Using the Arcuate Eminence–Trigeminal Notch Line to Localize the Anterior Wall of the Internal Auditory Canal in a Subtemporal Approach: An Anatomical Study

Abstract

Background  This article aims to describe the regional anatomy of the anterior end of the arcuate eminence, the lateral end of the trigeminal notch, and the line connecting the two (i.e., the arcuate eminence–trigeminal notch line [ATL]) and to determine whether the ATL could be used as a landmark for localizing the internal auditory canal (IAC).

Methods  Twenty sides of the middle cranial fossae were examined. The anterior end of the arcuate eminence, the lateral end of the trigeminal notch, the ATL, and other crucial structures were exposed. The relevant distance and angle of related structures in the anterior wall of the petrosal bone were measured.

Results  The anterior end of the arcuate eminence and the lateral end of the trigeminal notch could be identified in all specimens. The anterior end of the arcuate eminence lay over the geniculate ganglia and the vestibule area, and could be visualized directly or determined from the intersection of the long axes of the greater superficial petrosal nerve and arcuate eminence. On the petrous ridge, the lateral end of the trigeminal notch was also the transitional point of the suprameatal tubercle and trigeminal notch. The ATL corresponded to the projection of the anterior wall of the IAC on the anterior surface of the petrous bone.

Conclusion  The ATL corresponded to the projection of the anterior wall of the IAC on the anterior petrous surface and could be used as an alternative landmark for localizing the anterior wall of the IAC.

Introduction

The subtemporal approach refers to a group of skull base approaches that are widely employed by neurosurgeons and is characterized by the use of the space between the temporal lobe and petrous bone. This approach has important variations such as the middle fossa approach, extended middle fossa approach, and anterior transpetrosal (Kawase) approach. These approaches are used for the resection of lesions involving the upper petroclival region and the medial petrous bone, such as petroclival meningiomas, trigeminal schwannomas, small vestibular schwannomas inside the internal auditory canal (IAC), and other middle–posterior fossa communicating tumors.

The localization and opening of the IAC is frequently necessary in these approaches and is regarded a critical technique in skull base surgery. However, the IAC is located deep inside the petrous bone and is surrounded by vital structures, including the cochlear, vestibule, semicircular canals, petrous segment of the internal carotid artery, facial nerve, geniculate ganglia, and greater superficial petrosal nerve (GSPN). This anatomical complexity causes substantial difficulties and poses a challenge to neurosurgeons regarding the correct localization and safe exposure of the IAC.

Since the 1960s, several techniques have been developed for localizing the IAC in subtemporal approaches ( Table 1 ). ^{1 2 3 4 5 6 7 8 9 10 11} These techniques require the exquisite dissection of the dura matter of the middle fossa to identify the GSPN and geniculate ganglia or the removal of the bone covering the geniculate ganglia, the superior semicircular canal, or the tympanic cavity. The procedures carry a risk of hearing loss and facial paralysis and require complex measurements of distances and angles that are inconvenient to perform intraoperatively. Therefore, a more reliable, direct, and easy-to-use method for localizing the IAC that can be applied to epidural and intradural subtemporal approaches is needed.

Table 1. Techniques localizing the IAC in subtemporal approaches.

Reporter	Year	Technique to localize IAC
House	1961	Sequential drilling of the bone overlapping GSPN, GG, labyrinthine segment of the facial nerve, and fundus of IAC
Fisch and Rüedi	1970	Drill and expose SSCC, the line ∼60 degrees centered over superior ampulla of SSCC
Garcia-Ibanez and Garcia-Ibanez	1980	The bisection line of the angle formed by GSPN and AE
Kawase et al	1985	The line extending from the external auditory meatus through the tegmen tympani eminence
Matsunaga et al	1991	Biauricular line in cases with useful hearing, drill, and expose malleus head to localize IAC in cases without useful hearing
Catalano and Eden	1993	The line going through the zygomatic root and malleus head
Sennaroglu and Slattery	2003	External auditory canal, or the bisection line of the angle between GSPN and AE or SSCC
Lee et al	2006	The line starting from the foramen spinosum and forming a right angle with the line between posterior origin of the zygomatic arch and foramen spinosum
Cheng et al	2009	The line going through the point 1 cm posterior to the posterior margin of trigeminal impression on the petrous ridge, and angling 135 degrees with petrous ridge
Lan and Shiao	2010	∼1 cm away from the tip of GG on the line angled at 96 degrees with GSPN
Sampath et al	2012	The line connecting the point 2.38 cm posterolateral to the petrous apex along the petrous ridge and the point 2.39 cm posterior to the foramen spinosum along the foramen spinosum–foramen ovale line

Abbreviations: AE, arcuate eminence; GG, geniculate ganglion; GSPN, greater superficial petrous nerve; IAC, internal auditory canal; SSCC, superior semicircular canal.

In this study, we selected the anterior end of the arcuate eminence, the lateral end of the trigeminal notch (TN), and the imaginary line connecting these two points (i.e., the arcuate eminence–TN line [ATL]) as the key landmarks for localizing the IAC. The position of the anterior end of the arcuate eminence, that is, at the lateral end of the TN, is fixed, and easy to recognize. Hence, we chose the ATL as the anatomical landmark. We examined the anatomical relationships between the ATL and IAC, cochlear, petrous segment of the internal carotid artery, and semicircular canals. We propose that the ATL can serve as an alternative reference landmark for localizing the anterior wall of the IAC in subtemporal approaches.

Methods

Both sides of 10 formalin-fixed normal Chinese adult cadaver heads with intact middle fossae were examined under a microscope (Mitaka MM51, Mitaka, Japan) at 1.9 to 46.6× magnification. The vessels of the specimen were injected with colored silicone, and the vertex and brain of each cadaver head was removed. The dura matter covering the middle cranial fossa was dissected and removed. The integrity of the GSPN and Meckel's cave was carefully preserved. The arcuate eminence, TN, petrous ridge, petrous tubercle, suprameatal tubercle, and trigeminal impression were identified.

The main anatomical landmarks were as follows: the anterior end of the arcuate eminence (point A), the lateral end of the TN (point B), the intersection of the petrous ridge and the perpendicular line across point A (point C), the intersection of the GSPN and the lateral edge of the trigeminal impression (point D), and the intersection of the petrous ridge and the projection line of the posterior wall of the IAC (point E). If the arcuate eminence was difficult to identify, the anterior end of the arcuate eminence was identified at the intersection of the long axes of the GSPN, and the arcuate eminence. The rhombic area enclosed by points A, B, C, and D was defined as the Kawase rhomboid. The line connecting points A and B was termed the ATL ( Fig. 1 ).

Fig. 1.

The relationship between the critical structures one the anterior wall of the petrosal bone. ( A ) Main anatomical landmarks and structures in the schematic illustration. ( B ) Main anatomic structures on the anterior wall of the petrosal bone. ( C ) The position of the cochlear and the petrous internal carotid artery. ( D ) Angles between the anterior and posterior wall of the internal auditory canal (IAC) and the petrous ridge. AE, arcuate eminence; GSPN, greater superficial petrous nerve; SMT, suprameatal tubercle; PICA, the petrous segment of the internal carotid artery; Point A: the anterior end of arcuate eminence; Point B: the lateral end of the trigeminal notch; Point C: the intersection of the petrous ridge and the perpendicular line across point A; Point D: the intersection of GSPN and the lateral edge of the trigeminal impression; Point E: the intersection of the petrous ridge and the projection line of posterior wall of the IAC; Point F: the intersection of the arcuate eminence–trigeminal notch line (ATL) and the medial cochlear margin; Point I: the intersection of lateral margin of the trigeminal impression with posterior margin of the petrous internal carotid artery; Point J: the medial limit of the trigeminal notch on the petrous ridge; ATL: the line connecting Point A and B was termed as the arcuate eminence–trigeminal notch line; ÐABJ: the angle between the anterior wall of IAC and the posterior surface of the petrous bone; ÐAEC: the angle between the posterior wall of IAC and the posterior surface of the petrous bone.

Neurosurgical power systems (Xishan DK-N-MS, Chongqing, China) and diamond drills with different sizes were employed for bone drilling. Bone removal was initiated at the lateral end of the TN and along the medial portion of the ATL. With the removal of the suprameatal tubercle, the dura matter covering the upper and medial margins of the internal acoustic meatus was revealed. Thereafter, drilling was performed to trace the dura matter into the IAC and to expose the superior and anterior walls of the IAC. After the medial portion of the IAC was identified, bone removal was expanded anteriorly to the horizontal segment of the petrous internal carotid artery; laterally along the upper wall of the IAC to its fundus, cochlear, geniculate ganglia, and superior semicircular canal; and medially under Meckel's cave and the trigeminal impression to the inferior petrous sinus. Finally, the posterior wall of the IAC was exposed.

Once bone drilling was completed, some additional landmarks were determined to examine the anatomical relationships between the ATL and relevant structures. These landmarks consisted of the intersection of the ATL and the medial cochlear margin (point F), the intersection of the lateral margin of the trigeminal impression with the posterior margin of the petrous internal carotid artery (point I), and the medial limit of the TN on the petrous ridge (point J).

The distances and meaningful angles were respectively measured with Vernier calipers (accuracy, ± 0.02 mm) and a protractor (accuracy, ± 0.01 degree) ( Fig. 1 ).

Results

In all specimens, the anterior end of the arcuate eminence, and the lateral end of the TN could be identified. The angle between the anterior wall of the IAC and the posterior surface of the petrous bone (ÐABJ) was a round obtuse angle measuring 134.42 ± 1.08 degrees ( Fig. 1 ). The turning of the two planes was just beneath the lateral end of the TN. On the petrous ridge, the lateral end of the TN was also the transitional point of the TN and the suprameatal tubercle. The anterior end of the arcuate eminence lies over the geniculate ganglia and the vestibule area. The imaginary ATL, which measured 18.25 ± 2.20 mm, corresponded to the projection line of the anterior wall of the IAC on the anterior surface of the petrous bone and was actually the diagonal of the Kawase rhomboid. The angle between the posterior wall of the IAC and the posterior surface of the petrous bone (ÐAEC) was an acute angle measuring 76.86 ± 7.38 degrees ( Fig. 1 ). The turning of the two planes was just below the lateral end of the suprameatal tubercle. The virtual line between the lateral end of the suprameatal tubercle and the anterior end of the arcuate eminence corresponded to the posterior wall of the IAC. The triangle formed by the suprameatal tubercle and the anterior end of the arcuate eminence corresponded to the projection of the IAC on the anterior petrous surface.

Drilling the bone centered on the lateral end of the TN to expose the medial portion of the internal acoustic meatus and IAC was efficient and relatively safe. Along the petrous ridge, the suprameatal tubercle, which has an average length of 12.13 ± 2.22 mm and an average height of 4.86 ± 1.17 mm, was located lateral to the TN, and rode on the superior margin of the internal acoustic meatus. The internal acoustic meatus could be safely exposed after drilling out the suprameatal tubercle, and the anterior wall of the IAC could be exposed after drilling the bone along the ATL. The medial side was relatively safe because the cochlear, which is located at the corner formed by the ATL, geniculate ganglia, and GSPN, could only be encountered at the lateral portion of the ATL. Anteriorly along the lateral margin of the trigeminal impression, the distance between the lateral end of the TN, and the posterior margin of the horizontal petrous internal carotid artery was 9.64 ± 1.73 mm. The span of the TN was approximately 7.02 ± 1.03 mm, and the medial end of the TN continued with the petrous tubercle.

Table 2 shows the detailed information about the measurements.

Table 2. Distance and angle of related structures in the anterior wall of the petrosal bone.

		Mean ± SD
A-B	Distance between the outer edge of the trigeminal nerve notch and point of the intersection of the apex of the arcuate eminence with the initial point of GSPN	18.25 ± 2.209 (mm)
B-C	Distance between the outer edge of the trigeminal nerve notch and the intersection of the perpendicular line through the apex of the arcuate eminence	14.81 ± 2.63 (mm)
B-D	Distance between the outer edge of the trigeminal nerve notch and the intersection of trigeminal nerve and the GSPN	11.46 ± 1.76 (mm)
B-E	Distance between the outer edge of the trigeminal nerve notch and the point of the projection of the superior wall of the IAC on the petrous ridge	12.13 ± 2.22 (mm)
B-F	Distance between the outer edge of the trigeminal nerve notch and point of the intersection with the inner side of the cochlear and the ATL	9.20 ± 2.20 (mm)
B-I	Distance between the outer edge of the trigeminal nerve notch and the point of the intersection with the petrous segment of the internal carotid artery and the trigeminal nerve	9.64 ± 1.73 (mm)
B-J	Distance between the outer edge of the trigeminal nerve notch and the point of the inner edge of the trigeminal nerve notch	7.02 ± 1.03 (mm)
C-H	Distance between the point of intersection of the petrous ridge with the perpendicular line across the point A and the point of the projection of the posterior semicircular canal on the petrous ridge	12.32 ± 2.97 (mm)
D-G	Distance between the point of the intersection of trigeminal nerve and the GSPN and point of the intersection of the inner side of the cochlea with the GSPN	7.96 ± 2.03 (mm)
Thickness of point K	Thickness of the midpoint of BE	4.86 ± 1.17 (mm)
Thickness of point L	Thickness of the midpoint of FK	4.79 ± 1.26 (mm)
Angle ABJ	Angle between the anterior wall of the IAC and the petrous ridge	132.42 ± 11.08 (°)
Angle AEC	Angle between the posterior wall of the IAC and the petrous ridge	76.86 ± 7.38 (°)

Abbreviations: ALT, arcuate eminence–trigeminal notch line; GSPN, greater superficial petrous nerve; IAC, internal auditory canal; SD, standard deviation.

Discussion

We used the lateral end of the TN, the anterior end of the arcuate eminence, and the ATL as alternative landmarks for localizing the anterior wall of the IAC. In the past 60 years, several techniques and landmarks have been developed and used in subtemporal approaches to localize the IAC ( Table 1 ). However, some disadvantages are associated with these methods. The GSPN and foramen spinosum are covered by dura matter and cannot be directly visualized intradurally. The superior semicircular canal, geniculate ganglia, and proximal segment of the GSPN are covered by dura mater and bone. The unroofing of these structures carries a risk of deafness, facial paralysis, and alacrimia. So, this method must be performed correctly to avoid these complications. The malleus head is protected by the tegmen tympani, and opening the middle ear is not only unnecessary in most neurosurgery cases but also poses a potential danger of hearing loss, cerebrospinal fluid leakage, and meningitis. The zygomatic root and external auditory meatus are far from the IAC and are difficult to focus on a specific point. The direction of the biauricular line cannot be precisely defined when the head is draped and rotated during the operation. The accurate measurement of the distances and angles is tedious and difficult to achieve intraoperatively, particularly on the perpendicular petrous surface with the subtemporal approach. Furthermore, the IAC is a bony canal with a certain width and length, specifically in its medial portion. Existing methods are effective at determining the orientation of the long axis of the IAC but fail to precisely locate the anterior or posterior wall. By using our method, the ATL and its two endpoints can be readily identified both intradurally and extradurally without the need for drilling or measurement. Consequently, most of these problems can be avoided.

The arcuate eminence is an arc-like bony protrusion on the anterior surface of the petrous bone. It is evident in more than 85% of cases and is a commonly used landmark in subtemporal approaches. ^{12 13 14 15} The bisection technique of Garcia-Ibanez, ^{16 17 18 19 20} which is based on the arcuate eminence and GSPN, is still one of the most popular methods. The anterior end of the arcuate eminence lies over the vestibule area and can be visualized directly or determined from the intersection of the axes of the GSPN and the arcuate eminence. ⁷ The TN is a depressed segment between the petrous tubercle and the suprameatal tubercle; it represents the posterior margin of the trigeminal impression and forms the lower border of the trigeminal porus. ²¹ On the petrous ridge, the lateral end of the TN is also the medial end of the suprameatal tubercle. The anterior wall of the IAC turns medially to the posterior surface of the petrous apex just below the lateral point of the TN, and the two planes form an approximately 135-degree angle. Therefore, the ATL corresponds to the projection of the anterior wall of the IAC ( Figs. 2 and 3 ). Under the guidance of ATL, the anterior wall of the IAC was exposed with removal of the suprameatal tubercle. In individuals with obscure arcuate eminence, we can also utilize the imaginary line drawn from the lateral end of the TN at a 135-degree angle away from the petrous ridge to trace the anterior wall of the IAC. Cheng et al ⁹ reported that the lateral end of the TN can be used as the original landmark. They advocated using the line starting 1 cm posterior to this point on the petrous ridge to determine the direction of the long axis of the IAC.

Fig. 2.

The relationship between the ATL and the anterior wall of the IAC. ( A ) Exposure of the AE, the GSPN, and the location of the ATL. ( B ) Cochlear locates at the initial part of the ATL. ( C ) The IAC is exposed and the ATL is parallel to the anterior wall of the IAC. ( D ) Course of the ATL and the IAC. AE, arcuate eminence; ATL, arcuate eminence–trigeminal notch line; FS, foramen of spinosum; GSPN, greater superficial petrous nerve; IAC, internal auditory canal; SMT, suprameatal tubercle.

Fig. 3.

A cerebellopontine angle (CPA) schwannoma of the left side was removed by the left temporo-occipital epidural approach. ( A ) Preoperative magnetic resonance imaging (MRI) scan. ( B ) Preoperative computed tomography (CT) scan. ( C ) Exposure of the AE and the GSPN. ( D ) Drilling along the ATL. ( E ) The IAC is exposed and the ATL is parallel to the anterior wall of the IAC. ( F ) Postoperative MRI scan. AE, arcuate eminence; ATL, arcuate eminence–trigeminal notch line; GSPN, greater superficial petrous nerve; IAC, internal auditory canal.

We emphasize the importance of the anatomical relationships between the ATL and surrounding structures when drilling the medial petrous bone ( Figs. 2 and 3 ). First, starting the drilling along the ATL is an efficient approach for identifying the anterior wall of the IAC ( Fig. 3 ). In a real surgical anatomy, the anterior wall of the IAC could be exposed after drilling the bone along the ATL. Some authors have demonstrated that drilling in a medial-to-lateral fashion is preferred for the opening of the IAC. ^{3 7 8 9 19} However, it should be noted that the cochlear is located at the corner of the GSPN, the geniculate ganglia, and the ATL. Although the IAC itself is approximately 18 mm long, the safe distance from the lateral end of the TN to the medial cochlear margin is only approximately 9 mm. Second, the suprameatal tubercle is approximately 12 mm long and is located lateral to the lateral end of the TN; it forms the upper border of the internal acoustic meatus. When drilling the suprameatal tubercle, the internal acoustic meatus can be rapidly identified from above, and the upper and anterior walls of the IAC can be easily exposed after further drilling along the ATL. Third, the posterior margin of the petrous internal carotid artery is approximately 9 mm anterior to the lateral end of the TN. Great care should be taken to avoid accidental damage to the internal carotid artery when drilling the bone anterior to the ATL.

When using the ATL to locate the IAC, anatomical variants and bony destruction should be considered. We believe that careful examination of the preoperative computed tomography images and three-dimensional imaging reconstruction techniques may help surgeons correctly assess the bony structures of the petrous apex. In addition to the ATL, a series of anatomical landmarks exist in the middle cranial fossa as reported in previous studies. In complicated cases with obvious anatomical variations or bony destruction, it is better to use the ATL in combination with other landmarks rather than to rely on a sole anatomical landmark. If available, an image-guided navigation system can also be helpful for the accurate localization of the IAC.

Conclusion

We confirmed that the ATL corresponded to the projection of the anterior wall of IAC on the anterior petrous surface. We suggest that the ATL can be used as an alternative landmark for localizing the anterior wall of the IAC using the subtemporal approach.