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.