Facial Nerve Localization: Is Triangulation the Key?
Royer MC1, Moore MG1, Cordes S1, Weisberger E1, Kokoska MS1,2
1Department of Otolaryngology-Head and Neck Surgery, Indiana University School of Medicine, Indianapolis, IN 2Otolaryngology-Head & Neck Surgery, Richard L. Roudebush VA Medical Center, Indianapolis, IN
INTRODUCTION
METHODS AND MATERIALS
CONCLUSIONS
DISCUSSION RESULTS
REFERENCES
Figure 3A. Facial nerve
(arrow) identified using
planar intersection.
Figure 3B. Identifying planar
intersection of the TM suture
and DM.
ABSTRACT
Acknowledgement:
The authors would like to acknowledge
Ms. Gudrun Carlson for her outstanding
work on the medical illustrations.
Objectives: To offer a novel
framework that provides residents and
practitioners with a reliable and
surgically relevant method for
identifying the facial nerve trunk
(CNVII) after exiting the skull base
during parotidectomy and to describe
the effect on Otolaryngology
Residents’ dissection time, accuracy
and confidence when taught this
technique compared to traditional
methods and their baseline
competency.
Methods: The currently available
literature and textbooks do not
completely address the challenges
trainees encounter in learning how to
identify CNVII during parotidectomy. In
response to this gap in the literature
and potential associated deficits in
surgical training, we developed an
approach that integrates multiplanar
intersections (based on mathematical
principles) with surgical anatomy to
create a reliable method for CNVII
identification during parotidectomy.
This method was then taught to
residents randomly divided into groups
and their confidence, accuracy and
dissection times were subsequently
recorded.
Results: The multiplanar intersections
shown via our medical illustrations and
surgical photography demonstrate the
application of the triangulation concept
to improve the accuracy and efficiency
in surgical localization of CNVII. The
prospective arm showed no significant
difference in cadaveric dissection time,
but did result in a subjective increase
in confidence for safely, efficiently and
accurately identifying the facial nerve.
Conclusions: The integration of
multiplanar localization with surgical
anatomy provides a reliable method
for a surgeon to consistently and
rapidly identify the CNVII, which
inherently reduces the risk for
inadvertent injury to the CNVII. While
no improvement in dissection time was
found, the increase in confidence
afforded by this technique argues for
incorporating this method into the early
training of surgeons.
One can visualize imaginary planes formed by both the Tympano-
Mastoid (TM) suture and Posterior Digastric Muscle (DM), (Figure 2).
The 2-dimensional plane which lies on the lateral surface of the DM
forms the deep plane (or floor, in a sagittal orientation), localizing the
depth for the CN VII trunk. A second plane, which lies in the plane (in an
axial orientation) of the TM suture, forms an intersecting plane to the DM
plane. The resulting intersection forms a line. This line invariably
corresponds to the location of the facial nerve trunk. Because the CN VII
trunk is a linear structure corresponding to a line formed by the
intersecting planes, only two intersecting reference planes are needed to
localize the CNVII trunk. If we were interested in a specific point along
the nerve then a third intersecting anatomic plane could be added.
A dissected facial nerve during a standard parotidectomy is seen in
Figure 3A. The tines of the angled nerve dissector are sitting within the
TM suture and the DM is visualized. Figure 3B shows the imaginary non-
parallel planes formed by these landmarks and the resulting intersection
(resultant line) overlying the main trunk of the facial nerve. Reviewed
textbooks provide a general region for finding the facial nerve, which is
non-specific (a neighborhood) and no targeted localization can be
extrapolated. Additionally, several articles use radiological data to
describe anatomical relationships between CN VII and nearby
structures. Many of these studies use CT and MRI localization of CN VII
by measuring distances from reference points5,6. However, these studies
are not practical from a surgeon’s perspective, because the reference
points are not usually accessible in the surgical field.
The literature and common Otolaryngology textbooks were
reviewed to assess the reported methods that are used to
teach surgical localization the facial nerve trunk. As a result of
the identified lack of a precise and reproducible surgically
relevant method in publications for locating the CNVII during
parotidectomy, the attached schematics were developed
(Figures 2 & 3, description in discussion). IRB approval was
then obtained to perform a prospective study of
Otolaryngology Residents attending a Head and Neck
Anatomy Course. Otolaryngology residents were randomly
divided into two groups. Both groups received CNVII
localization lecture and prosection instruction on traditional
approaches prior to their initial dissections. Prior to their
second dissection on the contralateral side, The control group
received traditional method instruction again, whereas the
experimental group received the triangulation method
instruction. A survey was completed by the participant pre-
dissection and post-dissection assessing various aspects of
the dissection method (see results), including their confidence
at reliably identifying CNVII and their time to identify the CNVII
was measured before and after the instruction.
We should optimize how we teach and learn surgical
approaches and techniques to ensure efficiency,
consistency and accuracy. The application of triangulation
(or multiplanar intersection) to surgical approaches is a
novel concept. Our method of targeted localization using
the intersecting planes of the tympanomastoid suture and
posterior digastric muscle is a unique, efficient,
reproducible, and understandable technique to teach
Otolaryngology residents and fellows how to reliably
localize the CN VII. While no increase in the speed of
dissection was found, residents did benefit from an
increase in confidence of reliably finding CNVII when
compared to controls.
The potential complications of parotidectomy are well-
known and widely reported1,2. These complications range
from gustatory sweating, periauricular numbness,
infection and hematoma, to severely debilitating facial
paralysis resulting from injury to the facial nerve (CN VII)1-
3.
Although retrograde dissection of a CNVII branch is an
option to locate the nerve trunk, most otolaryngologists
prefer to identify the nerve initially at the trunk, just distal
to its exit from the stylomastoid foramen. The reasons for
this preference include the small caliber of the distal nerve
branches (which increases its risk for injury during
dissection) and a reduction in the length of unnecessary
dissection along the nerve branch (most parotid masses
are located proximal to the distal ends of the nerves).
Identification and careful dissection of the CN VII nerve
trunk early in the procedure is critical to preventing injury.
We offer a reliable and efficient technique of targeted
localization of the CN VII trunk, which in turn will increase
the confidence of a surgeon during parotid surgery in
general, and parotidectomy, in particular.
1. Cummings CW, et al. Otolaryngology-Head and Neck Surgery, 4th Edition.
St. Louis: Mosby, Inc, 2005.
2. Bailey BJ, Johnson JT, Newlands SD. Head and Neck Surgery-
Otolaryngology, 4th Edition. Philadelphia: Lippincott Williams & Wilkins,
2006.
3. Robertson MS, Blake P. A method of using the tympanomastoid fissure to
find the facial nerve at parotidectomy. Aust. N.Z. J. Surg. 1984; 54:369-373.
4. Lore JM, An Atlas of Head and Neck Surgery, 3rd Edition. Philadelphia: W.B.
Saunders, 1988.
5. Pather N, Osman M. Landmarks of the facial nerve: implications for
parotidectomy. Surg Radiol Anat. 2006; 28(2):170-5.
6. De Ru JA, et al. The location of parotid gland tumors in relation to the facial
nerve on magnetic resonance. Journal of Oral and Maxillofacial Surg. 2002;
60(9):992-995.
X = X0 + t a
The dissection time after receiving instruction for the experimental and control groups
was not significantly different (experimental group improvement: 5.4 minutes; control
group improvement: 5.8 minutes).
Figure 1 displays a graphical representation of the positive effect seen after residents
received teaching in the triangulation method of localizing CNVII.
Residents reported increases in their agreement with all the statements on the
questionnaires. The statements/questions were as follows:
Q1-I am confident I can accurately find CNVII.
Q2-This technique is easily to understand.
Q3-This technique is easy to perform.
Q4-I am confident I can reliably find CNVII.
Q5-I am confident I can quickly find CNVII.
Questions 3-5 showed a statistically significant percentage increase in
agreement/confidence (those receiving only traditional teaching). Question 4 showed
a statistically significant increase greater than the control group, while all other
questions showed no significant difference when compared to the control group.
Figure 2. Artist’s illustration depicting
localization of the facial nerve using the
intersecting planes of the TM suture and DM.
1
2
3
Q1 Q2 Q3 Q4 Q5
Str
on
gly
Dis
ag
ree
(1
) to
Str
on
gly
Ag
ree
(5
)
Pre-Instruction
Post-Instruction
Figure 1. Graph showing the increase in
agreement/confidence before and after receiving
the triangulation method training. The increase
for questions 3-5 were statistically significant.