Keywords bicoronal incision - modified bicoronal incision - frontal branch of STA - hairline
- superficial temporal artery - temporal peak
Introduction
The bicoronal scalp incision for a transcranial approach was first introduced in 1907.[1 ] Also, the bicoronal incisions are frequently used in craniofacial surgeries before
1970.[2 ]
[3 ]
[4 ]
[5 ]
[6 ]
[7 ]
[8 ] The classic bicoronal incision (also known as a Souttar incision) begins just above
the zygomatic arch and curves slightly anteriorly at the midline to meet the same
point above the opposite zygomatic arch ([Fig. 1 ]). The starting point needs to be within 1 cm of the tragus to preserve the superficial
temporal artery (STA). The incision then follows a course of 3 cm posterior to the
hairline.[5 ]
[8 ]
[9 ]
[10 ]
Fig. 1 Classic (dashed white line) and modified (dashed yellow line) bicoronal incisions.
The classic incision begins just above the zygomatic arch and curves slightly anteriorly
at the midline to meet the same point above the opposite zygomatic arch. The modified
incision begins just posterior to the temporal peak (red arrow) on both sides and
extends just posterior to the hairline (black line) toward the widow peak (blue arrow).
To date, there have been no reports of the STA preservation rate after classic bicoronal
incisions. However, Suanchan et al found a mean distance from the STA to the anterior
edge of the ear cartilage of 0.6 cm at the level of the superior border of the zygomatic
root. They reported a mean angle between the orbitomeatal line and the axis of the
parietal branch of the STA (pSTA) of 88.8 degrees (75–95 degrees).[11 ] These small distances suggest that the classic bicoronal incision carries a high
risk of injury to the pSTA and the main trunk of the STA (mSTA).
Several studies[12 ]
[13 ]
[14 ]
[15 ]
[16 ] have described a basal interhemispheric approach (IHA) via a modified transbasal
bifrontal craniotomy[9 ] for clipping aneurysms of the anterior communicating artery (AcoA)using a modified
bicoronal incision. This incision begins at the temporal (temple) peak and curves
superiorly just posterior to the hairline toward the widow peak, where it meets the
equivalent incision coming from the opposite temporal peak. At the midline, the incision
follows the widow peak ([Fig. 1 ]).[12 ]
[13 ]
[14 ]
[15 ]
[16 ] In the modified craniotomy, the skull flap incorporates the anterior wall of the
frontal sinus and the superior part of the superior orbital rim. This successfully
facilitated unlimited access to lesions at the anterior base of the skull.[9 ]
Although there have been numerous cadaveric and imaging studies of the STA,[17 ]
[18 ]
[19 ] the exact course of the fSTA has not previously been described, especially in relation
to the hairline. This study aimed first to determine the course of the fSTA in relation
to the hairline in cadaveric heads; and, second, to evaluate the modified bicoronal
incision and its rate of fSTA preservation in clinical cases.
Materials and Methods
Sixteen sides of adult cadaveric heads were dissected to determine the course of the
fSTA in relation to the temporal and frontal hairline. The distance between the most
anterior part of the fSTA and the temporal peak of the hairline was measured. If the
most anterior part of the fSTA was located anterior to the temporal peak, the distance
was reported as a positive value, and vice versa. The distances from the STA bifurcation
to the zygomatic arch were also measured ([Fig. 2 ]).
Fig. 2 The relationship between the most anterior part of the fSTA and the temporal peak.
(A ) The hairline (white line), temporal peak (yellow dot), widow peak (black arrow),
and sideburn (blue arrow) were identified. The red lines show the courses of the mSTA,
fSTA, and pSTA. The distances between the most anterior part of the fSTA and the temporal
peak (a) were measured. (B ) After the skin was reflected through the subcutaneous plane, the temporal peak (yellow
dot) was marked and the fSTA (dashed red line), pSTA, and mSTA were dissected. The
distances between the most anterior part of the fSTA and the temporal peak (a) and
between the STA bifurcation and the zygomatic arch (b) were measured. fSTA, frontal
branch of the STA; mSTA, main trunk of the STA; pSTA, parietal branch of the STA;
STA, superficial temporal artery.
Patients with AcoA aneurysms who underwent clipping via a modified transbasal bifrontal
craniotomy[9 ] and IHA using the modified bicoronal skin incision ([Figs. 1 ] and [3 ])[13 ]
[16 ] between June 2017 and January 2022 were enrolled. Patients who did not undergo postoperative
computed tomography angiography (CTA) and those who had previously undergone a pterional
craniotomy were excluded. The preservation of the fSTA was defined as the continued
existence of the distal segment of the fSTA (ascending frontal artery).[20 ] This was identified on postoperative CTA, where it could be seen distal to the edge
of the skin incision ([Fig. 3 ]). The preservation of the pSTA was also assessed.
Fig. 3 Modified transbasal bifrontal craniotomy. (A ) Computed tomography angiography image, showing the skull flap, the temporal line
(dashed white line), the supraorbital foramen (arrow), and the skin incision (staple
line). (B ) Intraoperative image taken after elevation of the craniotomy flap. The scalp flap
(blue arrow), pericranial flap (black arrow), temporalis muscle (yellow arrow), temporal
line (dashed white line), and frontal sinus (arrowhead) are visible in the image.
Operative Technique
Using a scalpel, the modified bicoronal incision was begun just posterior to the temporal
peak on both sides and extended just posterior to the hairline toward the widow peak
([Fig. 2 ]). Doppler ultrasonography was not utilized during the operation. A two-layer technique
was used to separate the scalp flap from the underlying pericranium with suprafascial
dissection.[21 ]
[22 ] The scalp flap was reflected inferiorly to the superior orbital rims, with preservation
of the supratrochlear and supraorbital neurovascular bundles. After elevation of the
pericranium, the modified transbasal bifrontal craniotomy[9 ] was performed ([Fig. 3 ]).[13 ]
[14 ]
[15 ]
[16 ]
Results
In our study of 16 sides of cadaveric heads, the mSTA and fSTA were identified in
all 16 (100%) specimens; however, no pSTA was seen in one (6.25%). The mean distance
from the STA bifurcation to the zygomatic arch was 3 (1.5–4.5) cm. The most anterior
part of the fSTA passed through the temporal peak in 2 specimens (12.5%), passed anterior
to the temporal peak in 9 (56.25%) specimens, and passed posterior to the temporal
peak in the remaining 5 (31.25%) specimens ([Table 1 ]). The average distance from the fSTA to the temporal peak in the anterior and posterior
aspects was 0.44 (0.2–0.7) cm and 0.52 (0.3–0.8) cm, respectively ([Table 2 ] and [Fig. 4 ]).
Table 1
The results of cadaveric head dissection
No.
fSTA anterior to temporal peak (cm)
STA bifurcation to zygomatic arch (cm)
1
−0.5
4.5
2
−0.8
3
3
0.4
3
4
0.4
2.5
5
0.2
3.5
6
−0.5
1.5
7
0.5
3
8
0.5
2.2
9
0.5
3.5
10
0
4.5
11
0
4
12
0.4
–
13
0.4
3.4
14
−0.3
2.7
15
−0.5
2.2
16
0.7
2
Average
–
3
Abbreviations: -: no parietal branch of STA; fSTA, frontal branch of superficial temporal
artery; STA, superficial temporal artery.
Table 2
The relationship between the most anterior part of the frontal branch of superficial
temporal artery and the temporal peak
fSTA
Anterior to TP
Through TP
Posterior to TP
Number
9 (56.25%)
2 (12.5%)
5 (31.25%)
Range (cm)
0.2–0.7
–
0.3–0.8
Average (cm)
0.44
–
0.52
Abbreviations: fSTA, frontal branch of superficial temporal artery; TP, temporal peak.
Fig. 4 The results of our cadaveric study of the course of the fSTA. The blue dot represents
the temporal peak, the solid white line represents the hairline, and the dashed white
line represents the superior border of the zygomatic arch. fSTA, frontal branch of
the STA; mSTA, main trunk of the STA; pSTA, parietal branch of the STA; STA, superficial
temporal artery.
The sample in our clinical study comprised 19 patients who underwent clipping via
IHA for AcoA aneurysms using a modified bicoronal incision. Of these patients, 9 (47.4%)
were male and 10 (52.6%) were female. The average age was 56.9 (30–82) years. In 16
(84.2%) of the patients, subarachnoid hemorrhage was the presenting symptom. Our evaluation
of the postoperative CTAs found that the right fSTA was preserved in 14 (73.7%) patients
and the left fSTA in 16 (84.2%) patients. The overall rate of fSTA preservation was
78.9% (30/38). In 13 (68.4%) of the patients, both fSTAs were preserved. The pSTA
was preserved in all patients (100%) ([Tables 3 ] & [4 ]). Frontalis paralysis was not observed in any patient.
Table 3
The characteristics of patients and the preservation of the frontal branch of superficial
temporal artery after modified bicoronal incision
Case no.
Age (y), gender
Rupture
Frontal branch of STA
Parietal branch of STA
Preoperative CTA
Postoperative CTA
Preoperative CTA
Postoperative CTA
R
L
R
L
R
L
R
L
1
82, F
Y
Y
Y
Y
Y
Y
Y
Y
Y
2
53, F
Y
Y
Y
Y
N
Y
Y
Y
Y
3
57, M
Y
Y
Y
Y
Y
Y
Y
Y
Y
4
60, F
Y
Y
Y
Y
Y
Y
Y
Y
Y
5
30, M
Y
Y
Y
Y
Y
Y
Y
Y
Y
6
64, M
Y
Y
Y
Y
Y
Y
Y
Y
Y
7
63, F
N
Y
Y
Y
Y
Y
Y
Y
Y
8
62, M
Y
Y
Y
Y
Y
Y
Y
Y
Y
9
65, F
N
Y
Y
Y
Y
Y
Y
Y
Y
10
40, M
Y
Y
Y
N
Y
Y
Y
Y
Y
11
69, M
Y
Y
Y
N
N
Y
Y
Y
Y
12
40, M
Y
Y
Y
N
Y
Y
Y
Y
Y
13
57, F
Y
Y
Y
Y
Y
Y
Y
Y
Y
14
52, F
Y
Y
Y
Y
Y
Y
Y
Y
Y
15
67, F
N
Y
Y
Y
Y
Y
Y
Y
Y
16
60, F
Y
Y
Y
N
Y
Y
Y
Y
Y
17
66, F
Y
Y
Y
N
N
Y
Y
Y
Y
18
43, M
Y
Y
Y
Y
Y
Y
Y
Y
Y
19
52, M
Y
Y
Y
Y
Y
Y
Y
Y
Y
Abbreviations: CTA, computed tomography angiography; F, female; L, left side; M, male;
N, no; R, right side; STA, superficial temporal artery; Y, yes.
Table 4
The preservation of the frontal branch of superficial temporal artery after modified
bicoronal incision
Preservation of fSTA
Right side (n = 19)
Left side (n = 19)
Both side (n = 19)
Average
(n = 38)
14 (73.7%)
16 (84.2%)
13 (68.4%)
30 (78.9%)
Abbreviation: fSTA, frontal branch of superficial temporal artery.
[Figs. 3 ] and [4 ] present cases 9 and 10, respectively, as illustrative examples. While both sides
of the fSTA were preserved in case 9 ([Fig. 5 ]), the incision caused the loss of the right fSTA in case 10 ([Fig. 6 ]).
Fig. 5 Illustrative case (case 9). (A , B ) Preoperative CTA images showing both sides of the fSTA and the ascending frontal
artery (arrow). (C , D ) The modified bicoronal incision is represented by the line of staples. The preservation
of the fSTA on both sides was confirmed through the identification of the ascending
frontal artery (arrow) on the postoperative CTA. CTA, computed tomography angiography;
fSTA, frontal branch of the superficial temporal artery.
Fig. 6 Illustrative case (case 10). (A , B ) Preoperative CTA images showing both sides of the fSTA and the ascending frontal
artery (arrow). (C , D ) The modified bicoronal incision is represented by the line of staples. The preservation
of the left fSTA and the sacrifice of the right fSTA were confirmed through the identification
of the ascending frontal artery (arrow) on the postoperative CTA. CTA, computed tomography
angiography; fSTA, frontal branch of the superficial temporal artery.
Discussion
Blood Supply to the Frontal Scalp and Bicoronal Scalp Flap
The fSTA, supraorbital artery, and supratrochlear artery provide blood supply to the
forehead area of the scalp. After classic bicoronal incisions, which frequently sacrifice
the STA, the bilateral supratrochlear and supraorbital arteries are often left to
provide the entire scalp flap blood supply and must be preserved during scalp flap
elevation.[23 ]
[24 ] Because of this, there is a risk of ischemia in the bicoronal scalp flap when the
STA is sacrificed. The preservation of the STA provides a better blood supply to the
scalp flap, especially in cases where the supratrochlear and supraorbital arteries
are absent or injured. It is also possible that STA preservation would reduce postoperative
scalp complications but further research is needed to compare the rates of such complications
in those with and without postoperative STA preservation.
Previous Anatomic Studies of the STA
Studies of the anatomy of the STA have contributed various insights. Tayfur et al
studied 33 cadaveric heads and found that the STA bifurcation was located above the
zygomatic arch in 62%. The mean distance between the STA bifurcation and the superior
border of the zygomatic arch was 2.3 cm.[25 ] Jean-Philippe et al used the head and neck CTA of a patient sample to study the
STA. They found that the length of the fSTA ranged from 23 to 101 mm, with a mean
of 58 mm. Its angle in relation to the oculo-meatal line ranged from 0 to 60 degrees,
with a mean of 28 degrees. The STA bifurcation was located above the zygomatic arch
in 61.54% and was nearly located above the oculo-meatal line in 99.04%. The distance
between the STA bifurcation and the zygomatic arch ranged from 0 to 47 mm, with a
mean of 12 mm.[17 ]
Pinar and Govsa studied the anatomy of the STA in 27 sides of cadaveric heads. The
fSTA was present in all of their specimens and ran forward to the front of the head,
parallel with the upper corner of the orbicularis oculi muscle. It then returned to
the galea to supply blood to the frontalis muscle. The diameter of the fSTA was greater
than that of the pSTA in 55% of their specimens.[19 ] Tubbs et al studied the STA anatomy of cadaveric heads in relation to deeper brain
structures. They found the STA bifurcation to be located an average of 3 cm superior
to the tragus. Many deep brain structures could be identified using the branches of
the STA, but the courses of these branches should first be confirmed by palpation
or Doppler identification.[26 ] Kim et al used three-dimensional CTA to study the anatomy of the STA. Three landmarks
(the posterior margin of the mandible condyle, the superior margin of the zygomatic
arch, and the keyhole) were used as reference points for localization of the STA bifurcation.
They found that 82.6% of STA bifurcations occur above the zygomatic arch at a mean
distance of 21.7 mm.[27 ]
Koziej et al studied 419 STAs on head CTAs. They found a mean distance between the
lateral angle of the orbital rim and the fSTA of 36.6 mm. Frontal and parietal branches
were detected in 98.1 and 90.7% of patients, respectively. The STA bifurcation was
located above the zygomatic arch in 75.6%, below in 14.7%, and on the zygomatic arch
in 9.7%. The mean distance from the center of the zygomatic arch to the STA bifurcation
was 16.8 mm.[18 ] In a meta-analysis of STA morphology, the STA bifurcation was located above and
on the zygomatic arch in 79.1 and 11.1% of instances, respectively. The frontal and
parietal branches of the STA were present in 97.6 and 96.4%, respectively. The fSTA
was found to have a significantly larger mean diameter than the pSTA, suggesting that
the fSTA is the main branch.[28 ]
Kleintjes determined the lateral orbital rim to be the key visible or palpable landmark
required to predict the course of the fSTA. The fSTA was present in 70% of their specimens
and coursed anterosuperiorly from the temporal area to the forehead, turning superoposteriorly
at an acute angle from the forehead as the ascending frontal artery or transverse
frontal artery neared the lateral orbital rim. However, the exact location of this
fSTA turning point is not well described in their study.[20 ]
Despite these previous anatomical studies, the anatomical landmarks of the fSTA have
not been established. Our cadaveric study demonstrated the close relationship between
the most anterior part of the fSTA and the temporal peak. All of the fSTA passed just
anterior or posterior (within 0.5 cm) to the temporal peak.
The Risk of STA Injury Using the Modified Bicoronal Incision
The traditional bicoronal incision resembles the pterional incision, which is performed
on both sides of the skull. Suanchan et al[11 ] found that standard pterional incisions pose a significant risk of injury to the
STA, particularly affecting the pSTA. Consequently, the risk of vascular injury is
also substantial with the classic bicoronal incision.
Previous anatomical studies of the STA have shown that the majority of the fSTA follows
a path that is close to the oculo-meatal line in the temporal region. It then extends
forward toward the forehead, running parallel to the upper edge of the orbicularis
oculi muscle.[17 ]
[19 ]
[20 ] Consequently, employing a modified bicoronal incision may pose a risk of injury
to the STA, particularly concerning the fSTA.
There have been no previous clinical studies of the preservation of the STA after
modified bicoronal incisions. Our clinical study showed a high rate of fSTA preservation
(78.9%) and 100% rate of pSTA preservation after modified bicoronal incisions. This
was in accordance with our cadaveric finding that the majority of fSTAs (68.75%) pass
through and just anterior to the temporal peak.
The Risk of Facial Nerve Injury Using the Modified Bicoronal Incision
Shin et al studied 55 sides of cadaveric heads and found that the temporal branch
of the facial nerve (TFN) generally courses 1 to 2 cm anteriorly and inferiorly to
the fSTA in the temporal region. However, in 3.6% of their specimens, the TFN ran
just beneath the fSTA.[29 ] This study found that a scalp incision on or anteroinferior to the fSTA risks injury
to the TFN. To avoid this, the incision should be superior and posterior to the fSTA.
Because the majority of fSTAs passes through and just anterior to the temporal peak
and the modified bicoronal incision begins just posterior to the temporal peak, the
risk of TFN injury using this incision is low.
Limitations of the Modified Bicoronal Incision
The traditional or classic bicoronal flap provides critical exposure of the maxillofacial
area with an aesthetic outcome.[5 ]
[23 ]
[24 ] In the modified bicoronal incision, the skin incision is shorter and a smaller area
of bone exposure is required. This modified incision provided adequate exposure for
the modified transbasal bifrontal craniotomy, allowing access to the anterior and
inferior surfaces of the frontal lobe, the anterior skull base, and the frontal air
sinuses. Since the inferior limit of exposure using the modified incision is the superior
orbital rim, inferior access to the maxillofacial area is not possible. This limitation
can be corrected by extension of the incision inferior to the temporal peak along
the hairline, but the fSTA is then frequently transected. The lateral limit of this
incision is just lateral to the temporal line. When temporal lobe or lateral frontal
lobe access is needed, the classic bicoronal incision is necessary.
Complications of Bicoronal Incisions
Long-term alopecia has been reported as a postoperative complication of 7 to 18% of
bicoronal incisions. The risk factors found to contribute to this complication were
the use of a Raney clip for a prolonged period to control bleeding and Colorado tip
monopolar cautery of cutaneous and subcutaneous incisions.[2 ]
Although postoperative alopecia was not assessed in the present study, we routinely
use Raney clips and cold steel scalpel for cutaneous and subcutaneous incisions. This
is because the likelihood of this complication is not expected to be high with modified
bicoronal incisions as the incision is performed just behind the hairline and the
high preservation rates of the STA and the supraorbital and supratrochlear arteries
suggest a robust blood supply to the scalp flap. Nevertheless, this issue should be
studied further in the future.
Strength and Limitations of Our Study
The retrospective and descriptive nature of our study was its main limitations. The
relatively small numbers of cadavers and patients were also drawbacks. The clinical
outcomes, such as scalp flap ischemia, delayed wound healing, wound infection, and
alopecia, were not assessed in this series. However, it can be assumed that these
complications should be minimized when the arterial supply is preserved to the greatest
extent possible.
To the best of our knowledge, this is the first study to simplify the localization
of the fSTA during surgery through the determination of its location in relation to
the hairline.
Conclusion
The most anterior part of the fSTA was located very close to the temporal peak of
the hairline. The modified bicoronal skin incision for bifrontal craniotomy and modified
transbasal craniotomy was an effective means of STA preservation.