Topographic Facial Nerve Transplantation

Presentation This is a clinical report describing the case of performing a topographically correct sural nerve transplantation in the extracranial facial nerve stem using the intraneural facial nerve stem topography. Proposed by Meissl in 1979, this is the first case of successful fascicular nerve grafting of the facial nerve stem following extensive laceration. It was performed on a young male patient who had suffered a deep traumatic soft tissue laceration to the left preauricular area because of a high velocity car accident. In the surgery performed immediately upon presentation, the facial nerve stump could not be identified at the stylomastoid foramen and was therefore not reconstructed. Subsequently, he suffered complete total left facial nerve paralysis. Three days after the initial presentation, when the swelling of the trauma area had subsided, a high resolution ultrasound was conducted and the transected facial nerve stump was located at the stylomastoid foramen, confirming the indication of secondary facial nerve reconstruction. Surgical exploration identified the distal facial nerve stumps within the parotid gland. There was a significant gap between the nerve stumps due to trauma mechanism. A tension-free direct nerve repair could not be deduced since the gap extended from the extracranial nerve stem to the pes anserinus of the facial nerve within the parotid gland.


IDEAS AND INNOVATIONS
2 of 3 expected to identify the nerve stumps in a second-look surgery which would allow ipsilateral repair instead of a cross-face nerve graft, which would be the alternate option in case of failure to repair the transected facial nerve.
We performed a fascicular repair of the proximal facial nerve stump at the stylomastoid foramen using five autologous grafts from the sural nerve of approximately 6 cm length ( Figure 2). They were arranged according to the topography proposed by Meissl for the restoration of distal continuity of the facial nerve.

Outcome
Follow-ups were conducted after eight months and four years post trauma. Both showed excellent clinical outcomes with good symmetric mimic muscle contractions and minimalsynkinesias ( Figure 3, Figure 4, Video 1: https://youtu. be/doppARxcr5k). The Sunnybrook Facial Grading System [2] scores at the initial presentation and at the time of follow-up after eight and 48 months are summarized in Table 1. The patient was satisfied with the result and there was no need for further corrective interventions. In this way, additional procedures like nerve or muscle grafting, as well as myectomy or botulinum toxin injection were avoided.

DISCUSSION
Direct tension-free nerve coaptation is generally preferred in nerve repair surgery [3,4]. In this case, nerve coaptation was not a viable option because of the extent of the nerve gap created by the laceration and the initially unlocated proximal nerve stump. The typical reconstructive approach in this case would have been a cross-facial nerve grafting to provide innervation from the contralateral facial nerve or the ipsilateral extra-facial nerve transfer to a free muscle graft [5][6][7]. In contrast to these methods, we restored the physiological muscle innervation by direct fascicular nerve repair of the proximal and distal facial nerve stumps [8][9][10]. Although other authors have reported on the fascicular structure of the facial nerve [11,12], Meissl's contribution is unique as it describes the method of correlating the fascicles of the nerve stem with target muscles. However, his description of the facial nerve intraneural topography is only a rough outline of the mimic muscle innervation pattern. Hence the procedure of topographic facial nerve transplantation carries the risk of mass movements, aberrant innervations and synkineses. Although these risks may be manageable with additional corrective interventions, further preclinical studies are necessary to refine and improve the knowledge of intraneural topography of the facial nerve.

CONCLUSION
This is the first clinical report that describes the application of Meissl's findings on the intraneural topography of the extracranial facial nerve. We were able to successfully reconstruct a facial nerve stump at the stylomastoid foramen according to the topographic pattern as proposed by him. The clinical out-

IDEAS AND INNOVATIONS
comes were excellent as was evident in the follow-up examination after four years of surgery. However, current knowledge on the topography of the facial nerve is still coarse and its clinical use restricted to seldom events. Therefore, we plan further preclinical research to reinforce Meissl´s findings and refine the topography of the fascicular architecture of the facial nerve.