they cut the medial ring and
proceed posteriorly following the traject of the interclinoid ligament
towards the posterior clinoid process.
the Dolenc approach (3), the cut
of this ring permits
of the anterior bend of the intracavernous carotid artery,
facilitating the access to the anterior part of the venuos compartment
of the cavernous sinus and the sellar region.
distal ring was first
described by Pernecsky in 1985 (10), this ring is more firmly attached
to the lateral aspect of the carotid artery than over its medial side,
where it can be found a semilunar
small space between the edge of the ring and the artery,
corresponding to the entrance of the carotid cave described by
Kobayashi (6) as a dural pouch. We didn’t find any redundant dura at
the level of the carotid cave, but just a virtual space between the
carotid artery and the endosteal dura of the carotid sulcus.
clinical importance of the carotid cave is that this virtual space may
be occupied by an aneurysm.
dealing with aneurysms of the paraclinoid region it’s necessary
to cut the distal ring to gain more surgical field , Dolenc
(1), Perneczky (10).
connective tissue layer covering the lateral aspect of the internal
carotid artery in the
clinoid space, that unites the medial and distal ring, is considered
for us as transitional dura between the dura over and under lying the
anterior clinoid process (deep layer of the superior wall of the
cavernous sinus). Knosp (5) and Perneczky (11) refer to the
connective tissue surrounding the carotid artery as corresponding to a
remnant of the abundant connective tissue in fetal cavernous sinuses.
continuous development of
microsurgical techniques to treat
tumors in the cavernous sinus and
vascular lesions of the internal carotid artery cavernous and
clinoid segments, emphasizes the need for an accurate understanding of
the microanatomy of the dural folds that surround the internal carotid
artery in the parasellar region.
fundamental knowledgment of the dural complexity from the parasellar
area, allows us surgery strategies procedures with the use of the
above anatomical landmarks described.
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