The porcine model for intratemporal facial nerve trauma studies DAVID M. BARRS, MD, CREIGHTON J. TRAHAN, DVM, KENNETH CASEY, MD, and DANIEL BROOKS, MD. San Antonio, Texas, Bethesda, Maryland, Farmington, Connecticut, and Aurora, Colorado
Facial nerve anatomy was compared in the rat, rabbit, cat, and pig in an effort to develop a model for facial nerve trauma within the temporal bone. The porcine model was found to have the most suitable anatomy. landmarks for the nerve were excellent. The pig had a definite facial nerve mastoid segment that was vertical, as in the human, and long enough to allow for performance of sequential procedures on the nerve. It was also large enough for grafting and electrical testing. A detailed description of the advantages of the pig model and the anatomy of the surgical approach to the facial nerve in the porcine model is presented. [OTOLARYNGOL HEAD NECKSURG 1991;105:845.)
M o s t information and treatment recommendations for facial nerve trauma have been derived from human facial nerve clinical studies, from research on the extratemporal facial nerve in animals, or from extrapolations of extremity peripheral nerve research. ‘-I5 This is also true for metabolic and regeneration studies, which show the peak of nerve regeneration occurring at approximately 21 days after injury. Temporal bone trauma, unfortunately, provides a unique environment of blood, disrupted air cells, mucosa, and bone that surrounds the traumatized facial nerve. Little experimental work has been done on trauma to the true intratemporal portion of the facial nerve. In an effort to remedy this situation, an animal model that would meet several specific criteria for intratemporal facial nerve trauma was sought. First, the mastoid segment of the facial nerve needed to be long enough to allow conductance of several procedures on the intratemporal facial nerve. Because earlier research of
From Otologic Associates and The University of Texas Health Science Center at San Antonio (Dr. Barrs); the Department of Laboratory Animal Medicine Uniformed Services, University of the Health Sciences (Dr. Trahan); the Division of Neurosurgery (Dr. Casey), University of Connecticut; and the Department of Pathology (Dr. Brooks), Fitzsimons Medical Center, Aurora. The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the United States Army or the Department of Defense. Presented at the Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery, San Diego, Calif., Sept. 913, 1990. Received for publication Jan. 9, 1991; accepted March 21, 1991. Reprint requests: David M. Barrs, MD, Otologic Associates, 771 1 Louis Pasteur, Suite 504, San Antonio, TX 78229. 231 1129903
delayed grafting of transected facial n e r ~ e ~ . ’ ~has ”’ demonstrated extensive scarring in the proximal stump, the mastoid segment of the facial nerve needed to be at least several millimeters long to allow proximal resection to viable nerve tissue. Second, access to the mastoid segment of the facial nerve had to be achieved without injuring the external auditory canal, the tympanic membrane, the labyrinth, or other otologic structures. Damage to any of these would greatly increase the risk of infection or would preclude comfortable, long-term survival of the animal. Third, it was recognized that the donor nerve used for grafting to the traumatized facial nerve should be comparably sized. The loss of this donor nerve could not seriously affect the function of the animal. Fourth, the animal needed to be large enough to allow needle electrode monitoring of facial nerve evoked electromyography (EEMG) and compound nerve action potential ( N A P ) . Finally, prominent bony landmarks were required for locating and delineating facial nerve buried in scar tissue, as is always the case in delayed reparative surgery subsequent to facial nerve trauma. Using these criteria, the authors considered the pig the best animal model for intratemporal facial nerve trauma. This article describes the use and benefits of the porcine model for intratemporal facial nerve trauma. It describes disadvantages of several other animal models. ANIMAL SUBJECTS
The first part of this study was a preliminary investigation to determine the most appropriate species for intratemporal facial nerve trauma studies. The second part of the study involved use of the porcine animal model for a study of delayed grafting of transected facial nerves. The anatomic and histologic descriptions, a45
OtolaryngologyHead and Neck Surgery
846 BARRS et a1
PIG
A
1 RABBIT
Fig. 1. A, Schematic drawing of the long vertical mastoid segment of the facial netve in the pig compared to the rabbit facial nerve, which curves almost directly from the parotid to the tympanic segment. 6, lntraoperative view of the right rabbit facial nerve (thick white mows) curving directly from the parotid to the area of the middle ear and stapes (thin white arrow].
measurements, and electrophysiologic readings described in this article were taken from the control side (nongrafted side) during this latter study. Both phases of this study were conducted in a facility accredited by the American Association for Accreditation of Laboratory Animal Care, and in strict accordance with the National Institutes of Health Guide for the C m e crnd
Use of Luboralot-y Aiiitmds'n and Aiiiinnl Welfore Act, (7 USC 2131-2157) (PL 89-544, 91-597, 94-279, and 99-198. as amended). For studies in novel research areas, selection of an appropriate animal model should be from the lowest phylogenetic species deemed capable of meeting the research needs.18 The first animal considered was the
Volume 105 Number 6 December 1991
Porcine model for intratemporal facial nerve trauma studies 847
Fig. 2. Placementof postauricularand upper cervical incision is shown (black arrows]. Eye and snout electrodes (white arrows) are placed approximately 8 cm apart for EEMG monitoring. rat, but this animal was deemed inappropriate because its diminutive size did not allow for multiple surgical procedures. The rabbit, which was the second animal model to be evaluated, satisfied the majority of the criteria listed. While under general anesthesia, five rabbits, weighing 5 kilograms (kg) each, were subjected to terminal surgical exploration of the parotid, mastoid, and tympanic facial nerve. Despite a relatively narrow facial nerve ( 1 mm average), identification and electrophysiologic monitoring of the nerve were satisfactory. The facial nerve curved, however, in a transverse plane, directly from the parotid to the tympanic segment, to lie immediately under the superior portion of the tympanic membrane (Fig. 1 , A and B ) . There was no discrete vertical segment of the nerve that might correspond to the mastoid segment in the human. Because of this horizontal orientation, and the lack of a true mastoid segment, it was decided that multiple operations on the true intratemporal nerve could not be completed without risk of damage to the tympanic membrane or to the labyrinth.
A study of induced facial nerve paralysis in the cat. by Greer et al. ,I'' prompted investigation of this animal as a possible suitable model. Dissection of a cat cadaver, however, revealed anatomy similar to that of the rabbit. Five pigs. weighing 25 to 50 kg each. were evaluated in a similar fashion. The facial nerve of this species was easily identified in the parotid and mastoid because of a very large styloid process, and a constant, prominent stylohyoid ligament that inserted just superior to the stylomastoid foramen.?" The long vertical mastoid segment was of sufficient length and appropriate orientation to allow multiple procedures and grafting. EEMG was accomplished without difficulty. Initially, the primary problem in the pig was the lack of identification and designation of an adequate donor sensory nerve for grafting. The greater auricular nerve was always sacrificed during exposure to the facial nerve. Although available for immediate grafting. this transection eliminated the use of the greater auricular nerve in any type of delayed-repair grafting. The surd nerve
848 ELWRS et 01.
OtolaryngologyHead and Neck Surgery
Fig. 3. The superficial dissection and retraction of the anterior flap (fbick white arrows) expose the cervical musculature and stylohyoid tendon (tbin white arrows).
Fig. 4. Deeper dissection exposes the mastoid (M), tip of the styloid process (cuwed arrow), and the stylohyoid tendon (thin arrows) inserting into the skull base. The facial nerve (thick arrow) exits the stylomastoid foramen just inferior to the insertion of the stylohyoid tendon. The facial nerve is further demonstrated by the surgical instrument just anterior to the main trunk.
Volume 105 Number 6 December 1991
Porcine model for intratemporal facial nerve trauma studies 849
Fig. 5. The facial nerve (black arrows) is completely exposed in the mastoid segment and further dissected into the parotid, showing the first major bifurcation (thick white arrow). The external auditory canal [thin white arrow) anterior to the facial newe is purposely opened in this specimen to show the proximity of the external auditory canal to the mastoid facial nerve. even its most superior portion. The facial nerve will dive rapidly at the mastoid genu. to lie medial to the tympanic membrane in the tympanic segment of the facial nerve.
Fig. 6. Photomicrograph of a typical proximal facial nerve at the tympanic segment. The facial newe is compact with few discreet fascicles (Masson trichrorne stain; original magnification x 32.)
850 W R S e t a1
OtolaryngologyHead and Neck Surgery
Fig. 7. Photomicrographof the focial nerve (surrounded by arrows] in the distal mastoid segment
shows multiple discreet fascicles, chorocteristic of the nerve ot this level (Masson trichrome stain; original magnification x 32.)
was found to be smaller than the facial nerve. and often proved difficult to locate and identify. The posterior tibial nerve was approximately the same as the facial nerve, although the posterior tibial nerve was often in two discrete bundles. Stimulation of the exposed distal posterior tibial nerve, with 0.2 to 0.5 milliampere (mA) electrical current, elicited plantar flexion of the toes. Because of the minimal disruption of normal locomotor activities that was caused by sacrifice of this particular nerve, the lower section of the posterior tibial nerve could be safely harvested and used for grafting purposes. Porcine Facial Nerve Anatomy
Twenty 3-month-old (20- to 40-kg each) male, inbred micropigs (Yucatan Micropigs, Charles River Laboratories, Inc., Fort Collins, Colo.) were selected for the study because of their size and ease of handling. The left side was used for a grafting study (reported elsewhere”); the right (or control side) was used for the photographs and measurements that are presented in this study. The orientation of the swine external auditory canal is in a superior-to-inferior direction, whereas that of the human is in the lateral-to-medial direction. The mastoid segment of the facial nerve is always lateral
to the tympanic membrane annulus. An approach to the horizontal segment of the facial nerve therefore risks entry into the external auditory canal. with subsequent contamination of the research-induced facial nerve trauma site. Because of this, the pig is recommended as a suitable model for intratemporal facial nerve trauma to the mastoid segment, as opposed to trauma induced in the tympanic segment. The porcine facial nerve anatomy. therefore, will be presented in two portions. The first will be that of an approach to the mastoid segment, whereas the second will be of a terminalprocedure approach to the tympanic segment of the facial nerve. Approach to the Mastoid Segment
A postauricular and upper cervical incision is made through the skin and the thick layer of subcutaneous fat (Fig. 2 ) . The superficial cervical muscles are exposed and the anterior cervical flap reflected forward (Fig. 3). At this point the very prominent styloid process, which is easily palpated, acts as a guide for further dissection. The bright white stylohyoid tendon. which is much more prominent than in the human (Fig. 3), is the major landmark for locating the facial nerve. The facial nerve lies just inferior to the insertion of this tendon at the skull base. Deeper dissection allows com-
Volume 105 Number 6 December 1991
Porcine model for intratemporal facial nerve trauma studies 851
PAROT 10 FAC IAL NERVE
TRANSECTED
EAR CANAL TEMPORALIS
BONE
MUSC LE
STY LOHYOID TENDON
FAT
A
SUPERFICIAL I)ISSE
Facial nerve anatomy was compared in the rat, rabbit, cat, and pig in an effort to develop a model for facial nerve trauma within the temporal bone. The porcine model was found to have the most suitable anatomy. landmarks for the nerve were excellent. The pig had a definite facial nerve mastoid segment that was vertical, as in the human, and long enough to allow for performance of sequential procedures on the nerve. It was also large enough for grafting and electrical testing. A detailed description of the advantages of the pig model and the anatomy of the surgical approach to the facial nerve in the porcine model is presented. [OTOLARYNGOL HEAD NECKSURG 1991;105:845.)
M o s t information and treatment recommendations for facial nerve trauma have been derived from human facial nerve clinical studies, from research on the extratemporal facial nerve in animals, or from extrapolations of extremity peripheral nerve research. ‘-I5 This is also true for metabolic and regeneration studies, which show the peak of nerve regeneration occurring at approximately 21 days after injury. Temporal bone trauma, unfortunately, provides a unique environment of blood, disrupted air cells, mucosa, and bone that surrounds the traumatized facial nerve. Little experimental work has been done on trauma to the true intratemporal portion of the facial nerve. In an effort to remedy this situation, an animal model that would meet several specific criteria for intratemporal facial nerve trauma was sought. First, the mastoid segment of the facial nerve needed to be long enough to allow conductance of several procedures on the intratemporal facial nerve. Because earlier research of
From Otologic Associates and The University of Texas Health Science Center at San Antonio (Dr. Barrs); the Department of Laboratory Animal Medicine Uniformed Services, University of the Health Sciences (Dr. Trahan); the Division of Neurosurgery (Dr. Casey), University of Connecticut; and the Department of Pathology (Dr. Brooks), Fitzsimons Medical Center, Aurora. The opinions and assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the United States Army or the Department of Defense. Presented at the Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery, San Diego, Calif., Sept. 913, 1990. Received for publication Jan. 9, 1991; accepted March 21, 1991. Reprint requests: David M. Barrs, MD, Otologic Associates, 771 1 Louis Pasteur, Suite 504, San Antonio, TX 78229. 231 1129903
delayed grafting of transected facial n e r ~ e ~ . ’ ~has ”’ demonstrated extensive scarring in the proximal stump, the mastoid segment of the facial nerve needed to be at least several millimeters long to allow proximal resection to viable nerve tissue. Second, access to the mastoid segment of the facial nerve had to be achieved without injuring the external auditory canal, the tympanic membrane, the labyrinth, or other otologic structures. Damage to any of these would greatly increase the risk of infection or would preclude comfortable, long-term survival of the animal. Third, it was recognized that the donor nerve used for grafting to the traumatized facial nerve should be comparably sized. The loss of this donor nerve could not seriously affect the function of the animal. Fourth, the animal needed to be large enough to allow needle electrode monitoring of facial nerve evoked electromyography (EEMG) and compound nerve action potential ( N A P ) . Finally, prominent bony landmarks were required for locating and delineating facial nerve buried in scar tissue, as is always the case in delayed reparative surgery subsequent to facial nerve trauma. Using these criteria, the authors considered the pig the best animal model for intratemporal facial nerve trauma. This article describes the use and benefits of the porcine model for intratemporal facial nerve trauma. It describes disadvantages of several other animal models. ANIMAL SUBJECTS
The first part of this study was a preliminary investigation to determine the most appropriate species for intratemporal facial nerve trauma studies. The second part of the study involved use of the porcine animal model for a study of delayed grafting of transected facial nerves. The anatomic and histologic descriptions, a45
OtolaryngologyHead and Neck Surgery
846 BARRS et a1
PIG
A
1 RABBIT
Fig. 1. A, Schematic drawing of the long vertical mastoid segment of the facial netve in the pig compared to the rabbit facial nerve, which curves almost directly from the parotid to the tympanic segment. 6, lntraoperative view of the right rabbit facial nerve (thick white mows) curving directly from the parotid to the area of the middle ear and stapes (thin white arrow].
measurements, and electrophysiologic readings described in this article were taken from the control side (nongrafted side) during this latter study. Both phases of this study were conducted in a facility accredited by the American Association for Accreditation of Laboratory Animal Care, and in strict accordance with the National Institutes of Health Guide for the C m e crnd
Use of Luboralot-y Aiiitmds'n and Aiiiinnl Welfore Act, (7 USC 2131-2157) (PL 89-544, 91-597, 94-279, and 99-198. as amended). For studies in novel research areas, selection of an appropriate animal model should be from the lowest phylogenetic species deemed capable of meeting the research needs.18 The first animal considered was the
Volume 105 Number 6 December 1991
Porcine model for intratemporal facial nerve trauma studies 847
Fig. 2. Placementof postauricularand upper cervical incision is shown (black arrows]. Eye and snout electrodes (white arrows) are placed approximately 8 cm apart for EEMG monitoring. rat, but this animal was deemed inappropriate because its diminutive size did not allow for multiple surgical procedures. The rabbit, which was the second animal model to be evaluated, satisfied the majority of the criteria listed. While under general anesthesia, five rabbits, weighing 5 kilograms (kg) each, were subjected to terminal surgical exploration of the parotid, mastoid, and tympanic facial nerve. Despite a relatively narrow facial nerve ( 1 mm average), identification and electrophysiologic monitoring of the nerve were satisfactory. The facial nerve curved, however, in a transverse plane, directly from the parotid to the tympanic segment, to lie immediately under the superior portion of the tympanic membrane (Fig. 1 , A and B ) . There was no discrete vertical segment of the nerve that might correspond to the mastoid segment in the human. Because of this horizontal orientation, and the lack of a true mastoid segment, it was decided that multiple operations on the true intratemporal nerve could not be completed without risk of damage to the tympanic membrane or to the labyrinth.
A study of induced facial nerve paralysis in the cat. by Greer et al. ,I'' prompted investigation of this animal as a possible suitable model. Dissection of a cat cadaver, however, revealed anatomy similar to that of the rabbit. Five pigs. weighing 25 to 50 kg each. were evaluated in a similar fashion. The facial nerve of this species was easily identified in the parotid and mastoid because of a very large styloid process, and a constant, prominent stylohyoid ligament that inserted just superior to the stylomastoid foramen.?" The long vertical mastoid segment was of sufficient length and appropriate orientation to allow multiple procedures and grafting. EEMG was accomplished without difficulty. Initially, the primary problem in the pig was the lack of identification and designation of an adequate donor sensory nerve for grafting. The greater auricular nerve was always sacrificed during exposure to the facial nerve. Although available for immediate grafting. this transection eliminated the use of the greater auricular nerve in any type of delayed-repair grafting. The surd nerve
848 ELWRS et 01.
OtolaryngologyHead and Neck Surgery
Fig. 3. The superficial dissection and retraction of the anterior flap (fbick white arrows) expose the cervical musculature and stylohyoid tendon (tbin white arrows).
Fig. 4. Deeper dissection exposes the mastoid (M), tip of the styloid process (cuwed arrow), and the stylohyoid tendon (thin arrows) inserting into the skull base. The facial nerve (thick arrow) exits the stylomastoid foramen just inferior to the insertion of the stylohyoid tendon. The facial nerve is further demonstrated by the surgical instrument just anterior to the main trunk.
Volume 105 Number 6 December 1991
Porcine model for intratemporal facial nerve trauma studies 849
Fig. 5. The facial nerve (black arrows) is completely exposed in the mastoid segment and further dissected into the parotid, showing the first major bifurcation (thick white arrow). The external auditory canal [thin white arrow) anterior to the facial newe is purposely opened in this specimen to show the proximity of the external auditory canal to the mastoid facial nerve. even its most superior portion. The facial nerve will dive rapidly at the mastoid genu. to lie medial to the tympanic membrane in the tympanic segment of the facial nerve.
Fig. 6. Photomicrograph of a typical proximal facial nerve at the tympanic segment. The facial newe is compact with few discreet fascicles (Masson trichrorne stain; original magnification x 32.)
850 W R S e t a1
OtolaryngologyHead and Neck Surgery
Fig. 7. Photomicrographof the focial nerve (surrounded by arrows] in the distal mastoid segment
shows multiple discreet fascicles, chorocteristic of the nerve ot this level (Masson trichrome stain; original magnification x 32.)
was found to be smaller than the facial nerve. and often proved difficult to locate and identify. The posterior tibial nerve was approximately the same as the facial nerve, although the posterior tibial nerve was often in two discrete bundles. Stimulation of the exposed distal posterior tibial nerve, with 0.2 to 0.5 milliampere (mA) electrical current, elicited plantar flexion of the toes. Because of the minimal disruption of normal locomotor activities that was caused by sacrifice of this particular nerve, the lower section of the posterior tibial nerve could be safely harvested and used for grafting purposes. Porcine Facial Nerve Anatomy
Twenty 3-month-old (20- to 40-kg each) male, inbred micropigs (Yucatan Micropigs, Charles River Laboratories, Inc., Fort Collins, Colo.) were selected for the study because of their size and ease of handling. The left side was used for a grafting study (reported elsewhere”); the right (or control side) was used for the photographs and measurements that are presented in this study. The orientation of the swine external auditory canal is in a superior-to-inferior direction, whereas that of the human is in the lateral-to-medial direction. The mastoid segment of the facial nerve is always lateral
to the tympanic membrane annulus. An approach to the horizontal segment of the facial nerve therefore risks entry into the external auditory canal. with subsequent contamination of the research-induced facial nerve trauma site. Because of this, the pig is recommended as a suitable model for intratemporal facial nerve trauma to the mastoid segment, as opposed to trauma induced in the tympanic segment. The porcine facial nerve anatomy. therefore, will be presented in two portions. The first will be that of an approach to the mastoid segment, whereas the second will be of a terminalprocedure approach to the tympanic segment of the facial nerve. Approach to the Mastoid Segment
A postauricular and upper cervical incision is made through the skin and the thick layer of subcutaneous fat (Fig. 2 ) . The superficial cervical muscles are exposed and the anterior cervical flap reflected forward (Fig. 3). At this point the very prominent styloid process, which is easily palpated, acts as a guide for further dissection. The bright white stylohyoid tendon. which is much more prominent than in the human (Fig. 3), is the major landmark for locating the facial nerve. The facial nerve lies just inferior to the insertion of this tendon at the skull base. Deeper dissection allows com-
Volume 105 Number 6 December 1991
Porcine model for intratemporal facial nerve trauma studies 851
PAROT 10 FAC IAL NERVE
TRANSECTED
EAR CANAL TEMPORALIS
BONE
MUSC LE
STY LOHYOID TENDON
FAT
A
SUPERFICIAL I)ISSE
