American Society for Peripheral Nerve

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Correlation of the Axonal Capacity with the Microscopic Diameter of Relevant Facial Nerve Branches for Facial Reanimation in 788 Specimens: A Microscopic and Histologic Study
Marc Ruewe, Medical Student1; Veronika Mandlik, MD1; Simon Udo Engelmann, Medical Student1; Christian Taeger, MD1; Lukas Prantl, Prof 1; Ernst Tamm, Prof1, Ronald Bleys, Prof2; Andreas Kehrer, MD3
1University of Regensburg, Regensburg, Germany, 2University Medical Center (UMC) Utrecht, Utrecht, Netherlands, 3Department of Plastic, Hand, and Reconstructive Surgery; University Hospital Regensburg, Regensburg, Germany

Background
Peripheral facial palsy causes severe functional, aesthetic and psychological impairments. Restoration of function and facial expressions is possible through cross-face-nerve-grafts (CFNG) and functional muscle transplantation. Various criteria must be taken into account to choose the right donor nerve for coaptation. Previous studies have shown that an axon capacity exceeding 900 axons is correlated with strong functional results. The aim of our study is to correlate axon numbers with the diameter of zygomatic and buccal facial nerve branches to facilitate donor nerve selection.

Methods
Antegrade microsurgical dissection was performed on 106 hemifaces of fresh unpreserved cadavers. Nerve biopsies were taken in clinically important donor nerve regions of the zygomatic and buccal system. Level I and level II branches were classified as relevant for CFNG coaptation. Nerves were PPD-fixed, sectioned, and stained for digital semi-automated axon quantification. Cross sections were measured by two orthogonal vectors using Zeiss AxioVision software which was first calibrated with a micrometer scale. Nerves in situ were presumed to be round. The measurement included nerve sheath structures and perineurium comparable to a clinical setting.

Results
A total number of 788 branches were evaluated. Quality standards for semi-automated axon analysis and diameter measurement were met by 495 specimens. A diameter of one millimeter correlated with 1834 693 axons (n= 68; r= 0.66; p= 0.0001) in major zygomatic branches and for major buccal branches 1851 913 axons (n= 58; r= 0.37; p= 0.004). In downstream zygomatic branches 1067 531 axons / mm (n= 161; r= 0.61; p= 0.0001) could be found. Downstream buccal branches showed 1208 530 axons / mm (n= 208; r= 0.58; p= 0.0001). Axonal density in the buccal system was consequently higher than in the zygomatic system (p= 0.006). Overall axon density decreased from the facial main trunk to the periphery.

Conclusion
In order to achieve a cut-off value of greater than 900 axons, the donor nerve branch in the zygomatic system is required to have a diameter of a minimum of 0.84 mm, and the buccal branches are required to have a diameter of a minimum of 0.74 mm.


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