Landmark structures to approach the internal auditory canal: a dimensional study related to the middle cranial fossa approach

The M-Point A Novel Landmark in Middle Fossa Surgery: A Cadaveric Morphometric Study

BACKGROUND

The arcuate eminence (AE) is an anatomically consistent bony protrusion located on the upper surface of the petrous bone that has been previously studied as a reference for lateral skull base approaches. There is a paucity of information in the neurosurgical literature seeking to improve the safety of the extended middle cranial fossa (MCF) approach using detailed morphometric analysis of the AE.

OBJECTIVE

To evaluate the use of the AE as an anatomical landmark to help with early identification of the internal acoustic canal (IAC) in MCF approaches by means of a cadaveric study, using a new morphometric reference termed the "M-point."

METHODS

A total of 40 dry temporal bones and 2 formalin-preserved, latex-injected cadaveric heads were used. The M-point was established as a new anatomic reference by identifying the intersection of a line perpendicular to the alignment of the petrous ridge (PR), originating from the midpoint of the AE, with the PR itself. Subsequent anatomical measurements were performed to measure the distance between M-point and IAC. Additional distances, including PR length and the anteroposterior and lateral AE surfaces, were also measured.

RESULTS

The mean distance between the M-point and the center of the IAC was 14.9 mm (SD ± 2.09), offering a safe drilling area during an MCF approach.

CONCLUSIONS

This study provides novel information on identification of a new anatomic reference point known as the M-point that that can be used to improve early surgical identification of the IAC.

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

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.

Three-Dimensional Changes of the Auditory Canal in a Three-Year Period during Adolescence Using CBCTs

Purpose

There is a lack of identifying suitable regions in the head that can be used for three-dimensional superimposition techniques. For this reason, with the use of cone-beam computed tomography (CBCT), the ear canals were analyzed to verify changes during a period of three years in the adolescent years.

Methods

CBCTs from fifty-six patients (ages: 10 to 20) were used to landmark the anatomy of the ear canals. Each patient was analyzed using two CBCT reconstructions that were taken approximately three years apart. AVIZO® software was used to locate 28 landmarks distributed following the ear canal path and foramina (ovale, spinosum, rotundum, etc.) in the cranial base to obtain spatial relationships. Three-dimensional coordinates were obtained from the landmarks, and the average distance between various landmark pairings was calculated. The repeated measure ANCOVA was used to determine statistical significance.

Results

In the main data set, the largest mean distance change was found to be 4.37 mm ±  18.29 mm between the left foramen ovale and the left superior medial ear canal opening. The smallest mean distance change was 0.18 mm ± 3.25 mm between the right inferior lateral ear canal opening and the right inferior medial ear canal opening.

Conclusions

During the adolescent years, the ear canal presents dimensional changes. Even though in different areas throughout the canal, the average distances were minor, still, large standard deviations were present; thus, caution should be taken when trying to use this structure for superimposition of CBCTs.

Localization of the internal auditory canal by identifying the intersection of the posterior border of the trigeminal ganglion and the superior petrosal sinus in cadavers

The identification of the internal auditory canal (IAC) has relied on visualization of the arcuate eminence (AE). However, it is not uncommon that the topographic markers on the middle cranial base are featureless and difficult to identify, including the AE. "Point T", the intersection of the posterior border of the trigeminal ganglion (TG) and the superior petrosal sinus (SPS) has been presented as a marker to localize the IAC. Thirty-four sides from 17 dry skulls and five formalin-fixed latex-injected cadaver heads were studied. In the dry skull, the imaginary line of the IAC was defined by connecting the uppermost point of the rim of the external auditory canal and the uppermost point of the porus acousticus on the petrosal ridge. Point T was defined as the posterior margin of the trigeminal impression on the petrosal ridge. For cadaver heads, a standard middle fossa approach was performed, and the line of the IAC was defined by joining the tip of Bill's Bar and the midpoint of the dura on the porus acousticus. Point T was expressed as the intersection of the posterior border of the TG and the SPS. The distance between point T and the medial end of the IAC was termed "segment TI", and the angle spanning from segment TI to the IAC was "angle theta (theta)". In dry skulls, segment TI (mean+/-standard deviation [SD]) measured 9.74+/-0.71mm and angle theta was 135.56+/-3.21 degrees ; in cadaver heads, segment TI measured 10.25+/-0.58mm and angle theta measured 133.43+/-2.00 degrees . An alternative for localization of the IAC is proposed when the AE is difficult to identify in the middle cranial fossa. As a mnemonic, the IAC can be located by identifying point T first, and then tracing 1cm posteriorly along the SPS and turning laterally 90 degrees plus half of 90 degrees (135 degrees total).

Petrous anatomy for middle fossa approach

OBJECTIVE

The objective was to describe the relationship of anatomical landmarks required for the middle fossa approach to lesions of the petrous apex and internal auditory canal (IAC). Landmarks for safe identification of the IAC are defined, as are two zones (safe zones I and II) anterior and posterior to the IAC.

STUDY DESIGN

Temporal bone anatomical study.

METHODS

Ten temporal bones underwent high-resolution computed tomography followed by wet bench dissection. A set of 39 different distances between vital structures was measured, where possible, from both radiology films and under the microscope. Mean, median, minimum, and maximum measurements of all distances were determined. Angular measurements were made using a specially designed instrument.

RESULTS

Measurements were made in an anatomical position to mimic the middle fossa surgical technique. Measurements are presented for the superior surface of the temporal bone from the sigmoid sinus and inner table to vital structures, from the anterior and posterior petrous apex, and from the IAC.

CONCLUSIONS

Petrous apex anatomical knowledge is required for the safe middle fossa approach to the IAC and petrous apex. Two safe zones are defined that should prevent damage to the cochlea and the superior semicircular canal.

Use of reconstructed sagittal computed tomography images to plan middle cranial fossa surgery

OBJECTIVE

To facilitate planning in temporal bone surgery for the middle cranial fossa approach by using sagittal reconstructed temporal bone computed tomography images.

STUDY DESIGN

Comparison of anatomic measurements on random high-resolution, reformatted computed tomography scans of the temporal bone.

METHODS

High-resolution computed tomography of 10 normal temporal bones in the axial and coronal planes was obtained, and two-dimensional sagittal reconstructions were performed using a commercial software program. Eight anatomical relationships between neural and/or vascular structures were measured. Representative images were inverted to recreate the plane of the middle cranial fossa approach.

RESULTS

Anatomical relationships among the vestibule, superior semicircular canal, internal auditory canal, internal carotid artery, and middle cranial fossa exhibited a high SD in the 10 subjects. The sample size and the large range for the eight anatomical relationships precluded the detection of a significant difference between right and left temporal bones or sex and age of the patient.

CONCLUSION

The present report presents a novel, practical measurement protocol for rapidly evaluating important individual anatomical differences in patients before middle cranial fossa surgery. Inverted sagittal reconstructions facilitate presurgical planning for the middle cranial fossa approach by 1) assessing critical anatomical relationships before surgery and 2) providing customized measurements between vital landmarks and the first in vivo measurements. This decreases the likelihood of surgical mishaps and improves teaching by providing the first in vivo measurements of practical anatomical relationships in the sagittal plane.