Keywords
anterior clinoid process - optic strut - clinoidectomy - keyhole approach - aneurysm
Introduction
Modification of a classic pterional approach into the keyhole approach with craniotomy
sizes from 3 to 5 cm has been developed to address craniometric volume challenges
brought up by the classic pterional approach. However, the tight skull-base operating
corridor still poses a formidable challenge in terms of mobility, and proximal vascular
control for the neurosurgeon.
The combination of a minipterional approach with an extradural resection of the anterior
clinoid process (ACP) makes it possible to expand the volume of the working operating
field (WOF) on the skull base area.
The ACP represents the terminal portion of the lesser wing of the sphenoid bone and
forms the anterior aspect of the lateral wall of the optic canal. Anatomically, the
ACP is attached to the wing of the sphenoid bone by three bony structures (pillars):
a lateral pillar, which is formed by a wedge-shaped ridge and is bounded from below
by the superior orbital fissure, the medial pillar that forms the roof of the optic
canal, and the inferior pillar (or optic strut [OS]), which separates the supraclinoid
portion of the internal carotid artery (ICA) inferolaterally from the superior medial
optic nerve.[1]
As ACP covers the roof of the cavernous sinus and the paraclinoidal segment of the
ICA, anterior clinoidectomy becomes mandatory in approaching cavernous sinus, and
in optic canal decompression.[2]
Thus, ACP is a key access to the sellar/parasellar area, anterior and anterolateral
circle of Willis, middle cerebral artery, upper basilar artery, and anterior cranial
fossa. Removal of the ACP allows to reach all these regions and expands the range
of indications for neurosurgical interventions in the skull base area, particularly
at keyhole approaches, where every cubic millimeter of freed space has a crucial role
for neurosurgeons.[2]
The technique of removing the ACP and OS has practical importance for a neurosurgeon.
In our clinic, we use sequential drilling of the lateral and medial pillar, with further
breaking, drilling, and cutting off the OS. The option of removing the OS depends
on its morphometric parameters.[1]
Therefore, in this article, we decided to carry out a morphometry of ACP and OS and
offer our own surgical classification of the OS to choose the appropriate method for
removing it, and evaluate the percentage of the expansion of the volume of the WOF
after the ACP removal.
Material and Method
The present cross-sectional observational study was done in the Department of Neurosurgery,
of the Federal State-Financed Institution “Federal Centre of Neurosurgery” of the
Ministry of Health of the Russian Federation (Tyumen). The study was conducted on
47 computed tomography (CT) of the patient's head, included 31 females (65.96%) and
16 male adults (34.04%) whose age ranged between 28 and 79 years, all heads were measured
bilaterally.
The CT measurements of length and width of ACP and OS was made with a Canon medical
system Aquilion One 640 multislice CT scanner with 0.5 mm thick on CT RadiAnt DICOM
Viewer (ver. 2020.1.1). The average basal width shows the distance between the lateral
margin of the optic canal and the lateral edge of the ACP, the average basal length
shows the distance from the middle of the base of the ACP to the apex of the ACP,
and the average basal width of the inferior pillar (OS) shows the distance from anterior
to posterior edge of the OS, all heads were measured bilaterally.
The CT measurements of the ACP volume and volume of WOF were conducted by three-dimensional
(3D) reconstruction in the Syngo.via Siemens program. The ACP volume and the volume
of the WOF were measured by tracing the points in different slices of 0.5 mm thick.
The volume of the WOF was a polygonal figure limited by the lines connecting the following
anatomical landmarks: the middle of the sulcus chiasmatis, the middle of the dorsum
sella, the posterior clinoid process, the trigeminal depression of the pyramid of
the temporal bone, the lateral edge of the base of the ACP, the base of the ACP, and
the medial edge of the base of the ACP ([Fig. 1]).
Fig. 1 The working operating field (WOF). WOF is a polygonal figure limited by the lines
connecting the following anatomical landmarks: 1, the middle of the sulcus chiasmatis;
2, the middle of the dorsum sella; 3, the posterior clinoid process; 4, the trigeminal
depression of the pyramid of the temporal bone; 5, the lateral edge of the base of
the anterior clinoid process (ACP); 6, the medial edge of the base of the ACP.
Determination of the percentage of expansion of the operative field after removal
of the ACP was estimated on 5 fixed human cadaver heads. Microsurgical anatomical
dissections and measurements were performed by the exoscope VITOM 3D (Karl Storz,
Germany), the holding pneumatic arm Mitaka (Japan), the camera system, the cold xenon
light source, the control unit (Karl Storz, image1 pilot), and the 3D 4K monitor.
Images and video sequences were recorded in 3D and 4K quality with Karl Storz AIDA
recorder system.[3] We demonstrated the possibilities of combined approach by using a clinical case
and 3D reconstruction of the CT in the Meshmixer 3.5 program ([Fig. 2]).
Fig. 2 (A) Reconstruction of the anterior clinoid process (ACP) before aneurysm clipping. (B) Three-dimensional (3D) reconstruction of the ACP after aneurysm clipping.
Results
According to the CT measurements of digital indicators the length of the right ACP
was 11.31 ± 2.76 mm, the left ACP was 11.54 ± 2.86 mm, and the width of the right
ACP was 7.70 ± 1.66 mm, left ACP was 7.64 ± 1.67 mm. Average index of ratio of ACP
width to ACP length (iacp) was 0.67, the value of the minimum iacp in our study was 0.45, the value of the maximum iacp in our study was 0.90. The width of the OS varied in the range from the narrowest
OS 1.37 mm to the widest OS 4.75 mm. There was no correlation between the length and
width of the ACP and the width of OS due to the variability of the values. The average
volume of the right ACP was 0.71 ± 0.16 cm3, the left ACP was 0.71 ± 0.15 cm3. The volume of the right WOF was 3.26 ± 0.74cm3, the left WOF was 3.20 ± 0.76 cm3. Removal of the right ACP expanded the right WOF by 22.21 ± 3.88%. Removal of the
left ACP expanded the left WOF by 22.78 ± 5.50% ([Figs. 1] and [3]).
Fig. 3 The computed tomography (CT) measurements of the anterior clinoid process (ACP) volume
and volume of the working operative field were conducted by three-dimensional (3D)
reconstruction in the CT Radiant program. The ACP volume and volume of the operative
field (OF) were measured by tracing the points in different slices of 0.5 mm thick.
The volume of the operative field was a polygonal figure limited by the lines connecting
the following anatomical landmarks: the middle of the sulcus chiasmatis, the middle
of the dorsum sella, the posterior clinoid process, the trigeminal depression of the
pyramid of the temporal bone, the lateral edge of the base of the ACP, the base of
the ACP, and the medial edge of the base of the ACP. 1, The volume of the left operative
field = 2.99 cm3. 2, The volume of the right operative field = 2.74 cm3. 3, The left ACP volume = 0.54 cm3. 4, The right ACP volume = 0.55 cm3.
The cadaver dissection using the exoscope VITOM 3D demonstrated sequential removal
of the lateral, medial, and inferior pillar of the ACP. The extradural resection has
increased the volume of the operative field by approximately 25% and allowed to visualize
previously unidentified structures as the posterior clinoid process, basilar artery,
superior cerebellar artery, posterior cerebral artery, and cranial nerve III ([Fig. 4]).
Fig. 4 Cadaver dissection of anterior clinoid process, step by step. (A) Anatomical structures before anterior clinoid process (ACP) removal. (B) Three-dimensional (3D) anatomical structures before ACP removal. (C) Anatomical structures after partial drilling of ACP. (D) 3D anatomical structures after partial drilling of ACP. (E) Removal ACP. (F) 3D image of ACP removal. (G) Operative field after removal of ACP. (H) 3D operative field after removal of ACP. (I) Operative field before ACP removal. (J) 3D operative field before ACP removal. (K) After removal of ACP the posterior clinoid process, basilar artery, superior cerebellar
artery, posterior cerebral artery, and CN III are exposed. Red line-opened field after
ACP removal. (L) 3D after removal of ACP the posterior clinoid process, basilar artery, superior
cerebellar artery, posterior cerebral artery, and CN III are exposed. APC, anterior
clinoid process; BA, basilar artery; CN (cranial nerve) II, optic nerve; CN III, oculomotor
nerve; CN V1, trigeminal nerve ophthalmic branch; DM, dura mater; ICA, internal carotid
artery; OC, optic canal; OS, optic strut; PCA, posterior cerebral artery; R, ruler;
RAPC, removal anterior clinoid process; S, suction; SCA, superior cerebellar artery;
SOF, superior orbital fissure.
During cadaveric dissection and removal of ACP, we assessed the technical difficulties
in removing the ACP. The complexity of ACP dissection directly depends on the ratio
of width to length of ACP. When the iacp was more than 0.90, the ACP was short and wide, and there was no problem with dissection
of the ACP tip and we used the extradural technique of removing. When the iacp was less than 0.45, the ACP was long and narrow, it was difficult to skeletonize
the tip and we had to use combine technique (extradural fragmentation with intradural
removal of the tip).
Moreover, during cadaver removal of the ACP, we found that mainly technical difficulties
arise on stage of OS removal. The inferior pillar is the most significant in neurosurgical
operations and requires special neurosurgical equipment. Incomplete OS removal can
lead not only to damage the optic nerve or the ICA but decrease the proximal vascular
control.
Taking into account the variability of the anatomical variations of iacp and the width of the bone of OS, we proposed a surgical classification of complicated
and uncomplicated ACP in [Table 1].
Table 1
Parameters of anterior clinoid process by CT scans and frequency of occurrence of
uncomplicated and complicated ACP in our study
|
Anterior clinoid process
|
iacp
|
N (%)
|
OS
|
N (%)
|
Pneum
|
N (%)
|
|
Uncomplicated ACP
n = 30
|
iacp ≥ 0.67
|
31 (66)
|
Narrow optic strut ≤ 2.5 mm
|
35 (74)
|
Without pneumatization ACP
|
39 (83)
|
|
Complicated ACP (if one of the parameters is present)
n = 17
|
iacp 0.45 ≤ 0.67 mm
|
9 (19)
|
Medium optic strut
2.5 mm ≤ 4.0 mm
|
7 (14)
|
With pneumatization ACP
|
8 (17)
|
|
iacp ≤ 0.45
|
7 (14)
|
Wide optic strut ≥ 4.0 mm
|
5 (10)
|
|
|
Total
|
47 (100)
|
|
47 (100)
|
|
47 (100)
|
Abbreviations: ACP, anterior clinoid process; CT, computed tomography; iacp, index of ratio ACP width to ACP length; OS, optic strut; Pneum, pneumatization.
In our morphometric study of 47 CT we found out that there was 63% uncomplicated ACP
and 37% complicated ACP cases. Complicated ACP was associated more with iacp (66%) than with a wide OS (10%) and pneumatization (17%).
This classification has practical application and can be used by neurosurgeons to
choose the method of the dissection and removal of ACP. In obedience to this classification,
we have developed our own algorithm of ACP dissection and removal.
The possibilities of the combined approach were demonstrated in a clinical case.
A Clinical Case
A 44-year-old woman ([Video 1]) was admitted to the Federal Centre of Neurosurgery in Tyumen, without a history
of previous subarachnoid hemorrhage, presented with symptoms of weakness and headaches.
Neurological examination did not reveal any neurological pathology. The CT angiography
and selective cerebral angiography showed a saccular aneurysm of the left ICA-posterior
communicating artery (PCommA). Microsurgical clipping of the aneurysm of the left
ICA-PCommA via left minipterional craniotomy with extradural anterior clinoidectomy
using the microscope Carl Zeiss OPMI PENTERO 900 was done.
Video 1 Extradural part: meningo-orbital band incision; the medial, lateral pillar sawing;
optic strut removing with specialized bone rongeurs (Muranaka, Japan), and a drill.
Intradural part: microsurgical clipping of the left internal carotid artery-posterior
communicating artery (ICA-PCommA) aneurysm with 3 clips; indocyanine green (ICG) control.
Extradural anterior clinoidectomy was performed in this case, step by step after soft
tissue dissection and meningo-orbital band coagulation. According to preoperative
CT RadiAnt DICOM Viewer measurements the length of the ACP was 10.01 mm, and the width
of the ACP was 7.7 mm. iacp was 0.77, OS was 2.73, without pneumatization; iacp allowed to make skeletonization without technical difficulties.
The lateral pillar was removed during the resection of the lesser wing of the sphenoid
bone and part of the roof of the superior orbital fissure. The medial pillar was removed
by drilling with the Stryker system rotary drill.
Removal of the optic septum was performed using specialized bone rongeurs (Muranaka,
Japan) and a drill, because according to the preoperative CT RadiAnt DICOM Viewer
measurements, the dimension of the OS width was 2.73 mm. In obedience to our classification,
this OS was classified as medium OS. After resection of all bone pillars, the ACP
becomes mobile and is fixed mainly due to the petroclinoidal and interclinoidal ligaments.
A complete dissection of the ACP is performed by sharp dissection of these ligaments
with microscissors or a specialized sickle-shaped microdissector (Feather, Japan).
The proximal part of the ICA was opened after ACP removal, this condition contributed
to improving the proximal vascular control and expanding the operative field. After
demobilization of the ICA the microsurgical clipping of the aneurysm of the left ICA-PCommA
was performed by three special clips: two straight clips and one curved fenestrated
clip. Then a standard closure of wound, and hemostasis was done. Postoperative control
was made by CT angiogram. The result was downloaded to the Meshmixer program for 3D
reconstruction ([Fig. 5]). The patient was discharged 6 days after the operation.
Fig. 5 Algorithm of anterior clinoid process (ACP) dissection with evaluation of index of
ratio ACP width to ACP length (iacp).
Discussion
Since the conception of aneurysm surgery introduced by Norman Dott in 1933, and then
developed by Yaşargil et al[4] in 1975, by using the microneurosurgical methods, different approaches for aneurysm
clipping have been innovated with good improvement in surgical outcomes. The pterional
craniotomy with the frontotemporal Sylvian fissure opening has been the hallmark of
these surgeries.[5] However, postoperative complications such as mandibular dysfunction, chronic pain,
and alterations in the facial sensory components frequently associated with temporal
atrophy and injury of the frontal branch of the facial nerve and trigeminal branches
have been reported.[6]
[7]
[8] As a result, constant modifications to the classical pterional approach have been
done with the desire to reduce the volume of craniotomy, and also to reduce the traumatism
caused during the intraoperative and postoperative complications.
Technological progress, the emergence of operating microscopes, VITOM 3D, endoscopes,
and the description of microsurgical techniques have made it possible to reduce the
volume of the surgical access to the keyhole approaches. The minipterional keyhole
craniotomy has been developed as one of the alternative modalities.[9] However, modification of a classic pterional approach into keyhole with craniotomy
sizes from 3 to 5 cm, limits the scope of surgical interventions in the skull base
area and has some safety issues, especially when an intraoperative rupture occurs.
Achieving the hemostasis within a small corridor poses great challenge for the neurosurgeon.[9]
[10]
[11]
The principle of minimally invasive techniques entails a balance between minimal tissue
trauma and maximum anatomic exposure.[12] In this study, the rationale for an extradural anterior clinoidectomy as a keyhole
approach was illustrated by morphometry of the ACP. ACP is the key of the access to
the sellar and parasellar area, anterior and anterolateral circle of Willis, middle
cerebral artery, upper basilar artery, and anterior cranial fossa. In our study, the
combination of a minipterional approach with extradural resection of the ACP expanded
the volume of the operative field of the skull base by 25%. It is a very important
digital indicator, particularly at keyhole approaches, where every millimeter of freed
space has a crucial role for neurosurgeons. This extended operative corridor aids
neurosurgeons improve proximal vascular control and expands the range of indications
for neurosurgical interventions in the skull base area.
In the last decades, there has been an increasing interest and publication in ACP
morphometry. In a study done by Dagtekin et al, the average basal width, length, and
thickness of the ACP were found to be 7.3, 9.7, and 5.4 mm, respectively.[13] Similar observations were reported in Nepal, Indian, Japan, and Korean skulls, respectively.[14]
[15]
[16]
[17] Cecen et al classified ACP into three types: type I ACPs short, wide, and wide-angled;
type II ACPs long, narrow, and narrow-angled; and type III ACPs.[18]
In our study the length of the right ACP was 11.31 ± 2.76 mm, the left ACP was 11.54 ± 2.86 mm,
the width of the right ACP was 7.70 ± 1.66 mm, the left ACP was 7.64 ± 1.67 mm, which
means that the length of ACP of skulls is longer and the width is less than the average
calculated in other countries.[18] This finding therefore correlated to a type II ACP classification (long, narrow,
and narrow-angled) which require more challenging surgical techniques according to
Cecen et al.[18] In our study, the average iacp was 0.67, the value of the minimum iacp in our study was 0.45, and the value of the maximum iacp in our study was 0.90. The complexity of ACP dissection directly depends on the ratio
of width to length of ACP. According to those parameters we created our own algorithm
of ACP dissection, which is shown in [Fig. 5].
Moreover, during the cadaver removal of the ACP, we found that mainly technical difficulties
arise on stage of OS removal. The inferior pillar is the most significant in neurosurgical
operations and requires special neurosurgical equipment.
There are a lot of studies devoted to the OS removal during the surgery in the sellar
and parasellar regions, as incomplete removal can lead to damage to the optic nerve
or the ICA. The objective of the study done by Kapur and Mehić was to quantify dimensions
of the OS and ACP, and to determine variations in positions and forms of these structures.
They find out that the average width of the OS was 3 mm on the skulls belonging to
males, and 2.8 mm on those belonging to females; moreover, the OS was most commonly
attached to the anterior two-fifths on the lower side of the ACP.[19] In our study, the width of the OS varied in the range from the narrowest OS 1.37 mm
to the widest OS 4.75 mm and there was no correlation between the length and width
of the ACP and the width of OS due to the variability of the values.
According to these findings, we created our own algorithm of OS removal, which is
shown in [Fig. 6].
Fig. 6 Algorithm of anterior clinoid process (ACP) removal.
Along with morphometric measurements, the literature describes variants of pneumatized
ACP, which can complicate the surgical treatment as a result of unintentional opening
of the sphenoid and ethmoid sinus opening during clinoidectomy.[20]
[21] In our previous study, we proposed our own method of removing the ACP by drilling
the lateral, medial pillar with sequential removal of the inferior pillar.[1] We offer a method of removal of OS (inferior pillar) using a special technique described
in this article. In our opinion, a thorough study of the anatomy of the ACP with CT
will allow to choose the optimal access for sequential removal of ACP, which contributes
to performing a gentle surgery without intraoperative complications.
Conclusion
The knowledge of morphometry of ACP and OS is very important for neurosurgeons for
finding the safest technique of extradural anterior clinoidectomy. In our series,
removal of the right ACP expanded the right WOF by 22.21 ± 3.88%, the left ACP expanded
the left WOF by 22.78 ± 5.50%. The extradural resection performed on the cadaver specimen
increased the volume of the operative field by approximately 25%. The results of cadaver
dissection correlated with the results of the real surgery, which we illustrated in
our clinical case. Taking into account the variability of the ACP and OS, we would
like to propose our own surgical classification of complicated (iacp ≥ 0.67; medium OS 2.5 mm ≤ 4.0 mm; wide OS ≥ 4.0 mm; ACP with pneumatization) and
uncomplicated ACP (iacp 0.45 ≤ 0.67 mm; iacp ≤ 0.45; narrow OS ≤ 2.5 mm; ACP without pneumatization). This classification has
practical application and can be used by neurosurgeons to choose the optimal method
of dissection and removal of the ACP. Basing on the aforementioned classification,
we created our own algorithm. Detailed study of ACP anatomy and our neurosurgical
technique of exradural resection of ACP will allow surgeons expand the volume of the
narrow operative corridor in the skull base area by 25%, it will also help neurosurgeons
to improve proximal vascular control, avoid complications, and increase the range
of indications for neurosurgical interventions in the skull base area.
